RESULT OF RAPID COASTAL EROSION ASSESSMENT IN MUNICIPALITY OF SAGAY, CAMIGUIN
On November 18-November 27, two typhoons namely, Lando and Mina (internationally coded as Hagibis and Mitag respectively) hit the country with heavy downpour and strong winds. The strong winds and low pressure associated with these typhoons pushed the ocean’s surface to the level higher than ordinary sea level and caused storm surges along most coastal areas of the Philippine Islands.
Storm surges occurred along coastal barangays in Pangasinan, the Zamboanga Peninsula, Northern Mindanao and the Autonomous Region in Muslim Mindanao last November 27 at 7 pm resulting to coastal erosions, floods and damage to properties and deaths. According to PAG-ASA, the big waves that hit these towns were due to the strong winds triggered by the northeast monsoon and the two typhoons.
The two typhoons that hit the Philippine
How does a storm surge occur?
A storm surge or storm tide is the result of piling up of water on the oceans surface as it is pushed by strong winds associated with a low pressure weather system such as tropical cyclones. The water during storm surge is higher than the ordinary sea level, high tide or low tide. However, flooding and coastal erosion due to storm surge are huge when it occurs during high tide. The figure below illustrates a typical scenario during a storm surge.
In computing for the storm surge height, it is important to note of the normal high or low tide, mean sea level and the actual height of the wave. The difference between the height of the storm tide and the normal tide is the surge height. The height of storm surge caused by landfalling tropical cyclones (or typhoons in the case of the Philippines) is a function of a number of factors such as speed, intensity, size of the radius of maximum winds (RMW), radius of the wind fields, angle of the track relative to the coastline, the physical characteristics of the coastline, and the bathymetry of the water offshore.
For the purpose of this blog, these factors shall be lightly discussed in an attempt to explain how the storm surge caused erosion and damage to properties along the coastal barangays of Sagay, Camiguin.
The Study Area
The island of Camiguin is located 10 Kilometers off the coast of Misamis Oriental, Northern Mindanao. It can be reached by taking a bus to Balingoan Port in Balingoan, Misamis Oriental and from there by taking a barge that ply to Benoni Port in Guinsiliban, Camiguin. Coastal barangays can be reached through the circumferential road around the island.
Sagay is a 5th class municipality in the Province of Camiguin. It is politically divided into nine barangays namely, Alangilan, Bacnit, Balite, Bonbon, Bugang, Cuna, Manuyog, Mayana and Poblacion. The Municipality of Sagay is bounded on the east by Bohol Sea, on the west by Guinsiliban and Mahinog, on the north by Catarman and on the south by Bohol Sea. Specifically, the coastal barangays of Sagay are approximately bounded by the coordinates North 9° 05 12’, East 124° 45’ 26’’ and North 9° 06’ 37’’, East 124° 42’ 07’’.
Based on the Corona Climatic Classification, the Island of Camiguin is under type II which means that it has no pronounced dry season but has very pronounced rainy season from November to January. Camiguin has an annual temperature of 23.64 degrees centigrade with the lowest temperature averaging at 23.41 degrees centigrade between April and August. The average humidity of the island is 76.77% all-year round. Rainfall distribution of the province indicates that the northeastern part of the island receives an average of 2,300 mm of rainfall, due to the northeast monsoons. The western part of the island receives a slightly lower rainfall of 2,000 mm. Typhoons coming from southeast usually occur between the months of October and January.
Camiguin is a volcanic island whose geology and topography is a product of activities of its seven volcanoes, wave action along the coast and constant erosion by rainfall and wind action. The island is the northern extension of the Misamis Peninsula and is part of the Northern Mindanao earthquake epicenter. The highest peak in the island is Mt. Timpoong which rises to about 1, 580 masl.
In general, the topography of the island can be divided into two sections: 1) the steep volcanic terrains in the interior middle of the island with highland plateaus and 2) the narrow and flat lowland terrains along its circumference. There are two islets that surround Camiguin namely, White Sand and Mantigue. Mantigue Islet which is fringed by white sand and coral reefs is located offshore of Mahinog. White island is technically a sand bar that is likewise fringed with coral formations.
The Municipality of Sagay is underlain by three types of rocks: Lava Flow deposits, Airfall Pyroclastics and Recent Alluvium. Outcrops of the lava flow deposits and airfall pyroclastics can be observed along the road from Barangay Liong to Bugang, Sagay. The said deposits were observed to be highly altered to orange-brown soil. In most parts, parent rocks are relatively unaltered or sometimes occur in combination with soil which often forms scree on the foot slopes.
