Saturday, November 27, 2021

Special Report: Green shield

Mangroves proven to slow storm surges, but in Cebu, wrong species planted

EXTREME weather conditions no longer just happen in the movies. They have become real.

The effects of powerful earthquakes in the country and other parts of the globe have heightened public awareness on disaster preparedness.

However, few have looked at how protecting the environment is a critical factor in protecting communities against these phenomena.

The 7.2 magnitude earthquake generated by the North Bohol Fault on October 15, 2013 caused the upliftment of at least four kilometers of shoreline from Barangay Punta Cruz in Maribojoc to Tangnan in the adjacent town of Loon, Bohol.

The phenomenon caused the death of healthy corals, seagrasses and organisms that consider these areas their habitat. Mangroves that are not supposed to remain dry for long are under threat.

Dr. Fernando Siringan of the University of the Philippines-Marine Science Institute said that in Punta Cruz in Maribojoc, the emergent portion stretches by as much as 50 meters from the original coastline.

The tidal flat rose during the earthquake, depriving seagrass of water and killing it. The water drops at the edge of this new beach.

“The water goes 1.5 meters deep even in low tide then drops to five to six meters. It would take time for the seagrass to recover (considering this depth). This has implications on fisheries,” Siringan told Sun.Star Cebu.

The same changes can be observed in the neighboring four barangays in Loon town.

Seagrass beds provide home and food to young fish, starfish, sea urchins and other organisms.

Less than a month after the 7.2 magnitude earthquake, Yolanda, a category 5 typhoon, hit the Visayas and caused a storm surge with catastrophic effects on Tacloban City, Leyte, leaving more than 6,000 persons dead. A storm surge was also observed in some islets in northern Cebu, but there were no reported casualties as coastal residents were able to seek higher ground.

Illustration: Storm surge's effect on mangroves
Mangroves can serve as a line of defense against storm surges. A storm surge is a sudden rise in the sea level above the normal high tide on the coast driven by the winds of a storm. Every kilometer of mangroves can reduce water levels by five to 50 centimeters, according to a report published by Nature Conservancy and Wetlands International in 2012. (Sun.Star Cebu Graphics/Rigil Kent Ynot)

Build up defenses

In the light of recent disasters, the National Economic and Development Authority 7 updated the Regional Development Plan for Central Visayas to include environmental protection and nature conservation in its goals and priorities.

A report prepared by Anna McIvor, Tom Spencer, Iris Möller and Mark Spalding for the Cambridge Coastal Research Unit Working Paper and published by the Nature Conservancy and Wetlands International in 2012, reveals “that mangroves can reduce storm surge water levels by slowing the flow of water and reducing surface waves.”

The report says storm surge reduction through mangroves has been measured to range from five to 50 centimeters water level reduction per kilometer of mangrove width (distance into the mangroves). “In addition, surface wind waves are expected to be reduced by more than 75 percent over one kilometer of mangroves.”

The report says mangroves are more effective at reducing the water levels of fast moving surges than those of slow moving surges.

“The model (numerical model of Keqi Zhang, et. al. in Estuarine, Coastal and Shelf Science Journal 2012) also indicates that water level reduction through mangroves is non-linear, with the greatest reduction in surge height occurring near the seaward edge of the mangroves,” it says.

Hard evidence

Evidence of the value of mangroves in shoreline protection exists in other areas where cyclones are also part of daily life.

Ruchi Badola and S. A. Hussain, in “Valuing ecosystem functions: an empirical study on the storm functions of Bhitarkanika mangrove ecosystem” (2005), found that in the mangrove-protected village in Orissa, India, damage to houses and other adverse effects were lowest, while crop yields were least affected.

The Ramsar Convention on Wetlands based in Switzerland pointed out that coastal wetlands -- such as reefs, mangroves and salt marshes -- act as frontline defenses against devastation.

It noted that in areas devastated by the 2004 Indian Ocean tsunami, over a quarter of mangrove forests had been destroyed between 1980 and 2000.

On the other hand, the coastal protection services of the 200-hectare Rekawa mangrove and lagoon ecosystem in Sri Lanka was estimated in 2005 to be worth US$700,000 to $2.2 billion.

