See the 2012 Interim Standards here.

See the 2012 Interim Standards here.
Marion County Engineering Standards
Draft Interim Stormwater Quality Treatment
Engineering Standards for the Storm Water
Management Area
Marion County Public Works
5155 Silverton Road NE
Salem, OR 97305-3802
www.co.marion.or.us
February 2012
G:\Engineering\SHARED\EngStandards\IIDesign\Drafts\Chapter5-Drainage\review copies\interim for SWMA\revised_SWQT
ES 2-22-12.doc
Revised 2/22/12
Interim Stormwater Quality Treatment Engineering Standards
TABLE OF CONTENTS
Interim Stormwater Quality Treatment Engineering Standards
Section
1
Page
GENERAL......................................................................................................................... 1
A.
JURISDICTION ................................................................................................................. 1
B.
APPLICABILITY ................................................................................................................ 3
C.
OBJECTIVES.................................................................................................................... 3
D.
OPERATIONS AND MAINTENANCE............................................................................... 3
2
STORMWATER QUALITY................................................................................................ 4
A.
GENERAL CONSIDERATIONS........................................................................................ 4
B.
WATER QUALITY DESIGN FLOWS AND VOLUMES ..................................................... 5
C.
TREATMENT FACILITY DESIGN STANDARDS ............................................................. 5
D.
SITE DEVELOPMENT DESIGN BMPS .......................................................................... 12
E.
IMPERVIOUS COVER REDUCTION BMPS .................................................................. 13
F.
SPECIAL POLLUTANT SOURCES ................................................................................ 15
G.
MULTIPLE USE OF DETENTION FACILITIES .............................................................. 15
3
HYDROLOGIC ANALYSIS ............................................................................................. 16
A.
GENERAL RUNOFF CALCULATIONS .......................................................................... 16
B.
RATIONAL METHOD...................................................................................................... 16
C.
HYDROGRAPH METHODS ........................................................................................... 22
4
DETENTION REQUIREMENTS ..................................................................................... 28
5
STORMWATER DISCHARGE ........................................................................................ 28
6
CONVEYANCE SYSTEM DESIGN................................................................................. 28
7
DRAINAGE SUBMITTALS ............................................................................................. 28
A.
SUBMITTAL REQUIREMENTS ...................................................................................... 28
B.
LANDSCAPE PLAN REQUIREMENTS .......................................................................... 30
C.
EXISTING SITE DRAINAGE PLAN REQUIREMENTS .................................................. 30
D.
PROPOSED ON-SITE STORMWATER DRAINAGE PLAN REQUIREMENTS ............. 30
E.
SUPPORTING DATA...................................................................................................... 31
F.
OPERATIONS AND MAINTENANCE SUBMITTAL........................................................ 31
8 REFERENCES ....................................................................................................................... 32
9 DEFINITIONS ......................................................................................................................... 32
10 ABBREVIATIONS................................................................................................................. 34
Marion County Public Works
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List of Tables
Table 5-1 Facility Types Approved for County Maintenance ........................................................ 4
Table 5-2 Seed Specifications for Biofiltration Swales ................................................................. 7
Table 5-3 Simplified Method Worksheet .................................................................................... 10
Table 5-4 BMPs for Site Design Considerations ......................................................................... 13
Table 5-5 Rational Method Runoff Coefficient “C”....................................................................... 17
Table 5-6 Runoff Coefficient Adjustment Factors ........................................................................ 17
Table 5-7 NRCS 24-Hour Type 1A Hyetograph Distribution ...................................................... 23
Table 5-8 Curve Number Values ................................................................................................. 25
Table 5-9 Manning’s Roughness Coefficient ‘n’ for Sheet Flow.................................................. 26
Table 5-10 Allowed Pipe Materials, Applications, and Characteristics ........................................ 28
Table 5-11 Manning’s Roughness Coefficients for Open Channels ............................................ 28
Table 5-12 Checklist of Calculations to be Included in Stormwater Management Report........... 29
List of Figures
Figure 5-1 Marion County Stormwater Management Area............................................................ 2
Figure 5-2 Rainfall Zone Map for Marion County........................................................................ 18
Figure 5-3 Rainfall Intensity - Duration - Recurrence Curve for Zone 5...................................... 19
Figure 5-4 Rainfall Intensity - Duration - Recurrence Curve for Zone 7...................................... 20
Figure 5-5 Rainfall Intensity - Duration - Recurrence Curve for Zone 8...................................... 21
Figure 5-6 Average Velocity of Shallow Concentrated Flow ....................................................... 27
Appendix A Maintenance Information
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1 GENERAL
These stormwater quality treatment standards are applicable within the Marion County
Storm Water Management Area (SWMA) as designated by Oregon Department of
Environmental Quality (DEQ) and are in addition to all applicable standards specified for
Drainage in Section V of the 1990 Marion County Engineering Standards.
A. JURISDICTION
Oregon Revised Statutes (ORS) require that the design and construction of all facilities
within County rights-of-way be approved by the respective boards of county commissioners
(ORS 374.305). Marion County Ordinance 671 designated the Director of Public Works to
undertake these activities on behalf of the Board. In addition, the Marion County Roads and
Rights of Way and Zoning codes require compliance with certain applicable County
Engineering Standards on private property. Within Urban Growth Boundaries certain city
requirements may also apply.
These Standards are intended to be consistent with the most currently adopted provisions
of all applicable local, State, and Federal codes and regulations and the Marion County
National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm
Sewer Systems (MS4) permit issued by the DEQ. Please bring any apparent
inconsistencies to the attention of the Director.
Drainage facilities within the Marion County SWMA are required to be constructed to meet
the standards of this chapter per the following authorities:
• Publicly owned and maintained storm drainage facilities within Marion County
jurisdiction, including those associated with County maintained roads in unincorporated
Marion County (rural) and under County maintenance jurisdiction within incorporated city
limits (urban) - required by Public Works policy
• Privately owned and maintained drainage facilities inside a County right-of-way or
discharging to Marion County maintained facilities - required by Public Works policy or
Marion County Code 11.20, allowed by ORS 374.305
• Drainage facilities outside the building envelope and within an urban growth boundary -
required by Marion County Code (MCC) 16.30.140, 16.33.160 and 16.33.320
• Rural drainage facilities outside the building envelope - required by Marion County Code
MCC 17.110.830, 17.172.140, 17.172.320, 17.172.430
• Environmental Protection Agency (EPA) Phase II Rule for Small MS4s (40 Code of
Federal Regulations 122.30 through 122.37) and the Clean Water Act (33 U.S. Code
1342)
A map of the Marion County SWMA is shown on Figure 5-1. The map may periodically be
updated; the current version will be available at
http://www.co.marion.or.us/PW/ES/waterquality/strmwtr.htm
Marion County Public Works
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FIGURE 5-1 MARION COUNTY STORMWATER MANAGEMENT AREA
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B. APPLICABILITY
Stormwater quality treatment is required for new development and redevelopment within
the Marion County Storm Water Management Area (SWMA) if the project will disturb one
acre or more or is less than an acre if the project is part of a larger common plan of
development or sale that will ultimately disturb one acre or more. This applies to the
following types of projects:
•
Transportation-related projects within the public right-of-way, including road construction
and reconstruction
•
Development associated with partitions and subdivisions.
•
Commercial, industrial, and residential development and redevelopment.
The SWMA is shown in Figure 5-1. Boundaries may be subject to change, as defined by
DEQ or in the Marion County Stormwater Management Plan.
Exceptions to SWMA water quality treatment requirements:
• Agricultural field drains.
• Discharges regulated by the Department of Agriculture.
The Director reserves the right to modify the Standards herein on a case-by-case basis.
Standards for drainage inside the building envelope are administered by the Marion County
Building Official within Public Works and are not addressed within these standards.
Responsibility for review and approval of storm drain design is typically separated into two
categories:
• New and re-development projects that are funded by private sources or sources other
than Marion County are reviewed and permitted by the Land Development Engineering
and Permits Section of Public Works.
• Projects funded by or through Marion County are reviewed and approved by the Capital
Projects Section of Public Works.
C. OBJECTIVES
The objectives of these Standards include the following:
•
Maintain and improve the water quality in Marion County’s waterways, lakes, wetlands,
and other natural resources.
•
Meet the requirements associated with the County’s NPDES Municipal Separate Storm
Sewer System permit and other applicable laws and regulations.
•
Construct facilities that are safe, economical to maintain, and have a long design life to
minimize costs to Marion County.
D. OPERATIONS AND MAINTENANCE
Maintenance of stormwater facilities shall be the responsibility of the private property owner
unless the facility is in a public right-of-way or in an easement dedicated to, and formally
accepted by, the County or appropriate government body.
The County will maintain stormwater management facilities as follows:
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•
A stormwater management facility that is within a public right-of-way shall typically be
County maintained. However, there may be circumstances under which the County will
not be responsible for maintenance of facilities within the right-of-way (for instance, if the
right-of-way is not a County-maintained road). In those cases, as separate maintenance
agreement assigning maintenance responsibility may be required.
•
The County shall maintain stormwater management facilities serving residential
developments provided an easement dedicated for stormwater management and
inclusive of site access is conveyed to the County, the facilities are constructed under a
Public Works permit, adequate access is provided, and the facility goes through a
warranty period prior to transfer to County maintenance.
Table 5-1 lists the types of stormwater quality treatment facilities the County will maintain
within a stormwater easement if they are designed and constructed according to these
Standards. Table 5-1 also lists facility types that the County will not maintain. If County
maintenance is anticipated for any facility not included on the list, it must be approved by
the Director prior to design and construction.
TABLE 5- 1 FACILITY TYPES APPROVED FOR COUNTY MAINTENANCE
Facilities the County will maintain
Biofiltration swales with grasses, grass-like plants, reeds, and/or sedges
Vegetated Infiltration Facilities (rain gardens)
Pre-approved proprietary devices
Facilities the County will not maintain
Vegetated swales with shrubs or trees on bottom or side slopes
Pervious pavement
Underground Injection facilities subject to DEQ regulation
A site specific Operations and Maintenance (O&M) Manual must be submitted for review
and approval with the design submittal for stormwater and drainage facilities. The Manual
must include a list of the items to be inspected, inspection schedules, self-inspection
checklists, instructions for inspection documentation, and routine maintenance procedures.
More information is provided in Appendix A.
2 STORMWATER QUALITY
For new and redevelopment projects as described in Section 1B, stormwater quality
treatment will be required according to this Section.
Stormwater pollutants are generally separated into the following categories: suspended
solids, oxygen-demanding pollutants, temperature, bacteria, organic carbon, hydrocarbons,
metals (i.e., lead, copper, zinc, and cadmium), nutrients (i.e., nitrogen and phosphorous),
and toxins (i.e., pesticides, chemical toxins). These stormwater quality standards are
intended to reduce the discharge of the above pollutants to waters of the state.
A. GENERAL CONSIDERATIONS
Methods used to reduce pollutants in stormwater and to preserve stormwater quality are
often referred to as Best Management Practices (BMPs). BMP refers to operational
activities and physical controls that are applied to reduce the discharge of stormwater or
pollutants and to minimize their impacts to the receiving waters. In general, BMPs refer to
both structural and nonstructural practices that have direct impacts on the release,
transport, or discharge of pollutants.
Proposals to place stormwater quality treatment facilities in a public right-of-way or special
setback must be pre-approved by the Director. No stormwater quality treatment facilities
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will be allowed in the public right-of-way or special setback area unless they are outside the
footprint of the ultimate design section of the road.
