Singapore has reclaimed about 800 ha of land at the north-western tip of Pulau Tekong, with the completion of main construction works for the country’s first polder, announced the Housing & Development Board (HDB) and national water agency PUB in a joint press release. This reclaimed land will be used for military training purposes, freeing up space on the mainland for other uses, such as developing new homes and amenities for the residents.
The completion of main construction works for the polder via the empoldering method marks the first time Singapore has reclaimed land below the mean sea level. Compared to the traditional land reclamation method of infilling with sand to reclaim land above sea level, the empoldering method uses less sand. This has resulted in close to 50% of savings in sand used for the reclamation.
Using this method, a tract of dry land below the surrounding sea level has been created by constructing a coastal dike and a network of drains and water pumping systems, as well as a stormwater collection pond. Together, these features protect the area from rising sea levels and more intense rainfall due to climate change:
Robust coastal dike
The 10-km-long coastal dike runs along the perimeter of the polder and protects the low-lying polder land from the sea. It stands at up to 6 m above mean sea level at its highest point, and allows for the polder land to lie about 1.2 m below mean sea level. The coastal dike height can be raised if needed, to adapt to future increase in sea levels.
This coastal dike was the first key infrastructure to be completed, as it was necessary to fully enclose the area being reclaimed to manage water levels within the polder. A road that runs along the entire crest of the coastal dike has also been built to facilitate maintenance works. This road, together with another 21 km of roads constructed within the polder land, allow vehicles to safely navigate the polder.
To ensure its robustness and prevent erosion of its sand core, the coastal dike is constructed with a variety of materials and structures:
A). Cement bentonite inner wall
Within the coastal dike is a cement bentonite wall, which forms a barrier to keep any seepage of seawater through the coastal dike to a minimum. Any seawater seepage is collected in a seepage drain and pumped out of the polder. This prevents seawater from entering the freshwater drainage network on the polder and helps to maintain the salinity levels in the stormwater collection pond
B). Nature-based solutions
The seaward side of the coastal dike is covered with a layer of natural rock which is effective in absorbing wave-impact to ensure the coastal dike remains robust, and also cost-effective for both construction and maintenance. Additionally, the naturalistic look of the rock enables the polder’s coastline to blend in more sensitively with its natural surroundings.
Meanwhile, the landward side of the coastal dike slope has been designed with a verdant covering. This helps to reduce ambient heat and beautify the landscape. The selected Bermuda grass has a deepreaching root system which allows it to hold the soil firmly together. This mitigates the erosion of the coastal dike during rain, or in the unlikely event of water from large waves flowing over the top of the coastal dike and into the polder land. The Bermuda grass is also a hardy plant which grows well in Singapore’s hot tropical climate and can tolerate a saline environment.
Comprehensive water management system
Due to Singapore’s high rainfall and the polder’s low-lying nature below mean sea level, managing stormwater runoff requires special attention. The polder’s water management system comprises a stormwater collection pond, two pumping stations, and an extensive 45 km of drainage network with more than 30 water control, inlet and outlet structures with gates to manage rainfall.
During dry weather, water is recirculated by pumping water from the stormwater collection pond to the polder drainage network to prevent stagnation.
The outlet structures run through the coastal dike and help to channel excess water (collected within the polder) out to the sea. As these outlet structures had to be built on-location, they required a dry environment to construct. However, this posed a challenge as the structures had to lie below mean sea level, and had to be constructed first before the corresponding segment of the coastal dike could be built.
To overcome the challenge, temporary water-retaining structures (cofferdams) were installed to keep the construction areas of these outlet structures sufficiently dry, before the coastal dike was built over the completed outlet structures.
The polder’s cofferdams were made primarily of steel sheet piles, which were driven up to 26 m deep into the ground to ensure they were strong enough to keep seawater out of the construction area. Temporary pumps were also deployed to pump stormwater out of the area, until the drainage pumping station was ready for operation. After the outlet structures and coastal dike were completed and able to keep the seawater out of the polder, the cofferdams were removed.
The drainage system on the polder is monitored and operated by PUB through the fully automated Supervisory Control and Data Acquisition (SCADA) system at the operation control centre located within the drainage pumping station. The system is directly connected to PUB’s joint operations centre to allow for remote monitoring. PUB has also installed more than 170 CCTVs and over 260 sensors to monitor the dike as well as the water quality and water level within the drainage network.
Stormwater collection pond and water pumping stations
When it rains, runoff within the polder and the Pulau Tekong catchment area is conveyed to the 116-ha stormwater collection pond to mitigate flooding within the polder. Once the water level in the stormwater collection pond reaches a pre-set operating level, excess water is channelled to the drainage pumping station to be pumped out into the sea. This stormwater collection pond is designed to hold up to 5 mil cu m of water, equivalent to 2,000 Olympic-sized swimming pools.
The central pumping station located next to the stormwater collection pond circulates water in the drains to ensure good water quality, while the drainage pumping station located next to the coastal dike helps to pump excess rainwater into the sea when the stormwater collection pond is full.
Reducing sand use
Over 10 mil cu m of clayey soil, enough to fill about 4,000 Olympic-sized swimming pools, was dredged from the construction of the polder’s stormwater collection pond. This presented HDB the opportunity to further reduce the use of sand by repurposing some of the clayey soil as infilling material for the polder land.
Due to the high water content of the soil, improvement works to strengthen the soil had to be carried out first, to ensure minimal land settlement. For example, after infilling an area with a layer of clayey soil, prefabricated vertical drains (PVDs) were inserted at regular intervals into the soil to create drainage paths. The soil was then overlaid with a layer of sand to compress it and force the water in the soil to drain upwards through the PVDs. The process transformed the soft soil into sturdy usable land, ready for development.
In designing the coastal dike and its safety provisions, HDB and PUB drew on the deep experience of the Netherlands, which has the world’s highest standards for dike safety, to ensure the long-term viability and safety of the polder land.
All images: HDB
