Vegetation effects on the morphological behavior of alluvial channels

This study examined the morphological behavior of a channel with vegetation by means of both laboratory experiments and a numerical simulation. Two sets of laboratory experiments were carried out to elucidate the influence of riparian vegetation on braided rivers with erodible banks. One was for a braided river without vegetation from an initially straight channel and the other was for a river with vegetation, where the density of alfalfa was controlled to form vegetated channels. A numerical simulation was also conducted to investigate the effects of vegetation on the channel morphology considering bank erosion, and the results were compared with those of the laboratory experiments. The experimental results showed that the water depth increased and the width decreased with vegetation. The mainstream was also developed to a high meandering channel and then bifurcated to two channels due to vegetation downstream. This was due mainly to a change of flow direction by the blockade of secondary channels near the vegetated zones and backwater effects near the vegetated banks. As the vegetation density increased, the correlation coefficient of cross-sectional data increased in both the numerical and experimental results, and good agreement between the numerical and experimental results was obtained. This indicates that the channel mobility decreased with increasing vegetation density. The numerical model generated the features of the bed topography obtained in experimental channels.