UM-2/96-97 : Flood estimation for large catchments using deterministic approach

Abstract

The proper planning of a project needs the proper study of hydrology. The flood problem is not only a concern of the branch of engineering but is important to a number of other branches as well. To avoid destruction, any hydraulic structure constructed must have adequate protection against the maximum flood expected. This maximum flood that any such structure can safely pass is called the design flood. Design flood. estimation is 'one of the most important aspect of project hydrology. As per the design criteria, if the hydraulic structure has storage capacity more than 60 Mni , then probable maximum flood estimate or 1000 year flood is required. However, for the storage structures having capacity less than 60 Mni , estimate of standard project flood or 100 year flood is needed. The estimates for return period floods are made by carrying out flood frequency analysis. Such estimates are only peak flood estimate. Probable maximum peak flood or standard project flood estimates are the estimates of flood hydrograph. These estimates not only provide the peak flood but also time to peak, volume and other characteristics of the flood hydrograph. Unit hydrograph approach is one of the most popular approaches available for estimation of design flood hydrograph. But its applicability is normally limited to small to medium sized catchments (size less than 5000 sq. km.) due to inherent limitation of non-uniform rainfall in large sized catchments. For large size catchments, the flood estimates are carried out applying unit hydrograph techniques together with flood routing through channel and existing or proposed reservoirs if any. The procedure involves sub- dividing the catchment into sub-catchments and preparing the network model for the whole catchment. As the computations involved are tedious and lengthy, therefore the designers are often discouraged for

Design Flood Estimation Package 1

opting the network modelling approach in order to estimate the design flood for large sized catchments. Keeping in mind the requirement of the designer, an interactive software has been developed which may not only provides design flood estimates for large as well as small sized catchments but also helps the users to analyse their historical rainfall-runoff records for calibration and validation of the various techniques for part or whole of the catchment and river reaches.