HYDROLOGICAL STUDY OF NYANDO BASIN

ABSTRACT
A hydrological study of Nyando basin which is located in Western Kenya was conducted. The
objective of the study was to evaluate and analyze the Nyando basin’s rainfall and stream flow.
Nyando basin was selected for study because of the frequent flooding cases that have occurred
in the region over the past years.
A systematic approach for attaining the objective was used. At first, data was visually
scrutinized. Missing data was filled through interpolation or station correlation. Areal rainfall
over Nyando basin was determined by Thiessen polygon method. Mean monthly rainfall,
rainfall trends over periods of 20-30 years and probability analyses of rainfall were carried out
as well. Flood analysis, storage analysis and frequency distributions of rainfall were determined.
The areal rainfall was obtained as 1608mm per annum and according to the moving averages
curves there was an increase in rainfall trend. The catchment experienced a bimodal seasonal
pattern of rainfall i.e. long rains in March-May and short rains in October-December or JulySeptember.
Rainfall frequency curves were plotted to reflect rainfall at different probabilities.
Flood analysis showed that the region experiences heavy floods. The stream flow analysis
revealed an increasing trend. The flood for a 20 year return period at station 1GD03 was 595
m3/s.

 

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