Combined Fluvial and Pluvial Flooding in an Urban Catchment: A Hydrodynamic Modeling Approach of Davao River, Philippines

Flooding is a significant natural hazard affecting millions of people worldwide, with both fluvial (accumulated rainfall as river runoff) and pluvial (accumulated rainfall directly ponding in the floodplain) sources contributing to the inundation risk of an area. Recently, Davao City, a city with a vast land area spanning Mt Apo in the Philippines, experienced severe flooding on January 16~19, 2024, and January 31, 2024. It not only caused aggravated flooding in low-lying areas but also triggered other related hazards in the city, affecting 214,100 families in the entire area. Traditional flood modeling approaches often focus on fluvial and sometimes on pluvial flooding being modeled separately, neglecting the interactions between these two or more flood drivers, often leading to less accurate flood representations. Hence, this study suggests an integrated methodology for fluvial and pluvial compound flooding, aiming to enhance a better comprehension of the dynamics of flood risk within mixed forested and urban settings. This includes the analysis of the flood morphology employed by using (QGIS) in isolated cases of fluvial and pluvial and its combined scenarios in varying return periods (5, 10, 25, 50, and 100) to assess its impact on the flood inundation of an area. The research utilizes a hydrologic-hydraulic modeling framework to simulate fluvial, pluvial, and combined flooding processes through (HEC-RAS) to estimate the depth and extent of flood waters over an area and verify its results with historical flood events. The results concluded a very pronounced flood-impact zones in combined scenarios due to the effect on water volume and flood depth Hence, the proposed methodology highlights the urgency of understanding flood behavior as experienced by recent flood events to be better prepared for the potential of future multi-hazard risks exacerbated by the changing environment we have nowadays.