Conveyance estimation for channels with emergent vegetation boundaries

Abstract

Emergent vegetation strips occur commonly in rivers, especially along the banks, and have a significant influence on channel conveyance. A simple approach is presented for estimating the conveyance in such rivers, and the method is tested against laboratory and field data. Field data are also used to determine a realistic value of the resistance coefficient for marginal vegetation boundaries. Laboratory experiments have shown that the resistance of vegetation strips depends on the number of vegetation/ clear channel interfaces, and that the overall Manning resistance coefficient n varies strongly with flow depth. For practical purposes, conveyance can be estimated by summing the discharges in vegetated and clear channel zones, calculated separately but with allowance made for the interzonal interaction. The clear channel discharge usually makes the major contribution and can be reliably determined by application of familiar composite roughness formulae, accounting for different bed and side boundary resistance. Field data indicate a Manning's n of about 0·13 for a typical emergent vegetation side boundary. The vegetated zone discharge is invariably underestimated by using a basic vegetation resistance equation without accounting for interzonal interaction effects, especially for narrow strips, but its contribution to the total channel discharge is usually relatively small.