Assistance during Disasters (Public Service) – Naga City Landslide

On 20 September 2018 at 5:54 am, a massive landslide devastated Barangays Tinaan and Naalad in Naga City, Cebu. The landslide claimed the lives of 78 Nagahanons, while 18 were injured and 6 remain missing. Because of clear and present danger immediately after the event, families were moved to evacuation centers for safety purposes and was at its highest on 7 October 2018 numbered at 1,947, equivalent to 8,091 Internally Displaced Persons (IDPs). As of 28 November 2018, the number of evacuees decreased by nearly half the original number because of cash and relocation assistance from the Naga City Government. The remaining IDPs occupy 10 evacuation centers because their homes either fall at the “No Permanent Habitation” or “Regulated and Restricted Zones” and the uncertainty of when they can go back to their homes subject to the final assessment of the DENR-MGB.

The immense and enormous consequences of the DENR-MGB Delineated Zones to the communities and city in general is beyond the capacity of the City Government. Based on the Naga City Administration’s calculations, resources to comply with DENR-MGB delineated danger zones will last until 31 December 2018. Because of Naga City’s dilemma, a plea was made for a reassessment which DENR Secretary Cimatu has responded to favorably. The reassessment is crucial because it may improve necessary interventions and the LGUs capacity with respect to risk and disaster management of the 2018 landslide disaster in Naga City, Cebu. To know the risks and potential hazards of its communities, the Naga City administration engaged technical experts to do a reassessment of the landslide event and delineation of danger zones. The output of the engagement is envisioned to complement existing data and information provided by DENRMGB and other agencies to identify not only the hazardous areas but also places that are considered as safer grounds or less dangerous places where relocation and sectoral development can be introduced. Apart from helping in post-disaster needs assessment, the added information gathered will be useful to enhance knowledge for better decision-making to provide well-grounded solutions to the current dilemma of Naga City, Cebu.


Aerial photo of the Naga Landslide



The methods used in the reassessment of the Naga City landslide area and vicinities, include: Field interview surveys, geological field work; drone surveys; remote sensing analysis; CCTV review; high-resolution Digital Surface Model (DSM) and Digital Terrain Model (DTM) interpretation for the presence of sinkholes; Factor of Safety Analysis using SSAP2010, numerical modeling of shallow landslides using Soil Stability Index Mapping (SinMAP) software; deep-seated or structurally-controlled landslide analysis using Coltop 3D and Matterocking; landslide-runout modeling with Coltop 3D; debris ow modeling with FLO-2D; and Permanent Scatterer Interferometry (PSInSAR) for ground deformation including subsidence. In addition, riverine and urban flood scenarios were modeled with FLO-2D while storm surge scenarios were simulated with WXTide, the JMA Storm Surge Model and FLO-2D. Other hazards such as those related to earthquakes and volcanoes from nearby islands were assessed using secondary data from PHIVOLCS, MGB and others sources gathered from the review of related literature. Analysis of the Naga City, Cebu landslide, was improved with sharing of data and information that were gathered with an international community of landslide and geotechnical experts from France, United Kingdom, Italy and Mexico.

Landslide Analysis

The landslide started as a simple translational slide, where failure along a south southeast dipping bed of siltstone allowed a large part of the mountain composed of limestone to move downhill. The CCTV shows that the landslide lasted for a minute and traveled at 36-72 km/hour. It also shows that the whole landslide mass moved as one whole block to start with no significant backward regression. The fractures that developed prior to the collapse and observed at the top of the landslide suggest that the whole block started to move some time before the main acceleration of the sliding mass. Because the sliding mass moved as a whole means a larger volume was available favoring increased runout of the landslide measured up to 1.2 km long from the head scarp (240m elevation in Sitio Tagaytay) to its toe (40 m elevation in Sitio Sindulan). This is in contrast to a retrogressive landslide with multiple rotated blocks, where each individual unit would have smaller volumes, thus not promoting a longer runout distance. Once in motion, the front of the sliding mass accelerated, stretching the sliding Limestone body to create the Debris Avalanche. Often a landslide converts to a debris ow as water escapes, which is a downward moving mass similar to wet-concrete. The absence of debris flows suggests that the Limestones did not hold much internal water, and allowed any to drain away rapidly.

