TR(BR)-4/99-2000 : Development of regional flood formula using L-moments for north Brahmaputra river system

Abstract

Estimation of magnitudes of likely occurrence of floods is of great importance for finding solution of a variety of water resources problems such as design of different types of hydraulic structures, urban drainage systems, flood plain zoning and economic evaluation of flood protection works etc. As per Indian design criteria, frequency based floods find their applications n estimation of design floods for almost all the types of hydraulic structures viz. small size darns, barrages, weirs, road and railway bridges, cross drainage structures, flood control structures etc., excluding large and intermediate size dams. For design of large and intermediate size dams probable maximum flood and standard project flood are adopted, respectively. Whenever, rainfall or river flow records are not available at or near the site of interest, it is difficult for hydrologists or engineers to derive reliable flood frequency estimates directly. In such a situation, the regional flood frequency relationships or the flood formulae developed for the region are one of the alternative methods which provide estimates of design floods, especially for small to moderate size catchments.

In this study, annual maximum peak flood data of 12 gauging sites of North bank tributaries of river Brahmaputra have been considered. Screening of the data has been carried out for assessing the suitability of the data for using for regional flood frequency analysis by computing the Discordancy measure (D,) in terms of the L-moments. Also, homogeneity of the region has been tested using the L-moment based heterogeneity measure, H. To establish what would be the expected inter-site variation of L-moment ratios for a homogeneous region, 500 simulations were 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 homogeneous region. Hence, the data of these 10 sites have been used in this study. Catchment areas of these sites vary from 148 to 30100 square kilometres and their mean annual peak floods range from 99.6 m3/s to 8916.1 m3/s. Comparative regional flood frequency analysis studies have been carried out using the various L-moments based frequency distributions viz. Extreme value (EV1), General extreme value (GEV), Logistic (LOS), Generalized logistic (GLO), Normal (NOR), Generalized normal (GNO), Exponential (EXP), Generalized Pareto (GPA) and five parameter Wake by (WAK). 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 range of hydrologic applications, L-moments provide simple and efficient estimators of characteristics of hydrologic data and of a distribution's parameters (Stedinger et al., 1992). Hosking (1997) presented state of the art application of L-moments in frequency analysis.

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. For estimations of floods of various return periods for the gauged catchments of the study area, the regional flood frequency relationship has been developed using the L-moment based GEV distribution. 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 peak flood and catchment area.