Urban Lakes as a WSUD System

Chapter 13

Urban Lakes as a WSUD System Christopher Walker1, 2 and Terry Lucke1 1

Stormwater Research Group, University of the Sunshine Coast, Sippy Downs, QLD, Australia;

2

Covey Associates Pty Ltd, Sunshine Coast, QLD, Australia

Chapter Outline 13.1 13.2 13.3 13.4

Introduction Urban lakes and urban development Urban lakes and stormwater quantity Factors impacting urban lake health 13.4.1 Key causes in urban lake degradation 13.4.2 Consequences of urban lake degradation 13.5 Urban lakes and stormwater treatment 13.5.1 Historical view of urban lakes and stormwater treatment

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13.6 Evolution of urban design principles, management, and role in WSUD 13.6.1 Contemporary design principles 13.6.2 Urban lake management 13.7 Urban lakes and ecosystem health 13.7.1 The Ecosystem Health Paradigm 13.8 Conclusions References Further Reading

276 276 278 279 279 282 282 284

ABSTRACT Constructed waterbodies serve a variety of purposes. Urban lakes are a contemporary constructed waterbody, which often serve to increase amenity and property values, fulfill a flood mitigation purpose, and may provide a source of land fill for development purposes. Although popular, urban lakes have degraded in many areas as a result of: (1) poor quality of inflow; (2) significant water level variation; (3) persistent stratification; (4) long stormwater residence times; and (5) high loading of organic carbon. The failure of many urban lakes to remain as a resilient and healthy ecosystem stems from poor design and a lack of pretreatment to runoff. Recent design guidelines have better informed designers as to the key design elements for urban lakes; however, management of these waterbodies remains reactive, rather than proactive. Utilizing the Ecosystem Health Paradigm as a template for urban lake management may allow for a more holistic assessment of urban lake health and better inform management requirements. This chapter looks at the history of urban lakes and how such systems are integrated and managed in the urban context. Keywords: Ecosystem health; Eutrophication; Stormwater treatment; Urban lakes; Urban runoff.

13.1 INTRODUCTION Constructed waterbodies are artificial water features that have played a significant role in history, serving to fill a range of functions (Mays et al., 2007), including the following: l l l l

Potable water storage; Aquaculture; Detention; and Esthetic value.

Simply put, these waterbodies are a means to retain or detain a desired volume of water to satisfy a need or needs. Constructed waterbodies have been utilized by civilizations as far back as 3000 BC (Yang et al., 2005).

Approaches to Water Sensitive Urban Design. https://doi.org/10.1016/B978-0-12-812843-5.00013-7 Copyright © 2019 Elsevier Inc. All rights reserved.

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Constructed waterbodies have also been built purely for ornamental and recreational purposes. In 18th and 19th century Europe, numerous shallow lakes were excavated for ornamental purposes, for aesthetics, or to attract waterfowl for game hunting (Sayer et al., 2008). With the industrial revolution and the creation of a wealthy mercantile class affluent landowners often built ornamental lakes to buffer their estates from the encroachment of urbanization (Binnie, 2004). Modern constructed waterbodies built within development areas are commonly referred to as “urban lakes.” Urban lakes are defined as a permanent body of open water of significant size (>0.1 ha) (Gibson et al., 2000). The size, depth, and retention time (i.e., the time it would take for runoff to displace the stored volume of water) of an urban lake vary according to the design requirements. The key roles of urban lakes are linked to aesthetics (Huser et al., 2016) and stormwater management (Hagare et al., 2015). The desire for waterside living and the need for stormwater management have seen urban lakes become increasingly incorporated into urban developments. Urban lakes help to mitigate the density of urban developments by creating passive open space areas. They are generally designed to maximize the open water area, as this often results in increased property values (Walker, 2012; Walker et al., 2013; Sikorska et al., 2017). In Australia during the 1980s and 1990s, urban lakes were often incorporated into developments to treat stormwater, with the assumption that the lakes would provide treatment functions typically associated with sediment ponds and constructed wetlands. This was a common assumption in South East Queensland (SEQ), Australia. This approach led to significant water quality issues, as the urban lakes were often not capable of processing nutrient loads typical of urban ecosystems. Urban lakes are now generally implemented as a “receiving environment” with stormwater treatment occuring upstream of the waterbody. This chapter will explain how urban lakes fit into the water sensitive urban design (WSUD) philosophy and the design principles and requirements that are critical for lake management and maintainance of ecological health in such waterbodies. The term health, which has often been used ambiguously, will be assessed under the “Ecosystem Health” Paradigm (EHP). Ecosystem health expands aquatic health beyond physicochemical parameters, and incorporates biological and social indicators to determine overall health of an ecosystem (Walker, 2012).

13.2 URBAN LAKES AND URBAN DEVELOPMENT Urban lakes provide a “pseudo-natural” aspect to urban developments, fulfilling a desire for a connection or proximity to nature and water. Furthermore, properties within close proximity to urban lakes often have higher values (Walker, 2012; Walker et al., 2013; Sikorska et al., 2017). Natural waterfront property is generally out of reach for most property investors (Cook et al., 2016) and urban lakes can provide a satisfactory alternative to natural waterfront property in many cases. Urban lakes have been established in the past to appeal to a broader range of investors. The appeal of an urban lake often has a positive effect on the entire development and not just on the adjacent properties. Studies conducted in North America (Michael et al., 1996; Gibbs et al., 2002) found a strong correlation between property prices and lake clarity. This indicated that aesthetics were a significant driver of property value. In another study, Sikorska et al. (2017) found that lake accessibility played a more important role than water quality in property values in Poland. Although it may be arguable that water quality does not influence property value, it is clear that proximity and accessibility to open water does influence property value. If stakeholders can easily access and utilize the waterbody passively or actively, it will inherently increase values. From a practical perspective, urban lakes can also provide a significant source of land fill for developers, which is extremely valuable for developments that need to be raised above a specific flood level. Urban lakes are typically constructed close to the lowest elevation point of a development for managing the increased flows (Schueler and Simpson, 2001; Walker, 2012) from the increase in impervious areas (CSIRO, 2006). Alternatively, a series of lakes can be constructed throughout a development to manage flows from smaller subcatchments. Either approach generally requires the waterbodies to be excavated, with the material often utilized for fill to ensure flood protection in other parts of the development. Removing or reducing the need to import fill into a development can be a significant cost benefit. Furthermore, in SEQ, achieving adequate flood protection needs to be unambiguously demonstrated prior to receiving a development approval. Apart from economic drivers, urban lakes can provide intangible benefits such as fostering community well-being or social capital (Walker et al., 2012). Stakeholders have historically ranked proximity to natural settings, particularly water, as very desirable and associate them with a higher quality of life (Horwitz et al., 2001; Ogunseitan, 2005; Walker et al., 2013). Quantifying links between health, well-being, and urban lakes is difficult, but there is sufficient evidence that welldesigned and established green spaces (e.g., parks, urban lakes, playing fields) have a positive impact on community health and well-being (Lee and Maheswaran, 2010).