The Naga City Rockslide-Debris Avalanche is an event that is rare compared to other Philippine landslides (debris flows not included) because of its runout distance. There are only two Rockslide-Debris Avalanche events that have occurred among the approximately 13,000 landslides that happened in the Philippines since 2003. The other Rockslide-Debris Avalanche happened in Guinsaugon, Leyte, in 2006 where a mountain scarp 700 m high slid along planar structures of the Philippine Fault and generated a Debris Avalanche 4km long. Apart from having the same deposit features (Torevas, Megablocks, Hummocks and Jigsaw Cracked boulders), the height to length ratio of the runout of the Guinsaugon Debris Avalanche is 1:6, which is nearly the same height to length ratio of the Naga City, Cebu Rockslide-Debris Avalanche. Such type of landslides called a Debris Avalanche also happen in other places in the world. Comparable events include the 1998 Debris Avalanche in Casita, Nicaragua, the 2014 Oso Debris Avalanche in Washington, USA, and the 2013 Bingham Mine Debris Avalanche in Utah, USA.

Fractures that were reported as early as August 2018 rapidly grew and expanded days before the 20 September 2018 Rockslide-Debris Avalanche and prompted authorities to evacuate villagers near the source head of the landslide. Unfortunately, the extremely large sliding mass, which was the reason for the long runout of the landslide, was unexpected, causing the inability to anticipate the evacuation of residents in Sitio Sindulan, Tinaan and the base of a cliff at Barangay Naalad, both located near the landslide toe, where many houses-with people still sleeping- were buried. Perhaps the biggest lesson in the Naga Disaster is there are such types of landslides, although more the exception than the rule, that are massive and have long runout lengths. Not all unstable slopes that collapse fall within the vicinity of the foot slope. Developing fractures, such as those reported prior to the 2018 Naga City, Cebu disaster, should have been closely monitored and investigated by landslide experts familiar with the behavior and dynamics of extremely large mass wasting events. Such kind of monitoring have been done in other landslide areas of the world (Hungr et al., 2005) such as in the Chilean Andes and the Swiss Alps. Fractures that developed at Sitio Tagaytay, Naga City, Cebu, indicated a factor of safety below 1 and served as the warning for the landslide. Communities could have benefited from monitoring of ground deformation not just in possibly predicting the time of likely failure but also in identifying probable causes of instability by comparing it with natural events (e.g. 26 February 2018 M3.0 Naga City earthquake, rainfall, subsidence) and anthropogenic activities (e.g. excavations). The absence of such monitoring of the quarry area makes it diffcult to pinpoint the exact final trigger of the landslide amongst the compounding possibilities. Other more modern techniques to monitor mm scale deformation or movement of the ground can be done through the use of Radar Interferometry (PSInSAR) as mentioned in this report.

The CCTV video which clearly shows a whole sliding mass from the base of the Quarry benches in the APO Tenement to the linear fractures in Sitio Tagaytay necessitated modeling to determine instability and the Factor of Safety (FOS). The result of numerical simulations using SSAP2010 shows a very low factor of safety (0.7-0.74) for the mountainous area that collapsed, with the lowest factor of safety about 210 meters upslope of the Limestone Quarry benches in the APO Tenement. Why the landslide chose to fail 630 meters further upslope may be due to fractures or faults in the landslide source area. Lineament interpretation of time-series satellite images show the presence of northeast trending linear structures whereas field investigation showed the presence of northeast-trending scarps, faults and evidence of oblique-slip faulting at the head of the landslide. The combined geometry of the fracture/fault structures passing through Sitio Tagaytay and the southeast dipping siltstone bed at the base of the Limestone mass provided the perfect conditions for a large block of mountain to slide and generate a debris avalanche as it started to break apart and accelerate.

