Bankfull Transport Capacity and the Threshold of Motion in Coarse-Grained Rivers

Abstract

The threshold stress for bed sediment transport exerts a primary control on the geometry and stability of coarse-grained rivers (diameter ≥ 5 mm). Understanding how riverbed mobility couples to channel form is a key mechanistic link for predicting river response to external perturbations such as land use practices and changing climate. Unfortunately, determination of a representative threshold stress is notoriously difficult in the field. Empirical studies have observed that the critical dimensionless shear (Shields) stress (τ*c) is correlated with channel slope, a property that is substantially easier to estimate. Mechanistic models have been developed to explain the observed correlation; however, limited field data preclude the widespread application of these models. For practical reasons, the empirical regressions between slope and τ*c are utilized as predictive models. Through a large compilation of field data, we demonstrate that there are two significant problems with using the empirical regressions: (1) they are based on a partial sampling of the observed parameter space of coarse-grained rivers, and (2) they do not capture the covariation between the bankfull Shields stress (τ*bf) and τ*c. These regressions provide spurious predictions for the bankfull transport capacity (τ*bf/τ*c) of gravel-bed rivers. When site-specific empirical measurements of τ*c are made, coarse-grained rivers exhibit a remarkably constant transport capacity that is in close agreement with equilibrium channel theory (τ*bf = 1.2τ*c). From these data we advocate that, in the absence of measurements, τ*c can be reasonably estimated from the τ*bf using equilibrium channel theory.