An assessment of local scour at floodplain and main channel of compound channel section

Scour around structure is a well known problem for engineers around the world. Most of the rivers in Bangladesh are compound and alluvial in nature. Pier like structures are often constructed at floodplain of compound channel, also subjected to local scour and may cause failure of structure. A compound channel of 22 m long and 20 cm deep having one adjacent floodplain of 80 cm wide with 165 cm wide main channel was constructed to conduct this study considering relative depth ratio (Yr) 0.26. General behavior of local scour at floodplain and main channel have been investigated for commonly used circular and round nose shaped structures by using three locally available bed materials with three discharges and four length-width ratios. Scour map, bed profile, velocity profile, flow intensity contour map and 3D perspective plots were analysed for both floodplain and main channel to evaluate comparison between floodplain and main channel scour. Generally scour behaviour are near about same at flood plain and main channel for different bed materials, discharges, structure shapes and length width ratio. But flow velocity and scour depth are always higher in main channel than floodplain. The scour depth around circular structure was highest as compared with scour depth around other shapes of structures both at flood plain and main channel. Floodplain velocity distribution is associated with vortex generation as mid of channel.