Transport of nonsorbing solutes in a streambed with periodic bedforms

Abstract

Previous studies of hyporheic zone focused largely on the net mass transfer of solutes between stream and streambed. Solute transport processes within the bed has attracted less investigation. In this study, we combined flume experiments and numerical simulations to examine solute transport processes in a streambed with periodic bedforms. Solute originating from the stream was subjected to advective transport driven by pore water circulation due to current-bedform interactions as well as hydrodynamic dispersion in the porous bed. The experimental and numerical results showed that advection played a dominant role at the early stage of solute transport, which took place in the hyporheic zone. Downward solute transfer to the deep ambient flow zone was controlled by transverse dispersion at the later stage when the elapsed time exceeded the advective transport characteristic time t c (= L/u c with L being the bedform length and u c the characteristic pore water velocity). The advection-based pumping exchange model was found to predict reasonably well solute transfer between the overlying water and streambed at the early stage but its performance deteriorated at the later stage. With dispersion neglected, the pumping exchange model underestimated the long-term rate and total mass of solute transfer from the overlying water to the bed.Therefore both advective and dispersive transport components are essential for quantification of hyporheic exchange processes.

Inter‐disciplinary perspectives on processes in the hyporheic zone

“…Similar simulations could be conducted for streambed ripples. Jin et al (2010) investigated advective-dispersive transport in hyporheic flow caused by bedforms. They found that neglecting dispersion in advection based pumping exchange models can lead to an underestimation of long-term exchange rates and volumes.…”

Effects of salinity variations on pore water flow in salt marshes

“…Prior to each experiment, the sand was first thoroughly rinsed with deionized water (four times and 1 h each time) following the method of Jin et al [2010] to remove impurities. It was then washed with salt water of 15 ppt salinity for a sufficiently long time (four times and 1.5 h each time) to ensure that the pore water in the sand had the initial salinity of 15 ppt.…”

Effect of bed permeability and hyporheic flow on turbulent flow over bed forms

“…Additionally, the wall‐normal Reynolds stresses (Figure d) associated with the shear layer are lessened for the dune on a permeable bed (Figure d), and the zone of high Reynolds stress does not impinge on the bed as in the cases for the impermeable bed dunes. Upwelling flow in the leeside can be seen to be supplied by a positive vertical flow within the underlying pores in this region (Figures d, d, and d (bottom)), with subsurface flow downwelling on the stoss side and upwelling in the leeside (Figure d), as has been documented in previous studies of hyporheic flow and bed forms [ Cardenas and Wilson , ; Janssen et al ., ; Hester et al ., ; Thibodeaux and Boyle , ; Jin et al ., ; Harrison and Clayton , ].…”

“…However, while progress has been made into refining the original advective pumping model and extending it to more complex geometrical configurations [ Bardini et al ., ; Stonedahl et al ., ], past research has largely neglected the nonlinearity of the turbulent flow above the bed. Recent numerical models typically treat the surface and subsurface flows as separate, assuming turbulent flows above the bed and Darcian flows beneath [ Cardenas and Wilson , , ; Cardenas et al ., ; Jin et al ., ; Janssen et al ., ]. The coupling is obtained through continuity on mean pressure terms, neglecting the existence of a transition region between the two regions, where unsteady and nonlinear interactions are reasonably expected.…”