Iligan river basin located in Northern Mindanao, Philippines covers 165.7 km<sup>2</sup> of basin area. In December 2011, tropical storm Sendong (Washi) hit Iligan City, leaving a trail of wrecked infrastructures and about 490 persons reported dead. What transpired was a wake up call to mitigate future flood disasters. Fundamental to mitigation is understanding runoff behavior inside a basin considering that this is the main source of flooding. For this reason, the present study evaluated total runoff volume, peak discharge and lag time given land cover scenarios in four different years- 1973, 1989, 1998 and 2008. IFSAR and LIDAR DEM were integrated to generate the basin model in ArcGIS. HEC-HMS was used in simulating models for each scenario with Soil Conservation Service Curve Number (SCS CN) as the loss parameter method. Four simulation models of the runoff with varying CN values were established using RIDF as rainfall input with 5 year, 10 year, 25 year, 50 year and 100 year Rainfall Return Period (RRP). Total Runoff volume, peak discharge and lag time were progressively higher from 1973 to 2008 with 1989 land cover as exception where runoff parameters was its lowest. The total runoff volume, peak discharge and lag time is governed by vegetation type. When vegetation is characterized predominantly with woody perennials, runoff volume and peak time is lower. Conversely, when the presence of woody perennials is minimal, these parameters are higher. This study shows that an important way to mitigate flooding is to reduce surface runoff by maintaining vegetation predominantly composed of woody perennials.
This study aims to assess the results of runoff volume, peak flow and the lag time between peak rainfall and peak river discharge or peak flow when no proper land use management is done and another is when sound land use management is adopted. Hence, two (2) land cover/use scenarios were created. The first scenario is the Projected Land Cover. This scenario was created using a Trend Analysis function from MS Excel derived from the 1973, 1989, 1998, 2008 and 2010 land cover scenarios. The second scenario is the Desired Land Use wherein it makes use of slope as the basis in assigning the different land uses. Specifically, agriculture and built-up were assign to 0-18% slope, 18-30% slope for agroforestry, 30-50% for production forest and >50% for protection forest. Limitedly available LiDAR DEM strips was integrated into the IFSAR DEM to generate a detailed basin model and slope in GIS. The HEC-HMS was used for simulating runoff models. The Projected Land Cover has a higher total runoff volume and peak flow and shorter Lag time as compared to the Desired Land Use scenario in all the four (4) Rainfall Return Period these are 5 years, 25 years, 50 years and 100 years. The latter has twice as much forest vegetation that the projected land cover scenario, it has better forest cover quality and plus the presence of agroforestry. Such condition helps improve soil infiltration and thus reduces runoff volume and peak time. The study shows when land cover conditions are left by itself without any intervention, the impact of flood disaster is more likely to be magnified. The study also shows that flood disaster can be mitigated if the Desired Land Use scenario will be adopted as one of the course of action in flood disaster risk reduction management.