Sustainable Urban Stormwater Management in the Tropics: an Evaluation of Singapore’S ABC Waters Program ⇑ H.S

Journal of Hydrology 538 (2016) 842–862

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Journal of Hydrology

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Review papers Sustainable urban stormwater management in the tropics: An evaluation of Singapore’s ABC Waters Program ⇑ H.S. Lim a, , X.X. Lu b,c a College of Science and Engineering, James Cook University, Queensland 4870, Australia b Department of Geography, National University of Singapore, 1 Arts Link, Kent Ridge, Singapore 117570, Singapore c Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore, 469C Bukit Timah Road, Singapore 259772, Singapore article info summary

Article history: The Active Beautiful Clean (ABC) Waters Program was implemented in 2006 as part of Singapore’s Received 30 December 2015 stormwater management strategy and reﬂects the country’s move towards Water Sensitive Urbanism Received in revised form 4 April 2016 through the adoption of Low-Impact Development (LID) ideology and practices. It is the ﬁrst holistic Accepted 28 April 2016 and comprehensive LID program in the tropics and holds promise for extension to other tropical cities. Available online 7 May 2016 This paper presents a comprehensive summary of the goals, LID practices (ABC design features) and This manuscript was handled by Geoff Syme, Editor-in-Chief design considerations as well as results of several monitored sites, including a constructed wetland, two rain gardens, green roofs and three canal restoration projects. We evaluate the ABC Waters Program based on these initial results and consider the challenges, issues and the research needs for it Keywords: ABC Waters Program to meet its hydrological and water quality remediation goals. So far, the ABC design features evaluated Urban stormwater management perform well in removing particulates. Performance in nutrient removal is poor. With over 60 projects Tropics completed within 10 years, post-project monitoring and evaluation is necessary and complements on- Singapore going laboratory and modelling research projects conducted by local academic institutions. Ó 2016 Elsevier B.V. All rights reserved.

Contents

1. Introduction ...... 843 2. Urban stormwater runoff and Low Impact Development (LID)...... 843 3. Stormwater best management practices (BMPs) ...... 843 4. Singapore’s Active Beautiful Clean (ABC) Waters Program ...... 844 4.1. Goals, objectives and framework of the ABC Waters Program ...... 844 4.1.1. Regulatory and administration reform ...... 844 4.1.2. Technical development and implementation ...... 844 4.1.3. Building capacity ...... 844 4.1.4. Building social capital ...... 845 5. ABC Waters Design Features ...... 845 6. ABC Design considerations ...... 846 6.1. Location and sizing ...... 846 6.2. Growing media ...... 850 6.3. Plant selection ...... 851 6.4. Performance targets (treatment objectives) ...... 851 7. Examples of ABC projects – case studies ...... 851 7.1. Constructed wetlands – Grove Drive ...... 851 7.2. Rain gardens – Balam Estate and Nanyang Junior College (NYJC) ...... 852 7.3. Green roofs ...... 853 7.4. River/canal restoration – three examples ...... 854 8. Current knowledge, challenges, opportunities and research needs...... 855

8.1. The challenge of climate change ...... 857 8.2. The opportunity for innovation ...... 857 8.3. Research needs ...... 857 8.3.1. Understanding influent runoff quality better ...... 858 8.3.2. Factors that influence ABC design feature performance ...... 858 8.3.3. Performance during extreme events ...... 859 9. Beyond Singapore – LID practices in the tropics/sub-tropics ...... 859 10. Conclusions...... 859 Acknowledgements ...... 859 References ...... 860

1. Introduction moves polluted stormwater out of urban areas to avoid flooding. This reactive, single-purpose approach merely shift the problems The Active Beautiful Clean (ABC) Waters Program was launched downstream without restoring natural hydrological and geomor- in April 2006 by Singapore’s water agency, Public Utilities Board phic processes that are essential to healthy catchment-ecosystem (PUB). The Program’s main goal is urban stormwater management functioning (Brierley and Fryirs, 2008; Brierley and Fryirs, 2009). and flood control using LID practices that had become popular in End-of-pipe treatment systems are expensive to maintain and do Europe and USA. Over 60 BMP projects were completed in the last not consider values of waterways throughout the catchment 10 years under this Program and more than 100 projects are sched- (Collins et al., 2010; Roy et al., 2008). uled for completion by 2030 (Source: http://www.pub.gov.sg/ Nowadays, urban stormwater runoff is viewed as a resource. abcwaters/ABCcertified/Pages/ABCcertifiedProjects.aspx). Its suc- Cities are increasingly designed to retain stormwater runoff to cess is reflected in the number awards won by PUB for some key augment water supply, enhance thermal comfort and provide projects. ecological habitats and aesthetic landscape features that function Much has been written about Singapore’s approach to water as recreational and community-building spaces (Davis et al., management, from the water supply and quality management 2010; Hamel et al., 2013). The move towards water sensitive angle (e.g. Tortajada, 2006; Khoo, 2009; Tortajada et al., 2013; cities is part of a greater philosophy of smart urban growth Tortajada and Joshi, 2014). Malone-Lee and Kushwaha (2009) where cities are more sustainable and resilient to the impacts wrote about the ABC Waters Program from an urban design/infras- of extreme events and climate change (Novotny, 2009; CIRIA, tructure perspective whilst Irvine et al. (2014) used the program as 2013). an example in Singapore’s holistic approach to water resources This new approach adopts a whole-system, catchment-based management. There is no recorded published literature examining view to restoring the pre-development flow regime by bringing the entire ABC Waters Program from the perspective of the LID back natural hydrological (infiltration, storage, evapotranspiration) practices adopted and the corresponding field and modelling and geomorphic processes (sediment supply, channel planform data/results. Such information is useful for managing water- migration) in the urban environment (Vietz et al., 2012). More related issues in other tropical cities (see Irvine, 2013). recent discussions on stormwater management practices focus At its 10th year anniversary, this paper seeks to provide a com- on restoring critical components of the natural flow regime; flow prehensive summary of the ABC Waters Program in terms of its magnitude, duration, timing, frequency and variability of both high project goals, framework and types of BMPs adopted. We collate and low flows (Poff et al., 1997). Attention is also given to increas- the results of published and unpublished work related to LID prac- ing connectivity between different parts of the urban landscape, tice in Singapore to evaluate LID performance. Using this informa- both within the fluvial system (upstream–downstream linkages) tion, we evaluate the challenges, opportunities and research needs and the catchment (terrestrial–fluvial linkages) and treating pollu- for this Program to make more significant contributions to tants in urban stormwater runoff (Wohl et al., 2005; Fletcher et al., stormwater management in Singapore and elsewhere within the 2013). These ideas and management practices are known as Low tropics. Impact Development (LID), Water Sensitive Urban Design (WSUD) and Sustainable Urban Drainage Systems (SUDS) in different parts of the world and are encapsulated in legislative frameworks in the 2. Urban stormwater runoff and Low Impact Development (LID) US (USEPA, 1972), Europe (EU Water Framework Directive, 2000) and Australia (ANZECC, 2000). Impervious surfaces alter the hydrological flow regime and have a high pollutant transport capacity even when the percentage 3. Stormwater best management practices (BMPs) of impervious cover is as low as 5–15% (Brabec et al., 2002). Changes in flow regime, poor water quality, channel erosion and The underlying principle of LID/WSUD is to return the pre- degraded fluvial habitats are common problems found in urban development flow regime to an urban site and remove urban streams (Walsh et al., 2005). stormwater pollutants by adopting the following ideas (adapted Hydrological changes associated with urbanisation are from Prince George’s County, 1999; CIRIA, 2013): increased storm runoff volumes and peak flows (Qp), faster flow velocities and shorter time of concentrations. A reduction in infil- Use hydrology as an integrating framework. tration generally leads to less groundwater recharge and baseflow. Adopt microscale projects distributed throughout the The flashy response results in tremendous stresses for the urban catchment. stream and downstream receiving areas (Walsh et al., 2005). Control stormwater at its source. Pollutants in urban stormwater runoff originate from roofs Use simple technologies and employ natural hydrologic, chem- (heavy metals, organics, pathogens), road and parking lot runoff ical and biological processes to reduce stormwater volume and (heavy metals, hydrocarbons), parklands (nutrients, organics, remove pollutants. pathogens) (Laurenson et al., 2013). The traditional response is to Create multi-functional landscapes and infrastructures. construct a well-connected drainage network that efficiently 844 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

