SYNTHETIC UNIT HYDROGRAPH ITS LIMITATION AND APPLICATION

Abstract

Regional hydrometeorological approach is usually addpted for estimation of floods in small catchments because of absence or inadequacy of observed storm rainfall, storm runoff data. Central Water Commission in India has developed representative Synthetic Unit Hydrographs (SUH) for 23 subzones covering entire geographic area of the country. For this purpose observed rainfall runoff data at 329 railway bridge sites in the country has been used to develop simple regression relationships (y=axb) between SUH parameters and geomorphological parameters (Area, Length, Slope etc). CWC study for different subzones has been carried out over the period 1978 to 1995. In this dissertation, CWC reports were studied and available data has been compiled for further research. Limitations of the prevailing SUH practice in the country have been stated. Hydrologic behavior of watersheds (similarity, non linearity etc) can be identified through correlation study between geomorphological and hydrographic parameters. This is illustrated through a case study of upper Narmada & Tapi basin comprising of 18 watersheds.Correlation between 14 parameters were worked out. In addition Peak Discharge Relation with A, L/A, LS/A have been derived and statistical tests have been performed. With the availability of computer software, it is possible to attempt several different regression equations and adopt most appropriate equation on the basis of rigorous statistical tests. In this study, detailed procedure for derivation of synthetic unit hydrograph has been presented based on case studies for two basins in India and Indonesia. Computer based multiple nonlinear regression technique termed as SPSS method has been applied to (i) Upper Narmada Tapi subzone in India and Bengawan Solo basin in Indonesia. It is seen that SUH derived by SPSS method considering six subcatchments is more close to representative UH for Sakker river compared to 18 subcatchments. The SPSS method uses multiple nonlinear regression between ) Abstract SUH parameters and physiographic parameters and the best fit is based on rigorous reliability tests instead of using single correlation coefficient criteria. It is therefore no more necersary to sacrifice reliability of the SUH model for the sake of simplicity on convenience. With availability of computer softwares, SUH derivations for various subzones in India can be made more reliable. Land use and land cover are important factors influencing storm rainfall-runoff process. Geomorphological approach in development of SUH does not take this important aspect into consideration. Further due to accelerated human interference, land use and land cover are getting modified to a significant level particularly in small catchments. Development of synthetic unit hydrographs model in such catchments would need frequent revision. The SUH which is to be used • for estimation of design flood should be based on predicted land use and land cover condition in future.