Why landslides occur in Sirao, Busay, Naga?

Landslide is one of nature’s most powerful and destructive forces – it strike wherever the rock and soil are loosened by rain or by gravity.  

Torrential rains usually trigger widespread landslides in areas where slopes are steep, where bedrocks are weathered, fractured and loose. It also frequently occurs in areas where people have altered the natural landscapes or stripped its vegetation.

In Central Visayas, several landslides have occurred over the past days after continuous downpour since tropical depression Auring. One of the most massive and damaging landslides to have occurred were in the upland barangays of Cebu City. Close to a hundred families were evacuated in barangay Sirao, and ten vehicles were buried in sitio Garahe, in barangay Busay after two separate landslide incidents.

At least 38 households were also evacuated in barangay Uling, Naga after a minor landslide occurred.

Although there are only five houses that were directly affected by the landslide path, all the other households living within the danger zones were already asked to go to a safer ground near the barangay hall.

Barangay Uling has a history of mass movement thus the bureau recommended for continuous monitoring of the tension cracks as the toe of the landslide is still moving.

In barangay Sirao, where its lithology is a combination of sandy and clayey and a weathered bedrock, it is typical that a mass movement will occur. When rain pours, it saturates the ground and causes slope failure.

The ground is unstable making it unfit for human settlement.  Our geologists have noticed old landslide areas that have already grown vegetation. Slope failures are everywhere, exhibiting visible cracks and slumps.

After the rains, the ground may compress and stabilize and residents may go back to their homes but the ground will remobilize after another downpour and another landslide may occur.

The incident in Garahe, Busay in Cebu City was another case of mass wasting caused by heavily weathered volcanic rocks.

While a riprap was constructed to protect and stabilize a slope, this could not hold a massive collapse of altered volcanic rocks. Not even vegetation could stop the landslide.

The presence of trees or any vegetation does not mean the slope is spared by a landslide. Trees according to MGB geologists can only hold up to four meters deep as their roots could not reach the solid bed rock. It may help stop erosion but if the underlying rock is highly fractured, weathered and loosened, it is still vulnerable to landslide.

It is important to understand the geological condition of a property before proceeding to any development.

The Mines and Geosciences Bureau have completed its geological hazard mapping in the country. Its coverage is specific down to the sitio level. Several information, education and communication campaigns have been launched in the region including Cebu. Among those involved in the geohazard workshops are the barangay captains, barangay councilors, local disaster risk reduction units, teachers, barangay tanods, and some officials of the people’s organization. Copies of the geohazard maps, including map coordinates of specific geologically hazardous areas were distributed.

In Central Visayas, landslides occur in almost all towns and cities but some landslides are more dangerous than others.

Disasters such as landslides have happened before in the areas specified and may happen again. People who live in these locations will continue to live with the threat of landslides. While the bureau recommends total avoidance in areas that have high disaster susceptibility, some may insist to stay for various reasons that may be personal to them.

MGB-7 Director Loreto B. Alburo consistently reminded local government officials to be guided by the geohazard maps, implement early warning signs and conduct preemptive evacuation.

The bureau, as a recommendatory agency, does not have a mandate to stop construction of any private development. Government projects, however, cannot proceed without approval and certification from MGB.

 

Residents advised to avoid landslide and flood prone areas

The Mines and Geosciences Bureau (MGB) reiterates its call to local government units through their disaster and risk reduction team to implement preemptive evacuation as the weather bureau forecasts more rain this week.

“The effects of landslide and flooding on people and structures can be minimized by total avoidance of landslide and flood hazard areas or by imposing conditions in areas that were already identified as landslide-prone,” said MGB-7 Regional Director Loreto B. Alburo.

He added that local governments units can reduce geohazard effects through land-use policies and regulations and by helping the bureau continue its education and information campaign to the residents on the past hazard history of a specific site.

The Philippine Atmospheric Geophysical and Astronomical Services (Pagasa) has forecasted moderate to occasional heavy rains and thunderstorms which may trigger flashflood and landslides.

MGB-7 has identified 41 barangays in Cebu City that are susceptible to flooding and 63 barangays, mostly hinterland villages that are vulnerable to landslide. In barangay Sirao, 13 sites are identified as susceptible to rain-induced landslide including portion in Kamandagan, Kambyo-os, Langub, Proper, and Tawagan II.

Mandaue City has 28 barangays that are vulnerable to flooding and five barangays whose areas may have a possibility of a landslide.

There are also 16 low-lying barangays in Lapu-Lapu, 10 barangays in Consolacion, 17 in Talisay and 16 in Minglanilla that may experience flooding based on its geological location and flood history.

Landslide and flooding are closely related because both are mostly induced by rain. Slope saturation by water is a primary cause of landslides. This effect can occur during an intense rainfall.

“We advise that the residents in hazardous areas would coordinate with their local officials if they would be asked to temporarily or permanently evacuate especially during a long period of rain to avoid casualties and damage to properties,” Alburo said.

The local officials are also advised to check the 1:10,000 geohazard maps provided. The public can also check their locality’s vulnerability, up to sitio level, by checking the map online posted in gdis.denr.gov.ph/mgbgoogle.

 

MGB-7 cautions 789 villages in Central Visayas on possible landslide and flooding

The Mines and Geosciences Bureau has identified 789 barangays in Central Visayas that are highly susceptible to landslide and flooding brought by latest weather disturbance.

MGB-7 Regional Director Loreto B. Alburo urged the local government units to implement preemptive evacuation in areas that are critical to these geological hazards.

