RN-48 : Regional approaches for flood estimation in mountainous area

Abstract

Floods in mountainous areas are the catchment's response due to precipitation and are influenced not only by catchment and stream characteristics but also storm characteristics and vegetal and land use changes. More often, hydraulic structures in mountainous and hilly areas are to be constructed at ungauged locations. For the estimation of design flood at the ungauged locations in mountainous areas, several regional techniques are developed which account for geomorphological and meteorological factors of the watershed to compute the peak flood in mountainous areas. A review of the regional flood formulae is carried out in the report.

Various regional approaches used in the mountainous areas of India may be classified as: Regional empirical formulae Rational formula, Envelop curve methods, Regional unit hydrograph, Regional flood frequency analysis and Graphical correction technique.

Each of the regional techniques has got its own advantages and limitations. Empirical formulae, while easy to use, do not generally give any idea about the flood frequency.

The rational formula is difficult to apply unless the return periods for rainfall runoff are assumed to be equal. In the case of envelop curve method, it may be used as an approximation that the order of frequency of flood derived from the curve would be somewhat greater than the longest period of record. But, this approach also has same limitation as in case of empirical formulae that it takes into consideration only one basin characteristic, i.e. catchment area only. The shape and slope of the catchment, rainfall geology etc. are not accounted for. Regional unit hydrograph and regional flood frequency approaches are mostly used for design flood peak computation in mountainous areas.

Frequency analysis of available data are generally done to find out the expected maximum flood for a particular return period. However, uncertainty prevails in the estimation of peak floods because of availability of limited data for mountainous areas, non-homogeneity in data, difference in hydraulic characteristics, effect of man-made and natural changes etc..

Future studies need to be aimed at developing such regional flood formulae which would be able to take into account the catchment characteristics, stream characteristics, vegetation cover, and land use, etc. which largely influence formation of floods in mountainous areas.