DISCUSSION - MACROSCALESURFACE ROUGHNESS AND FRICTIONAL RESISTANCE INOVERLAND-FLOW BY LAWRENCE,D.S.L

Lawrence argued that the inundation ratio Lambda, defined as the mean flow depth d divided by the roughness height k, is the dominant contro l of flow resistance f and should be used as the primary variable when evaluating the hydraulics of overland flow on rough surfaces. Lawrenc e defined three flow regimes on the basis of ii and developed an expre ssion for f in terms of Lambda for each regime. Common sense, however, suggests that f is independent of Lambda where Lambda < 1 because whe n roughness elements protrude through the flow, the value of f for the flow is the same regardless of the height of the elements. The error appears to have crept in as a result of Lawrence's representation of r oughness elements by hemispheres. Lawrence found that f proportional t o d/k, which she interpreted to mean f proportional to Lambda. However , in her model the length dimension denoted by k is in fact half the b readth b/2 of the roughness elements. The distinction between k and b/ 2 is important, especially for roughness elements where k not equal b/ 2, Thus, contrary to Lawrence's claim, f is not generally a function o f Lambda. Instead, f is a function of Lambda only where Lambda > 1. Wh ere Lambda < 1, f is a function of d/(b/2) or d/b. (C) 1998 John Wiley & Sons, Ltd.