Hydrodynamically Smooth Flows Over Surface Material in Alluvial Channels

Abstract

It is a general practice to treat the flow in alluvial channels in two scales: the microscale over bed materials (sand grains), and the macroscale over bed forms. Contrary to common thinking, the flow over bed materials appears to be in the hydrodynamically smooth regime, or at most in the transition region. This explains why the roughness-resistance coefficient of grains still varies with the Reynolds number. Such an identification is helpful in understanding flow and transport in alluvial channels.