GEOHAZARD IDENTIFICATION REPORT OF THE PROPOSED RELOCATION SITES IN BOSTON, DAVAO ORIENTAL

INTRODUCTION Recent natural disasters caused by geologic hazards have been frequently occurring bringing huge damage and enormous number of fatalities. As a step in adapting to climate change, understanding the characteristics and behavior of the area’s geologic and climatological setting gives important information to engineers in designing appropriate structures and projects. In view of this, the government mandated all developments to undergo geological assessment to address the problems involving geohazards.

In response to the request of Hon. Rebecco B. Rosit Sr., municipal mayor of Boston, geological site assessments were conducted on the proposed relocation sites at Barangays Cawayanan (So. Proper, Boundary, Baclinan), Carmen, Cabasagan, and Poblacion. LOCATION AND ACCESSIBILITY Municipality of Boston, Davao Oriental is located on the eastern seaboard of Mindanao Island approximately centered at geographic coordinates of 7°52'9.26"N, 126°22'31.58"E. It is the northernmost municipality of the Province of Davao Oriental which is about 150 km north of Mati City. It is accessible by any means of transportation through either routes at Surigao-Davao Coastal Road or Compostela-Cateel Road. It can be reached in about 3-4 hours vehicle ride from Mati City and a 5-6 hours from Davao City.

Figure 1 Location of Municipality of Boston showing Compostela-Davao Oriental route (red line) and Surigao-

Davao Coastal route (blue line)

Proposed relocation sites are within the jurisdiction of Municipality of Boston,

which are accessible by a 4-wheel vehicle through the unpaved barangay roads in about 5-20 minutes depending on their distance from the Municipal Hall. Location of the proposed relocation sites are listed below:

Table 1 Geographical coordinates of the proposed relocation sites as plotted in Google Earth

Figure 2 Location of the proposed relocation sites showing the access roads from the Municipal Hall of Boston

CLIMATE AND VEGETATION

Boston falls under Type IV climate based on the Modified Corona’s Classification Map of the Philippines characterized by more or less even distribution of rainfall throughout the year. According to the Koppen-Geiger Climate Classification, Boston is situated in a tropical rainforest (Af) identified as having no dry season and at least 60 mm average rainfall per month. Places having this type of climate are dominated by the Intertropical Convergence Zone. Average annual temperature in Boston is 26.7°C and average annual rainfall reaches to about 3980 mm.

Latitude Longitude

Cawayanan Boundary 7°53'30.72"N 126°21'32.30"E 11.06

Cawayanan Proper 7°53'54.61"N 126°20'36.04"E 12.19

Cawayanan Baclinan 7°53'1.44"N 126°21'21.97"E 9.82

Carmen 7°52'2.29"N 126°20'57.73"E 5

Poblacion 7°51'51.19"N 126°22'54.21"E 1.1

Cabasagan 7°50'23.89"N 126°22'8.59"E 4.26

Coordinates (Google Earth)

Barangay Sitio

Approx.

Distance from

Municipal Hall

(km)

Figure 3 Climate Map of the Philippines (Modified Coronas Classification)

Figure 4 Climate Map of the Philippines (Koeppen-Geiger Classification)

Figure 5 Climate Graph of Boston, Davao Oriental

Table 2 Climate Data of Boston, Davao Oriental

Figure 6 Temperature Graph of Boston, Davao Oriental

Vegetative covers on the proposed areas consist of sparse amount of coconut and secondary grown trees with scattered distribution of grasses.

Figure 7 Coconut trees left standing after Typhoon Pablo at the proposed relocation site in So. Boundary, Brgy.

Cawayanan

Figure 8 Grasses and few coconut trees starting to grow again at the proposed relocation site in So. Proper,

Brgy. Cawayanan

Figure 9 Cultivated portion of the proposed relocation site in So. Baclinan, Brgy. Cawayanan w/ few coconut

trees

Figure 10 Few coconut trees left standing in Brgy. Carmen relocation site

Figure 11 Revegetated area at the proposed relocation site in Brgy. Poblacion showing some coconut trees left

standing after Typhoon Pablo

Figure 22 Logged area at the proposed relocation site in Brgy. Cabasagan after coconut trees were swept by

Typhoon Pablo

TOPOGRAPHY AND DRAINAGE

Municipality of Boston, Davao del Oriental is characterized by variable topography ranging from flat low-lying to steep mountainous areas. Major drainage systems such as the Dacongbanwa River, Baclinan River, and Cabasagan River drain eastward to the Philippine Sea. Detailed geophysical setting of the proposed relocation sites are discussed below: So. Boundary, Brgy. Cawayanan Characterized by a moderately sloping denudational hill with high erosion rate evident on mass movements and gullies. Surface runoff drains downslope towards Tigbawan Alluvial Plain, Baclinan Alluvial Plain, and the Philippine Sea.

