Water Table and Overbank Flow Frequency

Evan Christianson
Gustavus Adolphus College
In Cooperation with
Nora Matell
Williams College
Advised by Dr. Greg Hancock
College of William and Mary
Stream Incision, part II
Nora Matell
Discharge
Natural vs. Urban Stream Response
Precipitation
Discharge Natural
Discharge Urban
Tim e
Following stream incision:
•What happens to the riparian water table?
• How much does it change?
•At what rate does it change?
•What happens to the channel?
•What happens to floodplain/channel interaction?
•What happens during and after storms?
What happens to
the channel
following
incision?
Changes in channel shape and size
distance across stream (m)
distance across stream (m)
0
2
4
6
8
0
10
8
10
1
depth (m)
1
1.5
1.5
2
2
2.5
2.5
3
3
just downstream of knickpoint
upstream of knickpoint
distance across stream (m)
distance across stream (m)
2
4
6
8
0
10
0
0
0.5
0.5
1
1
depth (m)
depth (m)
6
0.5
0.5
0
4
0
0
depth (m)
2
1.5
2
2
4
6
8
1.5
2
2.5
2.5
3
3
further downstream of knickpoint
farthest downstream of knickpoint
10
What happens to floodplain/channel interaction
following incision?
stilling well
staff gauges
Correlation between stilling well and stream stage of transect B
45
staff gauge height (cm)
40
y = 0.5023x - 2.8714
R2 = 0.9582
35
30
25
20
15
10
30
40
50
60
stilling w ell height (cm)
70
80
90
Changes in flooding frequency
%of storms that stream
overflows its bank
UPDATE GRAPH
WITH 5
90
far upstream
80
upstream
70
downstream
60
far downstream
50
farther downstream
40
30
20
10
0
negligible
small
medium
size of storm
large
What happens to the water
table when it storms?
Wells with pressure transducers installed
Transect B
Transect E
Time – precipitation occurs from hour 3 to 5
upstream
downstream
Comparative affect of a large storm on
upstream and downstream water tables
upstream
100.6
0.8
well B1
100.4
0.7
well B2
well B4
100.2
0.6
well E1
100
well E2
well E4
99.8
0.4
precipitation
99.6
0.3
99.4
0.2
99.2
99
0.1
downstream
98.8
7/7/04 12:00
0
7/7/04 18:00
7/8/04 0:00
tim e
7/8/04 6:00
7/8/04 12:00
precip/15 min
elevation (m)
0.5
Upstream, near-stream response to a large storm
100.32
100.3
elevation (m)
100.28
100.26
well B1
100.24
100.22
100.2
100.18
7/7/04 12:00
7/7/04 18:00
7/8/04 0:00
7/8/04 6:00
7/8/04 12:00
time
Downstream, near-stream response to a large storm
99.7
99.6
elevation (m)
99.5
99.4
well E1
99.3
99.2
99.1
99
98.9
7/7/04 12:00
7/7/04 18:00
7/8/04 0:00
time
7/8/04 6:00
7/8/04 12:00
Groundwater/stream interactions during and following a large
storm in an unincised channel
specific discharge
0.03
100.65
100.60
100.55
100.50
100.45
100.40
0.02
0.01
100.35
100.30
100.25
100.20
0
-0.01
7/7/04
12:00
7/7/04
18:00
7/8/04 0:00 7/8/04 6:00
tim e
7/8/04
12:00
Flux f rom w ell B1 to stream
stream elevation
elevation of w ell B1
q = -K (dh/dl)
100.15
100.10
7/8/04
18:00
Groundwater/stream interactions during and following a
large storm in an incised channel
0.02
99.8
99.7
specific discharge
99.6
99.5
0
99.4
f lux f rom w ell E1 to stream
99.3
stream elevation
99.2
elevation of w ell E1
99.1
99
98.9
-0.02
98.8
7/7/04 7/7/04 7/7/04 7/8/04 7/8/04 7/8/04 7/8/04 7/8/04
9:36
14:24 19:12
0:00
4:48
9:36
14:24 19:12
tim e
Specific discharge,
based on Darcy’s Law:
Channel incision contributes to exacerbating
the “urban” hydrograph:
•Channel volume increases significantly
•Storm flow contained within channel – floodplain
inundation rare
•Lowered groundwater levels allow for significant storage
of storm water – but this storage is only for the short term
Effects of Urbanization
Discharge
Natural vs. Urban Stream Response
Precipitation
Natural Dischargel
Time
Effects of Urbanization
Increased impervious surface
• Higher peak flow
• Reduced lag time
• Reduced base flow
Natural vs. Urban Stream Response
Greater discharge often
leads to channel incision
Discharge
Lag time
Precipitation
Natural Dischargel
Urban Discharge
Time
What Are The Effects Of Channel Incision?
