Sustainable Management of the Centennial Parklands Ponds

Transcription

1 Sustainable Management of the Centennial Parklands Ponds AUTHORS Mal Brown, Storm Consulting Stephen Nichols, Centennial Parklands George Freeman, Storm Consulting ABSTRACT Centennial Parklands in Sydney s inner eastern suburbs provide a popular recreational resource with over 3 million visitors per annum. Within the parklands are 12 ornamental ponds providing a range of values. Management of Centennial Parklands ponds is vested with the Centennial Park and Moore Park Trust (the Trust). The Trust maintains and improves Trust lands and ensures environmental protection. Through the early to mid 1990s, the water quality of Centennial Parkland s ponds deteriorated to the extent that blue-green algal blooms occurred in most ponds perennially over several years. The blooms were attributed to long term pond degradation as a result of ongoing stormwater pollution, and pond design and function which did not cater for the pollution inputs, and the presence of carp in ponds. Any degradation of the ponds environment or water quality has far reaching implications for the Park s amenity and its environmental and heritage values. The pollution of the Park s waterways is of utmost concern to the Trust. Park visitors rank cleanliness of the ponds as a high priority. In 1997, the Trust prepared the first plan to manage and improve the ponds. The Ponds Restoration Program Strategy Plan 1997 provided a comprehensive discussion of issues facing the ponds, and the resultant management recommendations to remedy them. Initial works on the ponds were completed in five stages from 1998 to about 2003 with works in seven of the 12 ponds. The works included design refinements to facilitate management of stormwater pollution inputs, and others to improve the ecological values of the ponds. In the Trust renewed its focus on the ponds. They amended and revised the previous Plan to create a Ponds Management Plan 2012 which included the identification of new works based on a critical review of the previous works. They invested significant resources to establish the most costeffective pollution trapping, removal and disposal strategies and they implemented these on two of the ponds. The future value and amenity of the Centennial Park ponds is intrinsically linked to the ongoing management of stormwater pollution inputs from external catchments. This paper describes the extraordinary effort and investment made by the Trust to maintain and improve one of their key assets.

2 1. INTRODUCTION Located in the inner eastern suburbs of Sydney, Centennial Parklands is an iconic Sydney landmark. First established in 1888 as a commemoration of the centenary of European settlement in Australia, the parklands have a rich and complex cultural history. Extending over approximately 365 hectares, the parklands exist as an invaluable asset within Sydney. The parklands comprise of three separate parks, namely Centennial Park, Moore Park and Queens Park (Figure 1). Together the parks offer a popular recreational resource for upwards of 3 million visitors annually. Situated within the inner metropolitan area of the Sydney, the Centennial Parklands encompass 365 hectares of mixed use public space. They feature open parkland, sports fields, equestrian grounds, formal gardens, ponds and grand avenues amongst a variety of other recreational facilities including walking tracks and a golf course. FIGURE 1 - CENTENNIAL PARKLANDS SHOWING WEST TO EAST MOORE PARK, CENTENNIAL PARK AND QUEENS PARK The parklands perform key environmental functions, most notably providing habitat and a refuge for native flora and fauna while also facilitating urban stormwater treatment in what is an increasingly urbanised catchment. The catchment boundary and flow paths are shown in relation to the ponds in Figure 2. The ponds are predominantly situated at the lower portion of Centennial Park and act as the receiving waters for runoff generated from a 600Ha catchment. FIGURE 2 - CENTENNIAL PARKLANDS PONDS CATCHMENT CONTEXT SHOWING CATCHMENT BOUNDARY AND FLOW PATHS

3 The pond system consists of 12 ornamental ponds in total with a combined surface area of approximately 26 hectares. There are five major stormwater inflows to the pond system and a complex series of flow paths (Figure 3). In addition, the ponds are connected to groundwater in the underlying Botany Aquifer. Pond water is extracted from the lower pond (Kensington Pond east) and is used to intensively irrigate 55Ha of sports fields and golf course. Stormwater flows out of the system in the south, and on to the Botany Wetlands and Botany Bay. FIGURE 3 - CENTENNIAL PARK POND CONFIGURATION AND FUNCTION IN RELATION TO STORMWATER (KIPPAX LAKE NOT SHOWN) Centennial Parklands is managed by the Centennial Park and Moore Park Trust (the Trust), a statutory body which was established under the NSW Centennial Park and Moore Park Trust Act (the Act) of The Trust has the role of ensuring the maintenance and improvement of Centennial Parklands, including the Centennial Parklands Ponds. The Trust is responsible for facilitating environmental protection and continued public amenity of the parklands, developing and upholding strategies to increase the recreational, historical, scientific, educational, cultural and environmental value of the lands. The Trust first prepared a plan to manage and improve the Centennial Parklands Ponds in 1997 with the development of the Ponds Restoration Program Strategy Plan Since that point management plans and strategies have been adapted to allow for the continued refinement of pond designs and associated works in order to facilitate the management of stormwater pollution inputs and the enhancement of the ecological values of the ponds. The preparation of the Ponds Management Plan 2012 represents the most recent strategy for the sustainable ongoing management of the Centennial Parklands pond system. The plan provides a holistic and comprehensive planning document with aims of guiding future pond restoration works and maintenance activities over the period The plan has focused on achieving a physically and ecologically robust pond system through contemporary management measures that reflect best practice and are considerate of life-cycle costs of implementation. This paper presents the current stormwater-related problems facing the Centennial Parkland Ponds and how these are proposed to be managed using the Ponds Management Plan PROBLEMS WITH THE PONDS