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In the early adoption of WSUD in Australia, urban lakes were utilized as a means of stormwater treatment. However, uncontrolled stormwater runoff has caused widespread adverse impacts on urban lake health (e.g., eutrophication). As problematic urban lakes become more common and research (Leinster, 2006; Walker et al., 2013; Walker, 2012) identified that urban runoff was the primary cause of lake degradation, urban lakes were no longer considered to be a viable means of treating urban stormwater runoff. This aspect will be discussed further within this chapter.

13.3 URBAN LAKES AND STORMWATER QUANTITY Urban lakes/dams have historically played a significant role in regional flood management. For example, urban lakes can be used as stormwater retention/detention basins to attenuate stormwater runoff rates from urban developments. Stormwater basins collect stormwater and slowly release it at a controlled rate (attenuation) such that downstream areas are not flooded or eroded. This type of urban lake is generally constructed to reduce the cost of downstream channel upgrades; to reduce downstream flooding impacts; or to meet regulatory requirements limiting urban catchment outflow peaks (i.e., non worsening). Stormwater basins are usually designed to mitigate floods up to the 1% annual exceedance probability (AEP) flood level (i.e., a 1% AEP flood). These basins also provide community benefit such as recreation areas, scenic pond, and pollution control. There are two general types of stormwater basins: detention basins and retention basins. Detention basins are usually dry when not being used for flood storage, whereas retention basins usually have a permanent pool of water, even when not acting as flood storage. Detention basins are landscaped areas formed by the construction of simple dam walls or by excavation below ground level. They can also be designed to use any existing natural swales or depressions. These areas primarily serve to capture and temporarily store stormwater runoff from impervious surfaces, such as rooftops, roads, and pavements, to prevent excessive runoff and channel erosion in receiving environments. Detention basins are also effective at removing particulate-based contaminants via sedimentation but less effective for treatment of soluble pollutants where biological uptake of nutrients is required. Pollutant removal through sedimentation relies on strong affinity of metals, nutrients, and hydrocarbon contaminants to adsorb onto particulate matter such as clay and silt. Pollutant removal efficiency increases with increasing hydraulic residence-time (Persson and Wittgren, 2003; Hossain et al., 2005). Retention basinetype urban lakes also treat stormwater runoff. They have a permanent ponding level of water in the lake at all times and as such, were often used in favor of dry detention basins, as the value of lakes to development/ communities is often considered to be higher. The water level in a stormwater pond is established by a low flow outlet. Generally, the outlet is incorporated as part of a metal or concrete structure called a riser. The outlet of a detention basin is usually level with the bottom of the basin so that all of the water eventually drains out and it remains empty between storms, whereas retention basins have an outlet or a weir at a higher point so that it retains a permanent pool of water. One of the most important elements of maintaining basins is making sure the outlet is not blocked or clogged. Other maintenance activities include repairing erosion within the basin, removing sediment, and managing the vegetation. Repairing erosion early can save significant costs, especially the resulting sedimentation that will need to be removed from the basin. The consequences of a storm exceeding a detention basin’s or retention basin’s operating design storm should be considered in the design and operation of a stormwater basin. Although such an occurrence may be rare, it is still possible, and the consequences of failure can be catastrophic because of the proximity to urban areas. The design approach should be selected based on the target pollutants and site and economic constraints. Forebays (or sediment traps) at the inflow points to the detention area can be used to capture coarse sediment, litter, and debris, which will simplify and reduce the frequency of maintenance. Forebays can either be sized to hold the expected sediment volume between maintenance cycles or be designed to have sufficient capacity to detain or infiltrate (where possible) frequently occurring storm events (typically < 1-year Average Return Interval) without discharge to the main detention area. Detention basins must also be designed to minimize the risk of mosquito breeding, which can be achieved by ensuring the basins are free draining.

13.4 FACTORS IMPACTING URBAN LAKE HEALTH Many urban lakes, along with natural waterbodies, are considered degraded, with eutrophication (i.e., excessive concentrations of nutrients causing accelerated plant/algae growth) being a significant global problem (Wagner and Erickson, 2017). This widespread degradation of urban lakes has been broadly researched and is consistently linked to the

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impacts of urban runoff (Mitsch and Gooselink, 2000; Bayley and Newton, 2007; Walker et al., 2013). More specifically, three key factors associated with urban lake degradation are: l

l

l

Catchment runoff attributes (e.g., increased impervious area, loss of infiltration capacity, increased directly connected drainage infrastructure); Poor quality of inflow (e.g., high nutrient concentrations, high organic carbon load, high heavy metal concentrations); and Poor design attributes (e.g., lake depth, wind alignment, lack of riparian environment).

These factors have caused many urban lakes to shift from a stable state, dominated by submerged macrophytes and low turbidity, to a deteriorated, eutrophic state, explained by the “alternate stable state” theory (Scheffer and Carpenter, 2003). This alternate state is dominated by floating macrophytes, phytoplankton and/or cyanobacteria, potential odor issues, and turbid water (Scheffer and Carpenter, 2003). The mechanisms of recovery from the alternate stable state are difficult because of hysteresis effects (i.e., a transition which is not easily reversed), which presents difficulties in transitioning from the degraded state back to the stable ecosystem state (Beisner et al., 2003).

