Kindaruma and Kiambere Dams on Flood Frequency and Risk on River Tana, Kenya
Omari, Herbert L
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Floods are a pertinent problem whose efforts to solve should consider tangible technical consideration based on fairly accurate flood observations or forecasts. The current study was done in the upper Tana basin which mainly covers and with its headwaters in the south or on slopes of Mount Kenya and the eastern parts of the Nyandarua Ranges. Despite its high development potential for irrigated agriculture and hydro power generation and tourism, the Tana system - the largest river basin in Kenya, experiences extensiv0 flooding. Substantial control efforts by means of damming have however been effected. This study was set out to investigate the effect, if any, of the Masinga, Kamburu, Gitaru, Kindaruma, and Kjambere dams on the flood frequency and risk on the Tana river and the implications of such effect on the future development of the floodplain below the dams. The degree of success of such efforts could be of great use in the stemming the flood problem elsewhere in the country. Both river flow and rainfall data recorded in the basin were analysed in this study. Annual flood max imo and mean annual flows at four regular gauging stations (RGS) upstream of the dams and two others downstream - including the control one - were used. The control station, RGS 4G1 at Garissa, had a record period of 58 years (1933-90). The data was tested for homogeneity and I consistency using graphs and the t-statistic and then applied to three approaches to Flood -Frequency and Ri.s k Analysis (FF & RA) namely: Gumbel type I, 2-parameter Log-Normal and Log-Pearson xi type III, with a view of examining each ones adaptability to the pre- and post- each of the damming floodflows on the Tana river. Using the Mean Absolute Deviation Test (MADT), the 2 parameter Log-Normal distribution was found to be the mare appropriate for the purpose of flood data prediction, analysis and inferences in this study. The Chi-Square statistic was utjlized to determine the significance of the effect of each dam on the flood experiences in the lower Tana basin. Analysis of the generated data revealed that; , except for the Kamburu/Gitaru, the dams have actually snpp ressed the various pre-dam T-year floods with Kindaruma having the greatest effect (~ 47%) followed by Masinga (~ 32 %). Kamburu/Gitaru was found to have aggravated the flood problem cd ready buffered by Kindaruma - by about 46%, bringing the net effect of Masinga to a mere 1% with regard to the Kindaruma floods. By the completion of Masinga the pre-dams floodflows of th(~ 'I'n na river had been abated by approximately 48%. Though signif icant, Kindaruma' s suppression of the floods was purely incid0nt~l, just as was the Kamburu/Gitaru aggravation not totally unex pert ed because, except Masinga, all the other dams considered were designed primarily for hydro-electric power generation and th0rerore with secondary consideration of flood control aspects. From the study it was concluded that, should the predicted flood abatement trend continue, then it is likely to appreciably influ- I ence the la~duse development patterns and policy downstream of the reservoirs: one, development will be encouraged to very close ranges of the river reach thereby exposing them to a lot of risk; xii the building of the dams, thus, has not completely eliminated the dangers posed by the pre-dams floods. Two, the cost of flood mitigation efforts will be much lower than in the pre-dams period. Three, perennially swampy areas in the lower parts of the floodplain - towards the delta, will easily be reclaimed and communications networks improved but there will be an inevitable ecological change to contend with. Recommendations on the basis of the study, include: building of minor floodwalls along the river stretches franking urban areas like Garissa, Ho l.a , (;arsen etc. identification and creation of "washland" areas upstream of urban and agricultural areas like the northern outskirts of Garissa town to hold local contribution of storm waters. river channel improvement in the vic in i t.y of developments already done flood-zoning of the entire lower Tana floodplain to guide its future development policy. education of the floodplain popu:tace on the flood dangers they are still ~xposed to. employment of dam-based flood contra I ]n the other areas similarly prone to flooding. persuing further research in the current topic, using partial flood series and examining other probability distributions not considered.