Hyporheic exchange with heterogeneous streambeds: Laboratory experiments and modeling

Abstract

[1] Hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone. In reality, streambed sediments have a heterogeneous structure, and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns. To assess the basic effects of sediment structure on hyporheic exchange, we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negativeexponential correlated random hydraulic conductivity fields. Dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high-and low-permeability regions of the streambed. As no existing model could represent these effects, we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form-induced boundary head distribution and the heterogeneous sediment structure. A numerical particle-tracking approach was then used to assess the resulting hyporheic exchange. The combined flowtransport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t % 30 hours. The heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed. The layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth. As a result, solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed. The combination of faster near-surface transport and shallower solute penetration produced a shorter mean hyporheic residence time. On the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate.

Quantifying biologically and physically induced flow and tracer dynamics in permeable sediments

“…Spatial heterogeneity can be important under natural conditions (e.g. the permeability in a sand ripple can be markedly different from the sediment below), yet its importance for flow patterns and tracer exchange has only been addressed in a few pioneering studies (Salehin et al, 2004), and hence, this is clearly a topic for future research. Furthermore, we assume that changes in the tracer concentration do not significantly influence the pore water properties (density, viscosity).…”

“…Relatively few studies have compared tracer data with the output of corresponding tracer models (Khalili et al, 1997;Rena andPackman, 2004, 2005;Salehin et al, 2004;Meysman et al, 2006a). Such a "thorough" validation of flow and tracer models is definitely needed to build up confidence in transport modules of sandy sediment models.…”

“…We further assume that the sediment is homogeneous, and so the permeability, porosity, and pore water viscosity remain uniform over the model domain (see Salehin et al, 2004 for a more complex model formulation in heterogeneous sediments). Under the condition of homogeneity, expression (4) reduces to the classical Laplace equation…”

“…Rasmussen et al, 1998), (2) saturating the pore water with tracer, and measuring the appearance of the tracer in the overlying water (Huettel and Gust, 1992a;Meysman et al, 2006a), (3) adding dyes to localized zones of pore water (spots or layers), and visualizing the migration and dispersion of these dyed patterns (Huettel and Gust, 1992a;Salehin et al, 2004), and (4) adding tracer to the overlying water, and measuring the decrease of the tracer in the overlying water (e.g. Glud et al, 1996;Salehin et al, 2004).…”

Transport of nonsorbing solutes in a streambed with periodic bedforms

“…It is a dynamic zone that experiences active exchange of mass (solute) and energy (heat) with the overlying stream water [13,16]. At the local scale, the exchange is largely affected by stream water flows, stream network morphology and bedforms, and sediment properties [17][18][19][20][21][22][23]. Solute exchange between the hyporheic zone and the stream affects significantly the stream water quality and ecology.…”