Wetlands, Otters and Urbanization in Singapore

By Paul Nettleton on 14 July 2021

During my evening run the other day, I stumbled across a family of otters happily rolling around in the grass and diving into the wetland ponds at Singapore’s Alexandra Canal Wetlands. I was startled by what I thought was a rare occurrence in a city, amazed by how unafraid the otters were of us bystanders, and anxious for the otters and pet dogs that were having a standoff as the dog-owners cautiously backed away. 


It is no secret that wetlands are hotspots for biodiversity, instrumental in flood protection, and help to enrich the beauty of our natural world. However, as urbanization takes hold of many countries, vast areas of wetlands have been filled in to make way for roadways, buildings, canals, and cities. In the 20th century alone, the global wetland area has diminished by 64-71%, and in regions such as Asia this figure is even higher [1]. This drastic change in landscape causes a major shift in how water is managed and how nature functions.

Although creation of urban environments inevitably depletes wetland environments, is there a way to bridge the gap and get the best of both worlds?

The answer to this question is not straightforward, but for the sake of my otter friends and their wetland habitat, I will do my best to unpack this topic.

The beauty of wetlands

Simply put, wetlands are places where water meets land in the natural environment. The soil in wetlands has adapted to changing water levels, resulting in unique characteristics in plants and animals that live in the wetlands, such as roots that dissipate wave energy, skin that breathes, and long gangly legs for wading through water. This is also why otters have relatively flat heads. A flat head enhances hydrodynamics for swimming and minimizes its visibility when the otter wants to poke its head out of the water to look around, take a breath of fresh air and listen to what is going on around it.

Wetlands are one of the world’s most essential ecosystems. Not only are they vital for the survival of many plant and animal species, but they also provide numerous economic benefits and contribute to human well-being.

Wetlands are a sink for flood waters, temporarily retaining stormwater during heavy rainfall events and slowly releasing water to the downstream systems after the rainfall subsides. Wetland plants uptake nutrients (e.g. nitrogen and phosphorous) from water, purifying the water to enable other forms of life to thrive in the surrounding ecosystems.

A brief history of Singapore's mangrove forests

Singapore is a prime example of how urban development impacts wetland environments. The coastlines and rivers of Singapore are home to a special type of wetland – mangrove forests. Back in 1819, mangrove forests covered an estimated area of 75km2 in Singapore [2], amounting to 10-13% of the country’s land area [3].

Figure 1: Vegetation types in primeval Singapore [2]

Due to charcoal and firewood exploitation, rapid urban development, conversion to prawn ponds, land reclamation, and damming rivers, the area covered by mangrove forests reduced by approximately 93-94%, to 4.83km2 in 1992 [2]. Thanks to various government and NGO mangrove restoration efforts, the mangrove forest area in Singapore has increased to approximately 6.59km2 in 2010 [2]. However, this is still a long way from the 75km2 of mangrove forests that were present two centuries ago.

Figure 2: Mangrove forest sites in modern-day Singapore [2]

The tradeoffs of urbanization

Traditional urbanization sees stormwater as a nuisance. To keep the urban population happy and safe, large concrete canals and drains are built to evacuate stormwater from cities as efficiently as possible.

However, where greenery is replaced with asphalt and concrete, stormwater will flow much faster as the smooth hard surfaces provide little resistance to the flow of water. This can cause an overload of downstream water systems and flooding of surrounding areas. In addition, asphalt and concrete surfaces do not uptake nutrients from water in the same way wetland plants do, resulting in a buildup of pollutants in urban water systems.

Three ways to bring wetlands back into Singapore's urban fabric

Although wetland environments in Singapore have depleted due to urbanization, in the last few decades, Singapore has started creating pockets of wetland habitats within the city’s urban fabric. These urban wetlands improve Singapore’s liveability, by absorbing excess rainfall, reducing floods, enhancing water quality, moderating temperatures and increasing biodiversity.

Here are three ways that wetlands have been merged back into Singapore’s urban environment.

1. Converting a canal into a natural river

From 2007 to 2012, the large concrete canal that ran along the south side of Bishan-Ang Mo Kio Park was transformed into a natural river that meanders beautifully through the park. This was no simple feat, and required careful engineering and creativity. Since the natural river was restored, the Bishan Otter Family and a variety of wetland animals, from kingfishers to dragonflies, have seen a resurgence along the river.

Figure 3: Aerial view of Bishan-Ang Mo Kio Park [4]

There are multiple factors to consider when embarking on a river restoration project, such as the impact to surrounding developments, public health, and erosion control. Bishan-Ang Mo Kio Park is an excellent example of how these factors are addressed using integrated solutions to create an urban riverine park that serves as a bridge between wetlands, people and urban environments.

