Study on the effects of oblique flow on protected and unprotected river banks by using mathematical model

Abstract

River bank erosion is an endemic and recurrent natural disaster in Bangladesh, deltaic country located at lower part of the basins of the three mighty rivers of the world. The huge amount of discharge and heavy sediment load cause the rivers to be unstable and the channels are constantly migrating laterally. As a result there are active bank erosions all over the country at different reaches of rivers. Bank erosion along with pronounced local scour could be more serious when it undergoes an oblique flow from a chute channel. With the objective, to observe and assess the hydro-morphological behavior including the bank erosion pattern of the main channel due to oblique flow from a chute channel, mathematical modeling has been carried out by using MIKE21C based on the input data collected from the physical model set up at River Research Institute (RRI), Faridpur. Mathematical model has been developed by using different input data such as initial and final bed level of the cross section, velocity, water surface slope, flow line, scour pattern and flow on bank line, collected from different physical model runs. As physical model is a scaled model, before using in mathematical model, all the data, except grain size, have been converted according to appropriate equations. In physical model effect of oblique angle 200, 400 and 600 with different discharge ratios 1:1.2, 1:1.4 and 1:1.6 have been investigated. Mathematical models have been calibrated for all the oblique flow angles against data from physical model. The impact of oblique flow angles on the unprotected river bank near the meeting place of main and chute channel have been studied. At protected and unprotected bank condition the influence of oblique flow angles as well as discharge ratios on the near bank velocity, bed scour are also observed and compared with the result obtained from physical model. Hydro-morphological phenomena like the bank erosion, bed scour depend on many parameters, for instance change in flow ratio, upstream and downstream condition, incoming and existing sediment load, bank property, channel layout etc. Among various parameters, influence of different flow ratio along with different angles on the bank erosion at protected and unprotected banks have been investigated with this study. Though minor but it is expected this research work done with the help of mathematical model would add to knowledge bank and guide to other researchers who intend to go forward.