Scree deposits that originated from these rock units and recent alluvium underlie most of the coastal barangays of Sagay. The alluvium deposits along the beach were noted to be made up of well-rounded cobbles, pebbles and very coarse sand in decreasing amount respectively. The coarse-sized nature of the beach deposits usually indicates high energy environment and therefore suggests that strong wave or current action usually hit the coastline.
Based on the Geohazard Susceptibility mapping done by previous workers from the Mines and Geosciences Bureau-10, Sagay is susceptible to at least three types of geohazards: Landslide, Floods and Mudflow. Occurrences of tsunami or seismic waves on the island were also reported by PHIVOLCS and the locals. In February 8, 1990, movement along Alicia Thrust Fault in Bohol resulted to magnitude 6.0 earthquake which triggered tsunami or seismic waves that inundated five towns on the southeast part of Bohol. The said event triggered anomalous big waves along Camiguin which is about 50 Kilometers southeast of Bohol. This event would indicate that Camiguin Island is also susceptible to tsunami whenever there are strong tsunamigenic earthquakes along its vicinities.
The team which is composed of Ms. Maria Gracia Collantes and Ms. Joy Christine V. Asis, geologists from the Mines and Geosciences Bureau Regional Office 10 set out to Sagay, Camiguin to conduct a rapid assessment of the coastal erosion and damage due to storm surge. Each coastal barangay was assessed in terms of the area inundated by coastal flood, number of houses and other structures damaged by storm surge, area affected by erosion and the physical characteristic of the coastline. Locations were taken using a GPS and plotted on a digital map. Pictures were also taken to document the assessment and the damage caused by the storm surge.
Results and Discussions
Occurrence of storm surges is quite common in the Philippines considering its geographic location, proximity to large bodies of water and its archipelagic nature. The Philippine Islands lie within the typhoon belt along western pacific such that about 19 typhoons strike the country every year. The different bathymetries on its more than 7,000 islands become an important factor as to the degree of damage and devastation caused by storm surges. When assessing storm surges due to tropical cyclones, it is important to note of the typhoon’s speed, intensity, size of the radius of maximum winds (RMW), radius of the wind fields, angle of the track relative to the coastline, the physical characteristics of the coastline, and the bathymetry of the water offshore. Since the team was not able to collate enough data on the two typhoons Lando and Mina, this discussion will focus primarily on what transpired during the two typhoon events, on the physical characteristics of the coastline as observed during the assessment and secondary data on bathymetry of Camiguin if available.
It should be noted that when Lando left Philippines Islands towards South China Sea, its maximum sustained winds increased to about 95 Kilometers per hour. While Lando was getting strong, Typhoon Mina (at 140 Kilometers per hour near the center and gusts of up to 170 Kilometers per hour) was devastating portions of northern Luzon on Sunday afternoon, November 25, 2007. However, the devastation was further intensified when Mina “pulled” Lando by doing a dramatic U-turn from the South China Sea and landfalling on Palawan Island. This phenomenon where two tropical cyclones interact with one another is known as the Fujiwhara Effect. This interaction of Mina and Lando and the northeast monsoon resulted to strong winds that caused surface of the sea to pile up and hit coastal barangays as storm surges.
The storm surges in Sagay resulted to damage in properties and seawalls, partially to totally uprooted trees and brought debris towards the road. It is noteworthy that the coast of Sagay is narrow and dominantly lined with well-rounded cobbles. In some parts of Sagay, sea stacks were likewise noted. These two observations indicate that the coast of Sagay is often hit by strong waves and therefore is very prone to erosion. Strong winds coming from southwest and northwest will expectedly result to strong wave currents.
Coastal defense or management is seemingly the best way to mitigate the effects of anomalous big waves such as tsunami and storm surges. An appropriate and well-designed coastal defense will reduce coastal erosion and flooding. The following methods are the components of coastal management:
1) Abandon Coastal Erosion and Flood Prone Areas
This method involves desertion of houses and other properties and let the coast take care of itself. It is environment-friendly and probably the cheapest way to allay the damage of coastal erosion and flood which is by taking people and their properties away from hazard-prone areas. This kind of set-up however may also result to loss of livelihood and money from tourism. In addition, finding a resettlement area is quite difficult in the case of Sagay considering that there are portions within the area that are prone to other geologic hazards such as mudflow and landslide. So in considering for a resettlement area, these hazards should also be taken into account.
