Roughness into the Channel Create the Flood Situation - A Case Study at Lower Reach of Shilabati River Basin, Paschim Medinipur, West Bengal

Abstract

Roughness into the channel of streams and rivers determines several important hydraulic parameters must be properly represented in models for estimating water discharge and sediment transport. Hydraulic parameters mainly gradient, hydraulic radius, width, width-depth ratio, wetted perimeter and cross sectional area are considered for analysing the ability of channels to carry discharge but resisting forces are opposing component of this movement. This paper reviews to evaluate the different approaches used to identify the roughness and to understanding of how flow resistance is created by roughness in open channel flows. The Shilabati river basin embraces a portion of the eastern fringe of the Chotonagpur plateau. South west monsoon is the principal source of rainfall in this area. After the heavy storm, the low lying depression in the lower reach of this catchment attracts huge flood water and sediments from it upper catchment, sub-tributaries and larger catchments of neighbouring rivers mainly Damodar, Darakeswar, and Kangsabati. The upper courses are steeper (0.84) and rocky facilitating easy drainage, that gradually concentrates at lower reach as it lost its capacity due to sudden reduction of gradient (Gadghat, 0.08; Bandar, 0.02). In the Shilabati river the cross sectional area changes through time in response to fluctuations in discharge. Channel slopes near these gauging stations are surveyed, that shows continuously decreasing tendency towards lower catchment. Capacity of channel is measured by conveyance of flow. Roughness coefficient value at three gauging station (Banka 0.50, Gadghat near Ghatal 0.82 and at estuary, Banka,1) measuring by the Manning’s equation depicts that the resistance to flow increases steadily at downstream. Channel substrate mainly vegetation and netting also influences the boundary roughness and flow resistance. Friction from sediment load mainly grain roughness are responsible for generating resistance which steadily declines movement and create jamming of water which ultimately leads to flood and water storage situation.