4 Between the period , the ponds were used as Sydney s second water supply because the Tank Stream in the CBD became too polluted. Then in turn development and inappropriate land use in the pond s catchment caused water pollution and this supply also had to be abandoned. Therefore, water quality and degradation of the ponds are not new issues in the history of Centennial Parklands. On 1 January 1901, the Parklands became the focus of the nation as the site of the inauguration of the Australian Federation. At this time, the ponds had been converted into a series of flood mitigation and storage dams and later, to the ornamental pond system that stands today. In a period between the early to mid 1990s, it became evident that the capacity of the ponds to function in light of increasing catchment development pressures was becoming severely impeded. Extensive and prolonged drought caused pond water quality to deteriorate to the extent that perennial blue-green algal blooms occurred in the majority of the 12 ponds causing them to turn a vivid green colour. Signs were erected at ponds warning people not to make contact with the water due to the toxins released by the algae. As the algal blooms died off they floated and rafted then decayed causing serious odours. The situation was dire. The algal blooms were symptomatic of a pond system that had become physically, chemically and biologically degraded over time and it was apparent that management practices would need to be altered to restore the health and ecological functions of the pond system, to support amenity to park users. At this time, the ponds became the focus of an inter-governmental committee to investigate water quality and to make recommendations on how to improve it. All of this early work has lead to an understanding among Parklands managers that the values and amenity provided by the ponds is reliant on them being healthy, attractive and stable. None of these outcomes can be achieved without effective water quality management across the pond system. The following issues were identified as being the most significant to manage in relation water quality in the ponds, and this is still relevant today: the pond system has a long hydraulic retention time meaning water can stagnate, especially in dry times pond stormwater inflows allow pollutants to spread, making retrieval impractical some ponds short circuit creating stagnant zones pond outflows do not allow for water level control pond sediments are nutrient-enriched and unstable making them susceptible to re-suspension the ponds have extensive weed macrophytes as opposed to desirable reeds. These weeds die off seasonally and contribute vast quantities of decaying organic matter into ponds. As this decomposes, it strips the water of oxygen which affects the biota present European carp feeding behaviour constantly churns up sediments which re-releases nutrients, and they cycle nutrients within the ponds. They also prey on other aquatic biota.

5 As a response to the pond water quality problems in the mid 1990s, the Trust commissioned and implemented the Ponds Restoration Program Strategy Plan In the subsequent period capital works were undertaken in several of the ponds to improve their water quality function and aesthetics. The works provided considerable improvement in pond condition as evidenced by the fact that in a subsequent extended drought in the mid 2000s, no blue-green algal blooms persisted. However, the lack of effective gross pollutant trapping at inflows was causing the accumulation of significant quantities of pollution near pond inflow zones. It was clear this pollution was in a position to again spread through the pond system causing widespread problems. In 2010, the Trust investigated options to retrieve and dispose of these accumulated pollutants (sediment, organic debris and litter) and were staggered by the high costs involved. It became evident that the best strategy was to prevent pollutants from getting into ponds, and as a result they commissioned the preparation of a comprehensive pond condition assessment with associated options for sustainable management. This was to inform the preparation of the Ponds Management Plan PLANNING RESPONSE In response to the review of the pond system, the Ponds Management Plan 2012 was developed with aim to provide a holistic and comprehensive planning level document that will guide pond restoration works and maintenance activities between 2012 and Consistent with this, the management objective for the whole pond system was to continue works to create a sustainable pond system that could be maintained in such a state into perpetuity. It was identified that the only way to achieve this objective was to centre design improvement works on creating physically and ecologically robust ponds through measures that restore and/or enhance the ponds water quality, while also having regard for life cycle costs of the implementation. 3.1.PONDS MANAGEMENT OBJECTIVES Objectives for each pond were established based on their unique position and function within the pond system (Table 1). TABLE 1 INDIVIDUAL POND OBJECTIVES IDENTIFIED IN THE PONDS MANAGEMENT PLAN 2012 Pond Stormwater inflow treatment, GPTs and stilling basins Irrigation extraction for the Parklands Fine particle filtering, after gross pollutants are removed Aquatic weed removal, extensive Nymphaea spp Oxygenation of flows, for ponds that are fed by low O 2 springs 1. Model Yacht 2. Fly Casting 3. Musgrave