B. WATER QUALITY DESIGN FLOWS AND VOLUMES
The Water Quality Flow (WQF) for water treatment facilities shall be determined in
accordance with the hydrologic calculation methods contained in Section 3 of these
Standards, using the following design storm depths or intensities:
• When permitted in Section 3A to use the Rational Method, the water quality design
storm intensity shall be 0.17 inches per hour
• When required in Section 3A to use a hydrograph method, the water quality design
storm event shall be 0.83 inches per 24-hour period using the NRCS Type 1A 24-hour
rainfall distribution.
When the Water Quality Volume (WQV) is required for stormwater quality treatment facility
design, it shall be based on the volume of runoff from the water quality design storm event
of 1.38 inches per 24-hour period1 using an analytical method acceptable to the Director.
C. TREATMENT FACILITY DESIGN STANDARDS
Required water quality treatment facilities shall conform to the following standards.
1. Biofiltration Swales
Biofiltration swales are long, narrow grassy or vegetated channels engineered to convey
and treat stormwater runoff, allowing pollutants to settle and filter out as the water flows
through the facility. In addition to providing pollution reduction, they can also manage flow
rates and volumes. They may be seeded or planted with grasses (grassy swale) or
herbaceous plants (vegetated swale).
Hydraulic Criteria
•
Design flow: WQF
•
Minimum hydraulic residence time: 9 minutes
•
Maximum water quality design depth: grass: 4 inches for grass; other
vegetation: ¾ vegetation height when established, up to a maximum 8 inches
•
Minimum longitudinal slope: 0.5%
•
Minimum freeboard: 0.5 feet (for facilities not protected from high flows)
•
Manning’s ‘n’ (for sheet flow) value: reference Table 5-9 for “vegetated swale”
•
Maximum velocity: 1.0 feet per second at WQF
Other Structural Criteria
•
Provide an energy dissipater at the entrance to swale, with a minimum length of
4 feet. It will be designed to reduce velocities and spread the flow across the
swale cross section.
•
Install intermediate flow spreaders at a minimum of 50 foot intervals.
•
Minimum length: 100 feet
•
Minimum bottom width: 2 feet
•
Side slope: 3H:1V or flatter
•
Provide an approved outlet structure for all flows
1
The 24-hour rainfall depth for the WQF and WQV are different because they are based on different
assumptions for water quality design storms.
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•
Where a swale turns 180-degrees forming parallel channels, freeboard shall be
provided between each of the parallel channels. A 1-foot (above ground surface)
wall may be used above the treatment area to provide freeboard while enabling
a narrower system. As an alternative, a soil-based berm may be used. The berm
shall have a minimum top width of 1 foot and 2.5H:1V or flatter side slopes.
•
Where swales are designed with ditch inlets and outlet structures and design of
maintenance access to such structures may be difficult due to swale location,
swales may be designed as flow through facilities with un-sumped structures.
Soil Amendment and Planting Requirements
•
Provide a minimum depth of 12-inches of topsoil or amended soil to support
plant growth.
•
Seed, sod, or plants are required to completely cover the swale bottom and side
slopes.
o
Suggested seed specifications are shown in Table 5-2. For the swale flow
path, approved native grass mixes are preferable to sod, but are not
required, and may be substituted for standard swale seed mix. Certified
weed-free seed shall be applied at the rates specified by the suppliers.
Plant selection and placement at different levels in the swale must take
into account:
ƒ Plant tolerance for different moisture regimes, including depth and
frequency of flowing water and level and duration of standing water
ƒ Plant tolerance for sun and shade
ƒ Plant height at full growth, if there are sight distance concerns
ƒ Ease and timing of plant establishment
ƒ Ease and cost of maintenance
o The minimum plant material quantities per 100 square feet of facility area
shall be as follows:
ƒ At least 50 percent of the groundcover plantings shall be grasses or
grass-like plants.
ƒ Two large shrubs/small trees (for swales not within the ROW or
adjacent to streets): 3-gallon containers or equivalent.
ƒ Six shrubs or large grass-like plants: 1-gallon containers or equivalent.
ƒ Herbaceous plants: 1 per 12 inches on center, triangular spacing, for
the sideslope planting area only. Minimum container: 4-inch pot.
Plants, either from seed or pots, shall be established at the time of facility
completion (at least 3 months after seeding).
If this is not feasible,
biodegradable erosion control matting (e.g., jute mat, coir) appropriate for
expected flow velocities shall be installed in the flow area. No runoff shall be
allowed to flow in the swale until either vegetation is established or matting has
been provided.
o
•
•
Trees and shrubs may be allowed in the flow path within swales if the swale
exceeds the minimum length and widths specified.
•
The design for plantings shall minimize the need for herbicides, fertilizers,
pesticides, or soil amendments at any time before, during, and after construction
and on a long-term basis.
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TABLE 5-2 SEED SPECIFICATIONS FOR BIOFILTRATION SWALES
Std Swale
Sowing
Moisture
Sun
Height
Scientific Name
Common name
mix
Season
Requirement
Req’ment
(inches)
Grasses– soil moisture as indicated into summer months
Spike bentgrass
10%
EF / S
Saturated/wet
Sun
20 to 40
Agrostis exarata
Redtop bentgrass
EF / S
Moderate/dry
Sun
to 24
Agrostis gigantea
Water foxtail
5%
F/S
Inundated/wet
Sun
to 24
Alopecurus geniculatus
American sloughgrass
F/S
Inundated /wet
Sun
to 36
Beckmannia syzigachne
California brome
10%
EF / S
Moderate/dry
Sun
18 to 46
Bromus carinatus
Sitka brome
10%
EF / S
Wet/moderate
Sun/shade
18
Bromus sitchensis
Columbia brome
F/S
Moderate/dry
Shade
to 12
Bromus vulgaris
California oat-grass
15%
F/S
Moderate/dry
Sun
12 to 28
Danthonia californica
Tufted hairgrass
10%
F/S
Saturated/wet
Sun
8 to 24
Deschampsia cespitosa
Slender hairgrass
5%
EF / S
Wet/dry
Sun
16 to 32
Deschampsia elongata
Blue wild rye
EF / S
Moderate/dry
Sun /shade
24 to 60
Elymus glaucus
Slender wheatgrass
EF / S
Moderate/wet
Sun
24 to 30
Elymus trachycaulus
Western fescue
10%
F/S
Moderate/dry
Sun
12 to 36
Festuca occidentalis
Roemer's fescue
F/S
Moderate/dry
Sun
15 to 40
Festuca romerii
Red fescue
Moderate
Sun/shade
to 24
Festuca rubra
w/other grasses
Tall mannagrass
F/S
Saturated/moderate
Shade
30 to 54
Glyceria elata
Western mannagrass
F/S
Saturated/wet
Sun
30 to 60
Glyceria occidentalis
Meadow barley
25%
EF / S
Moderate/wet
Sun
16 to 40
Hordeum brachyantherum
Junegrass
F/S
Moderate/dry
Sun
6 to 24
Koeleria macrantha
Rice cutgrass
F/S
Inundated/ wet
Sun
35 to 48
Leersia oryzoides
Perennial rye grass
EF / S
Wet/moderate
Sun
to 36
Lolium perenne
Western panicgrass
Panicum occidentale
Annual bluegrass
Moderate
Sun
to 4
Poa annua
Pine bluegrass
Moderate/dry
24 to 48
Poa secunda
Sedges and Rushes – soil moisture as indicated into summer months
Dense sedge
F/S
Wet
Sun
to 20
Carex densa
Slough sedge
F/S
Inundated/moderate
Sun/shade
24 to 60
Carex obnupta
Pointed broom sedge
F/S
Wet/moderate
Sun
to 30
Carex scoparia
Sawbeak sedge
F/S
Inundated/moderate
Sun
to 42
Carex stipata
Ovate spike rush
F/S
Inundated/wet
Sun
to 30
Eliocharis ovata
Creeping spike rush
F/S
Inundated/wet
Sun
4 to 27
Eliocharis palustris
Taper-tipped rush
F/S
Inundated/wet
Sun
to 36
Juncus acuminatus
Toad rush
F/S
Wet/moderate
Sun
to 12
Juncus bufonius
Soft rush
Wet/inundated
Sun
to 72
Juncus effusus
F/S
Spreading rush
F/S
Wet/moderate
Sun/shade
to 24
Juncus patens
Herbaceous Plants
Western yarrow
F
Wet/moderate
Sun
8 to 16
Achillea millefolium
Water plantain
F/S
Inundated/wet
Sun
12 to 36
Alisma media
Western columbine
F
Wet/moderate
Sun
to 36
Aquilegia formosa
Common camas
F
Wet/moderate
Sun
12 to 36
Camassia quamash
Great camas
F
Wet/moderate
Sun
36 to 48
Camassia quamash spp quamash
Blue-eyed mary
F/S
Moderate/dry
Sun
to 18
Collinsia rattanii
Large flowered collomia
F/S
Moderate/dry
Sun
4 to 36
Collomia grandiflora
Dense spike primrose
F
Wet/moderate
Sun
6 to 40
Epilobium densiflorum
Wooly sunshine
F
Wet/moderate
Sun
4 to 24
Eriophyllum lanatum
Blue gilia
F/S
Moderate/dry
Sun
12 to 24
Gilia capitata
Oregon iris
F
Moderate/dry
Sun
12 to 14
Iris tenax
Spanish clover
F
Moderate/dry
Sun
6 to 20
Lotus purshianus
Sickle keel lupine
F
Moderate/dry
Sun
30 to 60
Lupinus albicaulis
Small flowered lupine
F
Moderate/dry
Sun
8 to 32
Lupinus micranthus
Stream lupine
F
Moderate/dry
Sun
30 to 42
Lupinus rivularis
Popcorn flower
F/S
Inundated/wet
Sun
6 to 18
Plagiobothrys figuratus
Selfheal
F/S
Wet/moderate
Sun/shade
12 to 24
Prunella vulgaris var.
Western buttercup
F
Moderate/dry
Sun
12 to 24
Ranunculus occidentalis
Checkermallow
F
Moderate/dry
Sun
36 to 48
Sidalcea campestris
Goldenrod
F
Moderate/dry
Sun
12 to 60
Solidago canadensis
American vetch
F
Moderate/dry
Sun
6 to 30
Vicia americana
Sowing seasons: EF – Early Fall: Sept 1 – Sept 30 F - Fall: Sept 1 to Oct 31 S – Spring: Mar 1 to May 15
Moisture requirements: Wet or inundated - more suited for swale bottom; dry - more suited for side slopes and top
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Maintenance Considerations for the O&M Manual
•
Inspect annually and after large flow events for debris and reduction in flow
capacity.
•
Establish and maintain 70% or greater vegetative cover.
•
Remove and prevent establishment of invasive weeds. Avoid the use of
pesticides, herbicides, and other chemicals in stormwater quality control
facilities. If chemical control is necessary, use a licensed pesticide applicator.
2. Proprietary Engineered Treatment Devices
The use of proprietary engineered treatment devices is permitted on a case-by-case
basis with approval by the Director.
The devices will be sized in accordance with the manufacturer’s recommendations to
handle the water quality.
Technical submittals from the manufacturer are required, including hydraulic design
criteria, particulate removal efficiency, operations and maintenance requirements and
schedules, and an installation list of similar uses in Western Oregon and Western
Washington.
Maintenance Considerations for the O&M Manual
Inspect and perform maintenance according to manufacturer’s recommendations.
3. Vegetated Infiltration or Filtration Facilities
A vegetated infiltration or filtration facility, also called bioretention facility, is a flatbottom, shallow depressed landscape area designed to capture and filter or infiltrate the
water quality volume (WQV). These facilities include rain gardens or planter boxes.