To restore natural hydrologic processes, BMPs attempt to: its point of origin (source approach). The pathway approach includes the traditional stormwater drainage network and canal 1. Minimise impervious areas. restoration projects (e.g. Kallang River). Protection measures such 2. Modify drainage flow paths to increase infiltration and water as flood barriers were also installed at locations prone to flooding loss through evapotranspiration; e.g. increase flow path dis- (receptor approach). This approach increases a catchment’s ability tances and roughness, decrease slope, change channel shape to cope with rainfall events that are greater than the drainage net- and patterns, promote vegetation. works’ design storm. It reflects PUB’s push for a water sensitive city 3. Improve water quality through sedimentation, filtration, by acknowledging natural hydrological processes and flow path- adsorption, precipitation, ion-exchange, infiltration and bio- ways in urban stormwater management. transformation (Sansalone and Hird, 2003). Figs. 1 and 2 show the timeline of important ABC Waters Pro- gram activities and a map of the completed projects respectively. A successful LID BMP achieves storm hydrograph attenuation The success of the Program is reflected in the number of interna- and water quality improvements. Typically, peak flows are reduced tional awards won by PUB as an agency (Global Water Awards while low flows are enhanced, although the LID-altered flow for the Utility Performance Initiative, 2012) and for individual pro- regime may not return to pre-development levels. jects; Waterfront Awards Program (2012) for the Kallang River- Each BMP is designed to fulfil slightly different roles in restoring Bishan Ang Mo Kio Park (hereafter referred to as the Kallang River) hydrological processes and water quality treatment, although a and the Asia Pacific Regional International Water Association Pro- BMP can be designed to be multi-functional (e.g. bioretention ject innovation Award (2012) for the Alexandra Canal project. basin) (Sansalone and Hird, 2003; Hamel et al., 2013). Green roofs mainly retain and delay roof runoff to reduce Qp (reductions 4.1. Goals, objectives and framework of the ABC Waters Program between 60% and 80% have been reported) (Trinh and Chui, 2013). They are more effective for small events due to their limited The ABC Waters Program includes three components. The Active storage capacity (Carter and Jackson, 2007). Bioretention systems component creates new community spaces around waterbodies are designed to retain and treat stormwater by promoting infiltra- and encourages environmental stewardship among citizens. The tion, groundwater recharge and evapotranspiration losses. They Beautiful component develops waterways and reservoirs into are very effective in reducing peak flows (44–63% reduction) and vibrant and aesthetically-pleasing spaces. The Clean component provide pollutant removal, via the growing media and vegetation, aims to improve the quality of water and to educate the public to despite their small size (Davis, 2008; Davis et al., 2009). The choice reduce water pollution. The construction of BMPs throughout Sin- of BMPs, their location and spatial arrangement within a catch- gapore falls under this component (PUB, 2014). The program draws ment determine their effectiveness. heavily from Australia’s Water Sensitive Urban Design (WSUD) framework for integrated stormwater management. The Singapore 4. Singapore’s Active Beautiful Clean (ABC) Waters Program framework includes the following components (Wong, 2011).

Singapore, a small urban island-state 137 km north of the Equa- 4.1.1. Regulatory and administration reform 2 tor (716 km ), is touted as a success story for its innovative Catchment planning policies, stormwater management plans at approach to urban water management in the face of severe envi- various scales and the development of performance targets and ronmental constraints (Lim, 1997). Rainfall is high (2338.5 mm/ compliance tools1 fall under this category. The ABC Waters Master year) and occurs throughout the year (178 rain days/year) (Source: Plan was launched in 2007. It adopts a 3P (People, Public, Private) http://www.weather.gov.sg/climate-climate-of-singapore/). Rain- approach involving cooperation between public agencies, private fall patterns are influenced by the monsoon system with more companies and the community. rainfall recorded during the Northeast monsoon season (end Octo- ber–March) than during the Southwest Monsoon season (June– 4.1.2. Technical development and implementation September) (Beck et al., 2015). Yet, Singapore is considered a This category involves research and development between gov- water-scarce country despite abundant rainfall. It ranks 170 out ernment agencies as well as tertiary institutions. Field experi- of 190 countries in freshwater availability due to a high population ments, monitoring activities at pilot demonstration sites (e.g. and limited water resources (UNESCO, 2006). The Singapore Balam Estate rain garden, Grove Drive constructed wetland) pro- Government plans to expand the water supply catchment areas vide important field data to check the performance and modelling to 90% of the country’s entire area, which includes many urbanised results of these BMPs. PUB published the ABC Waters Design areas that produce poorer runoff quality. Guidelines (2009, 2011, 2014), Handbook of Managing Urban Run- Singapore’s approach to stormwater management involves the off (2013) and Engineering Procedures for ABC Waters Design Fea- construction of dense networks of drains and canals that transport tures (2009, 2011a,b). The engineering procedures provide stormwater into reservoirs and the sea. Natural drainage pathways comprehensive technical information that includes design plans were previously lined, straightened and enlarged when necessary (including checklist tables), maintenance requirements and (Lim, 1997). LID practices were practiced in Singapore as early as worked examples for each BMP. the 1970s where stormwater retention ponds captured stormwater runoff and reused it for public greenery irrigation (Centre for Liv- 4.1.3. Building capacity able Cities, CLC, 2015). The ABC Waters program, signals a shift PUB builds technical expertise through its training and certifica- in thinking, favouring widespread LID practices for a more sustain- tion programs; ABC Certification (2010), ABC Professionals Pro- able approach to stormwater management. The larger goal is to gram (2011) and the ABC Professionals Registry (2013). The integrate waterways with the urban landscape to provide a more curriculum of local academic institutions now includes the princi- livable and sustainable urban environment (Irvine et al., 2014). ples and design of the ABC Waters Program (CLC, 2015). The recent flash floods in 2010 and 2011 further highlighted the role of BMPs in managing urban stormwater runoff. 1 Regulations for development sites require installing on-site measures to retain The ABC Waters Program initiated a source-pathway-receptor rainwater and surface runoff from entering the drainage network. Developers can approach to stormwater management (PUB, 2013, 2014). BMPs discharge the amount of water equivalent to a 10 year/4-h event (Water and were constructed throughout the country to treat stormwater at Wastewater Asia W.W., 2014). H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 845

ABC Professional 3rd edition of Registry ABC Water launched. Design Guidelines Handbook of published. Managing Urban Runoff Developers to published. reduce total discharge of Regulatory stormwater measures for runoff to open developers to roads to that of a ABC Waters use on-site 10year-4-hr Professional measures to event. Program slow down launched. surface runoff PUB announces ABC Waters Kallang River- implemented. 24 ABC projects Program ABC 2nd edition of Bishan Ang Mo completed and exhibition at ABC Waters 1st edition of Certification the ABC Design Kio Park project First river 48 projects Asian Master Plan Singapore ABC Design Program Guidelines completed ($76 classroom certified. Civilisations launched. International Guidelines launched. published. million). launched at Museum Water Week published Sungei Ulu Geyland River (Febuary). PUB identifies (SIWW) (June). Inter-Agency Alexandra Canal ABC projects at Pandan. upgrade 100-150 launched. Drainage project Sungei Pandan, completed (23rd ABC Waters locations for Sungei Review completed ($34 Sungei Ulu NYJC rain ABC project). Rochor Canal Program projects in the First rain Serangoon Committee million). It is the Pandan and garden ABC project launched next 10-15 garden built ABC project (IADRL) 14th ABC Geylang River completed Pang Sua Pond (April). years. (Balam Estate). completed. completed. launched.

2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

Fig. 1. Timelines of ABC Waters Program activities and key projects.

Fig. 2. Map of ABC projects located throughout Singapore (redrawn after PUB, 2014).

4.1.4. Building social capital of the Engineering guidelines (PUB, 2009, 2011b) do not have any A very important component of the ABC framework is social design information for green roofs and vertical greenery, even capital; the People component in the 3P approach. PUB invested though they are part of a holistic/integrated approach to stormwa- heavily in creating the public’s environmental stewardship from ter management in Singapore (PUB, 2011a, 2014). the onset of the ABC Waters Program through information signs ABC design features are designed to treat a 1-in-3 month aver- at ABC project sites, publications (e.g. PURE magazine), educational age return period storm event. All overﬂows are discharged into activities and encouraging citizen involvement in monitoring drains that are sized with a 10-year return period. They provide activities at ABC sites (Plate 1)(PUB, 2014). the following functions (PUB 2014):

5. ABC Waters Design Features Cleansing: all ABC design features. Detention: vegetated and bioretention swales, bioretention The Engineering Procedures for ABC Waters Design Feature basins, cleansing biotopes. (PUB, 2009, 2011b) provide guidelines for seven BMPs, now Conveyance: vegetated swales, bioretention swales. referred to as ABC design features. Table 1 summarise their pur- Inﬁltration: bioretention basin, bioretention swales, inﬁltration poses, treatment processes and maintenance issues. Both versions systems. 846 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

Plate 1. (a) Information signs found at each ABC design feature/site, (b) PURE magazine. Source: http://www.pub.gov.sg/mpublications/Pages/PureMagazine.aspx.

A survey of certified projects on the PUB website between 2010 groundwater table. One has to consider carefully the impacts of and 2014 reveals that the bioretention systems (bioretention swale increased groundwater recharge on underground infrastructure and bioretention basins/rain garden, n = 41), green roofs (n = 27) which are common in Singapore. In Syracruse, New York, unlined and green walls (n = 18) are the most popular ABC design features bioretention basins can cause groundwater mounding that lasts (Fig. 3). The most ambitious ABC project is the Kallang River pro- up to 40 days to recover to original levels for 1 and 2 years design ject, involving restoration of a concrete canal into a meandering storms respectively (Endreny and Collins, 2009). Modelling results river with an adjoining floodplain. This project is listed amongst for the Marina Catchment in Singapore found that the groundwater worldwide river restoration projects aimed at returning space to table can rise up to 1–2 m during the wet season due to recharge rivers; e.g. Room for the River (The Netherlands, https://www. from bioretention basins (Trinh and Chui, 2013). Frequent heavy ruimtevoorderivier.nl/english/) and Making Space for Water (UK, rainfall during the monsoon seasons may raise the groundwater Wilby et al., 2008). table even higher and pose a danger to underground infrastructure. While the underground transport system and pipes are generally below the groundwater table and designed for submerged condi- 6. ABC Design considerations tions, other underground infrastructures such as basement parking lots, connecting walkways and shops may be affected by ground- This section on design considerations deals mainly with vege- water table fluctuations. BMPs should therefore be carefully sited tated ABC design features since they are the most popular features with respect to local groundwater conditions. in Singapore. The size of BMP is affected by factors such as space availability, design objectives, climate, soil properties and vegetation. In Singa- 6.1. Location and sizing pore, sizing is controlled mainly by land availability. PUB recommends integrating ABC features into Singapore’s urban design to The engineering guidelines (PUB, 2011b) recommends that ABC minimize the use of land; integrating features within streetscapes design features are located at areas with slopes 1–4% for maximum or acting as an interface between urban development and the effectiveness. Infiltration-based design features such as bioreten- canals (PUB, 2011a). The size of ABC features can be as large as tion swales should be sited at least 1 m above the seasonal high 5%, but is generally 2% of the catchment area (PUB, 2011a,b). Table 1 Information about key ABC features; design, environmental processes and design issues (collated from PUB’s Engineering Procedures for ABC Water Design Features, 2011b, ABC Design Guidelines, 2014).

