DETERMINATION OF FLOOD MITIGATION PLANNING PARAMETERS IN AN UNGAUGED RIVER BASIN: HYDROLOGIC AND HYDRAULIC ASPECTS

Flood mitigation planning involves the development of a comprehensive set of structural and non-structural measures to minimize the adverse effects of flooding. Implementing this process in ungauged locations poses particular challenges due to the lack of relevant hydrologic and hydraulic parameters necessary for making informed decisions. This paper presents a methodology for determining such parameters using a hydrologic-hydraulic model in a pilot site, specifically the Mapatos River Basin in Camarines Sur, Philippines. The model applied a combination of well-established theoretical and empirical equations, including the shallow-water equations, SCS Curve Number Method, SCS Unit Hydrograph Method, Convolution Equation, and Muskingum-Cunge Equation. To extract the physical characteristics of the study area, high-resolution topographic data was utilized, with a horizontal resolution of 1 meter in the river and immediate floodplains and 5-m resolution for the rest of the basin. The validity of the model was assessed using data from a recent flood event, resulting in a Nash-Sutcliffe Efficiency value of 0.54, indicating satisfactory results. It should be noted that the simulated flood depths tended to be higher than the observed values, which is appropriate for design studies as a margin of safety must always be incorporated into planning and design. For the design consideration of 25-year, 50-year, and 100-year average storms in the basin, the hydrologic design parameters, such as rainfall excess, time lag, peak discharge, and total hydrograph, were quantified. Similarly, the hydraulic aspects, such as flood extent and flood depths at critical locations in the downstream floodplains, were determined. This methodology can be readily applied to other ungauged locations, provided that similar or more sophisticated model validations are conducted.