Among the towns identified as critical include Alicia, Anda, Batuan, Bien Unido, Bilar, Buenavista, Calape, Candijay, Carmen, Catigbian, Dagohoy, Danao, Dimiao, Duero, Garcia Hernandez, Guindulman, Inabanga, Jagna, Loon, Mabini, Pilar, Pres. Carlos P. Garcia

Sagbayan, San Isidro, San Miguel, Sierra Bullones, Talibon, Trinidad, Ubay, and Valencia in Bohol; Alcantara, Alegria, Aloguinsan, Argao, Asturias, Badian, Balamban, Barili, Bogo City, Boljoon, Carcar, Carmen, Cebu City, Compostela, Consolacion, Daanbantayan, Dalaguete, Danao City, Dumanjug, Ginatilan, Liloan, Malabuyoc, Mandaue City, Medellin, Moalboal, Oslob, Pinamungahan, Poro, Ronda, Samboan, San Francisco, Sibonga, Sogod, Tabuelan, Toledo City, Tuburan, and Tudela in Cebu; and Amlan, Ayungon, , Bacong, Bais City, Basay, Bayawan City, Bindoy, Dauin, Dumaguete City, Guihulngan City, Jimalalud, La Libertad, Pamplona, San Jose, Santa Catalina, Siaton, Sibulan, Tanjay City, Tayasan, Valencia, and Zamboanguita in Negros Oriental.

Chief Geologist Al Emil Berador asked the public to continuously monitor the latest weather bulletin from the Philippine Atmospheric Geophysical and Astronomical Services Administration (Pagasa).  Another weak low pressure area (LPA) was spotted in Mindanao. On Wednesday, the weather bureau will know whether it would develop into a tropical depression to be named, “Besing”.

Although tropical depression Auring has weakened into an LPA, it will continue to bring rains in the Visayas area. Berador warned that continuous downpour will trigger possible landslide and flooding in low-lying areas.  

There are factors that contribute to flooding which include proximity to perennial river creek system, poor drainage facility, inherently low-lying area, proximity to coastal area, heavily silted river or creek, and encroachment along natural drainage channels.

Preemptive evacuation is highly encouraged to save lives and properties. Alburo reiterates the call for local disaster risk reduction units in every barangays to help in the evacuation and be guided by the 1:10,000 geohazard maps provided by the bureau.

There are 41 barangays susceptible to flooding and 22 barangays that are prone to landslide in Cebu City; 28 barangays are critical to flooding and five villages prone to landslides in Mandaue City.

Toledo City, the hardest hit among the localities in Cebu by tropical depression Auring have nine barangays that are prone to flooding and 17 barangays that are critical to landslide.

 

MGB-7 cautions LGUs on possible flood and landslide

Low-lying villages and those living in areas declared as hazard-prone in Cebu and Bohol are advised to take precautionary measures as moderate to occasional heavy rains are expected in the next few hours.

The Philippine Atmospheric and Geophysical Astronomical Services Administration (Pagasa) has lifted the tropical cyclone warning signal number 1 over Bohol, Siquijor, Negros provinces, Cebu and other areas in the Visayas in their 11 a.m. weather bulletin. The tropical depression has weakened into a low pressure area.

While Pagasa said the winds brought by the low pressure area may not be destructive, they warned of the possible effects of rainfall. Moderate to occasional heavy rain is expected in the southern towns of Cebu including Toledo City, Pinamungahan, Aloguinsan, Barili, Dumanjug, Ronda and Alcantara. 

Mines and Geosciences Bureau – Central Visayas (MGB-7) Regional Director Loreto B. Alburo urge the local disaster and risk reduction management units to undertake preemptive evacuation efforts in geologically hazardous areas. There are over 502 sitios in Cebu that were rated as "high" and "critical" areas for rain-induced landslide and flooding based on the 1:10,000 geohazard map.

The regional office has furnished the local government units in Central Visayas copies of the geohazard maps and conducted workshops to identify safe relocation sites.

Sinkhole opens in Badian farmland

The Mines and Geosciences Bureau – Central Visayas has confirmed a sinkhole formation in an agricultural land in the Bugas, Badian town, southwest of Cebu.

A section of a farm in sitio Purok 2 recently collapsed into a sinkhole.

A team from MGB-7 led by supervising geologist Josephine T. Aleta conducted an investigation in the area and found a big cavern hollow measuring three to five meters below the ground surface has formed. The sinkhole is seven meters deep with an elongated hole of about five to seven meters.

There was no water found in the sinkhole.

The town of Badian and its neighboring towns sit on a limestone area and is prone to sinkholes.

“Sinkholes are natural formations that occur when limestone deposits gradually dissolve after a contact with rain or circulating ground water,” chief geologist Al Emil Berador explains.

MGB-7 Regional Director Loreto B. Alburo asked the local government unit of Badian to continue monitoring the area and set up warning signs to warn the public of possible hazards near the sinkhole area.

Sinkholes, considered as a geological hazard happen when carbonate rocks are dissolved by water. This slow geological process, is often unnoticeable.

The dissolved carbonate rocks create cavernous space underground forming caves. When there is no longer underground support, the ground from above will collapse. It can also occur if a corroded limestone area is disturbed by natural tremors such as earthquakes or by human-induced activities, including blasting.

Berador said majority of the areas in Cebu have limestone deposits. About 60 to 70 percent of Cebu including areas that are habitable such as urban areas, is composed of limestone.