Channel Forms and Patterns

Abstract

Channel forms and patterns are important determinants of ease of movement of water and sediment and immediate clearance of materials from up slope. Twenty five (25) cross-sections have been drawn along the lower reach of the Rupnarayan River, by measuring river depth using Echo Sounder and collecting stuff readings using leveling instrument, to understand the channel forms and patterns. Most of the cross-sections, except two (at Kolaghat-AA^′ and Geonkhali-YY^′) are asymmetrical in nature. Depth is more near Kolaghat (maximum 10.5 m) and Geonkhali (maximum 11.5 m) than the middle of these two extremes (<8 m). Most of the portions near two banks and around the mid channel bar have the depth less than 2 m. Width-depth ratio is less near Kolaghat (38.09) and Geonkhali (130.43), but in the middle portion width-depth ratios are more than 300 and becomes 1111.10 in cross-section QQ^′. Sudden expansion and widening of the channel near Kolaghat (width is 400 m at AA^′, 1650 m at BB^′, 2900 m at CC^′) leads to flow separation , reduction of energy and deposition of sediment (during low tide) and near Geonkhali sudden constriction (bottle neck shape) of the channel (width-depth ratio—130.43) hinders free draining of ebb tide water leading to ponding effect and reduction of velocity and stream energy to drain the sediments and initiates sedimentation.