The flood models of the project areas were developed according to the framework that consisted of two components.
The first component deals with the upstream watershed hydrology, wherein a hydrological model is developed to estimate how much runoff is produced during a rainfall event. For this component, HEC HMS was used.
The second component deals with the river and flood plain hydraulics which aims to determine the behavior of water coming from the upstream watershed as it enters the main river and travels downstream towards the sea. With these two component combined, a flood model is developed and can be used in a variety of applications: as a flood forecasting tool, to reconstruct past flood events, to generate flood hazard maps, and to generate flood inundation information in near real-time.For this component, HEC RAS was used.
Using a 10-m SAR DEM provided by UP Diliman Phil-LiDAR 1 Team, the upstream watersheds and the floodplains of the project areas were delineated using watershed delineation algorithm in a GIS. The hydrological model for the upstream watersheds were developed using HEC HMS. Soil type and land-cover related parameters of the HEC HMS model were derived through analysis of latest medium resolution satellite images, and from the DA-BSWM soil maps, respectively. So far we have developed, HEC HMS models of Cabadbaran, Mainit-Tubay, and Tago River Basins.
The hydraulic models for the flood plain were developed using HEC RAS. At present, we were able create geometry files (x-section, river banks, and river centerlines) of the model. Ideally, the geometry files that have been created should be parameterised (e.g., to assign elevation values from) using the edited LiDAR DTMs and river bathymetry/river bed topography data. However, river bathymetry data are not yet available and will be provided by UP Diliman Phil-LiDAR 1 – Data Validation and Bathymetry Component (DVBC) by next year. Based on recent information from DVBC (as of 11/11/2014), the conduct of validation and bathymetric surveys in the Year 1 project areas will be conducted next year.
The flood model which is the combination of HEC HMS and HEC RAS will be used to run actual flood events in the project areas, as well as to simulate flooding due to hypothetical extreme rainfall events of various return periods (e.g., 2-year, 5-year, 25-year, 50-year, 100-year).
HEC HMS Models
2D Flood Models
In addition to HEC HMS and 1-D HEC RAS flood routing models,we are also trying to develop two-dimensional or “2D” flood models to simulate detailed channel flow, unconfined overland flow and street flow over complex topography. The advantage of using 2D flood models over 1D models is that we can be able to visualize how flood water propagates from one place to another. For example, 2D flood models can help us view how a river overflows and where does the flood water goes after the overflow. An example 2D flood simulation of the January 2014 Typhoon Agaton flood event in Cabadbaran River is shown below. The simulation was generated using the Environmental Fluid Dynamics Code or EFDC with inflow hydrographs computed using HEC HMS and tidal data as boundary conditions.