Effects of Trees on the Safety of Dams

Effects of Trees on the Safety of Dams
A TECHNICAL MANUAL ON THE
EFFECTS OF TREE AND WOODY
VEGETATION ROOT PENETRATIONS
ON THE SAFETY OF EARTHEN DAMS
A TECHNICAL MANUAL ON THE
EFFECTS OF TREE AND WOODY
VEGETATION ROOT PENETRATIONS
ON THE SAFETY OF EARTHEN DAMS
EDITED BY:
Dr. B. Dan Marks, P.E.
Dr. Bruce A. Tschantz, P.E.
COMPILED BY:
Marks Enterprises of NC, PLLC
One Palatka Drive
Arden, North Carolina 28704
(828) 684 – 9804
December 2002
DISCLAIMER
Publication of this Manual was made possible by a Federal Emergency Management
Agency (FEMA) funded Project administered by the Association of State Dam
Safety Officials (ASDSO) through a Contract with Marks Enterprises of NC, PLLC.
Although the publication of this Manual has been financed by federal funds, the
presentation of methods, procedures, products, techniques, and trade names in this
Manual does not constitute endorsement by FEMA, ASDSO, Marks Enterprises of
NC, PLLC, or the authors (editors) of this Manual.
TABLE OF CONTENTS
Page
Preface
i
Acknowledgements
ii
Glossary
v
Chapter 1:
Introduction
1-1
Chapter 2:
Problems with Tree and Woody
Vegetation Growth
2-1
Chapter 3:
Tree Growth and Tree Root
Development Requirements
3-1
Chapter 4:
Earthen Dam Safety Inspection
and Evaluation Methodology
4-1
Chapter 5:
Controlling Tree and Woody
Vegetation Growth on Earthen Dams
5-1
Chapter 6:
Dam Remediation Design Considerations
6-1
Chapter 7:
Economics of Proper Vegetation
Maintenance
7-1
PREFACE
Damage to earthen dams and dam safety issues associated with tree and woody
vegetation penetrations of earthen dams is all too often believed to be a routine
maintenance situation by many dam owners, dam safety regulators, and engineers.
Contrary to this belief, tree and woody vegetation penetrations of earthen dams and their
appurtenances have been demonstrated to be causes of serious structural deterioration and
distress that can result in failure of earthen dams. For the first time in the history of dam
safety, a Workshop on Plant and Animal Impacts on Earthen Dams was convened
through the joint efforts of the Association of State Dam Safety Officials (ASDSO) and
the Federal Emergency Management Agency (FEMA) in November 1999 to bring
together technical resources of dam owners, engineers, state and federal regulators,
wildlife managers, foresters, and academia having expertise in these areas of technology.
The ASDSO/FEMA Workshop highlighted the realization that damage to earthen dams
resulting from plant and animal penetrations was indeed a significant dam safety issue in
the United States. The purpose of this Technical Manual on the Effects of Tree and
Woody Vegetation Root Penetrations on the Safety of Earthen Dams is to convey
technology assembled through the ASDSO/FEMA Workshop by successful completion
of four objectives. These objectives are as follows:
1.
Advance awareness of the characteristics and seriousness of dam safety
problems associated with tree and woody vegetation growth impacts on earthen dams;
2.
Provide a higher level of understanding of dam safety issues associated with tree and
woody vegetation growth impacts on earthen dams by reviewing current damage control
policies;
3.
Provide state-of-practice guidance for remediation design considerations associated with
damages associated with tree and woody vegetation growth on earthen dams; and
4.
Provide rationale and state-of-practice techniques and procedures for management of
desirable and undesirable vegetation on earthen dams.
i
ACKNOWLEDGEMENTS
Development of A Technical Manual on the Effects of Tree and Woody Vegetation
Root Penetrations on the Safety of Earthen Dams is a success story relative to the
expenditure of federal funds for promotion of dam safety in the United States. The editors
of this Manual wish to acknowledge the support of involved dam safety organizations
and agencies, and the diligent work of many dedicated individuals that have made
significant contributions to the contents of this Manual.
Sincere appreciation is extended to past and present members of the Subcommittee on
Dam Safety Research of the Interagency Committee on Dam Safety (ICODS) for their
support of the proposal to convene a Workshop on Plant and Animal Impacts on
Earthen Dams, and to the members of ICODS for recommending the allocation of
federal funding for the Workshop through the Federal Emergency Management Agency
(FEMA).
Appreciation and sincere gratitude are extended to the members, and especially to the
full-time staff, of the Association of State Dam Safety Officials (ASDSO) for their
support and coordination of the federally-funded project that culminated with convening
of the Workshop on Plant and Animal Impacts on Earthen Dams at the University of
Tennessee in Knoxville, Tennessee during the period of November 30 through
December 2, 1999. The editors of this Manual are especially appreciative of the
continued support, patience, and dedication of Susan Sorrell and Sarah Mayfield of the
ASDSO staff. Without the dedication and hard work of these individuals the Workshop
on Plant and Animal Impacts on Earthen Dams could not have been achieved with the
quality and success evident in this historical gathering of individuals with expertise in
diverse technology from many disciplines directed toward the common interest of dam
safety in the United States.
ii
The Steering Committee of the ASDSO/FEMA Workshop on Plant and Animal
Impacts on Earthen Dams was comprised of the following individuals who contributed
significantly and diligently to make the Workshop a truly historical dam safety
technological event:
Dr. B. Dan Marks, P.E. (Chairman)
Charles Clevenger, MS (Deceased)
Dr. Bruce A. Tschantz, UT Knoxville
William L. Bouley, USBR
David K. Woodward, NCSU
Sarah M. Mayfield, ASDSO
Susan A. Sorrell, ASDSO (Project Coordinator)
Participants in the Workshop on Plant and Animal Impacts on Earthen Dams brought
together diverse technologies, experiences, and scientific developments to create a
significant contribution to dam safety in the United States. The editors of this Manual
acknowledge the valuable contributions of the following Workshop participants:
Matthew A. Barner, Wright State Univ.
Douglas E. McClelland, USDA Forest Service
William L. Bouley, USBR
Dr. James E. Miller, USDA-CSREES/NRE
Charles Clevenger, MS (Deceased)
Dr. Dale L. Nolte, USDA-APHIS/WS/NWRC
Dr. Kim D. Coder, Univ. of Georgia
Richard D. Owens, USDA-APHIS/WS
Gary Drake, Reemay, Inc.
Tom Renckly, Maricopa County, AZ
Edward Fiegle, GA Dam Safety
Dr. David Sisneros, USBR
James K. Leumas, NC Dam Safety
Boris Slogar, OH Dam Safety
Dr. B. Dan Marks, Marks Enterprises
Susan A. Sorrell, ASDSO
Sarah M. Mayfield, ASDSO
Dr. Bruce A Tschantz, UT Knoxville
Dr. Marty McCann, NPDP-Stanford
David K. Woodward, NCSU
iii
The contents of this Manual were significantly enhanced by editorial reviews and comments
of the ASDSO Manual Overview Committee. The editors of the Manual are sincerely
appreciative of contributions made by the following individuals:
Timothy G. Schaal, SD Water Rights Program
Daniel M. Hill, Burgess & Niple, Ltd.
R. David Clark, MA Office of Dam Safety
James K. Leumas, City of Raleigh, NC
Lori Spragens, ASDSO Executive Director (Project Coordinator)
Because of the efforts of the many individuals previously mentioned, the editors are
confident that users of this Manual will develop a better understanding and gain a greater
appreciation of the seriousness and magnitude of problems associated with the effects of tree
and woody vegetation root penetrations on the safety of earthen dams and their
appurtenances.
iv
GLOSSARY
This glossary provides the definitions of some of the basic terms used in this Manual and
is not intended to be a comprehensive glossary of terms associated with dam safety. A
more extensive resource of dam safety terms and definitions is available through the
many references provided at the end of each chapter of the Manual.
Absorption - the process of being taken into a mass or body, as water being taken in by
plant roots.
Abutments - the interface between the sides of a valley containing a dam and the dam
embankment. Right and left abutments are referenced by viewing the dam
while facing downstream.
Adsorption – the adhesion of an extremely thin layer of molecules to the surface of solid
bodies or liquids with which they are in contact.
Appurtenances – structures associated with dams such as spillways, gates, outlet works,
ramps, docks, etc. that are built to allow proper operation of dams.
Berm – a horizontal step or bench in the embankment slope of an earthen dam.
Biological Barrier – an herbicidal releasing system, device, or material designed to
exclude root growth and/or penetration of plants into a protected
underground zone (such as a dam embankment).
Boil – a typically circular feature created by the upward movement of soil particles by
seepage flowing under a pressure slightly greater than the submerged unit weight
of the soil through which seepage is occurring.
Breach – a break, gap, or opening in a dam that typically allows uncontrolled release
of impounded water.
Capillary Rise – the rise of water in the voids of a soil mass as a result of the surface
tension forces of water.
Clearing – the removal of trees and woody vegetation by cutting without removal of
stumps, rootballs, and root systems.
v
Crest – the near horizontal top surface of an earthen dam, or the control elevation of a
spillway system.
Diameter at Breast Height (dbh) – the diameter of a tree measured at about four feet
(breast height of average person) above the ground
surface.
Drainage System – graded and/or protected pervious aggregates in a dam designed to
collect, filter, and discharge seepage through the embankment,
abutments, or foundation.
Earthen Dam – a dam constructed of compacted natural soil fill materials selected to
minimize embankment seepage while maximizing workability and
performance.
Embankment – an earthen or rockfilled structure having sloping sides constructed of
select compacted fill materials.
Failure – a (dam) incident that results in the uncontrolled release of water from the
impoundment of a dam.
Freeboard – the vertical distance from the normal operating water level of an
impoundment to the crest (top) of the dam.
Grubbing – the removal of stumps, rootballs, and lateral root system of trees and woody
vegetation. A construction operation that is typically done following the
clearing operation.
Herbicide – a chemical substance or mixture designed to kill or maintain undesirable
Plants that may include herbaceous plants, vines, brush, and trees.
Hydraulic Height (of a Dam) – the vertical distance from the normal operating water
level of the impoundment to the invert of the outlet
works or downstream outlet channel.
Hydro-seeding – the technique of applying grass seeds, fertilizer, agricultural lime, and
seedbed mulch to seeded area in a pressurized aqueous mixture.
Lateral Root System – roots of trees and woody plants that extend laterally from the tap
root and/or rootball to provide lateral support and nutrient
uptake for the plant.
vi
Line of Saturation – the leading boundary of the progression of saturation of soil in an
embankment exposed to an increasing head (source) of water
(impoundment).
Line of Wetting – the leading boundary of the progression of wetting (partial saturation)
of soil in an embankment exposed to an increasing head (source) of
water (impoundment).
Maintenance – routine upkeep necessary for efficient inspection, and safe operation and
performance of dam and their appurtenances. Labor and materials are
required; however, maintenance should never be considered to comprise
dam remediation.
Mowing – the cutting of grass, weeds, and small-diameter woody vegetation by
mechanical devices such as mowers, bush hogs, and other vegetation cutting
machinery.
Mulching – the application of protective material such as straw, fiber matting, and
shredded paper to newly seeded areas.
Operation (of a dam) – activity by a dam owner for the necessary and safe use and
performance of a dam, the appurtenances of a dam, and the
impoundment.
Owner – any person or organization that owns, leases, controls, operates, maintains, or
manages a dam and/or impoundment.
Phreatic Surface – the upper boundary (surface) of seepage (water flow) zone in an
embankment.
Piping – the progressive downstream to upstream development of internal erosion of soil
as a result of excessive seepage that is typically observed downstream as a hole,
or boil, that discharges water containing soil particles.
Remediation – restoration of a dam to a safe and intended design condition.
Revegetation – restoration of desirable ground cover vegetation (i.e. grasses) to
disturbed areas designed to prevent embankment surface erosion.
Rootball – the root and soil mass portion of a tree or woody plant that is located directly
beneath the trunk or body of the tree that provides the primary vertical
support for the tree or woody plant.
vii
Root Penetration – intrusion of plant roots into a dam embankment so as to interfere
with the safe hydraulic or structural operation of the dam.
Root System – roots contained in the rootball and the lateral root system collectively
comprise the root system of trees and woody plans and provide both
lateral and vertical support for the plant as well as providing water and
nutrient uptake for the plant.
Seeding – application of a seeding mixture to a prepared seedbed or disturbed area.
Seepage – the flow of water from an impoundment through the embankment, abutments,
or foundation of a dam.
Seepage Line – the uppermost boundary of a flow net, or the upper surface (boundary) of
water flow through an embankment (see Phreatic Surface).
Slump – a portion of soil mass on an earthen dam that has or is moved downslope,
sometimes suddenly, often characterized by a head scarp and tension cracks on
the crest and embankment slope.
Spillway Systems – control structures over or through which flows are discharged from
the impoundment. Spillway systems include Primary or Principal
Spillways through which normal flows and small storm water flows
are discharged and Auxiliary or Emergency Spillways through which
storm water flows (floods) are discharged.
Stripping – the removal of topsoil, organic laden materials, and shallow root systems by
excavating the ground surface (surficial soil stratum) after grubbing an area.
Structural Height (of a Dam) – the vertical distance from the crest (top) of the dam to
the lowest point at the toe of the downstream
embankment slope, or downstream toe outlet channel.
Stump – that portion of the trunk or body of a tree or woody plant left after removal by
cutting during timber harvesting and/or clearing of trees and woody plants.
Stump Diameter – the diameter of a tree or woody plant at the ground surface.
Tap Root – the primary vertical root in the rootball that is the origin of development for
the rootball and lateral root system growth.
viii
Toe of Embankment – the point of intersection of the embankment slope of a dam with
the natural ground surface.
Weeds – shallow-rooted, non-woody plants that grow sufficiently high as to hinder dam
safety inspections and do not provide desirable embankment slope protection
against surface runoff.
Woody Vegetation – plants that develop woody trunks, rootballs, and root systems that
are not as large as trees but cause undesirable root penetration in
dams.
Zone of Aeration – the partially saturated zone of a soil mass above the zone of
saturation (above the height of capillary rise of water in a soil mass).
Zone of Saturation – the saturated zone of a soil mass above the phreatic surface defined
by the height of capillary rise.
ix
Chapter 1
Introduction
At the time Joyce Kilmer dedicated his famous poem “Trees” to Mrs. Henry Mills Alden,
he was undoubtedly inspired by the beauty of a healthy living tree, and rightly so. For those
that do not remember, the first verse of this famous poem is as follows: “I think that I shall
never see / A poem lovely as a tree.” Most people are inspired and impressed by the
splendor of trees; however, dam owners, operators, inspectors, dam safety regulators,
engineers, and consultants might find the following verse more nearly appropriate. “I think
that I shall never see / A sight so wonderful as a tree / Removed from an earthen dam /
Whose future safety we wish to see.” This paraphrased verse is not intended to debase the
great works of Joyce Kilmer; but rather, is intended to draw attention to the fact that trees
and woody vegetation growth have no place on the embankment of an earthen dam.
Dam safety regulators and inspectors, engineers, and consultants are frequently
confronted with grass roots resistance in the issue of removal of trees and woody
vegetation from earthen dams. This resistance is often associated with sentimental,
cultural, ecological, legal, and financial issues. A fundamental understanding and
technical knowledge of potential detrimental impacts of trees and woody vegetation
growth on the safety of earthen dams is necessary in order to address these issues.
Purpose
The purpose of this Manual is to provide the dam owner, operator, inspector, dam safety
regulator, engineer, and consultant with the fundamental understanding and technical
knowledge associated with the potential detrimental impacts of tree and woody
vegetation growth on the safety of earthen dams. In addition to objectives related to
raising the knowledge level of detrimental effects of trees and woody vegetation growth
on the safety of earthen dams, the contents will provide the user of this Manual with an
1-1
Chapter 1
Introduction
understanding of the methods, procedures, and benefits of maintaining a growth of
desirable ground covering vegetation on the embankments of earthen dams.
Scope
The editors of this Manual have organized the contents in a sequential manner in order
that the reader and user of this Manual can develop the desired fundamental
understanding and gain the technical knowledge associated with the detrimental impacts
of tree and woody vegetation growth on earthen dams. Chapter 2 deals with the problems
associated with tree and woody vegetation growth on earthen dams. Chapter 3 presents
some common misconceptions about tree growth and tree root development. These
misconceptions are contrasted with factual data about tree growth and tree root
development.
Chapter 4 presents a recommended earthen dam inspection protocol and procedures for
determination of potential impacts of tree and woody vegetation growth on earthen dams.
Chapter 5 begins the presentation of proper vegetation management on earthen dams. The
user of this Manual is presented with methods and procedures for maintaining desirable
vegetation growth, while also controlling tree and woody vegetation growth.
Chapter 6 presents a number of remediation design considerations associated with the
removal of trees and woody vegetation from the embankments of earthen dams. This
chapter also presents a recommended phased-remediation procedure for removal of
undesirable vegetation (trees and woody vegetation) from earthen dam embankments.
Chapter 7 is a succinct factual presentation of costs associated with either continual
proper vegetative maintenance or long-term dam remediation construction after tree and
woody vegetation removal. The contents of this chapter should make every dam owner
cognizant of the need for proper operation and maintenance relative to vegetative growth
on earthen dams.
1-2
Chapter 1
Introduction
Implementation
While this Manual may not be considered highly technical relative to the presentation of
complex engineering calculations for the solution of potentially serious earthen dam
safety problems, this Manual does present a combined sixty-five years of research and
practice in dam safety engineering associated with tree and woody vegetation growth
impacts on earthen dams. This Manual is presented in a manner to be beneficial to the
entire dam safety community (dam owners, dam operators, dam safety inspectors, dam
safety regulators, dam safety engineers and consultants). Dam safety engineers and
consultants can utilize this Manual as a reference for recommendations for
proper
maintenance of desirable vegetation growth, control of undesirable vegetation growth,
and remediation dam design associated with the removal and control of trees and woody
vegetation growth on earthen dams. Dam safety regulators and dam safety inspectors can
utilize this Manual as a guideline for the inspection of earthen dams relative to tree and
vegetation growth dam safety issues and for the direction of dam owners and operators in
the proper method and procedures for maintaining earthen dams without detrimental
vegetative growth. Dam owners and operators can utilize this Manual to establish proper
operation and maintenance programs to promote the growth of desirable vegetative
growth on earthen dams and/or remove and control the undesirable tree and woody
vegetation growth on earthen dams.
The last verse in the famous poem Trees by Joyce Kilmer is as follows: “Poems are
made by fools like me / But only God can make a tree.” Again, the author will
paraphrase this last verse, not to debase the great works of Joyce Kilmer, but to make a
distinct point. “Only God can make a tree / But not removing trees from dams / Is
done by fools like me.”
1-3
Chapter 1
Introduction
There is yet much research and study to be done relative to the growth of proper
vegetative cover on earthen dams. However, there is no doubt that trees and woody
vegetation have no place on the embankment slopes of an earthen dam. The authors of
this Manual intend to continue technological development in the area of controlling tree
and woody vegetation growth on earthen dams. The authors would appreciate
documentation of unusual cases of tree and woody vegetation growth related to safety
issues associated with earthen dams. Documentation of these issues can be communicated
through ASDSO and/or directly to the authors of this Manual.