Purpose Environmental Location Design considerations Maintenance activity Maintenance Comments processes frequency (a) Sedimentation basins Trap sediments Settling (trap 70– Construction sites Settling pond Monitor sediment Once every 5 years Need more data on 90% of sediment in accumulation (or earlier) sediment load for size range of Singapore urban 125 lm) catchments Detain stormwater As a pre-treatment Size is determined by Fair and Greyer De-water and dredge before constructed equation for sedimentation the sediments wetlands Reduce stormwater flow Protect inlet from velocity erosion Check that outlet is not blocked with debris (b) Swale/buffer Provide a buffer between Delay flow to Mainly along streets, Vegetated Maintain good Frequency not stormwater generation and reduce stormwater parks, carparks, vegetation growth specified receiving streams to reduce volume and flow footpaths (remove weeds, prune

downstream erosion velocity, sediment vegetation, etc.) 842–862 (2016) 538 Hydrology of Journal / Lu X.X. Lim, H.S. deposition via vegetation Part of treatment Requires mild slopes (1–4%) Routine inspection of Maintenance train providing pre- vegetation should be treatment for conducted after wetlands and major storm event bioretention basins 2 Inspect inlet and outlet Ideal to treat: areas up to 0.01 km points for scour and blockage, etc. Sizing is based on treating minor (2– Remove litter and 10 yr ARIa) or major (50–100 yr ARI) debris events (c) Bioretention swale Provide treatment and Provides: Similar to swale but Flow must be evenly distributed across Make sure system Frequency not Consider hydraulic conveyance functions at the end of the filter media surface. Maintain low doesn’t experience specified conductivity and depth treatment train of flow velocities to prevent scour. (0.5 m/s excessive erosion or of filter media and of swales or buffers for 2–10 year, 2 m/s for 50–100 ARI deposition especially at surrounding soils events respectively) the inlet where scouring and erosion are more likely to occur Recover percolated Sedimentation, 600 mm filter media (sandy loam), Maintain vegetation Loh (2012) recom- Type of materials used stormwater for discharge infiltration through 100 mm transition zone, 150 mm growth mends more for filter media is into receiving waters via soil, detention and drainage zone (gravel zone, 2 mm size) intensive mainte- important to achieve perforated underground biological uptake nance during the pollutant removal, pipes below the drainage mainly achieved in first year of opera- plant growth layer (baseflow the filter media tion when plants augmentation) or storage layer are establishing to for reuse as potable water prevent excessive (i.e. exfiltration into weed growth surrounding soils discouraged) Anoxic zone is sometimes included Refer to Loh and Hunt Design largely based on (2013) Australian guidelines which need to be corrected for Singapore conditions such as

optimal sizing, SAZ 847 (include or not?), depth of ﬁlter media, etc. (continued on next page) 848

Table 1 (continued)

Purpose Environmental Location Design considerations Maintenance activity Maintenance Comments processes frequency Effluent should be <5-year ARI event K s of filter media should be less than 500 mm/h for optimal plant growth (preferably 50–200 mm/h) Sizing varies on site-basis and can be determined via modelling (MUSIC model) (d) Bioretention basins Same as bioretention swale Same as Implemented at Similar to bioretention swale Same as bioretention Same as Same as bioretention or rain gardens with main focus on bioretention swale various scales from swale bioretention swale swale detention of floodwaters via planter boxes to with main focus on ponding streetscape rain detention of gardens floodwaters via ponding Usually incorporated Submerged zone ensures system Refer to Loh and Hunt with pre-treatment performance during dry periods (2013) ..Lm ..L ora fHdooy58(06 842–862 (2016) 538 Hydrology of Journal / Lu X.X. Lim, H.S. measures at the (maintain soil moisture and plant growth entry to the system and nitrogen removal) Lined or unlined, depending on in-situ soil Ks Ponding is usually to a level of 0.3 m above filter media surface (e) Constructed Shallow extensively Sedimentation, fine Standalone or part of Consider the interaction between Vegetation checks and Inspection every Design for adequate wetlands vegetated water bodies that filtration, biological a treatment train for wetland hydrology and hydrodynamic maintenance to ensure 3 months during detention time for treat urban stormwater processes for floodwater detention behaviour with the various physical, healthy growth (e.g. first year of environmental quality pollutant uptake purposes. The latter chemical and biological treatment weeding, planting, establishment, processes to remove function requires the processes. These include: debris removal) thereafter every pollutants wetland to be uniform distribution of flow velocity 6 months located upstream of hydrologic regime (wetness gradient, other treatment baseflow and inundation depth), elements uniform vertical flow velocity, scour protection Design flow for the inlet zone is the 1- year ARI event using the Rational Formula or flood routing method if the wetland is large or part of a treatment train Includes an inlet zone Recommended detention time in Mosquito control (coarse sediment removal), macrophyte zone is 48–72 h to macrophyte zone (heavily adequately remove pollutants vegetated area for (particularly nutrients). Detention depth sedimentation and should be approximately 0.5 m deep for pollutant uptake) and high shallow to deep plants and open water flow bypass channel (to protect macrophyte area) Focus is to remove fine to Clayey soils required for macrophyte Water quality colloidal particulates and zone monitoring dissolved contaminants Plants should be chosen carefully to ensure 70–80% cover with 2 growing seasons (see Yong et al., 2010) Provide mosquito predators and adequate water depth for their survival to prevent mosquito vector diseases (e.g. dengue) (f) Cleansing biotopes Similar to constructed Particularly good at Highly flexible Detention time (function of size) is Same as constructed Not stated Sustain oxygen wetlands. Soil media is breaking down important wetlands availability for nutrient poor and planted organic pollutants biological with aquatic vegetation that (particulate and transformation will offer pollutant uptake dissolved) via processes sedimentation and oxic biological transformation (via bacterial community) High performance for It can be located at Constant and evenly-distributed flow Vegetation should be slightly unpolluted outdoor areas (e.g. throughout the system required for diversified and able to stormwater runoff (PUB did parks, playgrounds, optimal performance (i.e. inflow volume withstand fluctuating not define what slightly ponds, lakes), on the and loading are important design water levels. polluted means), low roof or underground considerations) Indigeneous vegetation maintenance and with deep roots are aesthetically-pleasing preferred Usually has different sections that are Performance should connected to each other for treatment improve over time as and regeneration functions of each vegetation and 842–862 (2016) 538 Hydrology of Journal / Lu X.X. Lim, H.S. section bacterial community establishes itself Recover the treated Substrate media and vegetation should stormwater for discharge be determined on a case-by-case basis into receiving waters depending on site characteristics (baseflow augmentation) or storage for reuse as potable water (g) Infiltration systems To capture stormwater Infiltration It includes an area to detain water (above To ensure the system is Routine inspection Performance curves are Leaky wells runoff and encourage or underground) and an infiltration area not clogged, inlet required obtained from MUSIC Infiltration infiltration into in-situ soils (the interface between detention and points should not be modelling trenches/soakaways/ and groundwater surrounding soils) blocked. Till the surface basins if the surface is clogged. Maintain vegetation Water quality treatment is Groundwater The infiltration We need empirical not an objective recharge zone should be data to test these inspected every 1– curves to see if they are 6 months applicable across Singapore 2 Size of catchment treated: ) Leaky Infiltration wells (<0.001 trenches km (<0.001–0.1 km2) Infiltration soakaway (<0.001–0.1 km2) Infiltration basins (>0.1 km 2) Soils with hydraulic conductivity between 3.6–360 mm/h preferred Design flow for the inlet zone is the 1-year ARI event using the Rational Formula or flood routing method if the wetland is large or part of a treatment train a Average Recurrence Interval (ARI) refers to the estimated time period between storm events of a given magnitude. Typical ARIs used by PUB are 1 in 2-year, 5-year, 10-year and 50-years (PUB, 2014). 849 850 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

10 Number of projects 5

Fig. 3. Number of certified projects for the various ABC design features, 2010–2014. Data source: http://www.pub.gov.sg/abcwaters/ABCcertified/Pages/ABCcertifiedProjects. aspx.