Figure 33 Moderately sloped hill at So. Boundary relocation site

So. Baclinan, Brgy. Cawayanan

Generally gently sloping denudational hill bounded by Baclinan Alluvial Plain from north to south except on the western portion towards the Maharlika Highway which grades to moderately dipping slopes.

Figure 44 Gently sloping hill of So. Baclinan relocation site viewing the Dacongbanwa Aalluvial Plain at its

background

So. Proper, Brgy. Cawayanan

Flat-topped mountain with steep denudational slopes and well-defined gullies and ravines. Surface runoff drains towards Baclinan River in an eastward direction and minor tributaries on its west. Runoff also drains towards So. Tigbawan which is an accumulation zone of debris from So. Proper Cawayanan.

Figure 55 Flat-topped mountain characterizes the topography of So. Proper, Brgy Cawayanan relocation site

with steep to moderate slopes

Brgy. Carmen

Gently to moderately sloping denudational hill bounded by the Dacongbanwa Alluvial Plain from north to south except on the southwestern portion which is moderately dipping slopes. Surface runoff mostly drains eastward to Dacongbanwa River.

Figure 66 Gently to moderately sloping hill at Brgy. Carmen relocation site showing the Maharlika Highway

Brgy. Poblacion

Characterized by a moderately dipping denudational slopes with cliffy portions along the coastline. Surface runoff drains eastward through gullies to the Philippine Sea. Wetlands were also observed at the northern portion catching water and sediments before draining towards the open sea.

Figure 77 Gently to moderately sloping hill of Brgy. Poblacion relocation site showing the swamp at the

foreground which was excluded on the proposal

Brgy. Cabasagan

Part of the Cabasagan Alluvial Plain which is flat bounded by the Cabasagan River to the north, where most of the water and sediments drain, and the mountainous areas of Boston-Cateel boundary. The proposed site is also bounded by creeks at its east and west which both drain towards Cabasagan River.

Figure 18 gently undulating terrain at the Brgy. Cabasagan relocation site

GEOLOGY

The island of Mindanao is a result of complex tectonic processes mostly influenced by large faults and trenches such as the Sulu Trench, Cotabato Trench, Philippine Trench, and the Philippine Fault Zone. The study area is located on the central portion of the Eastern Mindanao Cordillera situated east of the Philippine Fault Zone and west of the Philippine Trench. Features along these mountain ranges are strongly influenced by tectonic and volcanic activities generated by the said major geologic structures. Stratigraphy

Municipality of Boston is underlain by eight formations emplaced from Cretaceous to Recent Age based on the studies conducted by this office and other agencies but only six were covered on the study area (refer to Geologic Map).

Agtuuganon Formation- MMAJ-JICA (1973) described this formation from the thick coralline limestone occupying Mt. Agtuuganon in Compostela Valley. It consists of a lower bedded portion and a massive limestone member. This conformably overlies the Dacongbanwa Formation and described as young limestone. Alluvium- unconsolidated fluvial, alluvial, coastal, and flood sediments of quaternary age. Barcelona Formation/Barcelona Group- volcanic and sedimentary suite in the Bislig-Lingig coastal area consisting of basalt flows with intercalated agglomerates, breccias, and highly indurated clastic sedimentary rocks. Pillow and columnar

structures are also noted on basalts and described as a basement rock of Cretaceous-Paleogene in age. Bislig Formation- a sequence of conglomerate, sandstone, shale, limestone, pyroclastic rocks, and basaltic lava flows which conformably overlies the Mangagoy Formation. This is also exposed in headwaters of Panusugon and Cateel rivers. Its age ranges from Late Oligocene-Early Miocene. Dacongbanwa Formation- used by MMAJ-JICA (1973) to describe the massive Middle Miocene coralline limestone at the northwestern slope of Mount Agtuuganon. This unconformably overlies the Bislig Formation. Kaban Formation- described as andesitic rocks intercalated with clastics and formed by three igneous cycles in different sedimentary basins. Mangagoy Formation- conformably overlies the Kaban and Barcelona Formations consisting of volcanic conglomerate, siltstone, mudstone, and an upper part of thick limestone. This was dated Late Oligocene probably corresponding to the Mabuhay Formation in the northern portion of the Mindanao Pacific Cordillera. Soil Layer Soil regolith on the proposed relocation site in So. Boundary is mostly composed of wet clay and can easily be penetrated by a force of finger. On the other hand, clay soil layers in So. Baclinan, So. Proper Cawayanan and Brgy. Carmen sites are mostly ferruginous and stiff. Overburden in Brgy. Poblacion proposed relocation site is mostly organic clay and is thinner compared to other sites. In Brgy. Cabasagan, soil are mostly unconsolidated silt, sand, and clay with minor distribution of gravel.