Planter boxes differ from rain gardens in that they are structural reservoirs and typically
are designed to receive less runoff than a rain garden.
Vegetated infiltration and filtration facilities may be engineered to provide full infiltration
or they may use with an underdrain system. Facilities with underdrains provide filtration
through amended soil but allow water that doesn’t percolate into the underlying soil to
be piped off-site.
The design of an infiltration facility may provide detention for volumes exceeding the
WQV and may meet the detention (flow control) requirement. For this, the design must
provide flood control volume above the WQV water surface elevation of the facility, with
flows bypassing the facility’s filtering or infiltration function by overtopping into an inlet
designed to restrict the peak flow for larger events.
General
A drain system and overflow to an approved conveyance system and discharge point is
required.
Infiltration planter boxes or rain gardens may be used when:
• Native soils infiltrate at more than 0.5 inches per hour, and
• They can be located at least 10 feet from building foundations and
• They are not immediately upslope of building structures or pavement sections.
Otherwise, use filtration facilities with an adequately designed underdrain system.
Infiltration Testing Requirements
Infiltration rates are required in order to size infiltration stormwater management
facilities.
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Testing shall be conducted or observed by a qualified professional. This professional
shall either be a registered professional engineer in the State of Oregon, or a soils
scientist or geologist licensed in the State of Oregon.
All infiltration testing must be conducted in the immediate vicinity of the facility at a
depth corresponding to the proposed facility depth. At least one test for every 1,000
square feet (or fraction thereof) of proposed rain garden surface area must be
conducted. The test location must be representative of the predominant soils within the
proposed rain garden footprint.
An Infiltration Testing Report, submitted with the facility design, must include boring or
test pit logs, a site plan showing the location and elevation of the infiltration tests, a
description of the subsurface conditions encountered, the number and depth of
infiltration test holes, a summary of the infiltration testing method and how it was
conducted, all infiltration testing results, and recommended design infiltration rate(s). A
minimum factor of safety of at least 2 must be applied to the measured infiltration rate to
obtain the design infiltration rate (e.g., design infiltration rate may be no more than onehalf the measured infiltration rate). The design infiltration rate may not exceed 2.5
inches per minute.
Facility Design Criteria
Sizing
• The size and depth of the facility shall be based on the area required to retain and
infiltrate the WQV without overtopping.
• The square-footage is determined at the peak water surface prior to overflow.
• Facilities that receive runoff from impervious areas 10,000 square feet or less
may be designed using the Simplified Method (Table 5-3)
• Facilities not sized using the Simplified Method must be designed to drain the
WQV in less than 18 hours after the end of the 24 hour event
Inlets
• In streets or parking lots, stormwater may flow directly into the vegetated facility
via curb openings.
• Provide energy dissipation for facilities with concentrated points of inflow.
Dimensions
• Planter boxes
o Minimum width is 30 inches if designed as an infiltration facility; 18 inches if
underdrain is provided
o Minimum ponding depth is 6 inches
o Maximum ponding depth is 12 inches
• Rain gardens
o
o
o
o
o
Minimum bottom width is 3 feet.
Maximum side slopes shall be at a ratio of 3 horizontal to 1 vertical.
Minimum ponding depth is 6 inches for the water quality storm.
Maximum ponding depth is 12 inches
Minimum freeboard (vertical distance between the design water surface
elevation and overtopping) is 2 inches
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Table 5-3 Simplified Method Worksheet
For facilities treating runoff from impervious areas 10,000 square feet or less
1
Instructions
Subbasin or tributary area or designation
Enter the impervious area contributing flow to the
vegetated facility.
Site Information
(1) Impervious area
required to be
mitigated
Select stormwater management facilities from Rows A
through D in Column 1 below. Enter the square
footage of impervious area that will flow into each
facility type in Column 2.
3 Multiply each impervious area from Column 2 by the
corresponding sizing factor in Column 3, and enter the
result in Column 4. This is the facility surface area
required.
4 Total Column 2, Rows A through D and enter the
(4) Total Impervious
resulting Impervious Area Managed on Line (5)
Area Managed
5 Subtract (4) from (1) and enter the result in line (5).
(5) Remaining
This must be zero or less. Submit this form with the
Impervious Area
Stormwater Management Report.
Column 1
Column 2
Column 3
Stormwater Management
Impervious Area
infiltration rate *
sizing
Facility
Managed
factor
______________ sf
<0.75 in/hr
0.09
A Infiltration planter
______________ sf
0.75 - 1.25 in/hr
0.08
______________ sf
1.25 - 1.75 in/hr
0.07
______________ sf
>1.75 in/hr
0.06
B Filtration planter
______________ sf
0.05
______________ sf
<0.75 in/hr
0.1
C Infiltration Rain Garden
______________ sf
0.75 - 1.25 in/hr
0.09
______________ sf
1.25 - 1.75 in/hr
0.08
______________ sf
>1.75 in/hr
0.07
D Filtration Rain Garden
______________ sf
0.06
__________ sf
2
__________ sf
__________ sf
Column 4
Facility Surface
Area
____________ sf
____________ sf
____________ sf
____________ sf
____________ sf
____________ sf
____________ sf
____________ sf
____________ sf
____________ sf
E Total
______________ sf
* Provide documentation for infiltration rate selected.
Piping
• Piping must meet Oregon Plumbing Specialty Code requirements
• Underdrains (required for filtration facilities):
o
Minimum depth of perforated pipe under facility surface is 18 inches
o
Minimum underdrain pipe diameter is 6 inches
o
Maximum underdrain pipe spacing is 20 feet on center
o
Provide cleanouts to enable maintenance of the underdrain. Cleanouts can
also be used to conduct an inspection (by camera) of the underdrain system
ensure that the pipe was not crushed or disconnected during construction.
o
Overflow pipes
ƒ
ƒ
Overflow drains shall allow at least 6 inches but not more than 12 inches
of water to pond in the planter prior to overflow.
On private property, this overflow drain and piping must meet Plumbing
Code requirements and shall direct excess stormwater to an approved
discharge point as identified on the engineered design drawings.
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ƒ
Within the public street right-of-way, this overflow drain and piping must
meet County Public Works Standards and shall direct excess stormwater
to an approved discharge point.
Setbacks
• Infiltration facilities and filtration facilities with no liners:
o
Shall have a 10 foot minimum setback from building structures.
o
Shall be set back a minimum of 5 feet from property lines.
o
Shall not be located immediately upslope of building structures.
• Filtration facilities that are lined with waterproofed concrete or 60 millimeter PVC
liner generally do not have a required setback.
• Other building setback requirements may prevail.
Soil Amendment and Mulch
• Amend the native soil with a mix of one part organic one part coarse sand, such
that there are equal parts compost, sand, and native soil. If there is surface
ponding, add organic compost and sand and re-till until infiltration performance is
enhanced.
• Minimum depth of soil amendment is 18 inches.
• A 2-inch layer of shredded bark mulch (not bark dust or bark chips) shall be used
over the amended soil and between the plantings.
Planting, Landscape and Irrigation Requirements
The entire facility area must be planted with vegetation. Selected plant materials
should be appropriate for soil, hydrologic, and other facility and site conditions. For
plants suitable to vegetated infiltration and filtration facilities, refer to Portland
Stormwater Management Manual Appendix F4 (Portland BES, 2008), City of Eugene
Stormwater Management Manual Appendix G (Eugene, 2008) or the Oregon Rain
Garden Guide (Oregon Sea Grant, 2010).
• Planter Boxes plantings criteria
o
1-gallon containerized plants or larger shall be planted at a maximum spacing
of 12 inches on-center throughout the facility surface area, unless otherwise
approved by the Director.
• Rain gardens plantings criteria
o
A 2-year warranty period from the time of plant installation shall be provided.
At the end of the first year and again at the end of the 2-year warranty period,
all plants that do not survive must be replaced. The warranty period may be
extended if satisfactory plant survival is not achieved within the first two years.
o
Establishment procedures, such as control of invasive weeds, animal and
vandal damage, mulching, re-staking, watering, and mesh or tube protection
replacement, shall be implemented to the extent needed (as determined by the
Director) to ensure plant survival.
o
Facilities located in the public right-of-way are not permitted to use evergreen
trees to meet landscaping requirements.
o
The design for plantings shall minimize the need for herbicides, fertilizers,
pesticides, or soil amendments at any time before, during, and after
construction and on a long-term basis.
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o
Plants shall be selected and planted to minimize the need for mowing, pruning,
and irrigation.
o
Certified weed-free native grass or native wildflower seed shall be applied at
the rates specified by the suppliers.
o
If plant establishment cannot be achieved with seeding by the time of
substantial completion of the stormwater facility portion of the project, the
contractor shall plant the area with wildflower sod, plugs, container plants, or
other means to complete the specified plantings and protect against erosion
before water is allowed to enter the facility.
o
Irrigation schedules should be adjusted during the growing season to provide
the minimum water necessary to maintain plant health and to maintain the
available pore space for infiltration.
Maintenance Considerations for the O&M Manual
The site-specific Operations and Maintenance (O&M) Manual submitted with the facility
design must include an inspection and maintenance schedule and a program
description that addresses vegetation management, sediment and debris removal,
erosion, pipe and orifice maintenance, and structural repair or replacement. Owners are
required to provide all-weather access to the County and check their facilities regularly
to determine maintenance needs. See Section 1D and Appendix A for O&M Manual
requirements.
4. Other Treatment Facilities
The Director will accept stormwater quality treatment facilities other than those listed in
Sections 2C1 through 2C3, including ponds and constructed wetlands, designed
pursuant to the standards specified by Portland (Bureau of Environmental Services),
City of Eugene, or Clean Water Services (in Washington County). See Section 8
References.
The Director may, on a case-by-case basis, accept facilities for which water quality
treatment is demonstrated to the Director’s satisfaction and for which the design is
signed and stamped by a professional engineer registered in Oregon.
D. SITE DEVELOPMENT DESIGN BMPS
Many stormwater pollutants can be significantly reduced by simple design considerations
and/or construction procedures that retain or preserve natural hydrology. Sites subject to
water quality treatment requirements shall incorporate the considerations shown in Table 54 BMPs to the maximum extent feasible (MEF) for site and project specific conditions and
constraints.
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Table 5-4 BMPs for Site Development and Road Design
Best Management Practice (BMP)
1.
2.
3.
4.
5.
6.
7.
Preserve the natural vegetation on site to the maximum extent
practicable. The developed area should be situated to minimize the
clearing of existing groundcover, maximize the preservation of
wetlands, and buffer waterway corridors (riparian areas). If feasible,
the preserved area should be located down slope from the
developed area, since flow control and water quality are enhanced
by flow dispersion through undisturbed soils and native vegetation.
Define distinct development envelopes and protected areas.
Impervious development areas should be compact.
Minimize direct connections of impervious areas to underground
drainage systems by allowing stormwater to permeate into the soil
(within the confines of DEQ Underground Injection Control (UIC)
program regulations and prevailing site and soil conditions) and/or
be filtered by vegetation before being collected into catchbasins
(i.e., vegetated swales).
Use drainage facilities as a benefit to the site. Create multi-use
facilities and natural drainage pathways where possible.
Provide at least one foot of permeable soil in landscaped areas to
disperse and infiltrate water, support plant growth and filter and
decompose pollutants. Such dispersion must be carefully designed
to avoid groundwater contamination, erosion, landslide hazards,
building setbacks, and other similar features; and must adhere to
DEQ UIC program regulations.
