Importance of vegetation in riverbank resilience: An experimental approach

Abstract

Floodplain (FP) vegetation determines the flow structures in river channels based on its shape, type, spacing and root‐soil binding capacity. The present study focuses on experimental approach to understand the role of FP vegetation in determining the flow patterns and morphological changes in the channels. Flow properties like streamwise and transverse velocity, along with secondary flow and turbulent kinetic energy, were compared between rectangular and compound channels considering partial vegetation cover. The presence and absence of dip phenomenon in the main channel (MC) of compound and rectangular channel respectively suggest that the dip phenomenon is influenced by both vegetation emergence in FPs and differential cross‐sectional depths of compound channels. The secondary flow direction in the slopes and MC of compound channel is towards the FP region which is opposite to that observed in rectangular channel where secondary flow is towards MC showing the effect of non‐uniform cross‐section of compound channels. The morphological changes were analysed by performing experiments in unsymmetrical erodible riverbank channels with bankfull condition considering artificial rigid FP vegetation (Phragmites karka) and naturally growing flexible vegetation (Oryza sativa). After more than 24 h of continuous experimental runs, the O. sativa remains intact in the soil because of development of strong root‐soil bond whereas; all the P. karka gets uprooted. The channel cross‐section in the downstream channel becomes almost uniform for P. karka, whereas differential cross‐sectional height is observed for O. sativa. This study shows that along with the shapes and size of vegetation, root‐soil binding capacity also determines the morphological changes in river channels. Furthermore, it also shows the importance of growing vegetation in the laboratory to properly simulate vegetation observed in river channels.