Roughness coefficients and velocity estimation in well-inundated sheet andrilled overland flow without strongly eroding bed forms

Even with the flow of water over a soil surface in which roughness elements are well inundated, and in less erosive situations where erosional bed for ms are not pronounced, the magnitude of resistance coefficients in equation s such as those of Darcy-Weisbach, Chezy or Manning vary with flow velocity (at least). Using both original laboratory and field data, and data from t he literature, the paper examines this question of the apparent variation o f resistance coefficients in relation to flow velocity, even in the absence of interaction between hydraulics and resulting erosional bed forms. Resis tance equations are first assessed as to their ability to describe overland flow velocity when tested against these data sources. The result is that M anning's equation received stronger support than the Darcy-Weisbach or Chez y equations, though all equations were useful. The second question addressed is how best to estimate velocity of overland flow from measurements of slope and unit discharge, recognizing that the ap parent flow velocity variation in resistance coefficients is probably a res ult of shortcomings in all of the listed resistance equations. A new method ology is illustrated which gives good agreement between estimated and measu red flow velocity for both well-inundated sheet and rill flow. Comments are given on the predictive use of this methodology. Copyright (C) 1999 John W iley & Sons, Ltd.