The engineering guidelines provide sizing curves for each ABC surface or at depth, degradation and saturation of pollutant uptake design feature based on modelling results using the Model for capability over time (Hatt et al., 2007; Siriwardene et al., 2007; Li Urban Stormwater Improvement Conceptualisation, MUSIC (PUB, and Davis, 2008; Le Coustumer et al., 2009). Le Coustumer et al. 2011b). These sizing curves are a function of the surface area of (2009) measured 37 biofilters in Melbourne, Sydney and Brisbane the design feature (as a percentage of impervious catchment) ranging between 0.5 and 3 years old and found that Ks decreased against the percentage reduction of target pollutants (Fig. 4). 50% with age. This problem may be counteracted by over-sizing, Recent modelling work using COMSOL for a rain garden in north- using coarser materials for the filter media or taking advantage eastern Singapore also produced graphs that showed the relation- of root systems of planted vegetation to maintain surface infiltra- ship between rain garden surface area and expected overflow tion rates. Over-sizing a system is not an attractive option in Singa- volume and vertical exfiltration rate (Mylevaganam et al., 2015). pore’s case. The coarse nature of ASM soil, presence of a pre- treatment sedimentation basin and vegetation should however, 6.2. Growing media reduce the impacts of clogging and prolong the life of the ABC design features. Field evidence is contradictory for two rain gar- The composition of the growing media used for ABC design fea- dens in Singapore. At the Balam Estate rain garden, soil texture tures must maintain a balance of (a) providing infiltration and (b) for the top 10 cm exhibited trends of fining over a 6-year period contain the necessary physico-chemical properties for pollutant (sandy loam to sandy clay loam) (Kernan, 2014). Measurements removal and nutrients for plant growth (Clark and Pitt, 2012). of Ks similar to Le Coustumer et al. (2009) at another rain garden Singapore generally has clayey and poorly drained soils that are (Nanyang Junior College, NYJC rain garden) found that Ks decreased not entirely suitable for the above purposes (PUB, 2011a,b). The by an order of magnitude during the first year of operation (from commonly used soil mix in Singapore (Approved Soil Mixture, 407 to 53 mm/h) but recovered in the second year, presumably ASM soil) for roadside greenery and ABC design features include due to the establishment of plant cover (Tan, pers. comm; Guo the following composition; clay (5–30%), silt (5–60%) and sand et al., 2014). (20–75%) (NParks, https://www.nparks.gov.sg/~/media/nparks- Because the growing media treats a wide variety of stormwater real-content/partner-us/developers-architects-and-engineers/de- pollutants, its design requires a clear understanding of the poten- velopment-plan-submission-requirements/greenery.pdf?la=en). tial sources and concentrations of pollutants and their hydraulic PUB (2011b) recommended sandy loam material for ABC design loading and removal mechanisms (see Clark and Pitt, 2012). features (PUB, 2011b, based on FAWB (2007) guidelines). This Organic materials are generally added to provide a carbon source mix provides adequate stormwater detention and contact time and nutrients for plant growth but they may leach out nutrients for pollutant removal and water for plant growth and is commonly and remobilize metals such as copper (Hatt et al., 2008; Lim used elsewhere (e.g. Davis et al., 2009). The soil characteristics for et al., 2015). Although PUB adopts design media based on Aus- bioretention systems according to PUB (2011b) are: tralian guidelines (FAWB, 2007), research at local institutions are now looking at using recycled or waste materials available locally Clay: not stated specifically. to amend the filter media used in ABC design features. This is due Silt: not stated specifically. to the scarcity of sand in Singapore and also the move towards sus- Sand: not stated specifically. tainability through recycling waste material. Laboratory experi- Organic matter: 3–10%. ments using compost, coconut fibre, residuals from the water pH 5.5–7.5. treatment process (WTR) and concrete waste found that WTR is Salt content: <0.63 dS/m. the best material to use because it does not leach organics or nutri-

Saturated hydraulic conductivity (Ks): 50–200 mm/h. ents and is relatively stable over time (Guo et al., 2015; Lee et al., 2015; Lim et al., 2015). Columns ﬁlled with a combination of sand Problems associated with the growing media in BMPs are (80%), silt (5%), WTR (10%) and compost (5%) led to high removals decreased performance due to sediment clogging, either at the of TSS and TP (93.4% and 92.7% respectively) and relatively high TN H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 851

Fig. 4. Sizing and performance curves for a bioretention system showing the relationship between bioretention surface area and total nitrogen (TN) removal for different ponding depths (no ponding, 100 mm, 200 mm, 300 mm ponding) when Ks of the ﬁlter media is 360 mm/hr Source: Fig. 6.6, PUB (2011b).

removals (59.8%) when stormwater was flushed through them concentrations or loads are compared with their effluent counter- (Guo et al., 2015). This work resulted in a patent2 for engineered parts. In Singapore, targets are set for 3 water quality parameters; soil for Singapore. Initial results from the NYJC rain garden using a total suspended solids (TSS), total phosphorus (TP) and total nitro- combination of sand (35%), compost (5%), WTR (10%), silt (10%) gen (TN) to the levels shown in Table 2 for 90% of storm events and local topsoil (40%) performed reasonably well during the first (PUB, 2014). These performance targets are similar to the Aus- one and a half years of operation. Effluent was within the limits of tralian Runoff Quality Objectives (Burns et al., 2012). the treatment objectives, with TSS, TN and TP exceedance probabil- Table 2 gives a summary of TSS, TN and TP concentrations in ities of 23.5%, 11% and 29% (Guo et al., 2014). Singapore’s urban runoff for comparison with the ABC performance targets. Singapore’s runoff quality is generally comparable with 6.3. Plant selection other sites around the world and nutrients are generally on the low-end. TSS concentrations are quite variable and concentrations Vegetation in BMPs enhance permeability in the growing on the high-end are often due to localised discharge of construc- media, slows surface flow to encourage sedimentation and pollution waste into the drainage network. Nutrient concentrations tant removal. Native species are encouraged because they are are typically low or slightly above the ABC target concentrations adapted to local climate and soil conditions. There are guidelines with the exception of samples from the Stanford Canal (Chui, for plant selection for rooftop greenery and waterbodies published 1991, 1997). The calculated removal efficiency value may be low by National Parks Board (NParks)’s Centre for Urban Greenery and when inflow concentrations are low, giving a false impression of Ecology (CUGE) (Tan and Sia, 2008; Yong et al., 2010; Tan and poor performance (Strecker et al., 2001). Exceedance probability Chiang, 2011). curves provide a better indicator of performance, showing the Research on tropical plants, especially native species, suggests probability that a pollutant exceeds its treatment objective that they can successfully remove nutrients from urban stormwa- (Fassman, 2012). ter runoff. A study of 30 species of tropical plants planted in bioretention columns found that almost all of the species effectively 7. Examples of ABC projects – case studies removed nitrate. 24 species and 11 species reported removals of 60% and 85% respectively. Arundodonax var. versicolor and This section presents preliminary work on a range of ABC design Bougainvillea ‘Sakura Variegata’ achieved 95% removal. Phosphate features including constructed wetlands, rain gardens, green roofs removal was achieved mainly by the growing media (Loh, 2012). and river restoration projects from published journal articles and Native trees (e.g. Elateriospermumtapos) effectively removed nitro- student theses. gen from spiked runoff by storing it in their leaves, which can be removed by regular maintenance (Chen et al., 2014). 7.1. Constructed wetlands – Grove Drive 6.4. Performance targets (treatment objectives) The Grove Drive constructed wetland receives stormwater run- Removal efficiency or percentage load reduction is the most off from a predominantly residential catchment. Treated water commonly-used metric for BMP performance where influent drains into a major stormwater canal (Plate 2). The wetland is undersized (512 m2, 0.06% of its catchment area) when compared to recommended sizing for constructed wetlands (0.5–2%) (Tilley 2 J.Y. Hu, S.L. Ong, S.H. Chew, C.Y. Tan, H.L. Guo, L.Y. Lee, F.Y. Lee, B.L. Ong, X. Chen, T. H. Chang, H.S. Lim. ‘‘An engineered soil composition and a method of preparing the and Brown, 1998). This system consists of a sedimentation basin same” (Singapore Patent Application No: 201308272-2). and a free-water surface wetland with three consecutive treatment 852 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

Table 2 Comparison of ABC treatment objectives (TSS, TN, TP) with urban stormwater quality from various sites around Singapore and elsewhere.

TSS (mg/l) TN (mg/l) TP (mg/l) ABC Waters Program treatment objectives <10 <1.2 <0.08 Summary of the urban runoff studies in Singapore (details given below) 0–9145 0.07–3.51 0.02–2.3 Summary of urban runoff concentrations worldwide (Makepeace et al., 1995) 4–1223 0.82–16.0 0.02–0.82 Specific studies around Singapore Stanford Canal (Chui, 1991, 1997) 18–223 – 0.9–2.3 Bedok Catchment (Lee, 2001) 20–947 – – Queenstown (Lim, 2003) 0–2785a 0.91–2.48a 0.048–0.89a 30–550b 0.26–3.51b 0.048–0.57b Marina Catchment (Lam, 2008) 0–2959a –– 81.5–323.8b Jurong (Lee, 2009) 1.01–8459 – – Ang Mo Kio (Lee, 2009) 0.99–9145 – – Average inflow into Grove Drive constructed wetland (Lee, 2012) 1.6–114 – – Average inflow concentrations into Balam Estate rain garden (Ong et al., 2012) 14.5–35.9 0.83–1.93 0.09–0.19 Average inflow concentrations into NYJC rain garden (Guo et al., 2014) 1.1–8.3 0.07–0.75 0.02–0.12

a Baseﬂow samples. b Storm samples.

cells, each having different configurations of aquatic vegetation (Cyperus haspen, Eleochaus dulcia, Lepironia articulata, Scirpus gros- sus) and open water. The hydraulic retention time is 72-h. Water samples were taken approximately 1.5 years after construction of the wetland (Lee, 2012). Baseflow samples were taken every 2 weeks from 30 November 2011 to 8 February 2012. Sam- ples were taken from five storm events and tested for TSS, nitrate