Use construction techniques and landscape designs that minimize
soil compaction. Provide protection from compaction by defining
vehicle pathways; using appropriate landscape plant selection and
placement; and using amendments such as organic matter, coarse
sand, pumice, granulated rubber, and similar soil components.
Minimize use of impervious surfaces consistent with site design.
Site
Development
Design
Use to MEF
Road
Design
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Use to MEF
Key: MEF – maximum extent feasible
E. IMPERVIOUS COVER REDUCTION BMPS
The applicant can reduce the amount of impervious area and thus reduce water quality and
quantity treatment requirements by installing pervious pavement and/or building green
roofs.
The hydrologic calculations submitted for these facilities must reflect the current and
proposed pervious and impervious cover for the site.
1. Pervious Pavement Requirements
•
Pervious pavement will not be allowed inside the curb line of County maintained
streets.
•
Pervious pavement design must meet all applicable State and County building
codes.
•
Proprietary porous pavement systems must be installed per manufacturer
specifications.
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•
Pervious pavements in the public right-of-way must be pre-approved by the
Director.
•
Pervious pavement will not be given full water quality treatment credit at slopes that
exceed 3%.
•
A drain system and overflow to an approved conveyance system and discharge
point is required to prevent ponding in the event that the surface is clogged with
sediment or debris.
•
Installation of signs prohibiting seal coating of the pavement must be part of the
site design.
•
Acceptable pervious pavement types:
o
Porous concrete mix, open-graded mix placed over an open-graded base
rock layer
o
Uni Eco-Stone® pavers placed over an open-graded base rock layer
o
Uni Ecoloc® pavers placed over an open-graded base rock layer
o
SF RIMATM pavers placed over an open-graded base rock layer
Other paving systems may be reviewed on a case-by-case basis for porous
designation and appropriate use. They must show the ability to pass water quickly
through the pavement layer but not present a gravel or soil surface to traffic,
which could generate dust.
•
Maintenance Considerations: The O&M Manual submitted with the site design
must address frequency and type of pavement sweeping, cleaning and jetting. The
County will not be responsible for maintaining pervious pavement on privately
owned property.
2. Green Roof Requirements
• Slopes should be less than or equal to 10%.
• A drain system and overflow to an approved conveyance system and discharge
points is required.
• High quality waterproofing material must be used on the roof surface.
• Soil of adequate fertility and drainage capacity at depths of 2 to 6 inches, and weight
of 10 to 30 pounds per square foot, shall be applied. The building structure must be
shown to be adequate to hold the additional weight.
• Soil coverage to prevent erosion shall be established immediately upon installation
by using mulch, vegetation mats, or other approved protection methods.
• Temporary irrigation to establish plants is recommended. A permanent irrigation
system using potable water may be used, but an alternative means of irrigation, such
as air conditioning condensate or other non-potable sources is recommended.
Alternative sources should be analyzed to determine if the irrigation source water
has chemicals that might harm or kill the vegetation.
• Vegetation shall conform to the following:
o
Drought-tolerant, requiring little or no irrigation after establishment
o
A growth pattern that allows the plant to thoroughly cover the soil. At least
90% of the overall surface shall be covered within 2 years.
o
Self-sustaining, without the need for fertilizers, pesticides, or herbicides
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o
Able to withstand heat, cold, and high winds
o
Very low-maintenance, needing little or no mowing or trimming
o
Perennial or self-sowing
o Fire resistant
A mix of sedum/ succulent plant communities is recommended because they
possess many of these attributes. Herbaceous plants, grasses, and other low
groundcovers can also be used to provide additional benefits and aesthetics;
however, these plants may need more watering and maintenance to survive and
keep their appearance.
•
Operations and Maintenance Requirements: The O&M Manual submitted with
the site design must provide procedures for watering, control of invasive weeds,
and vegetation reestablishment.
3. Other Vegetated Facilities
The Director may approve other vegetated facilities that reduce or disconnect
impervious areas, such as vegetated filter strips, on a case-by-case basis when
adequate hydrologic and engineering analysis have been provided.
F. SPECIAL POLLUTANT SOURCES
Runoff from the following types of higher risk facilities requires special treatment. These
facilities shall be covered where practical and hydraulically isolated from stormwater unless
the Director issues a discharge permit, which will require pretreatment.
•
Petroleum dispensing and storage facilities
•
Aboveground storage of liquids harmful to aquatic life
•
Solid waste storage areas, containers, and compactors
•
Exterior storage of bulk materials with potential for toxic runoff
•
Equipment and/or vehicle washing and maintenance facilities
•
Interior floor drains
•
Other facilities determined by the Director to have a high potential for discharge of toxic
or oily water.
The designers of treatment facilities for these sources must contact the County’s Public
Works Land Development Engineering and Permitting section to determine what, if any,
special design considerations or treatment facilities are required.
G. MULTIPLE USE OF DETENTION FACILITIES
Historically, detention facilities have been used primarily for the management of stormwater
discharge flow rate. However, these facilities can and should provide a combined benefit of
controlling both stormwater quality and quantity in most cases. Detention facilities that
incorporate vegetative or infiltrative properties acceptable to the Director may be approved
as meeting the water quality treatment requirement.
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3 HYDROLOGIC ANALYSIS
Stormwater quality design flows and volumes required in accordance with Section 2 shall
be conducted according to the methods described in this Section.
A. GENERAL RUNOFF CALCULATIONS
•
Drainage Areas: All designs shall identify drainage areas within and upstream of the
development. In undeveloped urban drainage areas, drainage calculations shall
separately consider existing drainage patterns and post-developed drainage patterns.
•
Analytical Methods: The engineer is not limited to any one method for calculating
runoff. For drainage areas less than ten acres in size, the Rational Method may be
used. Areas ten acres or more must be analyzed using a method acceptable to the
Director, such as the Santa Barbara Urban Hydrograph (SBUH) Method, Natural
Resource Conservation Service (NRCS) TR20 or TR55, or other similar methods.
B. RATIONAL METHOD
1. General
The rational method for analyzing small drainage basins may be used with the following
limitations:
•
Only for use in predicting a conservative peak flow rate to be used in determining the
required capacity for conveyance elements
•
The time of concentration shall be five minutes when computed to be less than five
minutes.
2. Rational Equation
Q = Cf ×C × I × A
Where
Q = Peak flow (cubic feet per second)
Cf = a runoff coefficient adjustment factor to account for reduction of
infiltration and other losses during high intensity storms.
C = a runoff coefficient determined by ground cover. The engineer must
document the methodology used in determining the value proposed.
I = rainfall intensity in inches per hour for the storm duration. For the
Rational Method, the basin time of concentration is used as the storm
duration. Intensities for given durations are shown in Figures 5-3
through 5-5. The time of concentration must first be calculated (see
Section 3C5, Time of Concentration, below).
A = the basin area in acres.
3. Runoff Coefficient “C”
The runoff coefficient is difficult to estimate because it represents the interaction of many
complex factors including surface ponding, infiltration, antecedent moisture, ground cover
conditions, ground slopes, and soil type. Table 5-5 represents typical average values for
different land uses and topography.
The actual runoff coefficient for a given drainage basin can best be approximated by
calculating a weighted average of all distinct surface types as follows:
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C av =
Where
∑C
x
Ax
Atotal
Caf = the weighted average C-value for the drainage basin
Cx = individual C-values for distinct surface types within a subbasin
Ax = individual areas for distinct surface types within a subbasin
Atotal = total area of the drainage basin.
TABLE 5-5 RATIONAL METHOD RUNOFF COEFFICIENT “C”
Land Use
Developed Surface Types:
Impervious surfaces (pavement and roofs)
Gravel surfacing (parking lots, storage areas, roads)
Pervious hardscapes (e.g., grasscrete)
Landscape areas (except lawns)
Lawns
Undeveloped Surface Types:
Meadow, pasture, or farm
Mixed
Woodland and Forest
Development Types:
Commercial Development
Industrial Development, Heavy
Dense Residential (over 6 units/acre)
Industrial Development, Light
Normal Residential (3 to 6 units/acre)
Light Residential (1 to 3 units/acre)
Parks
Ground slope
0% to 2%
2% to 10%
Over 10%
0.9
0.85
0.5
0.3
0.17
0.9
0.85
0.55
0.35
0.22
0.9
0.85
0.6
0.4
0.35
0.25
0.15
0.1
0.3
0.2
0.15
0.35
0.25
0.2
0.8
0.7
0.7
0.6
0.5
0.35
0.15
0.85
0.8
0.75
0.7
0.55
0.4
0.2
0.9
0.9
0.8
0.8
0.6
0.45
0.25
4. Runoff Coefficient Adjustment Factor
The coefficients in Table 5-5 above are applicable for 10-years or less recurrence interval
storms. Less frequent, higher intensity storms require adjusted runoff coefficients because
infiltration and other losses have a proportionally smaller effect on runoff. Runoff coefficient
adjustment factors (CF) for storms of different recurrence intervals are listed in Table 5-6.
TABLE 5-6 RUNOFF COEFFICIENT ADJUSTMENT FACTOR
Recurrence Interval
10 years or less
25 years
50 years
100 years
Marion County Public Works
Runoff Coefficient
Adjustment Factor (CF)
1.0
1.1
1.2
1.25
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5. Rainfall Intensity “I”
The peak rainfall intensity shall be derived from ODOT’s rainfall intensity-durationrecurrence (IDR) curves for a given zone. Rainfall zones for Marion County are shown in
Figure 5-2. The IDR curves for Zones 5, 7, and 8 are shown in Figures 5-3 through 5-5. The
design storm duration is typically based on the longest time of concentration for the
drainage area.
FIGURE 5-2 RAINFALL ZONE MAP FOR MARION COUNTY
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FIGURE 5-3 IDR CURVES FOR ZONE 5
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FIGURE 5-4 IDR CURVES FOR ZONE 7
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FIGURE 5-5 IDR CURVES FOR ZONE 8
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C. HYDROGRAPH METHODS
The physical characteristics of the site and the design storm shall be used to determine the
magnitude, volume and duration of the runoff hydrograph. The SBUH is the primary
acceptable hydrograph method. However, other acceptable methods include the NRCS TR20 method, the TR-55 method, or other similar methods. If a software package is used,
documentation of the software's processing and methodology shall be submitted with the
results. All input and assumptions shall be clearly documented. The typical input information
needed for the hydrograph method is:
•
24-hour rainfall distribution
•
Total 24-hour rainfall depth
•
Basin area characteristics
•
Curve Number (CN)
•
Time of concentration
1. Rainfall Distribution
The rainfall distribution to be used within the County is the design storm of 24-hour duration
based on the NRCS 24-hour Type 1A rainfall distribution (Table 5-7).
2. 24-Hour Rainfall Depth
The 24-hour rainfall totals for the given return intervals shall be in accordance with the
rainfall depths shown in NOAA Atlas 2. Precipitation-Frequency Atlas of the Western United
States. Volume X – Oregon.
3. Basin Area Characteristics
To obtain the highest degree of accuracy in hydrograph analysis, proper selection of
homogeneous basin areas is required. Significant differences in land use within a given
basin must be addressed by dividing the basin area into subbasin areas of similar land use
and/or runoff characteristics. Hydrographs should be computed for each subbasin area and
superimposed to form the total runoff hydrograph for the basin.
All pervious and impervious areas within a given basin or subbasin shall be analyzed
separately. This may be done by either computing separate hydrographs or computing the
precipitation excess. The total precipitation excess is then used to develop the runoff
hydrograph. By analyzing pervious and impervious areas separately, the cumulative errors
associated with averaging these areas are avoided and the true shape of the runoff
hydrograph is better approximated.
