The effect of bed mobility on resistance to overland flow

When sediment grains are transported as bed load in overland flow, there is a net transfer of momentum from the flow to the grains. When these grains collide with other grains, whether on the bed or in the flow, stream-wise flow velocity decreases and resistance to flow increases. Resistance to flow generated in this manner is termed bed-load transport resistance. Resistance to flow f over a plane bed may be partitioned into grain resistance f(g) and bed-load transport resistance f(bt). We use the symbols f(btf) and f(btm) to denote f(bt) for flows over fixed beds and over mobile beds, respectively, and we compute the effect of bed mobility on flow resistance f(mob) by subtracting f(btf) from f(btm). The data for this study come from 54 flume experiments with fixed beds and 38 with mobile beds. On average f(mob) is approximately equal to half of f(btm), which is about one-quarter off. Hence, f(mob) is about one-tenth off. Predictive equations are developed for f(btf),and f(btm) and f(mob) using dimensional analysis to identify the relevant independent variables and regression analysis to evaluate the coefficients associated with these variables. Values of f(mob) are always positive which implies that mobile beds offer greater resistance to flow than do fixed beds. Evidently bed-load grains colliding with mobile beds lose more momentum to the bed than do grains colliding with fixed beds. In other words, grain collisions with mobile beds are less elastic than those with fixed beds. Copyright (c) 2005 John Wiley & Sons, Ltd.