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Flooding is a perennial problem in the Philippines, which, according to PAGASA, experiences the greatest number of tropical cyclone visits than anywhere else in the world. With a yearly average of 20 tropical cyclones, the country has its share of major floods, including those from Typhoon Ulysses in November, which caused 67 deaths and agricultural damage worth 16 billion pesos.

But with climate change in the picture, the frequency and effects of disasters continue to rise. According to a report from the United Nations Office for Disaster Risk Reduction, major floods across the globe has doubled in the last 20 years. Among all the climate-related disasters, storms and floods were the most common events. 

 

Flash flood in Oriental Mindoro last Dec. 4 due to heavy rains (photo by Shen Francisco Fabon)

 

Types of Floods

PAGASA Hydrologist Rosalie Pagulayan explains that floods can be classified according to their location and how they occur. 

 

River flooding is caused by the swelling of rivers, also called water basins. “Just like basins, our rivers have limited carrying capacities,” she says. “If the water level exceeds these capacities, water overflows and causes floods.”

 

Urban flooding occurs in highly-urbanized areas. Paved roads seal off soil, which absorbs rain. Pagulayan mentions a foreign study stating that in urban places, 50% of rainfall becomes run-off water. In cases of excessive rain, drainage systems and culverts are not enough to store water, creating floods.

 

Meanwhile, flash floods are one of the most dangerous flood types because of how fast they rise. Possibly occurring mere minutes after rainfall, flash floods leave little or no time for residents to prepare. “These usually happen when nearby rivers are too narrow and have limited carrying capacities. Flash floods can carry harmful debris, such as rocks,” says Pagulayan.

 

Coastal flooding happens in coastal areas, and can be attributed to storm surges caused by tropical cyclones pushing water toward the shore. Tsunami, produced by offshore earthquakes, may also generate coastal flooding and even more devastating impacts.

Though technically, landslides are not a flood type, Pagulayan feels compelled to mention them as a significant impact of strong rainfall. “When there’s excessive rain in mountainous areas, water saturates soil and loosens it, which may lead to landslides. The steeper the slope, the faster loose soil can roll down and create devastation.”

 

Marikina flood after Typhoon Ulysses (photo by Jilson Tiu/Greenpeace)

 

Causes of floods

Rains are the primary cause of floods, but the flood intensity depends on numerous factors such as the affected area’s landscape and its river’s carrying capacity. “Another factor is the siltation in rivers. These sediments may lessen our rivers’ capacity to carry water, which is why the Department of Public Works and Highways carries out desilting operations.” When river protection structures such as dikes break, this leads to the sudden rush of water which creates floods. Tidal patterns also induce floods, especially in low-lying areas like Malabon and Navotas. 

But a major flood contributor unrelated to meteorology is human intervention, which is a “very glaring factor” according to Pagulayan. “It’s basic science. Canopies and good vegetal cover intercept rains, protecting the soil from water saturation and erosion. Now we see that the water flowing down from the mountains are heavily silted, which means there aren’t enough trees to hold in the soil.” 

River garbage is also a significant factor. “Typhoon Ondoy in 2009 showed the overwhelming amount of trash in our rivers. Rivers are a major source of our livelihood, and throwing garbage in them makes them a source of devastation.”

 

Typhoon Ulysses caused the Marikina River to swell and submerge vehicles in flood. (photo by Jilson Tiu/Greenpeace)

 

Flood Mitigation

To lessen the possibility of floods in the urban setting, Pagulayan suggests simple measures. “In other countries, they use cement mixed with semi-permeable asphalt so the soil underneath can absorb some of the rain. Other LGUs (local government units) put bricks on walkways instead of cementing them. I think little things like these can make a big difference in the long run.”

As for Filipinos building residences and structures near rivers, Pagulayan states, “We actually have a law that addresses this. The Water Code of the Philippines provides guidelines on how far away structures should be built from bodies of water. In urban areas, easement should be more or less 3 meters. In agricultural areas, it is 20 meters, while forest areas should have an easement of 40 meters.” 

To manage coastal areas, Pagulayan emphasizes the importance of mangroves, which protect against storm surges. “Mangroves also facilitate fish development. When there are plenty of fish near the shore, our fishermen don’t have to go far.”

She shares that PAGASA carries out flood mitigation measures in two ways. “We have the structural and the non-structural. Structural involves moving the water away from the people, while non-structural is moving the people away from the water. Structural measures include the building of dams, dikes, and river walls. The main concern with the structural aspect is that it’s very expensive. To make your structures sturdy, the more you have to spend.” 

Meanwhile, an example of non-structural measures is PAGASA’s early warning system, which aims to translate data into actionable information. “If we foresee a 100-millimeter rainfall, we can’t just release that data to local governments and communities. We have to give them a clear picture of the impact. Instead, we say that the projected flood is knee-deep. This way, people can better understand the situation and prepare for it.” 

Aside from conducting education campaigns and public information drives, PAGASA also releases weather forecasts and tropical cyclone warnings, flood bulletins and advisories, hydrological, climatological and farm weather forecasts.  “The structural and non-structural measures should complement each other. Hazard maps highlight danger and safe zones, so people will be aware where to build their homes, and where to seek refuge when they need to evacuate. With all these kinds of information, we hope to reach out to more people,” Pagulayan ends.

 

Watch Panahon TV Reports: Understanding Floods  for more information.

Agay Llanera

An hour before midnight on October 26, 1978, Super Typhoon Kading was at its peak in the country, bringing torrential rains that caused the water level at Angat Dam to swell. Because  stored water, when reaching a critical high level, might cause dam structures to break, the National Power Corporation (NPC) opened all its three floodgates to 8 meters. As the rains continued to pound, adding to the dam’s water level, NPC decided to further open the floodgates to its maximum height of 14 meters by 6 a.m. the next day.