1-4
Chapter 2
Problems with Tree and Woody Vegetation Growth
According to the 1998-99 National Inventory of Dams (NID) data, there are approximately
76,700 dams of significant size1 and hazard category in the 50 states (USCOE, 1999). Most
of these dams are regulated by the jurisdictional states, but many are not because of specific
exemption clauses or different size or hazard restrictions. Because some states have lower
size definitions for their dams than used for the NID count, the actual number of stateregulated dams is much higher (about 94,000). In Tennessee over 40 percent of the
approximately 1000 inventoried dams not subject to regulation because of statutorily named
county exclusions or agricultural use exemptions. Most of these unregulated dams and some
of the regulated dams in Tennessee have troublesome trees and brush growing on their faces
and crests. Some states estimate that as many as 95 percent of their regulated dams have
trees. Figure 1 illustrates the general magnitude and range of the tree growth on regulated
dams in 48 states where this information is reported (ASDSO, 2000). About half of the
state-regulated dams are estimated to have excessive tree growth.
Figure 1. Estimated percentages of state-regulated dams having trees.
1
Inclusion in the National Inventory has been defined under P.L. 99-662 and P.L. 92-367 to include dams
that are at least 25 ft. high or 50 acre-feet of storage (excluding low hazard dams less than 6 ft. high or 15
acre feet of storage) and dams that due to location may pose a significant threat to human life or property in
event of failure.
2-1
Chapter 2
Problems with Tree and Woody Vegetation Growth
Most dam safety engineers, including state and federal officials, consultants, and other experts
involved with dam safety, agree that when trees and woody plants are allowed to grow on
earthen dams, they can hinder safety inspections, can interfere with safe operation, or can even
cause dam failure. In the past, engineers and dam safety experts have not always been in
agreement about the best way to prevent or control tree growth, remove trees, or repair safetyrelated damages caused by trees and woody vegetation. However, all dam engineers agree that
a healthy, dense stand of low-growing grass on earthen dams is a desirable condition and
should be encouraged.
From November 30 - December 2, 1999, a joint ASDSO/FEMA-sponsored workshop was held
in Knoxville, Tennessee, for the purpose of inviting a panel of experts to discuss various
problems, policies, and practices associated with plant and animal penetrations of earthen
dams. Much of this manual follows up the work and recommendations produced by the
workshop participants for engineers and owners to use in managing problems associated with
both plant and animal intrusions. This chapter will discuss the consensus of current attitudes,
issues, and policies involving woody vegetation penetrations of earthen dams, by state and
federal officials, researchers, and practitioners active in dam safety.
Attitudes Toward Woody Plant Growth on Dams
The Association of State Dam Safety Officials (ASDSO) sent out survey questionnaires to dam
safety officials in all 50 states and to federal representatives to the Interagency Committee on
Dam Safety (ICODS) to determine state and federal agency attitudes about the effects of trees
and woody plant growth on dams under their jurisdiction (ASDSO, 1999).
In this survey the state and federal agency representatives were asked (1) if they considered
vegetative growth to be a problem on dams, (2) if they had specific policies or operating
procedures for removing unwanted vegetation and trees on dams and if they didn’t, how did
they handle such problems, (3) what legal, financial, environmental or other constraints did
2-2
Chapter 2
Problems with Tree and Woody Vegetation Growth
they have in dealing with unwanted vegetation problems, (4) to provide documented evidence
and examples where vegetation has negatively affected the safe operation or has contributed to
the failure of dams, (5) to provide references to current or past research regarding the effects of
plants and trees on dam safety, and (6) to provide example cost and other information related to
rehabilitation and remediation of dams having problem woody plant growth. This chapter
summarizes the collective state and federal attitude, and practice toward woody plant growth
on dams.
Problems Caused by Trees and Woody Plants
Of the 48 states that responded to the above seven questions (Alabama and Delaware did not
reply), all state dam safety officials indicated that they consider trees and plant growth on dams
to be a safety problem. One eastern state dam safety engineer goes so far to say that trees are
probably the major problem that he has to deal with. He notes further that most of the trouble
occurs because owners (and some engineers) do not recognize trees as problems and become
complacent as trees slowly grow into serious problems. Both state and federal officials agree
that trees have no place on dams. Federal agencies like the
Corps of Engineers, U. S. Bureau of Reclamation, and
TVA, which own, operate and maintain their own dams, do
not allow trees to grow on their structures. Figure 2 shows
a problem dam in Nebraska where tree roots have been
reported to penetrate the chimney drain and thus affect the
operation
the dam.
of
Figure 2. Example dam with problematic
trees in Nebraska.
The problem most commonly noted by state
officials is that trees, woody vegetation, briars,
and vines interfere with effective safety
inspections. Figure 3 illustrates this problem
Figure 3. Example dam with inspectionhindering trees in Tennessee.
for a dam located in Tennessee.
2-3
Chapter 2
Problems with Tree and Woody Vegetation Growth
Figure 4 gives a breakdown of the percentage ranges of regulated dams where the 48 reporting
state dam safety officials shown in Figure 1 estimate that trees and brush hinder safety
inspections in their respective states (ASDSO, 1999). While half the states report having only
20 percent or fewer dams with significant trees and woody vegetation that hinder inspections,
vegetation on an estimated 30,000 or nearly a third of the collective state-regulated dams, is
reported to obstruct effective dam safety inspections.
Figure 4. Estimated percentages of state-regulated dams where trees and
brush are considered a deterrent to effective safety inspections.
2-4
Chapter 2
Problems with Tree and Woody Vegetation Growth
Other dam safety problems caused by woody vegetation growth are:
• Uprooted trees that produce large voids and
reduced freeboard; and/or reduced x-section for
maintaining stability as shown in Figure 5.
• Decaying roots that create seepage paths and
internal erosion problems.
• Interfering with effective dam safety monitoring,
inspection
cracking,
and
maintenance
sinkholes,
for
slumping,
seepage,
Figure 5. Serious damage by uprooted tree to
embankment stability at a dam in Oregon.
settlement,
deflection, and other signs of stress
• Hindering desirable vegetative cover and causing embankment erosion
• Obstructing emergency spillway capacity
• Falling
trees
causing
possible
damage
to
spillways and outlet facilities
• Clogging embankment underdrain systems
• Cracking,
uplifting
or
displacing
concrete
structures and other facilities
• Inducing local turbulence and scouring
around trees in emergency spillways and
Figure 6. Tree root induced scouring on crest and
downstream face of Coffey dam in Kansas.
during overtopping as shown in Figure 6.
• Providing cover for burrowing animals
• Loosening compacted soil
• Allowing roots to wedge into open joints and cracks in foundation rock along abutment
groins and toe of embankment, thus increasing piping and leakage potential.
• Root penetration of conduit joints and joints in concrete structures
2-5
Chapter 2
Problems with Tree and Woody Vegetation Growth
Current Policies and Procedures
Twenty-four of the 48 responding states noted that they had formal policies and/or operating
procedures for addressing tree and woody plant growth issues. These policies usually include
one, or some combination, of the following:
•
Trees are not allowed to grow on dams or near toe and abutment
•
All trees and stumps must be removed, but roots may be left
•
All trees, stumps, and roots must be removed
•
All trees must be removed, but root systems of "small" trees may be left; root systems of
"large" trees must be removed
•
Dams are treated on a case-by-case basis -- usually under the direction of a qualified
professional engineer.
For those states that choose to distinguish between "small" and "large" trees, the definition
basis ranges from two to eight inches in diameter; most use a size of four or six inches in
carrying out their policies.
Of the remaining 24 states indicating that they have no formal policies or procedures, the range
of recommended procedures to dam owners varies widely. Some states evaluate dams on a
case-by-case basis, while other states require owners either to maintain their dams, to remove
vegetation for inspection, or to use other means for dealing with plant problems such as
requiring a qualified engineer to be retained, depending on the dam hazard classification.
2-6
Chapter 2
Problems with Tree and Woody Vegetation Growth
In summary, states follow several schools of thought and considerations in dealing with trees
and vegetation on existing and new dams:
Existing Dams:
Distinguish between “small” trees and
“large” trees
Remove all trees, stumps, and roots from
dam embankment
Cut trees to ground level, but leave stumps
and roots
Cut trees, remove stumps, but leave roots
Consider case-by-case basis
Breach, remove, or decommission dam
Figure 6. Trees cut prior to removing stumps
and roots from dam.
Require retention of a qualified engineer by
owner
Do nothing.
Chapter 4: Dam Remediation Design Considerations
presents recommended procedures for removal of
trees and dealing with tree and woody vegetation
related problems.
Figures 6 and 7 illustrate extensive efforts necessary
to restore a heavily wooded earthen dam to a
Figure 7. Completed remediation job after removing stumps, seeding, fertilizing & mulching.
desirable vegetated and maintained condition.
New Dams:
Establish effective ground cover and hope for the best in continual maintenance
Use vegetative barriers such as bio-barriers,
or use silvicides/herbicides/chemical treatment.
2-7
Chapter 2
Problems with Tree and Woody Vegetation Growth
Constraints to Removing Trees and Plants
Several state and federal dam safety officials reported constraints to removing and/or
controlling unwanted trees and other vegetation. Constraint categories explicitly cited by state
dam safety officials (number of states in parentheses) are given below:
Financial limitations by owners (13 states)
Environmental regulations and/or permits (10 states)
Legal issues (6 states)
Aesthetics (5 states)
Threatened/endangered species issues (2 states)
Media (1 state)
Sentimental reasons (several).
States indicated that the greatest constraint to removing unwanted trees and plants and
repairing a structure infested with roots is limited financial capability by the owner. States
such as Kentucky try to work with the owner to minimize the financial burden without
threatening public safety. Ohio has recently established two low-cost loan programs to assist
qualified public and private dam owners in funding safety-related improvements to their dams,
including repairs mandated by the state dam safety program.
Environmental constraints range from limitation of the use of certain herbicides or chemicals
for controlling vegetation and for treating stumps and/or roots near water bodies; to prohibition
of, or air quality concern for, burning cleared vegetation. Unless exempted, vegetation removal
and maintenance around dams may conflict with wetland protection regulations.
In
Washington, environmental issues can pose a major hurdle to removing trees, but ultimately,
public safety takes precedence over environmental concerns. In Arizona, problems with timeconsuming environmental permit requirements for larger plant removal projects are sometimes
encountered.
2-8
Chapter 2
Problems with Tree and Woody Vegetation Growth
Some states have limited legal power to force owners to remove trees and vegetation from
dams. This lack of authority may cause delays and expensive and time-consuming litigation to
obtain an order. Other states, like Maine, do not have specific laws that force owners to
remove vegetation from their dams, and removal orders have yet to be tested. One state, South
Carolina, notes that if the owner will not voluntarily cut or remove unwanted vegetation, the
only course is to start legal action against the owner. Because legal help is limited, such help is
normally requested for the "most extreme cases." This means that only a few owners can be
forced to do something about their vegetation. In New Hampshire, legal assistance is
sometimes necessary to perform enforcement functions. In Oregon, if there is a problem with a
recalcitrant owner, a Proposed Order can be initiated by the Oregon Dam Safety Program to
correct the situation if it is determined to be an immediate threat to the integrity of the
structure.
However, this process can be rather lengthy and expensive when staff time,
materials, and attorney fees are included in the costs of preparing for a contested case hearing.
In the end, most dam owners have the right to contest state directives to remove trees and other
plants through administrative and legal processes and judicial appeals.
In some states, concerns have arisen when dams are located in parks or environmentally
sensitive areas, especially when endangered or threatened species habitat is involved, in turn
creating legal constraints.
Aesthetics and sentimental reasons are often used by dam owners and their neighbors to resist
removing trees and undesirable vegetation.
This is particularly true if owners have
intentionally planted ornamental trees and shrubs on their dams to provide shade or fruit, or to
improve looks. Some owners believe that the more woody vegetation on a structure, the better
-- thus making it very difficult for state dam safety officials to request its removal.
The power of the press has had major influence on tree removal programs in some cases,
especially where the target dam is owned by a poor or downtrodden citizen or insolvent
municipality. Heated controversy between public safety interests and private owners or
2-9
Chapter 2
Problems with Tree and Woody Vegetation Growth
interest groups was generated through various newspaper stories and letters to the editor in
1990 over the removal of 500 mature cottonwood trees on two dams owned by an 85-year-old
widowed rancher who at the time was suffering from serious illness. The news stories, which
cast the owner as being targeted because she was vulnerable, influenced the owner's neighbors
to encourage her to take a stand against further removal of 500 remaining trees because they
felt that enforcement of the state dam safety act "would cause more harm than good."
While these constraints affect the ability of many states to enforce their regulations, some states
such as Arkansas, Georgia, Colorado, Iowa, Maryland, Montana, New Jersey, North Carolina,
and Tennessee report no major constraints to enforcement and consider the safety of dams to be
of primary importance.
Federal agencies appear to have fewer constraints than states relative to mandating the upkeep
and maintenance of jurisdictional dams. However, some federal agencies noted that they must
make sure that they comply with the National Environmental Policy Act and the Endangered
Species Act prior to initiating tree and plant control and management. Isolated constraints at
the National Park Service involving funding priorities, historic preservation, and disruption of
visitor services may override safe operation and maintenance needs at some dams. Local
watershed districts that are often poorly funded are responsible for the operation and
maintenance of many of the USDA/NRCS flood control dam projects.
Vegetation-Caused Problems and Failures
Twenty-nine states indicated documented evidence
where vegetation on dams has either caused dam
failure or negatively affected their safe operation.
Sixteen states had no documented evidence and five
states had no response. Several states provided photos
(Figure 8) and information on tree caused failures or dam
2-10
Figure 8. Exposed tree roots in overtopped
dam.
Chapter 2
safety problems.
Problems with Tree and Woody Vegetation Growth
The most recent documented dam failure due to tree root penetration
occurred in May 1999 at an unnamed Air Force Academy dam near Colorado Springs. Here,
an approximately 13-ft. high dam with a pond capacity of less than 5 acre-feet of horse stable
waste water failed, releasing its contents and injuring a horse in a stable located about 100
yards downstream. The failure occurred after more than 7 inches of rain had fallen in the
previous 72 hours. The dam had several pine trees on its crest and faces, and the breach
opening exposed an extensive, deep root system. Roots up to 4 inches in diameter were found
in the breach area. Figure 9 shows an example of a large root exposed in the bottom of the
channel at the breach. The dam had not overtopped, and the failure was attributed to internal
erosion of the decomposed granite embankment material along the roots. A tree had been
located directly over the breach.
Figure 9. Large pine tree root located in the channel of the breach opening of a failed
Air Force Academy waste lagoon pond dam (David Eyre, Senior Civil
Engineer, Air Force Academy, Colorado, 1999).
2-11
Chapter 2
Problems with Tree and Woody Vegetation Growth
At the Federal level, USDA/NRCS referred to documented cases where dam failure has been
determined to be caused solely by trees, and noted that trees have also masked other more
serious seepage problems, which went undetected.
Past and Current Research
Other than a few references to the University of Tennessee Tree Growth Report (Tschantz,
1988), only one or two other citations for tree or woody plant-related research were identified
by the state dam safety officials (USDA/SCS, 1981). The surveyed Federal agencies had
relatively little to offer in the way of references to current or past research regarding the effects
of tree and plant growth on dam safety. The Corps of Engineers referred to geotechnical and
other related program research conducted at the Waterways Experiment Station, published as a
technical report series, Repair- Evaluation-Maintenance-Rehabilitation (REMR). One recent
study for the St. Paul District showed that a hole formed by a blown-down tree in the
downstream toe area can produce a potentially dangerous increase in hydraulic seepage
gradient and internal erosion or piping problems in dikes (Duncan, 1999). The USDA/NRCS
referred to the 1950's research work done at the ARS Hydraulics Laboratory in Stillwater,
Oklahoma, on Flow in Vegetative Channels, which could have application to some emergency
spillways.
A recent literature review, sponsored by ASDSO/FEMA and conducted for the Steering
Committee on Plant and Animal Penetration of Earthen Dams, researched available material on
the effects of woody plants on dam safety (Tschantz et al, 1999). All types of sources and
searches were inventoried, including ASDSO conference and workshop proceedings, ASCE
technical journals and articles, USCOLD, direct e-mail and telephone contacts of selected
federal and state agency officials, universities, research laboratories and other data bases
accessible through the National Technical Information Service (NTIS) and National
Performance of Dams Program (NPDP). While only a few references were found on recent or
2-12
Chapter 2
Problems with Tree and Woody Vegetation Growth
current research of tree and plant effects on dam safety, several references on federal and state
practices, policies, and procedures for dealing with trees and vegetation were cited in such
topical areas as:
woody plant physiology
documented examples of woody plant-caused dam failures, operation, and
maintenance problems
case histories related to tree-caused dam failures
current and past federal, international, and other research activities
federal, state, international, and other organizations' policies, procedures and
practices for preventing and remediating woody plant problems, and
federal, state or private cost documentation for removing or controlling trees and
woody plants.
Costs of Removing Trees and Tree Related Remediation
Limited cost information for removing trees and brush or for repairing damages caused by
vegetation at dams was available from the states or federal agencies. Most state dam safety
officials indicated either that they did not have the data or that the owner or his consultant
would have that information. Virginia reported that, while costs can be nominal, in extensive
tree growth situations where grubbing is required, $10,000 to $20,000 per dam is common and
that at one dam; the tree-clearing cost was about $40,000. Missouri reported that such costs
could range from $1,000 to $10,000 depending on how badly the dam is overgrown with trees.
A prominent North Carolina geotechnical engineering firm stated that ten different contractors,
working in North Carolina, South Carolina, and Georgia, reported recent bid prices ranging
from about $1500 to $3000 per acre for cutting trees at ground level, removing stumps and root
balls, and grubbing the area to remove perimeter roots. Contractors were advised that clearing
2-13
Chapter 2
Problems with Tree and Woody Vegetation Growth
and grubbing would be done on embankment slopes ranging from 1.5:1 (Horizontal to Vertical)
to 4:1 (Horizontal to Vertical), within possible wet areas in the lower 1/3 to 1/2 of the
downstream slopes, and on earthen dams ranging in height from 25 to 50 feet. Table 1
compares cost experiences reported by state dam safety officials in different regions of the
country for clearing and grubbing trees from dams.
Reporting
State
Number
*Cost
of Dams
per acre
Survey
Comments
Based on consultants' feedback; cost varies depending
More than 25
Georgia
Oklahoma
South Dakota
$1,000 to
on dam face conditions such as slope steepness, degree
$5,000
of wetness and tree density.
1
$900
2 acres of d/s slope over 2-1/2 day period
1
$1,150
3-1/2 acres, current proposal estimate.
Several
$100 to $200/Acre
Usually 10 - 20 trees per dam
Based on 3 hourly laborers working for 2 weeks on
Nevada
1
$532
3.25 acres of willow & mesquite removal on d/s dam
face (~1995)
Michigan
General DNR
$3,500
Light clear/grub (diam.<6")
construction
$6,000
Medium clear/grub (diam.<12")
cost experience
Tennessee
Texas
7
1
$12,000
Heavy clear/grub (diam<24")
$1,540 (Ave.)
