Local and Reach-Scale Hyporheic Flow Response From Boulder-Induced Geomorphic Changes

Stream hydromorphology regulates in-stream water flow and interstitial flow of water within streambed sediments, the latter known as hyporheic exchange. Whereas hyporheic flow has been studied in sand-bedded streams with ripples and dunes and in gravel-bedded streams with pool-riffle morphology, little is known about its characteristics in plane bed morphology with subdued streambed undulations and sparse macroroughness elements such as boulders and cobbles. Here, we present a proof-of-concept investigation on the role of boulder-induced morphological changes on hyporheic flows based on coupling large-scale flume sediment transport experiments with computational fluid dynamics. Our results show that placement of boulders on plane beds increase the reach-scale hyporheic median residence time, tau(50), by 15% and downwelling flux, q(d), by 18% from the plane bed. However, reach-scale hyporheic exchange changes are stronger with tau(50) decreasing by 20% and q(d) increasing by 79% once the streambed morphology reached equilibrium (with the imposed upstream sediment and flow inputs on boulders). These results suggest that hyporheic flow is sensitive to the geomorphic response from bed topography and sediment transport in gravel-bedded streams, a process that has been overlooked in previous work.