In this study, regional flood frequency relationships have been developed using L-moments for South Bihar, in the alluvial plains of Indo-Gangetic basin and Kaimur-Chhotanagpur Santhal Pargana plateau. The annual maximum peak flood data of twenty-two gauging sites whose catchment areas vary from 11.7 to 3171 sqkm have been used. The mean annual peak floods of these sites vary from 29.15 cumec to 1293.2 cumec. Comparative regional flood frequency analysis studies have been carried out using some of the commonly used frequency distributions viz Extreme Value (EV1), General Extreme Value (GEV), Normal, Log Pareto (GPA) and Wakeby based on L-moments approach.

For planning and design of various types of water resources projects, estimation of flood magnitudes and their frequencies has been engaging attention of engineers the world over since time immemorial. L-moments of a random variable were first introduced by Hosking (1986). They are analogous to conventional moments, but are estimated as linear combinations of order statistics. In a wide of range of hydrologic applications, L-moments provide simple and efficient estimates of characteristics of hydrologic data and of distribution parameters.

Based on the L-moment ratio diagram and Z^Dist statistics criteria, Pearson Type-III (PT-III) distribution has been identified as the robust distribution for the study area. For the estimation of floods of desired return periods for the gauged catchments, the regional flood formula has been developed by using the Pearson Type-III distribution based regional flood frequency curves derived by utilizing the L-moments approach.

For estimation of floods of desired return periods for the ungauged catchments, the regional flood formula has been developed by coupling the regional flood frequency curves of the L-moments based Pearson Type-III distribution and regional relationship between annual maximum peak flood catchment area.

Thus, for estimation of floods of various return periods for the gauged catchments, the derived regional flood formula or its graphical representation may be used for estimation of floods of derived return periods for the ungauged catchments of study area.

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