Transverse hyporheic flow in the cross-section of a compound river system

Previous studies of bedform-induced hyporheic flow focused on processes driven by longitudinal hydraulic head variations at the sediment-water interface (SWI) due to river flow and bedform interactions. By way of numerical simulations, we examined the transverse hyporheic flow in a river system with compound sections, comprising a main river channel and a floodplain connected by a bank. The results show that head fluctuations across the compound beds associated with the transverse secondary flow can cause significant lateral hyporheic flow through two pore water circulating cells. Such a 2-D compound bedfrom have a higher flux across the SWI than the 2-D longitudinal dunes since the 2-D compound cross-section form induces more drag. The circulating flow cells originate from the bank surface and terminate at the main channel and floodplain bed surfaces, respectively. The hyporheic exchange concentrates on the bank region below the SWI where an intensive solute transfer concurrently occurred. The head gradient along the SWI is sensitive to the bank slope angle, which consequently determines transverse hyporheic flow dynamics and solute transport processes. Larger bank slope angles induce higher hyporheic fluxes and smaller hyporheic areas and residence time. The effects of increasing flux on solute transport in the compound section are partly influenced by a sharply decreasing hyporheic area. Thus the solute consequently transfers downward within the hyporheic zone with a lower rate at larger bank slope angle. Larger river discharges may promote both the transverse hyporheic flow and solute transformation across the bed surface. These physical processes may have implications for the biogeochemical processing within the hyporheic zone across a compound river system.