13.4.1 Key causes in urban lake degradation Research on constructed urban lakes has identified five key causes of urban lake degradation (Taylor et al., 2007; Walker, 2012). These causes are as follows: Poor quality of inflowdUntreated urban runoff is typically high in pollutant loads and is often of poor quality. Sediment and nitrogen and phosphorus species are largely the issue in Australia; however, heavy metals also pose an issue in many countries. The causes of poor runoff quality in urban catchments are varied but are predominantly associated with fertilizer use on lawns and gardens, higher sediment loads associated with erosion, and a lack of infiltration. Impervious surfaces can warm stormwater runoff as they absorb and retain heat at a higher capacity than natural ground cover. Therefore, stormwater runoff is often at a higher temperature than its receiving environment. This influences several biochemical processes (e.g., increases biological oxygen demand) and creates a more hospitable environment for microbiological contaminants (Morrison et al., 2005). In urban developments, which contain commercial and light industrial uses (e.g., grocery stores and vehicle service centers), nutrients, sediment, heavy metals (e.g., due to construction materials, wearing of vehicle parts, and breakdown of paints), and other anthropogenic pollutants are often higher than in residential urban developments. Commercial and light industrial areas (e.g., car service centers, supermarkets) are increasingly incorporated into residential developments and serve to increase the quantity and variety of pollutants (Eves, 2007; Wagner and Erickson, 2017). In SEQ, Australia, where rainfall is infrequent and storm events often intense, this can cause significant impacts to waterways from catchment areas where pollutants have accumulated during dry periods (CSIRO, 2006). It is well understood that directly connected stormwater drainage infrastructure intensifies the impacts of urban development. Hard stand areas directly connected to stormwater drainage infrastructure (e.g., concrete pipes) reduce the potential for infiltration through the soil and increase the volume of untreated stormwater discharged directly to a receiving environment (Walsh et al., 2007; Wong et al., 2006). Without a buffer or pretreatment systems that provide infiltration or stormwater treatment, receiving environments such as urban lakes are degraded over time. Water level variationdRiparian vegetation is impacted by both prolonged inundation and reduction in water levels. Both events can lead to a loss of riparian vegetation, which, when it decays create conditions hospitable to nuisances such as flies and mosquitoes (Taylor et al., 2007). The riparian zone also influences ecological processes by providing critical habitat, regulating stream temperature, and serving as a source of leaf and woody litter, essential for many terrestrial and aquatic invertebrates (Ewel et al., 2001; Bond et al., 2008). The loss of the riparian system essentially removes critical habitats and buffers for urban runoff, thus reducing the resilience of the urban lake to eutrophication. Persistent stratificationdStratification is the formation of two distinct layers in a lake, the epilimnion and hypolimnion (Fig. 13.1). The different layers form as a result of differing water density, associated with a temperature or salinity gradient (Walker, 2012). Although a common occurrence in natural lakes, stratification can cause issues in urban lakes if the design depth is greater than 3 m, particularly in subtropical and tropical climates. The hypolimnion layer (cooler and denser) in degraded urban lakes is typically of poor quality, as it often acts as a sink for organic matter and sediment causing low levels of dissolved oxygen (DO). More specifically the layers are defined as follows: l

Epilimnion: The surface layer of the lake. This layer may be up to 3 m deep, subject to lake clarity. The water temperature in this layer is determined by sunlight and wind mixing.

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Epilimnion (approximately 0–3 m deep) Thermocline (approximately 3–4 m deep, varies with temperature) Hypolimnion (>3–4 m deep)

FIGURE 13.1 Schematic of typical stratification layers in a lake.

l

l

Thermocline: This layer is defined by an abrupt transition to cooler temperatures. The thickness of this layer is determined by temperature and seasonal variation. Hypolimnion: The bottom layer of the lake. This layer is generally >3 m deep. In degraded systems, this layer is often hypoxic because of ongoing decay of organic matter. The low levels of DO are associated with the decay of organic matter by aerobic (oxygen consuming) bacteria, and if water clarity is low, it can also result in reduced photosynthetic activity in submerged macrophytes. In low DO conditions, nutrients and heavy metals are typically desorbed from sediments and become bioavailable in the soluble form (e.g., NOx, PO4) (Walker, 2012). In excessive concentrations, soluble nutrients can lead to, or exacerbate, lake eutrophication.

Stratification can be disrupted either by a severe wind event or by seasonal changes (e.g., air temperature cools the epilimnion below that of the hypolimnion). In such events, the water profile inverts and nutrient-rich waters within the hypolimnion rise to the surface of the waterbody and exacerbate the eutrophic state. Heavy metal concentrations may also impact the health of higher level species (e.g., macroinvertebrates, crustaceans, and fish) (Scheffer et al., 2001; Melbourne Water, 2005; Taylor and Breen, 2006; Taylor et al., 2007), which may lead to widespread mortality within an aquatic ecosystem. Stormwater residence timedStormwater residence time in urban lakes relates to the flushing frequency of the lake. Urban lakes have an optimal stormwater residence times that ensure that stormwater treatment is most effective. Urban lakes in cooler climates have an optimal stormwater residence time of up to 50 days, whereas lakes with a higher summer time water column temperature require a much shorter stormwater residence time (15e20 days) to limit the risk of pollution buildup and algal blooms, as higher algal growth rates are associated with higher water temperatures (Water by Design, 2012). Algal blooms are significant threats to both environmental and public health, as some species of algae and cyanobacteria are potentially toxic to mammals, notably, the Anabaena spp. (NHMRC, 2008). An urban lake with limited flushing capacity is likely to experience a number of potential water quality issues. The length of the residence time is directly linked to the likelihood of an algal bloom, as algal cells that are not flushed from a waterbody can can multiply exponentially if nutrient concentrations are not limited (Burge and Breen, 2006; Taylor and Breen, 2006; Water by Design, 2012). Climatic conditions influence the rate of growth in algal species with warmer water, increasing metabolic activity (i.e., uptake of bioavailable nutrients by algal species). Prolonged stormwater residence time in urban lakes is usually a consequence of poor design. Poor designs often fail to take the catchment area and quantity of runoff from frequent events into account when sizing the lake system, e.g., small contributing catchment and large lake volume.

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High organic carbon loadingdIn natural lake systems, organic matter is critical to microbial processes as it provides a food source for heterotrophic organisms (Ewel et al., 2001; Bond et al., 2008). Aquatic ecosystems not influenced by urban development are able to process organic carbon loads and do not generally create nutrient loads that are beyond the capacity of the lake to process and remain healthy. In urban lakes, organic carbon loads, particularly those sourced from floating macrophytes, add to nutrient loads that are already potentially high because of the impacts of urban runoff. Microbial decomposition of floating macrophytes depletes DO levels within the hypolimnion (Fig. 13.1), which in turn creates higher concentrations of soluble nutrients and heavy metals (due to solubilization). Over time, a high mass of decomposing material may provide a continuous supply of nutrients to an urban lake, resulting in persistent eutrophication or algal blooms, regardless of the water quality entering the system (Taylor et al., 2007; Walker, 2012).

