Assessment and Analysis of t he Floodplain of Cagayan De Oro River Basin

Abstract

A technical approach using 1-dimensional steady flow model, GIS and remote sensing were used to analyze the Cagayan de Oro River Basin with a catchment area of 1400 sq. km., located in the northern central part of the island of Mindanao. Precipitation frequency analysis was done for Cagayan de Oro Rainfall Station, the only station with historical records starting from the year 1950. Probable point rainfall for the said station was used in the frequency analysis for 2, 5, 10, 25, 50, and 100 years return period storms. Results from precipitation frequency analysis were then used to construct the Rainfall-Intensity-Duration-Frequency (RIDF) Curves. Design precipitation hyetograph with their respective return periods were established using the alternating block method. A hydrologic model was built in HEC-HMS using ArcGIS and HEC GeoHMS. Physical attributes of the watershed were extracted using the above-mentioned tools and then exported to the HEC-HMS model. Using the design storms calculated during the precipitation analysis, the HMS model yields peak flows for different return periods. The hydraulic model was established in the river section starting from the river delta up to 9 kilometers upstream. In ArcGIS, river geometry was extracted from Triangulated Irregular Network (TIN) derived from Digital Elevation Model and ground survey. The river geometry was then exported to HEC-RAS for hydraulic analysis. Water levels computed in the hydraulic analysis were then exported back to ArcGIS for inundation mapping. The flood inundation results show that the city of Cagayan de Oro is extremely exposed to flood hazards. It is observed that the channel capacity is only capable of handling 2-year and 5-year storms without flooding portions of the city.