Bank permeability in an Australian ephemeral dry‐land stream: variation with stage resulting from mud deposition and sediment clogging

Percolation of flood waters into the bed and banks of ephemeral streams provides one of the key mechanisms responsible for transmission loss. However, there are very few published estimates of the rates at which water can enter stream‐bank sediments, and little is known about the variation in bank permeability with elevation above the bed and the resulting effects on transmission loss in floods of different magnitudes. This paper presents the results of 69 field determinations of bank infiltrability made on Fowlers Creek, an ephemeral dry‐land stream located in arid western New South Wales, Australia. Fowlers Creek carries high concentrations of suspended sediments, which are deposited as mud drapes on the bed, banks and floodplain. Results demonstrate that infiltration rates are lowest at the base of the banks, and tend to increase steadily with elevation on the bank, even above the apparent upper limit of mud drapes. In parallel, the texture of the bank sediments (assessed from samples of the uppermost 10 cm) becomes coarser with elevation above the bed. This pattern is inferred to relate to the delivery of silts and clays into pore spaces in the bank sediments by percolating flood waters. The patterns of infiltration rate and sediment texture mapped in the field are reasoned to be the product of many clogging episodes in past flood events having different peak stages. The increase in infiltration rate and mean particle size up the banks reflects lower frequencies of submergence and clogging of the upper banks by large floods, and more frequent inundation and clogging of the lower banks by sub‐bank‐full flows. The stage‐related changes in bank permeability provide a mechanism that can drive variations in transmission loss among floods having different peak stages and hydrograph shapes. Copyright © 2007 John Wiley & Sons, Ltd.