13.4.2 Consequences of urban lake degradation Constructed urban lakes, react to increased nutrient loading and availability similar to that in natural lakes (Carpenter et al., 1998a, 1999). Scheffer et al. (2001) argued that natural lakes may degrade as a result of internal processes and external factors, but this may take centuries under natural conditions. In an urban lake environment, the shift to the alternate stable (degraded) state may take as little as 2 years (Bayley and Newton, 2007; Taylor et al., 2007; Walker, 2012). Urban lakes typically begin in the stable state, which is relatively undisturbed (Scheffer et al., 2001; Walker, 2012). Over time, urban lakes with suboptimal designs will likely shift to an alternate stable state once some threshold point is reached. This alternate state is dominated by floating macrophyte growth or cyanobacteria and/or phytoplankton blooms and turbid water, which is consistent with a eutrophic to hypereutrophic system (Scheffer, 2004; Taylor et al., 2007; Walker, 2012). Urban lakes follow the five-stage pattern of eutrophication described in Fig. 13.2. The alternate stable state may represent a progression to what Taylor and Breen (2006) described as a “catastrophic ecosystem change.” At the final stage (Stage V), eutrophication is evident and the lake is degraded to the point where it may be no longer be capable of supporting higher trophic level species. Algal and cyanobacteria blooms become more frequent, as the loss of submerged macrophytes effectively removes competition for nutrient resources (Burge and Breen, 2006; Sirivedhin and Gray, 2006; Taylor et al., 2007). Once a lake shifts to the alternate stable state, it can be extremely difficult to shift back to the original state. The resistance to recovery is described as the hysteresis effect (Beisner et al., 2003). A contributing factor to the hysteresis effect is that urban lakes act as a “sink” for pollutants, particularly for nutrients. Phosphorus, a limiting nutrient for algae and cyanobacteria, can buildup in such high concentrations within lake sediment that internal recycling can occur over decades. This can result in a long recovery time, regardless of the management strategy implemented (Coveney et al., 2005). Research has found that lakes in the alternate stable state may be resistant to recovery strategies, as many remediation efforts have had limited effectiveness (Carpenter and Cottingham, 1997; Carpenter et al., 1998a, 1999; Coveney et al., 2005; Wagner and Erickson, 2017). As the mechanisms of urban lake degradation are now well understood and underpinned by the alternative stable state theory, a closer examination of the historical and contemporary role of urban lakes and stormwater treatment is warranted. This will be addressed in the next section.

13.5 URBAN LAKES AND STORMWATER TREATMENT 13.5.1 Historical view of urban lakes and stormwater treatment In the early adoption of WSUD principles in Australia, urban lakes were utilized as stormwater treatment systems. The view was that urban lakes could provide treatment similar to that of sediment ponds (where sediment would settle out and be retained) or constructed wetlands (where submerged macrophytes process the nutrients from urban runoff). Subsequently, many existing urban lakes were installed as the only means of urban runoff treatment, following the the now widely discredited mantra that “dilution is the solution to pollution” (Varis and Somlyody, 1997). Utilizing an urban lake allowed for more open water area to be realized, and offline treatment systems (e.g., bioretention basins and constructed wetlands) were not considered necessary. Greater areas of open water were considered to be a significant incentive to investors, given the higher aesthetic and economic values associated with greater areas of easily accessible open water (Walker, 2012; Walker et al., 2013; Sikorska et al., 2017). An urban lake may initially be able to process pollutants associated with urban runoff, as described in Fig. 13.2, but this ability tends to be very brief. The often rapid nature of the shift to the alternative stable state demonstrates that the

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I. Initial state: urban lake recently constructed. Benthic sediment main nutrient source and most nutrient is in the organic form. organic carbon/sediment accumulation minimal. system dominated by submerged and/or riparian macrophytes. water clarity high with limited phytoplankton. nutrient and sediment loads increase with ongoing exposure to urban runoff. Initial nutrient loading promotes rapid growth of submerged macrophytes. II. Transition to eutrophic state: ongoing exposure to nutrient loads from urban catchments begin to reduce ability of benthic sediment to bind nutrients. some formation of floating plants and algal mats begins, limiting light penetration. sediment loads from runoff also reduce water clarity. submerged macrophytes still present, but no longer able to process nutrients. Ill. Eutrophication: sediments are now saturated and prolific growth of algal mats and floating macrophytes has impacted population of submerged macrophytes. inorganic nutrients present within the water. floating plants and phytoplankton respond positively to availability of inorganic nutrient. dissolved oxygen levels begin to reduce due to blanketing of lake surface by plants and algae. IV. Catastrophic ecosystem shift: submerged macrophytes now eliminated from system and nutrient concentrations are very high within water column. the system is now dominated by phytoplankton and algal blooms are frequent. nutrients are largely in the soluble form due to lack of oxygen and in the inorganic form. decaying organic matter a continuous source of nutrients.

FIGURE 13.2 The stages of eutrophication in a degrading urban lake. Modified from Taylor, S., Breen, P., 2006. Chancellor Park Lakes e Validating Conceptual Models of Lake Behaviour: Monitoring Program Data Interpretation & Lake Management Recommendations, Ecological Engineering Pty. Ltd., pp. 1e117.

