Surface Water and Groundwater Hydraulics, Exchange, and Transport During Simulated Overbank Floods Along a Third-Order Stream in Southwest Virginia
Restoring hydrologic connectivity between the channel and floodplain is a common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact biogeochemical processing and potential nutrient removal, yet rigorous field evaluations of surface and groundwater flows during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection. Experimental floods entailed pumping stream water onto an existing floodplain swale, and were conducted throughout the year to capture seasonal variation. Each set of experimental floods entailed two replicate floods occurring on successive days to test the effect of varying antecedent moisture. Water levels and specific conductivity were measured in surface water, shallow soils, and deep soils, along with surface flow into and out of the floodplain. Total flood water storage increased as vegetation density increased and or antecedent moisture decreased. Hydrologic flow mechanisms were spatially and temporally heterogeneous in surface water, in groundwater, as well as in exchange between the two and appeared to coexist in small areas. Immediate propagation of hydrostatic pressure into deep soils was suggested at some locations. Preferential groundwater flow was suggested in locations where the pressure and electrical conductivity signals propagated too fast for bulk Darcy flow through porous media. Preferential flow was particularly obvious where the pressure signal bypassed an intermediate depth but was observed at a deeper depth. Bulk Darcy flow in combination with preferential flow was suggested at locations where the flood pressure and electrical conductivity signal propagated more slowly yet arrived too quickly to be described using Darcy's Law. Finally, other areas exhibited no transmission of pressure or conductivity signals, indicating a complete lack of groundwater flow. Antecedent moisture affected the flood pulse arrival time and in some cases vertical connectivity with deeper sediments while vegetation density altered surface water storage volume. Understanding the variety of exchange mechanisms and their spatial variability will help understand the observed variability of floodplain impacts on water quality, and ultimately improve the effectiveness of floodplain restoration in reducing excess nutrient in river basins.