Effective diffusivity and mass flux across the sediment‐water interface in streams

Abstract

The exchange of water between a stream and its hyporheic zone (defined as the sediment beneath and immediately adjacent to a stream) underpins many ecological and hydrological functions in turbulent streams. Hyporheic exchange can be parameterized in terms of an effective diffusion coefficient Deffand considerable effort has gone into developing process‐based models and empirical correlations for predicting the value of this transport parameter. In this paper we demonstrate previous laboratory estimates forDeffcan be biased by as much as a factor of 10, due to errors in the equations and/or ambiguities in the variables used to reduce data from transient tracer experiments in flow‐through and recirculating flumes. After correcting these problems, an analysis of 93 previously published flume experiments revealsDeffdepends on properties of the tracer (molecular diffusivity), flow field (shear velocity, kinematic viscosity), and sediment bed (permeability and depth). The shear velocity depends implicitly on the Darcy‐Weisbach friction factor, which captures the influence of bed roughness and bed forms on hyporheic exchange in both laboratory and field studies. The dependence ofDeffon sediment bed depth is consistent with the hypothesis that coherent turbulence in the water column drives mass transport across the sediment‐water interface. Furthermore, the dependence ofDeff on sediment bed depth raises the possibility that hyporheic exchange rates measured in the laboratory are not representative of hyporheic exchange rates in the field.