What is Hidden in Hiding Functions? The Influence of Packing on Size-Selective Transport

Abstract:

Studying landscape evolution and developing defensible land management plans require predictions of sediment flux in rivers. Bed load transport equations often use hiding functions to represent the relative mobility of different grain sizes, which is assumed to be caused by hiding effects. Hiding effects describe the tendency for small particles to have lower projection into the flow and higher friction angles than large grains, decreasing their mobility. Hiding function exponents (b) are fit using reach-scale measurements, whereas hiding effects operate at the grain scale. Therefore, the relation between b values and hiding effects has not been explicitly demonstrated. Hiding effects are also commonly measured for grains placed on bed surfaces rather than for in situ grains that experience the packing and burial that are typical of natural streambeds. To examine this issue, we measured resisting forces and projection for in situ and surface grains in a set of laboratory flume experiments. We used these values in a force balance equation to calculate critical shear stresses and then fit hiding functions to the predicted stresses. Lower projection and higher resistance of in situ grains caused their critical shear stresses to be much higher than those for surface grains. In situ grains also had much lower hiding effects because fine and coarse in situ sediment exhibited relatively similar projection values. We compared b values calculated for in situ and surface grains to those obtained from bedload transport measurements in our experiments. Only in situ sediment with relatively weak hiding effects predicted the observed b value from bedload observations. Our results demonstrate that the relative mobility of fine and coarse sediment was largely controlled by packing resistance and that hiding effects are less significant than previously thought.