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TABLE 5-7 NRCS 24-HOUR TYPE 1A RAINFALL DISTRIBUTION
Time
from
start of
storm,
minutes
Cumulative
% Rainfall
Time
from
start of
storm,
minutes
Cumulative
% Rainfall
Time
from
start of
storm,
minutes
Cumulative
% Rainfall
Time
from
start of
storm,
minutes
%
Rainfall
%
Rainfall
%
Rainfall
%
Rainfall
Cumulative
% Rainfall
10
0.40
0.40
370
0.95
22.57
730
0.72
67.40
1090
0.40
86.00
20
0.40
0.80
380
0.95
23.52
740
0.72
68.12
1100
0.40
86.40
30
0.40
40
0.40
1.20
390
0.95
1.60
400
0.95
24.47
750
0.72
68.84
1110
0.40
86.80
25.42
760
0.72
69.56
1120
0.40
87.20
50
60
0.40
2.00
410
0.40
2.40
420
1.34
26.76
770
0.57
70.13
1130
0.40
87.60
1.34
28.10
780
0.57
70.70
1140
0.40
88.00
70
0.40
2.80
430
1.34
29.44
790
0.57
71.27
1150
0.40
88.40
80
0.40
90
0.40
3.20
440
1.80
31.24
800
0.57
71.84
1160
0.40
88.80
3.60
450
1.80
33.04
810
0.57
72.41
1170
0.40
89.20
100
110
0.40
4.00
460
3.40
36.44
820
0.57
72.98
1180
0.40
89.60
0.50
4.50
470
5.40
41.84
830
0.57
73.55
1190
0.40
90.00
120
0.50
5.00
480
2.70
44.54
840
0.57
74.12
1200
0.40
90.40
130
0.50
5.50
490
1.80
46.34
850
0.57
74.69
1210
0.40
90.80
140
0.50
6.00
500
1.34
47.68
860
0.57
75.26
1220
0.40
91.20
150
0.50
6.50
510
1.34
49.02
870
0.57
75.83
1230
0.40
91.60
160
0.50
7.00
520
1.34
50.36
880
0.57
76.40
1240
0.40
92.00
170
0.60
7.60
530
0.88
51.24
890
0.50
76.90
1250
0.40
92.40
180
0.60
8.20
540
0.88
52.12
900
0.50
77.40
1260
0.40
92.80
190
0.60
8.80
550
0.88
53.00
910
0.50
77.90
1270
0.40
93.20
200
0.60
9.40
560
0.88
53.88
920
0.50
78.40
1280
0.40
93.60
210
0.60
10.00
570
0.88
54.76
930
0.50
78.90
1290
0.40
94.00
220
0.60
10.60
580
0.88
55.64
940
0.50
79.40
1300
0.40
94.40
230
0.70
11.30
590
0.88
56.52
950
0.50
79.90
1310
0.40
94.80
240
0.70
12.00
600
0.88
57.40
960
0.50
80.40
1320
0.40
95.20
250
0.70
12.70
610
0.88
58.28
970
0.50
80.90
1330
0.40
95.60
260
0.70
13.40
620
0.88
59.16
980
0.50
81.40
1340
0.40
96.00
270
0.70
14.10
630
0.88
60.04
990
0.50
81.90
1350
0.40
96.40
280
0.70
14.80
640
0.88
60.92
1000
0.50
82.40
1360
0.40
96.80
290
0.82
15.62
650
0.72
61.64
1010
0.40
82.80
1370
0.40
97.20
300
0.82
16.44
660
0.72
62.36
1020
0.40
83.20
1380
0.40
97.60
310
0.82
17.26
670
0.72
63.08
1030
0.40
83.60
1390
0.40
98.00
320
0.82
18.08
680
0.72
63.80
1040
0.40
84.00
1400
0.40
98.40
330
0.82
18.90
690
0.72
64.52
1050
0.40
84.40
1410
0.40
98.80
340
0.82
19.72
700
0.72
65.24
1060
0.40
84.80
1420
0.40
99.20
350
0.95
20.67
710
0.72
65.96
1070
0.40
85.20
1430
0.40
99.60
360
0.95
21.62
720
0.72
66.68
1080
0.40
85.60
1440
0.40
100.00
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4. Runoff Curve Numbers
Runoff curve numbers were developed by the Natural Resources Conservation Service
(NRCS) (formerly referred to as the Soil Conservation Service (SCS)) after studying the
runoff characteristics of various types of land. Curve numbers (CN) were developed to
consolidate diverse characteristics such as soil type, land usage, and vegetation into a
single variable for computing runoff. The approved runoff curve numbers are included in
Table 5-8.
The following are important criteria/considerations for selection of CN values:
•
Many factors may affect the CN value for a given land use. For example, the movement of
heavy equipment over bare ground may compact the soil so that it has a lower infiltration
rate and greater runoff potential.
•
CN values can be area weighted when they apply to pervious areas of similar CN (within 20
CN points). However, high CN areas should not be combined with low CN areas (unless the
low CN areas are less than 15 percent of the subbasin).
•
Antecedent soil moisture values should be considered. A normal antecedent moisture
condition should generally be assumed for design.
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TABLE 5-8 RUNOFF CURVE NUMBERS
CN For Hydrologic Soil Group
A
B
C
D
Cover Description
Urban Areas
Open space
Poor condition (grass cover <50%)
Fair condition (grass cover 50% to 75%)
Good condition (grass cover >75%)
Impervious Areas
Paved parking lots, roofs, driveways (excluding right-of-way)
Streets and roads
Paved: curbs and storm sewers (excluding right-of-way)
Paved; open ditches (including right-of-way)
Gravel (including right-of-way)
Dirt (including right-of-way)
Urban districts
Commercial and Business
Industrial
Residential districts by average lot size:
1/8 acres or less (town houses)
¼ acre
1/3 acre
½ acre
1 acre
2 acres
Agricultural Lands
Pasture, grassland, or range- continuous forage for grazing
<50% ground cover or heavily grazed with no mulch
<50 to 75% ground cover and not heavily grazed
>75% ground cover and lightly or only occasionally grazed
Meadow-continuous grass, protected from grazing and generally mowed
for hay
Brush – weed-grass mixture with brush as the major element
<50% ground cover
<50 to 75% ground cover
>75% ground cover
Woods-grass combination (orchard or tree farm)
Woods
Forest litter, small trees, and brush are destroyed by heavy grazing or
regular burning
Woods are grazed but no burned, and some forest litter covers the soil
Woods are protected from grazing a, and litter and brush adequately
cover the soil
Impervious Surface Reduction Facilities
Type
Pervious Pavement
Trees
New and/or existing evergreen
New and/or existing deciduous
Green roof
Roof Garden
Contained Planter box
Infiltration and Flow-through Planter box
Source: NRCS TR55 Table 2-2a (1986)
% impervious
68
49
39
79
69
61
86
79
74
89
84
80
98
98
98
98
98
83
76
72
98
89
85
82
98
92
89
87
98
93
91
89
85
72
89
81
92
88
94
91
92
93
65
38
30
25
20
12
77
61
57
54
51
46
85
75
72
70
68
65
90
83
81
80
79
77
92
87
86
85
84
82
Source: NRCS TR55 Table 2-2c (1986)
Hydrologic
Condition
Poor
Fair
Good
68
49
39
79
69
61
86
79
74
89
84
80
-
30
58
71
78
Poor
Fair
Good
Poor
Fair
Good
48
35
30
57
43
32
67
56
48
73
65
58
77
70
65
82
76
72
83
77
73
86
82
79
Poor
45
66
77
83
Fair
36
60
73
79
Good
30
55
70
77
Source: Portland Stormwater Management Manual (2008)
Hydrologic
condition
Good
Good
Good
Good
76
85
89
n/a
36
36
n/a
n/a
n/a
n/a
60
60
61
48
48
48
73
73
n/a
n/a
n/a
n/a
79
79
n/a
n/a
n/a
n/a
n/a – not applicable
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5. Time of Concentration “Tc”
Calculations for time of concentration should be divided into segments: sheet flow, shallow
concentrated flow, and channel/pipe flow. The time of concentration is calculated as the sum
of the travel times for each discrete segment of the longest flow path.
•
Overland sheet flow is shallow flow over a plane surface. It occurs in the furthest upstream
segment of the drainage path, which is located immediately downstream from the drainage
divide. The length of the overland sheet flow segment is the shorter of (1) the distance
between the drainage divide and the upper end of a defined channel, or (2) a distance of
300 feet. The sheet flow time of concentration can be calculated with overland flow
kinematic wave equation, using roughness coefficients shown in Table 5-9.
TABLE 5-9 MANNING’S ROUGHNESS COEFFICIENT ‘N’ FOR SHEET FLOW
Surface Type
n
Impervious Areas
0.014
Gravel Pavement
0.02
Developed: Landscape Areas (Except Lawns)
0.08
Undeveloped: Meadow, Pasture, or Farm
0.15
Developed: Lawns
0.24
Developed: Vegetated swale: Mowed grass
0.20
Unmowed grass
0.24
Other dense vegetation
0.30
Undeveloped: Mixed
0.30
Undeveloped: Woodland and Forest
0.40
•
For overland flow distances greater than 300 feet, sheet flow typically becomes shallow
concentrated flow until it finds a defined channel. The average velocity is a function of
watercourse slope and surface type and can be approximated using Figure 5-6. For slopes
less than 0.005 feet per feet, the following equations can be used to determine the average
flow velocity of the shallow concentrated flow.
For unpaved surfaces:
V = 16.1345 x S0.5
For paved surfaces:
V = 20.3282 x S0.5
Where:
V = Velocity in feet per second
S = Slope in feet per foot
•
Manning’s equation for channelized flow shall be used to calculate velocities in channels
and pipes. Manning’s roughness coefficients for pipes and channels are shown in Table 510 and 5-11, respectively. Note that new PVC or HDPE pipe likely have a manufacturer's ‘n’
value of approximately 0.009. However, regardless of pipe material, sand, grit, and slime will
build up on pipe walls. This results in true ‘n’ values over time of approximately 0.013. As a
consequence, a Manning’s roughness coefficient of 0.013 shall be used for design of PVC
or HDPE piping systems. If an alternative piping material is approved, either the pipe
manufacturer's recommended coefficient shall be used or an ‘n’ value of 0.013, whichever is
greater.
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FIGURE 5-6 AVERAGE VELOCITY OF SHALLOW CONCENTRATED FLOW
(source: ODOT Hydraulics Manual, originally from the 1972 Soil Conservation Service
Handbook)
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TABLE 5-10 ACCEPTABLE PIPE MATERIALS, APPLICATIONS, AND CHARACTERISTICS
Allowed
Allowed
Minimum
Other
Material Type
Minimum Cover
Sizes
Slope
Manning’s ‘n’
Considerations
Corrugated Aluminized
54” and
18” (design
0.025
Soil resistivity
Steel (unlined)
greater
required)
Ductile Iron
All
6”
0.013
Smooth bore HDPE
Non reinforced
concrete (NRCP)
Reinforced concrete
(RCP)
Polyvinyl Chloride
(PVC) ASTM 3034
Up to 18”
18”
Up to 18”
All
1% Min.
0.010
Water tight joints
12”
0.013
Water tight joints
12”
0.013
Water tight joints
0.010
No sun exposure
Up to 18”
TABLE 5-11 MANNING’S ROUGHNESS COEFFICIENTS FOR OPEN CHANNELS
Description
‘n’ value range
Concrete lined channel
0.012 to 0.020
Rock lined channel
0.030 to 0.045
Vegetated swale w/short grass
0.045 to 0.090
Vegetated channel w/maintenance
0.030 to 0.120
Natural waterway
0.035 to 0.220
4 DETENTION REQUIREMENTS
Comply with 1990 Marion County Engineering Standards Section V.
