The Trailing Suction Hopper Dredger (TSHD) has a suction tube with a drag head that is lowered into the seabed to suck up sand, clay or gravel into its hold. Large TSHD vessels can carry 30,000 tonnes of soil, sucked up in an hour. When used for land reclamation, the TSHD can drop the soil via underwater cargo doors beneath the ship, pump the load via a floating pipe, or spray the slurry of soil and water onto the target site via a nozzle at the ship’s bow.
The trailing suction hopper dredger (TSHD) is a mind-blowing piece of marine engineering that could best be described as the largest vacuum cleaner on Earth.
Looking like the flat head brush and extender pipes of vacuum cleaners in your home, the ship’s suction tube with a drag head lowers into the seabed and sucks up sand, clay or gravel into its hold.
Large TSHD vessels can carry 30,000 tonnes of soil. If this feat were copied on land, that would be like using 3,000 10-tonne lorries to transport soil at once. Assuming the army of lorry drivers and myriad logistics could be so organised, the traffic congestion would be unimaginable.
One of these ships can suck up all that in about an hour.
When used for land reclamation, the TSHD can drop the soil via underwater cargo doors beneath the ship, pump the load via a floating pipe or spray the slurry of soil and water onto the target site via a nozzle at the ship’s bow.
Singapore has employed fleets of TSHD to suck up the seabed off Cambodia and Vietnam shores, and bring the material to the island republic’s coastline to reclaim land since 2003 after Malaysia and Indonesia banned seabed dredging in its waters.
“Since independence, Singapore’s physical constraints have made it a challenge to balance its competing needs and reclaiming land had been done since her colonial days. Land reclamation is also an important activity in small countries such as Belgium and The Netherlands. It has become an industry and a fine art,” Tommy Koh and Jolene Lin write in the 2006 Singapore Year Book of International Law and Contributors.
Bunds and silt curtains
Modern engineering marvels pertaining to land reclamation nowadays are also evident in the environmental management side of the business.
Before the start of a reclamation, the site is enclosed with a cofferdam. Massive containment bunds made from rocks or geo-textile bags of sand several kilometres long are constructed to barricade and push the sea away. This allows the reclamation site to be pumped out before the TSHDs deposit millions of tonnes of soil into it.
The setup prevents suspended silt from escaping and causing the surrounding sea to become turbid.
Without the bunds, silt curtains and other contraptions, land reclamation would be a destructive process, says marine ecologist Dr Aileen Tan from Universiti Sains Malaysia.
At least two silt curtains outside the containment bunds are also default equipment to keep the floating material from spreading.
“Without the bunds, silt curtains and other contraptions, land reclamation would be a destructive process. The earthworks will churn up silt at the reclamation site and nutrients trapped within the seabed will fill the water column of the sea,” says marine ecologist Dr Aileen Tan from Universiti Sains Malaysia.
She explains that silt in the seabed is a depository of millions of tonnes of nitrogen, potassium, phosphorus and myriad minerals – potent plant fertiliser.
“When all that is kicked up and mixed into the seawater, you can expect algal blooms larger than Mother Nature can ever create,” she says.
Algal blooms might be the worst thing that can happen to fish, short of being caught. The churned up nutrients cause marine microorganisms to flourish astronomically and turn the sea into a thick soup of green or red containing tens of millions of cells per litre of the sea.
Commonly called red tides, they last from a few weeks to about a year, subsiding and reccuring erratically. Some of them are made of microorganisms that produce toxins, killing all marine life in the zone and causing humans to become seriously ill if they eat infected seafood.
Even without the toxins, the algal blooms deplete the dissolved oxygen of the sea at night (when the cells are not doing photosynthesis) to such an extent that marine life still die or leave the area.
“This is the most immediate ecological harm of land reclamation work,” she says.
Dr Tan cautions that in spite of all the containment technologies, “the sea is a moving entity. There will be seepages through the containment bunds and contractors need to shift or reinforce them over time” as the reclamation work progresses.
Even after reclamation projects are completed, contractors need to continue monitoring the surrounding area for two to three years, she says.
“But this must be enforced and government authorities must go after them for reports to make sure they comply.
“I support land reclamation because we need land. But the quality of the reclamation must be at a sustainable level,” she adds.
Malaysian Nature Society adviser Datuk Dr Leong Yueh Kwong considers the environmental impact of reclamation earthworks a short-term problem.
“Whatever we humans do to the sea’s marine life, it will recover over time,” says the environmentalist and academician.
What must also be considered are long-term environmental, social and ecological aspects, he says.
Dr Leong stresses that governments have to take social needs into account when reclaiming land, and allocate sufficient public open spaces on the new land before ceding portions of it to investors and developers.
“The impact of the whole area must be considered, too. Not just the project site. These new tracts of land will cause changes to nearby parts of the sea and shore and if there are negative impacts, then the environmental consultants should be made liable for rectification costs.”
He also proposes that environmental impact assessments be reviewed by a panel of independent experts.
“No matter how land reclamation is done, there will be consequences particularly for coastal communities such as fishing villages,” he says.
Conventional compensation paid out in the past to fishermen affected by reclamation projects brought only short-term relief.
“They spent all the money in a short time,” says Dr Leong.
He adds that what is more important is to set up a community development fund to help affected communities adjust to new ways of life post-reclamation.
“If every possible social and environmental angle is addressed, everyone will welcome land reclamation,” he says.http://www.star2.com/living/living-environment/2016/01/15/land-reclamation-raising-land-from-the-sea/