Assessment of flood hazard along the N-S section of North Bengal plains, India

The extent and impact of floods in the North Bengal plains show spatial variation from the foothill to the southern part of the floodplain. This phenomenon is further modified and accelerated by anthropogenic impacts such as bridge construction. According to the flood hazard rating chart, there is a chance of flooding both upstream and downstream of bridges, even in the far downstream area close to the confluence of the rivers Lish, Gish, and Chel with river Tista. Depth (m), velocity (m/sec) and debris factor are considered for the computation of the flood hazard rating map. This is further justified by the flood susceptibility map and validated by the Area Under the ROC (Receiver Operating Characteristic curve) Curve (AUC curve). The fuzzy Analytic Hierarchy Process (FUZZY AHP) has been used to assign weights to the selected parameters for the flood susceptibility map. Both the results justify the fact that the downstream section of Chel is more affected than its upstream section. Even the extent of the flood is higher along river Chel in comparison with rivers Lish and Gish. River Gish is affected by flooding throughout its upstream and downstream reaches. However, near bridges flood rate increases and then alteration is found. Mostly agricultural lands in lower flood plains, tea gardens in alluvial fan areas and settlements are under the flood coverage area. It discloses the probable factors like distance from the foothill to the bridge, sediment grain size (D50), volume of discharge (cumec) etc.