Total clearing, grubbing & reseeding cost for 7 dams =
(approx. range =
$16,705 @ ~1.5 acres per dam. Jobs included range of
$1030 to $3290)
tree sizes & heavy brush. (1995-98)
$5,500
Part of overall site clearing and grubbing contract for
new dam in East Texas (1995)
Cost included clearing, grubbing, mulching and
Ohio
1
$10,000
seeding. Heavily wooded; hundreds of trees removed
Minnesota
Current
$1350
Clearing brush with brush saw - no grubbing
estimates from
$2800
Clearing brush by hand - no grubbing
Minnesota
$4475
Clear and grub brush, incl. stumps
consultant
$4225
Cut & chip up to 6" trees; grub/remove stumps
$6775
Cut & chip up to 12" trees; grub/remove stumps
$960
16 m-hrs @ $60/hr to clear and grub small trees
from d/s slope (1999)
Small Projects
(diam. < 6") for less than one acre projects
*Reported costs not indexed
Table 1. Cost Comparisons for Clearing, Grubbing and Removing Trees from Dams.
2-14
Chapter 2
Problems with Tree and Woody Vegetation Growth
While the range of remedial costs varies widely, depending on several factors, it appears that
about $1,000 - $5,000/acre may be a reasonable baseline to use for rough estimating purposes,
with the lower figure applicable to small and low-density tree growth and the larger figure
appropriate to mature, very dense tree stands.
A typical 25-foot high by 750-foot long earthen dam having 3:1 (Horizontal to Vertical)
embankment slopes, a 15-foot crest width, and a freeboard of 10 feet above normal pool has
approximately two acres of exposed crest and face area for potential tree growth. Total costs
for clearing and grubbing trees for such a dam would be in the range of $2000 to $10,000
depending upon the local site conditions.
Several site-specific factors can influence tree removal costs. These include size and type of
trees, growth density, total job size (number of acres), location of growth (crest and/or both
faces), embankment slope steepness, slope condition (such as degree of wetness or surface
texture), degree and type of required surface treatment (backfilling, use of herbicides or biobarriers, mulching, seeding, fertilizing, etc.), and regional labor and construction indices.
The U. S. Bureau of Reclamation reported detailed cost data using three herbicidal application
methods (aerial, cut-stump, and ground-based foliar-application) in its 1987-93 program to
control salt cedar along waterways in seven states of the Upper Colorado Region. Application
costs ranged from about $60/acre for aerial spraying to about $1000/acre for cut-stump and
spray methods (Sisneros, 1994). The National Park Service indicated that it has done tree
removal with the assistance of the U. S. Bureau of Reclamation, but cost information is not
readily available.
2-15
Chapter 2
Problems with Tree and Woody Vegetation Growth
Summary
Trees appear to be a major dam safety issue for many states. Based on recent survey responses
from 48 states, it is estimated that about one half of the state-regulated dams have trees
growing on them. The same reporting states estimate that an average of nearly a third of the
dams that they regulate have sufficient trees, brush and other growth to hinder effective safety
inspections.
Current state and federal policies, procedures, and practices relating to tree and woody plant
removal, control, and management for dam safety are generally fragmented and inconsistent
among state and federal dam safety agencies. However, all state and federal agency dam
safety officials and experts agree that trees have no place on dams and need to be managed
and controlled on both existing and new dams for at least three important reasons: (1) trees
and dense vegetation hinder effective dam inspections; (2) tree roots can cause serious
structural instability or hydraulic problems, which could lead to dam failure and possible loss
of life; and (3) trees and brush attract burrowing animals, which can in turn cause serious
structural or hydraulic problems.
The fragmentation among state and federal agencies applies only to procedures about how and
to what extent the trees and their roots should be removed and resulting cavities remediated to
ensure a hydraulically and structurally safe dam. Other chapters in this Manual present
methods and practices for controlling trees and woody plants and for remediating damage
caused by trees and other woody plants.
While limited information is available, a sampling of state dam safety officials and other
experts report that the cost of removing trees and brush from the face of a dam can broadly
range from about $1,000 to $10,000 per acre, depending on several factors. Typically, the cost
of clearing and grubbing trees from dams falls into the $1,000 - $5,000 per acre range. The
2-16
Chapter 2
Problems with Tree and Woody Vegetation Growth
broad range of costs is not surprising as most dam safety engineers agree that tree removal
costs are very much site specific. Controlling vegetation annually is relatively inexpensive, but
removing trees on and repairing damage to neglected dams may cost owners several thousand
dollars.
Most dam safety experts agree that research needs to be done on determining the relationship
of plant and tree species to root penetration of artificial environments such as embankment
dams; the interaction between root systems and the phreatic zone and surface; and development
and understanding of various types of physical, biological, and chemical treatment and barriers
for controlling root growth. Because many existing dams exhibit dense growths of trees and
woody vegetation with deep-penetrating root systems, engineering methods need to be
developed for understanding, predicting, and stabilizing the effects of these root penetrations to
minimize internal erosion and failure. Dam safety experts agree that both technical and nontechnical pamphlets and brochures, practice manuals, web-based documents, workshops, and
guidance materials need to be developed for educating dam owners about the problems caused
by trees and woody vegetation. Engineers, dam safety officials and inspectors, developers, and
contractors must be provided technical training and information relative to the control and/or
safe removal of trees and other undesirable woody vegetation from earthen dams.
2-17
Chapter 2
Problems with Tree and Woody Vegetation Growth
References
1. Association of State Dam Safety Officials (ASDSO), State Survey: Animal and Vegetative
Impacts on Dams, Part I - Vegetation on Dams (7 questions), September 1999.
2. Association of State Dam Safety Officials (ASDSO), State Survey, Percentage of Trees on
State-regulated Dams (2 questions), January 2000.
3. Soil Conservation Service (SCS), U. S. Department of Agriculture, Technical Note 705 –
Operations and Maintenance Alternatives for Removing Trees from Dams, South Technical
Center, Fort Worth, April 1, 1981, 8 pages.
4. Tschantz, B. A. and Weaver, J. D., Tree Growth on Earthen Dams: A Survey of State Policy
and Practice, University of Tennessee, Civil Engineering Report, November 1988, 36 pages
plus Appendices A and B.
5. Tschantz, B. A., Wagner, C. R., Jetton, J. W., and Conley, D. C., Bibliography on the
Effects of Woody Vegetation on Dams, compiled for the Association of State Dam Safety
Officials (ASDSO) Steering Committee on Plant and Animal Penetration of Earthen Dams,
University of Tennessee, September 1999, 18 pages.
6. Tschantz, B. A., Overview of Issues and Policies Involving Woody Plant Penetrations of
Earthfilled Dams, Presentation and Proceedings, ASDSO/FEMA Specialty Workshop on
Plant and Animal Penetrations on Dams, November 30 - December 3, 1999, 8 pages.
7. Duncan, J. M., Review of Corps of Engineers Design for Rehabilitation of the Perimeter
Dikes around Cross Lake, Minnesota, Report submitted to St. Paul District, Corps of
Engineers and R. Upton, Ad Hoc Committee Chair, Cross Lake, July 14, 1999, 16 pages
plus Appendices A through C.
8. Sisneros, D., Upper Colorado Region Salt cedar Cost Analysis/Evaluation, U. S. Bureau of
Reclamation, Research and Laboratory Services Division, Environmental Sciences Section,
Denver, Co, Final Report, Memorandum No. 94-2-2, February 1994, 272 pages.
9. U. S. Army Corps of Engineers (USCOE), in cooperation with the Federal Emergency
Management Agency (FEMA) and Association of State Dam Safety Officials (ASDSO),
National Inventory of Dams - 1998-99, CD-ROM NID-GIS, v. 1.0, with Information
Booklet, September 1999.
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Chapter 2
Problems with Tree and Woody Vegetation Growth
10. Marks, B. Dan, S&ME Engineering, Inc., Arden, N. C., Faxed communication on recent
contractor-bid clearing and grubbing costs, February 23, 2000.
11. Association of State Dam Safety Officials (ASDSO), Report on Specialty Workshop #1:
Plant & Animal Impacts on Earthen Dams, Knoxville, Tennessee, Nov. 30 – Dec 3, 1999,
June 2000.
2-19
Chapter 3
Tree Growth and Tree Root Development Requirements
The purpose of this chapter is to provide the reader and user of this Manual with a basic
understanding of plant physiology related to fundamental processes of tree growth and
tree root development. It is not the intent of this chapter to delve into a detailed biological
study of trees and woody vegetation, but to provide the reader with a fundamental
understanding of the requirements for tree growth and tree root development while
attempting to dispel some of the misconceptions and myths associated with tree and
woody vegetation growth, particularly as related to tree root development.
Common Myths and Misconceptions
There are many misconceptions and common myths relating to trees and woody
vegetation that have been accepted by many people without a scientific basis. Many of
these common myths and misconceptions relative to plant physiology have originated
from uneducated interpretations of observations associated with tree growth and tree root
development. Some of these myths and misconceptions associated with trees and woody
vegetation affect correct interpretation and understanding of the impact of such growth on
the safety of earthen dams. The more common myths and misconceptions must be
dispelled so that a new level of understanding about the impacts of trees and woody
vegetation on earthen dams can be properly developed. Trees and woody vegetation, like
all living things, must have oxygen, nutrients, and water (moisture) to survive. Without
these requirements, tree roots cannot continue development and tree growth cannot
continue. The root system of trees and woody vegetation is in simplified terms comprised
of two major components that are the root ball, typically directly below the trunk of the
tree, and the lateral or perimeter transport root system that typically extends beyond the
‘drip line’ or vertical projection of the canopy of the tree.
3-1
Chapter 3
Tree Growth and Tree Root
Development Requirements
Tree Tap Roots are thought by many to be the primary root system for all ages and types
of trees and woody vegetation. In fact, the taproot is the first root to develop from the
seed or reproductive source. This central root is an extension of the stem and differs from
the stem only in that the root contains nodes for development of additional roots. Once
the taproot has stabilized the young plant (tree), the root ball begins to develop and the
taproot becomes less important than other roots that grow laterally from the taproot. The
developing root ball provides vertical support for the tree as well as providing nutrients
and water (moisture) to the tree. Roots extending laterally from the root ball increase the
stability of the tree while functioning to collect and store nutrients, oxygen, and water for
the tree. While it is true that some trees have more clearly defined taproots, taproots of
most trees do not extend significantly far below the massive root ball of healthy trees.
However, taproots are more predominant in locations where trees grow in deep deposits
of loose dry soils.
Tree Root Soil Stabilization is likely the most common misconception associated with
tree growth and tree root development. How many times has the reader heard, or perhaps
mistakenly said, “If it were not for those trees and tree roots this slope would really
be eroded or unstable – those tree roots are really ‘holding’ that soil slope”. Many
otherwise knowledgeable and educated individuals believe the myth that tree roots
actually stabilize soil masses by ‘holding’ the soil together. This misconception leads
many people to believe that heavy tree and woody vegetation growth is actually
beneficial for steep embankment slopes. Tree root development that is necessary to
provide nutrients for tree growth and stabilize the tree actually loosens the soil mass.
Laterally extending tree roots could be thought of as being nature’s original application
of the geotechnical engineering design concept of soil nailing. Root penetration stabilizes
the tree and loosens the soil mass within which the tree roots are developing; the
converse is a myth and certainly not true.
3-2
Chapter 3
Tree Growth and Tree Root
Development Requirements
Groundwater Penetration by tree root systems is another common myth and
misconception believed by many otherwise knowledgeable individuals. Although
Cypress, Tulip Poplar, some Willow and Water Birch tree species appear to have root
systems that are submerged, nutrient root systems of trees cannot survive beneath the
water table or the phreatic surface (seepage line) in an earthen dam. Trees and woody
vegetation depend upon their transport root systems to provide the major portion of the
oxygen demand for continual tree growth and tree root development. Most species of
trees and woody vegetation quickly die of suffocation once the lateral transport root
system and root ball are inundated. This phenomenon can be visually observed in many
areas of Arkansas, Mississippi, and Louisiana where large tracts of timber have been
artificially flooded for duck hunting. If these flooded tracts of timber are not drained
seasonally, the timber (trees) die as a result of suffocation. Similarly, beaver activity
causes significant losses in the timber industry every year as a result of inundation of
harvestable timber. Tree roots do not penetrate the water table or the zone of saturation
where oxygen demands of the tree cannot be met. If the zone of saturation or water table
is raised above the level of tree roots for an extended period, the tree will die as a result
of suffocation. Tree root development and tree growth cannot occur when moisture
contents in the soil mass are greater than about forty percent.
Soil Moisture Uptake of many species of trees far exceeds that which most individuals
would estimate as a normal requirement of water for continual tree growth and tree root
development. It is not uncommon for most species of healthy mature trees to absorb 200
to 300 gallons of water per day if this amount of water is available to the lateral transport
root system. Reduced availability of soil moisture will curtail continual tree root
development until such time that the soil mass is replenished with sufficient moisture to
allow resumption of tree root development. Tree root development and tree growth
cannot occur in soil masses having moisture contents less than about twelve percent for
extended periods.
3-3
Chapter 3
Tree Growth and Tree Root
Development Requirements
Woody Vegetation Control Versus Dam Performance is an issue that is clearly
misunderstood by many dam owners, operators, inspectors, dam safety regulators,
engineers, and consultants. Tree and woody vegetation root penetration is not a beneficial
effect on the performance of earthen dams. As indicated previously, tree root penetration
does not stabilize a soil mass, particularly an embankment slope. Quite the contrary, tree
root penetration loosens the soil of an embankment slope and creates a condition more
conducive to surface water penetration and slope failure. Earthen dams are not unlike
other engineered structures in that they must be properly maintained in order to perform
as perceived in the original design of the structure.
When does routine vegetation maintenance and control become a dam safety and/or dam
performance issue? The author is of the opinion that vegetation maintenance and control
on an earthen dam ends, and the need for an engineered earthen dam rehabilitation plan
begins, when effects of an improper vegetation maintenance and control program create
conditions that are detrimental to the structural integrity of the earthen dam. For
example, an earthen dam that exhibits a dense growth of grasses and weeds that are waist
high, but is free of significant woody vegetation growth, is an earthen dam that is in need
of proper vegetation maintenance and control to allow proper inspection of the dam.
However, waist-high grasses and weeds would not typically affect the structural integrity
of the earthen dam. Conversely, an earthen dam that supports a dense growth of four to
eight inch diameter trees that preclude proper access for inspection is a dam safety and
performance issue. Dense growths of trees and woody vegetation not only present a
hindrance to proper dam safety inspection, but also are detrimental to the structural
integrity of the earthen dam. Proper removal of trees and woody vegetation from earthen
dams is a dam safety and performance issue that must be conducted in accordance with
properly designed dam remediation plans and specifications.
3-4
Chapter 3
Tree Growth and Tree Root
Development Requirements
Tree Root Characteristics and Requirements
As previously indicated, root systems of trees and woody vegetation consist of two
primary components that are the root ball and the lateral transport root system. While all
tree and woody vegetation roots have a primary function of providing oxygen, nutrients,
and water to the plant, they also provide stability for the plant. The root ball that is
typically directly below the trunk of the tree provides vertical support while the lateral
transport roots provide lateral support for the tree. Root systems of trees and woody
vegetation growing on dam embankment slopes will typically be asymmetrical as a result
of the need for the tree to be stabilized in the sloping embankment soil mass. The lateral
transport roots will typically be better developed on the uphill side of the tree than on the
downhill side of the tree. Dr. Kim D. Coder at the University of Georgia has conducted
extensive studies and research on tree growth and tree root development requirements
and characteristics. He has developed data from these studies and research programs that
relate tree trunk size to root ball diameter and lateral transport root system diameter.
These data are presented in Table 1 below.
Table 1: Typical Rootball and Root System Sizes for Various Tree Sizes
Tree Diameter, inches
Rootball Diameter, feet
Root System Diameter, feet
4 to 5
6
10 to 12
6 to 7
8
16 to 18
8 to 9
10
20 to22
10 to 11
12
26 to 28
12 to 14
14
30 to 32
15 to 18
16
38 to 46
19 to 23
18
48 to 58
24 to 36
20
60 to 90
37 to 45
22
92 to 112
3-5
Chapter 3
Tree Growth and Tree Root
Development Requirements
During the presentation of common myths and misconceptions about tree growth and tree
root development, requirements of trees and woody vegetation for continual growth and
root development were discussed. Based upon research and studies conducted by Dr. Kim
Coder, requirements for tree and woody vegetation growth and root development are
tabulated in Table 2.
Table 2: Root Growth Resource Requirements
Requirement
Minimum Value
Maximum Value
Soil Oxygen Content
2.5%
21.0%
12.0%
N/A
Soil Bulk Density (Clays)
N/A
87 pcf
Soil Bulk Density (Sands)
N/A
112 pcf
Soil Air Voids
Water Content of Soil
12.0%
40.0%
Limiting Soil Temperatures
40ºF
94ºF
Soil pH Values
3.5
8.2
Soil air void content is one of the most critical factors for continual tree root
development. This factor is critical since both soil density and soil oxygen content are
dependent upon the amount of air voids present in a soil mass. Because of the importance
of soil air void content, Dr. Coder conducted extensive research to determine limiting air
void contents for various soil types required for continual tree root growth (See Table 3).
Table 3: Limiting Soil Air Voids for Root Growth in Various Soil Types/Textures
Soil Type/Texture
Air Voids, %
Sand
24
Fine Sand
21
Sandy Loam
19
Fine Sandy Loam
15
Loam
14
Silt Loam
17
Clay Loam
11
Clay
13
3-6
Chapter 3
Tree Growth and Tree Root
Development Requirements
Utilizing weight-volume relationships for various soil types and textures, Dr. Coder was
able to determine the limiting (maximum) dry density of soil that would allow continual
tree root development. Results of these correlations between minimum soil air void
content and maximum soil dry densities required for continual tree root development are
presented in Table 4 below.
Table 4: Limiting Soil Dry Density for Root Growth in Various Soil Types/Textures
Dry
Soil Type/Texture
Density,
pcf
Sand
112.3
Fine Sand
109.2
Sandy Loam
106.1
Fine Sandy Loam
103.0
Loam
96.7
Silt Loam
90.5
Clay Loam
93.6
Clay
87.4
In an attempt to relate the research data developed by Dr. Coder to geotechnical
engineering data developed from over 200 earthen dam projects, the author has compiled
a comparative list of soil properties for various soils that have been found in earthen dam
embankments. The ranges given in the data presented in Table 5 below are associated
with soil in a loose condition and soil in a compacted state that might be required in the
construction or remediation of an earthen dam. The user of this Manual must be aware
that these soil parameters are typical values and should not be relied upon for design of
new earthen dams or design of remediation plans for existing dams.
