Development of Inundation Maps as Barangay-Level Risk Adaptation for Flooding in the Cagayan De Oro River

Abstract

Flooding is commonly blamed on an insufficient drainage system or its clogging due to debris. But in 2011, Cagayan de Oro River experienced intense flooding during Typhoon Sendong. This was due to high amount of precipitation unloaded from its vast watershed. Hard interventions such as the creation of levees may only displace water. “Soft” interventions, such as the modelling done in this paper, can be a better adaptation to these new-normal events. The study aims to establish a flood risk advisory at the barangay-level to assist in the planning of communities frequently affected by river flooding.

To this end, a watershed model was developed and calibrated using HEC-HMS. The rainfall-run-off process was recreated, using soil, landcover and terrain data for its initial parameters. After calibration, the 5-year return period rainfall scenario from PAGASA, the country’s meteorological agency, was entered into the model. The resulting outflow was used in the HECRAS floodplain model.

Using LiDAR data produced by the University of the Philippines-DREAM Project, a 1-meter resolution digital elevation model (DEM) was used in the inundation analysis. This accurately determined areas most susceptible to flooding due to its topography.

Maximum depth and area flooded per barangay were determined. A total of 17 barangays were found to be at risk from a 5-year return period rain event. Flooding covered 146 hectares, more than 50% was affected by high floods. The results can assist local government units in prioritizing high risk areas for future disasters.