1:10,000 Scale Coastal Vulnerability Assessment and Mapping in the Municipalities of Salug and Leon B. Postigo, Zamboanga del Norte

By Geohazard and Engineering Geology Section

The Philippines has one hundred forty-six (146) cities and one thousand four hundred eighty-eight (1,488) municipalities. Fifteen (15) of the former, and eight hundred thirty-two (832) of the latter line the country’s coast. About sixty (60) percent of the Philippine population inhabit these cities and municipalities, and approximately fifty million Filipinos, which is almost half of the country’s population, live in the low-lying coastal areas. The large coastal population, dense structural build-up, and diverse coastal livelihoods entail a towering number of elements exposed and vulnerable to climate change-aggravated coastal geohazards such as sea-level rise, coastal inundation, storm or monsoon surges, and beach scouring and erosion.

The 1:10,000-scale Coastal Vulnerability Assessment and Mapping (CVAM) is a Department of Environment and Natural Resources-Mines and Geosciences Bureau (DENR-MGB) study in line with the government’s effort at reducing, if not mitigating, the destructive effects and impacts of natural coastal geohazards to the populace. It aims to identify the coast’s physical vulnerability–its degree of incapability to cope with the consequences of climate change, climate extremes, and accelerated sea-level rises, such as coastal erosion, subsidence, and flooding–through the expert assessment of its relevant physical characteristics.

To identify the physical vulnerability of the study area, the CVAM utilized a modified version of the Coastal Vulnerability Index (CVI), first devised by Gornitz and Kanciruk in 1989, coupled with the Analytical Hierarchy Process. The method discretizes the coastline into various segments which are assigned ranking values based on the different physical parameters evaluated–Geomorphology, Shoreline change, Coastal alignment relative to the dominant wind direction, Mean tidal range, Coastal slope, Presence or absence of natural buffers, and the Presence of hard engineering structures. It is an effective approach that integrates diverse relevant parameters for the vulnerability assessment.

The Municipalities of Salug and Leon B. Postigo in the Province of Zamboanga del Norte are the target municipalities of the Coastal Vulnerability Assessment and Mapping for the month of February 2021. The CVAM Team consists of Supervising Geologist Paul Asthor D. Yecyec, Senior Geologists Abbygail B. Soco, and Ollyn Rey T. Balignot, and Geologist II Merill Jan T. Balignot conveyed and assisted by Administrative Aide III Donald F. Cenabre.

INITIAL FINDINGS

Municipality of Salug, Zamboanga del Norte

The Municipality of Salug is in the central part of the Zamboanga del Norte Province. The Municipality of Leon B. Postigo bound it in the east, the Municipality of Liloy and Godod in the west and south, and the Sulu Sea in the north. Salug lies along the regional road linking coastal towns and cities of Zamboanga del Norte. It generally has an undulating terrain, with flat to gently sloping coastal and fluvial plains in the north, and hilly to mountainous terrain in the south. Its north-northwest-facing coastline is about fourteen aerial kilometers long and consists of beach-lined rocky cliffs and bluffs in its central section, and wide flat low-lying sandy beaches and coastal plains in its western and eastern portions.

The municipality has twenty-three (23) barangays and eight of which line its coast. The coastal barangays of Mucas, Poblacion, Poblacion East, Caracol, Canawan, Bacong, Lipakan, and Ramon Magsaysay have a total of sixteen (16) coastal puroks all targeted for the assessment due to their susceptibility to coastal hazards (Table 1).

Previous Coastal Geohazard Assessment and Mapping completed in the municipality in 2018 showed that both coastal erosion and accretion are not very drastic along the shore of its coastal barangays. The former is exemplified by backshore scouring along coastal flats and rockfall along steep cliffs, and the latter is typified by minor sand deposition on beaches. This study compared shoreline data from the 1956 NAMRIA topographic map with the acquired shoreline data from the 2018 GPS shoreline tracking and Google Earth Landsat imagery and showed minor shifts of shoreline within the span of over sixty (60) years, with erosion and accretion rates of less than three meter per year. Though coastal erosion and accretion are not severe in the coastal barangays, communities along the coast are still susceptible to coastal flooding caused by storm and monsoon surges, floodwaters emanating from the river systems, and surface runoff as most of the coastal settlements are situated in low-lying areas.

In the current study, the team utilized the modified Coastal Vulnerability Index (CVI) and Analytical Hierarchy Process (AHP), which consider the municipality’s coastal geomorphology, shoreline shift, coastal slope, mean tidal range, orientation relative to the dominant wind direction, natural buffers, and hard engineering structures, to determine its coast’s physical vulnerability.

During the assessment, the team tracked the municipality’s shoreline to gather updated shoreline trend data for comparison with available ones from the previous 2018 study and 2020 and 2013 satellite images. Beach profiling was also done along the beach-lined areas of the coastal barangays to acquire data on the coastal slope. The team established a total of eighteen (18) profile lines along the municipality’s coast.

Table 1. List of coastal barangays and puroks of the Municipality of Manukan, Zamboanga del Norte.
Figure 1. Location and relief map of Salug’s coastal barangays and puroks.

Salug Beach Profile Data

The CVAM Team conducted beach profiling and established a total of eighteen profile lines along the beach-lined areas of the municipality.

Table 2. A list of beach profile points established along the coast of the Municipality of Salug, Zamboanga del Norte.

