Flood inundation assessment of Buayan-Malungon River Basin, Philippines using rainfall-runoff-inundation (RRI) model

Flooding is a natural disaster that is always present in the Philippines and while it cannot be entirely prevented in specific areas, it can be controlled by managing river overflow to mitigate adverse events. This study assessed the flood inundation of the Buayan-Malungon River Basin (BMRB) using Rainfall-Runoff-Inundation (RRI) model. Two significant rainfall events were employed for calibration and validation: the first in October 2020 for calibration, and the second in May 2021 for validation. Each scenario encompasses one-week period, including one to two days before and after the peak rainfall. Thus, the first scenario spans October 12-18, 2020, and the second scenario spans May 13-19, 2021, with rainfall data on an hourly basis. Calibration involved a sensitivity analysis where parameters were adjusted individually, and model efficiency was evaluated using three methods namely Nash-Sutcliffe Efficiency (NSE), RMSE-observations standard deviation ratio (RSR) and coefficient of determination (R sq). The calibration process determined that setting the vertical saturated hydraulic conductivity (Kv) to 8.33E-07, a value suitable for sandy clay loam, produced NSE, RSR, and R cu metrics of 0.60, 0.63, and 0.63, respectively, which fall within acceptable ranges for the October 2020 scenario. This parameter was further validated with the May 2021 scenario, yielding NSE, RSR, and R sq values of 0.66, 0.59, and 0.68, respectively, all of which are also acceptable. The flood simulation which resulted to inundation depth greater than 0.5 meter was considered flood-prone area. The October 2020 simulation revealed that 20 barangays [villages] in the provinces of Sarangani, South Cotabato, and Davao Occidental [Philippines] were flood-prone. The May 2021 simulation identified an additional 1 barangay as flood-prone. During October 2020 rainfall event, the total inundated area was 416.24 hectares, with Barangay Tamban experiencing the highest inundation at 84.7 hectares. In May 2021 rainfall event, the inundated area increased to 2086.04 hectares, with Barangay Tamban again having the highest inundation at 309.76 hectares. To address these findings, various prevention and mitigation strategies are recommended. Preventive measures include implementing land zoning ordinances, watershed protection, public awareness and education, and improved agricultural practices. Mitigation measures include constructing levees and dikes, implementing color-coded safety markers, developing emergency preparedness and response plans, and enhancing flood forecasting in agricultural areas.