If You Build It, They Will Come: Modifying Coastal Structures for Habitat Enhancement

Nhung Nguyen, Singapore.  Karenne Tun, Singapore.  Lena Chan, Singapore. 
October 5, 2016

Many voices. Greener cities. Better cities.

Since the founding of modern Singapore in 1819 by Sir Stamford Raffles, the small island nation has developed from a sleepy fishing village into a modern day metropolis, and has lived up to the adage, “if you build it, they will come”. Particularly over the last eight decades, Singapore’s coastal landscapes have gone through significant transformation. From an island skirted by mangroves and mudflats, this modern-day coastal city state has become one of the most populated metropolises in the world. Seawalls, headlands, and breakwaters were built along the coast to protect valuable land and inland assets from coastal erosion and inundation. The majority of Singapore’s contemporary coastline is thus reinforced by sea defences and other forms of coastal infrastructure.

The marriage of engineering and ecological knowledge is imperative to create or modify coastal structures that protect the coast and support biodiversity.

Sea defences to shore up the coastlines are a fundamental need for small coastal city states like Singapore, especially in the context of land scarcity, expanding population and impacts from climate change. While sea defences and coastal infrastructure are largely permanent engineered structures that are not purpose-built for supporting biodiversity, one strategy to enhance the capacity of these structures to compensate for and replace lost biodiversity without compromising their intended functions would also come through purposeful engineered modifications. This requires a mindset change that challenges us to understand the functions of coastal infrastructure beyond engineering goals and to explore opportunities for supporting and restoring biodiversity. Ecologically informed engineering in the design and construction of coastal infrastructure can reduce the loss of intertidal and shallow water biodiversity on artificial shorelines.

Biodiversity by chance

As it provides valuable economic, social and functional services, coastal infrastructure—such as jetties, floating pontoons, and seawalls—can also contribute directly and indirectly to habitat and biodiversity loss, as the construction of such infrastructure usually takes place in the shallow areas fringed by biodiversity rich intertidal and reef areas. However, as the character Dr. Ian Malcolm famously uttered in the 1993 movie “Jurassic Park”, “Life will find a way”, recent observations and studies in Singapore have shown that far from being barren, these artificially engineered structures can support unique assemblages of marine organisms. In particular, seawalls support a relatively high diversity of intertidal organisms and share several metrics with rocky shores, such as the number of species present and dominant species. For example, the presence of hard substrates, such as granite armour rocks used for shoreline reinforcement, can support the recruitment of biodiverse corals and other reef organisms in areas where reefs either used to exist, or could exist if suitable substrate were present. We observed this phenomenon along seawalls at reclaimed sites such as Pulau Semakau, East Coast, and Marina East, which continue to support rich assemblages of corals in less than a decade after the completion of reclamation works. We observed similar biodiversity revival within marinas, where the submerged walls of the floating pontoons used for berthing boats supported rich assemblages of marine organisms. In particular, the concrete coating used for the submerged walls provided suitable surfaces that encouraged the recruitment of marine organisms. However, the uniformity of seawall construction material, the inclination of their surfaces, and the lack of microhabitats such as holes, cracks, crevices and rock pools resulted in lower biodiversity assemblages compared to natural rocky shores.

Biodiversity by choice and reverse engineering

Our observations of biodiversity occurring by chance along artificially engineered coastal structures presented us with the perfect opportunity for studying the factors that facilitated their successful development, such as surface material, rugosity, slope gradient and hydrodynamic regimes, among others. We adapted and then applied these factors to intentionally enhance the biodiversity of other existing and future coastal structures. Recent investigations suggest that larval supply of marine organisms is not limited in Singapore. However, the availability of suitable habitat is limited in many areas. By introducing appropriate substrates in the right environment, coupled with effective management of human activities, we believe that marine biodiversity can be revived or enhanced along otherwise barren areas. One way to do this is through the reverse engineering of structures—i.e., the process of extracting design information from a manmade structures/objects and using this information to enhance other structures/objects—to understand the design and engineering aspects and environmental factors that facilitated the recruitment of organisms in the examples described above. We looked at the nature of the built structure from the type of material used, the methods of construction, surface complexity, inclination, hydrodynamic conditions, exposure to varying tidal regimes and anthropogenic activities, and the historical condition of the sites that contributed to their ability to host and support biodiversity. Based on those metrics, we investigated different strategies for biodiversity enhancement and developed the following framework to assess coastal structures and their capacity to host biodiversity [Figure 1].

figure-1_a-framework-to-assess-coastal-structures-and-their-friendliness-toward-biodiversity
Figure 1: A framework to assess coastal structures and their friendliness toward biodiversity.

Based on this framework, we identified and investigated strategies for increasing heterogeneity and complexity of built surfaces, introducing novel habitats such as tidal pools, enhancement units, and textured tiles, substrate manipulation, planting of coastal vegetation, and incorporating purpose built elements to coastal structures.

We found that for enhancement on existing seawalls and coastal structures, surface complexity is the most important and also the most easily manipulated amongst all assessment criteria. Complexity can be manipulated at different spatial scales, ranging from millimetre to metre, and targeting different organismal behaviour. We worked with four complexity parameters that were developed in a separate research project by our research collaborators from the National University of Singapore, namely (1) the number of object types; (2) the relative abundance of object types; (3) the density of objects; and (4) the variability and range in the objects’ dimensions, to design reverse engineered tidal pool units to be introduced along an existing stretch of seawall with a barren horizontal surface.

These tidal pool units consisted of purpose-designed and fabricated concrete modules measuring 1.5m x 1.0m x 0.3m. They were fabricated with concrete suitable for the marine environment using negative fibreglass moulds, and were designed to collect seawater during high tide and to retain it during low tide to mimic a tidal pool environment. These tidal pool units are expected to create habitats that are similar to natural rock pools, to provide additional niches, and to encourage more diverse assemblages of marine organisms to thrive within the area.

Janine M. Benyus’ description of biomimicry as “the conscious emulation of life’s genius” (1997) served as a source of inspiration for our design. To design the units, we first studied natural rock pool habitats to identify attributes that made them suitable for certain marine organisms to colonize and thrive, and found that a combination of crevices, grooves, and pits provided ideal niches and succession for a variety of marine organisms. These attributes were then incorporated in the design process, according to the four complexity parameters, to create conceptual designs that would most closely mimic natural tidal pool habitats. Multiple designs were created based on the different complexity combinations, and two designs were selected for testing.

The first design is a pool with a combination of evenly distributed grooves with pits of three sizes – 30 small pits (20mm diameter), 30 medium pits (40mm diameter) and 30 large pits (70mm diameter) [See Figure 2]. The multiple sizes of the pits enabled us to increase the spatial scale of this feature. Pits and groves are cast on an inverted topographic surface. This surface plan mimics a natural hilly landscape in Singapore (Central Catchment Nature Reserve), where the complex topography houses significant biodiversity.

The second design is a pool of the same surface area with a randomised arrangement of steps. The steps’ thickness was calculated based on the aforementioned complexity parameters [See Figure 2]. The angular edges and offset create niches for marine organisms. We also embedded some pits (3mm) into some of the units of this design to test out the combination of pits and steps.

figure-2a1

figure-2a2

figure-2a3

Figure 2A: (1) Design of the pool with pits and grooves [top]; (2) Cross section A-A [middle]; (3) Casted pool [bottom]

Janine M. Benyus’ description of biomimicry as “the conscious emulation of life’s genius” (1997) served as a source of inspiration for our design. To design the units, we first studied natural rock pool habitats to identify attributes that made them suitable for certain marine organisms to colonize and thrive, and found that a combination of crevices, grooves, and pits provided ideal niches and succession for a variety of marine organisms. These attributes were then incorporated in the design process, according to the four complexity parameters, to create conceptual designs that would most closely mimic natural tidal pool habitats. Multiple designs were created based on the different complexity combinations, and two designs were selected for testing.

The first design is a pool with a combination of evenly distributed grooves with pits of three sizes – 30 small pits (20mm diameter), 30 medium pits (40mm diameter) and 30 large pits (70mm diameter) [See Figure 2]. The multiple sizes of the pits enabled us to increase the spatial scale of this feature. Pits and groves are cast on an inverted topographic surface. This surface plan mimics a natural hilly landscape in Singapore (Central Catchment Nature Reserve), where the complex topography houses significant biodiversity.

The second design is a pool of the same surface area with a randomised arrangement of steps. The steps’ thickness was calculated based on the aforementioned complexity parameters [See Figure 2]. The angular edges and offset create niches for marine organisms. We also embedded some pits (3mm) into some of the units of this design to test out the combination of pits and steps.

figure-2b1

figure-2b2

figure-2b3Figure 2B: (1) Design of the pool with randomized steps [top]; (2) Cross section A1-A1 [middle]; (3) Casted pool [bottom]

We studied the hydrodynamic conditions of the site that may affect the service life of the tidal-pool structures. Through hydrodynamic modelling, we calculated mean current speed and changes in bed thickness per year to identify whether the seawall is subjected to strong erosion or accretion. Mean current speed is also an indicator that helps determine if the coast is subjected to strong hydrodynamic forcing, which might result in lateral movement or even dislodgement of the fitted tidal pool structures. While there are studies suggesting that introduced artificial structures can have a positive impact on sandy shoreline stabilisation, the introduction of these structures should not compromise the ability of the engineered coastal infrastructure to perform its primary function. In the case of seawalls, which are built for sea defence, the enhancement measures have to preserve sea wall structural integrity, as well as connectivity of coastal processes.

We are currently studying the relationship between the tidal pool designs and community assemblage and succession by assessing their ability to provide shade and regulate temperature using drained and un-drained units. In order to reduce bias and account for treatment or site effect, we positioned the different design configurations randomly along a linear stretch of seawall, and introduced control plots to assess the effectiveness of introduced structures versus no modifications. A control plot in this context is an empty plot on the seawall that is of the same size as the tidal pool units [See Figure 3]. Data collected on these control plots will act as a baseline against which the treatments/modifications will be compared.

figure-3_the-tidal-pool-units-positioned-in-a-randomised-layout-with-control-plots
Figure 3: The tidal pool units positioned in a randomised layout with control plots (empty slot without any tidal pool unit).

Currently, we are monitoring the units biweekly to gather data on recruitment and succession of fauna and flora, as well as environmental parameters such as temperature, conductivity, and irradiance. Preliminary results indicate that the tidal pools were occupied by turf algae within the first week after installation and, shortly after, this single species was replaced by an assemblage of algae including Bryopsis spp., Dictyota sp., Enteromorpha spp., Ceramiales spp. and Ceramium spp. Faunal diversity and abundance increased over time and, after several weeks, we recorded periwinkle and nerite snails, crabs, tube and fire worms, feather stars, and bead anemones. The performance of each tidal pool design and its complexity elements are also being monitored. The outcomes of this study are expected to provide a more comprehensive understanding of the combination of complexity treatments on species recruitment and biodiversity.

Projected sea level rise poses the most immediate threat to Singapore, and protecting our coastline has been identified as a priority in dealing with the effects of climate change. The current efforts to defend our coastal areas from erosion include the construction of walls and stone embankments. Thus, the marriage of engineering and ecological knowledge is imperative to create or modify coastal structures that both protect the coast and better support biodiversity. Ecologically informed engineering in design and construction of coastal infrastructure would reduce loss of intertidal biodiversity on our artificial shoreline.

Nhung NGUYEN, Karenne TUN, and Lena CHAN
Singapore

On The Nature of Cities

Karenne Tun

About the Writer:
Karenne Tun

Karenne’s current work at NParks covers issues related to the management and conservation of Singapore’s coastal and marine environment (CME) and the biodiversity they support. She leads a team that serves as Secretariat for the Technical Committee on Coastal and Marine Environment, an inter-agency committee that undertakes studies, provides technical inputs for policy making, and builds capacity in CME related issues.

Nhung Nguyen

About the Writer:
Nhung Nguyen

Nhung leads NParks’ initiative to improve coastal infrastructure for coastal and marine habitat enhancement. She is also managing research projects related to coastal biodiversity conservation and spearheading geographic database development.

Nhung Nguyen

Nhung Nguyen

Manager (Coastal and Marine)/National Biodiversity Centre, National Parks Board. Dr Nguyen obtained her PhD degree in Geography at Royal Holloway, University of London. She studied integrated coastal management with a focus on local governance and participatory mapping in coastal resource management. She joined the Coastal and Marine Branch, National Biodiversity Centre at the National Parks Board in 2010 to develop and implement Integrated Urban Coastal Management in Singapore. This involves bringing together and working with coastal stakeholders on cross-cutting issues of Singapore’s coastal and marine environment. Nhung is now leading NParks’ initiative to improve coastal infrastructure for coastal and marine habitat enhancement. She is also managing research projects related to coastal biodiversity conservation and spearheading geographic database development.

2 thoughts on “If You Build It, They Will Come: Modifying Coastal Structures for Habitat Enhancement

  1. The work described here is interesting. As stated in the last paragraph such work can increase intertidal biodiversity of coastal infrastructure and this is usually for low-energy coasts. But it should be noted that it cannot mitigate sea-level rise which needs different approaches, although in some cases, using natural means. There is a spectrum of ecosystem-based adaptation measures, including the living shorelines in the USA. An important point is sedimentation within the natural habitat, e.g. mangroves, is necessary to keep up with a rising sea level. For corals to be effective measures they should grow upwards sufficiently to keep with sea-level rise.

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