A Team composed of Geologists from the Geosciences Division of MGB-IX recently completed the updating of the 1:10,000 scale Rain-Induced Landslide and Flood Susceptibility Map for Tigbao Municipality in Zamboanga del Sur.
Covering eighteen (18) barangays with one hundred twenty-eight (128) puroks and their respective sitios that were clustered and divided into Northwestern Cluster, Southeastern Cluster, and Southwestern Cluster, this activity aims to appraise the geohazard susceptibility ratings of the former geohazard map produced from the previous assessment.
The general topographic description of Tigbao is rugged consisting 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 are within the two (2) fault lines: the Ban-Tao Fault and the Dumalinao Fault, both classified as active. While 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 divided by a northwest-southeast trending ridge resulting to a Moderate flood susceptibility as the prevalent rating for these areas.
The Municipality of Tigbao comprises 13,706.56 hectares of rated areas for the different landslide and flood susceptibilities. Fifty-eight (58) percent is rated with High susceptibility to landslide and 0.53% for High Flood Susceptibility while 1.01% of the total area is considered safe in flood and landslide hazards.
Meanwhile, 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, management and conservation of these areas will be enhanced by identifying and formulating counteracting measures for potential hazards.
The Nanga-Nangan scenic waterfall located on the northwestern edge of Barangay Nanga-nangan of Tigbao along the boundary of Pagadian City and Lakewood Municipality is the only point where the 2,814.60-hectare watershed drains all its surface run-off. Flash flooding is the most serious hazard in this area which can produce large volumes of water with high turbidity.
In addition, the Mount Timolan Protected Landscape (MTPL) is declared a protected area in this study area conveying 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.