Effects of Turbulent Hyporheic Mixing on Reach‐Scale Transport

Abstract

Turbulence causes rapid mixing of solutes and fine particles between open channel flow and coarse‐grained streambeds. Turbulent mixing is known to control hyporheic exchange fluxes and the distribution of vertical mixing rates in the streambed, but it is unclear how turbulent mixing ultimately influences mass transport at the reach scale. We used a particle‐tracking model to simulate local‐ and reach‐scale solute transport for a stream with coarse‐grained sediments. Simulations were first used to determine profiles of vertical mixing rates that best described solute concentration profiles measured within a coarse granular bed in flume experiments. These vertical mixing profiles were then used to simulate a pulse solute injection to show the effects of turbulent hyporheic exchange on reach‐scale solute transport. Experimentally measured concentrations were best described by simulations with a nonmonotonic mixing profile, with highest mixing at the sediment–water interface and exponential decay into the bed. Reach‐scale simulations show that this enhanced interfacial mixing couples in‐stream and hyporheic solute transport. Coupling produces an interval of exponential decay in breakthrough curves and delays the onset of power law tailing. High streamwise velocities in the hyporheic zone reduce mass recovery in the water column and cause breakthrough curves to exhibit steeper power law slopes than predictions from mobile‐immobile modeling theory. These results demonstrate that transport models must consider the spatial variability of streamwise velocity and vertical mixing for both the stream and the hyporheic zone, and new analytical theory is needed to describe reach‐scale transport when high streamwise velocities are present in the hyporheic zone.

Assessment of turbulence effects on effective solute diffusivity close to a sediment-free fluid interface

“…In this framework, it is noteworthy to observe that also the periodic advection of turbulent eddies across the sediment-water interface exhibits an exponential decrease with depth underneath the stream bed (Bottacin-Busolin and Marion 2010). These results are at the basis of current practices that rely on embedding an exponential reduction (along the vertical) of effective diffusion effects in modeling frameworks (Bottacin-Busolin 2019; Roche et al 2019). These studies have shown that assuming an exponential reduction of the diffusion coefficient (a) has relevant implications on the modeling of solute mixing in streams, (b) is consistent with model-based interpretations of observed reach-scale transport experiments.…”

“…Our work tackles the characterization of the spatial distribution of mixing and diffusion parameters in the hyporheic zone which is a key element for all of the above mentioned methodologies. To improve the quality of this characterization, propagation of turbulence from the river flow into the hyporheic region has been quantified through experiments (Chandler et al 2016;Higashino et al 2009;de Lemos 2005;Roche et al 2019) and numerical simulations (Bottacin-Busolin 2019; Breugem et al 2006;Sherman et al 2019;Chandesris et al 2013). Notably, in this context we refer to the high resolution experimental data collected by Chandler et al (2016).…”

The Sensitivity of Hyporheic Exchange to Fractal Properties of Riverbeds

“…As the influence of turbulence on hyporheic exchange is critical (Grant et al, 2018), we suggest that using the appropriate surface flow processes (but not systematically the complete Navier‐Stokes equations) will become essential to better understand and predict hyporheic exchange. Extending the turbulent domain into the porous space remains an additional important challenge in coarser materials (Roche et al, 2019).…”

A One‐Dimensional Model for Turbulent Mixing in the Benthic Biolayer of Stream and Coastal Sediments

“…t in a sediment of porosity θ. The effective diffusivity for solute transport in the sediment, D eff (L 2 T −1 ), encompasses contributions from (tortuosity-modified) molecular diffusion, D ′ m , dispersive mixing, D d , and turbulent diffusion, D t (Boano et al, 2011(Boano et al, , 2014Chandler et al, 2016;Grant, Azizian, et al, 2018;Grant et al, 2012;Grant & Marusic, 2011;Nagaoka & Ohgaki, 1990;O'Connor & Harvey, 2008;O'Connor & Hondzo, 2008;Packman et al, 2004;Reidenbach et al, 2010;Richardson & Parr, 1988;Roche et al, 2018Roche et al, , 2019Voermans et al, 2017Voermans et al, , 2018Zhong et al, 2016):…”