Historical Rainfall-runoff Modeling of River Ogunpa, Ibadan, Nigeria

Flooding in major cities like the city of Ibadan, the largest urban centre south of the Sahara, Africa, is a common phenomena. Flooding occurred in several areas of the city each time when Ogunpa River overflowed its banks. Flood damage mitigation measures were necessitated by increased runoff due to rapid urbanization of the catchment area coupled with inadequate runoff data along the river course. Ogunpa River gained its national and international notoriety when many lives and properties worth billions of Naira were lost in the floods of 1960, 1963, 1978, 1980 and 2011. This study was conducted at Queen Elizabeth gauging station to develop an historical rainfall-runoff model for River Ogunpa. The model developed was a linear regression approach considering the effects of previous and current rainfall on the flow of the effluent streams. Average daily net rainfall data and average daily rainfall were regressed against average daily runoff data. Using linear regression method Net rainfall values (R') and rainfall values (R) were regressed against the corresponding discharge (Q) arrived at correlation coefficients of 0.66975 and 0.71191989 respectively. Utilizing 101 years of rainfall records for Ibadan City, runoff data for Ogunpa River were derived. This data could serve as a veritable hydrologic input in the design of embankment flood mitigation structures for River Ogunpa. It is recommended that to find a lasting solution to the menacing frequent flooding more runoff gauging stations be provided along the river course of Ogunpa River within Ibadan metropolis.