TN-95 : Geomorphological instantaneous unit hydrograph studies

Abstract

The time distribution of the runoff and the estimation of the peak flood constitute very important component of the planning and design as well as operation and maintenance of a water resources project. unit hydrograph method is a simple and versatile technique which is widely used for prediction and forecasting of the incoming floods. The unit hydrograph for a catchment at a particular site is generally derived using the observed discharge and the rainfall data However, many of the small catchments are ungauged or even if they are gauged, adequate and reliable record of the required data are rarely available. In such cases, it becomes necessary to use a technique which does not require observed hydrological data for the derivation of the unit hydrograph, at least for the initial stages of planning process.

In the absence of rainfall-runoff data, geomorphological characteristics of the basins can be used to synthesise unit hydrograph. The geomorphological characteristics can be easily derived from maps/toposheets having details of stream network as well as contours. Such maps/toposheets are readily available and can be considered to be very reliable. Once a unit hydrograph is estimated, the climatological data can be used for the estimation of the design flood.

Effort have been made since very beginning for the derivation of unit hydrograph with the help of the physical characteristics of the basins. The first systematic analysis was reported by Snyder in 1938 who suggested a synthetic unit hydrograph which could be derived with the help of length of main channel and the distance of the centroid of the basin from the outlet. The synthetic hydrograph proved to be very popular and attracted a number researchers for further improvements.

Rodriguez-Iturbe and Valdes(1979) made pioneering attempt in the direction of coupling of quantitative geomorphological analysis with the most important hydrologic variable, namely the stream flow response to surface runoff of the geomorphological unity, the basin. The structure of the hydrologic response is found to be intimately linked to the geomorphologic parameters of a basin when the hydrologic response is represented by the unit hydrograph. The geomorphologic parameters have also been found to have very good relationship with the parameters representing instantaneous unit hydrograph. This theory was subsequently generalised by Gupta et al.(1980).

Rosso (1984) related the Nash model parameters to the Horton order ratios of a catchment. Panigrahi (1991) also developed an approach for deriving complete shape of the geomorphological instantaneous unit hydrograph(GIUH) by linking it to the Nash model parameters. Many other authors have also worked for the use of the geomorphological parameters for improvements in the hydrological models.

In this study, an attempt has been made to review the various works carried out by different researchers in respect of the use of geomorphological parameters for derivation of the unit hydrograph or the instantaneous unit hydrograph.