Characterization of Downstream Channel Morphology of a Monsoon Dominated Dwarkeswar River in West Bengal
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Characterization of Downstream Channel Morphology of a Monsoon Dominated Dwarkeswar River in West Bengal Sadhan Malik and Subodh Chandra Pal* Department of Geography, The University of Burdwan, Burdwan, West Bengal, India E-mail: [email protected]*; [email protected]
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. INTRODUCTION Channel morphology denotes the systematic variations of different channel parameters like the width, depth, area, velocity and discharge (Kale and Gupta, 2001) and when they are studied along a river downstream, called downstream channel morphology (Schumm, 1960). The channel morphology and channel flood plains are the results of diverse sedimentological character, climatic condition, flow behaviour, tectonics and topographic settings and nature of flood plain vegetation (Kemp, 2010). The study downstream channel morphology especially bankfull downstream channel morphology is very important for the people residing near the river, river engineer and policymakers (Schumm, Dumount and Holbrook, 2000). Numerous scholars have studied the different aspect of bankfull channel morphology (Park, 1977; Pitlick & Cress, 2002). Most of them have shown that bankfull channel cross-section area, width, depth and discharge increases downstream (Kemp, 2010; Knighton, 2014) and this is also found in ephemeral rivers although with the different rate (Kale & Gupta, 2001; Leopold & Miller, 1962; Park, 1977; Wolman & Gerson, 1978). But, some limited studies, particularly after 1980s, have showed that channel width, flow area, w/d ratio may decrease downstr
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