6 4. One More Shot 5. Willow 6. Duck 7. Randwick 8. Lily 9. Busbys 10.Kensington East 11. Kensington West 12. Kippax 3.2. CONTEXT FOR PONDS MANAGEMENT In selecting management recommendations and developing subsequent concept designs, parkrelated requirements and principles must be adhered to, including: Cultural and historic considerations - The pond system character has evolved with the parklands and was to be maintained wherever possible; Social considerations - The public has a perception of the Centennial Parklands Ponds which have developed around their present configuration and function. Proposed changes were to be consistent with what would be socially acceptable; Recreational amenity - The ponds are used for passive recreation by many people. It was important to consider minimising alteration or interruption to existing recreational uses, and where possible, consider the provision of new recreational opportunities; Visual amenity - While the existing visual amenity was generally high, there was scope for improvement. There was also a need to landscape newly designed elements for aesthetic or access restriction reasons; Ease of maintenance - The Trust must maintain the ponds in perpetuity and so suggested changes were to be consistent with developing opportunities to simplify and improve maintenance activities; Habitat - A range of habitats for aquatic biota exist in and around the ponds but there was scope to provide greater habitat diversity and in doing so, create greater opportunity for long term ecological sustainability STRATEGIES AND RECOMMENDATIONS Within the Ponds Management Plan 2012 a comprehensive list of pond management strategies and works were devised in response to the problems and context of the ponds, falling under the following categories:

7 Ongoing and improved liaison with catchment stakeholders, e.g. re-convene a Ponds water Quality Management Committee Investigations to fill information gaps, e.g. pond water quality, pond hydraulic behavior, Pollution Incident Management Plan Capital works to implement pond rehabilitation actions, e.g. new Gross Pollutant Traps, bank stabilistation, willow removal, reed beds, water level control devices, etc Life cycle maintenance activities, e.g. ongoing removal of collected pollutants, aquatic weed, carp removal, etc. Monitoring and review points to trigger maintenance activities and to provide a feedback loop on the relative success of implementation. Action tables were prepared for each pond with Kensington Pond west used as an example here (Table 2, view in association with Figure 4). TABLE2- EXAMPLE MANAGEMENT RECOMMENDATIONS FOR KENSINGTON POND WEST Problem Management recommendations Rationale At the entrance of channel inflow from Busbys and Randwick Ponds, large amounts of organic debris and sand have accumulated. Install a stilling basin at the inlet to Kensington Pond. Provide maintenance access ramp. Once stilling basin is installed, remove existing net pollution trap from the channel. While the net traps are effective at collecting debris, they also fill and clog quickly. Sediment builds up in the channel behind the net trap which has no effective means of removal in the narrow and constrained channel. Western inflow accumulates sediment and litter. Install a CDS GPT. Provide maintenance access for routine cleaning. Remove existing net pollution trap. The inflow requires a gross pollutant trapping strategy to catch all particles down to coarse sand size. Nymphaea spp. Aquatic weed is covering parts of the pond area Remove aquatic weed (Nymphaea spp) from the ponds. Nymphaea is an aquatic weed species that clumps, spreads and periodically dies off in parts providing a high organic load to the ponds. It should be removed and its re-growth should be controlled through ongoing removal.

8 For each pond, a concept design of works and strategies was prepared that depicts the type of information in Table 3 for each of the ponds. Figure 4 shows an example of one of the concept designs prepared for Kensington Ponds east and west. FIGURE 4 WORKS AND STRATEGIES CONCEPT DESIGN FOR KENSINGTON PONDS 3.4.LIFE CYCLE MAINTENANCE ACTIVITIES As already alluded to, maintenance will be a key feature of pond management to sustain them in a robust condition in perpetuity. The maintenance activity requirements on an annual basis are shown in Table 4. The consequences of not maintaining will be that the ponds will not achieve their objectives, they will revert to a degraded state, and they will be at risk of major water quality decline during any future drought sequences. TABLE 4 MAINTENANCE ACTIVITIES RECOMMENDED IN THE PONDS MANAGEMENT PLAN 2012, SUMMARISED BY POND AND ACTIVITY CLASSIFICATION Maintenance activity Item Frequency of maintenance Removal and disposal of pollutants from GPTs Model Yacht Pond CDS 6/annum Musgrave Pond CDS 6/annum Musgrave Pond Net trap 26x/annum Musgrave Pond Channel upstream of net trap 2/annum Kensington Pond West CDS 4/annum Kensington Pond East Net trap 26/annum Removal and disposal of accumulated sediments & organic matter from pond inflows Model Yacht Pond Accumulation zone 1/20 years Musgrave Pond Whole pond 1/10 years