stormwater treatment capacity in urban lakes is limited, transient, or absent entirely, with long recovery times between inputs (Leinster, 2006; Bayley and Newton, 2007; Walker, 2012). Shifts from the stable to alternate state have been frequently observed in urban lakes in Australia (Walker, 2012; Hagare et al., 2015). In as little as 2 years, an urban lake may be unable to process nutrient loads from urban runoff. One reason is that these waterbodies do not have suitable surface areas for biofilm growth beyond that of the riparian vegetation (e.g., plant stalks on the lake edges). Biofilm coverage is an essential requirement for the sequestration of nutrients from stormwater (Borne et al., 2013; Winston et al., 2013). Biofilm helps process nitrogen in the water through nitrification, ultimately allowing uptake by plant species of nitrate. More importantly excess nitrate is converted into nitrogen gas via denitrification, as plant uptake may be limited. Refer to Fig. 13.3 for detail. Phosphorus can also be retained through binding processes that occur within the biofilm (e.g., adsorption) and uptake of orthophosphate is achieved by vascular macrophyte species. However phosphorus is largely managed via immobilization (i.e., bound to sediment). As the key factors that cause a shift into the degraded (alternative stable) state were identified, the design and management of urban lakes evolved to better cater for the impacts of urbanization. For example, the Queensland state government, along with a number of Australian councils, has developed policies that designate urban lakes as “receiving environments” (DILGP, 2017). As a designated receiving environment, stormwater runoff must be appropriately treated prior to its discharge into an urban lake. In essence, it encourages an urban lake to be considered a natural aquatic ecosystem, and this affords it a similar level of protection. Such policy documents are now linked to urban lake design

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Aquatic nitrogen cycle Nitrate reduction by assimilation

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Excretion

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NH4+

N2 (gas)

A (d mm ec on om ifi po cat sit ion io n)

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Consumer and decomposer nitrogen

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Dissolved organic nitrogen Organic nitrogen

FIGURE 13.3 Schematic of sources, sinks, transformations and flows in the aquatic nitrogen cycle. Modified from http://www.esf.edu/efb/schulz/ Limnology/Nitrogen.html.

guidelines and management strategies. However, a remaining critical purpose of urban lakes is to provide flood attenuation. The contemporary design and management principles of urban lakes are discussed further in this chapter.

13.6 EVOLUTION OF URBAN DESIGN PRINCIPLES, MANAGEMENT, AND ROLE IN WSUD Initial design strategies for urban lakes sought to maximize property values by creating greater areas of open water and reducing subdivision construction costs by providing a ready source of fill. Early designs focused exclusively on the economic benefits to the development and ignored the ecological role of urban lakes. This approach did not align with the principles of WSUD. As a result, it can be argued that many existing lake designs are in direct conflict with the core principles of WSUD, which seek to minimize or prevent environmental degradation, while simultaneously offering a benefit to the community in terms of esthetic and/or recreational value (BMT WBM, 2009).

13.6.1 Contemporary design principles As the quality of many early urban lakes started declining, it became apparent that proactive, long-term strategies were required. As a result, key design principles for urban lakes were established to ensure that contemporary urban lakes in Australia are designed appropriately. Three well-known Australian guidelines include the following: l

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Melbourne Water Constructed Lakes (2005) (https://www.melbournewater.com.au/Planning-and-building/Applications/Documents/Shallow-lake-systems-design-guidelines.pdf) Mackay Constructed Lakes guidelines (2008) (http://www.mackay.qld.gov.au/__data/assets/pdf_file/0008/14786/15. 15_Constructed_Lakes_V2.pdf) Townsville Constructed Lakes (DesignFlow and RPS, 2010).

Although all three guidelines stress the importance of design, the Townsville guidelines (DesignFlow and RPS, 2010) also provide recommended steps for regulatory authorities to use when assessing urban lake design applications. The steps provide a framework to identify designs that avoid the issues associated with urban lake degradation. A summary of the key steps from the Townsville guidelines is provided in Table 13.1. As seen in Table 13.1, urban lake design within Australia now considers a wide range of variables to ensure the risks associated with the five key causes of urban lake degradation are avoided. Waterbodies are now designed to cater to the increased understanding of urban lake ecosystems, and this must be demonstrated as part of a development approval process. An issue that still needs to be addressed, however, is the short-term and long-term management of existing urban lake systems.

TABLE 13.1 Summary of Urban Lake Design Steps Purpose

Step 1

Design intent

To establish purpose of waterbody, i.e., amenity, fill, flood mitigation, and to identify if concept lake design takes critical design principles (e.g., depth, inflows) into account.

Step 2

Lake type

Identify if lake will be saline, brackish, or freshwater. Each lake type requires different management, with brackish lakes to be avoided due to high risk of cyanobacterial blooms.

Step 3

Soil and groundwater assessment

Seeks to avoid issues associated with soils (acid sulfate, well-drained, or dispersive soils) and groundwater (poor quality groundwater, high in nutrient).

Step 4

Lake inflows and outflows

Assesses quality of inflows and possible risks associated with prolonged stormwater residence time by assessing catchment area to lake volume ratio.

Step 5

Define preliminary lake layout

Takes Steps 1e4 into consideration, as well as surrounding environments, infrastructure, etc., to establish a preliminary design.

Step 6

Lake bathymetry

Identifies the depth and bed profile of the proposed urban lake to determine stratification/mixing issues.

Step 7

Water balance model

Identifies potential for water level variation within lakes and if the waterbody will require frequent top up from stored supply.

Step 8

Algal management

Takes the above into account to determine waterbody residence time and thus determines risk of algal blooms. This step also defines recirculation requirements and defines critical design requirements of recirculation systems (e.g., recirculating lake water through constructed wetlands to remove nutrients and algal cells).

Step 9

Define final lake layout

Utilize the outcomes associated with Steps 1e8 to develop a final layout which takes into consideration the key design principles.

Step 10

Design of inlets/outlets and other hydraulic structures

Ensures that the relevant structures are design appropriately for the catchment’s hydrology and lake functions (e.g., mixing) and that maintenance issues are minimized (e.g., blockage of outlets), and other issues, such as climate change impacts, are considered.

Step 11

Design of lake edge/batters

Ensures that appropriate edge profiles are established based on the lake type. For example, densely vegetated batters are encouraged for freshwater lakes.

Step 12

Vegetated lake edge treatments

Defines the structure and diversity of the lake edge treatments and the establishment methodology.

Step 13

Maintaining flood capacity

Ensure that specialist hydraulic advice in relation to the specific urban lake catchment is considered in the design.

Step 14

Lake maintenance access

Ensures that suitable accesses into and around the lake is provided to allow for maintenance (e.g., floating macrophyte harvesting).

Step 15

Vector control

Ensures that the design avoids habitat associated with mosquito breeding and considers approaches to manage risk.

Step 16

Bird control

Establishes that design has considered risks associated with bird nesting and bird overpopulations.

Step 17

Public safety

Lake design is established to minimize casual access or other associated risks to community.

