Hydraulic resistance in overland flow during partial and marginal surface inundation: Experimental observations and modeling

Hydraulic resistance as a function of surface roughness inundation was evaluated in a set of experiments designed to simulate overland flow on a rough, granular surface. The data are compared with an additive drag model based on the contribution of individual elements to flow resistance and a mixing length model for estimating bulk flow resistance. During partial inundation of the surface roughness the observed coefficient of drag per element is much higher than for an isolated element, and a model based on element form drag alone underestimates the observed friction. These high resistance values are strongly correlated with the hydrostatic wave drag estimated from the free surface deformation around elements. The mixing length model, incorporating a multiplier setting the hydraulically effective roughness height, reliably reproduces the trends in resistance during marginal inundation. This multiplier is shown to have a value similar to the root-mean-square of the surface height.