Abstract

Alternate bars migrate downstream during floods due to coupled erosion and deposition on both sides of alluvial river channels. During low discharge periods, vegetation can grow on the tops of these bars, reducing migration rates and increasing bar wavelengths and bar heights. We explore two specific effects of above-ground vegetation on flow and transport. First, above-ground roots and groundcover can reduce bedload transport rates due to near-bed roughness, an effect not explored in most previous studies. Second, vegetation bodies (i.e. the above-ground trunk, stem, branches, and leaves) generate hydraulic drag. We model vegetation influences on alternate bar evolution using previously proposed equations which consider both vegetation body and above-ground root effects. We investigated three scenarios: vegetation body effects only, above-ground root effects only, and the full vegetation system (i.e., body and above-ground roots together). We find that vegetation body and root effects both reduce the bar migration rate and increase the bar wavelength. Reduced flow velocities over the bars due to vegetation body effects tend to enhance velocities and localized erosion on the opposite side of the channel, which in turn increases relative bar heights. Bar morphology and migration rate are most sensitive to vegetation growth rates at lower flood discharges where bar-top vegetation persists from year to year and older vegetation has stronger impacts on flow and transport. Higher peak floods tend to remove and reset vegetation growth, resulting in little sensitivity to growth rate.