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Chapter 3
Tree Growth and Tree Root
Development Requirements
Table 5: Summary of Typical Soil Parameters
Soil
Specific
Void
Porosity,
Dry Density,
Permeability,
Type
Gravity
Ratio
%
pcf
cm/sec
Sand
2.62 to 2.66
0.40 to 0.90
30 to 45
90 to 115
0.01 to 0.0001
Silt
2.60 to 2.68
0.50 to 1.20
35 to 55
75 to 110
0.001 to 0.00001
Clay
2.66 to 2.72
0.60 to 1.40
40 to 60
70 to 105
0.0001 to 0.0000001
As one can see from the tabulated summary of typical soil parameters, continual tree root
development cannot occur in soils that are well compacted. One of the best methods of
controlling tree and woody vegetation growth on new earthen dams and existing earthen
dams where remediation requires placement of additional embankment fill soil is to
compact the embankment fill soils to a high degree of compaction. Increased compaction
of embankment fill soils reduces the air void content and limits the amount of surface
water that can infiltrate into the embankment slope. However, a good ground cover of
grasses can be established in well-compacted soils since the depth of grass root
penetration is minimal and the surficial soils will typically sustain the shallow grass root
penetration.
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Chapter 3
Tree Growth and Tree Root
Development Requirements
References
1. Association of State Dam Safety Officials (ASDSO), Report on Specialty
Workshop #1: Plant & Animal Impacts on Earthen Dams, Knoxville, Tennessee,
November 30 – December 2, 1999, June 2000.
2. Coder, K. D., Tree Root Growth Control Series: Root Growth Requirements and
Limitations, Univ. of Georgia, Cooperative Extension Service Forest Resources,
Publication FOR98-9, March 1998, 8 pp.
3. Coder, K. D., Tree Root Growth Control Series: Soil Constraints on Root
Growth, Univ. of Georgia, Cooperative Extension Service Forest Resources,
Publication FOR98-10, March 1998, 8 pp.
4. Coder, K. D., Engineered to Fail? Tree Root Management on Dams, Abstract,
University of Georgia, Athens, November 1999, 1 page.
5. Dickerson, William C., Integrative Plant Anatomy, Harcourt Academic Press,
New York, 2000
3-9
Chapter 4
Earthen Dam Safety Inspection and Evaluation Methodology
The purpose of this chapter is to illustrate dam behavior during the initial years of design
life and to present a suggested inspection and evaluation methodology. An example
earthen dam configuration will be presented in order to illustrate earthen dam behavior
and to develop the suggested inspection methodology.
Example Earthen Dam Configuration
The example earthen dam is assumed to be a high-hazard dam having a structural height
of about 33 feet and impounding a lake area of about three acres at normal pool elevation.
The contributing watershed of the lake is about 320 acres (0.5 square mile) with a base
flow of about one-half (0.5) cubic feet per second (cfs).
The configuration of the example earthen dam consists of an upstream slope of 2:1
(horizontal to vertical), a crest width of fifteen feet, and a 3:1 (horizontal to vertical)
downstream slope. The dam has a freeboard of four feet making the hydraulic height of
the dam about 29 feet. The dam is founded on relatively impervious (compared to the
embankment fill soil) material with a down gradient slope of about three percent. The
example earthen dam section has a key trench directly below the centerline of the dam
crest that has a bottom width of ten feet and side slopes of 1:1 (horizontal to vertical).
The dam crest has a two-percent slope toward the impounded lake and the upstream slope
has no protection system against tree and woody vegetation growth or wave erosion. The
embankment of the example earthen dam is assumed to be homogeneous. Figure 1 is a
representation of the example earthen dam configuration with the theoretical seepage line
intercepting the downstream slope at about one-third the hydraulic height of the dam.
Rule-of-Thumb: The phreatic surface intercepts the downstream slope of a
homogeneous earthen dam at a vertical distance of about one-third the hydraulic
height above the toe of the downstream slope, provided there is no internal drainage
system in the dam embankment.
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Based upon data provided for the example earthen dam, this dam would be listed on the
National Inventory of Dams (NID). In addition, the example earthen dam would be
classified as a small-size, high-hazard dam by most state dam safety regulations.
Figure 2 illustrates the example earthen dam with an embankment subdrain system
located within the downstream embankment slope. The subdrain or embankment drain
system is located at about the point of interception of the seepage line with the
downstream slope if there was no embankment toe drain system within the downstream
slope. As a result of the presence of the embankment subdrain system, the seepage line
through the dam embankment has been modified (lowered) from the location of the
theoretical seepage line for a homogeneous earthen dam embankment. The seepage line
within an earthen dam is often mistakenly considered to have a permanent location.
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
However, the location of the seepage line is continually changing as a result of many
influential factors. Fluctuations in the pool elevation, seasonal and long-term
climatological conditions, and the growth of trees and woody vegetation in close
proximity to the seepage line are some of the factors that influence changes in the
location of the phreatic surface within an earthen dam embankment.
Important moisture regimes other than the steady-state seepage line (phreatic surface) are
often not given proper consideration in the evaluation of the performance of earthen
dams. The zone of saturation is located immediately above the phreatic surface or
seepage line where embankment fill soils have become saturated as a result of capillary
rise caused by capillary forces in the soil voids. Figure 3 illustrates the presence of zones
of saturation associated with that of a theoretical seepage line location without an
embankment subdrain system as well as that of a modified seepage line location with an
embankment toe drain system. The height of capillary rise (thickness of the zone of
saturation) is directly dependent upon the effective mean diameter of soil voids within the
earthen dam embankment. The effective mean diameter of compacted soil is dependent
upon the effective particle size (De) of the compacted embankment fill soil. Soil within
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
the zone of saturation is completely saturated; however, there is no flow or gravity
induced movement of water unless some external force disturbs the soil. This
phenomenon is often observable during the inspection of downstream slopes of earthen
dams. Seepage and free flowing water can be seen on the downstream slope of an older
dam below the point of interception of the seepage line if no embankment subdrain is
present. Above the point of interception of the seepage line with the downstream slope,
the soil is saturated and the Zone of Saturation can be observed for a significant distance
above the seepage line intercept in some cases. In the Zone of Saturation, pore water may
be observed to fill tracks made in the water-softened embankment soil. However, once
the tracks are filled by pore water released from the disturbed soil there will be no
continued flow or seepage from the embankment. This condition is often confused with
the presence of embankment seepage. Installation of a subdrain location in this situation
may lower the phreatic surface relatively quickly; however, months or even years may be
required to drain the zone of saturation because of tensile forces or negative pore
pressures in the embankment fill soils.
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Embankment Wetting, Saturation, and Seepage
Prior to presentation of the behavior and performance of an earthen dam embankment
during the initial years of the design life, one must have an understanding of relationships
between various velocities of moisture movement and water flow through compacted
embankment soils. First, consider the relationship between the optimum compaction
moisture content of an embankment soil and other moisture content properties.
Rule-of-Thumb: The optimum compaction moisture content as determined by ASTM
D-698 (standard Proctor compaction test) is approximately two to four percent below
the Plastic Limit (PL) of most soils and about three to five percent below the saturation
moisture content of the same soils.
Compacted soils will typically increase in moisture content from the compaction
moisture content to about the PL of the soil relatively quickly after construction of an
earthen embankment. The rate of wetting is much greater in soils compacted dry of
optimum moisture content than in soils compacted wet of optimum moisture content.
Although compacted soils may undergo wetting or increase in moisture content relatively
quickly when exposed to a source of water, the rate of saturation is much slower because
air trapped in discontinuous soil voids must be dissolved in soil pore water during the
saturation process. Embankment wetting and saturation are not associated with seepage
or the flow of water through a homogeneous earthen dam; however, relative velocities of
wetting and saturation can be related to values of steady-state seepage velocity,
permeability, or hydraulic conductivity of compacted embankment soils.
Figure 4 is an illustration of the example earthen dam with relationships between various
soil water flow velocities and permeabilities. First, consider the relationship between the
vertical and horizontal permeability of a compacted homogeneous embankment soil.
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Rule-of-Thumb: The horizontal permeability of a compacted homogeneous
embankment soils are typically about nine times to ten times (one order of magnitude)
greater than the vertical permeability.
Variation between the horizontal permeability and vertical permeability is the result of
the internal structure of compacted soils. This variation does not account for poorly
compacted lifts since the embankment is assumed to be homogeneous. Consequently, if
laboratory permeability tests indicate that a compacted embankment soil exhibits a
hydraulic conductivity value of about 0.000004 centimeters per second (cm/sec) then the
horizontal permeability of this compacted embankment soil will be about 0.000036 to
0.00004 cm/sec. Second, consider Darcy’s Law that is the basis for all theories and
analyses associated with the flow of water through soil masses. Darcy did not account for
soil voids relative to soil solids in derivation of his equation. As a result, the area of
discharge is the total cross-sectional area through which flow is occurring. If one assumes
that the hydraulic gradient producing flow through a soil mass is equal to one (unity),
then the discharge velocity (Darcy’s flow velocity) is equal to the permeability value of
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
the soil. The actual flow velocity in the voids of the soil is often identified as the seepage
velocity and is approximately equal to the discharge velocity divided by the porosity
value (expressed as a decimal) of the soil. Assuming that the compacted embankment soil
in the example earthen dam has a porosity of forty (40) percent (0.40), the seepage
velocity of the soil would be about 2.5 times greater than the discharge velocity. Third,
consider the wetting velocity or the velocity of the line of wetting. The wetting velocity is
the rate at which soil increases in moisture content up to about the PL when exposed to a
free water source. The line of wetting can often be observed as it progresses through soil
masses, particularly soils that are dry of optimum moisture content. The wetting velocity
is the sum of the seepage velocity and the capillary velocity or the velocity of wetting
attributable to capillary forces in the soil.
Rule-of-Thumb: The wetting velocity or the velocity of the line of wetting through
compacted soil is about one order of magnitude (ten times) greater than the seepage
velocity.
Applying this factor to the previous comparison between seepage velocity and discharge
velocity, one finds that the wetting velocity is about 25 times greater than the discharge
velocity. Based upon the foregoing discussion of earthen dam embankment wetting,
saturation, and steady-state seepage velocities, consider the illustration in Figure 5. This
figure illustrates embankment wetting, saturation, and steady-state seepage during the
early years of the design life of an earthen dam. Assume that laboratory testing indicates
that embankment soils of the example dam embankment have a permeability or hydraulic
conductivity value of 0.02 foot per day. The discharge velocity would be about 0.008 foot
per day with a hydraulic gradient of about 0.4 resulting in a horizontal discharge velocity
of about 0.08 foot per day. The associated seepage velocity would be about 0.02 foot per
day with a soil porosity of about 40 percent and the horizontal seepage velocity would be
about 0.2 foot per day. The velocity of the line of wetting or the wetting velocity would
be about 2.0 feet per day.
4-7
Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Based upon the estimated normal inflow from the contributing watershed, the lake
retained by the example dam should reach about fifty percent volume in approximately
twenty days and reach normal pool elevation in about forty days. Solid lines in Figure 5
illustrate the location of the line of wetting at various time intervals. The line of wetting
should reach the downstream slope in about ninety days. Note: The compacted
embankment soils remain partially saturated after passage of the line of wetting.
Dashed lines in Figure 5 illustrate the line of saturation at various time intervals. The line
of saturation moves at the seepage velocity that is about one-tenth the value of the
wetting velocity. When the line of wetting has reached the downstream slope in about
ninety days, the line of saturation is still at about the vertical from the intercept of the
normal pool with the upstream slope. Based upon this rate of progression, the line of
saturation will not reach the surface of the downstream slope and steady-state seepage
will not be initiated for about 900 days (about 2.5 years), provided that no external
influences affect the rate of wetting and saturation.
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
The estimated maximum steady-state seepage rate for the example dam will be about 5.5
gallons per day per foot of dam. Before leaving Figure 5, imagine that the example dam
contains an embankment subdrain system as indicated in Figures 2 and 3. The rate of
progression of the line of wetting and the line of saturation will both be affected by the
presence of the subdrain system.
Even without the presence of an embankment subdrain system, the time required for the
line of wetting could encompass an entire growing season depending upon the time of
year that the dam was completed. More importantly, the time that is required for the line
of saturation to intercept the downstream slope might encompass two or three entire
growing seasons. Tree and woody vegetation growth can become quite dense and
relatively large within the initial two to three growing seasons if not properly controlled.
The initiation of tree and woody vegetation growth on the downstream slope begins the
soil moisture uptake cycle so that the line of saturation and the seepage line may never
completely develop and intercept the downstream slope. The condition represented by
Figure 6 might initially be considered to be beneficial to the stability of the dam
embankment. However, one must understand that as the tree and woody vegetation
growth continues compacted soils of the dam embankment are continually loosened by
the penetration of major tree and woody vegetation root systems. Furthermore, trees that
might appear healthy to an untrained inspector may be an unhealthy specimen and have a
premature death leaving penetrating root systems to rot inside the dam embankment.
Additionally, soil nutrients in the compacted soil embankment of an earthen dam may not
be sufficient for development of growth beyond which the tree cannot be properly
sustained without premature death. Regardless of the cause, trees and woody vegetation
do die and cease to uptake soil moisture that they previously used. This change in soil
moisture uptake will affect the zone of aeration, zone of saturation, and the location of
the seepage line in the vicinity of the unhealthy or dead trees and woody vegetation.
4-9
Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
The Mid-Life Crisis of an Aging Earthen Dam
Once an earthen dam embankment has become impregnated with numerous trees and
woody vegetation penetrations, routine and even major maintenance activities will likely
not be sufficient to regain the original design life of the dam. At this time in the life of an
earthen dam, previously identifiable maintenance problems have become serious dam
performance and dam safety issues. Restoration through an engineered dam remediation
design and remediation construction is typically required to bring the dam to acceptable
standards relative to dam safety requirements.
Figure 7 illustrates some of the problems and dam safety issues that can be created by
uncontrolled or non-maintained tree and woody vegetation growth in what has been
termed by the author as the ‘Mid-Life Crisis’ of an earthen dam. Seepage flow may be
emerging from rootball cavities of blowdowns (uprooted trees) because they are no
longer using soil moisture and the seepage line has adjusted upward toward the surface of
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
the slope. Removal of mature trees by woodcutters deletes the soil moisture uptake of the
removed trees thus further modifying the location of the seepage line closer to the surface
of the downstream slope. Rootballs and root systems of otherwise healthy trees located at
and beyond the toe of the downstream embankment slope become inundated by the
adjusted seepage line. Since trees cannot live through prolonged submergence of their
major root systems, these trees will become unhealthy and die leaving decaying rootballs
and root systems as serious penetrations in the earthen dam. Rootball cavities remaining
from blowdowns (uprooted trees) and their relationship to the seepage line create
conditions susceptible to potential slope failure of the downstream embankment slope.
Restoration of the example earthen dam illustrated in Figure 7 to a safe condition cannot
be brought about through routine maintenance activities. An engineered dam remediation
design and remediation construction will be required to restore this dam to a safe
condition and original design life.
4-11
Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Inspection and Evaluation Methodology
The effectiveness, economics, and constructability of dam remediation designs for
earthen dams begin and end with proper evaluations of the characteristics and seriousness
of deficiencies as related to dam safety issues. All tree and woody vegetation growth on
earthen dams is undesirable and has some level of detrimental impact upon operation,
performance, and safety of an earthen dam. However, not all tree and woody vegetation
growth on earthen dams imposes the same level of impact on operation, performance, and
dam safety. Dam owners, regulators, inspectors, and engineers must develop an
understanding of the impact of tree and woody vegetation growth relative to location on
the dam configuration. Proper evaluation of the seriousness of dam safety issues related
to tree and woody vegetation growth on earthen dams is typically associated with the
location of the undesirable plant growth on the dam embankment.
A few examples of the variability of seriousness of plant penetrations are presented
herein to begin the learning process. The presence of a twelve-inch diameter tree on the
downstream side of the crest of an earthen dam typically does not pose the same degree
of impact on potential dam safety as a twelve-inch diameter tree located in the lower
portion of the downstream slope. Conversely, a twelve-inch diameter tree in the upper
portion of the downstream slope does not typically create the same level of seriousness as
an unhealthy twelve-inch diameter tree on the upstream slope or front crest of a dam
having a narrow crest width. Ornamental shrubs having shallow root systems along a
wide roadway crossing the crest of an earthen dam will not impose the same level of
seriousness as similar shallow rooted woody vegetation growing on the lower portion of
the downstream slope.
The purpose of developing a well-defined inspection and evaluation methodology is to
allow the establishment of dam remediation design priorities. Most anyone having a basic
understanding of the seriousness of tree and woody vegetation growth to the safety of
4-12
Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
earthen dams can inspect an earthen dam and recommend removal of all trees, stumps,
and root systems. However, inspectors and dam engineers must develop a definitive
inspection and evaluation methodology in order to prioritize the seriousness of various
locations of tree and woody vegetation growth on earthen dams.
Many individual dam owners do not have economic resources to undertake extensive
dam remediation projects to bring an earthen dam into safe operation and performance
conditions if the dam exists in a severely deteriorated condition. These owners often have
to budget dam remediation projects over a scheduled maintenance and remediation
construction period. Dam safety regulators, inspectors, and engineers that have developed
and utilized a well-defined dam safety inspection and evaluation methodology can
communicate priorities to dam owners so that the needed dam remediation design
components can be completed in a prioritized manner. All too often dam safety regulators
and engineers overwhelm dam owners with dam deficiencies without consideration of
prioritization of deficiencies on dam safety, performance, and operation.
Dam Safety Inspection and Evaluation Zones
Five dam safety inspection and evaluation zones have been identified within the
geometric configuration of a typical earthen dam. The delineated zones, illustrated in
Figure 8, are not numbered in any implied order of seriousness relative to the impact of
tree and woody vegetation growth, but have simply been numbered from upstream to
downstream. The seriousness and potential impacts of tree and woody vegetation growth
within each inspection and evaluation zone will be discussed during the description and
identification of the delineated dam safety inspection and evaluation zones.
4-13
Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Inspection and Evaluation Zone 1 begins on the upstream slope of the earthen dam
embankment at about four feet below normal pool elevation. Zone 1 extends laterally to
the centerline of the crest of the dam. Tree and woody vegetation growth in Zone 1 is
more critical relative to dam safety in the case of dams having a narrow crest width than
those having a wide crest width. Zone 1 also includes the area subject to damage resulting
from wave erosion and frequently recurring rapid drawdown events.
Inspection and Evaluation Zone 2 includes the entire width of the crest of the dam.
Zone 2 overlaps Zone 1 by one-half the crest width. Overlapping a portion of Zone 1 with
a portion of Zone 2 was done to emphasize the critical portions of both zones. Zone 2 is
typically considered to be one of the least critical zones relative to dam safety issues
associated with tree and woody vegetation growth. However, careful inspection of Zone 2
often reveals evidence of serious dam safety issues such as tension cracks, slope failure
scarps, and erosion features that may or may not be related to tree and woody vegetation
growth originating in other dam safety inspection and evaluation zones.
4-14
Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
Inspection and Evaluation Zone 3 extends from the centerline of the crest of the dam to
a point on the downstream embankment slope that is about one-third of the structural
height below the crest of the dam. Zone 3 overlaps Zone 2 by one-half the crest width and
is typically considered the least critical zone relative to dam safety issues associated with
tree and woody vegetation growth. The seepage line and zone of saturation in this portion
of an earthen dam embankment are typically sufficiently far below the surface to allow
excavation of tree rootballs on the downstream slope of the dam without installation of a
drain or filter system. A portion of Zone 2 has been overlapped by Zone 3 to draw
attention to the most critical portion of Zone 3 that is the downstream portion of the crest
of an earthen dam.