5 STORMWATER DISCHARGE
Comply with 1990 Marion County Engineering Standards Section V.
6 CONVEYANCE SYSTEM DESIGN
Comply with 1990 Marion County Engineering Standards Section V.
7 DRAINAGE SUBMITTALS
Submittals for an on-site stormwater discharge permit must include the information included in
this Section.
A. SUBMITTAL REQUIREMENTS
•
Completed checklist of calculations to be included in Stormwater Management Report
(Table 5-12)
•
Stormwater Management Report, including
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Infiltration Testing Report (if applicable; see Section 2C3)
Complete existing and proposed on-site stormwater drainage plans
o
o
•
Landscape plan
•
Construction drawings
•
Operations and Maintenance Submittal
On sites with steep slopes or shallow groundwater, the County may require a geotechnical
report in order to evaluate the suitability of the proposed facility and its location.
Facilities must be designed using hydrologic analysis methods described in Section 3 or the
Simplified Method allowed in Section 2C3. All hydrologic submittals shall include the information
listed in Table 5-12, as applicable, and all data necessary to facilitate the County’s review.
A
B
TABLE 5-12 CHECKLIST OF CALCULATIONS TO BE INCLUDED IN THE
STORMWATER MANAGEMENT REPORT
STORMWATER FACILITY TYPE
C pervious pavement
biofiltration swales
D vegetated infiltration and filtration facilities
proprietary engineered devices
E
green roofs
Parameter or Calculated Value to be
Included in the Stormwater Report
Site Variables:
Site soil type (A, B, C, or D)
Contributing area (acres)
Pre-developed curve number CN
Pre-developed time of concentration (Tc, minutes)
Post-developed curve number CN
Post-developed time of concentration (Tc, minutes)
Infiltration rates from on-site testing and design infiltration rate
Distance from ground surface to max. height of seasonal
groundwater (feet)
Hydrographs:
Post-developed hydrograph for the WQ design storm,
including peak rates and total volumes, if routed through the
facility, at each design or discharge point
Facility Geometry:
A
B
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Facility Type
C
D
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes *
Yes *
Yes *
Yes *
Yes
E
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Table showing area and volume of the facility every 6 inches
in elevation
Yes *
Side slopes (horizontal to vertical, H:V or %)
Yes
Yes
Longitudinal slope (H:V or %)
Yes
Yes
Bottom width and length (feet)
Yes
Yes
Yes
Overall width and length (feet)
Yes
Yes
Yes
Hydraulic Controls:
Orifice or weir descriptions, sizes, and elevations, including
by-pass facilities
Elevation, size, and type of overflow spillway or pipe
Calculated Values:
Water quality flow
Water quality volume
Yes
Yes
Yes
Yes
Yes
Yes *
Yes
Yes
* Unless the Simplified Method is used for facility sizing. In those cases, include a completed
Table 5-3 for each facility.
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B. LANDSCAPE PLAN REQUIREMENTS
The landscape plan and facility design must include elements that ensure landscape plant
survival and overall stormwater facility functional success. Construction specifications and/or
drawings need to include the following elements:
•
Irrigation system to be used for the establishment period of approximately 2-years and longterm. Note that public stormwater management facilities shall be designed so permanent
long-term irrigation systems are not needed.
•
Landscape plan showing the location of landscape elements, including size and species of
all proposed plantings.
•
Plant list or table, including scientific name, size at time of planting, quantity, type of
container, evergreen or deciduous, appropriate planting season, native or non-native to
northwestern United States, and other information in accordance with the facility-specific
planting section and landscape industry standards.
•
Topsoil stockpile location, including source of topsoil, if imported.
C. EXISTING SITE DRAINAGE PLAN REQUIREMENTS
A map clearly defining existing conditions, at a minimum including:
•
Existing contours of the land at 2-foot intervals or as approved by the Director with the
location of existing buildings, structures, and public and private utilities on the property.
Location of any existing building or structure on adjacent property which is within 15 feet of a
proposed public stormwater facility
•
Existing land cover and soil types of all areas included in the hydrologic calculation
•
All areas within 250 feet of the site, improved or unimproved, lying upstream and draining to
or through the proposed development
•
Location of existing stormwater facilities which transport surface water or collect
groundwater onto, across, or from the site, including natural watercourses, artificial
channels, drain pipes, or culverts
•
Locations of springs or other subsurface water outlets
•
Arrows indicating stormwater direction of flows on all public and private property and for all
hydraulic conveyance systems
•
Outlines of and labels on areas and subbasins included in the hydrologic analysis
•
Longest flow path, if used to determine time of concentration, in each subbasin
D. PROPOSED ON-SITE STORMWATER DRAINAGE PLAN REQUIREMENTS
A topographic contour plan clearly defining proposed conditions:
•
Outline of areas that will be disturbed during project construction
•
Proposed contours of the land after completion of the project at two-foot intervals or as
required to clearly show topography. This shall include elevations, dimensions and location,
extent, and slopes of all grading work proposed to be done.
•
Identify cut and fill areas, erosion prevention and sediment control facilities, detention and
water quality facilities, interceptor waterways (channels), velocity check dams, soils,
topography, vegetation, and areas of proposed reseeding.
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•
Proposed land cover all areas included in the hydrologic calculation
•
Outlines of and labels on areas and subbasins included in the hydrologic analysis
•
Longest flow path, if used to determine time of concentration, in each subbasin
•
Location of proposed stormwater facilities which transport surface water across or from the
site, including natural watercourses, artificial channels, under drain pipes, and culverts.
•
Boundaries of all areas that will be paved or otherwise altered in a manner that will increase
surface water runoff and boundaries of all areas to remain in an existing or natural condition.
•
For subdivisions, the approved point of discharge (i.e., curb, pipe, or waterway) shall be
identified for each proposed lot.
E. SUPPORTING DATA
Time of concentration calculations shall be submitted along with a map showing the assumed
flow path, drainage basins areas and their pre- and post-development characteristics, rainfall
depth, duration, intensity, and recurrence interval, and other necessary information.
Background computations for sizing water quality facilities, other than those sized using the
Simplified Method, shall include:
•
Peak discharge and rate of runoff that will be generated within the subject property due to
the design storm after development occurs.
•
Peak discharge and rate of runoff that will be generated by the design storm at all naturally
occurring points of discharge from the property (cubic feet per second, predevelopment, and
post-development.)
F. OPERATIONS AND MAINTENANCE SUBMITTAL
Information on the components of the Operations and Maintenance Manual is included in
Appendix A.
1. Privately Maintained Facilities
Operations and Maintenance packets for privately maintained facilities shall include the
following component for each permit application.
1. Agreement that runs with the land (per Marion County Post Construction Runoff Control
ordinance)
2. Facility specific Operations and Maintenance Manual, including a site plan
The Agreement identifies the property as having a stormwater management facility on the
property. The Agreement must be reviewed and accepted by Marion County Public Works,
signed, notorized, and recorded at Marion County Recorder’s Office. The property description
on the Agreement must be a full legal description of the property and may not be a tax lot
number. Legal descriptions may be obtained from the County Assessor’s office.
Both the Agreement and the Operations and Maintenance Manual must be submitted and
approved prior to issuance of the on-site stormwater drainage permit that includes a stormwater
quality treatment facility.
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2. County Maintained Facilities
A facility-specific Operations and Maintenance Manual must be prepared for County-maintained
(as described in Section 1D) stormwater management facilities. For facilities constructed under
a Public Works permit, the plan must be submitted before construction as part of the Public
Works permit application.
8 REFERENCES
Clean Water Services. Design and Construction Standards.
http://www.cleanwaterservices.org/content/Permit/D&C%20Chapters%202007/Chapter%20
4.pdf
Eugene, City of. 2008. Stormwater Management Manual. http://www.eugene-or.gov/
Marion County Public Works. 1990. Engineering Standards.
http://www.co.marion.or.us/PW/Engineering/engineeringstandards/
National Oceanic and Atmospheric Administration. 1973. NOAA Atlas 2. PrecipitationFrequency Atlas of the Western United States. Volume X – Oregon. J.F. Miller, R.H
Frederick, R.J. Tracey. Prepared for U.S. Department of Agriculture, Soil Conservation
Service.
Natural Resource Conservation Service (NRCS). October 1993: Soil Survey Manual; at
http://soils.usda.gov/technical/manual/
Jurries, Dennis. 2003. Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for
Storm Water Discharge Pollution Removal . Oregon Department of Environmental Quality.
http://www.deq.state.or.us/wq/stormwater/docs/nwr/biofilters.pdf
Oregon
Department
of
Transportation.
2011.
Hydraulics
Manual.
http://www.oregon.gov/ODOT/HWY/GEOENVIRONMENTAL/hyd_manual_info.shtml
Oregon Department of Transportation. Oregon Standard Specifications for Construction
Oregon Department of Transportation. Oregon Standard Drawings
Oregon Sea Grant. 2010. Oregon Rain Garden Guide.
http://www.oeconline.org/ourwork/rivers/stormwater/low-impact-development/rain-garden-guide Oregon State University
Extension Service, Corvallis, OR. ORESU-H-10-001.
Portland Bureau of Environmental Services (BES) 2008.
http://www.portlandonline.com/bes/index.cfm?c=47952
Stormwater Management Manual
U.S. Department of Agriculture Natural Resources Conservation Service.
Hydrology for Small Watersheds TR55
Washington Department of Ecology 2005.
Manual. Volume V. Runoff Treatment
1986.
Urban
Western Washington Stormwater Management
9 DEFINITIONS
Agricultural Field
Drain
Drain pipes, tiles, ditches or other conveyances installed in agricultural
fields to remove excess water
Best
The schedule of activities controls, prohibition of practices, maintenance
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management
practice
procedures and other management practices designed to prevent or
reduce pollution. BMPs also include treatment requirements, operating
procedures, and practices to control stormwater runoff.
County
Marion County
County Road
A public road under the jurisdiction of the county that has been designated
as a county road through an order by the Board of Commissioners
pursuant to ORS 368. This designation causes the roadway to be
maintained by the county.
Development
Ground disturbing activity or the addition or replacement of impervious
surface.
Director
Director of the Marion County Department of Public Works
Impervious
Surface
Any surface resulting from development activities that prevents the
infiltration of water or result in more runoff than in the undeveloped
condition. Common impervious surfaces include: building roofs, traditional
concrete or asphalt paving on walkways, driveways, parking lots, gravel
roads, and packed earthen materials.
Maintenance
Project
Projects that preserve and extend the service life of the existing roadway or
structure. This includes, but is not limited to, minor non-structural overlays
without widening, chip seals, recycle in place, LMC overlays, crack sealing,
bridge and rockfall screening, detector loop repairs, and drainage
enhancements.
Maximum Extent
Feasible
Maximum extent feasible applies to conditions for given site and project
and is based on the physical limitations of the site, practical considerations
of engineering design, and reasonable considerations of financial costs and
environmental impacts.
New Construction
Projects constructed in a new location, new alignments, major additions, or
rebuilding an existing facility with major vertical or horizontal alignment
changes.
Public Road
A road which is within a public right-of-way. It may be maintained by either
private or public funds (state, city, or county).
