2019 VRA and Geohazard Updating in Labason Municipality, Zamboanga del Norte [Initial Findings]

The VRA and Geohazard Updating of MGB-IX Geosciences Division in the Municipality of Labason were conducted on July 2019. Labason is part of the targeted municipalities in Zamboanga del Norte which is located on the northern-most central portion of central Zamboanga facing the Sulu Sea. The municipality is bounded by the Municipality of Gutalac to the west, Municipality of Liloy east and Municipality of Kalawit to the south.

La Libertad is a 3rd class municipality in the province of Zamboanga del Norte and is politically divided into twenty (20) barangays namely: Kipit, Osukan, Imelda (Pob.), Gil Sanchez (Pob.), Atonino (Pob.), Lopoc (Pob.), Dansalan, La Union, Bobongan, Malintubuan, Ubay, San Isdro, Lapatan, Balas, New Salvacion, Immaculada, Patawag, Lawagan, Lawigan and Gabu.

The terrain within the area is generally typified by steep to moderately sloping to generally flat terrain towards the north-northwest direction. Based on the 1:50,000 topographic map of Liloy Quadrangle (sheet no. 3544 III) and Murcielagos Islands (sheet no. 3444 II), the most proximal topographic high in the area is Mount Dansalan with elevation of 747 meters above mean sea level. Dendritic drainage pattern is noted within the vicinity of Mount Dansalan.

The two major river systems bounding the municipality are Patawag River towards the east and Kipit River towards the western limit of the municipality. Other drainage lines directly draining the study area the Labason River, Osukan Creek, Bobongan Creek and Dansalan Creek. These drainage lines are propagating northward, draining towards the Patawag Bay.

INITIAL FINDINGS

1. Barangay Kipit

Kipit is the western-most barangay in the municipality of Labason. It has a total of seven (7) Puroks and is bounded by Sulu Sea to the north, municipality of Gutalac to the west, barangay New Salvacion to the south and barangay Osukan to the east. The area is drain by Kipit River.

Field obeservations:

  • The barangay is characterized by flat to gently rolling terrain toward the southern limit. Land area on the upland portion of the barangay is utilized into coconut plantation while even sections, including natural ravines are utilized into rice paddies. Flat segment of the barangay approaching the coastal area also have coconut plantations.
  • Meandering River of Kipit (northeast-trending) traverses the western limit of the barangay. This river is the primary contributor of flood within the area due to its large floodplain as it approaches near the river delta. Apparently, there are build-up areas alongside the waterway including Mosque.
  • Based on the MGB-IX 2012 geohazard map, the barangay fall under low to moderate landslide susceptibility rating. This is merely due to low lying hills on the southern section of the barangay. In terms of flooding, the barangay fall under moderate to high flood susceptibility rating. Though mitigating structure has already been installed (dike), alongside the Kipit River, intense monitoring upstream is still highly recommended during extreme weather disturbances.
(Looking north) A newly constucted dike, as mitigating stucture alongside Kipit River. During Typhoon Vinta in 2017, flood height reached up to this level. (as indicated by the red mark)

2. Barangay New Salvacion

New Salvacion is situated on the south-western section of the municipality. It has total of seven (7) Puroks and is bounded by barangay Kipit to the north, municipality of Gutalac to the west, barangay Immaculada to the south and barangay Balas to the east. The barangay is drain by intermittent creeks draining north and east which eventually drains toward Kipit River.

Field observations:

  • The barangay is characterized by rolling terrain on the central section and mountainous toward northeast. Overall, land area of the barangay is utilized into coconut plantation. The highest point of the barangay is a part of Purok 5.
  • Build-up area within the barangay concentrates on the south trending ridge including its barangay center and evacuation site. These houses are made-up mostly of light materials and some are on stilts. In addition, Amphibolites rocks are widespread within the area.The barangay is characterized by rolling terrain on the central section and mountainous toward northeast. Overall, land area of the barangay is utilized into coconut plantation. The highest point of the barangay is a part of Purok 5.
  • Based on the 2012 MGB-IX geohazard map, the central section of barangay fall under low landslide susceptibility rating and going moderate on the outer section. The above-mentioned point within Purok 5 is considered high susceptible to landslide. Monitoring of these areas is highly recommended during inclement weather disturbances.The barangay is characterized by rolling terrain on the central section and mountainous toward northeast. Overall, land area of the barangay is utilized into coconut plantation. The highest point of the barangay is a part of Purok 5.
  • Build-up area within the barangay concentrates on the south trending ridge including its barangay center and evacuation site. These houses are made-up mostly of light materials and some are on stilts. In addition, Amphibolites rocks are widespread within the area.The barangay is characterized by rolling terrain on the central section and mountainous toward northeast. Overall, land area of the barangay is utilized into coconut plantation. The highest point of the barangay is a part of Purok 5.
An abandoned nipa hut within the steep slopes near the barangay center. Neighbouring houses on the opposite side has the same situation.

3. Barangay Immaculada

Immaculada is the south-westernmost barangay in the municipality of Labason It has a total of four (4) Puroks and is bounded by barangay New Salvacion to the north, municipality of Gutalac to the west and barangay Balas to the east. The northwest trending Immaculada creek drain on the northern section relative to the barangay center.

Field Observations:

  • Overall, the barangay is situated within a rolling topography where built up area concentrates within the ridge. It is surrounded by steeply sloping hills that plunges toward the natural valley to the western and northern section.
  • Based on the actual observation, on stilt houses made up of light materials, dwells alongside the ridge where pigpens are also present. These acts can lead to water seepage and eventually triggers mass movement.Overall, the barangay is situated within a rolling topography where built up area concentrates within the ridge. It is surrounded by steeply sloping hills that plunges toward the natural valley to the western and northern section.
  • Based on the 2012 MGB-IX geohazard map, the barangay fall under moderate landslide susceptibility due to its rolling terrain. Meanwhile, northern and eastern sections of the barangay are categorized of having high landslide susceptibility merely due to its critical slope angles, unconsolidated soil types and other factors. Proper mitigating structures, adequate drainage systems and active monitoring of the critical areas are highly recommended.
Stilt houses made up of light materials are situated along the ridge where pigpens are also present. Improper drainage from this setting can lead to water seepage triggering mass movement.

4. Barangay Balas

Barangay Balas is located in the south-western section of the municipality. It has a total of eight (8) Puroks namely Puroks Mahayag, Matin-ao, Limon, Jay, Tinago, Masukod, Sibulan and Bitoon. The area is bounded by barangay Osukan to the north, Municipality of Kalawit to the south, barangay New Salvacion to the west and barangay Gabu to the east. The barangay is drain by northeast and northwest trending intermittent creeks.

Field Observations:

  • The area is typified rolling terrain on the central and going mountainous toward southwest.
  • Several minor creeks are found within the barangay. These west-northwest trending fluvial waters pass through spillways belonging to the barangay road. Anecdotal account claims that inundation could take at least an hour. Water level monitoring tool and warning signage’s within the spillways are highly recommended.The area is typified rolling terrain on the central and going mountainous toward southwest.
  • Based on ground verification and filed observation, Sibulan Paaralan ng Buhay of Sitio Sibulan is critical to flash flood. A developing creek on the upper and back section of Grades 3 and 4 buildings has presence of boulder-sized amphibolites. During Typhoon Vinta in 2017, the above mentioned classrooms were highly affected. MGB-IX personnel already issued threat advisory.
  • Generally, the barangay fall under moderate to high landslide susceptibility rating. This is due to the abrupt change of slope angle especially in southern and south-eastern portion of the barangay. Constant monitoring of these areas is during inclement weather disturbances is highly recommended. The area is typified rolling terrain on the central and going mountainous toward southwest.
  • Several minor creeks are found within the barangay. These west-northwest trending fluvial waters pass through spillways belonging to the barangay road. Anecdotal account claims that inundation could take at least an hour. Water level monitoring tool and warning signage’s within the spillways are highly recommended.The area is typified rolling terrain on the central and going mountainous toward southwest.
  • Based on ground verification and filed observation, Sibulan Paaralan ng Buhay of Sitio Sibulan is critical to flash flood. A developing creek on the upper and back section of Grades 3 and 4 buildings has presence of boulder-sized amphibolites. During Typhoon Vinta in 2017, the above mentioned classrooms were highly affected. MGB-IX personnel already issued threat advisory.
Developing gully on the backside of Grades 3 and 4 building in Sibulan Paaralan ng Buhay. This segment of the school is critical to flash flood.

5. Barangay Lawigan

Lawigan is located on the central section of the municipality of Labason. It has a total of four (4) Puroks namely Puroks Narra, Ipil-Ipil, Mahogany and Tugas. It is bounded by barangay Imelda (Poblacion) to the north, barangays Gabu and Balas to the south and barangay La Union to the east. The area is drain by intermittent creeks and headwaters of Osukan River.

Field Observations:

  • The area topography is typified flat to gently rolling in the central section while slightly steep on the north-eastern section near the barangay Imelda.
  • The major river of Osukan drains the north-eastern limit of the barangay (headwater of Osukan Creek) toward barangay Imelda. North and south of the barangay center has presence of northeast trending intermittent creeks that eventually drain toward Osukan River.
  • Based on the 2012 MGB-IX geohazard map, the barangay fall under low to moderate landslide susceptibility due to its flat to rolling terrain. This area is one of the identified evacuation site in the municipality. In fact, the newly donated evacuation center of the Office of Civil Defense (OCD-IX) is constructed in this area. Adequate drainage system alongside the barangay road is recommended for easy access during evacuation.
The handover of landslide, sinkhole and flood posters to the barangay officials of Lawigan, Labason, Zamboanga del Norte.

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