Specification of Flows for Flushing Surficial Sand from Cobble Riffles

Maintenance of habitat for benthic organisms and fish spawning may require flushing of sand from gravel or cobble river beds. An approach is proposed for specifying both the magnitude and duration of flow necessary to scour sand to a desired depth over a specified length of riffle bed. Scour is assumed to begin at the upstream end of the reach to be flushed and to progress downstream. The flow magnitude necessary for scour to an equilibrium depth was found from previously published laboratory data to be related to the median sand grain size and its movability number (the ratio of shear velocity to settling velocity). The required flow duration is determined by the time for scour to progress over the specified distance. The progression rate was determined through new laboratory experiments to be related to the flow velocity and depth, the distance advanced, the sand movability number and the channel slope. Application of the approach is illustrated by example. Copyright © 2015 John Wiley & Sons, Ltd.