In‐stream geomorphic structures as drivers of hyporheic exchange

Abstract

[1] Common in-stream geomorphic structures such as debris dams and steps can drive hyporheic exchange in streams. Exchange is important for ecological stream function, and restoring function is a goal of many stream restoration projects, yet the connection between in-stream geomorphic form, hydrogeologic setting, and hyporheic exchange remains inadequately characterized. We used the models HEC-RAS, MODFLOW, and MODPATH to simulate coupled surface and subsurface hydraulics in a gaining stream containing a single in-stream geomorphic structure and to systematically evaluate the impact of fundamental characteristics of the structure and its hydrogeologic setting on induced exchange. We also conducted a field study to support model results. Model results indicated that structure size, background groundwater discharge rate, and sediment hydraulic conductivity are the most important factors determining the magnitude of induced hyporheic exchange, followed by geomorphic structure type, depth to bedrock, and channel slope. Model results indicated channel-spanning structures were more effective at driving hyporheic flow than were partially spanning structures, and weirs were more effective than were steps. Across most structure types, downwelling flux rate increased linearly with structure size, yet hyporheic residence time exhibited nonlinear behavior, increasing quickly with size at low structure sizes and declining thereafter. Important trends in model results were observed at the field site and also interpreted using simple hydraulic theory, thereby supporting the modeling approach and clarifying underlying processes.

First-Order Contaminant Removal in the Hyporheic Zone of Streams: Physical Insights from a Simple Analytical Model

“…Fourth, at a larger scale geomorphic features of a stream such as riffle-pool sequences, debris dams, meander bends, and regional groundwater flow all influence hyporheic exchange. 1,2,35,36 Finally, many contaminants of practical interest are removed in the sediment by reactions and processes that do not conform to first-order kinetics. 37 Keeping these limitations in mind, below we utilize our simple model to evaluate the distances over which stream contaminants might be removed by hyporheic exchange through reactive sediments.…”

Spatio‐temporal variations of hyporheic flow in a riffle‐step‐pool sequence

“…The transfer of water as a result of actual flow processes can be classified into four categories according to the control: (1) turbulent diffusion is caused by the transfer of turbulent momentum between stream and pore-water flow (Zhou and Mendoza, 1993;Packman and Bencala, 2000); (2) hydrodynamically induced advection, known as bedform-/flow-induced advection, current-obstacle interaction or pumping exchange, is caused by the acceleration of flow over bedforms that gives rise to pressure variations, thus inducing flow in and out of the bed (Thibodeaux and Boyle, 1987;Elliott, 1990); (3) hydrostatically induced advection results from spatial variations of the hydraulic gradient caused either by geomorphological features such as stream meanders (Wroblicky et al, 1998;Boano et al, 2006) and instream structures, e.g. debris dams, step-pool sequences (Gooseff et al, 2006;Lautz et al, 2006;Hester and Doyle, 2008), or hydrogeological characteristics, primarily permeability distribution (Woessner, 2000;Cardenas et al, 2004) and ambient groundwater discharge (Larkin and Sharp, 1992;Winter, 1999);and (4) what may be considered as transient exchange is the transfer driven by the fluctuations of stage and groundwater, for example through bank storage (Sauer and Pinder, 1970;Konrad, 2006).…”

The role of organisms in hyporheic processes: gaps in current knowledge, needs for future research and applications

“…1), but very few data are available at larger scales. Research combining flow modeling (Cardenas et al, 2004;Hester and Doyle, 2008;Peyrard et al, 2008) and field measures (Sa´nchez-Pe´rez et al, 2003a, b;Gooseff et al, 2006) is still needed to understand the relationship between sediment mobility, channel shape, and water fluxes between the river and its substratum. This is especially important for low-land sandy rivers, where hydrological exchanges are difficult to measure but are essential to maintain an oxygenated level inside hyporheic sediments (Dahm et al, 1987;Lefebvre et al, 2005).…”