Flood Frequency Analysis at the Downstream of Ogbese Catchment: A Life-Threatening Environmental Hazard

Flood is a global natural disaster that has great effect on the environment and it is a threat to lives and properties. This study is on a comprehensive flood frequency analysis of the Ogbese River catchment, located in Ondo States, south-western Nigeria. The specific objectives are to provide quantitative estimates of flood magnitudes for various return periods and the associated environmental and health hazards/challenges in the river catchment. The study analyzed annual discharge data from 1989-2019 obtained from Benin-Owena River Basin and the Nigeria Meteorological Agency (NiMet) office, Akure. It adopted the Gumbel’s distribution technique - a probability distribution model for river discharge – and carried out a linear regression analysis to establish a relationship between discharge and reduced variate. Results revealed annual maximum discharges ranging from 284.77 m³/s (1990) to 713.19 m³/s (1995), with a mean of 450.77 m³/s and standard deviation of 88.87 m³/s. High flood magnitudes dominated the early–mid 1990s, with record events in 1991 (665.17 m³/s), 1992 (582.14 m³/s), and 1995 (713.19 m³/s). Conversely, 2000–2019 recorded lower average magnitudes, though significant floods occurred in 2003 (517.88 m³/s), 2010 (577.10 m³/s), 2014 (554.42 m³/s), and 2019 (524.18 m³/s). This temporal pattern suggests possible non-stationarity in the flood series, potentially influenced by climate variability and land use changes. The sharp contrast between the lowest and highest values highlights strong inter-annual variability typical of tropical monsoon systems. The Gumbel distribution provided an excellent fit, with regression analysis yielding R² = 1.0, confirming model reliability. The flood destroyed houses, farmlands, displaced residents, caused food insecurity and health challenges like waterborne diseases (typhoid, cholera, and diarrhea), vector-borne diseases (malaria), trauma and hypothermia. The study underscores significant flood variability and provides critical baseline data for hydraulic infrastructure design and flood risk planning. It emphasized the need for government and community adoption of modern hydrological techniques, proactive land use management, and sustainable flood prevention strategies to protect lives and property in the study area.