Effects of regional groundwater discharge on transverse hyporheic exchange in compound channels

Abstract

Previous studies of transverse hyporheic exchange in compound rivers mainly focused on processes driven by riverbed morphology, and neglected the effects of regional groundwater discharge (RGD). This study numerically investigated the transverse hyporheic exchange subjected to the joint influences of bedform and RGD in a conceptual compound channel. The results show the existence of an interfacial exchange zone (IEZ) across compound riverbeds even in the presence of RGD. A stagnation zone forms at the intersection of hyporheic flow and discharging groundwater flow when RGD flux is large, implying localized inactive nutrient processing within this zone. RGD sharply reduces the scale of IEZ whereas the flow rate of IEZ decreases slightly, consequently resulting in shorter residence times. The flux, depth and mean residence time of IEZ linearly, exponentially and exponentially decrease with RGD, respectively. RGD is likely to hinder solute transport depth and mass accumulation through IEZ depth and control transfer rate by mean residence time of IEZ. Therefore, a larger RGD would decrease the maximum solute transport depth, accompanied by less amount accumulation and a lower transfer rate in the bed. For a given bedform, a power model can describe how solute amount accumulation depends on hyporheic fluxes, while the solute accumulation dependence can be described by a linear model when the RGD condition is fixed. Findings from this study would help better understand the various mechanisms underlying hyporheic zones in compound channels.