EXPERIMENTAL STUDIES ON PERFORMANCE OF BANK PITCHING ON GUIDE BUNDS

Abstract

An alluvial river generally flows in a wide belt between high banks called khadirs with meandering and braiding characteristics and it is not economical to construct a hydraulic structure such as a weir, a barrage or a bridge etc. spanning the whole river width. Therefore the width is narrowed down at the site of structure with the help of Guide Bunds to economize the cost of hydraulic structures across the river. But confining an alluvial river flow within a reasonable waterway leads to significant enhancement of the boundary shear stresses and thus the scouring action around the guide bunds. Guide bunds form one of the major and vital constituents of river training works and substantial cost of project is spent on them. The major cost in almost all river training works is due to the cost of stone pitching. Thus it is imperative that the stone pitching must be designed and protected in the most economic and efficient manner. It is common practice to use stone riprap for protecting or strengthening the sloping surface of a guide bund. Conventionally, the size of revetment material is computed by using the thumb rules and empirical relations available in the codes of practices and standard manuals of various organizations, which are based on the knowledge of past experiences. Sometimes designer are faced with the problem of obtaining a suitable solution from a range of equally valid equations that gives quite different results. For a conservative design approach, the largest stone size is adopted which may be uneconomical. Several studies have been made by researchers on the various hydraulic and material parameters influencing the performance of bank pitching in the straight and meandering channels. But these are not specific to guide bunds and generally apply to other river training works. Little efforts have been made in the past to verify the suitability of the available alternative methods for the design of bank pitching, thus emphasizing the need of more systematic studies in this direction. In the present dissertation work, efforts have been made to investigate the performance of the bank pitching around the guide bunds through experimental study and to check the factor of safety available in the different design formulae for the bank pitching and thus, to evolve more rational criteria for the design of bank pitching. Also, the experimental data has been used to develop a new relation which gives better criteria for determination of the size of riprap material to be used in guide bunds.