FLOOD CHARACTERISTICS AND POTENTIAL RESERVOIR CAPACITY OF RIVER OSUN AT APOJE GAUGING STATION

Abstract

Hydrological data namely: the total annual discharges and the gauge heights. spanning 18 years (1982-1999) were obtained from Ogun-Osun River Basin and Rural Development Authority, Abeokuta. The hydrograph and rating curve was drawn for the station. The flood frequency analysis was done using the "Annual maximum series" (AMS). Three statistical methods namely; the Normal, Log Normal and Log Pearson Type (111) distributions were fitted to the annual maximum discharges and flood frequencies were determined using seven plotting positions. The method of moment (MOM) was used in determining the mean, standard deviation and the coefficient of variation of the annual maximum discharges. On normal probability paper, the Observed and the predicted maximum annual discharges using the three distributions were plotted against their probabilities of non-exceedence. This procedure was repeated for the Weibull's, Blom's, Cunnane's, California, Gringorton's and the Chegodayev's plotting positions. At 95% confidence interval, the Upper and the lower frequencies of the maximum annual discharges were. The estimated upper and lower flood limits were determined The time series of the annual hydrograph showed that the maximum annual discharges vary from169 m3/s to 400m3/s within duration of 18 years. The predicted annual maximum discharges under the Log Pearson Type (111) distribution using the Weibull's plotting position varies from 433, 440 and 444 m3/sw at return periods of 25, 50 and 100 years with coefficient of correlation of 0.99. A skew coefficient of -0.5 was estimated for the maximum annual discharges of River Osun at Apoje sub-basin. For each year, the monthly cumulative volumes were plotted against their correspondent months. The reservoir and the spillway capacities in the months of each year were estimated by calculating the maximum departure of the mass curve from the year were estimated by calculating the maximum departure of the mass curve from the assumed uniform rates of withdrawal per month before and after spillage. The maximum reservoir capacity in each year was determined by adding the reservoir and the spillway capacities together. The annual cumulative volumes vary from 68,000 million m3 to 305,550 million m3 .The monthly demand varies from 5,030million m3 to 25,400 million m3 of water. The mean and coefficient of variation of the cumulative annual volume at Apoje sub-basin were 125.08 x 109m3 arid 0.42 respectively. The estimated maximum reservoir capacity for Apoje sub-basin was 22,000 million rrr'.