Simulating wet front evolution along a sinuous riverbank to understand impact of bank slope on hyporheic flow and transport

Hyporheic exchange flow (HEF) is one important driver determining the spatial and temporal evolution of the biochemical characteristics predominant in the hyporheic zone (HZ) of rivers. As such, better understanding of HEF patterns will allow us to improve our quantitative estimates of the biochemical reaction potential in the HZ and the wider river corridor. While HEF has been found to be impacted by riverbank morphology (including sinuosity and bank slope) past studies have specifically neglect the effect of bank slope in combination with river sinuosity.

 

Here we simulate and assess the impact of bank slope on the spatial extent of the HZ of a meandering river and on the evolution of HEF and residence times (RT) into the alluvial aquifer under varying aquifer transmissivity conditions. We use a 2-D numerical finite element model set up in COMSOL and implement variations in lateral bank slope by using a deformed geometry method (DGM) to simulate the wet front evolution into the alluvial aquifer during a dynamic flood event. Different scenarios were run for varying combinations of bank slope angle and aquifer transmissivity.          

 

Our results show that the impact of bank slope on HEF and wet front evolution was more pronounced in aquifers with lower transmissivity. Furthermore, the impact of bank slope can lead to both shorter and longer residence time of river water in the alluvial aquifer, depending on whether HEF is infiltrating a point bar or cut bank.

 

Aquifers with high transmissivity were more impacted by bank slope during the flood event, whereas aquifers with lower transmissivity were less impacted by bank slope during the flood event but while this impact lasted much longer into the post-flood-event phase. Overall, our study shows that river sinuosity and bank slope should be considered when assessing HEF and RT in river corridors.