Estimation of hydrodynamic pattern change of Ichamati River using HEC RAS model, West Bengal, India

The physically-based hydrodynamic model can simulate the water flow dynamics of a stream network against time varying boundary conditions that can be implemented for the case of Ichamati River. Such river models are often an important component of flood forecasting, tidal fluctuation system that forecasts river levels in flood prone regions of the middle stream of Ichamati river. The measurements of river bed depth/slope, water quality (river cross section), floodplain mapping and boundary condition flow are essential for the set up of a river model. But one can use proxy approaches relying mostly on remote sensing data from space platforms for the purpose. In this study, we set up the one dimensional river analysis system (RAS) model of the hydrologic engineering center (HEC) over the stream network of the Ichamati river basins. Good quality in situ measurements of river hydraulics (cross section, slope, flow) were available only for the upstream and middle stream flood prone region of the basin. The de-siltation of the stretch of Ichamati river under survey i.e. middle part showed impacts which were positive in ways more than one. The river velocity in the top strata is 0.3 m/s from model and the river regained its ecological flow. Also, the increased salinity of the river helped in cultivation of different species of fishes, which are socio economically relevant in the adjoining areas. Due to the increased capacity of the channel in the middle part possibility of flood has also decreased. The total volume of silt accumulated has been estimated 277,589.92 m(3). The rate of sedimentation was then calculated and found to be 16,328.82 m(3)/month silt deposition during monsoon and 11,109 m(3) silt deposition during non monsoon period. It may be concluded that out of 6 % annual silt deposition, 5 % silt deposition took place during the month between June and October and 1 % silt deposition during the month between November and May. Silt excavation along 20 km stretch of Ichamati river showed distinct rejuvenation of the river stretch. The program of silt excavation along other stretches of river Ichamati study area would certainly bring positive impact on the river ecology and environment. Ecological flow needs to be maintained in rivers in the upstream due to chocking the silt deposition. Ecological flow in Ichamati river is absent in many stretches of the river due to heavy silt deposition.