The study develops regional flood frequency relationship and flood formulae based on recently developed efficient techniques of flood frequency analysis to estimate floods of desired return periods for small to moderate size gauged and ungauged catchments of the middle Ganga plains (sub-zone 1F) encompassing parts of Uttar Pradesh, Bihar and West Bengal. A regional flood formula has been developed by coupling the L-moment based regional flood frequency relationship with the regional relationship between mean annual peak flood and the catchment area.
It uses data of eleven gauging sites and assesses the suitability of the data for using regional flood frequency analysis by computing the discordancy measure (Di) in terms of L-moments. The homogeneity of the region has been tested using the L-moment based heterogeneity measure, H. The inter-site variation of L-moment ratios for a homogenous region has been computed through five hundred simulations carried out using the four parameter kappa distribution. Kappa distribution includes as special cases the GLO, GEV and GPA distributions and is capable of representing many of the frequency distributions.
The choice of the method primarily depends on the design criteria and can be applied in estimation of design floods for almost all the types of hydraulic structures viz., small dams, barrages, weirs, road and railway bridges, cross drainage structures, flood control structures etc., excluding large and intermediate size dams. However, for design of large and intermediate size dam, probable maximum flood and standard project flood are adopted, respectively.
The homogeneity tests indicate that data of eight of the eleven sites constitute a homogenous region and hence were used in the study. The record length of the data varies from 23 to 33 years. The catchment areas of these sites range from 32.9 to 447.8 sqkm and the mean peak floods range from 24.3 cum/second to 555.2 cum/ second. Comparative regional frequency analysis studies were carried out using the various L-moments based frequency distributions.
The study helps estimate magnitudes of likely occurrence of floods and is important for solving a variety of water resources problems, such as design of various hydraulic structures, urban drainage systems, flood plain zoning and economic evaluation of flood protection works.
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