Inspection and Evaluation Zone 4 extends from a point on the downstream
embankment slope that is about one-third the structural height of the embankment to the
toe of the downstream embankment slope. Zone 4 is one of the two most critical zones
relative to dam safety issues associated with tree and woody vegetation growth as well as
other potential dam safety issues. This zone typically contains the interceptions of both
the zone of saturation and the seepage line with the downstream slope. The close
proximity of the zone of saturation and seepage line to the surface of the downstream
embankment slope in this zone is a critical factor relative to dam safety issues associated
with tree and woody vegetation growth. Tree and woody vegetation growth in this Zone
4 must be of major concern to everyone associated with the safety of an earthen dam
and must be evaluated carefully relative to prioritization of dam remediation
requirements.
Inspection and Evaluation Zone 5 extends from the mid-height of the downstream
embankment slope to a distance of one-half the structural height beyond the toe of the
downstream embankment slope. This zone typically contains the interception of the
seepage line with the downstream embankment slope and potential boiling (soil piping)
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
action beyond the toe of the downstream embankment slope. As such, this zone is critical
relative to long-term, steady-state seepage stability considerations for an earthen dam.
Tree and woody vegetation growth in this zone rapidly develops into serious conditions
that directly affect the safety of an earthen dam. Zone 5 overlaps Zone 4 to draw
attention to the more critical portions of both Zone 4 and Zone 5. As in the case of Zone
4, Zone 5 is typically considered to be one of the two most critical zones relative to dam
safety issues associated with tree and woody vegetation growth. Tree and woody
vegetation growth in Zone 5 must be a concern to all involved in the safety of an earthen
dam. Maintenance and/or engineered dam remediation must be undertaken
immediately in the event that tree and woody vegetation growth is significant within
Zone 5. Control of tree and woody vegetation growth well beyond the toe of the
downstream embankment slope cannot be over-emphasized. This area of an earthen dam
is critical to overall stability and potential dam safety issues associated with embankment
and foundation seepage.
The dam safety inspection and evaluation methodology set forth herein can be easily
modified and/or extended to meet the needs of specific dam owners, dam safety
regulators and inspectors, and engineers. This proposed methodology for dam safety
inspections and evaluations should provide a basic plan that will allow the reader to
customize and/or improve existing dam safety inspection and evaluation programs.
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Chapter 4
Earthen Dam Safety Inspection
and Evaluation Methodology
References
1.
Casagrande, Arthur, “Seepage Through Dams”, Contributions to Soil
Mechanics: 1925 – 1940, Boston Society of Civil Engineers, pp 295-336
(Originally Published in the Journal of New England Water Works
Association, Volume LI, No.2, June 1937.
2.
Cedegren, Harry R., Seepage, Drainage, and Flow Nets, John Wiley &
Sons, New York, 1967.
3.
Marks, B. Dan, “The Behavior of Aggregate and Fabric Filters in Subdrain
Applications, Research Report, Department of Civil Engineering,
University of Tennessee, Knoxville, Tennessee, February 1975.
4.
Means, R. E., and Parcher, J. V., Physical Properties of Soils, Charles E.
Merrill Publishing Company, Columbus, Ohio, 1963.
5.
Parcher, J. V., and Means, R. E., Soil Mechanics and Foundations, Charles
E. Merrill Publishing Company, Columbus, Ohio, 1968.
6.
United States Department of Agriculture (USDA), Natural Resources
Conservation Services (NRCS), (formerly Soil Conservation Service,
SCS), Technical Note 705 – Operations and Maintenance Alternatives for
Removing Trees from Dams, South Technical Center, Fort Worth, Texas,
April 1981.
7.
United States Department of Agriculture (USDA), Natural Resources
Conservation Services (NRCS), (formerly Soil Conservation Service,
SCS), Technical Engineering Notes – OK-08 (Revised) RE: Control of
Trees and Brush on Dams, Oklahoma State Office, Stillwater, Oklahoma,
February 1990.
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Chapter 5
Controlling Trees and Woody Vegetation on Earthen Dams
The establishment and control of proper vegetation on an earthen dam are essential to
maintaining a safe dam. Effective, shallow-rooted, vegetative cover is necessary to
reduce and prevent embankment slope erosion. Trees and other undesirable deep-rooted
vegetation should be prevented from being established for the following reasons:
•
•
•
•
•
•
•
•
Permit effective inspection and monitoring of embankment crest and faces
Allow for adequate access to dam for normal and emergency operation
Prevent structural damage from embankment piping and internal erosion, unstable
slopes from toppled trees, concrete wall/slab joint cracking/displacement, and
other problems
Reduce possibility of root-blocked drains
Prevent blockage of spillway channel
Discourage rodent and other animal activity by eliminating food source and
habitat
Eliminate expensive tree and brush removal and remediation costs
Reduce impression of owner neglect
Consequently, dam owners should observe these four important rules:
1. Existing trees should be removed and not be allowed to mature on earthen dams,
abutment groins, or around water conveyance structures
2. Trees or shrubbery should never be planted on or around new or existing dams
3. Existing trees should be watched closely until they are removed
4. Grasses and shallow-rooted native vegetation are the most desirable surface
covering for an earthen dam.
Dam owners should be especially aware of dangerous or potentially hazardous tree
conditions such as decaying or dead branches; lightening-caused splits; stripping or
breakage; leaning, uprooted or blown-down trees; and seepage around exposed tree roots
located along embankment slopes, especially in vulnerable downstream toe or abutment
areas. Outward leaning trees may result from a slumping embankment condition that can
be an indicator of slope instability. Any of these conditions warrants immediate attention
by the owner and a qualified engineer.
5-1
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Woody vegetation and tree growth creating undesirable root penetrations in earthen dams
can be controlled or prevented by proper management of root growth into new dams and
dams that have previously been cleared of trees by proper removal procedures. In this
manual, some of the characteristics of woody vegetation and tree growth are presented
relative to the aging of an earthen dam. Remedial dam repair design procedures and
construction techniques are presented for proper removal of trees of various sizes in
various areas of the geometric configuration of an earthen dam. Proper management and
control of woody vegetation on new and previously repaired dams (tree removal projects)
are based upon an understanding of soil conditions that limit root growth, factors that
affect or promote root growth, and various procedures and techniques that can be used to
stop, redirect, and/or reduce the rate of root elongation.
The purpose of this chapter is to provide a basic understanding of requirements for
healthy root elongation, and to provide an introduction to some techniques and
procedures that can be utilized to manage and control undesirable woody vegetation and
tree growth on earthen dams. Development of a basic level of tree-literacy combines
basic understanding of soil properties and characteristics with basic understanding of
requirements and characteristics of healthy tree root elongation and tree growth into a
single conceptual understanding of management and control.
Healthy Tree Growth Requirements
The primary requirement for healthy tree growth is an environment for continual
elongation of tree roots. Continual elongation of tree roots is essential to healthy tree
growth for the following reasons: 1) respiration that requires a continual flow of oxygen
to root tissue through soil pores; 2) soil moisture uptake that requires continual
availability of soil pore water that can be captured by root tissue; 3) nutrition that requires
root systems to make continuously renewed soil/root surface contact to provide needed
elements and nutrients for healthy tree growth; and 4) support and stabilization that
requires soil-to-root surface contact to resist externally applied loads.
5-2
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Managed tree root growth control is required to prevent or minimize dangerous impacts
on dams. To constrain root growth, identification of soil attributes and it’s supporting
environment that promote or limit growth is required.
By understanding what soil
conditions limit growth, various tools and techniques can be used to stop, redirect, or
inhibit tree root growth and elongation.
The following discussion on root growth
requirements, limitations and mechanics is based on a series of publications authored and
furnished by Dr. Kim Coder of the University of Georgia Cooperative Extension Services
(Coder, FOR98-9, -10, -11, & -13, 1998). The reader is referred to these well-referenced
publications for further and more detailed information.
Trees are not much different from all living organisms, relative to biological needs. Trees
must have (1) oxygen gained through respiration, (2) water gained through adsorption
and absorption, and (3) nutrition gained through adsorption and absorption, and (4) a
stable foundation to withstand external forces.
General root growth resource
requirements are summarized in Table 1. Roots utilize soil spaces for access to water and
essential element resources, and soil mass to provide structural support. Soil minerals
surround the water-filled and air-filled voids or pores. These pores are continually filling
and draining with water and air, depending upon the availability of water, water uptake,
and atmospheric air. Root growth follows pathways of interconnected soil voids. Such
voids result because of space between soil particles, between soil structural units (i.e.,
blocks, plates, aggregated soil, etc.); along soil fracture lines, lenses, joints, and various
interstitial interfaces; and through paths of biological origins such as decayed or shrunken
roots, animal burrows, etc. Better means of controlling growth can be developed by
understanding resource levels that encourage and limit root growth (Coder, FOR98-9,
1998).
5-3
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Root Resource
Requirements
Minimal
Maximum
Oxygen in soil atmosphere
2.5%
21%
Air pore space in soil (for root
growth)
12%
-
Soil bulk density restricting root
growth
-
1.4 g/cc (clay)
-
1.8 g/cc (sand)
Penetration strength (water content
dependent)
(note: 1 g/cc = 62.4 pcf)
0.01 kPa
2
3 MPa
(note: 1 kPa = 1kN/m = 10 mbar
= 0.145 psi)
Water content in soil
12%
21%
Root initiation (O2% in soil
atmosphere)
Root growth (O2% in soil
atmosphere)
Progressive loss of element
absorption in roots (O2% in soil
atmosphere)
12%
21%
5%
21%
15%
21%
Temperature limits for root growth
40oF/4oC
94oF/34oC
PH of soil (wet test)
pH 3.5 (acidic soils)
pH 8.2 (alk. soils)
Table 1. General list of tree root growth resource requirements (After Coder, FOR98-9, 1998).
Roots survive and proliferate where adequate water is available, temperatures are warm,
oxygen is present and other essential resources are concentrated. They generally tend to
be shallow, limited by available oxygen and water saturation in deeper soil. However,
near the base of the tree, deep-growing roots can be found, but are aerated by soil fissures
and cracks and around roots where mechanical forces exerted by wind loads on the tree
loosen the soil.
The ability of primary root tips to enter soil pores, open soil pores and elongate through
pores is dependent upon the force generated by the root and the soil penetration
resistance. As the diameter and length of an expanding root increase, its strength to resist
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
structural failure and its expansive force it can generate both increase. The chance for
structural failure increases with longer and smaller diameter roots, while short and thick
roots generate significant force but minimize structural failure. Radial expansion of the
root structure immediately behind the tip also helps to fracture or reduce penetration
resistance in the soil ahead of the elongating root tip.
Roots use the mass of the tissues behind the tip, including root hairs, lateral root
formation, and microbial entanglements to minimize the length over which root
elongation force (or pressure) is expressed, thus reducing structural failure potential. As
the root elongates, only root tissue within about six root diameters behind the tip is
involved with force generation. Root tissue further back will act as an anchor and
support base against the soil. Root tip pressure can be enormous and can range up to 915 MPa (9,000-15,000 mbars, 130-215 psi, or 18,700 – 31,000 psf)), with 1MPa or about
15 psi being most cited. Thus a typical root tip diameter of one millimeter is capable of
generating up to about a 0.25-pound force. While tree roots cannot produce enough
pressure to penetrate concrete, pipes, and most plastics or metals, they do take advantage
of cracks, holes, joints and faults already in materials and exacerbate cracks and faults by
growing root mass within, beneath, or around materials. When water supply is short, or
when temperatures increase, diameter of roots are sacrificed to facilitate more elongation.
Roots can lose more than one-third of their diameter under dry conditions, leaving roots
thinner and elongating at a slower rate. Such conditions can generate passageways and
set up the possibility for piping and internal erosion conditions in an earthen dam.
Additionally, the loss of root contact with the soil and potential for mechanical failure of
the elongating root system can lead to poor tree support, thus making a tree vulnerable to
wind forces and possible upending. Tree roots are opportunistic in the colonization and
control of resource space. The attributes that make a root an ideal resource gatherer for
the tree conspire to make roots soil matrix explorers and fault exploiters. To prevent,
control or eliminate roots from the soil infrastructure, dam owners and dam design
engineers need an understanding of environmental conditions that limit and promote root
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
growth. The foregoing discussion is summarized in terms of the four main requirements
and conditions for tree growth and tree root development as follows:
Trees need to breathe. Oxygen is required for healthy tree growth through continual root
elongation. In order for proper root respiration to occur, oxygen must continually move
through soil pore spaces to the root tissue. Tree roots are not the only living things in the
soil pore system that is competing for oxygen. As oxygen flows toward an otherwise
healthy root system, enormous numbers of aerobic organisms can utilize portions, and
perhaps all, of the available soil pore space oxygen before it can be utilized by root
systems. If all of the oxygen is used before reaching the root system, changes must occur
in the characteristics and growth rate of the root system. Trees have the ability to generate
energy for short periods using carbohydrates in low or non-oxygen environments.
However, this process is taxing on tree growth, and is approximately twenty times more
inefficient than under normal oxygen availability and respiration conditions (Rendig &
Taylor, 1989; Coder, FOR98-10,1998). Air-filled voids in soil must be of sufficient size
and continuity to allow carbon dioxide to move away from the root system and oxygen to
move to the root system in order to sustain healthy root elongation and tree growth.
Water-filled voids resulting from saturated soils around roots inhibit this process at a rate
10,000 times less than air-filled voids (Rendig & Taylor, 1989; Coder, FOR98-10, 1998).
When oxygen drops below two to five percent of atmospheric content, root growth and
the root’s ability to generate elongation force significantly declines (Souty &
Stepniewshi, 1988). Table 2 summarizes air void content requirements of various soil
texture and types that limit root elongation.
The table data shows that, for most
embankment soils, trees need at least 10-25% air-filled voids in order to promote healthy
growth. In summary, Roots that cannot breath die, resulting in unhealthy, unstable,
and/or dead trees.
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Chapter 5
Soil Texture
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Root-limiting % pores normally filled with
air
Sand
24%
Fine sand
21
Sandy loam
19
Fine sandy loam
15
Loam
14
Silt loam
17
Clay loam
11
Clay
13
Table 2. Root growth limiting air-pore space values by soil texture (After Coder, FOR98-10, 1998)
Trees need to drink. Second behind the need for oxygen is a tree’s requirement for
water.
Water uptake of trees occurs both by adsorption and absorption. In the same
manner as that described for oxygen supply, tree root systems depend upon the flow of
soil pore water to the root system to continually uptake sufficient water to sustain healthy
root elongation and tree growth. Soil voids that are sufficiently small to prevent continual
flow of pore water can limit the amount of water that elongating roots can use within the
soil matrix. Often, the moisture uptake is typically lower than that required for root
elongation and healthy tree growth. As noted in Table 1, root elongation and healthy tree
growth cannot be sustained where average soil moisture contents are less than about 12
percent nor greater than about 40 percent. Soils that restrict free moisture movement
preclude healthy root elongation (penetration) and healthy tree growth. Compacted
soils limit pore space and therefore tend to limit supplies of both oxygen and usable
water to trees.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Trees need nourishment. Third, roots systems must provide nutrition for healthy root
elongation and tree growth. Root elongation is required to encounter needed minerals,
nutrients, and companion microorganisms in the soil mass. Root elongation must be
continuous since replenishment of nutrients in soil is a long-term process that will not
meet the requirements of stationary root systems and trees. Elongating or growing root
systems continually encounter soil pores of various sizes. Soil pores that are larger than
root tips create little resistance to root elongation; however, as soil pore sizes approach
the size of root tips and/or become smaller than root tips resistance to root elongation
increases significantly. Soil pores that are much smaller than root tips may be deformed
in weak or soft soils; however, these small soil voids will reject root penetration in dense
or strong soil masses. Roots cannot ‘squeeze’ into small, rigid soil pores within soil
masses where soil strength and density preclude soil deformation and, therefore, growth
is inhibited. High strength, dense soil masses containing limited required nutrients for
healthy root elongation will not sustain healthy tree growth.
Trees need foundation support. Tree stabilization and support is provided by both
components of the tree root system. The root plate (root ball) provides vertical support
for the weight of the tree much the same as a shallow foundation system provides support
for a building column. However, tree root systems must also resist laterally applied
external loads (i.e., wind loads). Lateral root systems provide required lateral support
capacity against horizontal forces through development of soil-to-root frictional forces
(nature’s own application of “soil nailing”). Inadequate root elongation results in
reduction of base and lateral support, resulting in an unstable tree that becomes unhealthy
and/or subject to failure under laterally applied loads. Dense, compacted soil masses
preclude proper lateral root elongation thus creating unstable, unhealthy trees that are
subject to premature failure.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
In summary, whether in design of new dams or in maintenance of older existing dams,
engineers and dam owners need to appreciate the forces, conditions and resources that
control and affect the health and stability of trees so as to prevent or discourage trees
from growing on new or re-constructed dams or to understand why/how trees respond to
given and changing conditions on existing earthen dams.
Tree Root Elongation Management and Control
There are at least eight well-documented methods and tools available to control and limit
tree (root) growth through the application of tree root elongation processes, resource
availabilities, and soil preparation characteristics. These methods take advantage of
depriving the tree roots of ideal resource needs for healthy growth discussed above.
While these methods have been primarily used in urban or agricultural applications and
settings, some methods are directly applicable to use on earthen dams and include the
following methods described by Coder (FOR98-11):
1.
Intelligent designs and applications that include techniques and materials based upon knowledge
of tree growth and root development requirements. Here, minimizing available soil material faults
or interfaces and tree root spaces are the preferred means for controlling and discouraging tree
growth with the philosophy ‘Build it correctly and they will not come!’
2.
Root kill zones utilizing cultivation methods, sawing and cutting, trenching, vibratory plows, and
chemicals to control, discourage, and remove root structure. However, these methods often result
in damaging or killing the tree that, perhaps, should have been removed in the first place.
3.
Root exclusion zones utilizing soil structure changes, soil compaction, water/aeration, stress,
anaerobic conditions, soil injections and slurries, soil additives, and chemicals to prevent roots
from colonizing the soil structure areas due to applied physical or chemical changes to the soil.
Changing the soil structure, pore space volume or drainage/aeration matrices can generate a soil
environment that roots cannot effectively grow and sustain. A variety of physical- or chemicalbased soil altering materials (i.e., soil injected clay slurry or cement solutions) can be effective, at
least over the short term if adequate soil volume is treated. Compacting soils appear to be a very
good way to prevent root colonization. High density soils increase the resisting strength of these
soils to root penetration and deprive the roots from needed oxygen and available water. Certain
types of clay soils, freeze-thaw cycles, biological activity, and poor soil compaction can, over
time, produce root-accessible pore space. Soil or infrastructure building material additives that
neutralize or sterilize the available minerals and nutrients such as nitrogen gas, sulphur, sodium,
zinc, borate, salts, or herbicides may produce serious environmental consequences, short-lived
results, and non-targeted damage potential. Other methods or additives may be cost-prohibitive.
See Figure 6 at end of chapter for root clearance zones.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
4.