(NO3-N), ammonia (NH3-N) and orthophosphate (PO4-P). The constructed wetland was more efficient in removing TSS than nutrients, for baseflow and stormflow conditions (Table 3). Nitrate and ammonia removal efficiency are more variable between baseflow and storm conditions. The wetland performed considerably worse in phosphate removal and becomes a source of this nutrient. Negative removal efficiencies are reported for baseflow (À58.4%) and storm samples (À333.2%). Chua et al. (2012)’s study of a floating wetland (Chrysopogon zizanioides (Veti- ver grass), Typha angustifolia and Polygonum barbatum) in another Plate 2. The Grove Drive constructed wetland runs parallel to and drains into the Sungei Ulu Pandan canal. Source: http://watersensitivecities.org.au/singapore- part of Singapore found that nitrogen (TN: 7.8–67.5%) was more launches-its-abc-water-professional-certification/. effectively removed than phosphorus (TP: 19.1–46%), similar to the patterns observed at Grove Drive, although their system was more effective in TP removal. Even with the preliminary results of the Grove Drive study, it is 7.2. Rain gardens – Balam Estate and Nanyang Junior College (NYJC) clear that better performance is achieved for nitrogen removal than for phosphorus for both wetlands. The negative removal effi- Balam Estate is the first ABC project built in 2008. It is a rain ciencies for orthophosphate at Grove Drive and large standard garden (240 m2, 4% of its catchment area) that drains a highly deviations reflect the very wide range of removal efficiencies for urbanised public residential catchment (Ong et al., 2012). The fil- baseflow and stormflow samples ranging from positive to negative tered water is discharged into a storm drain that flows into the values. Negative removal efficiencies may be due to anaerobic con- Marina Reservoir, Singapore’s first urban reservoir. The filter media ditions which release particulate-sorbed phosphorus back into the depth of this rain garden is shallower than recommended due to water column or additional sources of phosphorus entering the the higher depth of the culvert and outlet pipe connecting the sys- wetland system from surface runoff via the adjacent slope (Lee, tem to the drainage canal. The system consists of a detention pond- 2012). Saturation of the wetland growing media also results in ing layer (100 mm), filtration layer (400 mm), saturated anaerobic phosphorus release especially during larger rainfall events (Lucas zone (SAZ) (400 mm) and drainage layers (150 mm). Anaerobic et al., 2015). A recent review of constructed wetland performance bacteria in the SAZ convert soluble nitrogen to nitrogen gas via in the United Kingdom reported a wide range of removal efficiency the process of denitrification (e.g. Dietz and Clausen, 2006). Table 4 values for nitrate and orthophosphate, similar to the Singaporean summarises the performance of the rain garden during the moni- results (refer to Table 3, Lucas et al., 2015). At this point, there is toring period. not enough data to evaluate the effectiveness of constructed This rain garden performs best in removing TSS and TN. Per- wetland systems in improving Singapore’s stormwater quality. centage load reductions are higher than concentration reductions. The large space requirement for constructed wetlands and their Ong et al. (2012) attributed the comparably higher removals for TN potential for mosquito breeding presents problems for their wide- (vis-à-vis TP) to the SAZ. However, a study conducted when the spread use across Singapore compared to other LID systems such as rain garden was more than 2 years old reported that there was a green roofs and bioretention/rain garden systems unless systems net export of nitrogen forms (ammonia, Total Kjeldahl nitrogen) such as floating wetlands are constructed on existing ponds or from the SAZ due to leaching or release of nitrogen captured from reservoirs. previous events. Most nitrate removal occurred in the top 50 mm H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 853

Table 3 Comparison of removal efﬁciency (concentration) of water quality parameters for baseﬂow and storm samples, Grove Drive (Lee, 2012) with results from other studies.

Removal efﬁciency (%) TSS TN NO3-N NH3-N PO4-P Baseﬂow conditionsa 91.6 ± 5.6 – 65.4 ± 35.3 42.7 ± 26.4 À58.4 ± 205.7 Storm conditionsb 78.9 ± 33.5 – À39.7 ± 238.6 62.2 ± 32.0 À333.2 ± 726.0 Chua et al. (2012)c – 7.8–67.5 – – Zheng et al. (2016) – 56.3 – 57.5 69.2 Lucas et al. (2015) –UK À21 to 66 À13 to 88 Lucas et al. (2015) – other parts of the world À18 to À48 17 to 51

a n = 6, mean ± standard deviation. b n = 9, mean ± standard deviation. c The removal efﬁciencies are based on the results from 3 types of vegetation tested in their ﬂoating wetland study.

of the anaerobic zone (400 mm) leaving the majority of this zone designs are found in Singapore, though intensive versions are more (Ritter, 2013). Kernan (2014)’s study when the rain garden was common3 (Mithraratne, 2013). 6 years old reported ammonia and phosphate leaching from the Field and modelling studies of green roofs in Singapore have SAZ. been conducted at the roof plot and catchment scale mainly using The NYJC rain garden, by contrast, does not have an anaerobic modular systems with a variety of growing media depths. A study zone. Its size (14%) is larger than the recommended 2% size for in Western Singapore reported average rainfall retention of 57% for ABC design features (Guo et al., 2014). Monitoring results during the entire study period for a setup with 120 mm growing media. its first year after construction found that influent concentrations This green roof system failed to retain rainfall once storage was of TSS, TN and TP are low when compared to PUB treatment filled and did not supply baseflow during dry spells (van objectives (see Table2), with exceedance probabilities of 12%, Spengen, 2010). Individual rainfall retention from another study 3.6% and 14%, respectively. Effluent TN concentrations decreased conducted nearby ranged between 62.6% and 68.1% for a green roof over time within the first year. No trend was observed for TSS setup with 7-l reservoir storage and growing depths of 7.5, 12.5, and TP removal. Effluent concentrations for all 3 pollutants were and 17.5 mm (Lim, 2012). The average retention of green roofs in higher than influent samples for some storm events, attributed Singapore (57–68%) is consistent with values obtained from stud- to leaching from the soil media which was still stabilising ies elsewhere (50–89%) (e.g. Carter and Rasmussen, 2006; post-construction. Mentens et al., 2006; Voyde et al., 2010; Berndtsson, 2010; Speak These two examples, although different in terms of design and et al., 2013; Razzaghmanesh et al., 2014). The limited storage age, highlight common issues to bioretention/rain garden systems capacity of green roofs once they are saturated means that these in Singapore; design (SAZ or not) and leaching problems associated systems have a minimal role in runoff mitigation (and flood con- with the growing media. The leaching phenomena seen at the trol) during the monsoon season when rainfall is frequent and/or Balam Estate site (ammonium and phosphorus) is a common intense, or for extreme events (e.g. Lee et al., 2013b). Given these occurrence observed for bioretention systems with SAZs (e.g. scenarios, the hydrological retention role of green roofs in Singa- Hunt et al., 2006; Zinger et al., 2013; Lee et al., 2013a). Leaching pore can be increased by increasing their spatial coverage within of nutrients at Balam Estate could be due to the decomposition a catchment or using deeper growing media. VanWoert et al. of materials added as a carbon source for denitrying microrganisms (2005) found that increasing the media from 25 to 40 mm led to in the SAZ. PUB (2011b) guidelines suggest a carbon source of retention increases of less than 3 percent for their extensive green mulch (5%) and hardwood chips (5%) in bioretention systems (by roofs. Their growing media depth is shallower than green roof sys- volume). A local study conducted by Lee et al. (2013a) achieved tems tested in Singapore (7.5–17.5 mm) and elsewhere (e.g. higher TN removal with a carbon source of vermicompost from between 20 and 150 mm, average of 100 mm for extensive green grass clippings and vegetable waste and a longer contact time roofs, Mentens et al., 2006; Berndtsson, 2010). Apart from a deeper (3 h) than a column system without vermicompost (100% sand) growing media, higher air temperatures and greater plant biomass and shorter contact time (1 h). Ideally, decomposition of the car- in tropical areas should increase evapotranspiration losses, bon source material should be equal to microorganism uptake. increasing rainfall retention by green roof systems in the tropics The continued leaching even after 6 years of construction at this compared to temperate climates. site is worrying and signals a need to re-evaluate the role of SAZ The hydrological impact of green roofs at the catchment scale in tropical systems as well as the materials used in the media. A was modelled using MIKE-SHE (Trinh and Chui, 2013). Green roofs judicious choice of carbon source and contact time may bring out (14% of the catchment) reduced Qp by 30–50% and delayed runoff the TN removal potential of a SAZ. Alternatively, rain garden by 2 h. To test green roof performance in mitigating extreme rain- designs without SAZ may perform equally well in nitrogen removal fall events in Singapore, van Spengen (2010) combined HYDRUS- especially if locally-optimised soil mixes are used (e.g. NYJC) site. 1D and SOBEK models to a small catchment in Western Singapore This hypothesis can only be tested with longer-term monitoring where 18.8% of total catchment was covered by green roofs (Sunset data from both sites. Way catchment). The green roof system reduced runoff by 0.77% and 6.78% for dry and wet conditions respectively, relative to a bare roof. Runoff reduction for driest conditions when retention 7.3. Green roofs was expected to be highest was modest (37%) for a 5-year design storm. Green roofs are rooftops that have vegetation growing on some soil mixture. Green roof design is classified as either extensive or 3 intensive, depending on the depth of growing substrate/media. PUB makes a distinction between green roofs and roof gardens in the Managing Urban Runoff Handbook (PUB, 2013). Green roofs are similar to extensive roofs Extensive green roofs have shallower growing media, typically less whereas roof gardens are akin to intensive roofs, which have a thicker growing media than or equal to 100–160 mm whereas intensive green roofs have and are more accessible, thereby creating multi-functional spaces. We use the term deeper growing media (Carpenter, 2008; Berndtsson, 2010). Both ‘green roofs’ in this paper to refer to both intensive and extensive systems. 854 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

Table 4 Concentration and load reduction of TSS, TN and TP in stormwater efﬂuent, Balam Estate rain garden, August 2009 to July 2010 (Ong et al., 2012) compared with similar systems studied elsewhere.