2) Construction of Structures for Defense Against Wave Action
There are two types of structures that can be used to reduce the power of wave action: the Hard Construction Techniques and the Soft Construction Techniques. The list below includes all the structures that seem appropriate in the case of Sagay.
A. Hard Construction Techniques
Groynes are any concrete or wooden barriers or walls constructed at right angle to the shoreline. This structure can interrupt or decrease wave energy associated with longshore drift coming at an angle to the coast of Sagay. When longshore drift approaches a groyne structure,it loses energy and deposits sand materials on the updrift side. Once the updrift side has accumulated enough sand wave energy can be further absorbed and thus reduce erosion on portions of the coast protected by the groyne. However, by constructing groyne the normal erosion and deposition process along the coast is interrupted. In the downdrift side, no sand materials are deposited such that there is an imbalance in the depths of water on each side of groynes. In addition, construction of groynes may be expensive as they are usually built in pairs. Groynes can also affect the aesthetics of the coast.
A seawall is a form of hard coastal defense constructed parallel to the shore and usually built on the inland part of the coast. A well-designed seawall is effective in reducing the effects of strong waves. In constructing seawalls various materials are used such as steel or wire cages filled with pebbles (gabions), reinforced concrete and boulders. Seawalls were observed along portions of Sagay coast. However, in Barangay _______ the seawall was not able to withstand the storm surge that occurred last November 27, 2007. This was probably due to the poor design of the seawall. Seawalls should be monitored all the time as their bases are often attacked by onrushing waves. Seawalls in Sagay can be improved by increasing their heights and using curved or stepped seawalls. Steps and curves on seawalls increase its capability of deflecting and dissipating waves. A pile of boulders can also be added along the base of the seawalls to protect their base from constant attack of waves.
· Rock Armor
To prevent cliffs from eroding and forming sea stacks, a pile of boulders naturally found along the coast of Sagay should be put along their foot slopes to protect them from wave action.
In constructing gabions, boulders and cobbles of rocks are wired into mesh cages and usually put along the coast most vulnerable to erosion or built perpendicular to the coast similar to groynes. When the seawater breaks on the gabion, the water drains through leaving sediments, also the rocks absorb a moderate amount of the wave energy. Abundant amount of these cobbles and boulders can be found along the coast of Sagay. Therefore, designing and building appropriate gabions appear to be a cost-effective method in reducing coastal erosion in the area.
· Offshore Breakwater
This is probably one of the costliest structure that could be built offshore of Sagay. In constructing breakwater, enormous concrete and rock boulders are sunk offshore to filter the wave energies. Waves break further offshore and therefore erosive power is reduced.
B. Soft Construction Techniques
· Beach Nourishment
This technique is done by incorporating or introducing more sand into the beach. Sand is an effective absorber of wave energy such that if a beach line is predominantly sand, coastal erosion is less. The type of sand to be used should be similar to the pre-existing sand in affected areas in Sagay. In addition, the source of sand should be proximal to the affected areas so that the cost of transportation is minimal. However, this technique is only effective if used on the beach where sand is the dominant material. Adding sand to cobble-dominated beach lines is costly as the volume of sand to be added on these should be enormous to serve the purpose of protecting the coast from erosion. Furthermore, sand materials do not naturally adhere to cobbles and boulders being of different properties so that during storm surge these are easily washed away which defeats its purpose.
In areas where the beach is composed of silt to sand, planting mangroves can be done. When these mangroves are mature enough, their roots and trunks can help protect the coast from erosion. In addition, mangroves are important habitat for young fishes and crustaceans so that not only are coastal erosion reduced but marine environment is also enhanced. However, this technique will expectedly take time before the outcome is experienced.
Besana, G. M., Daligdig, J.A., Abigania, M.T., Talisic, J.E. Insights on the 1990 Bohol Tsunamigenic Earthquake, Bohol Island, Philippines (internet)
The authors would like to extend their warmest gratitude to PENRO Gaudioso B. Malaton and his staffs for accommodating and guiding us during the assessment, and for introducing us to barangay officials and locals. We would like to thank also the locals for having spent some time during our interviews. Finally, thanks to Mr. Abner Padrique, geologist of Crewgold Philippines, for sharing his technical knowledge during the assessment.
Photos Taken During the Field Assessment of Sagay, Camiguin