资源环境科技发展态势分析平台

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 (tau(*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 tau(*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 (tau(*bf)) and tau(*c). These regressions provide spurious predictions for the bankfull transport capacity (tau(*bf)/tau(*c)) of gravel-bed rivers. When site-specific empirical measurements of tau(*c) are made, coarse-grained rivers exhibit a remarkably constant transport capacity that is in close agreement with equilibrium channel theory (tau(*bf) = 1.2 tau(*c)). From these data we advocate that, in the absence of measurements, tau(*c) can be reasonably estimated from the tau(*bf) using equilibrium channel theory.