Modeling effects of floods on streambed hydraulic conductivity and groundwater-surface water interactions

摘要

Flood events can induce temporal changes in streambed elevation and particle-size composition, which may influence the bed's hydraulic properties and stream-aquifer fluxes during and after an event. This study combines a set of previously developed modeling approaches to create a synthetic flood event during which bed sediment is entrained and deposited as a function of hydraulic conditions and particle size. One simulated river reach in a state of approximate dynamic equilibrium is chosen to investigate the impacts of size-selective sediment transport on stream-aquifer interaction. Along this reach, the preferential entrainment of fine sediment during the flood's rising limb leads to overall bed coarsening, and increases in vertical hydraulic conductivity (K_(bv)) and downward fluxes of floodwater into the streambed. Progressively finer sediment layers are deposited during the event's falling limb, causing the redevelopment of a colmation (clogging) layer on the bed surface and a decline in overall K_(bv) by the event's conclusion. This reduction in K_(bv) leads to prolonged retention of event water in the streambed (after the reach reverts from losing to gaining river conditions) when compared with what is expected if pre-event K_(bv) values are used to estimate river-aquifer exchanges. This process of sequential bed coarsening and fining during a flood event provides a mechanistic explanation for the event size-and-duration threshold, inferred in some systems, that must be exceeded for significant amounts of flood recharge to occur. The major consequences of these processes-enhanced infiltration and prolonged floodwater retention-have potentially major implications for groundwater-surface water interactions, water quality, contaminant transport, and riparian biogeochemistry.