TSS TN TP Concentration reduction by the system (%)a 57.0 ± 16.2 41.1 ± 16.8 21.5 ± 0.01 Load reduction by the system (%)a 73 ± 12.3 64 ± 15.5 53.1 ± 14.0 Dietz (2007)b À170 to 96 40–59 À240 to 87 Davis et al. (2009)b 54–99 32–94 À240 to 79 Koch et al. (2014)c –29–

a Data based on 6 storm events. b Summary of several studies on bioretention systems. Numbers are reported as load reductions. c Based on results of 7 swales.

Table 5 Comparison of nutrient concentrations in rainwater and green roof outﬂows between a Singapore study and other green roof systems against the ABC treatment objectives.

Rainwater (mg/l) Green roof outﬂow (mg/l)

TN TP NO3 PO4 TN TP NO3 PO4 ABC treatment objectives 1.2 0.08 Singapore Vijayaraghavan et al. (2012)a – – 1.2–1.5 3.3–4.3 – – 0.34–0.86 19.8–40.0 Teemusk and Mander (2007) – – 0.18 0.004 – – 0.42–0.80 0.006–0.066 Hathaway et al. (2008) 0.06–0.95 0–0.5 – – 0.7–6.9 0.6–1.4 – – van Seters et al. (2009) 0.70 0.72 Berndtsson et al. (2009) 2.65 0.04 1.03 0.02 2.31c 0.31c 0.07b 0.27b 0.59d 0.01d 0.11c 0.00c Gregoire and Clausen (2011) – – – – – – 1.64 0.06 Seidl et al. (2013) – – – – – – 1.1d 3.8d 5.0e 6.0e Razzaghmanesh et al. (2014)b – – – – – – 2.2–39.2 0.2–2.2 Beecham and Razzaghmanesh (2015) – – – – – – 1.0–40.0f 0.03–2.37f 1.01–100g 0.04–4.39g

a The growing media is 15 cm deep using local garden soil or a commercial substrate, DAKU, consisting of inorganic volcanic material, compost and organic and inorganic fertiliser. b Extensive roof. c Intensive roof. d 6 cm growing media depth. e 16 cm growing media depth. f Growing media: brick. g Growing media: scoria mix.

From a water quality perspective, green roofs do not necessarily floodplain in 2011 (Plate 3, Hauber, 2014)(Plate 3). The re- improve urban stormwater quality due to leaching from its grow- created floodplain stores runoff during high-flow periods and ing media (e.g. Gregoire and Clausen, 2011; Speak et al., 2014).Two becomes a recreational area during low flow periods (Hauber, studies on green roofs in Western Singapore reported elevated 2014). Cleansing biotopes treat stormwater runoff and release it levels of nutrients (nitrate and phosphorus) and metals (Ca, Mg, back into the Kallang River. These improved conditions saw the Na, K, Fe, Cu, Al) leached from the growing media used recovery of some ecological communities (e.g. otters) but the sys- (Vijayaraghavan et al., 2012; Vijayaraghavan and Joshi, 2014). tem is not completely self-functioning as fish-kills occur due to Nutrient outflow concentrations from green roofs in Singapore unknown reasons (e.g. illegal discharge upstream or low flows are close to the ABC treatment objectives for nitrogen but exceed and/or high temperatures that reduce oxygen levels in the canal4). the allowable values for phosphorus by an order of magnitude Alexandra Canal is a 2.35 km long canal ($34 million) that was and are higher than concentrations reported from other studies redesigned so that 1.2 km of its middle stretch included several (Table 5). This is probably a function of the growing media that ABC design features (wetlands, bioretention swale, rain garden included a variety of fertilisers (Vijayaraghavan et al., 2012). and sediment bays) and landscaped gardens (cascading falls and Nitrate concentrations are comparable or below those obtained a rockscape garden) (Plate 4). Qualitative observations and inter- from other studies (Table 5). These results highlight the important views with local users of this space reported poor maintenance. role of growing media composition in controlling the nutrient out- Vegetation was overgrown, the water was turbid with traces of flow from green roof systems and may have an important role in oil and was described as ‘smelly and dirty’ (Ng et al., 2014). Stu- determining whether eutrophication occurs in urban drainage net- dents collected baseflow samples from the Alexandra Canal and works or not in areas where green roofs are installed extensively in compared it with another canal that did not have any ABC design the catchment. features (Bedok Canal). Baseflow nitrate levels were high at both sites (greater than 10 mg/L) but looked green at the Alexandra Canal, corroborating the results of the nitrate field kit. Field obser- 7.4. River/canal restoration – three examples vations suggest that the Alexandra Canal had a greater range of biodiversity. Fish, terrapin, snails and dragonflies were only spot- Three river projects are presented to showcase the different ted at the Alexandra Canal. The ABC design features at Alexandra levels of complexity of canal restoration projects in Singapore. The Kallang River restoration project is the most ambitious and expensive ABC project in Singapore ($76.7 million SGD). A 2.7 km 4 http://www.channelnewsasia.com/news/singapore/bishan-park-water-quality/ long concrete canal was transformed into a more natural river sys- 1999276.html, http://www.straitstimes.com/singapore/hundreds-of-dead-fish- tem over a course of 3 years into a 3 km meandering river with a found-in-bishan-ang-mo-kio-park-river. H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 855

Plate 3. Photos of the Kallang River (a) before river restoration and (b) after restoration. Source: Hauber (2014).

Canal had a positive impact on biodiversity but did little to functions. Continuous monitoring of dissolved oxygen levels (DO, improve water quality. % saturation) during baseflow conditions at a Queenstown drain The third example, Pang Sua Canal, involves softscaping 100 m revealed significant fluctuations in oxygen levels over a diurnal of the concrete canals by vegetating the river banks and placing basis. Minimum values dropped to less than 50% but increased to large boulders in the canal to create a more natural setting (Plate over 100% over the course of 24 h (Lim, 2003). This together with 5). A viewing and fishing deck was constructed near the boulders low flows may result in stressful conditions for aquatic organism. for residents of the area. The recent dry spell in January–February Costa et al. (2016)’s work on an urban river (Ciliwung River, 2014 caused many plants to wither and exposed soil which became Jakarta) in Indonesia found that weirs (turbulent mixing of flows) prone to remobilization at the next storm event (Lee and Lee, and small floods restored the self-healing capacity of rivers 2014). through flushing, dilution and re-oxygenation. Applying this to From these 3 examples, the Alexandra Canal and Pang Sua canal Singapore’s canal restoration projects, the reduced oxygen levels sites are the more typical PUB-style canal restoration projects that due to high temperatures may be counteracted by increasing flow are repeated at other locations. They provide aesthetic functions, or turbulence in the canal system through careful engineering. require a relatively high level of maintenance (e.g. Alexandra More research needs to be conducted into the cause of frequent Canal) and show little understanding of fluvial geomorphic pro- fish skills and the relationship between the flow regime of rehabil- cesses. The large boulders in the Pang Sua Canal are incongruent itated canals and water quality in terms of water temperature and with the size and flat topography of the canal which cannot sustain dissolved oxygen levels. Otherwise, these projects are merely gar- the movement of such huge boulders. The short lengths of these dening efforts around urban infrastructure, serving as park rivers projects also make it hard to reproduce intended effects of biotic (Bernhardt and Palmer, 2007; Woo, 2010). Their large capital assemblage restoration (Wohl et al., 2005). investment requires that they fulfil their hydrological and ecolog- The Kallang River project provides an interesting case for fur- ical roles in addition to being park spaces. ther study on river restoration efforts in a tropical context. Accord- ing to Bernhardt and Palmer (2007), river restoration should restore channel form, maintain channel stability and improve bio- 8. Current knowledge, challenges, opportunities and research logical communities and ecological functioning. There is no pub- needs lished data on the hydrological, water quality and ecological impacts of canal restoration in Singapore, even though the prelim- Most information on the impact of LID practices on Singapore’s inary data suggests poor performance in their water-cleansing urban hydrological response comes from field and modelling 856 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

Plate 4. Alexandra Canal (a) before restoration and, (b) after restoration with ABC design features and landscaping. Source: http://livebettermagazine.com/article/ch2m-hill- singapores-canals-reconnecting-people-to-waters-life-ﬂow/.