Actions Made by the LGU Prior to the 2018 Naga Landslide

A complete history of the actions made by authorities regarding compliance on existing laws to protect lives and property prior to the landslide is detailed in the MGB report. A day before the landslide, residents of Sitio Tagaytay in the area where fractures developed were evacuated. However, residents more than a kilometer downslope in Sitio Sindulan, Barangay Tinaan, were not included in the evacuation order. Inability to evacuate the residents in Sitio Sindulan may have been brought about by the inability to foresee the long-runout of the landslide with height to runout ratio of 1:6. There are only 2 such cases (debris flows excluded) out of the nearly 13,000 landslide events since 2003 in the inventory of Project NOAH. These are the Guinsaugon, Leyte, and the Naga, Cebu, Rockslide Debris Avalanches. Although this unusual type of massive landslide with a long runout has happened elsewhere in the world, detailed monitoring and investigation is critical to be able to accurately predict their occurrence. Such detailed investigations showing the direction and possible runout more than a kilometer away from the landslide source (i.e. 2018 Naga, Cebu Landslide), is not re-ected in the Disaster Risk and Management (DRM) Plan of the City of Naga, which was approved and certified on 14 December 2017 by the Office of Civil Defense (OCD) after undergoing review by the technical working group composed of DRRM-mandated agencies such as the Department of Science and Technology (DOST), Department of Interior and Local Government (DILG), Department of Social Welfare and Development (DSWD), National Economic and Development Authority (NEDA), Philippine Atmospheric Geophysical and Astronomical Services (PAGASA), Mines and Geosciences Bureau (MGB), Philippine Institute of Volcanology and Seismology (Phivolcs) and Commission on Higher Education (CHED). Moreover, the same DRRM Plan of the City, which complies to the provisions of RA 10121 (Philippine DRRM Act of 2010), did not contain the appropriate technical information on landslide hazards for the City Administration of Naga to have acted accordingly to save the lives of the victims residing at Sitio Sindulan.

Imminent Landslide Threats at Ground Zero

Northeast-trending fractures parallel to the head scarp of the Naga Landslide are still developing. These have been modeled and show the potential of collapse when triggered by heavy rainfall or ground shaking towards the landslide debris field and possibly to the west towards Barangay Mainit. The volume of material that remains at the head scarp in Sitio Tagaytay has significantly decreased and the likelihood of a landslide with a large runout is much less. A map of the possible runout and deposit planform of a future landslide emanating from the developing fractures in Sitio Tagaytay has been prepared for the Naga City Administration for guidance. We also recommend that the area of the head scarp where fractures are still developing be clearly marked to prevent people from passing or visiting as the potential for a landslide in that area is extremely high.

The deposits at ground zero are loose and can be remobilized if saturated. Debris flow simulations using post-disaster topography have been prepared to show areas where these can likely happen. Removal of the loose debris can help decrease the potential of debris flows, which can be dangerous. A hazard map was prepared for the Naga City Administration for guidance on this type of hazard. Continuous monitoring of the topography is recommended if these loose deposits are removed as the new data on the landscape will be useful to determine future debris flow paths.

There is a river that was dammed by the landslide at its toe, which generated danger to the communities at the distal part of the landslide. However, numerical simulations of floods during extreme rain show that the water river can divert its course due to the relatively at terrain east of the landslide deposit in Sitio Sindulan. This will cause the increase in height of the floods in these areas and endanger settlers there. On the other hand, there is a large amount of loose material in the river course, which could be mobilized during severe weather conditions. Furthermore, long term aggradation of the river, may cause long term changes downstream. We recommend that these deposits in Sitio Sindulan be removed.

Subsidence is a main consideration in increasing the “No Permanent Habitation” or “Regulated and Restricted Zones”. Being a Karst terrain, subsidence has been identified in earlier reports as a major reason that initiated the Naga landslide. However, this study shows that it is more likely that the geometry of the underlying Siltstone at the base of the Limestone mass plus the presence of fractures/faults at the location of the head scarp was the primary cause of instability, which culminated into a Rockslide-Debris Avalanche at Naga on 20 September 2018. The Factor of Safety (FOS) prior to t e collapse of the Limestone mass above the Siltstone in the APO Tenement was very low (0.70 0.74) even without quarrying (0.800.84) and was primed for sliding/collapse. This is not to mention further destabilization that may have been induced by the 2013 Bohol Earthquake and the shallow focus Magnitude 3.0 earthquake in Naga City on 26 February 2018. After identifying all the sinkholes in LiDAR images and comparing them with the situation at Tagbilaran Bohol, which has far more sinkholes in the area, we find no justifiable reason to mark 650 hectares of

land in Naga City as highly susceptible to subsidence unless the whole area of Tagbilaran, Bohol is also declared as “No Permanent Habitation” or “Regulated and Restricted Zones”. Instead, we recommend that PSInSAR, a method to detect ground deformation from space covering large areas of land be used to monitor places for possible subsidence such as those sinkholes that were identified using LiDAR. The use of Ground Penetrating Radar (GPR) is good for locating hollow portions underground but their interpretation for potential subsidence should not be extended beyond the survey locations where there is no data from GPR.