Step 18

Life cycle costing

Provides the asset manager with long-term maintenance costs associated with the urban lake.

Adapted from DesignFlow and RPS, 2010. Townsville Constructed Lakes Design Guideline. Prepared for Townsville City Council.

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Design Steps

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13.6.2 Urban lake management Although the reasons for urban lake degradation are well understood, the approach to managing urban lakes has failed to adequately characterize them for what they are: dynamic, integrated ecosystems. Urban lake monitoring programs are typically limited to measuring physicochemical indicators in a static state, which presents a monodisciplinary, incomplete measurement of lake health. This approach makes frequent use of short-term solutions (e.g., macrophyte harvesting) and is largely reactive. Management essentially requires a system to fail in the eyes of stakeholders before any intervention occurs. This approach does not consider the links between biotic and abiotic factors. The approach also ignores the local community as a component of the urban lake ecosystem, notwithstanding the inevitable interaction between the community and lake. Reactive management intervention may even exacerbate problems in urban lakes. Many recovery strategies have actually been shown to reinforce the alternative stable state. For example, dredging lake sediment is one option, but it is an expensive exercise that causes a major disruption to both the lake environment and the urban community, and it potentially also impacts downstream ecosystems. Marotta et al. (2009) argued that while dredging removes the nutrient-laden sediment from a system, it also removes the aquatic macrophytes that take up nutrients and compete with algae and floating macrophytes. Furthermore, any uncollected dead floating macrophyte becomes a significant source of organic carbon on decay. The removal of these aquatic macrophytes, coupled with continuous nutrient laden input, can actually intensify eutrophication rather than limit it (Marotta et al., 2009). Many current urban lake recovery strategies often involve retrofitting upstream catchment areas with stormwater quality improvement devices (SQIDs) to pretreat urban runoff. Some examples of typical SQIDs are shown in Fig. 13.4. SQIDs serve a range of uses, from treating primary pollutants (e.g., gross pollutant traps removing anthropogenic litter) to tertiary treatment (e.g., constructed wetland and bioretention basins remove nutrient from runoff). Although commendable, these screening measures only serve to reduce future nutrient loads, and donot address nutrient cycling within the lake itself. Unfortunately, effective lake treatment strategies are limited.

FIGURE 13.4 Typical stormwater quality improvement devices. From top left, clockwise: dry detention/sediment basin, gross pollutant trap, constructed wetland, and bioretention basin.

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Although valuable, physicochemical indicators serve to measure only a component of lake health, not the entirety. Management strategies for urban lakes derived from physicochemical monitoring alone and will often result in repeated interventions (e.g., macrophyte harvesting). Basing management strategies on such limited physicochemical observations rather than patterns of behavior limits the effectiveness of remediation strategies and may fail to identify the key issues driving the broader ecosystem health of the urban lake. As urban lakes are often key features of residential developments, such an approach should be inclusive of the surrounding community in the assessment and indeed may help establish more adaptable and long-term management strategies (Foden et al., 2008; Walker, 2012). Although current guidelines assist in ensuring that important processes are considered in the urban lake design process, adaptive and ecosystem-based management principles for urban lake health are somewhat lacking.

13.7 URBAN LAKES AND ECOSYSTEM HEALTH With the rapid increase in new urban developments, such as that occurring in SEQ, it is unlikely that the popularity of urban lakes will decline. The impacts of urban runoff on urban lakes have been evident for many years and will persist without holistic monitoring to ascertain the linkages (e.g., causal relationships) between biogeochemical indicators. Lake management is often reactive to one specific problem (e.g., prolific floating macrophyte growth) and common solutions often do not address the underlying causes of the issues. Many problems in urban lakes are not linked to a single factor. but rather are the result of complex interactions in the aquatic ecosystem. Lake health is not dependent on a single parameter or indicator set, so effective monitoring and management of such systems may be more complex. There are a broad range of indicators that can be used to assess, and hence manage, the overall state of urban lake systems. Physicochemical indicators such as pH, DO, turbidity, and nutrients, are valuable, but provide an incomplete picture. Despite this, such indicators are the conventional norms for assessing urban lake health. Studies of aquatic ecosystems routinely conceptualize one or two indicators of ecosystem health, such as chemical or physical parameters, or assess a system from an economic or engineering perspective, rather than taking an integrated view (Rapport, 1998a; Walker, 2012; Wiegand et al., 2013). While the trophic status of an aquatic environment can be assessed using traditional physicochemical indicators, other indicators, such as macroinvertebrate diversity, constitute a significant component of biodiversity and are critical of a well-functioning aquatic ecosystem (Chessman et al., 2002). Furthermore, research on urban lakes almost universally fails to link community well-being to urban lake ecosystem health (Walker et al., 2012; Wiegand et al., 2013). As urban lakes are often key features of residential developments, a more holistic approach should also be inclusive of the surrounding community and the links between quality of life and urban lake ecosystem health. When discussing ecosystem heath, it can be useful to compare it with the complexities of human health. Using physicochemical indicators as the sole measure of health is analogous to a medical doctor providing a diagnosis based on a patient’s blood pressure. A conclusion is based on an incomplete assessment. This is typical to urban lake management, particularly in terms of community linkages. A simple approach to quantifying the link between a community and urban lake quality is to conduct a survey to investigate the values a community places on such a system (Walker et al., 2013; Hagare et al., 2015). There is a substantial amount of research on integrating biological, chemical, physical, and social attributes (Rapport, 1998a,b,c; Howard and Rapport, 2004; Xu et al., 2004, 2005; Foden et al., 2008). However, little of this research is linked to urban lake ecosystems (Walker, 2012; Walker et al., 2013; Wiegand et al., 2013) and none is particularly recent. An approach that integrates the various disciplines associated with aquatic ecosystem health in urban lake research may provide novel and ultimately more effective solutions for the management of urban lakes. Broad assessments may also be costly, if not carefully planned. Water quality analysis alone may result in significant costs. Furthermore, it may be difficult to source the broad expertise to guide more holistic assessments.

