Impact of Seasonal Flooding on Flexible Pavement Performance in Southwestern Nigeria

Seasonal flooding significantly undermines the performance and longevity of flexible pavements in Southwestern Nigeria, where intense rainfall, poor drainage, and weak subgrade materials exacerbate pavement deterioration. This study investigates the effects of flood events on pavement structures through field surveys, deflection testing, and geotechnical analyses in Lagos, Oyo, and Ogun States. The research identifies key distress mechanisms such as rutting, cracking, and subgrade softening caused by prolonged water exposure, confirming that flood duration and intensity directly correlate with decreased structural integrity. Laboratory tests reveal that post flood California Bearing Ratio (CBR) values declined by up to 45%, while Falling Weight Deflectometer (FWD) deflections increased by 30%, signaling substantial strength loss. A multivariate regression model was developed to predict Pavement Condition Index (PCI) based on flood variables, achieving a strong correlation (R² = 0.87). The study also evaluates economic impacts, estimating that flood damaged roads incur 40–50% higher rehabilitation costs. Mitigation strategies, including enhanced drainage design and use of resilient materials, are recommended. Findings support the need for policy reforms and adaptive pavement design to improve infrastructure resilience in flood prone areas of Nigeria.