In this study, an attempt has been made to develop a regional flood formula using L-moments for the north Brahmaputra river system. The annual maximum peak flood data of twelve gauging sites of the north bank tributaries of the river have been considered. The screening of the data has been carried out for assessing the suitability of the data for regional flood frequency analysis by computing the Discordancy Measure (Di) in terms of L-moments. Also, homogeneity of the region has been tested using the L-moment based heterogeneity measure, H.

The L-moments of a random variable were first introduced by Hosking (1986). They are analogous to conventional moments, but are estimated as linear combination of order statistics. In a wide range of hydrologic applications, L-moments provide simple and efficient estimates of characteristics of hydrologic data and of distribution parameters.

The expected inter-site variation of L-moment ratios for a homogenous region has been established by 500 simulations carried out using the four-parameter Kappa distribution for computing the heterogeneity measure H. Based on this test, it has been observed that the data of 10 out of 12 sites constitute a homogenous region. Hence, the data of these 10 sites have been used in this study. The catchment areas of these sites vary from 148 to 30100 sq kms and their mean annual peak floods range from 99.6 to 8916.1 cumecs.

Comparative regional flood frequency analysis studies have been carried out using the L-moments based frequency distributions viz Extreme Value (EV1), General Extreme Value (GEV), Logistic (LOS), Generalised Logistic (GLO), Normal (NOR), Generalised Normal (GNO), Exponential (EXP), Generalised Pareto (GPA) and five-parameter Wakeby (WAK).

Based on the L-moment ratio diagram and Z^Dist statistics criteria, GEV distribution has been identified as the robust distribution for the study area. The regional flood frequency relationship has been developed using the L-Moment based GEV distribution for estimation of floods of various return periods for the gauged catchments of the study area.

Also 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 relationship with the regional relationship between mean annual maximum flood and catchment area.

Download the report here: