The Geosciences Division of the Mines and Geosciences Bureau (MGB) Regional Office No. IV-A (CALABARZON)
conducted geological hazard assessment and mapping in the first half of 2011 in seven (7) municipalities in the Province of Quezon using 1:10,000 scale maps. The municipalities that were evaluated for geohazards are Real, Infanta, General Nakar, Mauban, Sampaloc, Lukban and Tayabas which are all located in the eastern side of Quezon Province. The activity was undertaken in response to the abrupt changing weather patterns being experienced by the country for the past several years together with the attendant devastating effects and the instruction from MGB Central Office Director Leo Jasareno to concentrate the regular geological hazard activity in Quezon Province, especially those municipalities located in the eastern seaboard.
The study areas are specifically within pre-established blocks with each block measuring 3 minutes by 3 minutes (3’ x 3’). This assessment and mapping project did not deviate much from the earlier 1:50,000 scale mapping by the Geosciences Division, except that more details are to be shown on 1:10,000 scale maps. Although focus was made in areas susceptible to landslide, flooding hazard assessment and mapping was also carried out.
Geohazard Assessment in Real, Infanta and General Nakar Municipalities (REINA group)
Fifty-five (55) barangays were assessed for geohazards in the municipalities of Real, Infanta, General Nakar (REINA group) last March 2011.
The REINA group is approximately circumscribed by geographical coordinates 14°32’ 00”- 14°50’00” North Latitude and 121°31’ 00” and 121°40’ 00” East Longitude (see figure below) and covered the following barangays:
(Poblacion Barangay 2)
(Poblacion Barangay 3)
The surveyed areas in Real are characterized by highly sloping terrain with small patches of flat lands that are limited to coastal areas, narrow floodplains and valley floors. Barangays Maragondon, Biga, Tanauan, Tignoan, Kiloloron and portion of Poblacion in Real, Quezon are found highly susceptible to mass movements, specifically rockslides, rockfalls and debris flows. Old and new landslide escarpments were noted measuring from 3 meters wide to 20 meters wide along the stretch of the Siniloan-Real-Infanta Road. Slide materials consist primarily of very loose soil, plant debris and boulders. It is safe to say that anthropogenic factors, such as the construction of the national road, contributed a lot to the instability of the slopes. However, geohazard occurrences are prevalent even with minimal human intervention. These connote the presence of generally non-cohesive soil and loose rock materials consisting of unconsolidated clay, silt and gravel including plant debris that can become more unstable with the onset of rains or earthquakes.
It is noteworthy to mention that retaining walls and small benchings (vegetated) are already in place. However, some of the retaining walls to mitigate slope failures are already partially destroyed.
In Barangay Maragondon and Biga, not only the relatively steep road cut is found to be in danger of landslides, but also the roadsides where gullying has already undercut a significant portion of the highway. Affected houses built along roadsides number from twenty (20) to forty (40).
General Nakar, Quezon
Gen. Nakar is dominated by moderately to very steeply sloped ridges and it also occupied a narrow northern portion of the Infanta Delta. This condition and setting made ten (10) out of fifteen (15) barangays in the municipality highly susceptible to landslide, namely, Mahabang Lalim, San Marcelino, Pagsangahan, Maigang, Pesa, Batangan, Anoling, Magsikap, Catablingan, Poblacion, Minahan Sur and Minahan Norte. Parts of these barangays have slope gradients that are more than fifty percent (50%). Barangay Minahan Norte was rated moderately susceptible to landslide.
Old landslide area (note the presence of new vegetative cover). Red arrow points to the location of the former School in Barangay Mahabang Lalim, General Nakar that was affected by the mass movement.
In the year 2005, a number of areas in REINA group had been affected by landslides. An example is Sitio Sulok in Barangay Katablingan wherein the area was buried under 1.5 meters thick landslide of debris material. Minor landslides and creeps are normally seen along the road wall with moderate to steep slopes in Barangay Minahan Sur.
Geohazard Assessment in Tayabas, Lukban, Mauban and Sampaloc Municipalities
The municipalities of Tayabas, Lukban, Mauban and Sampaloc were assessed for geohazards in May 2011 and they are all found in the eastern footslope of Mount Banahao. These municipalities are approximately within geographical coordinates 14° 01’ 00” to 14° 13’ 00” North Latitude and 121° 30’ 00” - 121°45’ 00” East Longitude.
The areas covered in the municipality of Tayabas are situated mostly in flat to gently rolling terrain. However, the eastern part of the target area has elevations ranging from 100 meters to 352 meters above sea level (masl). Drainage system in the area exhibits a radial pattern which is typical in terrains characterized by volcanoes.
Thirty-four (34) barangays were assessed in the municipality. Two (2) barangays, namely, Silangan and Ibabang Palale were found to be susceptible to mass movements. Seven (7) barangays were appraised to be susceptible to flooding. The twenty-five (25) barangays not included in the table below have no obvious identifiable hazard; however, this condition may change in the future. Barangay Dapdap should be closely monitored during rainy season, especially during heavy rainfall and typhoon periods, since it was observed that the area has the possibility to be an accumulation zone of debris flow from Mt. Banahao.
