By Geohazard and Engineering Geology Section
The Updating of 1:10,000 scale Rain-Induced Landslide and Flood Susceptibility Map for the Siay Municipality, Zamboanga Sibugay, aims to appraise the geohazard susceptibility ratings of the former geohazard map produced from the previous assessment.
This study covered all twenty-eight (28) barangays and their respective puroks and sitios, with one hundred seventy-five (175). All barangays are clustered and divided into Central Cluster, Northern Cluster, and Sibuguey River Delta.
The topography of Siay is composed of mountainous sections, hilly landscape, and floodplains. The northern parts are the mountainous portions, central parts are hilly, conveying moderate to low elevated land features. On the northeastern portion is the Balagon River basin in which composes significant relatively flat terrain which conjuncts to the Sibuguey Valley. At the southern side, it comprises the Sibuguey River Delta. This delta was formed by deposition of sediment that is carried by the Sibuguey River as the flow leaves its mouth and enters slower moving water of Sibuguey Bay. Deltas are typical have low slope gradient as it approaches to the coastline. Areas within the delta are considered as a part of the Sibuguey River flood plain.
The Central Cluster has a persistent rating of High to Moderate Landslide Susceptibility due to the topographic nature of this area and its general underlying rock mass properties. Steep sloping grounds such as spurs, gullies, road cuts, modified slopes, and old landslides specially with human presence and settlements are rated in a stepped up as having Very High Susceptibility. Flooding is limited along the low-lying areas such as floodplains, creek/riverbanks, abandoned and active river channels and others; it may occur on this cluster but only limited to channels of active and intermittent creeks and gullies. Areas adjacent to the Suluan River, Balagon River and the Sibuguey River are considered having Moderate to High flood susceptibility ratings.
The barangays of the Northern Cluster are predominantly located on the highlands which has limited gently sloping areas and mostly composed of moderate and steep slopes. Areas belong to this cluster has Moderate to High Landslide Susceptibility. It includes the area of Balagon basin which has a relatively flat and low reliefs with Balagon River and the Sibuguey River navigates. Within this cluster the highly exposed to flooding are Camanga and Maniha.
SIBUGUEY RIVER DELTA
A river delta is a landform that is created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water . The Sibuguey River Delta is the result of the Sibuguey River’s sediment deposition towards Moro Gulf. Its terrain begins with a gently slopes and it grades as into flat terrain into coastline.
The barangays that are on the delta are Laih Miranda, Monching, Logpond, Balucanan, Salinding, Mirangan, Lagting and Sibuguey. Most of the barangay centers of this cluster are situated on gently sloping terrain, and residential dwellings are also located within such topography. Moderate and steep sloping grounds are limited while most they are cultivated for rice, corn, calamansi, vegetable crop and for aquaculture. Landslides in these areas may be limited to steep sloping grounds such as road cuts and steep-faced hills (likely modified-slopes).
Flooding is prominent hazard in this area given the nature of this landform. Recent reported heavy and widespread inundation is on November 17, 2021, with more than a meter flood depth. Inundations may not be limited to the Sibuguey River’s high discharge also river systems such as the Suluan River, Batu River and Coloran Creek can cause local floodings depending on the intensity and duration of rainfall.
Modeling of Sibuguey River through HEC- River Analysis System (RAS) it implies that it can hold as much as 310 to 360 cubic meter per second along its reaches to its bank full stage. As the volume exceed the given ranges inundations manifest and the magnitude varies as the discharge increases. On November 17, 2021, flooding event, the estimated and modeled discharge from the Sibuguey River (Upper Reach) range from 1,510 to 1,915 cubic meter per second. Thus, it exceeds as it approaches to the Siay area.
In this cluster, flood susceptibility ratings are Moderate to High along the low-lying areas which can be inundated by 1 to 2 meters while Very High to areas along and near the river channel with more than 2 meters, and to areas with high-water velocity.
The municipality of Siay has 27,663.46 hectares of rated areas for each landslide and flood susceptibility rating. Thirty-five (35) percent are rated with High susceptibility to landslide. 19.45% is rated with High Flood Susceptibility, and five (5) percent of the total area is considered safe in flood and landslide hazards.