Analysis of Rainfall and River Water Level Exceedance Probability at Lokoja Town, Nigeria: Implications for Flood Risks and Vulnerability

Abstract

This study analyzed the frequency of rainfall amount and water level of Benue River at Lokoja and its implications for flood risks and  vulnerability. Exceedance probability model was used as a tool to evaluate the frequency of flooding in the area. The data used were  obtained from daily rainfall and river water level measurements from 1971-2020 for rainfall and 1980-2021 for water level. The goodness- of-fit for the selected distributions was tested using the Chi-square and the Kolmogorov–Smirnov tests at three significant levels (α=5%,  10% and 20%). The result of the Kolmogorov-Smirnov goodness-of-fit test for exceedance probability model shows that Cumulative  Density Function (CDF) is the better model for the flood exceedance using both rainfall and peak water level at all the stations. The test  shows that closeness of linear relationship of CDF R² value of 0.98, at 20%, 10% and 5% significance level. Goodness-of-fit test indicates that the normal distribution provides a good fit to the rainfall and peak water level data in the basin. The probability of exceedance model  of rainfall (flood) events over Lokoja generally indicates higher rainfall events (1000mm and above) with higher exceedance  probability of between 60%- 80%. The exceedance probability analysis revealed that there is a high likelihood of occurrence of flooding  events, with a return period of 5-10 years. This implies that the area is highly vulnerable to flooding, and appropriate measures need to  be taken to mitigate the risks. The study recommends the need for regular monitoring of river water level, improvement of drainage  systems, and adoption of sustainable land use practices to reduce the impact of flooding in the area.