Use of regional index flood for estimation of design floods in ungauged catchments: a case study of lake Victoria basin in Kenya.

Reliable estimation of flood magnitudes corresponding to required return periods; vital for structural design purposes, is grossly beset by lack of hydrological data in the study area of Lake Victoria Basin in Kenya. Use of regional information, derived from data at gauged sites and regionalized for use at any location within a homogenous region, will step in to curb the current prevailing problem. Therefore, the regional index flood method has been applied. Based on data from 14 gauged sites, a delineation of the basin into two homogenous regions was grieved using elevation variation (90m DEM), spatial annual rainfall pattern and Principal Component Analysis of seasonal rain fall patterns (from 94 rain fall station). At site annual maximum series were modelled using the LN (3P), LLG, GEV and LP3 distributions; each estimated using the method of probability weighted moments. Goodness of at tests were applied and GEV was identified as a most appropriate model for each site. Based on the GEV model, flood quintiles were estimated and regional frequency curves derived from the averaged at site growth curves. Using the least squares recession method; relationships developed between the index flood and catchment characteristics, indicated area, mean annual rainfall and altitude as the three main variables that greatly influence the index flood. Thereafter, estimates of design floods in an ungauged catchment within a homogenous region would be obtained from the deduced equation for index flood and quantiles from the regional curves. Though, relatively small errors are noted between observed and estimated design flood values. These estimates will be of great importance in flood risk estimation and to support water management and engineering decisions and actions.