13.7.1 The Ecosystem Health Paradigm The idea of “health” has been extended to describe the state of many different ecosystems. However, the traditional methods attached to the concept remain the same. That is, health is measured through the degree of divergence from the “ideal” state (Pantus and Dennison, 2005). The EHP is an adaptable means of assessing system health and is defined on the case-by-case basis (Howard and Rapport, 2004). This increases the flexibility of the EHP approach, making it optimal for a variety of scenarios and not limited to any one particular area (Howard and Rapport, 2004); however, the approach is generally site specific, meaning results are not often translatable across systems. This approach has been used to assess the health of ecosystems such as rivers, lakes, and forests, typically linking human pressures on the ecosystem in the

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assessment, as well as the human uses and amenities derived from the system (Howard and Rapport, 2004; Xu et al., 2004). Prior to research by Walker (2012), the EHP had yet to be applied to subtropical urban lakes, despite the obvious similarity in fundamental ecosystem functions. It has been well accepted (Mageau et al., 1995) that the EHP is represented by three key measures; vigor, resilience, and organization. These measures are briefly discussed below in the context of urban lakes. Vigor often refers to the activity, metabolism, or production in an ecosystem. In the case of urban lakes, both high and low levels of vigor may be viewed as unhealthy states (Rapport, 1998b; Karr and Chu, 1999). Urban lakes that are eutrophic will be overly vigorous when systems that are oligotrophic (i.e., have very low productivity, such as glacial lakes) may be unable to sustain primary producers and benthic invertebrates that support diverse higher level species (Nürnberg and Shaw, 2006). Resilience is an ecosystem’s ability to cope with stressors and its ability to recover from disturbance. Despite disturbance, a resilient urban lake will display good biodiversity that is critical to many ecological functions. A healthy urban lake ecosystem has the means to buffer against stressors events (e.g., major storm events), minimize the disturbance to populations, and demonstrate ongoing resistance to shifting to the alternative (degraded) stable state. Organization in an urban lake is defined by its complexity and the level of interactions (i.e., symbiosis, mutualism, and competition). Diversity and species richness are often indicators of good health. Indicator species, such as aquatic macroinvertebrates, are often used to develop sensitivity indices, such as the Stream Invertebrate Grade Number Average Level (SIGNAL2) or the Swan Wetlands Aquatic Macroinvertebrate Pollution Score (SWAMPS). SIGNAL2 (stream) and SWAMPS (wetlands) are often used to link faunal sensitivity to pollution and organization (diversity and abundance). However, they are rarely applied to constructed waterbodies (Walker, 2012). Vigor, resilience, and organization all influence each other. These links characterize the complex interactions that occur in urban lakes. Given the setting of most urban lakes, in that they are close to residential communities, it is important to quantify the community as a part of the urban lake ecosystem and define the links between the two. A broader, multifocal approach to managing and monitoring urban lakes presents a greater suite of information for managers to access, enabling a decision-making process that is not dominated by any one discipline. It presents a holistic assessment that may assist in establishing proactive and more adaptive management strategies. A suggested approach to monitoring urban lakes is provided below; however, it should be noted that the approach is broad and requires the collection of substantial data. This approach should be adopted when there are critical drivers to management: l

l

l

Static and Dynamic State Water Quality MonitoringdRegular physicochemical water quality monitoring is critical, as it allows for pattern identification and presents an inference of vigor or productivity. It is important to monitor a lake in both static and dynamic states (i.e., during storm events) to assess pollutant concentrations within and entering an urban lake so that impacts can be better assessed and management options can be better informed. Such data serve to infer the state of urban lakes, or ecosystem vigor, to an extent (i.e., whether or not the lake is in the alternate stable state). Such monitoring can also help to address resilience to some degree, as lakes are measured before, during, and after disturbances, including seasonal turnover and storm events. Storm event monitoring data are scarce, largely because of cost, but it is important to enable these events that represent the dynamic environmental state to be characterized, particularly for degraded systems requiring remediation. Systems with appropriate design are unlikely to require such monitoring, as upstream measures are in place to treat runoff. As part of the 2010 review of the Ecosystem Health Monitoring Program (EHMP) in SEQ, the need to include event monitoring as part of the program was identified (BMT WBM, 2010). Floral/Faunal IndicatorsdFloral and faunal surveys are useful in providing both a quantitative and a qualitative means of assessing organization. It enables managers to evaluate the impacts of water quality on organization and also to identify potential risks to lake health, such as the presence of weeds or other noxious species. Screening Level Risk AssessmentdIdentifying actual and perceived risks to the community living in association with an urban lake is arguably a critical component to applying the EHP, as well as also one that has had limited application to date. While an urban lake may not have a specific recreational use, it cannot be assumed that it will not be used for this purpose. Therefore, it is valuable to assess these waterbodies against hazards to public health using guidelines such as those provided by the National Health and Medical Research Council (NHMRC, 2008). The need to include human health monitoring as part of the SEQ EHMP has been identified (BMT WBM, 2010) and it bears merit to apply such assessments to urban lakes, subject to the proposed use (e.g., primary vs. passive recreation) given the proximity to, and use by, residential communities. This approach serves to identify and initially characterize the links between community and urban lakes, although the link is only characterized in a limited, one-way impact driven sense (i.e., risks lakes pose to the community).

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Community IntegrationdThis is a fairly novel concept where urban lake monitoring is concerned. Community consultation and surveys are generally conducted in response to an existing issue within an urban lake, rather than as part of developing proactive, adaptable management strategies. This limits and inhibits community involvement, which can subsequently limit community responsibility and custodianship of the surrounding environment. Without including the community as key components of an assessment, managers cannot identify the attitudes and values the community has regarding urban lakes and cannot identify behaviors that may significantly impact lake health. This approach is important to fully characterize the link between communities and urban lakes and identify not only the impacts lakes may have on a community (and the reverse) but also the mutual benefits that can be realized. Two Australian studies (Walker et al., 2012; Hagare et al., 2015) surveyed the local community. Walker et al. (2012) found that the community placed a high value on an adjacent urban lake system and aesthetics were not the sole driver in determining value. Water quality and the creation of a sense of community were also two critical values that were important to residents. Hagare et al. (2015) found that not only did residents feel that the lakes added value to properties by increasing the aesthetic appeal, but also added value to the community by providing usable open space areas. Understanding and examining community values can allow for targeted and proactive maintenance strategies.

