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

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8.5

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9.71

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ul

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15

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g

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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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

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-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

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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

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-0.8

-0.6

-0.4

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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

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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?