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

Abstract

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 proofof-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, τ 50 , by 15% and downwelling flux, q d , by 18% from the plane bed. However, reach-scale hyporheic exchange changes are stronger with τ 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.

Engineered hyporheic zones: design and applications in stream health restoration – a review

“…The authors reported that the induced morphological changes had greater effect on hyporheic flows than boulders alone, concreting the influence of secondary bedform presented by Gordon et al (2013). Model limitations and simplification were reported in both Zhou & Endreny (2013) and Dudunake et al (2020) studies.…”

“…Flume studies could control environmental factors, but efforts required was higher. Numerical models could achieve in depth assessment of hyporheic processes, isolate environmental factors and predict future outcomes, but was limited by assumptions, accuracy of measured data and system feedback to applied changes (Dudunake et al 2020). Jaeger et al (2019) suggested adoption of multi-feature multidisciplinary approach of study to reduce inadequacies and maximize research outputs.…”

“…Limitations like assumption of constant model parameters, spatial homogeneity and stationary bedform dynamics prevents upscaling of modelling results into predictions at larger scale (Ward 2016;Lui & Chui 2018). Further, numerical studies lacked accountability of feedback from engineered structures on streambed morphological changes due to sediment transport (Dudunake et al 2020).…”

“…Flume studies have been useful in explorations of feature-based influences on hyporheic exchanges (HE). Studies conducted by Mutz et al (2007), Tonina & Buffington (2007), Zhou & Endreny (2013) and Dudunake et al (2020) have investigated the effect of engineered elements like log jam, gravel pool-riffle, boulders and cobble vanes bedform sequence on induced HEF. While Mutz et al (2007) reported that introduction of wood tripled flow resistance and increased the vertical water flux, mixing depth and sediment pore water volume, Tonina & Buffington (2007) observed that the exchanges induced by bedforms was controlled by discharge and topographic submergence of the structures.…”

“…The authors attributed that the low bed pressure created downstream of the structure counter-balanced the downwelling forces, thus reducing the penetration depth. Dudunake et al (2020) tried to comprehend the effect of morphological changes induced by boulders on HE and observed increased hyporheic residence time and downwelling flux rate at local and reach scales. The authors reported that the induced morphological changes had greater effect on hyporheic flows than boulders alone, concreting the influence of secondary bedform presented by Gordon et al (2013).…”