Study of general scour of alluvial rivers

Abstract

The present study is undertaken to give an understanding of general scour characteristics of alluvial rivers. The concerned rivers arc: Surma, Tcesta, Brahmaputra and Upper Meghna river. Hydrologic and river data were collected from SWMC and primary data was c-.;.lected for Surma and Teesta rivers from field. Dominant flow was estimated for all the concerned rivers with five year hydrological data. Channel patterns of the rivers were also assessed by different methods to understand the influence of scour on channel patterns. It may be worth while to note that the cancelled study rivers broadly fall within meander and braided forms. From analysis of cross-section data, influence of unit discharge at bank full level on the observed scour was estimated for all the study rivers. Predicted scour depth and width was calculated using Lacey, Simon and Albertson, Blench regime equation for dominant flows. Sub-soil information is an important factor for scour assessment and those information were collected from recent studies of BWDB and FAP. Predicted scour depth and width was calculated for variable discharge at 4 locations. Predicted depth and width was compared with maximum observed depth with the three regime equations. The predicted depth of Surma river is 9.18 meter, Upper Meghna river is 12.35 ulder and Brahmaputra river is 17.32 meter by using Lacey's equation which wqs found to be. close to observed average depth. But in case of Teesta river, pre~icted depth by using Blench equation is 4.15 meter which is close to observed average depth. The observed width of Surma riv~r is 215 meter and Lacey's predicted width of Surma river is 194 meter. Other equations give abnormally lower value for Surma river. In case of Teesta, Brahmaputra and Upper M:::ghna river Blench equation gives closer value. -~ It appears that while one regime equation is suitable for one river it may not be suitable for another river. Lacey's regime width and depth matches with observed width of the Surma river.- Blench equation gives better match with other rivers in respect of width assessment