Plate 5. Pang Sua Canal with (a) upstream view of the canal with ABC design features, and (b) and (c) view of the boulders looking downstream of the canal. Source: Lee and Lee (2014). experiments conducted on green roof systems out of the list of the and water quality data in Singapore’s river networks that show ABC design features. Hydrological modelling using a variety of an improvement in stormwater runoff volumes and quality as a models has provided signiﬁcant insight into the hydrological reme- result of the implementation of ABC design features (pre and diation of LID practices in Singapore especially to examine the post-ABC). However, modelling results show that the effect of impacts of BMPs on different components of the hydrological cycle ABC design features on Singapore’s urban hydrology is sometimes

(van Spengen, 2010; Trinh and Chui, 2013), to determine optimal positive; decreased Qp, runoff volume and increased infiltration BMP location (Trinh and Chui, 2014), the effects of combining dif- (Trinh and Chui, 2013). Pre-development hydrology is restored ferent BMPs (Trinh and Chui, 2013) and their role in mitigating when a combination of LID practices are used (green roofs and rain runoff produced during extreme rainfall events (van Spengen, gardens) and if bioretention systems were sited along waterways 2010). The authors of the paper could not find monitored flow (Trinh and Chui, 2013, 2014). Therefore, in the context of H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 857

Singapore’s land-constraints, optimal hydrological improvements within a catchment may increase the hydrological improvements will be achieved through a careful choice and/or combination of not observed in van Spengen (2010)’s study but this needs to be ABC design features and their appropriate location in the catch- tested with more field and modelling research. Floating wetlands ment. This may include extending green roof coverage and com- are good options for Singapore because they can be located on bining BMPs taps on the retention features of green roofs and the existing ponds and reservoirs. Modular systems such as bioreten- infiltration and water balance improvements offered by bioretention swales and trees are still under research though they are tion systems. already implemented at some sites (e.g. bioretention planter boxes From a water quality perspective, so far, data from ABC design at Eight River suites, River Safari and Senja Parc) (see also CIRIA, features were collected from short-term studies and during the 2013; USEPA, 2013). The green channel cover (similar to a green early phases of their lifespan. Their performance for the 3 perfor- roof but located above a drain) is another innovative attempt at mance targets fall within the range of removal efficiencies reported incorporating BMP design into existing infrastructure. Palanisamy by studies conducted elsewhere, mainly in temperate areas (Tables and Chui (2015) modelled the impacts of the hypothetical green

3 and 4). Nutrient removal is lower than TSS for almost all the ABC channel cover and found that they reduce Qp by as much as 14%. design features described above, including green roofs. Leaching is This reduction is not significant on its own but can provide addi- the main cause of poor performance since influent runoff quality is tional hydrological improvements when combined with other generally low when compared to the performance targets and is a ABC design features (e.g. green roofs). problem observed from field experiments of local rain gardens/ Innovative ways of combining ABC design features may sustain bioretention systems and green roofs. their hydrological and water quality performance during extreme Considerable research on water quality amelioration of BMPs weather conditions by drawing on the different functions of BMPs are also conducted in laboratories of local academic institutions (Bernhardt and Palmer, 2007). Combining green roofs, vertical using columns and tanks to examine the following: ideal composi- greenery, bioretention systems, stormwater harvesting and re- tion of the growing media, selection of native vegetation and their use techniques creates a multi-tiered system of stormwater reme- respective pollutant uptake rates and lifespan of various BMPs (Lee diation and other ecological benefits such as habitat creation and et al., 2013a, 2015; Guo et al., 2015; Lim et al., 2015). Experiments connectivity (see PUB, 2011a, 2014; CIRIA, 2013). The advantages in the design aspect of LID systems include modular-based BMPs of BMP combination are supported by field and modelling studies and modular bioretention systems, similar to modular street trees (Brown et al., 2012; Trinh and Chui, 2013). Loperfido et al. (2014) in the USA (USEPA, 2013). These experiments are similar to those found that distributed BMPs (above and below ground) arranged conducted elsewhere but with a focus on local soil/material and as treatment trains reduced runoff volume during an extreme vegetation conditions (Hu, pers comm). 1000-year rainfall event associated with a tropical storm. In Singa- pore, improved hydrological response was obtained by combining 8.1. The challenge of climate change bioretention basins with green roofs and/or porous pavements (Trinh and Chui, 2013; Chui et al., 2014). The growing maturity Land constraints control the type and size of BMPs that can be and creativity in incorporating LID features in Singapore’s urban built in Singapore. Climate change is also an important challenge design is reflected in newer developments with rainwater harvest- for BMP performance when more extreme weather conditions ing, stormwater recycling systems and ABC design features (e.g. become the norm (Chan et al., 2012; Chang and Irvine, 2014). A Firefly Park in Clementi). recent study using 30 years of weather data (1980–2010) around Singapore reported that daily rainfall is now more intense, lasts 8.3. Research needs longer and that the frequency of intense events has increased (Chan et al., 2012; Beck et al., 2015). The 5-year and 10-year design We find that even though monitoring has been conducted for storms exhibit the greatest percentage increases in rainfall inten- the oldest ABC design feature and newer features, the currently sity; 1.3–2.4% increase and 0.3–1.7% increase respectively (Chan available data (published and unpublished) is still very limited et al., 2012). Such events produce more flashy hydrological and obtained when the ABC design features were relatively young. responses with a higher probability of flash floods in low-lying There is a knowledge gap on the lifespan of BMPs and the domi- areas. nant factors controlling their performance over time. The same The unusual dry spell between January and March 2014 gives a can be said for river restoration projects (Bernhardt et al., 2005). glimpse of more extreme climatic conditions that present chal- Limited information suggests that 10-year old bioretention lenges for ABC design features, especially vegetated systems. systems still perform well although we need to know when their Research elsewhere and in Singapore suggests that BMPs are gen- performance starts to decline for different climatic regimes (e.g. erally more effective for smaller rainfall events, especially events of Hatt et al., 2011; Lucke and Nichols, 2015; Thomas et al., 2015). 1.5–2 year return periods (Carter and Jackson, 2007; Hunt et al., Speak et al. (2013, 2014)’s work on a 43-year old green roof is a 2008; Trinh and Chui, 2013; van Spengen, 2010). The poor perfor- good example of investigations on aged systems and should be mance of green roofs for a 5-year design storm points to the need replicated for other BMPs (Koch et al., 2014). for more research in this area especially since the 2010 and 2011 Because post-project long-term monitoring is rarely done in floods in Orchard Road were caused by a 5-year design storm countries even where LID practices are well-established, we now (Chan et al., 2012). Larger, more intense rainfall events will likely need to invest in long-term monitoring to study performance of cause more damage and ABC design features will need to be mod- ABC design features at key sites and develop standardised mea- ified to cope with the changing climate. surements of important environmental and hydrological variables. Key variables include BMP design specifications (volume, area, 8.2. The opportunity for innovation depth), location, construction date, maintenance schedule, rainfall, influent and effluent discharge and corresponding pollutant con- The ideal BMP design for Singapore is one that can be installed centrations (see Koch et al., 2014). We need to know if project onto existing urban infrastructure and is space-efficient. Bioreten- goals have been met, if current designs and performance targets tion systems, green roofs and rain gardens are popular because are suitable vis-à-vis field conditions and whether they should be they are multi-functional and/or use existing spaces within the updated or changed since stormwater runoff is a source of potable urban environment (Fig. 3). A greater coverage of green roofs water in Singapore (after Wohl et al., 2005; Palmer et al., 2007; 858 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862