An integrated, interdisciplinary approach may serve to better assess urban lakes as an ecosystem and better infer the state of health than the current approaches to urban lake monitoring and management. This is particularly the case for degraded systems that required remediation. Although indicator assessments may not be as in-depth as specific measures of ecological processes, the scope is broader and indicators are linked, which serves to better characterize an urban lake as an ecosystem, enables a more robust measurement of health, and allows for a more informed approach to management and remediation. Foden et al. (2008) provide a valuable critique on the assessment of natural aquatic ecosystems. In particular, they highlight the general lack of consistency and failure to conduct holistic, well-integrated assessments of such environments. However, there has been a substantial improvement in the monitoring and management of natural aquatic ecosystems in the last few years, a prime example being the ongoing evolution of the EHMP in SEQ. Fig. 13.5, adapted from Healthy Land & Water, characterizes the various integrated approaches taken in assessing aquatic environments to determine ecosystem health. The success of the EHMP demonstrates the value of a holistic assessment and the excellent outcomes achieved through such assessments. Given the potential social and economic values urban lakes can represent, it is paramount that urban lake managers use the EHMP approach as a viable, tailored approach to better manage urban lake ecosystems.

Invertebrate survey

Physicochemical monitoring

Fish survey

Nutrient analyses

Ecological processes assessments

FIGURE 13.5 Ecosystem Health Monitoring Program.

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13.8 CONCLUSIONS This chapter has discussed how urban lakes are integrated within the WSUD philosophy, and what design principles and requirements are critical for urban lake management and maintaining health in these waterbodies. Urban lakes are often included in developments for aesthetic benefits and stormwater detention purposes. They can also provide a significant source of land fill for developments where building pads need to be raised above a specified flood level. This chapter showed that the inclusion of an urban lake can have a positive effect on the appeal on an entire urban development, and not just on the immediately adjacent properties. Apart from economic drivers, urban lakes can provide intangible benefits such as fostering community well-being. In the early adoption of WSUD in Australia, urban lakes were installed as a means of stormwater treatment. However, it was soon realized that untreated stormwater runoff has serious impacts on urban lake health (e.g., eutrophication), which often led to a shift to the alternative stable state. Consequently, in Queensland, urban lakes are no longer considered to be a viable means of treating urban stormwater runoff. Under the Queensland State Planning Policy (DILGP, 2017), urban lakes must only receive stormwater runoff that has been treated to the required standard, which varies with location within the state. We identified three key factors that are associated with urban lake degradation: l l l

Hydrology of the urbanized catchment; Poor quality of inflow; and Poor lake design attributes.

Once an urban lake has degraded (to the alternative stable state), it can be extremely difficult to shift back to the original healthy state. As the key factors that caused lake degradation were progressively identified, the design and management of urban lakes evolved to better cater for the impacts of urbanization. Urban lakes in Australia are now often classified as “receiving environments,” which means that stormwater runoff must be pretreated prior to its discharge into the lake. Urban lakes are also designed to provide flood protection in that they collect stormwater, temporarily store it, and then slowly release it at a controlled rate such that downstream areas are not flooded or eroded. As the quality of many early urban lakes started declining, it became apparent that proactive, long-term strategies were required to improve urban lake design and management. As a result, key design principles for urban lakes were established, and a number of urban lake design guidelines were developed. Urban lake design within Australia now considers a wide range of variables to ensure the risks associated with the five key causes of urban lake degradation (Section 13.4.1) are avoided. While the reasons for urban lake degradation are now well understood, the approach used to manage urban lakes in the past has often failed to adequately acknowledge them for what they areddynamic, integrated ecosystems. Urban lakes require an interdisciplinary approach that looks beyond the physicochemical quality and seeks to identify the links and relationships, inclusive of biogeochemical and socioeconomic processes within the ecosystem itself. With the rapid increase in new urban developments in Australia cities, it is unlikely that the popularity of urban lakes will decline. Urban lakes are important assets that can play a key role in the social capital of urban developments. This chapter has shown that with careful design, management, maintenance, and understanding of lake dynamics, urban lakes can provide many long-term benefits to urban communities. Their widespread implementation should be encouraged in appropriate landscapes.

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FURTHER READING Birch, S., McCaskie, J., 1999. Shallow urban lakes: a challenge for lake management. Hydrobiologia 395/396, 365e377. Bolund, P., Hunhammar, S., 1999. Ecosystem services in urban areas. Ecological Economics 29, 293e301. Carpenter, S.R., Caraco, N.F., Correll, D.L., Howarth, R.W., Sharpley, A.N., Smith, V.H., 1998b. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8, 559e568. Findlay, C.S., Bourdages, J., 2000. Response time of wetland biodiversity to road construction on adjacent lands. Conservation Biology 14, 86e94. Helfield, J.M., Diamond, M.L., 1997. Use of constructed wetlands for urban stream restoration: a critical analysis. Environmental Management 21, 329e341. Hem, B., 2007. ‘Entrepreneurs fill a niche by keeping ponds in subdivisions sparking’. The Houston Chronicle. From: http://www.redorbit.com/news/ science/975505/entrepreneurs_fill_a_niche_by_keeping_ponds_in_subdivisions_sparking/index.html. Kenna, T.E., 2006. Consciously constructing exclusivity in the suburbs? Unpacking a master planned estate in Western Sydney. Geographical Research 45, 300e313.

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Lee, S.Y., Dunn, R.J.K., Young, R.A., Connolly, R.M., Dale, P.E.R., Dehayr, R., Lemckert, C.J., Mckinnon, S., Powell, B., Teasdale, P.R., Welsh, D.T., 2006. Impact of urbanisation on coastal wetland structure and function. Austral Ecology 31, 149e163. Naselli-Flores, L., 2003. Man-made lakes in Mediterranean semi-arid climate: the strange case of Dr deep lake and Mr shallow lake. Hydrobiologia 506e509, 13e21. Wong, T.H.F., Breen, P.F., Somes, N.L.G., 1999a. Ponds vs wetlands - performance considerations in stormwater quality management. In: From 1st South Pacific Conference on Comprehensive Stormwater and Aquatic Ecosystem Management Auckland, New Zealand, pp. 223e231. Wong, T.H.F., Breen, P.F., Somes, N.L.G., Lloyd, S.D., 1999b. Managing urban stormwater using constructed wetlands. In: CRC for Catchment Hydrology Industry Report Series. Yang, X., Wu, X., Hao, H., He, Z., 2008. Mechanisms and assessment of water eutrophication. Journal of Zhejiang University 9, 197e209.