Effect of instream sand mining on hydraulic variables of bedload transport and channel planform: an alluvial stream in South Bengal basin, India

Sand mining adversely behaves with changing river system, especially in the bedload transport process. This study aims to find out the relationships between river water flows, sediment transport regime in bedload transport and also tries to determine how instream mining affect the sediment inflow and channel planform change. The field study has been conducted in three selected sites on Kangsabati River situated in the lower, middle and upper course. Several established equations were applied to derive the results from water flow and sediment regime. GSTAT was used to prepare coarse medium (CM) diagrams and tractive current deposits to analyze the trend of sediment throughout the river course. Sediment and bedload transport were measured using Ackers and White (J Hydraul Eng Div ASCE 99(hy11):2041-2060, 1973) and Meyer-Peter-Muller methods (IAHSR 2nd meeting, Stockholm, appendix 2. IAHR, 1948). Shear stress and critical shear stress were computed using Shields (Application of similarity principles and turbulence research to bedload movement, 1936) and DuBoys equations whereas Friend and Sinha's method (Braiding and meandering parameters, vol 75, No 1, Geological Society, London, 1993) was used to detect irregular planform response caused by sand mining. Regression analysis denotes that mining intensity can be change the shear stress (R-2=0.972); velocity (R-2=0.683); grain diameter (R-2=0.555) and sediment concentration (R-2=0.997) on bedload transport. Mining activities can be changes of hydraulic responses as well as interruption of sediment and bedload transport throughout the channel. In context of sand mining, planform response denotes that thalweg shifting leads to massive sandchar deposition in upper course, huge channel incision causes river bank erosion in middle course and pool-riffle alteration expanded in channel and pit area in lower course.