Influence of bank slope on sinuosity-driven hyporheic exchange flow and residence time distribution during a dynamic flood event

Abstract. This study uses a reduced-order two-dimensional (2-D) horizontal model to investigate the influence of riverbank slope on the bank storage and sinuosity-driven hyporheic exchange flux (HEF) along sloping alluvial riverbanks during a transient flood event. The Deformed Geometry Method (DGM) is applied to quantify the displacement of the sediment-water interface (SWI) along the sloping riverbank during river stage fluctuation. This new model approach serves as the initial step to consider complicated floodplain morphologies in physics-based models for better predictions of HEF. Several controlling factors, including sinuosity, alluvial valley slope, and river flow advective forcing and duration of flow are incorporated in the model to investigate the effects of bank slope in aquifers of variable hydraulic transmissivity. Compared to simulations of a vertical riverbank, sloping riverbanks were found to increase the HEF. For sloping riverbanks, the hyporheic zone (HZ) encompassed a larger area and penetrated deeper into the alluvial aquifer, especially in aquifers with smaller transmissivity (i.e., larger aquifer hydraulic conductivity or smaller specific yield). Furthermore, consideration of sloping banks as compared to a vertical river bank can lead to both underestimation or overestimation of the pore water residence time. The impact of bank slope on residence time was more pronounced during a flood event for high transmissivity aquifer conditions, while it had a long-lasting influence after the flood event in lower transmissivity aquifers. Consequently, this decreases the residence time of HEF relative to the base flow condition. These findings highlight the need for (re)consideration of the importance of more complex riverbank morphology as control of hyporheic exchange in alluvial aquifers. The results have potential implications for river management and restoration and the management of river and groundwater pollution.