Reconstruction
Projects that upgrade the facility to acceptable geometric standards, and as
a result, provide greater roadway width. The improvements may be in the
form of additional lanes and/or wider shoulders and produce an
improvement in the level of service. This normally includes the following
types of works: Projects that alter the original subgrade, by constructing
major widening that result in the addition of a new continuous lane,
channelization for signal or left-turn refuges when not part of an overlay
project, structure replacement, and similar projects.
Redevelopment
A project on a previously developed site that results in the addition or
replacement of impervious surface.
Maintenance projects are not
considered redevelopment.
Rehabilitation
Projects that preserve and extend the service life of existing roads and
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enhance safety, using cost-effective solutions. Improvements include
extending pavement life for at least 8 years, safety enhancements, minor
widening, improvements in vertical and horizontal alignment, improvement
in superelevation, flattening of sideslopes and removal of roadside
hazards.
Right-of-Way
A legal right of passage over a piece of land, generally established by
either an easement or ownership.
Rural
Those county areas which do not meet the definition of urban below.
Time of
concentration
Travel time for a drop of water to travel from the most hydrologically remote
location in a defined catchment to the outfall for the catchment where
remoteness relates to time of travel rather than distance
Urban
Those areas within an adopted urban growth boundary (UGB) or having
urban characteristics, such as lot sizes smaller than one acre.
10 ABBREVIATIONS
ASTM
BMP
Cfs
CN
DEQ
HDPE
HEC-RAS
HSG
MCC
MEF
MS4
NPDES
NRCP
NRCS
ODOT
ORS
PVC
RCP
ROW
SCS
UIC
WQF
WQV
American Society of Testing Materials
Best management practice
cubic feet per second
Curve Number
Oregon Department of Environmental Quality
High density polyethylene
Hydrologic Engineering Center River Analysis System
Hydrologic Soil Group
Marion County Code
Maximum extent feasible
Municipal separate storm sewer system
National Pollutant Discharge Management System
Non-reinforced concrete pipe
Natural Resources Conservation Service
Oregon Department of Transportation
Oregon Revised Statutes
Polyvinyl chloride
Reinforced concrete pipe
Right of way
Soil Conservation Service
Underground injection control
Water quality flow
Water quality volume
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Appendix A
Maintenance Information
Contents:
Overview
• Appendix A1 Operations and Maintenance Manual Outline
• Appendix A2 Facility specific maintenance activities
• Appendix A3 Example of inspection and maintenance log
Overview of Maintenance Submittal Requirements
1. Private facilities: Submit a copy of the Operations and Maintenance Manual, the Stormwater
Treatment Facility Maintenance Agreement, the Stormwater Management Report and
engineering drawings with the permit application for an on-site stormwater discharge permit.
After receiving approval for the on-site stormwater discharge permit and a copy of the
Stormwater Treatment Facility Maintenance Agreement with Marion County approval
signature, record the Maintenance Agreement with the Marion County Assessor’s Office.
The Operations and Maintenance Manual will be on file with Public Works.
2. Public facilities constructed by others to be transferred to the County for maintenance:
Submit a copy of an Operations and Maintenance Manual with the Public Works permit
application. County recording of this manual is not necessary.
3. Public facilities constructed by the County: Prepare an Operations and Maintenance
Manual and include it with the project file
Marion County Public Works
Appendix A
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Appendix A
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Appendix A1
Operations and Maintenance Manual Outline
1. Description of Site and Stormwater Management Facilities
a. Summary of overall site stormwater management
b. Table identifying each stormwater facility, its size, the stormwater source to each facility,
square footage of impervious surface treated, and discharge point.
c. Site map or figure showing specific location of each stormwater facility
d. Identification of who will assume responsibility for ongoing operations and maintenance
2. Schedule
a. When and how often each facility will be inspected
b. Specific intervals between particular operations and maintenance tasks
c. Definition of what size storm require additional inspections
d. Irrigation schedule
3. Procedures
a. Specific procedures for each facility type
b. Likely deficiencies and corresponding corrective actions
c. Course of action for unexpected deficiencies
d. If vegetation is part of a facility, describe the vegetation management procedures to be
employed to ensure that adequate vegetation cover is preserved
4. Inspection and Maintenance Logs – These are required for facilities serving commercial
facilities, but not required for facilities serving only one single family residence.
a. Example and instructions for maintaining required logs.
Marion County Public Works
Appendix A1
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Appendix A1
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Appendix A2
Facility-specific Maintenance Activities
Facility Type............................................................................................................................Page
Biofiltration Swales........................................................................................................................ 2
Planters......................................................................................................................................... 4
Rain Gardens................................................................................................................................ 6
Pervious Pavement....................................................................................................................... 8
Green Roofs.................................................................................................................................. 9
Marion County Public Works
Appendix A2
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Biofiltration Swales Maintenance Activities
1. Description: Swales are planted or grassed open channels that trap pollutants by filtering and
slowing flows, allowing particles to settle out. The swale should drain within 48 hours of a storm event.
2. Schedule: All facility components, vegetation, and source controls shall be inspected for proper
operations and structural stability, at a minimum, quarterly for the first 2 years from the date of installation,
2 times per year thereafter, and within 48 hours after each major storm event. Other than for facilities
serving only one single family residence, the facility owner must keep a log, recording all inspection
dates, observations, and maintenance activities.
3. Procedures:
a. Required inspection and maintenance:
Swale Inlet (such as curb cuts or pipes) shall maintain a non-turbulent flow of water entering the swale.
• Source of erosion shall be identified and controlled when native soil is exposed or erosion channels
are forming.
• Sediment accumulation shall be hand-removed with minimum damage to vegetation using proper
erosion control measures. Sediment shall be removed if it is more than 4" thick or so thick as to
damage or kill vegetation.
• Inlet shall be cleared when capacity is reduced. Sources of sediment and debris shall be identified and
corrected.
• Rock splash pads shall be replenished to prevent erosion.
Side Slopes shall be maintained to prevent erosion that introduces sediment into the swale.
• Slopes shall be stabilized and planted using appropriate erosion control measures when native soil is
exposed or erosion channels are forming.
Swale Media shall allow stormwater to percolate uniformly through the landscape swale. If the swale
does not drain within 48 hours, it shall be tilled and replanted according to design specifications.
• Annual or semi-annual tilling shall be implemented if compaction or clogging continues.
• Debris in quantities that inhibit operation shall be removed routinely (e.g., no less than quarterly), or
upon discovery.
Swale Outlet shall maintain sheet flow of water exiting swale unless a collection drain is used. Source of
erosion damage shall be identified and controlled when native soil is exposed or erosion channels are
forming.
• Outlets such as drains and overland flow paths shall be cleared when 25% of the conveyance capacity
is plugged.
• Sources of sediment and debris shall be identified and corrected.
Vegetation shall be healthy and dense enough to provide filtering while protecting underlying soils from
erosion. Mulch shall be replenished as needed to ensure survival of vegetation.
• Vegetation, large shrubs or trees that interfere with landscape swale operation shall be pruned.
• Fallen leaves and debris from deciduous plant foliage shall be removed.
• Grassy swales shall be mowed to keep grass 4” to 9” in height. Clippings shall be removed to remove
pollutants absorbed in grasses.
• Nuisance and invasive vegetation shall be removed when discovered.
• Dead vegetation and woody material shall be removed when swale function is impaired. Vegetation
shall be replaced within 3 months, or immediately if required to maintain cover density and control
erosion where soils are exposed.
Check Dams shall control and distribute flow.
• Causes for altered water flow shall be identified, and obstructions cleared upon discovery.
• Causes for channelization shall be identified and repaired.
b. Other Required Actions:
Debris and Litter shall be removed to ensure stormwater conveyance and to prevent clogging of inlet
drains and interference with plant growth.
Spill Prevention measures shall be exercised when handling substances that contaminate stormwater.
Releases of pollutants shall be corrected as soon as identified.
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Interim Stormwater Quality Treatment Engineering Standards
Training and/or written guidance information for operating and maintaining swales shall be provided to
all property owners and tenants. A copy of the O&M Manual shall be provided to all property owners and
tenants.
Access to the swale shall be safe and efficient. Egress and ingress routes shall be maintained to design
standards. Roadways shall be maintained to accommodate size and weight of vehicles, if applicable.
• Gravel or ground cover shall be added if erosion occurs, e.g., due to vehicular or pedestrian traffic.
Insects & Rodents shall not be harbored in the swale. Pest control measures shall be taken when
insects/rodents are found to be present.
Marion County Public Works
Appendix A2
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Interim Stormwater Quality Treatment Engineering Standards
Planters (Vegetated Infiltration and Filtration Facilities) Maintenance Activities
1. Description: Planters are designed to allow runoff to filter through layers of topsoil (thus capturing
pollutants) and then either infiltrate into the native soils (infiltration planter) or be collected in a pipe to be
discharged off-site (flow-through planter). The flow-through planter is designed with an impervious bottom
or is placed on an impervious surface.
2. Schedule: All facility components and vegetation shall be inspected for proper operations and
structural stability. These inspections shall occur, at a minimum, quarterly for the first 2 years from the
date of installation, 2 times per year thereafter, and within 48 hours after each major storm event. Other
than for a facility serving only one single family residence, the facility owner must keep a log, recording all
inspection dates, observations, and maintenance activities.
3. Procedures:
a. Required inspection and maintenance:
Downspout from rooftop or sheet flow from paving allows unimpeded stormwater flow to the planter.
• Debris shall be removed routinely (e.g., no less than every 6 months) and upon discovery.
• Damaged pipe shall be repaired upon discovery.
Splash Blocks prevent splashing against adjacent structures and convey water without disrupting media.
• Any deficiencies in structure such as cracking, rotting, and failure shall be repaired.
Planter Reservoir receives and detains storm water prior to infiltration. Water should drain from reservoir
within 3-4 hours of storm event.
• Sources of clogging shall be identified and corrected.
• Topsoil may need to be amended with sand or replaced all together.
Filter Media consisting of sand, gravel, and topsoil shall allow stormwater to percolate uniformly through
the planter.
• The planter shall be excavated and cleaned, and gravel or soil shall be replaced to correct low
infiltration rates.
• Holes that are not consistent with the design and allow water to flow directly through the planter to the
ground shall be plugged.
• Sediment accumulation shall be hand removed with minimum damage to vegetation using proper
erosion control measures. Sediment shall be removed if it is more than 4 inches thick or so thick as to
damage or kill vegetation.
• Litter and debris shall be removed routinely (e.g., no less than quarterly) and upon discovery.
Planter shall contain filter media and vegetation.
• Structural deficiencies in the planter including rot, cracks, and failure shall be repaired.
Overflow Pipe safely conveys flow exceeding reservoir capacity to an approved stormwater receiving
system.
• Overflow pipe shall be cleared of sediment and debris when 50% of the conveyance capacity is
plugged.
• Damaged pipe shall be repaired or replaced upon discovery.
Vegetation shall be healthy and dense enough to provide filtering while protecting underlying soils from
erosion.
• Mulch shall be replenished at least annually.
• Vegetation, large shrubs or trees that limit access or interfere with planter operation shall be pruned or
removed.
• Fallen leaves and debris from deciduous plant foliage shall be raked and removed.
• Nuisance or invasive vegetation shall be removed when discovered.
• Dead vegetation shall be removed
• Vegetation shall be replaced to provide at least 90% coverage or when planter function is impaired.
Vegetation shall be replaced within a specific timeframe, e.g., 3 months, or immediately if required to
maintain cover density and control erosion where soils are exposed.
b. Other Required Actions:
Debris and Litter shall be removed to ensure stormwater infiltration and to prevent clogging of overflow
drains and interference with plant growth.
