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CHARACTERIZING FLOOD HAZARDS IN AN ALLUVIAL FAN DURING EXTREME RAINFALL EVENTS USING LIDAR AND NUMERICAL MODELING

Conference Paper

CHARACTERIZING FLOOD HAZARDS IN AN ALLUVIAL FAN DURING EXTREME RAINFALL EVENTS USING LIDAR AND NUMERICAL MODELING

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In this paper, we characterize the flood hazards in an alluvial fan in Asiga Watershed in Agusan del Norte, Mindanao, Philippines through the use of high resolution topographic information derived from LiDAR and a suite of hydrologic and two-dimensional (2D) hydraulic models. The alluvial fan, with its active portion traversed by Asiga River, is of particular interest due to its recent flooding events that occurred in January and December 2014 caused by extreme rainfall brought by tropical storms Lingling (" Agaton ") and Jangmi (" Seniang "), respectively. While it is known that flooding was caused by the overflowing of the Asiga River, the role and contribution of the alluvial fan to flooding was not yet fully understood. Using a calibrated hydrologic model based on the Hydrologic Engineering Center Hydrologic Modeling System (HEC HMS), we determined the inflows at the apex of the alluvial fan during rainfall events. Then, we used these inflows as inputs into a 2D hydraulic model based on the HEC River Analysis System (HEC RAS) to simulate the movement of water entering the alluvial fan. LiDAR-derived Digital Terrain Model was specifically used as input into the 2D model and made possible the generation of very detailed flood characteristics like flood extent, depth, velocity, and arrival time. The use of LiDAR data was very instrumental in mapping the distributary channels that contributed to the wide extent of flooding. Hydrologic and 2D hydraulic simulations were conducted to examine flood hazards in the alluvial fan under different scenarios of extreme rainfall events, e.g., rainfall events with return periods of 2, 5, 10, 25, 50 and 100 years. Spatial overlay analysis was also conducted to assess the exposure of infrastructures to the various levels of hazards within the alluvial fan under these extreme events which can become very important inputs in the development of flood adaptation and mitigation strategies of the concerned LGU and communities residing within the alluvial fan.
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