Effect of Combined Subsurface Structures and Steps on Hyporheic Exchange

Abstract

The hyporheic zone (HZ) is an area immediately below the river bed where surface and subsurface water mix together (Orghidan, 1959). The HZ is an ecological hotspot and it has been denoted as the river's liver (Fischer et al., 2005) because it causes the attenuation of specific pollutants such as nutrients or organic contaminants. The main feature of the HZ is the vertical water transfer between surface and subsurface flows that moderates fluctuations of water temperature and strongly influences nutrient cycles (Bakke et al., 2020). Hyporheic exchange is characterized by downwelling and upwelling fluxes (Martone et al., 2020) that are determined by variations in hydraulic head, hydraulic conductivity of the porous medium, and bed thickness (Tonina & Buffington, 2009a, 2009bVaux, 1968). The combination of these factors will affect the exchange rate, the residence times in the porous medium, the penetration depth of exchange, and the length of the streamlines in the streambed.Natural river morphologies such as meanders or bedforms induce hyporheic exchange due to differences in hydraulic head on the river bed. However, human management of rivers can also affect hyporheic flow. Flow restoration operations such as constructed riffles (Kasahara & Hill, 2006), cross-vanes (Daniluk et al., 2012; log dams (Lautz & Fanelli, 2008), and in-stream structures (Hester et al., 2016) can improve water quality due to the enhancement of hyporheic exchange. In spite of the positive effects of hyporheic processes on water quality, river restoration is rarely done with the main goal of increasing hyporheic exchange processes (Boulton, 2007;Ward et al., 2011). Studies on the hyporheic flow have focused traditionally on the exchange caused by the natural morphology of rivers: streambed topography (