Rainfall Trends and Flooding in the Sondu Miriu River Basin

Abstract

This study was focused on the rainfall trends and flooding in the lower Sondu-Miriu catchment which is one of the sub basins in the Lake Victoria basin in Kenya. The broad problem that was investigated is the relationship between rainfall in the upper catchment and flood frequency in the lower basin. The specific objectives of the study were to determine rainfall trends, frequency and the relationship between the rainfall trends and flood occurrence, within the Sondu-Miriu basin respectively. The study was based on the hypothesis that there was no relationship between the trends in rainfall in the upper catchment of the river and flood frequency in the lower basin of the Sondu-Miriu River. Data required to solve the stated problems and meet the objectives of the study included monthly rainfall data from 3 rainfall stations in the basin as available in meteorological department databases and, the monthly river discharge data at Nyakwere bridge station (1JG01) as available in the Water Resource Management Authority databases. For both rainfall and river discharge data, the period of record used was 1960-1990 that provided the least number of missing records and therefore the most consistent. To determine trends in rainfall and river discharge, time series analysis technique was used where trend computation was based on the long term mean variance. For flood frequency determination, the study used unit hydrograph technique. Relationship between rainfall and flood was measured using the simple linear regression statistical technique. Significance testing was in all cases at ɑ 0.05. The regression analysis had an r value of 0.509 which indicated an average relationship between the rainfall trends and the flooding in the lower plains. The results of data analysis were that rainfall in the Sondu-Miriu basin showed decline in the amount 1960-1990 period while flood frequency did not show any definite return periods hence posing danger to the low land regions of the basin. Two of the three rainfall stations showed a decline in the mean annual rainfall with Tenwek being the only station showing annual increase in its rainfall. The flood frequency within this catchment is random with most floods having a range of 2-10 year return period. The findings of this study can be used for flood forecasting, to establish appropriate relief services and the physical planning for the development in the lower Sondu-Miriu River.