On why gravel bed streams are paved

TLDR

Bedload transport in poorly sorted gravel bed streams is governed by the fact that coarser grains are intrinsically less mobile than fine grains, implying a need for a mechanism that equalizes mobility. The study hypothesizes that the pavement observed in gravel bed streams during low flow is present during typical transport events and uses field data to quantify this concept and develop a predictive relation for river pavement. Pavement serves as the equalizing mechanism by exposing proportionally more coarse grains to the flow. Size distributions of bedload and subpavement are similar, with the coarse half moving at a rate comparable to the fine half, and the model predicts pavement absence in most sand bed streams, consistent with observations.

Abstract

Bedload transport in poorly sorted gravel bed streams is considered. Bedload and typical bed material (subpavement) size distributions are observed to be similar; it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half. Since coarser grains are intrinsically less mobile than fine grains, it follows that some mechanism must act to nearly equalize mobility. It is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes. This pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow. Field data are used to quantify this concept and to develop a predictive relation for river pavement. The model indicates that pavement should be absent in most sand bed streams, in agreement with observation.