This result was a catastrophe. With the sudden dam release, several towns in Bulacan near Angat Dam was instantly flooded. But the damage in houses, farms and working animals amounting to millions of pesos was not the only tragedy; with the dam’s spillways being opened without prior warning while residents were asleep, almost 200 people died in the sudden floods.

Civil suits were filed against NPC. Complainants all attested that they had not been warned of the maximum water release. Lives could have been saved if the dam release operation was done earlier and more gradually, and if there were proper communication among the sectors involved. In 1993, the suit was won, and NPC settled by giving 5 million pesos to families of those who had died.

But the tragedy was a wake-up a call to the government, which recognized the protocol gaps in dam release. In 1983, it began the Flood Forecasting and Warning System for Dam Operation. Implementation began in 1990, while the system became fully operational in 1992. The project led to the establishment of the PAGASA Data Information Center, flood forecasting and warning systems in both PAGASA and dam offices, hydrological stations, warning posts, and repeater and monitoring stations.

MARIS Dam in Nueva Ecija releases water (photo courtesy of NIA-Magat River Integrated Irrigation System)

Giving a dam(n)

PAGASA aims to manage the flooding risk caused by dams with its daily-updated products and services:

Rosalie Pagulayan, a hydrologist at PAGASA, emphasizes the importance of risk management in harnessing the full potential of our dams. “Major dams are multipurpose,” she says. “They generate power, irrigate farms, provide potable water and controls flooding. During the rainy season, the dam stores excess water, keeping it from inundating the rivers and communities.” Angat Dam, in particular, provides 97% of Metro Manila’s water requirements, and irrigates 31,000 hectares of farmlands in Pampanga and Bulacan.

Though dams are meant to prevent flooding, extreme weather events that bring massive rains can force dam operators to conduct water releases.

But even the frequency of tropical cyclones in our country is a double-edged sword. “Many complain about the inconvenience of the rainy season, but upon studying our climate, a former PAGASA director was able to determine that 50% of our rain comes from tropical cyclones. If we don’t have tropical cyclones, our water supply suffers. Other weather systems are also important; the northwest and southwest monsoons bring 38% of our rain, while the ICTZ  [intertropical convergence zone] brings 12%.” She adds that rains are important in crop irrigation and pushing out atmospheric pollution.

Pagulayan, who has been working in PAGASA for 15 years, says that there are protocols before spillways are opened. “Even before a typhoon comes, we predict how much rainfall it will bring. We give the data to the different dam offices, which compute the expected increase in water levels. To accommodate the incoming rain, the dams conduct pre-release—but only according to the river’s capacity to prevent flooding.” She adds that aside from measuring the amount of rain in the watershed, PAGASA also measure the rain’s impact. “There’s a warning post from dam offices before they release water. There’s ample lead time, so for example, if they’re opening the gates at 4 p.m., they warn residents as early as 10 a.m. so they can prepare.”

How flooding risks can be better managed

In other parts of the world, river systems span countries, such as the Brahmaputra River, which comes from India but flows downstream to Bangladesh. The famous Danube River also comes from several European regions. Such a situation calls for strong international coordination when the source of the river swells, and flows down to various countries and territories in low-lying areas.

But Pagulayan says this is not a problem in our country. “We are blessed with big river systems. We manage our own rivers, and solely benefit from them. Our rivers traverse through various municipalities, provinces and even regions.”

In the left graph above, Pagulayan shows how water from Nueva Ecija flows down to the Pampanga river basin, and contributes to a portion of Tarlac and Bulacan. “Our river systems encompass a lot of areas so disaster managers in these zones should be seamless in their communication and info-sharing.”

Meanwhile, as shown in the right graph above, water from the Bicol river basin comes from Albay, Camarines Norte, and mostly Camarines Sur. “But the Camarines Norte source is very important because rains are strong there, and because of its elevation, water flows down very fast,” shares Pagulayan. “All these rivers are owned by the Philippines. We don’t share them with other countries or regions. We are the only ones who can harness their potential.”

In areas that receive a lot of rain, dams and water reservoirs are built in mountainous areas, which can hold excess water. But watershed deforestation is an issue, leading to its decreased capacity for water storage. This may lead  to flooding during the rainy season, and water supply shortage during the hot and dry season.

Risks also increase when people build their homes and livelihoods near waterways and flood plains. “They may find it convenient to set up livelihoods that require water, such as farms, in those areas, but once water is released, their safety is compromised,” explains Pagulayan. “Flooding also negates development efforts, and destroys agricultural products and infrastructure. But the worst risk of all is the loss of lives.”

MARIS Dam’s spillway in Nueva Ecija (photo courtesy of NIA-Magat River Integrated Irrigation System)

Moving forward

To foster good relations with dam offices and local government units (LGU), PAGASA regularly conducts information and education campaigns, public information drives, and communication drills. “For every disaster, the challenge, I think, is communication, so this is what we constantly develop. We can’t stop; it has to be continuous so we can improve,” Pagulayan shares.

By practicing and strengthening the information flow from PAGASA down to the regional and municipal levels, data accuracy is ensured, allowing LGUs to map out action plans to lessen flooding impacts. “We learn from every experience how to convey data and information. We are open to change and we adjust our activities, so we can improve both our services and community action,” ends Pagulayan.

Communities can also mitigate risks by knowing what to do before a tropical cyclone arrives and causes flooding.