Characterization of Downstream Channel Morphology of a Monsoon Dominated Dwarkeswar River in West Bengal

Abstract

Downstream changes in channel morphology and its flow were investigated over a monsoon dominated Dwarkeswar river from the western part of West Bengal. The basin has developed over the Proterozoic Granite Gneiss Complex to recent Holocene alluvium, forming three distinctive geomorphological regions of the basin e.g., dissected plateau, plate-fringe and alluvial plain area. Sixty cross-sections along the entire main stream were surveyed and bankfull channel parameters were measured. Sediment samples from each location were collected and Manning's roughness (n) value for respective reaches were estimated and flow velocity, discharge, stream power and shear stress were calculated. The result has shown that channel width and channel capacity in the dissected plateau are increasing downstream, in the plateau-fringe area the trends are relatively constant. But in alluvial plain, width (87.3%), flow area (81.3%), discharge (84.1%), W/D ratio (91.3%) decreases downstream. Downstream decreasing channel width and channel capacity are strongly related to elevation, slope, HSI, TSI and SSI. Extremely low slope, channel switching, cohesive bank materials and vast flood plain width facilitates the river flow to spill over from the channel and spread to the surrounding area resulting diminishing discharge and thereby the channel morphology in downstream. Frequency of flow crossing bankfull limit has increased downstream resulting frequent flood diminishing channel morphology with distance down the valley.