Kusiong Debris Flow and Landslides

As severe tropical storm Paeng (Nalgae) crossed the archipelago, residents of Sitio Tinabon, Brgy. Kusiong, Datu Odin Sinsuat in Maguindanao del Norte braced for its effects by preparing and executing their evacuation protocol. Witnesses said the barangay had sounded an alarm system around midnight of 27th Oct 2022. This prompted some of the residents and families to rush to the St. Peter & St. Paul Episcopal Chapel which was located at a higher elevation near the foothills of Mt. Minandar. The memories and experiences brought by the storm surge during Typhoon Frank (Fengshen) in June 2008 and the tsunami brought by the August 1976 Moro Gulf Earthquake prompted the residents and the local government unit to instinctively move away from the shoreline and towards higher ground to seek refuge in the face of imminent danger from hazards. The community itself has moved from the coastline inland towards an elevated relocation site two years ago, thinking it was the safest place. However, at around 1:00 AM the next day (28th Oct), a debris flow of rocks, soil and gushing water rushed from the mountain, covering and destroying everything in its path, including the chapel and most of the newly-established relocation site.

Using satellite images from Planet, the event was delineated and analyzed (Figure 1) – including the spread of the debris flow, the number of triggered landslides within the vicinity of the event, and the landslides within Mt. Minandar that contributed to the rocks, soil and debris that devastated Kusiong.

Figure 1. Before (Oct 22) and after (Oct 30) Planet satellite images showing the changes within the vicinity of the sitio.

Based on our analysis of pre and post-disaster satellite imagery (Figure 2) a total of 498 landslides were triggered during TS Paeng within Kusiong and surrounding barangays. Of these landslides, around 20 fed collapsed material into the stream flowing from the watershed of Mt. Minandar down to the devastated settlement. Debris flow simulations (Figure 3) and landslide hazard maps (Figure 4), show that the resettlement area and designated evacuation area are likely to experience the impacts of mass movement. Furthermore, all the landslides triggered within the vicinity was previously identified to have medium to high hazard susceptibility.

Figure 2. The normalized difference vegetation indices (NDVI) of the satellite images helped illustrate the change on the surface that Paeng had caused in the area. Previously vegetated areas are now bare earth due to triggered landslides and some previously dry areas became wet and submerged with flood. These surface changes were used to automatically detect the landslides that were triggered.

Figure 3. Debris flow simulation from UPRI – NOAH Center, showing that the devastated area is highly susceptible to debris flow events.

Figure 4. Landslides (blue dots) that were triggered during Paeng were within either the medium (orange) or high (red) hazard area within the NOAH simulations.

This event is a reminder of the importance of multi-hazard and multi-scenario risk assessments. Unfortunately for the residents of Sitio Tinabon, in an effort to reduce their exposure to coastal hazards, they fell victim to landslide hazards that emanated from high-relief areas.