Hydraulic Resistance by the River Red Gum,Eucalyptus camaldulensis,in Ephemeral Desert Streams

Channel roughness in desert streams is generated by the colonisation of beds and banks by trees including the river red gum, Eucalyptus carnaldulensis. Often growing densely over the bed surface, these trees contribute channel roughness which is distributed through the flow; the effects are heightened by substantial barriers of flood-transported debris which lodge against trunks and branches, offering additional obstruction to water movement. Bed scour often occurs around such obstacles. Field mapping of channels in the Barrier Range, western NSW, where E. camaldulensis growth is common, has permitted the quantification of channel roughness. Results suggest that just under half of the total channel roughness may be contributed by in-channel vegetation, the remainder coming from boundary friction effects. Site-to-site variations in vegetation density are great and assessment of flood discharges and sediment fluxes in these streams without due incorporation of a roughness correction could result in substantial errors of overprediction.