Ramsar also said the value of avoided damage due to coral reefs in St. Lucia in Colombo, Sri Lanka range from $28 million to $250 million. This does not include income generation through biodiversity and tourism.

Mangrove cover

The mangrove forest and coral cover in the Philippines, however, are declining, according to the Philippine biodiversity report on coastal and marine ecosystems on (the website of the Philippine Clearing House Mechanism).

After ground validation of satellite images, the Department of Environment and Natural Resources (DENR) reported that the country’s mangrove forests covered 210,497.62 hectares as of 2008. This data includes information from 804 coastal cities and towns and 23,492 barangays covering a coastal area of 34,014 square kilometers.

The figure is a far cry from the 450,000 hectares recorded in 1918. The report blamed the conversion of mangrove areas to agriculture, aquaculture, salt ponds, human settlements and coastal development for the declining mangrove forest cover.

Cutting of mangroves and conversion of mangrove areas continue even though these acts are prohibited by the Revised Forestry Code of the Philippines and presidential proclamations.

“Palawan (52,693 hectares) in Region 4B appears to have the largest mangrove area, followed by Northern Samar (9, 961.69 ha.) in Region 8, Surigao del Sur (16,865.14 ha.) in Region 13, and Bohol (14,156.37 ha.) in Region 7 (DENR, 2008),” the report states.

Low survival

The government, through the DENR, has implemented mangrove planting programs in different areas in the country since the 1970s. But Jurgenne H. Primavera and E. Esteban, in a 2008 review of mangrove rehabilitation programs, noted that major government-led projects with foreign funding had low survival rates of 10 to 20 percent, mainly because of the use of inappropriate species and sites.

“The favored but unsuitable Rhizophora (bakawan) are planted in sandy substrates of exposed coastlines instead of the natural colonizers Avicennia and Sonneratia. More significantly, planting sites are generally in the lower intertidal to subtidal zones where mangroves do not thrive rather than the optimal middle to upper intertidal levels,” the review said.

Among the projects that Primavera and Esteban reviewed was the $35 million Central Visayas Regional Project, which aimed to plant mangroves in about 1,000 hectares from 1984-1992. The review stated that mangroves planted in Cebu and Bohol under the project had a survival rate of 17 to 19 percent in 1995. But it also noted that the project was able to successfully engage communities through a stewardship mechanism.

Ideal ignored

Dr. Emma Melana, head of the DENR 7 ecosystems research division, said the ideal species to be planted at the seaward fringe of the middle to upper intertidal zones are bungalon (Avicennia marina) and Sonneratia (including firefly mangrove and pedada), but partner communities prefer to plant Rhizophora varieties.

“The fisherfolk have to be consulted, and they have many uses for the bakawan. They use it for fishing and in building their houses. They also find propagules of bakawan easier to get, while the seeds of pagatpat (Sonneratia alba) are difficult to get because the fruits are also the favorite of bats and beetles,” she said.

She added that bakawan seedlings are also easy to procure and grow faster than the Avicennia and Sonneratia species.

She said DENR maintains a nursery which grows Avicennia and Sonneratia seedlings. The seedlings are given for free to groups that want to plant mangroves.

Melana said that while DENR cannot compel partner communities to plant certain species, what is important for shoreline protection is that mangroves are planted.

She said that to have an effect on storm surge height, the mangrove forest has to be thick and wide. “You need at least 10 years for a mangrove plantation to be stable (against storm surge),” she added.

In Cebu, lush mangrove forests can be found in Argao, Sibonga and Camotes Island. Areas with tidal floods are ideal for mangrove planting.

Poor reefs

Meanwhile, the country’s coral reefs covered about 27,000 square kilometers as of 2004. Of the figure, about 70 percent is considered in poor or fair quality and quantity. Only five percent can be considered in excellent condition, says the report.

A study by Cleto Nanola Jr. in 2004 revealed that Philippine reefs may be in a “steady state of decline.”

The establishment of marine protected areas (MPAs) is seen as one way to address the problem. Many local governments have established MPAs to protect and rehabilitate coral reefs with the help of the National Government and foreign-funded programs, like the Coastal Resource Management Project and the Ecosystems Improved for Sustainable Fisheries. (Sun.Star Cebu)

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