Experimental study of bank protection work and launching behaviour for oblique flow on a straight river bank

Abstract

Bangladesh is an alluvial delta of the Ganges, the Brahmaputra and the Meghna River. Hydro-dynamically rivers are very active in nature in this country. Bank erosion, deposition, char development, flood etc is common scenario for rivers in Bangladesh. Among these, river erosion is a common threat in Bangladesh. The rivers are generally meandering and braided in nature. Normally erosion occurs in outflanking bank of meandering channels. There is no systematic pattern of erosion in the braided river system. Erosion pattern in a braided river have a unique nature due to presence of frequent bars and islands within the channel which creates oblique flow in the channel. Oblique flow changes and accelerates the erosion process in a braided river than meandering river. There are a number of variables in the braided river system and is very important to understand its behaviour to prevent erosion. Different studies on braided river are mostly qualitative. But there is a significant lack of quantitative studies in braided rivers. This study has been done to understand the fluvial process of braided river. To understand the behaviour of different processes in the river system physical model and numerical models are used. Physical modelling is a better way for perception of the nature of rivers due to flow current interactions. Different behaviour of river can be incorporated in physical models. The present study has been conducted by physical modelling to understand the behaviour of a part -of braided -river when it is under attack of oblique flow. The physical model for this study has been developed in River Research Institute (RRI), Faridpur which is funded by Institute of Water Modelling (IWM). The physical model is developed in a scale ratio of I :50 for a straight bank ~th variable oblique channel angle and discharge ratio which falls under twenty one setup. Each setup has been developed to understand the effect of oblique flow on main channel. Naturally river bank can be straight -or bend. Straight bank has been considered in the study. Effect on unprotected bank, bank protection work and launching behaviour of protection material due to oblique flow has been identified. Stone boulder and geobags has been used as river bank protection material. Bank shifting tendency and bed scour in unprotected bank has been observed and measured. Revetment work by stone boulder and geo-bags has been done with the analyzed results of unprotected conditions. Launching behaviour of both the materials has been analyzed in this study. Some factor for design of bank protection work has been reassessed according to the results obtained from the model. Relation of oblique flow angle and discharge ratio with respect to main channel on scour has also been established. Some recommendations for further research have been provided.