Wednesday, October 14, 2009

Weighing the costs of dredging the lake

By Clarice Colting-Pulumbarit, Karen Lapitan
Philippine Daily Inquirer
First Posted 20:48:00 10/14/2009

Filed Under: Ondoy, Health, Weather, Environmental Issues

MANILA, Philippines-Environmental science and agricultural engineering experts believe dredging can solve the rising water level of Laguna Lake but consequences must be weighed.

Dr. Macrina Zafaralla, a professor of environmental science at the University of the Philippines-Los BaƱos (UPLB), said although she has espoused the dredging of the lake since the 1970s, recent literature has shown that if lake water is disturbed, all toxic and nontoxic sediments at the bottom of the lake would also be disturbed, posing a health hazard.

For land and water resources expert Dr. Victor Ella of the Institute of Agricultural Engineering-UPLB, dredging can lower the lake’s water level at a faster pace but it should be part of a comprehensive plan that includes the management of watersheds surrounding it.

Heavy metals

Zafaralla has been conducting studies regarding the state of the Laguna Lake based on its ecosystem since the 1970s.

She was also part of the team that formed a millennium assessment on the lake in 2005.

“If we disturb all the sediments that had been deposited in the lake for years, even the nontoxic sediments may be transformed into something harmful,” she said.

Possible effects of dredging include the reduction of fish catch, release of noxious odor, and more of the heavy metals present in the lake may be dissolved and released in overlaying water.

“Dredging can make heavy metals available and may be absorbed by algae, algae is eaten by fish, fish is eaten by man. It can be transferred to man and could pose a danger to health,” Zafaralla said.

Heavy metals that have been known in previous studies to be present in the lake are mercury, cadmium and lead which are harmful to humans and can even be cancerous, she said.

“We don’t know the extent of the damage and for how long it will be because it will be a virgin experience for us, but all parts of the lake ecosystem will be affected,” she said.

Zafaralla cited a study of the dredging of a lake in China that changed the water chemistry and composition of algae growing in the lake.

Another study showed that dredging may alleviate the problem only for a time but would return if the entry of pollutants in the lake are not controlled.

Public information

“I espouse dredging but the public will have to be informed of possible consequences,” she said.

Zafaralla said she had also voiced out during a conference on the Laguna Lake before Tropical Storm “Ondoy” struck that people should be informed of how many percent of the time their place would be inundated.

“Government agencies would have already drawn a topography map of which areas around the lake are easily inundated and would be the first to be inundated. There is available data, it should be published,” she said.

“The government should inform people—every municipality should have a map showing the probability of inundation depending on the amount of rainfall.”

A study Zafaralla did in 1977 showed water hyacinth growth in the lake.

Water hyacinth anchors itself to soil, and when it dies, it contributes to natural reclamation of the lakeshore thus, growth shows which areas are bound to get shallower first.


In the map showing results, areas with high hyacinth growth could be seen at the Marikina, Rizal and Laguna’s fourth district side which includes the towns of Sta. Cruz and Pakil.

She noted that these were among the areas hardest hit by the overflowing lake during Ondoy.

Ella noted that overflowing of the lake was due to extreme rainfall but also believed it was a contribution of different factors: direct runoff downhill aggravated by deforestation and loss of land cover.

“Overflow is partly the effect of long-term deposition of sediments that is why the lake has become very shallow.”

He said dredging is a good strategy to remove sedimentation and increase the water capacity of the lake but is concerned about its feasibility because it will take a long time.

“Without dredging, outflow of lake water is only through evaporation because there is very minimal seepage of water into the soil. With evaporation, water outflow would only reach 10 mm/day depending on prevailing weather conditions.”

Ella emphasized the importance of land cover and how much it helps despite a large amount of rainfall.

“Through interception of rain, it reduces impact and erosion. Vegetation also increases evapotranspiration which reduces moisture content of soil and increases infiltration capacity of soil. It delays runoff and retards flow of water downhill. Land cover also improves soil aggregation due to increased organic matter content of soil, and this soil will not easily be eroded.”

Comprehensive solution

Ella said dredging should not be a band-aid solution, after which sedimentation and erosion would return.

“The solution should be long-term and comprehensive, both involving watershed and lake management. There should be an over-all agency that will implement an integrated management of the two.”

He added that there were also problems with land-use planning—faulty drainage systems, silted waterways, and residential areas (subdivisions) built onto floodplains.

Ella noted that the concept of a spillway was also a good engineering solution—a connection between Laguna Lake and Manila Bay to serve as a conduit to evacuate excess water from Laguna Lake.

He suggested the creation of a hydrologic model for watersheds surrounding the lake to be able to see and predict the effects of land use change and climate change on runoff and sediments. A hydrologic model is a computer simulation model that mathematically represents the various hydrolic processes like runoff, evapotranspiration, infiltration, percolation, steamflow, baseflow, groundwater flow, erosion, etc., that take place in a watershed in response to rainfall.


Laguna Lake Development Authority (LLDA) general manager Edgardo Manda said they were still in the process of planning the long-term dredging of Laguna Lake.

He clarified, however, that the agency plans to dredge the Napindan Channel first as it would help subside the flood around areas near the lake.

He said the dredging should be done regularly.

“LLDA will be responsible for the scientific aspect of the lake’s dredging, but the actual dredging will be done with the help of DPWH (Department of Pubic Works and Highways),” Manda said.

LLDA was still looking to fund the long-term dredging, he added.

He said dredging seemed to be one of the best ways of preventing excessive flooding.

“As for the heavy metal contents found in the lake, the proper disposal will also be part of our plan. People do not have to worry about the health hazards of such as we will carefully plan its disposal,” Manda said.

He said the agency’s experts were looking into the best ways to handle those harmful contents of the lake.

Aside from the sediments accumulated in the lake, the problem of illegal settling should also be addressed, according to Manda, adding that the people living near the lake add up to the problems of clogging the channels where water should freely flow.

“But it’s the solution on illegal settling that should be monitored by the local government units,” he said.

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