Abstract
Abstract. Silang-Sta. Rosa Subwatershed is experiencing rapid urbanization. Its downstream area is already urbanized and the development is moving fast upstream. With the rapid land conversion of pervious to impervious areas and increase frequency of intense rainfall events, the downstream of the watershed is at risk of flood hazard. The widely used freeware HEC-RAS (Hydrologic Engineering Center- River Analysis System) model was used to implement the 2D unsteady flow analysis to develop a flood hazard map. The LiDAR derived digital elevation model (DEM) with 1m resolution provided detailed terrain that is vital for producing reliable flood extent map that can be used for early warning system. With the detailed information from the simulation like areas to be flooded, the predicted depth and duration, we can now provide specific flood forecasting and mitigation plan even at community level. The methodology of using 2D unsteady flow modelling and high resolution DEM in a watershed can be replicated to other neighbouring watersheds specially those areas that are not yet urbanized so that their development will be guided to be flood hazard resilient. LGUs all over the country will benefit from having a high resolution flood hazard map.
Highlights
Rosa subwatershed is threatened by increasing surface runoff due to ongoing urban sprawl in the downstream to the upstream of the watershed
The upstream areas of the watershed are being converted into built-up areas, the perennial flooding in the downstream areas of the watershed is expected to worsen unless effective mitigation and adaptation measures are conducted
The Disaster Risk and Exposure Assessment for Mitigation (DREAM) program was formed by the Department of Science and Technology (DOST) and the University of the Philippines Diliman to produce finer-scale flood hazard maps and inundation models for 6-hour early warning system for the 18 major river basins in the country using the airborne LiDAR technology
Summary
With the help of high resolution maps, reliable projections and effective disaster and risk adaptation and mitigation activities can be implemented at the community level. The DREAM program was formed by the Department of Science and Technology (DOST) and the University of the Philippines Diliman to produce finer-scale flood hazard maps and inundation models for 6-hour early warning system for the 18 major river basins in the country using the airborne LiDAR technology. The downstream of the Silang-Sta. Rosa subwatershed have LiDAR data coverage, it is not yet included in the priority areas for flood hazard mapping of the Phil-LiDAR 1 project. Since the downstream of Silang-Sta. Rosa subwatershed is perennially flooded, the use of the available LiDAR data was requested and a separate flood modelling and research study was conducted. Unlike the methodology of the Phil-LiDAR 1 which uses only one-dimensional (1D) unsteady flow analysis of the HEC-RAS software, two-dimensional (2D) unsteady flow analysis was implemented for the majority of the downstream of the subwatershed
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