Forecasting Rainfall Induced Lanslide using High Resolution DEM and Simple Water Budget Model

English Abstract:

Philippines is hit by an average of 20 typhoons per year bringing large amount of rainfall. Monsoon carrying rain coming southwest of the country also contributes to the annual total rainfall that causes different hazards. Such is shallow landslide mainly triggered by high saturation of soil due to continuous downpour which takes up from hours to days. Recent event like this happened in Zambales province September of 2013 where torrential rain occured for 24 hours after a weeklong continuous downpour amounting to half a month of rain. Rainfall intensity measured by the nearest weather station averaged to 21 mm/hr from 10 pm of 22 until 10 am the following day. The monsoon rains was intensified by the presence of Typhoon Usagi positioned north and heading northwest of the country. Landslides due to this occured in 3 different municipalities; Subic, San Marcelino and Castillejos. The disaster have taken 30 lives from the province. Monitoring these areas for the entire country is but a big challenge in all aspect of disaster preparedness and management. The approach of this paper is utilizing the available forecast of rainfall amount to monitor and forecast possible landslide that could happen. A simple water budget model following the equation Perct=Pt-R/Ot-STt-AETt was implemented in quantifying all the water budget component. Computations are done in Python scripted grid system utilizing the widely used GIS forms. Results of successive runs will let percolation and change in water storage as indicators of possible landslide that could happen. This approach needs three primary sets of data; 1 weather data, 2 topographic data, and 3 soil parameters. This research uses 5 m resolution DEM (IfSAR) to define the topography of the study area. Soil parameters are from fieldworks conducted. Weather data are from the PAGASA. Initially applying to the identified landsline prone areas in Zambales and historical weather data from the PAGASA Iba Station, quantities of each of the water budget component are being evaluated . Validity of results shall be tested on the stream gauges downstream of the study area. This will account the runoff portion of rainfall. Any discrepancy with the method's computed runoff is utilized for the calibration of the model. Reliability of the results are as good as the data and is limited by the availability of such.