seasonal variation in surface- groundwater exchanges in an urban floodplain with active gravel-bar...
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Seasonal variation in surface-groundwater
exchanges in an urban floodplain with active gravel-bar formation
Dorothea Lundberg
Karen Prestegaard
University of Maryland
1
10
100
1 10 100
Drainage Area, sq. km
Widt
h, m
Pre-urban reference
2009 Survey
Urban Channel Adjustment Paint Branch Watershed
1. Initial pre-urbanization channel2. Channel widening due to urban runoff
mobilizes sediment3. Gravel bar deposition downstream modifies
channel Blanchet, 2009; Prestegaard et al., 2001
1
2
3
Wid
th,
m
Drainage Basin Area, km2
How do accreted gravel bars affect surface-groundwater exchange?
1.Stream water moves through permeable gravel bars (Purple).
2.Decreased bank height, localized overbank flooding (Red)
3.Chute channels convey streamflow onto floodplain (Blue).
Study Site is a bar-floodplain system
Active gravel bar Accreted gravel bar
Chute channels Upland Floodplain
Methods: Field, Lab, Analytical1. Topographic surveys
2. Sampled soils spatially & vertically
3. Stream Gage 4. Weekly monitoring of 51 piezometers5. Analyze head data
Time series analysis Equipotential maps Groundwater
probability curves
Surface Grain Size Distributions Active
Floodplain/Gravel BarsFine-Medium
Sand/Gravel
Chute ChannelFine & Med.
Sand/Gravel Highest K (Hydraulic
Conductivity)
Upland floodplainSandy silt
Lowest K
Gravel
Seasonal Reversals in GW Gradient
Gravel bar head was fairly constant Upland floodplain head responded to changes in
ET demand producing highest & lowest groundwater levels
Chute channel head mimic floodplain head but with higher peaks during storms
8.1
8.3
8.5
8.7
8.9
9.1
9.3
9.5
9.71
5-J
ul
15
-Au
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-Se
p
15
-Oct
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-No
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-De
c
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-Ja
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b
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r
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-Ap
r
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-Ma
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-Ju
l
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-Au
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To
tal h
ea
d, m
Gravel Bar Chute Channel Uplands
Equipotential Maps – Winter/Spring High heads in upland
floodplain generates
flow from uplands to
stream and through
the accreted gravel bar
Minor flow from
stream through
accreted gravel bar
ROAD
Dominate flow is from the stream through the permeable bars into the upland floodplain.
BUT some is captured in the chute channel and the topographic low and returns to the stream
Equipotential Maps – Summer/Fall
Seasonal change in head
Upland floodplain shows greatest seasonal change in head.
The lowest heads are in summer for all areas
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7.5
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8.5
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9.5
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0 5 10 15 19 23 28 32 37 42 47Distance (m)
Ele
vatio
n (m
)
Topography Fall Winter Spring Summer
C C’
C’
Groundwater Probability
Chute channel exceeds the surface 10% of the year Upland floodplain water table never reaches the
surface. Gravel bar shows the smallest variation and responds
quickly to the stream elevation
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60 70 80 90 100
% time water is at or above indicated value
Dep
th t
o w
ate
r,
m
Gravel Bar
Chute ChannelUplands
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60 70 80 90 100
% time water is at or above indicated value
Dep
th
to
water,
m
Gravel Bar
Chute ChannelUplands
Dep
th t
o W
ater
, m
Annual Precipitation
Precipitation occurs throughout the year.
Major storms occurred in both winter & summer
Precipitation >2.2cm initiated chute channel/overbank flow
0
1
2
3
4
5
6
7
8
7/15 9/15 11/15 1/15 3/15 5/15 7/15 9/15
2009-2010
Rain
fall (
cm
)
Minimum storm to initiate Chute Channel flow
Pre
cip
ita
tio
n, c
m
Little Paint Branch Stream Discharge
Groundwater Flow From the Channel
Significant groundwater flow to the floodplain occurs during summer storms
Winter groundwater flow from channel is limited to permeable gravel bar
In winter, stream width expands and water moves onto floodplain by chute channel flow and overbank flooding
Conclusions GRAVEL BARS ENHANCE MOVEMENT OF
STREAMWATER INTO FLOODPLAIN
SEASONAL VARIATIONS IN PROCESSES:Summer lowering of the water table (by ET) drives
groundwater flow from the channel to the
floodplain.
High winter water table in the floodplain drives
most GW flow towards the stream channel.
Winter stream flow moves onto the floodplain by:
Width expansion, chute channel, & overbank flow
Minor groundwater flow through the accreted gravel
bar
Implications Gravel bars are known to attenuate stream
temperature and for storage of reactive solutes
but for these processes to occur, the flow must
go through the gravel bars. This study
indicates that during the winter period and
major storms, the flow through the gravel bars
is limited.
The streamflow into the floodplain during the
summer months helps provides addition water
for evapotranspiration which helps to maintain
the riparian forest.
Thank you/Questions?