FLOOD HYDROLOGY OF SOME •CATCHMENTS IN WESTERN HIMALAYAN REGION

Abstract

The main objective of this study was to study the hydrological characteristics of some catchments in western Himalayan region and to investigate the criteria and procedures for selection and application of design flood for water resources structures with possible improvements. Towards this objective, practices related to both the design flood criteria and estimation procedures, currently being followed in India, in general, and in Western Himalayan region in particular, have been critically reviewed. A comparison has also been made with the practices being followed in some of the developed countries. It is practically impossible to gauge every river at every location where a dam, barrage, bridge or similar hydraulic structure may be sited. Hence, a regional approach is inevitable for flood estimation, for a majority of such structures. In the past, there have been various attempts at regionalizing flood hydrological parameters. Due to the mountainous nature of the catchments in the Western Himalayan region, procedures developed for plain areas cannot be directly applied for flood estimation in this area. Snowmelt runoff is found to have very good correlation in log space with area under snow-cover. Four major approaches, viz., Rational formula, empirical formula, regional flood frequency approach and -regionalized synthetic UG approach have been critically reviewed. The Rational formula represents the response behaviour of only very small catchments. The other three approaches directly or indirectly use some form of regression relations, which represent the average behaviour. Wide confidence bands of the regression relations restrict the use of such methods. Disregard to this aspect leads to unreliable estimation. Synthetic UG approach proposed in CWC report (CWC, 1994) needs modification before applying to mountainous upper catchments. Flood estimation for nine project sites in the study area (Upper Yamuna, Upper Ganga and Sarju basins) using various methods have been carried out. iii The UG approach assumes catchment linearity. In this dissertation, the linearity of some catchments in the region has been studied through study of volume-peak discharge relation in log-space. The applicability of UG method for catchments in the region has been examined in this study. Appropriate volume-peak discharge relation for the study area have been developed and used in flood estimation.