Integral Modelling Approach for Hyporheic Exchange due to Porous Log Jams: Comparison With Experiments and Sensitivity Studies

Abstract

Hydrodynamic models of surface water (SW)–porewater (PW) interactions often rely on coupled modelling approaches that may not be suitable for coarser sediments and flow through structures such as log jams. SW–PW interactions play a critical role in maintaining the ecological health of rivers by enhancing biogeochemical activity within the hyporheic zone. In the present study, we applied an integral modelling approach to investigate for the first time the hyporheic exchange caused by a channel-wide porous log jam (PLJ). First, a previously conducted laboratory experiment was replicated. A very good agreement was obtained for flow dynamics, such as water depth differences, surface flow velocities, SW–PW interactions, and subsurface flow paths. For hyporheic exchange flow (HEF) patterns, the model performed well at low Froude numbers. However, for cases with higher Froude numbers, limitations emerged due to the model's two-dimensional discretisation, which obstructs flow uniformly across the width. An investigation of higher flow velocities revealed a logarithmic relationship between HEFs and Froude number. At higher flow rates, a linear decrease in HEFs was observed with increasing Froude numbers. Further analysis of the PLJ's effective porosity demonstrated an exponential decay in water depth difference between upstream and downstream, which was also reflected in the HEF rate. The study showed that the integral modelling approach serves as a robust basis for future studies of porous flow obstacles, although the relatively high computational demand has to be taken into account.