By Geohazard and Engineering Geology Section
The Updating of 1:10,000 scale Rain-Induced Landslide and Flood Susceptibility Map for the Tigbao Municipality, Zamboanga del Sur, aims to appraise the geohazard susceptibility ratings of the former geohazard map produced from the previous assessment.
This study covered all eighteen (18) barangays and their respective puroks and sitios, with one hundred twenty-eight (128). All barangays are clustered and divided into Northwestern Cluster, Southeastern Cluster, and Southwestern Cluster.
The general topographic description of Tigbao is rugged consists of several high peaks, hilly terrain, and floodplains. Relatively flat terrains are along Lacarayan, Libayoy, Nilo, Maragang, Limas, Begong, and Longmot. The characterization of these areas is within the fault line. There are two (2) faults in the area; the Ban-tao Fault and the Dumalinao Fault, classified as active. There are four (4) major river systems in the study area: the Labangan River, Timolan River, Maragang River, and Dinas River.
The northwestern Cluster has high to very high overall landslide susceptibilities ratings. Areas with steep sloping grounds such as spurs, gullies, road cuts, and old landslides are appropriate for the rating. Libayoy is the evident flood-prone area among the barangays.
Moderate landslide susceptibility ratings are prevalent assessment rating for the Southeastern Cluster. Flooding may be limited to floodplain of Timolan River, portions of Barangay Tigbao and Timolan. Barangay Nilo is within the floodplain of Maragang River, several of its tributaries are traversing within the area.
Landslides in Southwestern areas may be limited to steep sloping grounds such as road cuts, gullies, and steep-faced hills. Moderate to High landslide rating is prevalent for this cluster. Flooding is prominent within the floodplain which serves as local catchment basins which are divided by a northwest-southeast trending ridge. Moderate flood susceptibility is the prevalent rating for these areas.
Municipality of Tigbao is 13,706.56 hectares of rated areas for the different landslide and flood susceptibilities. Fifty-eight (58) percent are rated with High susceptibility to landslide. 0.53% is rated with High Flood Susceptibility and 1.01% of the total area is considered safe in flood and landslide hazards.
The Geohazard Assessment for the Environmentally Critical and Tourism-oriented Area aims to determine the potential types of geohazards potent in the said locale. As a result, these areas’ management and conservation will be enhanced by identifying and formulating counteracting measures for these potential hazards.
The scenic waterfall is located on the northwestern edge of Barangay Nanga-nangan of Tigbao along the boundary of Pagadian City and Lakewood Municipality. Nanga-nangan waterfall is the only point where the 2,814.60-hectare watershed drains all its surface run-off. Flash flooding is the is the most serious hazard in this area. It can produce large volumes of water with high turbidity.
The Mount Timolan Protected Landscape (MTPL) is declared a protected area in this study area. It conveys a total area of 2,689.54 hectares. Eighty-four (84) percent, of the total MTPL is Highly Susceptible to Landslides and some areas are classified as possible landslide accumulation zones. These areas should be considered as no-built zones or developments.