**5. Modeling flood inundation**

The watershed is a very dynamic environment especially if it is being altered by anthropogenic activities. Human activities modify the hydrologic cycle to meet the demands of the population in order to survive. More often than not, the modifications we do to our watersheds lead to the deterioration and destruction of our natural environment impairing their natural purpose of efficiently conveying excess surface runoff downstream.

Because of the necessity to alter our watersheds to satisfy the needs of the society, it is imperative that we simulate the options that we want to do to minimize the problems that could be brought about by the modification that we will do. To alleviate the already existing flood problems we encounter in the floodplains we propose flood control measures that need to be simulated as well. The simulation of the flow of flood in a watershed or flood modeling is important as a decision support tool or system to achieve better options in solving our flooding problems given our limited resources.

In flood modeling, one of the most important inputs other than the rainfall, is the elevation. Water seeks its own level and therefore, elevation has to be accurately represented.

In the Philippines, extensive flood modeling of many of its riverbasin happened along with the introduction of LIDAR which stands for Light Detection and Ranging to create 3D digital elevation model (DEM). The LIDAR DEM generated has an accuracy of 20 cm in the vertical and 50 cm in the horizontal directions

generated from at least 2 data points averaged per square meter. A Digital Surface Model (DSM) kind of model is produced from this scan of the earth's surface. Sample DEM that shows purely terrain elevation called Digital Terrain Model or DTM is shown in **Figure 3**. It has a resolution of 1 m x 1 m as compared to Synthetic Aperture Radar (SAR) and Interferometric Synthetic Aperture Radar (IfSAR) DEMs which has a resolution of 30 m x 30 m and 10 m x 10 m or 5 m x 5 m, respectively.

Starting 2012, all of the 18 major river basins of the Philippines and its 247 principal river basins and other smaller river basins had their floodplains modeled using LiDAR DEM which was a priority project of the Department of Science and Technology (DOST) through its Nationwide Operational Assessment of Hazards or Project NOAH's Disaster Risk and Exposure Assessment for Mitigation or DREAM Component Project and the Phil LiDAR Program.
