Numerical modeling of bed form induced hyporheic exchange

Abstract

The hyporheic zone is a region beneath and alongside a stream, river, or lake bed where shallow groundwater and surface water mix. Field and experimental observations, along with modeling studies, indicate that hyporheic exchange occurs mainly in response to pressure gradients driven by the geomorphological features of stream beds. Flow over a pool-riffle sequence creates an irregular pressure gradient that drives hyporheic exchange. Currently to analyze the overall flow pattern in different types of pool-riffle structures, hyporheic exchange flow was analyzed using a fully coupled hydro-dynamic model. Simulation results showed that recirculation zones and stagnation points in the pool-riffle structures dominantly controlled the upwelling and downwelling patterns. Numerical simulations were analyzed for the velocity distribution, velocity vectors, and the streamline and flux of groundwater and surface water. Upwelling flow was dominated by a pressure gradient generated by the apex of riffles. Downwelling flow patterns were affected by the flow pattern formed in pools, which was related to the geometric shapes of the pools. The mixing pattern in the groundwater was also affected by the pool shape. The results will be applicable for river restoration projects and stream ecology related to hyporheic exchange, in the prediction and management of upwelling and downwelling flow induced by bed forms.