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HYDROLOGICAL MODELING STUDIES FOR PROPOSED FLOOD-CONTROL DAMS IN THE MARIKINA RIVER BASIN
Leonardo Q. Liongson
Team Energy Professorial Chair
& Professor, Institute of Civil Engineering and National Hydraulic Research Center, University of the Philippines, Diliman, Quezon City, Philippines
Tropical Storm Ondoy (Ketsana) crossed Metro Manila and the adjacent river basins in a late wet-season episode of 2009, starting in the evening of September 25, 2009 and continuing into the next day of September 26, 2009. TS Ondoy brought very intense and heavy rainfall to the region - record amounts of rains fell over a very short time period of 12 hours to 24 hours, which are estimated to occur at an average annual frequency of 1 in 100 years or even higher, depending on the measuring location in the region. The rains generated record-magnitude flood flows and inundation in the Pasig-Marikina River Basin in Metro Manila, and the Laguna de Bay region. This paper describes the application of the distributed watershed rainfall-runoff model SWATCH in order to simulate the TS Ondoy floods and other floods of lower return periods (2 years to 100 years) in the upper Marikina River Basin, using as input the spatially-interpolated rainfall network data and available topographic, soil/geology and river-geometry data. The SWATCH model consists of a tree-network of several variable rectangular surface/soil/aquifer storage nodes and overland/channel flow links, with accounting of soil moisture, evapotranspiration, infiltration, and recharge to groundwater, and routing of overland and channel flows, interflow and baseflow into aggregated total streamflows. Using the model-computed inflow flood hydrographs, the options of single or cascade of flood-control storage dams (with selected reservoir-storage and spillway capacities) for attenuation of floods peaks and volumes, and downstream flood hazard reduction are initially evaluated by flood-routing studies.