ABSTRACT:

Stream hydromorphology regulates in-stream water flow and interstitial flow of stream 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 macro-roughness 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 experiments on sediment transport with computational fluid dynamics. Our results show that placement of boulders on “fixed” plane beds increase the reach scale hyporheic median residence time, τ50, by 15% and downwelling flux, qd, 18% from the plane bed. However, reach scale hyporheic exchange changes are stronger with τ50 decreasing by 20% and qd 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 which has been overlooked in previous work.