Rheological Study on Frequency Analysis of Rainfall Data of Lower River Ogun, South-Western Nigeria

Abstract

Rainfall is the driving force for various engineering design such as hydraulic structures, road drainage system and bridges. The proper consideration of rainfall variability in time and space is the main challenge in hydrological design. The frequent occurrence of flood in Abeokuta has generated a lot of concern in Ogun State. These flood events have severe consequences on lives and properties. In this study, the rheological study on frequency analysis of rainfall data for Lower River Ogun was investigated. Ninety-eight (98) years rainfall data collected from 1920-2018 was obtained from Ogun-Oshun River Basin Authority Abeokuta, Nigeria. The rainfalls collected were analyzed statistically using the peak rainfalls. The peak rainfalls data were fitted to the three major statistical distributions namely normal, log-normal and log Pearson Type III model, while two plotting positions of Hazen and Weibull were used in determining the probabilities of exceedance with return period 25, 50 and 100 years respectively. The three models were evaluated statistically using Error analysis. The Weibull plot when matched with normal, log-normal and log Pearson Type III Distribution gave the highest coefficient of correlation of 0.959, 0.9377 and 0.9377 respectively. Hazen plot gave minimal error with Root Mean Square Error (RMSE) of 139.44, 151.85 and 151.85 for normal, log-normal and log-Pearson Type III distribution respectively. The Weibull plot matched with normal distribution gave the highest fit, most reliable and accurate predictions of the flood in the study area having the coefficient of determination R2 = 0.959 and root mean square error RMSE 141.37 mm respectively.