Figure 19 Wet clay soil layer at So. Boundary relocation site

Figure 80 Stiff clay soil layer at Brgy. Carmen relocation site

ASSESSED GEOHAZARDS

Mass Movement So. Boundary, Brgy. Cawayanan Proposed Relocation Site Based on the 1:10,000 Scale Geohazard Mapping and Assessment conducted by this office, the area is delineated as highly susceptible to landslide due to steep denudational slopes and thick soil layer. Active soil erosion along slopes and gullies were also noted wherein soil regolith are mostly wet clay. During the field verification, west portion of the proposed site along the roadcuts is an active landslide area which is also delineated as a critical area to mass movements. Eastern portion of the site adjacent to the sanitary landfill is generally moderate denudational slopes rated as moderately susceptible to landslide. So. Baclinan, Brgy. Cawayanan Proposed Relocation Site The area is rated as moderately susceptible to landslide to it’s moderately to gently sloping terrain. So. Proper, Brgy. Cawayanan Proposed Relocation Site The proposed site situated on a flat-topped mountain within the vicinity of the barangay site of Cawayan Proper is rated as moderately susceptible to landslide however north of the site is delineated as critical due to observed mass movements.

Brgy. Carmen Proposed Relocation Site The area lies on a gently to moderately sloping denudational hill which is delineated as moderately susceptible to landslide. Brgy. Poblacion Relocation Site The area is delineated as moderately susceptible to landslide due to its moderately dipping slopes and absence of drainage systems although minor gullies were observed. Wave-cut erosion on cliffs is less but moderate on Tagoan Point due to well indurated rocks. Brgy. Cabasagan Relocation Site

The site is situated in a low-lying area particularly on an alluvial plain and is delineated as having low susceptibility to landslide based on the 1:10K geohazard map. However during field verification, the area is observed to be a probable accumulation zone of debris flow materials due to presence of creeks both in east and west boundaries. Flood

So. Baclinan, Brgy. Cawayanan Proposed Relocation Site The area is not affected by flooding since it is elevated by about 10 meters relative to Baclinan River Floodplain to its north, east, and south boundaries. Brgy. Carmen Proposed Relocation Site

Flood is concentrated on the floodplain of Dacongbanwa River on the north,

east, and south boundaries of the proposed site. The area is elevated by about 20 meters relative to the floodplain. Brgy. Cabasagan Proposed Relocation Site Based on the 1:10K geohazard maps which was done prior to the LPA and Typhoon Agaton’s arrival, the area is not affected by flooding. During the field assessment after the severe weather, debris were noted along the creeks and flood deposits were also noted along the barangay road. Seismic Hazards Earthquake-generators such as the Philippine Fault Zone along the western flank of the Eastern Mindanao Arc and the Philippine Trench at the eastern flank located offshore at the Philippine Sea. Ground shaking will be felt from time to time on the proposed sites due to earthquakes generated by fault movements. Intensity may vary depending on the magnitude of the earthquake, distance from the epicenter and geology of the area which is greater in alluvial and floodplains such as in Cabasagan proposed relocation site and lesser on intact and indurated rocks such

as in Poblacion proposed relocation site. Earthquake-induced landslides are also common on areas delineated as critical and highly susceptible areas especially when there is heavy rainfall during the earthquake event. Lateral spreading may also occur on gently sloping terrain such as in So. Baclinan proposed relocation site. Liquefaction will be concentrated on areas with sandy sediments and shallow groundwater in alluvial, coastal, and floodplains such as in Cabasagan relocation site. Tsunamis and storm surges are concentrated along the coastal plains.

CONCLUSION AND RECOMMENDATION

Climatologic and geologic hazards contribute to the delimiting factors of the capability of land development in the Municipality of Boston. The following are recommended for the proposed relocation sites to wit;

Areas identified as high to critical areas to landslide and flood are strongly not recommended as relocation sites such as in So. Boundary and Proper of Brgy. Cawayanan.

Sanitary landfill in So. Boundary which is adjacent to the proposed site can also cause health problems for the settlers.

Proposed site in So. Proper Cawayanan can also be moved towards east instead of getting near the critical area and maintain an easement zone from the slope’s edge to avoid risks from landslide.

It is recommended that an easement from the low-lying area shall also be considered in So. Baclinan proposed site to avoid flooding and houses’ ground elevation shall be at least 5 meters higher relative to the floodplain’s elevation.

An easement from the slope’s edge shall also be considered in Brgys. Carmen and Poblacion proposed relocation sites.

Wetland at the north portion of the lot in Brgy. Poblacion proposed site shall not be utilized as a build zone.

It is also not recommended to push through the proposed site in Brgy. Cabasagan since it is identified as an accumulation zone of debris during flood and landslide events.

Below is the summary of the results of geohazard investigation for the

following proposed relocation sites.

Barangay Sitio Relocation Site Suitability

Cawayanan Boundary Not Suitable

Cawayanan Proper Suitable but an easement from the slope should be considered and development should be towards east.

Cawayanan Baclinan Suitable

Carmen Suitable but an easement from the slope should be considered. Easement from the floodplain should also be implemented.

Poblacion Suitable but an easement from the slope should be considered. Wetland portion should not be utilized for housing projects.

Cabasagan Not Suitable

Prepared By: Capter John M. Tubo Geologist