Derivation and Analysis of Dimensionless Hydrograph and S Curve for Cumulative Watershed Area

Abstract

Unit Hydrograph (UH) is the most popular and widely used method for analyzing and deriving flood hydrograph resulting from a known storm in a basin area. However, the non-availability of flood runoff details due to poor network of stream gauges in flood prone Indian river basins is a major concern. Traditional techniques for design flood estimation use historical rainfall-runoff data for unit hydrograph derivation. For ungauged catchments, unit hydrograph are derived either by regional unit hydrograph approach or alternatively Geomorphological Instantaneous Unit Hydrograph approach. Central Water Commission (CWC) derived the regional unit hydrograph relationships for different sub-zones of India relating to the various unit hydrograph parameters with some prominent physiographic characteristics. The study area is located between 12°30′ N and 13°05′ N Latitude and 77°15′ E and 77°40′ E Longitude which is derived from Carto DEM 30 m resolution. The study area covers an area of 349.06 km2, having maximum length of 21.36km and the study area elevation varies from 700 m to 900 m. The study area is extracted using methodology Fill sinks, Flow direction, Flow accumulation, Watershed pourpoints, Snap pourpoints and Delineate watershed. Watersheds are also delineated from SOI toposheet 57H/5, 57H/9, 57G/12 using Arc GIS software. The extracted study area from Carto DEM 30 m is used for extraction of hourly runoff by pixel method for cumulative watershed area, time area diagram is prepared and derivation of unit hydrograph is carried out using CWC method for both toposheet and DEM data set. Unit hydrograph is derived using the regional relationships as proposed by Central Water Commission (CWC), for Kaveri basin subzone-3(i). This information is useful to derive flood hydrograph along any reach of stream. It helps in getting more appropriate values of flood discharge and time to peak with minimum calculation by interpolation techniques.Unit hydrographs and S curve of watersheds are tried to analyze with the help of dimensionless Unit Hydrograph and dimensionless S curve to understand the parameters of unit hydrograph for cumulative watershed areas.