Air gap systems designed to provide temporary and permanent air spaces for root pruning and
lack of root support by use of large cobble stone barriers and drain systems. One of the more
effective means of controlling tree root growth is providing stone matrices that dry quickly, create
large air gaps, have poor water-holding ability, and are impermeable to systematic root
penetration. Gravel layers or areas having at least 3/4 inch stone size or clean, graded, mediumsized rubble (crushed brick remnants or recycled paving and other materials), provided it is not
covered or filled in with sand, are reported to produce large enough air gaps to discourage root
growth.
5.
Barrier systems using commercial root traps, root deflectors, containment devices, metals, screens,
plastics, paints, and inhibitors. One of the easiest and most available materials used to control root
growth are various types of 2D-type screens and barriers. While some barriers are not completely
effective, many types have been shown to be effective. A list of mechanical, biological and
chemical tree root growth control barriers, products and systems is shown in Table 3.
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
Copper sulfate-soaked, synthetic, non-woven fabric
Copper screen
Cupric Carbonate (CuCO3) in latex paint
Fiberglass and plastic panels
Fiber-welded geosynthetic fabric/mesh
Galvanized metal screen
Ground-contact preserved plywood
*Geomembranes and heavy rigid plastics
Infrastructure aprons and footings
Metal roofing sheets
Multiple layers of thin plastic sheets
Nylon fabric/screen
Permeable woven geosynthetics
Rock-impregnated tar paper/felt
*Slow-release chemical barriers
Thin layered bitumen & herbicide mixtures
Woven and non-woven slit-film plastic sheets
*Common commercial tree growth control products available
Table 3. Selected list of tree root growth control barriers (after Coder, FOR98-11, 1998)
The costs of these products will likely continue to decrease as the demand for these products
increases in the future. Of the barriers shown in the list, three types are most commonly used:
traps (root engaging and constricting), deflectors (walls), and inhibitors (chemical constraints).
Combined features of the barrier, the site, and barrier installation and maintenance are critical to
their effectiveness, but no barrier should be assumed to stop all roots under all conditions. Most
types of mechanical and chemical barriers have limited effectiveness lives and this should be
factored into any long-term cost analysis. The reader is directed to the Table 3 reference source
and other related publications for details on commercially-available root barriers.
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Chapter 5
6.
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Directed growth systems to concentrate roots in desired directions, guide root growth along
channels, allow root survival in desired areas, and create root culverts or layers. As noted earlier,
roots are opportunistic and grow and proliferate where there are good supplies of resources.
Understanding root elongation, colonization, and survival processes allows growth-favoring soil
layers, corridors, and areas to be designed for directing or deflecting roots away from infrastructures
where tree roots can be harmful. Several methods or systems are used to attract, deflect, channel or
lead roots in a direction or area as needed. One attraction method used is called “baiting” and
involves providing ideal essential soil condition resources in a direction away from an infrastructure.
The net result is a much higher survival and growth rate in that part of the root system as opposed
near infrastructures where root damage can occur. Water, growth nourishment elements, and oxygen
should be limited and compaction should be maximum near infrastructures.
Another method is to “shepherd” roots to desirable locations using trenches, channels, layers,
raceways, tunnels, and other devices that are surrounded by root control obstacles, barriers, or
resource constraints. Growth channels filled with rich, well-aerated, ecologically healthy growth
medium will encourage root colonization and survival in areas away from sensitive infrastructure
targets.
7.
Selection of desired species of trees that require lower soil oxygen environments, have improved
root morphology, and are more effective species for long-term solutions. This method focuses on
choosing and planting available tree species that can survive under rather limited or harsh
environmental conditions. Several tree species are available that are small in size, have shallow
and less aggressive rooting, and are slower growing. Dam owners, however, should be reminded
again that trees in general are not a good plant option and have no place on dams; instead, more
desirable, native grasses should be planted and maintained.
8.
Creating avoidance zones to separate tree growth from earthen dam embankments and dam
appurtenances where root damage may be critical thus establishing biological-free zones that
reduce potential problems. This method simply recognizes that there are places where trees are
acceptable and other places, namely dams, where they are not (see Figure 6).
The most practicable of these methods for use on earthen dams are those associated with
intelligent design development, exclusion zones, kill zones, and barriers. Within this
group of suitable methods the combination of intelligent design development and
exclusion zones are the most effective. With an understanding of the previous meshing of
soil properties with healthy tree root elongation, it is not difficult to develop an intelligent
design scheme for new dams and the remedial repair of existing dams. An intelligent
design philosophy associated with dam embankment design and construction would
involve proper embankment soil compaction as the means of exclusion of root
elongation.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
In summary, there are many tools, methods and options for minimizing or preventing tree
root-caused damage to earthen dams. The most important management (and design)
concept to understand is how tree roots are invited to be associated with interstitial
elements and colonize soil matrices and discontinuities, and resource availability areas.
Our responsibilities as owners and dam design engineers must lie with creating and using
any or a combination of the numerous root growth control tools and techniques that are
tree-literate so that trees do not have the opportunity to become a safety problem to
embankment dams and their appurtenances in the first place.
Exclusion by Embankment Compaction
Design and construction practices of using optimum compaction of embankment soils
reduce potential settlement of embankments, increases shear strength of the embankment
soils, decreases the permeability of the embankment soils, and minimizes long-term
changes in the physical and engineering properties of soils. When embankment soil
compaction results in the attainment of desirable objectives from a geotechnical
engineering behavior perspective of earthen slopes, compaction of embankment soils also
precludes tree root growth and elongation as a result of exclusion of most of the
requirements for healthy root elongation and tree growth. As has been previously noted,
densely compacted soils discourage root elongation through increased resistance, lowered
oxygen levels, and reduced available water. Traditional embankment soil compaction
specifications require that the soil be compacted to about 95 to 98 percent of the standard
Proctor maximum dry density as determined by ASTM D-698. Furthermore, most
properly written soil compaction specifications generally require that compaction
moisture contents be maintained about two percent below to three percent above
optimum moisture content. At these degrees of compaction and at these moisture
contents, soil oxygen content, water content, and soil pore size are not available for
healthy root elongation and tree growth. Even if there is sufficient moisture content in the
soil to otherwise sustain healthy root elongation, the soil pore sizes are so small that
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
available pore water cannot be effectively moved to the root system. Consequently, the
compacted dam embankment fill soil produces an exclusion system that mechanically
impedes healthy root elongation and tree growth. Table 2 provides a summary of
minimum air voids for various soil types required to impede root elongation for healthy
root and tree growth.
Maintenance Mowing and Kill Zones
The second most effective method of controlling woody vegetation and tree growth on
dam embankments is through the use of native grass or ground cover with maintenance
mowing, and using kill zones where necessary around critical structures to control trees
and other undesirable nuisance-types of vegetative growth. Maintenance mowing should
be done at least twice per year with one mowing scheduled for spring after initiation of
new spring growth and the second mowing scheduled for late fall immediately prior to
the first killing frost or freeze (See Chapter 7). The spring mowing should be a very close
cutting of all vegetation to allow maximum sunlight to penetrate to desirable grass cover
species. The fall cutting should not be as close as the spring cutting to provide maximum
resistance to surface runoff erosion and to provide cover for desired wildlife species
(quail, rabbit, grouse, songbirds, etc.).
In areas where regular maintenance mowing is not practical to control woody vegetation
and tree growth, the selective use of herbicides might become necessary to control small
woody vegetation and tree growth. There are many commercially available herbicides
that are environmentally safe to use in most applications. However, one must always be
careful in the use, or overuse of herbicides, because they are design to kill and/or impede
(slow) plant growth. Overuse of herbicides may contaminate areas of the dam
embankment to such an extent that desirable grass cover cannot be effectively grown.
One must always follow manufacturers recommendations when using herbicides, or
better yet, solicit the advice of the nearest USDA/NRCS agent prior to using herbicides to
control woody vegetation and tree growth on earthen dams.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Chemical Barrier Systems to Inhibit Root Growth
Commercially available barrier systems are effective in controlling root elongation and
growth; however, many of these barrier systems are relatively expensive and cannot be
justified for placement over the entire earthen dam embankment. These barrier systems
are often economical for placement on portions of earthen dams where accessibility is
difficult after construction and/or where particularly problematic and nuisance woody
vegetation and tree growth is likely to occur.
One typical biocide product, called “Biobarrier©” is marketed and promoted, among
other applications such as sidewalk and landfill cap protection, to prevent tree and plant
roots from penetrating dams. The product consists of long-term, slow release nodules
containing Trifluralin herbicide, that are bonded to a geotextile fabric as shown in
Figure1.
Figure 1. Chemical biocide barrier installation showing slow-releasing biocide nodules attached in a
woven fabric matrix and installed under a cover of soil, mulch, gravel or stone (Biobarrier©).
This particular barrier is environmentally acceptable to EPA and indicated to be effective
against all types of roots around pipes, hardscapes, and dams and levees. While the
product is guaranteed for 15 years, its life is inversely proportional to environmental
temperature conditions.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
For example, its effectiveness is expected to be about 40 years at 20oC (68oF) and about
100 years at 15oC (60oF). For deep soil cover, it is expected to last 100 years; for near
soil-surface weed control installations, where temperatures are higher and cycle daily, the
projected life is expected to exceed the guaranteed 15-year life. Figures 2 and 3 show an
application of this product on a 25-foot high and 350-foot long earthen dam to prevent
deep penetration of deep-rooting native trees and woody vegetation such as willows,
sagebrush, and chokecherries.
Figure 2. Earth dam installation of chemical
Figure 3. Installed chemical barrier on a
dam in Montana (Kershner, 1992)
Herbicidal Applications
Herbicidal delivery to control undesirable vegetation depends on several considerations
which include (a) types of plants and weeds (herbaceous, vines, trees, brush,
phreatophytes, etc.), (b) site conditions (geology/sinkholes/karst), topography, (c)
proximity to water bodies, (d) riparian land use, (e) sensitive environmental factors
(Federal, state & local regulations; potential off-site wind drift over water or land), and
(f) application factors (dosage, placement, retention time, plant growth stage,
physiological factors, and method of application). A very important consideration is for
the user to follow the herbicide manufacturer’s warnings and instructions. The user is
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
also encouraged to consult with a local county extension office or agent to obtain advice
on the best and safest herbicide to use and on what recommended application technique
to use. While there are several herbicidal delivery methods available, the most common
techniques are shown below in Figure 4.
•
•
•
•
•
•
•
Foliage spraying
Tree injection
Frill or girdle treatment (slash through bark then spray or paint)
Basal bark spraying
Cutting tree and poisoning stump
Soil treatment
Other
Figure 4. Herbicide delivery application methods
Some of these techniques and herbicides used are illustrated in Figures 5a – 5f. The U.
S. Department of Agriculture (SCS, now NRCS) published a useful methods, treatment
points, and time of treatment guidelines for controlling trees and brush on dams,
including some of the applications listed in Figure 4 (USDA, 1988). Table 4 summarizes
the USDA recommendations.
With the exception of Krenite, which is applied to the
foliage, 2,4-D is the only approved herbicide for poisoning trees on dams. 2, 4-D is
manufactured by several companies and is sold under several trade names. In all cases,
the user is cautioned again to follow the manufacturer’s instructions and should consider
the manufacturer’s label instructions to supercede recommended instructions in the
USDA table.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Vanquish
Weedone
Chopper
Pathway
Tordon
Arsenol
Figure 5a. Frill cut application.
Touchdown
Crossbow
Access
Lower 18”:
Ester derivatives
to penetrate bark
Figure 5b. Basal bark spraying.
Apply treatment
within an hour
of cutting tree
Crossbow
Pathway
Tordon
Weedone
Figure 5c. Hypohachet application.
Figure 5d. Cut-stump application.
Vanquish
Weedone
Weedmaster
Crossbow
Banvel
Roundup
Accord
Arsenol
Chopper
Pathway
Tordon
Touchdown
Spike
Garland
Figure 5e. Backpack foliar application on dam.
Figure 5. Applications and techniques for different herbicidal deliveries to trees and brush, with
example commercially-available herbicides listed.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Figure 5f. Tractor spraying application on dam.
Recommended Time
Method of Application
•
•
•
•
•
•
Cutting trees and poisoning stumps
Injection
Foliage spraying
Frill treatment (trees larger than 4” dbh)
Basal spraying (trees smaller than 6”
dbh)
Prescribed burning (trees smaller than
2” dbh)
•
•
•
•
•
Growing season
Anytime
Last two (2) months of growing season
Anytime
Growing season
•
See technical specifications
Table 4. Recommended methods and time of herbicide treatment application (USDA, 1988).
USDA recommends that trees killed by herbicide should be removed within the year
following treatment to prevent front slope from falling into the reservoir and plugging the
spillway. Downed trees on the back slope should also be removed to prevent potential
problems of seepage, erosion, burrowing animals, etc.
The reader is referred to the USDA guideline for detailed discussion on each of the six
treatment methods listed in the above table. These methods can be applied to establish
tree and woody plant clearance or avoidance zones on and around dams as illustrated in
Figure 6.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Groin
Groin
Slope
Toe
area
ConcreteSpillway
structure
Concrete
structure
Structures
Drains
Figure 6. Tree clearance zones for embankment dams and dikes.
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Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
Desert Plants
Deep-rooted desert plants, when left unchecked, can propagate rapidly on earthen dams
located in arid and semi-arid regions of the U. S. Some of these deep-rooted plants
include Desert Broom shown in Figure 7, Salt Cedar, Mesquite, Cypress, Cottonwood
and Paloverde. All of these species require considerable effort to control and should not
be allowed to become established anywhere on dams. Palm trees can be a problem in
that they are shallow-rooted, but develop a large root ball that can produce large cavities
when toppled during high winds. Upstream and downstream access roads, in place at
many dams, should be utilized to create a buffer zone between these species and the toe
of dams.
Figure 7. Deep-rooting Desert Broom Plant
The Maricopa County, Arizona, Flood Control District (MCFCD) recommends, in cases
where deep-rooted plants are two feet in height or less, that they be controlled with a 35% solution of Roundup® Pro (Renckly and Drake, 1999). If the plants are over two feet
in height they should be hand cut to ground level. The stumps should be treated within
the first five minutes by an almost straight mix of either Roundup Pro® or Garlon 3AGarlon 4®, depending on the temperature conditions. MCFCD recommends that when
5-20
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
treating Salt Cedar near waterways that Rodeo be sprayed at a 3-5% solution with six
ounces of Siltwet® per acre added. This is sprayed on plants two feet in height and
under. Plants over two feet are hand cut and the stump treated with an almost straight
solution of Rodeo® within five minutes of cutting the plant.
Revegetation on earthen dams is recommended to minimize erosion on the embankment
slopes and to provide natural landscaping for earthen dams.
MCFCD recommends
hydro-seeding over labor-intensive hand-seeding to revegetate dam embankments.
Figure 8 illustrates hydro-seeding operations on a floodway dam. Seed, water, tack
material and a wood fiber or paper mulch are mixed in a hydro-seeder and sprayed
directed onto the slopes. The seeds are encapsulated in the mulch and tack material until
enough moisture is present to begin the germination process.
Figure 8. Hydro-seeding operations on a floodway dam in Maricopa County, Arizona
(Renckly & Drake, 1999).
5-21
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
MCFCD has found that it takes 2 to 3 years before “significant” vegetative cover results
are achieved because of the arid climate and high degree of embankment compaction.
MCFCD determines the desirable seed mix by first laying out a test acre on the dam
embankment and a plant count is then taken of all the different plant species that are
native to the area and placed on the test acre. This plant count is converted by the seed
supplier into the amount of seed needed to germinate the desired amount of the species
per acre. The amount of pure live seed (PLS) applied for individual plant species also
varies by availability from the local seed supplier. Table 4 shows a seeding mixture
specified for one of the District’s dams and is typical of specified hydro-seeding mixes.
No deep-rooted species are allowed in the seed mix.
MCFCD has found that revegetation efforts have successfully reduced erosion problems,
but has attracted both desirable and undesirable animals.
SEEDING MIXTURE
Common name
Scientific Name
Purple three-awn
Indian Wheat
Needle Grama
Desert Marigold
Mexican Gold Poppy
Creosote
Brittle Bush
Bursage
Aristida purpurea
Plantago insularis
Bouteloua arstiodoides
Baileya multiradiata
Eschschotzia mexicana
Larrea tridentata
Encelia farinosa
Ambrosia deltoidea
Pounds of Seed Per Acre
4
3
1
1
1
8
2.5
2
** Note: Apply 1500 pounds of wood fiber mulch in Hydro-seed mix, plus 150 – 200
pounds tack material per acre.
Table 4. Typical seed list specified for a flood control dam managed by the Maricopa County,
Arizona, Flood Control District (Renckly & Drake, 1999)
5-22
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
References:
1. Coder, K. D., Tree Root Growth Control Series: Root Growth Requirements
and Limitations, Univ. of Georgia, Cooperative Extension Service Forest
Resources, Publication FOR98-9, March 1998, 8 pp.
2. Coder, K. D., Tree Root Growth Control Series: Soil Constraints on Root
Growth, Univ. of Georgia, Cooperative Extension Service Forest Resources,
Publication FOR98-10, March 1998, 8 pp.
3. Coder, K. D., Tree Root Growth Control Series: Methods for Root Control,
Univ. of Georgia, Cooperative Extension Service Forest Resources,
Publication FOR98-11, March 1998, 9m pp.
4. Coder, K. D., Selected Literature: Root Control Methods, Univ. of Georgia,
Cooperative Extension Service Forest Resources, Publication FOR98-13,
March 1998, 4 pp.
5. Coder, K. D., Root Growth Control: Managing Perceptions and Realities,
Proceedings, Second International Workshop on Tree Root Development in
Urban Soils, International Society of Arboriculture, March 5-6, 1998, edited
by D. Neely and G. Watson, pp. 51-81.
6. Coder, K. D., Engineered to Fail? Tree Root Management on Dams, Abstract,
University of Georgia, Athens, November 1999, 1 page.
7. Rendig, V. V. and H. M. Taylor, Principles of Soil-Plant Interrelationships,
McGraw-Hill, 1989.
8. USDA-SCS, Technical Notes (OK-8), Control of Trees and Brush on Dams,
Stillwater, Oklahoma, April 5, 1988.
9. Sisneros, D., USDI-USBR, Res. And Lab. Serv. Div., Upper Colorado Region
Saltcedar Cost Analysis, Memo 94-2-2, February 1994.
10. USDI-USBR, Water Operation and Maintenance, Bulletin No. 150,
Guidelines for Removal of Trees and Vegetative Growth from Earth Dams,
December 1989.
11. Biobarrier©, Application Manual, Root Control System,BBA
Nonwovens/Remay, Inc. Product Information, August 1999, Old Hickory,
Tennessee.
5-23
Chapter 5
Controlling Tree and Woody Vegetation
Growth on Earthen Dams
12. New Hampshire DES Environmental Fact Sheet, Tree Growth on Dams, WDDB-8, 1997.
13. Ohio Department of Natural Resources, Dam Safety: Trees and Brush, Fact
Sheet 94-28, July 1999.
14. Pennsylvania DEP, Fact Sheet – Vegetation/Erosion Control on Dams, 31-40
FS,DEP1909,June1997,http://www.dep.state.pa.us/dep/deputate/watermgt/WE/
FACTS/fs1909.htm.
15. Renckly, T., Drake, G., Plant & Animal Management Practices on Flood
Control Dams, Maricopa Co., Arizona, November 1999.
