Landslide and sedimentation modeling in Wahig-Inabanga Watershed, Bohol, Philippines

The study applied two different models--logistic regression (LR) and Soil and Water Assessment Tool (SWAT)--to predict landslide and sedimentation in Wahig-Inabanga Watershed, Bohol. Specifically, logistic regression was employed to determine landslide-prone areas using several significant landslide-related instability factors, while the Soil and Water Assessment Tool was applied to predict stream flow and sediment yield. The applicability of the two models was evaluated, the risks associated to landslide occurrence were quantified, and the outputs of both models were integrated to generate landslide and sedimentation hazard map. The satisfactory results of model assessment and the very satisfactory performance of the model in accuracy evaluation justified the application of the LR model for landslide hazard assessment. The LR model revealed that only small portions of the watershed, 7,063 hectares or roughly 11% of its total are, were high and very highly prone to the occurrence of landslide. These were mostly situated in the upper elevations of the municipalities of Jagna, Pilar and Sierra Bullones. Quantitative risk analysis showed that landslide risk in Wahig-Inabanga Watershed is relatively low. The total expected annual damage estimated in crop production for the whole watershed ranged from P27,846.62 to P774,963.03. The biggest expected annual damage cost was computed from perennial crops which also had the largest area at risk. Total expected annual repair and reconstruction cost for built-up, residential and farm houses in particular, amounted to only P22,620.98 using the semi-quantitative method of risk estimation. The potential landslide risk estimates appear to be a minor importance to the communities residing in the watershed since the computed annual potential losses per hectare, particularly in crop production are more or less 10% only of their seasonal income, ranging from P119.36 to P3,695.04. The estimates on the annual expected damage cost for built-up are even more insignificant, from P29.99 to P276.87. Though the result of the SWAT model performance evaluation on stream flows was satisfactory, the model did not provide precise estimates of sediment yield in sub-basins and hydrological response units (HRU) with most of the land uses, especially corn, on flat to gently rolling terrain. Inaccuracy issue on sediment yield prediction deffered the sedimentation risk valuation. Integration of LR and SWAT models provided significant results and generated the landslide and sedimentation hazard map which are all offered as scientific and technical basis for sustainable watershed management and land-use planning.