Figure 4: View from the centre of the river at Bishan-Ang Mo Kio Park

2. Using vertical and horizontal flow wetlands to cleanse water

Vertical flow constructed wetlands are gravel/sand beds that are planted with wetland vegetation. Water filters through the soil layers as wetland plants uptake nutrients from the water. The purified water is collected in perforated pipes at the bottom of the gravel/sand bed and discharged.

In Singapore’s ABC Waters Design Guidelines [5], these systems are called ‘Cleansing Biotopes’. This type of system can be found in several locations in Singapore, including Jurong Eco-Garden, Bishan-Ang Mo Kio Park, and Jurong Lake Gardens. In these three locations, water is pumped from an adjacent water body to the surface of the cleansing biotope. The water then filters through the cleansing biotope and discharges back into the adjacent water body.

Figure 5: Vertical flow constructed wetlands [6]

The beauty of these nature-based systems, compared to industrial water filters, is that they provide multiple benefits, including improved water quality, beautification of the area for people to enjoy, and creation of havens for flora and fauna. Some days you will see a monitor lizard sitting on one of the cleansing biotope’s access manways basking in the sun. On other days, you will see dragonflies darting in and out of the wetland plants.

Figure 6: Cleansing Biotope at Bishan-Ang Mo Kio Park

The Lorong Halus Wetland is yet another great example of how Singapore has used wetland systems to manage water. This wetland is located on top of an old landfill site and is used to cleanse leachate (i.e. water that passes through the landfill waste). This innovative wetland system uses a series of horizontal subsurface flow wetlands, called reed beds, to purify the leachate [7].

Figure 7: Reed bed at Lorong Halus Wetland

After passing through the reed beds, the purified leachate goes through a final treatment step in free water surface wetlands, called polishing ponds, before the water is discharged into the used water network [8].

Figure 8: Polishing pond at Lorong Halus Wetland

3. Placing floating wetlands in urban water bodies

Floating wetlands are vegetated floating platforms that float on a free water surface, such as a pond or a reservoir. The roots of the plants are suspended in the water below the platform. The benefit of these systems is that they float with the rising and falling water levels caused by storm events.


Figure 9: Floating Treatment Wetlands [6]

These systems can be found in several locations in Singapore, including Pang Sua Pond, Jurong Lake Gardens, and Sengkang Floating Wetland. You will frequently see red-eared sliders swimming around the floating wetlands and fish that inhabit the root mats beneath the water surface.

Figure 10: Floating Wetlands at Pang Sua Pond

Back to the otters

Singapore’s otters have made a rebound in the last three decades [9], as have many other animal species in Singapore. It is no secret that healthy wetland ecosystems are part of the reason why the otters are back. Although urbanization inevitably encroaches into wetland areas, Singapore is an exemplary case of how wetlands can be reintegrated into cities in creative ways, helping to create healthy ecosystems and rejuvenate wildlife.


References
  1. Davidson, N.C. (2014) How much wetland has the world lost? Long-term and recent trends in global wetland area. Marine and Freshwater Research, 65(10): 936-941.
  2. Yee A. T. K., Ang, W.F., Teo, S., Liew, S. C. and Tan, H.T.W. (2010) The present extent of mangrove forests in Singapore. Nature in Singapore, 3:139–145.
  3. Low, J.K.Y. and Chou, L.M. (1996) Environmental Status of Singapore’s Costal Wetlands. Journal of the Singapore National Academy of Science, 22-24:103-108.
  4. Liao, K. (2019) The socio-ecological practice of building blue-green infrastructure in high-density cities: what does the ABC Waters Program in Singapore tell us? Socio-Ecological Practice Research, 1:67–81.
  5. Public Utilities Board (PUB) (2018) Active, Beautiful, Clean Waters Design Guidelines, 4th edn. PUB, Singapore.
  6. Stefanakis, A., Akratos, C.S. and Tsihrintzis, V.A. (2014) Vertical Flow Constructed Wetlands: Eco-engineering Systems for Wastewater and Sludge Treatment. Elsevier, p. 23-24.
  7. Cancelli, A.M., Gobas, F.A.P.C, Wang, Q., Kelly, B.C. (2019) Development and evaluation of a mechanistic model to assess the fate and removal efficiency of hydrophobic organic contaminants in horizontal subsurface flow treatment wetlands. Water Research, 151: 183-192.
  8. Ko, C. (2011) Latest wetland craze – Lorong Halus Wetland. Water Quality in Singapore. https://waterqualityinsingapore.blogspot.com/2011/04/latest-wetland-craze-lorong-halus.html
  9. Khoo. M and Lee, B. P. Y-H. (2020) The urban Smooth-coated otters Lutrogale perspicillata of Singapore: a review of the reasons for success. International Zoo Yearbook, 54: 1-12.