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Flow and infiltration during flash floods in ephemeral channels were investigated through scaling analysis and numerical experiments. Scaling of the equations governing flow has shown that momentum loss due to transmission losses (flow that infiltrates through the channel bed) during flash floods can be of the same order as momentum loss due to channel friction and can significantly affect the flow velocity. Numerical simulations were carried out using a shock-capturing MUSCL (monotonic upstream-centered scheme for conservation laws), which incorporates transmission losses as a sink term in the momentum and continuity equations. The wetted area of the channel bed during floods is the primary control on the volume of water that can infiltrate into the bed. The increased velocity of floods in narrow channels that adds wetted area to the channel bed due to greater flood propagation distance is not sufficient to overcome the reduction in wetted area due to reduced channel width; wider channels transmit a greater percentage of the flood volume that enters the channel reach to the bed sediments. Floods of the same total volume but different hydrograph shapes transmit different proportions of their volume to the bed sediments; the nature and magnitude of the differences will depend on the flood propagation distance. Increasing the total volume of the flood and decreasing the channel width increases the sensitivity of the total infiltration to the hydrograph shape. For reaches of the same bed area but different spatial distributions of channel width, differences in the rate of channel widening affect the spatial distribution but not the total volume of water that infiltrates into the bed sediments. (c) 2005 Elsevier B.V. All rights reserved.