Backwater Studies and Preparation of Flood Inundation Maps for Padalse Weir on River Tapi, Maharashtra

Backwater studies are important because of they bring out possible impact of reservoir on the areas close to upstream that may be flooded due to construction of hydraulic structures. The backwater phenomenon leads to an increase in the water surface level of upstream regions thereby imposing the threat of submergence during flood events and affecting the longitudinal extent of the river reach. In addition, an inundation map shows the spatial extent and depth of flooding at specific water level along an individual stream section is required to understand the effect of flooding in a particular area. This paper focussed on backwater studies for Padalse weir at different pond levels viz., High Flood Level (HFL) at RL +157.70 m and HFL at RL +156.00 m and Full Reservoir Level (FRL) at Stage-I (RL +152.38 m) for assessing the flood inundation in fringe areas of river Tapi and its tributaries. For this purpose, the 1-day maximum rainfall of eight rain-gauge stations located within study area of river Tapi was estimated by adopting method of moments of Extreme Value Type-1 (EV1) distribution and used. The adequacy of fitting EV1 to the annual 1-day maximum rainfall series used in extreme value analysis was evaluated through Goodness-of-Fit (viz., Anderson-Darling) test. The physiographic characteristics of the delineated catchments derived from ArcGIS tool and the weighted rainfall depth obtained from the estimated 1-day maximum rainfall by EV1 of eight rain-gauges was considered to compute the 25-year, 50-year and 100-year return period peak flood discharges (PFDs) by adopting synthetic unit hydrograph approach. With the estimated PFDs, the 1-D MIKE Hydro River model simulations were carried out with steady state condition at different pond levels of Padalse weir. By using the model simulation results, the flood inundation maps for Padalse Weir on river Tapi were prepared and presented.