Instability at Cantao-An School Grounds and Jail Facility

The geotechnical analysis of the factor of safety in this report includes the areas surrounding the school to determine likelihood of slope failure. Based on the results, the slopes are marginally stable. However, a recent MGB investigation reported the presence of tension cracks, soil creep, erosion and occurrence of a recent landslide event (2014) at the old quarry area behind the school grounds. Fieldwork conducted within the school grounds and old quarry area also showed lines of evidence that the school was built on top of an old landslide deposit. Joints and cracks were observed within the outcrops in the school grounds and normal faulting was observed near the old quarry site behind the school. Studies have shown that there is a higher chance of reactivation of landslide movement in areas of old landslide events. Because of these observations, modeling for deep-seated landslide instability and runout was conducted within the area of the school and it shows that the Cantao-an school is in a danger zone. Like in reports of fractures prior to the September 2018 Naga landslide, the reports of cracks and fissures in the school area is valuable to prevent loss of lives. The combined results of field observations and landslide models indicate that there is enough information to follow the recommendation of MGB to evacuate the school, a prudent move given the circumstances, which is to err on the side of caution.

A slope stability analysis was also done for Naga City Jail Facility to address the growing concern regarding potential landslide hazards in the vicinity of the jail facility as pointed out by MGB. Under the assumption that the rock mass in the vicinity of the jail facility has the same properties as the rock mass in the quarry area, it was found that the slopes near the jail facility are naturally unstable with a factor of safety value below 1.

Instability of the Naga City Mountain Range

Cebu City is at the top of the list of all provinces in the Philippines in terms of population exposed to landslides according to the study conducted by Project NOAH. It is clear from a look along the coastal range of Cebu and adjacent cities and municipalities that natural landslides have occurred on the mountains above the coast. Fault lineaments are also evident in places at the base of slopes and elsewhere. If the area is uplifting, it is also tilting and bending. The mountains of Naga City are inherently unstable and gravity is always at work. On top of the natural instability of the mountain slopes in the region, there is also anthropogenic activity. Mining on a natural unstable site is likely to aggravate landslide risk. However, mining activity can be done in such a way to stabilize naturally unstable slopes. If mining started at the top of the slope, and storage of material was made at the base, then anthropogenic and natural factors that destabilize slopes could be reversed.

The Naga City landslide highlighted the need to further study and monitor potential landslide hazards in Cebu Island. In the case of the 2018 Naga City disaster, there were clear indications of imminent collapse of the mountain through the manifestation of fractures in Sitio Tagaytay. Whenever there are such indicators, it serves as a clear warning of possible collapse that necessitates detailed investigation, monitoring, and corresponding action from the community to avert loss of lives and mitigate the impacts of the hazard. Guidelines provided in the Joint Memorandum Circular 2014-1 (JMC2014-1) can be used by Naga City. The guidelines provide recommendations for necessary actions that need to be executed both short term, when urgently needed (Naga City Landslide area and immediate vicinities), and long term at the soonest possible time. We take note that floods, landslides, and storm surges do not happen every day in Cebu and can have ample warnings and be monitored. Communities in these hazardous areas must know the risk in their neighborhood and should know exactly what to do and take the appropriate actions (e.g. move to nearby safe evacuation centers when there are accurate, understandable and timely warnings by authorities), especially during storms when hydro-meteorological hazard events are likely to take place. They can also transfer the financial risk to another party such as insurers. In the long term, it is strongly advised that development be focused in areas that are the safest places in the community as identified through hazards maps such as those presented in this report. Sectoral planning should also be conducted with current and future hazards (i.e Climate Change Impacts) as a primary consideration. These kinds of maps, produced scientifically and with the use of the latest technologies, not only depict the hazards but also identify the suitable areas for settlement and development, which allows for people in communities to manage and live in their land.

Monitoring of a larger area such as the whole of the island of Cebu can be done through Permanent Scatterer Interferometry (PSInSAR). This method can also be used to determine the overall deformation field in the area to sort out the background tectonic movement from natural instability and anthropogenic activity. Quarries in Cebu could also be monitored with this method. The United Kingdom and Italy have national monitoring now using Interferometry and should be done in the Philippines to save lives and property. These technologies and modern scientific methods enable us to monitor and give warning on imminent hazards that have been identified through combined field mapping and numerical modeling. Detailed 1:10,000 scale hazard maps specifically for landslides, floods, and storm surges were prepared for Barangays Tinaan, Inoburan, Cabungahan, Pangdan, Uling, Mainit, Naalad, and Cantao-an to serve as a scientific and technical guide for the Naga City Administration to cope with the short term and long term impacts of the 2018 Naga Disaster.

An assessment report was produced for the City of Naga. Multi-hazard maps for different barangays such as (B) Barangay Tinaan and (C) Barangay Naalad within Naga City were produced.