Seasonal variation in surface- groundwater exchanges in an urban floodplain with active gravel-bar formation Dorothea Lundberg Karen Prestegaard University of Maryland

Pre-urban reference 2009 Survey Urban Channel Adjustment Paint Branch Watershed 1.Initial pre-urbanization channel 2.Channel widening due to urban runoff mobilizes sediment 3.Gravel bar deposition downstream modifies channel Blanchet, 2009; Prestegaard et al., Width, m Drainage Basin Area, km 2

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, Analytical 1.Topographic surveys 2.Sampled soils spatially & vertically 3.Stream Gage 4.Weekly monitoring of 51 piezometers 5.Analyze head data  Time series analysis  Equipotential maps  Groundwater probability curves

Surface Grain Size Distributions Active Floodplain/Gravel Bars  Fine-Medium Sand/Gravel Chute Channel  Fine & Med. Sand/Gravel Highest K (Hydraulic Conductivity) Upland floodplain  Sandy 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

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 CC’

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 Depth to Water, m

Annual Precipitation Precipitation occurs throughout the year. Major storms occurred in both winter & summer Precipitation >2.2cm initiated chute channel/overbank flow Minimum storm to initiate Chute Channel flow Precipitation, cm

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.

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