Municipality of Leon B. Postigo, Zamboanga del Norte

The Municipality of Leon B. Postigo is also in the central portion of the Zamboanga del Norte Province. The Municipality of Bayog, Zamboanga del Sur bounds it in the south, the municipalities of Sindangan and Siayan in the northeast and southwest, the municipalities of Salug and Godod in the northwest and southwest, and the Sulu Sea in the north. The Ipil-Dipolog Highway, which links coastal towns in the province, traverses the northern part of the municipality and provides access to its coastal barangays. It generally has an undulating terrain, with flat to gently sloping coastal and fluvial plains in the north, and hilly to mountainous terrain in the south. Its northwest-facing coastline is about twelve aerial kilometers long and consists of beach-lined rocky cliffs and bluffs in the west, and wide flat low-lying sandy beaches and coastal plains in the east. During the assessment, the team observed that the municipality’s beach sediments grade in size from boulder and cobble in the west to gravel and sand in the east.

The municipality has eighteen (18) barangays and six of which line its coast. The coastal barangays of Palandok, Bogabongan, Manil, Delusom, Poblacion, and Talinga have a total of twenty-five (25) coastal puroks all targeted for the assessment due to their susceptibility to coastal hazards (Table 3).

Previous Coastal Geohazard Assessment and Mapping done in the municipality in 2018 showed that both coastal erosion and accretion are not very drastic along the shore of its coastal barangays. The former exemplified by backshore scouring along coastal flats and rock fall along steep cliffs, and the latter typified by minor sand deposition in beaches. This study compared shoreline data from the 1956 NAMRIA topographic map with the acquired shoreline data from the 2018 GPS shoreline tracking and Google Earth Landsat imagery, and showed minor shifts of shoreline within the span of over sixty (60) years, with erosion and accretion rates of less than three meter per year. Though coastal erosion and accretion are not severe in the coastal barangays, communities along the coast are still susceptible to coastal flooding caused by storm and monsoon surges, floodwaters emanating from the river systems, and surface runoff as most of the coastal settlements are situated in low-lying areas.

In the current study, the team utilized the modified Coastal Vulnerability Index (CVI) and Analytical Hierarchy Process (AHP), which consider the municipality’s coastal geomorphology, shoreline shift, coastal slope, mean tidal range, orientation relative to the dominant wind direction, natural buffers, and hard engineering structures, to determine its coast’s physical vulnerability.

During the assessment, the team tracked the municipality’s shoreline to gather an updated shoreline trend data for comparison with available ones from the previous 2018 study and 2020, and 2013 satellite images. Beach profiling was also done along the beach-lined areas of the coastal barangays to acquire data on the coastal slope. The team established a total of eighteen (18) profile lines along the municipality’s coast.

Table 3. List of coastal barangays and puroks of the Municipality of Leon B. Postigo, Zamboanga del Norte.
Figure 2. Location and relief map of Leon B. Postigo’s coastal barangays and puroks.

Leon B. Postigo Beach Profile Data

Table 4. List of beach profile lines established along the coast of the Municipality of Jose Dalman, Zamboanga del Norte.

PHOTO DOCUMENTATION

Municipality of Salug, Zamboanga del Norte

Barangay Mucas

Damaged houses and uprooted coconut trees in Purok 7, Mucas caused by monsoon surges of the Amihan Season that affected the barangay in the later part of January 2022.

Barangay Poblacion

Several houses on the beach of Purok 5, Poblacion. This coastal community was severely affected by flooding during Typhoon Vinta in December 2015.

Barangay Poblacion East

Coastal communities along the shore of Barangay Poblacion East. These houses are vulnerable to coastal erosion and strong, destructive wave action.

Barangay Caracol

Scouring of the backshore along the coast of Barangay Caracol. This may be attributed to strong wave action and coastal flooding.

Barangay Canawan

Low bluffs along the backshore of Purok Rosal. These are produced by strong waves that scour the shore.
A wave-cut platform in the foreshore of Purok Rosal produce from the gradual erosion of the Liloy Limestone exposure along the shore by wave action.

Barangay Bacong

Destructive waves continuously and gradually erode the rock unit exposed in the backshore of District 4, Bacong creating low cliffs with sea notches and producing sediments that compose the area’s beach.
District 4 community along the shore. These houses are vulnerable to destructive waves and coastal flooding.

Barangay Lipakan

Scouring in the flat to gently sloping beach of Purok 7.
Scouring along the coastal edge of moderately sloping hillsides in Purok 6.

Barangay Ramon Magsaysay

Houses in Purok 1, Ramon Magsaysay are vulnerable to destructive wave action. Though a short seawall line the area, it is already almost buried in sediments.

Municipality of Salug, Zamboanga del Norte

Barangay Palandok

Boulder beach along Sector 1, Palandok near the mouth of the Palandok River. The presence of large sized beach sediments along the shore indicates that the area is a high-energy environment.

Barangay Bogabongan

Hard engineering structures in Sectors 7 and 4. The groins in Sector 7, at the boundary of Bogabongan and Palandok, hamper the sediment migration along the beach causing accretion on the upwind side of the structure and erosion on the downwind side. The gabions in Sector 4, on the other hand, protect the eroding shore from further scouring caused by strong wave action. However, these structures are already damaged and are not very suitable for the environment since their iron wirings are easily corroded by saltwater.

Barangay Manil

Narrow boulder to cobble beach along Barangay Manil.

Barangay Delusom

Coastal houses community of Barangay Delusom. These houses are vulnerable to strong wave attacks and flooding.

Barangay Poblacion

The coastal community of Barangay Poblacion is mostly composed of light material houses that are vulnerable to strong wave attacks and coastal flooding.

Barangay Talinga

The backshore scouring in Barangay Talinga tilts and dislodges tress along the shore.

CLOSURE

The contents of this initial report are based on available historical data and data gathered from the field as of March 2022.

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