Flood dynamics and its spatial prediction using open-channel hydraulics and hydrodynamic model in the dam-controlled river of India

Abstract The spatial prediction of flood inundation, in the monsoon months (July–October), around the downstream southern part of the Damodar River Basin (Hooghly and Howrah districts of West Bengal) is very uncertain due to analytical lack of database, channel hydraulics and steady or unsteady flow anomaly. Using techniques of flood hydrology and HEC-RAS module of flood prediction, this study estimated the critical hydrological range of recurrent floods (1651–1822 m3 s−1 peak discharge) which are annually observed in the low-lying active floodplains of lower basin. The four days heavy rainfall (52.5 to 268 mm), yielded a runoff range of 10–207 mm which triggered maximum discharge of 7035 m3 s−1, causing havoc flood at downstream. There is possibility of channel shifting and embankmnet breaching during floods in the Mundeswari and Damodar/Amta channel, as Unit Stream Power can exceed 30 Wm−2 in tweleve cross-section stations. During 5-year and 10-year flood event (6676 m3 s−1) the floodplain inundation depth can reach 4–12 m in the Khanakul, and Udaynarayanpur region, having maximum chance of overbank flow. The most vulnerable site of embankment failure and overbank flow is stretch between Rajbalhat and Udaynarayanpur where the flood depth can cross 10 m limit for the critical discharge of 1811 m3 s−1. The flood risk is aggravated because within 42.5 km stretch of lower Damodar the channel can accommodate maximum flow of 1378 m3 s−1 (average flow area of 687 m2) at upstream cross-sections, but at downstream cross-sections the carrying capacity of channel is reduced to 1081 m3 s−1 (21.55 percent reduction) due to increasing siltation and decreasing average flow area (478 m2).