Flood Mapping and Assessment During Typhoon Ulysses (Vamco) in Cagayan, Philippines Using Synthetic Aperture Radar Images

Abstract

Typhoon Ulysses (Vamco) brought about more than 300 flooding incidents in the Philippines in November 2020. The Cagayan Valley Region was reported with the highest worth of damage to infrastructure and agriculture. In this study, synthetic aperture radar images were used to analyze the effect of Typhoon Ulysses in the region. The VV and VH bands of Sentinel 1 were used to detect the flooded areas before (November 1) and after (November 13) the typhoon. The VH band was found more effective at detecting flooded and agricultural areas (72,722.30 ha and 37,736.70 ha, respectively) than the VV band (69,501.12 ha and 35,606.59 ha, respectively). The VV polarization was found to be more suited for monitoring vegetation, whereas the VH polarization is more suited for estimating flood extent, as based on the backscatter intensity of the two polarizations before and during flooding on representative places. The significantly high amount of rainfall (200–300 mm) received, low elevation and slope of the affected areas, and the volume of water released from Magat Dam were noted as the contributing factors to the massive flooding event. More than 35,000 ha of rice areas were flooded in Alcala town. Results showed that the standing crops I n Alcala increased after the typhoon (from 660.55 to 923.16 ha) when the flood subsided. This was determined using MODIS, Sentinel 1 and 2, and a 0.3 NDVI threshold. It was also noted that before the typhoon, most of the rice crops were at a stage of maturity and were ready for harvest a few days later.