16. USDA-SCS (NCRS), S. Tech. Serv. Ctr., Technical Note 705, Operations &
Maintenance Alternatives for Removing Trees from Dams, April 1, 1981.
17. Univ. of Tenn. Agr. Extn. Serv., R. Bullock, Chemical Vegetation
Management on Non-cropland, Bulletin PB-1538, December 1995.
18. Univ. of Tenn. Agr. Extn . Serv., G. Rhodes & G. Breeden, 2001 Weed
Control Manual for Tennessee, Bulletin PB-1580, December 2000.
19. Kershner, C., Geotextiles - It’s Only Natural, Land and Water, January 1992.
20. STS Consultants Ltd., ASDSO Working Group, Dam Safety Guidebook,
1985.
21. Association of State Dam Safety Officials (ASDSO), Report on Specialty
Workshop #1: Plant & Animal Impacts on Earthen Dams, Knoxville,
Tennessee, November 30-December 2, 1999, June 2000.
5-24
Chapter 6
Dam Remediation Design Considerations
Specific dam remediation design considerations, procedures, and techniques will be
considered for each of the previously identified dam safety inspection and evaluation
zones. Figure 1 presents these zones as a review prior to discussion of potential dam
remediation design considerations for each zone. Dam remediation design alternatives
presented herein should be considered examples. These remediation design examples
should not be considered the only alternatives for use in dam remediation design to
correct deficiencies associated with tree and woody vegetation growth on earthen dams.
Some additional dam remediation design alternatives presented for correction of tree and
woody vegetation growth related deficiencies also provide positive correction of other
deficiencies and protection against other types of earthen dam deterioration.
6-1
Chapter 6
Dam Remediation Design Considerations
Inspection and Evaluation Zone 1
Figure 2 illustrates potential problems that can occur in Zone 1 with respect to tree and
woody vegetation growth on earthen dams. This illustration also depicts the occurrence
of wave erosion, vehicle access, and surface runoff erosion. Potential problems illustrated
include instability of relatively large trees on the upstream slope and dam crest, and
alteration of the seepage line as a result of wave erosion.
Dam remediation design techniques necessary to address potential problems illustrated in
Figure 2 are illustrated in Figures 3 and 4. Dam remediation construction typically
requires lowering of the normal pool elevation and/or complete drawdown of the retained
reservoir. This is particularly true for dam remediation construction in Zone 1. The
normal pool elevation should be lowered as far ahead of the scheduled dam remediation
construction as practicable.
6-2
Chapter 6
Dam Remediation Design Considerations
6-3
Chapter 6
Dam Remediation Design Considerations
Tree and woody vegetation growth in Zone 1 must be undercut to remove all stumps,
rootballs, and root systems developed by tree penetrations as illustrated in Figure 3. The
required depth of undercutting typically extends to near the limits of Zone 1, which is
about four feet below normal pool elevation. In the case of earthen dams with narrow
crest widths, the backslope of the undercut area will typically extend to near the
centerline of the dam crest or the downstream limits of Zone 1. Subsequent to
undercutting affected areas of Zone 1, the undercut area must be thoroughly inspected to
confirm that all major root systems (greater than about one-half inch in diameter) have
been removed during the undercutting operation. Following inspection and approval of
the undercut area by the engineer, suitable backfill should be placed in the excavation and
properly compacted to the dam remediation design limits. Backfill should consist of
approved embankment fill material and should be compacted to a minimum of 95 percent
of the maximum dry density of the fill soil as determined by the standard Proctor
compaction test (ASTM D-698). In conjunction with the undercutting and backfilling, the
dam remediation design should include a slope protection system to deter future tree and
woody vegetation growth and reduce the potential for wave and surface runoff erosion.
Figures 4(a) through 4(c) illustrate various configurations of rigid (concrete) upstream
embankment slope protection systems. Figure 4(a) illustrates a concrete slab being placed
directly on the upstream slope from about three feet below to about two feet above
normal pool elevation. While this system is somewhat limited relative to the area of
protection, the most critical aspect of this system is that it provides no filtration and/or
drainage system beneath the concrete slab. Continual wave action and the buildup of
hydrostatic pressures beneath the concrete slab will eventually result in downward
movement of the slab. Figure 4(b) illustrates a better dam remediation design utilizing a
concrete slab slope protection system. This slope protection system has been improved
over the original system by covering a larger area of the upstream slope and by providing
a filter system beneath the concrete slab protection system. The author is of the opinion
6-4
Chapter 6
Dam Remediation Design Considerations
that the dam remediation protection system shown in Figure 4(c) is the most desirable
and cost effective design for use of reinforced concrete for a protection system. The
reinforced concrete wall provides a gentle slope to flat backfill area that can easily be
maintained by mowing to preclude tree and woody vegetation growth. In addition, this
dam remediation design alternative can be used to provide a wider effective dam crest
and provides excellent protection against wave erosion.
NOTE: Reinforced concrete wall and slab systems constructed on the upstream slope
must always be provided with filtration/drainage systems to reduce the potential for
development of excessive hydrostatic pressures and internal erosion and scour of soil
from beneath the structures. The referenced figures are presented for illustrative
purposes and should not be used for actual dam remediation design without proper
design analyses to confirm any indicated dimensions of the drawings.
Alternative flexible upstream slope protection system designs for use in Zone 1 are
shown in Figures 4(d) and 4(e). The author has utilized both of these flexible slope
protection systems effectively to reduce potential tree and woody vegetation growth on
upstream slopes and to provide resistance to wave and surface erosion. Figure 4(d)
illustrates a typical gabion wall system while Figure 4(e) illustrates the use of a
Mechanically Stabilized Earth (MSE) wall system for protection of the upstream slope of
an earthen dam.
NOTE: Granular backfill material used in design and construction of these flexible
wall systems must be protected against soil contamination and internal erosion of
retained soil by an effective geotextile filter/drainage material and/or a graded
aggregate filter. These figures are presented herein for illustrative purposes and should
not be used for actual design without proper design analyses to confirm any indicated
dimensions of the drawings.
6-5
Chapter 6
Dam Remediation Design Considerations
Inspection and Evaluation Zones 2 and 3
Potential problems associated with tree and woody vegetation growth on earthen dams in
identified Zones 2 and 3 are illustrated with dam remediation design procedures in Figure
5. Potential problems illustrated for Zone 2 include the growth of mature trees having
stump diameters greater than twelve inches. Mature trees having stump diameters greater
than eight inches are illustrated at various locations throughout Zone 3 and in the overlap
area of Zones 2 and 3.
6-6
Chapter 6
Dam Remediation Design Considerations
Two dam remediation design procedures are illustrated in Figure 5 for removal of trees of
various sizes. This illustration implies that trees located in the overlap area of Zones 2
and 3 having stump diameters less than about twelve inches could be cut flush with the
ground and left in place for future treatment of the decayed stump and rootball system.
However, removal of all stumps, rootballs and root systems is always the better and more
conservative approach to removal of mature trees. Subsequent to cutting of trees having
stump diameter less than about twelve inches in the overlap area of Zones 2 and 3, the
surface of the stump can be treated with a protective coating similar to polyurethane that
will prolong the decaying process. Conversely, the referenced illustration indicates that
any trees in Zone 2 upstream of the overlap area of Zones 2 and 3 having stump
diameters of twelve inches or greater should be treated by total removal of the tree,
stump, rootball, and root system. The suggested dam remediation design and construction
procedure suggested for complete removal of trees, stumps, rootballs, and root systems in
Zones 2 and 3 consists of the following activities:
1.
Cut the tree approximately two feet above ground leaving a well-defined
stump that can be used in the rootball removal process;
2.
Remove the stump and rootball by pulling the stump, or by using a
track-mounted backhoe to first loosen the rootball by pulling on the stump
and then extracting the stump and rootball all together (this is much the
same procedure a dentist would use in extracting a tooth);
3.
Remove the remaining root system and loose soil from the rootball cavity
by excavating the sides of the cavity to slopes no steeper than 1:1
(horizontal to vertical) and the bottom of the cavity approximately
horizontal; and
4.
Backfill the excavation with well-compacted soil placed in relatively thin
lifts not greater than about eight inches in loose lift thickness. Compaction
of backfilled soils in these tree stump and rootball excavations typically
requires the use of manually operated compaction equipment or
compaction equipment attached to a backhoe.
NOTE: All disturbed areas must be protected by seeding and mulching.
6-7
Chapter 6
Dam Remediation Design Considerations
Figure 5 further illustrates that trees located in Zone 3 that have stump diameters greater
than about eight inches should be treated by total removal. The removal procedure should
be the same as previously described for larger trees in Zone 2. Trees having stump
diameter of less than about eight inches could be cut flush with the ground and treated
with a waterproofing sealant similar to polyurethane to prolong the stump and rootball
decaying process. Again, complete removal of the stumps, rootballs, and root systems of
all mature trees is a better and more conservative method of remediation.
Inspection and Evaluation Zone 4
Figure 6 illustrates potential problems associated with tree and woody vegetation growth
in Zone 4 of an earthen dam with suggested dam remediation design and construction
procedures.
6-8
Chapter 6
Dam Remediation Design Considerations
Young immature trees having stump diameters less than about six inches can be removed
by cutting flush with the ground and treating the stump with a wood preservative and/or
sealant to prolong the decaying process. This procedure is based upon the fact that
immature trees of this size typically have not developed a rootball and/or root system that
will significantly impact the zone of saturation or the seepage line in Zone 4.
Trees having stump diameters greater than about six inches must be treated by complete
removal; however, the dam remediation design and construction procedure for total
removal of trees in Zone 4 is somewhat more complicated than total removal of trees in
previously discussed zones. Treatment of mature tree penetrations in Zone 4 involves the
following activities:
1. Cut the tree approximately two feet above ground level leaving a prominent
stump for use in the rootball extraction process;
2. Remove the stump and rootball by pulling the stump or extracting with a
track-mounted backhoe after loosening the rootball by pulling on the stump
from different directions;
3. Clean the rootball cavity to remove loose soil and the remaining root system
by excavating the rootball cavity with maximum 1:1 (horizontal to vertical)
side slopes and a horizontal bottom; and
4. Install a subdrain and/or filter system in the tree penetration excavation and
backfill with compacted soil placed in maximum loose lifts of eight inches.
Note: Backfill placed in all tree removal excavations must be compacted to a
minimum of 95 percent of the maximum dry density as determined by
ASTM D-698.
Note: Subdrain and/or filter systems installed in tree removal excavations in
Zone 4 may be incorporated into major subdrain systems to be installed in the
overlap area of Zones 4 and 5.
6-9
Chapter 6
Dam Remediation Design Considerations
Inspection and Evaluation Zone 5
The author identified Zone 5 as one of the two most critical zones for tree and woody
vegetation growth on an earthen dam. Figure 7 illustrates some of the problems that can
occur with tree and woody vegetation growth in Zone 5. The major adverse feature in
Zone 5 is typically the interception of the downstream embankment slope by the seepage
line. The author is a strong advocate of the installation of embankment subdrain systems
during dam remediation design and construction even though the earthen dam may have
been provided with an embankment subdrain system during original design and
construction.
One must understand the impact of tree removal in Zone 5 on the seepage line and the
quantity of seepage that will occur subsequent to dam remediation in this zone. As
indicated by Figure 7, trees in Zone 5 having stump diameters less than about four inches
can be cut flush with the ground and the stump treated with a waterproof sealant to
6-10
Chapter 6
Dam Remediation Design Considerations
prolong stump and rootball decay. Trees having stump diameters greater than about four
inches must be removed completely. If the embankment toe drain or subdrain system is
installed in advance of tree removal in Zone 5, the rootball cavity can be backfilled with
compacted soil, provided seepage does not emerge from the excavation and/or the tree is
located beyond the toe of the embankment slope. Tree rootball cavities existing beyond
the toe of the downstream embankment slope generally require the installation of a filter
system and in some cases a weighted filter system as indicated in Figure 7. The weighted
filter system may be converted to a weighted drain system by installing a drain and outlet
pipes connected to the outlet pipe of the embankment subdrain system.
Summary of Dam Remediation Design Considerations
A summary of dam remediation design considerations for treatment of tree and woody
vegetation on earthen dams is presented below. Dam remediation design procedures and
techniques are presented for treatment of various size trees in the identified dam safety
inspection and evaluation zones.
Remedial Repair Zone
Procedures and Techniques
Zone 1
Remove all trees, stumps, rootballs, and root
system; clean rootball cavity; and backfill with
properly placed and compacted soil backfill. Install
tree and woody vegetation and wave erosion
protection system on the upstream slope from about
four feet below normal pool elevation to about three
feet above normal pool elevation.
Zone 2
Cut trees in overlap area of Zone 2 and Zone 3
having stump diameters of twelve inches or less
flush with the ground and treat the stump with a
waterproof sealant to prolong stump decay.
6-11
Chapter 6
Dam Remediation Design Considerations
Completely remove trees having stump diameters of
about twelve inches and greater, and backfill
rootball cavity with properly compacted backfill
soil.
Zone 3
Cut trees having stump diameters of about eight
inches and less level with the ground and treat the
stump with a waterproof sealant to prolong stump
and rootball decay.
Completely remove all trees having stump
diameters greater than about eight inches and
backfill the cleaned rootball cavity with compacted
backfill soil.
Zone 4
Cut all trees having stump diameters of six inches
or less flush with the ground and treat the stump
with a waterproof sealant to prolong stump and
rootball decay.
Remove all trees having stump diameters greater
than about six inches, install subdrain and/or filter
systems, and backfill with properly compacted soil
around the filter/drain system.
Zone 5
Cut all trees having stump diameters of about four
inches and smaller flush with the ground and treat
the stump to prolong stump and rootball decay.
Install a major embankment
system to lower the phreatic
and discharge embankment
major subdrain with tree
removal where possible.
toe drain or subdrain
surface, filter, collect,
seepage. Incorporate
rootball and stump
Remove all trees located beyond the toe of the
downstream slope having stump diameters greater
than about four inches. Install weighted filters
and/drain systems in rootball cavities where seepage
boiling and soil piping is likely to occur.
6-12
Chapter 6
Dam Remediation Design Considerations
Tree and Woody Vegetation Growth Control Program
Many individual dam owners and small dam owner organizations are not financially
capable of undertaking comprehensive dam remediation projects in one major
construction contract. Therefore, they must undertake dam remediation programs in a
sequential manner. The following sequential dam remediation program for controlling
tree and woody vegetation growth provides the owner, regulator, and engineer with a
reasonable opportunity to effectively evaluate the condition of an earthen dam and to
prioritize dam remediation relative to observed dam safety issues.
1.
First Year:
Cut all tall grasses, weeds, underbrush, and trees and woody
vegetation having stump diameters of four inches or less
flush with the ground and treat all cut stumps with a
waterproof preservative to prolong rootball and stump
decay.
2.
Second Year:
Cut all trees in Zones 1 through 4 having stump diameters
of six inches or less flush with the ground and treat the
stumps to prolong stump and rootball decay. Keep all zones
mowed and/or maintained to preclude renewed growth of
previously cut woody vegetation. Repair most severe
animal penetrations that exhibit seepage flows and/or cause
unstable slope conditions on Zones 1, 4, and 5.
3.
Third Year:
Initiate comprehensive remedial dam repair investigations,
analyses, and preliminary design. Remove all trees from
Zones 1 through 3 having stump diameters less than about
eight inches by cutting flush with the ground and treating
the stump with a preservative to prolong stump and rootball
decay.
4.
Fourth Year:
Finalize remedial dam repair design and begin construction
of remedial repairs for all plant and animal penetrations
that require special remedial dam repair design
considerations.
6-13
Chapter 6
5.
Dam Remediation Design Considerations
Fifth Year:
NOTE:
Finalize remedial dam repair construction and begin an
operation and maintenance program that will preclude the
need for future remedial dam repair associated with plant
and animal penetrations of earthen dams.
Earthen dams that exhibit severe dam safety deficiencies
and dam safety issues that cannot be prolonged as a result
of potential imminent dam failure are not subject to the use
of this type of sequential dam remediation program!!!
6-14
Chapter 7
Economics of Proper Vegetation Maintenance
Regular maintenance on a dam, especially attention to trees and brush, is known to be critical to dam
safety for several reasons (Tschantz, 2000):
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Overturning or uprooting trees causing large voids and reduced freeboard; and/or reduced
cross-section for maintaining stability
Decaying roots of dead trees causing potential seepage paths and piping problems
Interfering with effective dam safety monitoring, inspection and maintenance for seepage,
cracking, sinkholes, slumping, settlement, deflection, and other signs of stress
Hindering desirable vegetative cover and causing embankment erosion
Obstructing emergency spillway capacity
Falling trees causing possible damage to spillways and outlet facilities
Clogging embankment underdrain systems
Cracking, uplifting or displacing concrete structures and other facilities
Inducing local turbulence and scouring around trees in emergency spillways and during
overtopping
Providing cover for burrowing animals
Loosening compacted soil
Allowing roots to wedge into open joints and cracks in foundation rock along abutment
groins and toe of embankment, thus increasing piping and leakage potential.
State and federal dam safety officials and other dam safety experts agree that trees have no place on
dams. Federal agencies and some states do not allow trees to grow on dams. However, it is
estimated that about a third of the nation’s 77,000 inventoried dams have sufficient woody
vegetation to hinder effective dam safety inspections (ASDSO, 2000; Tschantz, 2000). Most states
require dam owners to remove trees and undesirable vegetation, but the cost of clearing and
grubbing trees and restoring the dam embankment slopes and crest is often cost prohibitive for many
dam owners, usually running into thousands of dollars. It would seem that regular control of woody
vegetation and maintaining the surface on an earthen is relatively inexpensive, compared to
removing trees on and repairing damage from neglected dams such as shown in Figure 1.
7-1
Chapter 7
Economics of Proper Vegetation Maintenance
Figure 1. Restored Downstream Slope on Fishing Creek Dam, Maryland (1991-92)
Likewise, it is important that owners maintain desirable vegetation on their dams on a regular
schedule to avoid the expense of periodically removing undesirable heavy brush and mature trees.
Early control is generally viewed to be the most cost-effective means of avoiding potential adverse
effects on these structures from their continued growth (USBR, 1989). The bulk of maintaining a
dam usually involves keeping the grass mowed and brush trimmed. An important question arises,
“How much is a dam owner justified in spending to maintain a dam on a regular or annual basis
to avoid having to bear the heavy cost of removing trees?” A correlative question then follows,
“How often should a dam be mowed to control undesirable woody growth?”
This chapter attempts to answer these questions, but there are many variables and site-specific
factors which need to be considered. Some assumptions also need to be made.
Tree Removal Costs
The cost of clearing and grubbing a dam depends on the size and type of trees, growth density, total
job size (i.e., number of acres of trees), location of growth (crest and/or both faces?), embankment
face steepness, slope condition (such as degree of wetness or surface texture), degree and type of
required surface treatment (backfilling, use of herbicides or bio-barriers, mulching, seeding,
fertilizing, etc.), and regional labor and construction differences.