1.) How is the riparian water table effected?
How much does it change?
At what rate?
2.) Effects on channel geometry?
3.) Changes in the floodplain/channel interaction?
4.) Storm response?
The Watershed at Eastern State
Area of
Study
Unincised (above the knickpoint)
Area = 1.5 km2
•1.3 km2 upstream of knickpoint
~15% impervious
Incised (below the knickpoint)
Well Cap
Bentonite Seal
Riser 1 ¼”
PVC Pipe
Water
Table
Sand
Well Screen
Methods
34 Wells installed in 6 transects
Surveyed all wells, floodplain, and
stream
Wells measured with water level
meter ~3 times a week
Well Field at Eastern State
Direction of Water Flow
Typical Water Table Levels
Tran se ct B (ab o v e kn ickp o in t)
7/21/2004
1 0 1.5
Elevation (m)
1 0 1.0
Close to surface
1 0 0.5
W ater Table
1 0 0.0
Floodplain
9 9.5
Low Gradient
9 9.0
9 8.5
-5
5
15
25
35
45
55
Distance From Stream (m)
Transect E (below knickpoint)
7/21/2004
101.5
Lowered
Elevation (m)
101.0
100.5
W ater Table
100.0
Floodplain
99.5
99.0
98.5
-5
5
15
25
35
Distance From Stream (m)
45
55
Higher Gradient
Transect A
Transect B
Transect C
Transect D
Transect E
Transect F
Summary of Measured Water Table Levels
Upstream/unincised regions show:
•Water table close to surface
•Little variance in head
•Low Gradient
Depth Below the Surface
0
Depth Below Surface (cm)
20
Downstream/incised regions show:
•Lowered water table
•Greater variance in head
•Steeper gradient
40
60
80
100
0
5
10
15
20
Distance from stream (m)
25
30
35
Above Knickpoint (Transect B)
Below Knickpoint (Transect E)
Water table lowers near stream
first then slowly propagates out
•Wells farthest from stream
lowered the least
Analytical Model
Boussinesq Equation
∂ 2 h ∂ 2 h Sy ∂h
+ 2 =
2
∂x
∂y
Kb ∂t
One Dimensional
 ∂ 2 h Kb 
 2
∂t = ∂h
 ∂x Sy 
h = head
t = time
x = distance
K = hydraulic conductivity
b = aquifer thickness
Sy = specific yield
Hydraulic conductivity found by conducting slug
tests
•Values range from: 0.03-0.1 m/day
Aquifer thickness estimated to be 2 meters
Specific yield estimated to range from .10-.18
Model Results
K = 0.08 m/day
Sy = 0.15
b = 2m
Original Water Table based off of transect B
Summary of Model
Quick Response near the stream
As gradient is reduced lowering of the water table is slowed
Areas further from stream have a minimal and delayed response
M odeled Water T able Responce to Stream Incision
102
101.5
Floodplain
Natural W ater Table
1 Month
Elevation (m)
101
6 Months
1 Year
100.5
2 Years
3 Years
100
4 Years
5 Years
99.5
6 Years
99
0
10
20
30
Distance from Stre am (m)
40
50
60
Conclusions
Channel incision lowers the riparian water
table, and increases variance.
This lowering is most dramatic near the
stream and propagates inland through time as
the gradient is reduced.
Further Work
Continue to monitor wells
Calibrate model to more accurately simulate real
conditions
Use model to predict future changes and to
understand what conditions were like in the past
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