Flooding is a critical issue in coastal areas, causing significant damages to areas where economic development grows. This paper presents an integrated assessment of the impacts of tidal influence and flood control structures on inundation patterns using HEC-RAS model in Abra River Basin, a major river in the Philippines. The influence of tidal effects on flood extent and depth was explored, emphasizing the need for specific measures in coastal areas. Results have shown that with the effect of tides during Typhoon Ineng in 2015, inundated area tend to increase with 32.51% and water level of 56.9%. Using the calibrated and validated 2D HEC-RAS model, various flood scenarios were simulated, assessing the effectiveness of existing and proposed flood structures. Geographic information system (GIS) platform was used to generate inundation maps for each flooding scenario. Using the HECRAS 1D flood model, results showed that after integrating existing flood control structures, 371.28 ha of the floodplain under high level flood warning (>1.5 m depth) are effectively protected, yet there is increase in flood exposure of 82.95 ha and 43.03 ha classified under low (<0.50 m depth) and medium level (0.51-1.5 m depth) flood warning, respectively. Focusing in community level, the introduction of combined coastal protection structure (cantilever retaining wall and gabion) in Villamar, Caoayan, a high-level flood risk community within the basin exposed to inland and tidal inundation marked reduction of 32.27% (0.41 ha) and 1.49% (0.90 ha) exposed to low- and medium- level flood warnings, respectively. The findings highlight the importance of integrating the tidal influence on proposed flood control structures. It also highlights the impact of structures on areas of varying scope under protection. The results provide valuable insights for optimizing flood management practices in areas where tidal impacts are significant.

1. Flood hazard maps with and and without tidal effect
2. Flood hazard maps with and without existing structures
3. Flood extent with and without the proposed coastal flood protection structure in Villamar, Caoayan, Ilocos Sur