EXPERIMENTAL INVESTIGATION INTO FLOW AROUND ISLAND

Abstract

Due to severe stream-induced erosion of the banks, large-sized islands like Majuli in the river Brahmaputra in the province of Assam, are facing imminent extinction. During high floods, these islands are subjected to debilitating erosion process and thus, protection of these islands has received a lot of attention. It has been found that there is lack of extensive study related to flow hydraulics around island. The present investigation basically attempts to fill this gap by undertaking study related to flow characteristics and scour pattern around island. Understanding of scour phenomenon is aimed at evolving flow control devices for protection of island. In the present study, elaborate experimentations have been conducted to investigate flow division around islands having width to length ratio between 0.15 to 1.0 and placed at different eccentricities, varying from 0% to 40%. 45 experimental runs were carried out under rigid bed flow conditions and 24 experimental runs were carried out under mobile bed flow conditions and the Froude number ranged from 0.105 to 0.2. The relationship developed for flow division around island has been found to work well for both rigid bed and mobile bed flow conditions. The scour pattern around islands having different width to length ratios has been studied for three Froude numbers, viz, 0.16, 0.19 and 0.2. The maximum scour around island has been found to increase with the increase in width to length ratio of island. The energy based theoretical model developed in the present study has also supported the above finding. The model indicates that islands with lesser value of width to length ratio are less vulnerable to scour.