The wide alluvial plain in Barangay Valencia, Tayabas, Quezon is susceptible to sheet flooding. Normal flood in the area is about one foot or 30 centimeters in height based on verbal communication with the residents.
The scenic town of Lukban, Quezon which lies on the northeastern footslope of Mount Banahaw de Lukban was assessed for landslide and flooding susceptibility. The entire town comprising of twenty-two (22) rural barangays and ten (10) urban barangays was covered in the geohazard assessment.
The lone barangay with high susceptibility for both landslide and flooding hazards is Barangay Palola. This was primarily due to the landslide and accompanying flooding that occurred in the barangay last July 2010 during Typhoon Basyang (see photo below). Four (4) barangays were also rated high with regard to flooding susceptibility. However, only part of each barangay is rated high since the entire barangay was not flooded during the flooding events.
Photo shows the landslide materials along Makatipo River which are composed of boulders, gravels, sand and clay.
Raging floods along Makatipo River carried these huge tree trunks and other debris materials downslope.
Scoured and sloping road shoulder along the provincial road from the town of Lukban going to Mauban and Sampaloc.
Mauban is an elongate-shaped (north-south axis) municipality of Quezon that is situated southeast of Manila. It is bounded in the east by Lamon Bay, in the south by Atimonan, in the west by Sampaloc and in the north by Real, all within Quezon Province.
It is characterized by moderately rolling to rugged topography with highest elevation of 570 meters above mean sea level which is noted northwest of the town proper. A relatively wide alluvial plain cuts across the central portion of Mauban, while a narrow belt of flat terrain defines its eastern shoreline.
The Municipality of Mauban is well drained by a good number of rivers and creeks that form a dendritic drainage pattern. These surface drainages, including the main river called Maapon River, flow from the western elevated areas towards Lamon Bay in an east-northeast direction.
For landslide assessment, there are three (3) barangays that have high susceptibility, namely, Bgys. San Lorenzo, Luya-Luya, and Alitap and three (3) barangays with moderate susceptibility, namely, Bgys. San Isidro, Soledad and San Gabriel.
For flooding assessment, there are eleven (11) barangays that have high susceptibility, namely, Bgys. Santol, Concepcion, San Vicente, San Rafael, San Miguel, Baao, Lulutan, Macasinan, Sadsaran, Balay-balay, and Luya-Luya. There is one (1) barangay with moderate susceptibility, namely, Bgy. Tapucan. Two (2) barangays have low susceptibility, namely, Bgys. San Vicente and Remedios Uno.
Sitios Malaking Aluhin and Tantuco of Barangay San Lorenzo were assessed to be susceptible to differential settlement. Bgy. Sto. Anghel was found free of geohazards.
Rockfall /Rockslide along the road leading to Sitio Malapya and Quezon Power Plant located in Barangay San Lorenzo, Mauban, Quezon.
Houses standing on the downslope from the barangay hall of Barangay Soledad, Mauban, Quezon.
Unstable slope in Sitio Macapagal on the downside of the barangay hall of Barangay Soledad, Mauban, Quezon
The municipality of Sampaloc is characterized by moderate to rugged terrain with steep slopes dominating the southwestern portion. Relatively flat areas cover the broad river floodplain. The surveyed area is defined by geographical coordinates 121° 36’ to 121° 42’ East Longitude and 14° 07’ to 14° 13’ North Latitude.
The studied area lies within three (3) pre-established blocks with each block measuring 3 minutes by 3 minutes (3’ x 3’) on the 1:10,000 scale map. It encompasses all fourteen (14) barangays of the municipality.
With regard to the identification of a possible relocation site in the future, Barangay Taquico may serve the purpose. However, the site is presently being used as an agricultural land (ricefield).
Rockfall along the provincial road from Mauban to Sampaloc, Quezon (left photo). A closer view of the fallen agglomerate (right photo).
Old landslide face/escarpment along the barangay road between Brgy. Toquito and Brgy. Apasan, Sampaloc. Slide materials consist of plant debris, sand and gravel.
Meandering Maapon River in Sampaloc, Quezon. Swelling of this river in the past resulted in flooding that reached as high as 8 meters above the normal water level of the river.
Barangay San Isidro, Sampaloc is found susceptible to both flood and landslide hazards. Houses along the floodplain can be washed out in times of river overflow, while the houses upslope can be affected by landslide due to steepness of the slope and the presence of relatively loose slope materials.
Underscouring of the riprap structure along Maapon River (red arrow) in Barangay Owain, Sampaloc. The Maapon River exhibits a relatively wide floodplain.
Portion of the riprap structure in Barangay Taquico, Sampaloc has been destroyed by past flooding event along Maapon River.