Spill Prevention measures shall be exercised when handling substances that contaminate stormwater.
Releases of pollutants shall be corrected as soon as identified.
Marion County Public Works
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Interim Stormwater Quality Treatment Engineering Standards
Training and/or written guidance information for operating and maintaining stormwater planters shall
be provided to all property owners and tenants. A copy of the O&M Manual shall be provided to all
property owners and tenants.
Access to the stormwater planter shall be safe and efficient. Egress and ingress routes shall be
maintained to design standards. Roadways shall be maintained to accommodate size and weight of
vehicles, if applicable.
• Obstacles preventing maintenance personnel and/or equipment access to the stormwater planter shall
be removed.
• Gravel or ground cover shall be added if erosion occurs, e.g., due to vehicular or pedestrian traffic.
Insects & Rodents shall not be harbored in the stormwater planter. Pest control measures shall be
taken when insects/rodents are found to be present.
Marion County Public Works
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Interim Stormwater Quality Treatment Engineering Standards
Rain Gardens (Vegetated Infiltration and Filtration Facilities) Maintenance Activities
1. Description: A rain garden is a vegetated depression created by excavation, berms, or small dams
to provide for short-term ponding of surface water until it percolates into the soil. The basin shall infiltrate
stormwater within 24 hours.
2. Schedule: All facility components and vegetation shall be inspected for proper operations and
structural stability, at a minimum, quarterly for the first 2 years from the date of installation, 2 times per
year thereafter, and within 48 hours after each major storm event. Other than for facilities serving only
one single family residence, the facility owner must keep a log, recording all inspection dates,
observations, and maintenance activities.
3. Procedures
a. Required inspection and maintenance:
Basin Inlet shall assure unrestricted stormwater flow to the vegetated basin.
• Sources of erosion shall be identified and controlled when native soil is exposed or erosion channels
are present.
• Inlet shall be cleared when conveyance capacity is plugged.
• Rock splash pads shall be replenished to prevent erosion.
Embankment, Dikes, Berms & Side Slopes retain water in the rain garden.
• Structural deficiencies shall be corrected upon discovery:
• Slopes shall be stabilized using appropriate erosion control measures when soil is exposed/ flow
channels are forming.
• Sources of erosion damage shall be identified and controlled.
Overflow or Emergency Spillway conveys flow exceeding reservoir capacity to an approved stormwater
receiving system.
• Overflow shall be cleared when 25% of the conveyance capacity is plugged.
• Sources of erosion damage shall be identified and controlled when soil is exposed.
• Rocks or other armament shall be replaced when only one layer of rock exists.
Filter Media shall allow stormwater to percolate uniformly through the rain garden.
• If water remains 36 to 48 hours after storm, sources of possible clogging shall be identified and
corrected.
• Basin shall be raked and, if necessary, soil shall be excavated, and cleaned or replaced.
Sediment and Debris Management shall prevent loss of rain garden volume caused by sedimentation.
• Sediment and debris exceeding 4” in depth shall be removed every 2-5 years or sooner if performance
is affected.
Vegetation shall be healthy and dense enough to provide filtering while protecting underlying soils from
erosion.
• Mulch shall be replenished as needed to ensure healthy plant growth.
• Vegetation, large shrubs or trees that limit access or interfere with basin operation shall be pruned or
removed.
• Grass shall be mowed to 4”-9” high and grass clippings shall be removed no less than 2 times per
year.
• Fallen leaves and debris from deciduous plant foliage shall be raked and removed.
• Nuisance or prohibited vegetation shall be removed when discovered. Invasive vegetation contributing
up to 25% of vegetation of all species shall be removed.
• Dead vegetation shall be removed when rain garden function is impaired. Vegetation shall be replaced
within 3 months, or immediately if required to control erosion.
b. Other Required Actions:
Debris and Litter shall be removed to ensure stormwater infiltration and to prevent clogging of overflow
drains and interference with plant growth.
• Restricted sources of sediment and debris, such as discarded lawn clippings, shall be identified and
prevented.
Spill Prevention measures shall be exercised when handling substances that contaminate stormwater.
Releases of pollutants shall be corrected as soon as identified.
Marion County Public Works
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Interim Stormwater Quality Treatment Engineering Standards
Training and/or written guidance information for operating and maintaining rain gardens shall be
provided to all property owners and tenants. A copy of the O&M Manual shall be provided to all property
owners and tenants.
Access to the rain garden shall be safe and efficient. Egress and ingress routes shall be maintained to
design standards.
• Obstacles preventing maintenance personnel and/or equipment access to the rain garden shall be
removed.
• Gravel or ground cover shall be added if erosion occurs, e.g., due to vehicular or pedestrian traffic.
Insects & Rodents shall not be harbored in the rain garden. Pest control measures shall be taken when
insects/rodents are found to be present.
Marion County Public Works
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Interim Stormwater Quality Treatment Engineering Standards
Pervious Pavement Maintenance Activities
1. Descrption: Pervious pavement is a permeable pavement surface with an underlying stone
reservoir that temporarily stores surface runoff before infiltrating into the subsoil or being collected in
underlying drain pipes and being discharged off-site. Pervious pavement accepts only precipitation, not
stormwater runoff directed to it from other sources.
2. Schedule: All facility components, vegetation, and source controls shall be inspected for proper
operations and structural stability, at a minimum, quarterly for the first 2 years from the date of installation,
2 times per year thereafter, and within 48 hours after each major storm event. Other than for facilities
serving only one single family residence, the facility owner must keep a log, recording all inspection
dates, observations, and maintenance activities.
3. Procedures
a. Required inspection and maintenance:
Surface: The surface shall be kept clean and free of leaves, debris, and sediment. The surface shall not
be overlaid with an impermeable paving surface.
• Regular sweeping shall be implemented for porous asphalt or concrete systems.
Overflows are used in the event that the facility’s infiltration capacity is exceeded.
• Overflow devices shall be inspected for obstructions or debris, which shall be removed upon
discovery.
• Overflow or emergency spillways shall be capable of transporting high flows of stormwater to an
approved stormwater receiving system.
• Sources of erosion damage shall be identified and controlled when native soil is exposed near the
overflow structure.
Vegetation (where applicable)
• Vegetation and large shrubs/trees that limit access or interfere with porous pavement operation shall
be pruned.
• Fallen leaves and debris from deciduous plant foliage shall be raked and removed.
b. Other Required Actions:
Source Control measures prevent pollutants from mixing with stormwater. Typical non-structural control
measures include raking and removing leaves, street sweeping, vacuum sweeping, limited and controlled
application of pesticides and fertilizers, and other good house keeping practices.
Spill Prevention measures shall be exercised when handling substances that can contaminate
stormwater. Releases of pollutants shall be corrected as soon as identified.
Training and/or written guidance information for operating and maintaining pervious pavement shall
be provided to all property owners and tenants. A copy of the O&M Manual shall be provided to all
property owners and tenants.
Access to the pervious pavement shall be safe and efficient. Egress and ingress routes shall be
maintained to design standards. Obstacles preventing maintenance personnel and/or equipment access
to the porous pavement shall be removed.
Debris and Litter shall be removed to prevent clogging.
Signage may serve to educate people about the importance or function of the site's stormwater
protection measures. It may also discourage behaviors that adversely affect stormwater protection
measures. For example, if debris is a problem, a sign reminding people not to litter may partially solve the
problem. Broken or defaced signs shall be replaced or repaired.
Marion County Public Works
Appendix A2
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Interim Stormwater Quality Treatment Engineering Standards
Green Roofs Maintenance Activities
1. Description: Vegetated roofs are roof systems that retain and filter stormwater and provide aesthetic
and energy conservation benefits.
2. Schedule: All elements shall be inspected once a month from April through September. Other than a
green roof on the house, shed, or garage of a single family residence, the facility owner must keep a log,
recording all inspection dates, observations, and maintenance activities.
3. Procedures
All facility components, including soil substrate or growth medium, vegetation, drains, irrigation systems (if
applicable), membranes, and roof structure shall be inspected for proper operations, integrity of the
waterproofing, and structural stability throughout the life of the roof.
a. Required inspection and maintenance:
Soil Substrate/ Growing Medium shall be inspected for evidence of erosion from wind or water.
• If erosion channels are evident, they shall be stabilized with additional soil substrate/growth medium
and covered with additional plants.
Structural Components shall be operated and maintained in accordance with manufacturer’s
requirements. Drain Inlets shall be kept unrestricted.
• Inlet pipe shall be cleared when soil substrate, vegetation, debris or other materials clog the drain inlet.
Sources of sediment and debris shall be identified and corrected.
• Determine if drain inlet pipe is in good condition and correct as needed.
Debris and Litter shall be removed to prevent clogging of inlet drains and interference with plant growth.
Vegetation shall be maintained to provide 90% plant cover.
• During the Establishment Period, plants shall be replaced once per month as needed. During the long
term period, dead plants shall generally be replaced once per year in the fall months.
• Fallen leaves and debris from deciduous plant foliage shall be removed.
• Nuisance and invasive vegetation shall be removed when discovered.
• Dead vegetation shall be removed and replaced with new plants.
• Weeding shall be manual with no herbicides or pesticides used. Weeds shall be removed regularly
and not allowed to accumulate.
• Fertilization is not necessary and fertilizers shall not be applied.
• During drought conditions, mulch or shade cloth may be applied to prevent excess solar damage and
water loss.
• Mowing of grasses shall occur as needed. Clippings shall be removed.
b. Other Required Actions:
Irrigation can be accomplished either through hand watering or automatic sprinkler systems. If automatic
sprinklers are used, manufacturers’ instructions for operations and maintenance shall be followed.
• During the Establishment Period (1-3 years), water sufficient to assure plant establishment and not to
exceed ¼ inch of water once every 3 days shall be applied.
• During the long-term period (3+ years), water sufficient to maintain plant cover and not to exceed ¼
inch of water once every 14 days shall be applied.
Spill Prevention measures from mechanical systems located on roofs shall be exercised when handling
substances that can contaminate stormwater. Releases of pollutants shall be corrected as soon as
identified.
Training and/or written guidance information for operating and maintaining green roofs shall be
provided to all property owners and tenants. A copy of the O&M Manual shall be provided to all property
owners and tenants.
Access and Safety to the green roof shall be safe and efficient.
Egress and ingress routes shall be maintained to design standards. Walkways shall be clear of
obstructions and maintained to design standards.
•
Insects shall not be harbored on the green roof.
Marion County Public Works
Appendix A2
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Revised 2/22/12
•
Interim Stormwater Quality Treatment Engineering Standards
Standing water creating an environment for development of insect larvae shall be eliminated by
manual means. Chemical sprays shall not be used.
Marion County Public Works
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Interim Stormwater Quality Treatment Engineering Standards
Appendix A3
Stormwater Management Facility Inspection & Maintenance Log
(Sample)
Property Address:
Inspection Date:
Inspection Time:
Inspected By:
Approximate Date/Time of Last Rainfall:
Type of Stormwater Management Facility:
Location of Facility on Site (In relation to buildings or other permanent structures):
Water levels and observations (Oil sheen, smell, turbidity, etc.):
Sediment accumulation & record of sediment removal:
Condition of vegetation (Height, survival rates, invasive species present, etc.) & record of
replacement and management (mowing, weeding, etc.):
Condition of physical properties such as inlets, outlets, piping, fences, irrigation facilities, and side
slopes. Record damaged items and replacement activities:
Presence of insects or vectors. Record control activities:
Identify safety hazards present. Record resolution activities:
Other identified maintenance items and required corrective action.
Marion County Public Works
Appendix A3
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