Morandi et al., 2014). The EU and USA now include a list of priority pollutant uptake abilities. Local institutions are conducting pollutants such as heavy metals and polycyclic aromatic hydrocar- research in this area already in laboratory settings that need to bons in their regulations due to their toxicity and bioaccumulation be transferred to field experiments. effects (see Furumai et al., 2011; Kabir et al., 2014). A list of priority pollutants monitored by PUB for reservoir water is available from 8.3.2.2. Vegetation selection. We need to identify a variety of vege- Lim et al. (2011). It will be interesting to see the role of ABC design tation, preferably native species that will survive frequent or features in removing priority pollutants from urban stormwater extreme rainfall as well as potentially dry weather conditions. runoff. Long-term monitoring also allows us to examine performance 8.3.2.3. Performance over time. We need to understand the lifespan at different time scales, e.g. event shock loadings versus long- of the various ABC design features. How will performance change term fluxes to downstream systems (Koch et al., 2014), and the with vegetation maturity? How significant is clogging in vegetated longevity of the different ABC design features. The Balam Estate systems? and NYJC rain garden and Grove Drive sites are good choices for long-term studies given their different design configuration and 8.3.2.4. Location/size/coverage. The studies described in this paper catchment conditions and existing dataset for their performance. include undersized (Grove Drive constructed wetland) and over- The Kallang River provides an excellent site for much-needed stud- sized systems (NYJC rain gardens). We need to determine optimal ies into tropical canal restoration studies. sizing relative to pollutant and runoff loading in land-scarce Singa- Additional data not only allows us to develop locally-relevant pore. Spatial coverage is an important issue especially for green guidelines for Singapore’s climate and urban setting, it helps ABC roofs. Modelling studies can further examine the relationships practitioners to refine or include more criteria for project assess- between BMP size, location, combinations and respective pollutant ment in terms of its contribution to scientific knowledge, policy, reduction to produce reference curves that can be used by planners economic benefits and social–cultural development (see Morandi and engineers (e.g. Mylevaganam et al., 2015). A systematic et al., 2014). approach incorporating spatial information can be used to priori- The specific research needs based on preliminary data pre- tise hydrologically-sensitive areas for BMP construction (see sented in this paper include: Martin-Mikle et al., 2015), in addition to using distributed models to identify optimal location. 8.3.1. Understanding influent runoff quality better Information about the hydrological response behaviour and 8.3.2.5. Maintenance issues. Once we know the performance of the stormwater quality in Singapore is still limited in the number of various ABC design features, we are able to refine current mainte- water quality parameters examined, mainly sediment and nutri- nance protocol. These include watering needs, plant pruning and ents (refer to Table 2), although recent work now includes a wider media regeneration (when necessary). range of parameters; heavy metals (Joshi and Balasubramanian, Building on the data we already know from monitored rain gar- 2010), organic pollutants (Xu et al., 2011), pharmaceuticals and dens and green roofs, more specific research needs for rain gar- endocrine disrupting chemicals and other emerging organic com- dens, include long-term monitoring of the Balam Estate rain pounds (Pal et al., 2014; You et al., 2015) and pathogens (Escheri- garden to establish the extent of nutrient leaching from the SAZ chia Coli, Irvine et al., 2014). and research on a suitable carbon source that suits Singapore’s We need to understand the following areas through a combined conditions (e.g. Lee et al., 2013a). More work is needed to under- approach of field monitoring and modelling experiments (e.g. stand the water balance dynamics of rain gardens, such as NYJC, Ragab et al., 2003; Hatt et al., 2007; Phillips et al., 2008; Blecken to understand the hydrological and water quality remediation et al., 2009; Collins et al., 2010; Mullane et al., 2015): impacts of these systems. Groundwater monitoring should also be conducted on a long-term basis to see if pollutants travel pollutant chemistry, sources and loading of stormwater runoff through the growing media into groundwater table. in Singapore, For green roofs, we need to better understand their hydrological the controls of pollutant mobilisation since they may vary for role in Singapore and whether increasing their spatial coverage and different pollutants. growing depth will lead improve rainfall retention and baseflow Chua et al. (2009) found that TP mobilisation was controlled augmentation. We need to know the recommended substrate mainly by rainfall depth and average intensity, while TSS mobil- depth for optimal rainfall retention relative to the cost and weight isation was controlled by rainfall depth, average intensity and loading on the roofs (e.g. Seidl et al., 2013; Savi et al., 2015). maximum 5-min intensity for a mixed-land use catchment in Another key research need is whether green roofs add pollutants Singapore. to Singapore’s stormwater runoff through the growing media and the dynamic relationship between stormwater quality, rainfall the role of native vegetation in pollutant uptake (see CUGE publi- properties and antecedent conditions (length, frequency of cations cited earlier). Finally, PUB’s Engineering Guidelines need wet/dry periods), to include a section for green roofs. the partitioning of rainfall into various hydrological Restored canals have to be monitored to examine if they components. achieved hydrological and water quality improvements under the Trinh and Chui (2013) found that evapotranspiration is an ABC Waters Program. Key questions include whether the Kallang important component in Singapore’s urban hydrological cycle. River restored floodplain provides runoff storage and volume BMPs that increase ET losses will reduce stormwater runoff reduction during wet periods and if the boulders at Pang Sua Canal volumes. provide enough resistance to slow down stormwater flow. These questions can be answered through hydrodynamic modelling 8.3.2. Factors that influence ABC design feature performance (e.g. Niezgoda and Johnson, 2006; Zhou and Endreny, 2012). We 8.3.2.1. Media. We need to develop growing media that uses need to know if the ABC design features constructed at these canal locally-sourced materials that are able to retain runoff and encour- restoration sites play any role in water quality improvements and age pollutant uptake under the frequent rainfall and warm temper- the underlying causes of poor performance. Finally, the Kallang atures in Singapore. We need to know when the media saturates River site provides an excellent opportunity for research into river and the necessary maintenance issues required to prolong their geomorphology and adjustments post-restoration in a tropical H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 859 setting. Repeated channel cross-section surveys provide valuable (Lucas et al., 2015). The development of institutionalised design information on channel planform changes over time, with respect guidelines and performance targets is crucial for the successful to urban infrastructure (Miller and Kochel, 2010; Kristensen adoption of LID practices in other tropical cities to maintain consis- et al., 2014). This will be a valuable contribution to tropical fluvial tency in design and LID performance monitoring. geomorphology since post-restoration monitoring is rare even in Finally, local community involvement plays an important role countries where river restoration projects are common (e.g. USA, in the success of a LID project, as seen in Singapore’s case. We Bernhardt et al., 2005). strongly encourage collaboration between LID practitioners in Sin- gapore with others in countries with similar climatic conditions and urban growth patterns (e.g. Queensland, Australia, see 8.3.3. Performance during extreme events Biermann et al., 2012) as well as other tropical/sub-tropical cities There is only one study on BMP performance for extreme events where there LID practices are a new but an emerging opportunity in Singapore; van Spengen (2010)’s work on green roofs. We need a to improve stormwater flow and water quality problems. better understanding of the performance of ABC design features under more extreme rainfall conditions (amount and rainfall intensity) and dry periods. This can be achieved by modelling studies for 10. Conclusions more extreme rainfall conditions (5, 10, 25, 50, 100-year design storms) in tandem with field monitoring studies at the plot to Singapore’s stormwater management strategy combines catchment scale (e.g. Versini et al., 2015). We can then make mod- conventional stormwater drainage with LID practice through the ifications to the current recommended design guidelines in terms ABC Waters Program. The program has seen significant of sizing, growing media, design changes (e.g. SAZ, adding storage development over its first 10 years, evidenced by the number of zones for green roofs) and careful plant selection (type, diversity). projects completed and breadth and scope of BMPs constructed, ranging from small source-treatment sites to larger expensive canal restoration projects. The number of awards received for 9. Beyond Singapore – LID practices in the tropics/sub-tropics the program and projects testify to their success from the design and sustainability perspective. All this are possible because of the Tropical cities experience high population growth and uncon- strong political will and holistic framework of the Program. trolled urban expansion, often accompanied with the lack of proper We argue that the Program has achieved the Active and Beauti- waste disposal facilities and infrastructure. This has led to ful components of the Program but the Clean component and increased flooding and significant water pollution (Poerbandono hydrological remediation aspects require more data to check if et al., 2014; Costa et al., 2016). Singapore, by contrast, is a city the ABC design features meet their hydrology and water quality based on judicious planning and strict legislation on urban growth remediation goals. Most work on hydrological improvements in and environment pollution control. Despite differences in urban Singapore was achieved using hydrological models while informa- growth and management between Singapore and other tropical tion about water quality remediation was based more on labora- cities, the ABC design guidelines will be more adaptable than tory and field monitoring studies. The results of a few modelling guidelines taken from temperate urban areas where LID practices studies suggest that ABC design features successfully reduce peak first started due to similarities in climatic controls. For example, flows and stormwater volumes especially if a combination of dif- a 2-year event in Thailand is equivalent to a 10 or even 100-year ferent systems are built together as a treatment train. Field- rainfall event in the Buffalo, New York, USA. Using the LID design based monitoring studies for rain garden systems and constructed guidelines for a 10-year rainfall event based on USA climatic con- wetland show positive TSS removal but nutrient leaching is a prob- ditions will fail to treat a 10-year rainfall event in Thailand given lem despite generally low nutrient content in influent stormwater the differences in rainfall characteristics. Design storm modelling runoff. for LID designs in tropical areas have to be based on local rainfall Although the ABC Waters Program draws heavily from Aus- conditions, preferably drawing from the experience of a tropical tralian guidelines, it is now time for Singapore to develop locally- case study such as Singapore where climatic conditions are rela- relevant BMP designs and performance targets based on the expe- tively similar (see Irvine, 2013). Further, the problems faced by rience gained and complemented with continued field monitoring. the ABC design features such as leaching, poor nutrient removal We stress the importance of long-term monitoring at key sites. and loss of aquatic communities due to low flows and low dis- Important key questions involve the performance and lifespan of solved oxygen as a function of high temperatures will also be faced ABC design features under current climatic conditions. We encour- by other tropical sites. Solutions to these problems can be based on age research into innovative designs as well as clever ways of com- modifications of the Singapore design to suit local conditions bining ABC design features in land-scarce Singapore. More especially in the anticipation of climate change where higher tem- importantly, we need to consider the performance of these design peratures and increased rainfall intensity and seasonality are features under extreme conditions in order to make necessary expected. design modifications to adapt to the anticipated climate change At present, bioretention/rain garden systems and rain gardens conditions. Field monitoring conducted in tandem with ongoing are the most suitable LID features to use in land-scarce Singapore. laboratory and modelling projects puts PUB and associated scien- River restoration projects are common in the ABC Waters Program tists in a position to make significant contributions to scientific but such projects are expensive and require a considerable amount knowledge about LID practices for hydrological and water quality of technical expertise which may not be available in developing remediation within the tropics. tropical cities. Therefore, small-scale distributed LID features (e.g. rain gardens, green roofs) connected to each other and the urban drainage network hold the most promise in hydrological and water Acknowledgements quality remediation of these cities. The rapid pace in which ABC design features were constructed This paper is a result of many discussions with Professor Ong is due largely to having clear design guidelines that are enforced Say Leong, A/P Hu Jiangyong, Drs. Lee Lai Yoke, Guo Huiling, Tony by PUB. Even countries such as the UK, where LID practices have Lim, Tan Czheaw Yheaw and May Chui through the National been practiced for some time, lack unified design codes and perfor- University of Singapore-PUB project (R-706-000-020-490). We mance databases for LID practices such as constructed wetlands acknowledge the work of many students whose projects provided 860 H.S. Lim, X.X. Lu / Journal of Hydrology 538 (2016) 842–862 important information on the performance of various ABC design Dietz, M.E., Clausen, J.C., 2006. Saturation to improve pollutant retention in a rain features. We also thank Lee Li Kheng for drawing Figure 2. garden. Environ. Sci. Technol. 40, 1335–1340. Endreny, T., Collins, V., 2009. 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