7-2
Chapter 7
Economics of Proper Vegetation Maintenance
The reader is referred to Table 1 in Chapter 2 and Figure 2 below for unit area tree removal cost
comparison experiences reported in a survey by eight state dam safety officials in different regions
of the country. The survey data shows that the cost of clearing and grubbing trees and other woody
vegetation varies widely within and among states, but generally ranges from about $1000 to $5000
per acre, depending on site-specific conditions (Tschantz, 2000).
12000
11000
10000
9000
Minimum cost/acre
Maximum cost/acre
$Cost/acre
8000
7000
6000
5000
4000
3000
2000
1000
0
Figure
Georgia
Michigan
Minnesota
Nevada
Ohio
Oklahoma
Tennessee
Texas
2.
Comparison of tree removal costs per acre of dam surface area reported by 8 states.
These data compare favorably with the $1500 - $3000 bid price data for three Southeastern states
discussed earlier in Chapter 2 for cutting trees, removing stumps and rootballs, and grubbing the area
to remove roots for different dam conditions. While not included in the above Figure 2 chart data,
Massachusetts’ dam safety personnel reported in 2000 that, based on its own in-house experience,
some local consultants and other sources, “broad area” tree removal costs ranged from $5000 and
$6000 per acre or from about $800 to $1000 for individual 18-24 inch trees in their region. One dam
safety official, from Tennessee, provided detailed cost data for clearing trees from seven dams in
that state from 1995-1999. The cost for clearing and grubbing trees and for reseeding for one typical
dam in 1998 is described for the reader in Table 1.
7-3
Chapter 7
Economics of Proper Vegetation Maintenance
Dam Height
Length of Dam
Freeboard above Normal Pool
Density of trees ≤ 6 inches diameter primarily on downstream
face
Approximate surface area of downstream face
Approximate dam face slopes
Amount of brush cutting
Stumps grubbed out
Amount of hand work
Total job cost for clearing, grubbing & reseeding
Unit area job cost
Year job completed
22.3 ft.
830 ft.
8 ft.
“Moderate”
≅1.3 acres
3H:1V
“Moderate”
Yes
“Considerable
”
$4275
$3290/acre
1998
Table 1. Tree clearing/grubbing and reseeding cost for a “typical” dam
located in Fayette Co., Tennessee (Bentley, 2000)
For comparison purposes, general sitework cost information is available from various construction
cost books. General cost data for cutting and clearing out individual trees and for clearing wooded
area is shown in Table 2 from one source (BNi, 2001). Indices are normally provided for factoring
in regional cost differences. Other cost book sources provide detailed material, labor and equipment
requirements for estimating site clearing costs (Means, 2001).
Clear small size wooded area:
• Light density
• Medium density
• Heavy density
$3,607/acre
$4,900/acre
$5,880/acre
Cut trees & clear out stumps:
• 9 to 12 inches diameter
• To 24 inches diameter
• 24 inches and up
$290 per tree
$370 per tree
$490 per tree
Table 2. General tree cutting and clearing construction cost data (Bni, 2001).
Similar general tree clearing and grubbing, chipping, seeding, mulching and fertilizing data for
estimating construction costs in various regions of the country are also available from other sources
(Means, 2001; AC&E, 2002). For example, 2001 Means cost data gives tree cutting, chipping,
7-4
Chapter 7
Economics of Proper Vegetation Maintenance
clearing, and grubbing costs for trees 6-inches or less to be $2975/acre and stump removal to be
$1425/acre for a total unit cost of $4400/acre. For trees up to 12 inches the cost is $6925/acre, and
for trees up to 24 inches the cost is $15,250/acre (Means, 2001). If burning is allowed, the cut and
chip costs can be significantly reduced. Hydro or air seeding, including seed & fertilizer is
estimated to be 35¢/square yard (about $1700/acre) (Means, 2001). Mulching would add to this
cost.
Maintenance Costs
For most dams, maintenance means keeping the crest and dam embankment slopes mowed and
trimmed. The cost of mowing a dam depends on many factors, including geographical location,
accessibility, condition of slopes as discussed above, degree of public use and desired aesthetics,
type of vegetation and frequency of mowing. Cost also depends on whether the work is done
directly by private owners, subcontracted commercially, or done by in-house state or federal
maintenance crews. Table 3 summarizes these factors. The availability of slope mowers as
illustrated in
Table 3. Factors Affecting Dam Maintenance Cost
•
•
•
•
•
Region of country
Type of ground cover & vegetation
Accessibility to dam
Surface condition
Size of job (surface area)
•
•
•
•
•
Embankment slope steepness
Mowing frequency
Local labor costs
Type of maintenance provider
Degree of public use; aesthetics
Figure 3 illustrates the use of a slope mower for easing the burden of maintenance for state and
federal agencies and for other multiple or large dam owners.
Figure 3. Example of slope mower
(Terratrac© photo used with
permission from AEBI North
America, Inc.)
7-5
Chapter 7
Economics of Proper Vegetation Maintenance
Most public works dams usually get mowed at least twice a year, in the early fall and late spring.
Many subdivisions, homeowner associations, and/or residential developments typically mow dams,
located in high-visibility areas, about once a month to every six weeks. One geotechnical
consultant, who specializes in embankment dam rehabilitation, uses a “rule of thumb” mowing
estimate of about $100 per acre with a minimum fee of $200 to $250 per mowing job (Marks, 2000).
1998 bid prices for mowing general right-of-way areas along East Tennessee highways averaged
about $32 per acre, with a range of about $28 to $38 per acre for four jobs (TDOT-Region 1, 2000).
The U. S. Corps of Engineers, Nashville District, furnished recent annual mowing costs for three
District dams, including some proximate recreation zones, having total mowing areas ranging from
8 to 27 acres. The average mowing cost for these three dams was about $55/acre and ranged from
$43.42 to $78.24/acre (Corps, 2000).
The Tennessee Valley Authority furnished similar estimated in-house annual mowing cost data
associated with general dam safety grounds maintenance activities for its dams. However, TVA’s
annual cost data included labor, supervision, slope mower fuel, parts, equipment, etc. and averaged
slightly over $600/acre for 31 saddle and main embankment dams with a cost range from about $45
to $2000/acre (TVA, 2000).
A dam owner is advised that, in addition to mowing cost, the total annual maintenance expenditure
should also include the expenses of dam inspection(s), minor repairs and rehabilitation of various
structural components, removal of obstructions from emergency and service spillways, and other
safety or operational costs associated with maintaining a dam.
Example maintenance cost analysis
The following example illustrates a rational procedure for answering the two earlier questions: 1)
how much should a dam owner spend yearly to maintain a typical earthen dam to control trees and
woody vegetation growth while avoiding bearing the cost of removing mature trees at a later date?
7-6
Chapter 7
Economics of Proper Vegetation Maintenance
and 2) how often should an earthen dam be mowed to maintain acceptable ground covering
vegetation? Maintenance expense in this example is for mowing only. Assumptions for this
example are as follows:
1. Dam Description:
•
Length = 900 feet
•
Crest width = 15 feet
•
Embankment slopes (upstream and downstream) = 3:1 (horizontal to vertical)
•
Height = 35 feet
•
Normal pool = 10 feet below crest
•
Nearly vertical end abutments
2. Economic Analysis Assumptions:
•
30-year project analysis period
•
Annual rates of return rates = 4, 6, 8, 10, and 15%
•
Zero annual inflation on recurring costs
3. Maintenance Assumptions:
•
Assume that 10-year old brush and trees are mature enough to significantly
hinder effective inspection. Trees of this age can reach in size from 6 to 8
inches in diameter, depending upon species, tree density and other
environmental conditions
•
Mowing costs = $100 per acre (with a minimum fee of $250 per mowing)
•
Trees can grow on all exposed upstream and downstream embankment slopes
and the crest of the dam
•
Assume tree removal, including clearing and grubbing, costs = $2500/acre
•
Seeding & mulch not included in surface restoration costs.
7-7
Chapter 7
Economics of Proper Vegetation Maintenance
Economic Analysis Calculations
Charts have been prepared and attached at the end of this chapter as a tool in helping to estimate
mowing areas (or tree stand estimates) for different dam configurations. Chart 1 can be used to
determine dam embankment slope area in acres for four slopes ranging from 1.5:1 to 3:1 (horizontal
to vertical) and for dam lengths of 200 and 500 feet. Linear interpolations and ratio extrapolations
can be made for other slope configurations and dam embankment lengths, respectively. Note that
when determining the area of an upstream embankment slope, the equivalent dam height entered into
the chart is the vertical distance between normal pool and crest elevation. Chart 2 is used to estimate
dam crest area for three convenient lengths; crest areas for other actual dam crest lengths can be
calculated from direct ratios. A self-guiding Chart 3 is provided to allow for small abutment area
reduction corrections to be estimated and applied to slope area determined from Chart 1.
For the assumed example dam given above, make the following computations:
1. Use the attached charts to estimate total mowable and potential tree-covered dam area:
(a) Downstream Embankment Slope (35 ft. high, 3:1 slope, 900 ft. length):
A1 = 1.28 acres x 900/500 = 2.3 acres (Use Chart 1; no abutment area reduction
correction∗)
(b) Crest (15 ft. wide, 900 ft. length)
A2 = 0.17 x 900/500 = 0.31 acres (Use Chart 2)
(c) Upstream Embankment Slope (10 ft. high exposure, 3:1 slope, 900 ft. length):
A3 = 0.37 x 900/500 = 0.67 acres (Use Chart 1)
(d) Estimated total dam area to be restored ≈ 2.3 + 0.3 + 0.7 = 3.3 acres
2. Estimated 10-year cycle clearing and grubbing job costs, over a 30-year analysis period, starting
with end of 10th year:
Total Estimated Cost = 3.3 acres x $2500 per acre = $8250
∗ For this example, the abutment slopes are assumed vertical or 0o, but total slope area reduction for a 30o abutment
would be only ≈ 0.25 acres (see Chart 3).
7-8
Chapter 7
Economics of Proper Vegetation Maintenance
3. Find the annual break-even cost balance between mowing and recurring clearing and
grubbing,
using the sinking fund factor (SFF), assuming 4, 6, 8, 10, and 15% discount rates for a 30-year
period. A sinking fund is an equivalent annual amount to be set aside and left to grow at a
certain interest rate into a specified amount at the end of a predetermined time period.
It is assumed in this example that mowing and clearing and grubbing costs do not change over
the 30-year analysis period and that the dam safety inspections are not hindered for up to 10year tree growth. By assuming a zero inflation rate for these costs, the results of this exercise
are not dependent on the selected period of analysis; therefore, the annual values are valid for
a 50- or 100-year period as well as for a 30-year period.
♦ Annualized clearing and grubbing cost = $8250 x (SFF, i, N years)
where the SFF = i/[(1 + i)N – 1]
and i = discount rate (expressed as fraction)
N = time period, in years, between tree clearing and grubbing
♦ Mowing job cost = 3.3 acres x $100/acre = $330
♦ Equivalent number of mowings per year = (Annualized clearing & grubbing costs)/($330
per mowing)
The following Table 4 shows that the annualized clearing and grubbing costs and equivalent number
of annual mowings varies somewhat with the discount rate.
For this example, at a 6%
discount rate, this dam owner would be able to justify about two mowings per year at $330 per
mowing to avoid having to shell out $8250 every 10 years for clearing the dam of trees and woody
vegetation. The owner could afford to mow once or twice a year, even at a relatively high 10%
7-9
Chapter 7
Economics of Proper Vegetation Maintenance
Assumed discount rate, i
Annualized 10-yr frequency
clearing and grubbing cost
Equivalent number of
mowings per year
4%
$687
2.1
6%
$626
1.9
8%
$569
1.7
10%
$518
1.6
15%
$406
1.3
discount rate. By mowing on a regular basis, the owner would also realize side benefits of a more
aesthetically pleasing dam -- one that would be viewed more as a community asset than a liability,
be accessible and inspection friendly, and be less attractive to unwanted burrowing animals.
Table 4. Annualized Cost Comparison for Assumed $2500 per acre for a 10-Year Cycle
Clearing and Grubbing Payout.
If $5000 per acre or $16,500 for 3.3 acres, rather than the $2500 per acre and $8,250 per job, had
been assumed for tree clearing costs over the 10-year cycle control period, the justifiable annual
costs for mowing would double for the same discount rates. For this higher restoration cost, the
owner would be justified to mow 3 or 4 times per year, depending on the cost of money. The
following Table 5 illustrates this assumption.
Assumed discount rate, i
Annualized 10-yr frequency
clearing and grubbing cost
Equivalent number of
mowings per year
4%
$1374
4.2
6%
$1252
3.8
8%
$1138
3.4
10%
$1036
3.1
7-10
Chapter 7
Economics of Proper Vegetation Maintenance
15
$ 813
2.5
Table 5. Annualized Cost Comparison for Assumed $5000 per acre for a 10-Year Cycle
Clearing and Grubbing Payout.
For a more conservative 5-year tree growth cycle and a $2500 clearing and grubbing cost
assumption, the annualized clearing and grubbing costs would be $1523, $1464, $1406, $1351, and
$1224 for the same discount rates, respectively. The corresponding justifiable mowings would be
4.6, 4.4, 4.3, 4.1, and 3.7 per year. Similarly, justifiable mowings for an assumed $5000 clearing and
grubbing cost would double the justifiable mowings to 9.2, 8.9, 8.6, 8.2, and 7.4 per year. Figure
4 compares 5 and 10-year annualized costs for $2500/acre clearing and grubbing payouts.
Equivalent cost in number of mowings/year
5
5-year C&G cycle
4
3
10-year C&G cycle
2
1
0
0
2
4
6
8
10
12
14
16
D isco unt rate (% )
Figure 4. Comparison of Annual Tree Clearing and Grubbing
Costs for 5 and 10-Year $2500/acre payouts.
Realistically, unit area costs would likely be reduced substantially for more frequent clearing and
7-11
Chapter 7
Economics of Proper Vegetation Maintenance
grubbing or bush hogging of smaller growth. Obviously, the above values will be different if the
costs are assumed to escalate each year. For example, assuming a modest 3% annual inflation factor
results in an increase in the clearing and grubbing cost from $8250 to $14,900 for a 30-year analysis
period.
Summary
Cost data obtained from the private, state and federal sectors show that dam maintenance and tree
removal and dam restoration costs can vary widely, depending on several factors.
It has been demonstrated, by way of example and reasonable cost assumptions, that dam owners can
economically justify mowing their embankments 2 to 8 times a year, depending on local factors and
costs, to prevent trees and other woody vegetation from maturing to a point that could compromise
dam safety and require major capital outlays. It appears extremely economically efficient for dam
owners to control woody growth on at least an annual basis, to avoid the large cost of removing
mature brush and trees every 5 to 10 years and to comply with state inspection requirements.
So, how much should a dam owner spend on maintaining his dam? At least enough to keep it
mowed and trimmed a couple times a year – probably something in the neighborhood of $500 to a
$1000 annually for most dams, if contracted. Keeping a dam mowed a minimum of twice a year
does not appear to be an unreasonable financial burden for most small dam owners. A dam owner
must understand that spending a few dollars on annual vegetative maintenance and upkeep, such as
mowing, will pay dividends over the long run for an asset (and potential liability) such as a dam.
7-12
Chapter 7
Economics of Vegetation Maintenance
References
1. Architects, Contractors, and Engineers (AC&E), Guide to Construction Costs,
Division #2 – Sitework & Demolition, Cyber Classics, Inc., 2002.
2. Association of State Dam Safety Officials (ASDSO), State Survey: Animal and
Vegetative Impacts on Dams, Part I - Vegetation on Dams (7 questions),
September 1999
3. Association of State Dam Safety Officials (ASDSO), State Survey, Percentage
of Trees on State-regulated Dams (2 questions), January 2000.
4. Bentley, L., Memorandum: Cost of Dam Clearing on Seven Tennessee Dams,
February 2000.
5. BNi Building News, General Construction 2001 Costbook, Sections
02110.01-02110.50 (Sitework), 2001, p. 18.
6. Marks, B. D., S&ME Engineering, Inc., Arden, N. C., Faxed communication on
recent contractor-bid clearing and grubbing costs, February 23, 2000.
7. Means, R.S., Building Construction Cost Data 2001, Site Preparation Section
02230, 2001, pp. 42-43.
8. Soil Conservation Service, U. S. Department of Agriculture, South Technical
Service Center, Fort Worth, Engineering Technical Note 705, Operations and
Maintenance Alternatives for Removing Trees from Dams, April 1981, 8 pp.
9. Tschantz, B. A., Overview of Issues and Policies Involving Woody Plant
Penetrations of Earthfilled Dams, Presentation and Proceedings, ASDSO/FEMA
Specialty Workshop on Plant and Animal Penetrations on Dams, Nov. 30 - Dec.
30, 1999, 8 pp.
10. Tschantz, B. A., Current Problems, Practices and Policies on Tree and Woody
Plant Penetration of Dams, paper presented at ASDSO National Dam Safety
Conference, Providence, R. I., September 2000.
11. Tennessee Department of Transportation (TDOT), Region 1 Supervisor,
Mowing contract bid data – 1998, March 2000.
7-13
Chapter 7
Economics of Vegetation Maintenance
12. U. S. Bureau of Reclamation, U. S. Department of the Interior, Guidelines for Removal of
Trees and Other Vegetative Growth From Earth Dams, Dikes, and Conveyance Features,
Bulletin No. 150, Water Operation and Maintenance, December 1989, pp. 1-3.
13. U. S. Corps of Engineers, Nashville District, email information furnished by D. Williams,
March 2000.
14. Tennessee Valley Authority, Knoxville, TVA Maintenance Data, Email attachment
information furnished by J. Morse, April 2000 and August 2001.
7-14
Chapter 7
Economics of Proper Vegetation Maintenance
Chart 1. Dam face area for different geometries
120
L = 200 ft
Z= 1.5
Dam height - Hd (feet)
100
2
2.5
L = 500 ft
3
2
Z= 1.5
3
2.5
80
60
40
1
Z
20
Hd
Face slope: 1 vert : Z horiz.
0
0.00
0.50
1.00
1.50
2.00
Dam face area (acres)
7-15
2.50
3.00
3.50
4.00
Chapter 7
Economics of Proper Vegetation Maintenance
Chart 2. Dam crest area by width and length
60
L = 100 ft
L = 200 ft
L = 500 ft
50
Crest width (feet)
40
30
20
10
0
0.00
0.10
0.20
0.30
Crest area (acres)
7-16
0.40
0.50
0.60
Chapter 7
Economics of Proper Vegetation Maintenance
Chart 3. Abutment area corrections vs. dam height for 2.5H:1V & 3H:1V side slopes, with 45o abutment angle
and corresponding minimum dam crest length
500
Minimum crest length
line for given area correction
(for 2.5:1 or 3:1 slopes)
450
1
Z
400
4
A butm en t
an gle
HD
1
Enter dam height on right (55 ft)
2
Obtain abutment area correction (-.84 ac)
300
3
Project line up to min. crest length line
250
4
Project left to obtain minimum dam
crest length for using full area correction (380 ft)
Dam height - H d (feet)
Minimum crest length of dam (feet)
3
350
200
150
Abutment area correction lines
2.5:1
100
3:1
100
1
50
2
0
0.0
0.5
50
0
1.0
1.5
Area correction (-acres)
o
(Multiply by 58% for 30 abutments)
7-17
2.0
2.5
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