Development of flood risk assessment tool and early warning system for the municipality of Bay, Laguna, Philippines

An integrated approach was used in analyzing food risk in low-lying areas of Bay, Laguna [Philippines]. The study was specifically undertaken in Bay/Cambantoc, Calo and Molawin watersheds where rainfall-runoff monitoring was conducted to develop a unit hydrograph and calibrate the United State Department of Agriculture-Soil Conservation Service (USDA-SCS) runoff model as an input in estimating flood hazard. The derived 30-min 1-cm UH curve showed that Bay/Cambontoc watershed has the highest peak discharge of 41 cu m/s compared to 14 cu m/s and 10 cu m/s for Calo and Molawin watersheds, respectively. However, shorter time to peak was noticed in Bay/Cambantoc watershed which only took 0.5 to 1 hour while the other two watersheds had 1.5 hours. The computed initial abstraction ratios of 0.0307 for Molawin watershed, 0.0053 for Calo watershed and 0.003 for Bay/Cambantoc watershed were used to calibrate SCS runoff equation. Obtained h values were far below the 0.20 coefficient used by USDA-SCS but very close to the results of recent studies which ranged from 0.01 to 0.053. Higher percentage of rainfall converted to runoff was noted in Molawin watershed with 44% compared to only 8% in Bay/Cambantoc and 24% in Calo watersheds. Result showed that Tagumpay emerged as the most risky barangay [village] to flood with 0.26 index among the 10 barangays assessed. It is followed by barangay Maitim with 0.20 index. Barangays San Isidro, Dila, San Agustin, San Nicolas, San Antonio, Puypuy and Calo have moderate levels of risk with index ranging from 0.10 to 0.17. On the other hand, the least risky barangay is Santo Domingo with only 0.08. A flood hydrograph calculator was developed for real-time monitoring of water level of Bay river which is part of the proposed science-based early warning system for the Municipality of Bay.