DESIGN FLOOD ESTIMATION FOR TEHRI DAM USING DETERMINISTIC AND STATISTICAL APPROACHES

Abstract

Design flood is an important hydrological aspect which needs careful assessment for planning and design of a storage project., For these hydrological studies long term rainfall and discharge data are required. When the project for Tehri dam was first conceived very scanty data was available in respect of rainfall and river discharge at Tehri dam site. In absence of adequate data different statistical/conceptual approaches were used to make assessment as regards the water availability and maximum design flood at Tehri. Subsequently additional river discharge data has become available. Design flood has therefore been updated, taking into account this additional data. In this thesis the assessment about value of maximum design flood made earlier has been rechecked by deterministic and statistical approaches. The available daily data of rainfall and discharge is not sufficient for estimating design flood. In absence of hourly rain fall and discharge data, frequency analysis is applied to AFS to get 1000 years return period flood which equivalent to design PMF. In deterministic approach design storm is selected. After adjustment of design storm and its critical sequencing, it is applied to design unit hydrograph of the basin to get the direct surface run-off hydrograph. As short duration discharge data of flood is not available, SUH has been derived. Two approaches for generation of SUH have been considered and probable maximum flood peak of 13700 m3/s and 14260 m3/s were obtained by adding base flow/snow melt to the DSRO which are comparable to the previously adopted design value of 15540 m3/s: The PMF computed on the basis of statistical approach design flood of 10000 Yr return flood works out to 6886 m3/s. This value is corresponding to Pearson type III distribution which fits well to the AFS. Selected design flood value is 14260 m3/s and hence PMF needs no revision. In this study, with the help of ArcGIS software physiographic analysis has been done for the catchment area and all the parameters required for generating the synthetic unit hydrograph were obtained. Design storm value of 20.5 cm has been derived from analysis of 42 years daily rainfall data.