9 Kensington Pond East Whole pond 1/10 years Kensington Pond West Kensington Pond West Accumulation zone downstream of apron Stilling basin on channel from Busby/Randwick 1/20 years 2/annum Randwick Pond southeast inflow Accumulation zone 1/annum Randwick Pond - sthn inflow Trash rack 1/annum Busbys Pond Frog Hollow Drain Accumulation zone 1/annum General pond maintenance Clean up litter Pond edges and shallow margins Monthly Electrofish for carp Especially in the lower ponds Annually Spray Nymphaea aquatic weed Typically Kensington, Busbys, Randwick, Lily Each spring Remove/spray weeds Pond edges and shallow margins Each spring Operate aeration devices Lily and Duck Ponds TBD 3.5. COST CONSIDERATIONS Full costing was undertaken for investigations, works and strategies and for all maintenance activities. Recommendations were assigned priority based on the effect that they have on improving water quality function. The total estimated expenditure on High Priority ( ) recommendations across the pond system is $1,979,000. The total estimated expenditure on Medium-Low Priority ( ) recommendations is $772,500. The high priority works will be the subject of future ponds works funding bids. The total expenditure on pond life cycle maintenance activities is $173,600 per annum. The costs are high reflecting the overall value of the 12 pond stormwater assets in the various functions they provide to visitors, the environment and the broader community. The costs represent a realistic investment to maintain these functions such that the asset value of the ponds is increased and their life prolonged. Options were considered for gross pollution trapping, e.g. comparative analysis of litter net traps vs. installation of CDS GPT at major stormwater inlets. Capital and operational costs were used to determine life cycle costs which formed the basis of decision-making. Cost saving measures were identified for further consideration. As tipping fees are unaffordable in the medium-long term, it is imperative to have an option of disposing removed sediments and organic debris from the ponds within the Parklands. Classification of the waste occurred and a decision was made to dispose of it in landfill for works completed in Instead it is proposed in future to process the material and recycle it as compost to avoid tipping fees, to treat the material as a resource and to reduce the costs of imported mulches.

10 Options were identified that if implemented in one pond, they would provide benefits to several others concurrently. An example is Lily Pond which is fed by de-oxygenated spring water. This pond flows into downstream ponds and so it is essential to improve this water quality by oxygenating it. This will prevent the establishment of blue-green algal blooms by removing conditions which favour them from the outset. In this manner, five ponds benefit from the treatment of one. 4. WORKS COMPLETED The Trust invested $1.8 million (not included in the costs shown above) on pond rehabilitation works in two ponds during the period (Kensington and Model Yacht Ponds) and this included the following works: installation of two large CDS GPTs installation of a stilling basin on a minor inflow to Kensington Pond west re-instatement of a reed bed in Model Yacht Pond replacement of dilapidated stormwater infrastructure removal and disposal of over 1,500m 3 of accumulated sludge (sediment, organic debris) from both ponds willow removal from both pond edges removal of all Nymphaea aquatic weed from the two Kensington Ponds, and follow-up removal stabilization of over 500m of pond banks Where possible, some of the work was completed by the Centennial Parklands volunteer program. Volunteers re-planted the reed bed and associated soft landscaping of the GPT in Model Yacht Pond. The Plates shown below provide an insight into the problems and associated works undertaken in Kensington and Model Yacht Ponds Plate 1: Inlet to Kensington Pond east before works showing sediment accumulation, willows and poor visual amenity, Plate 2: Kensington Pond east after works, showing weed removal, bank stabilisation, willow removal, revegetation

11 Plate 3: Kensington Pond east before weed Plate 4: Weed removal process in pond, includes removal roots/rhizomes on bed Plate 5 LHS: Minor inlet to Kensington Pond east showing sediment and debris deposition moving into pond Plate 6 RHS: Stilling basin at same inlet with access ramp to facilitate removal of accumulated sediment/debris Plate 7: Sediment removed from pond for drying, Plate 8: Volunteers planting reed bed filter in before disposal. Model Yacht Pond

12 5. CONCLUSION Because the Centennial Parklands ponds are subject to major stormwater inflows from large contributing catchments, they are sinks for pollution. In the past, this uncontrolled pollution and the long time that water can spend in the pond system can cause stagnation and give rise to blue-green algal blooms. This causes ponds to turn green with floating scums that rot and stink. Therefore there is a history of rapid pond water quality degradation, especially during dry periods. This creates a need to act with proactive interventions that will prevent this system decline such that the pond system can maintain its levels of service to park visitors. With such a large and complex pond system to manage, a comprehensive and strategic approach is required that responds to the many often competing issues and constraints of working in the Parklands. The Plan developed for the pond system must not only provide water quality improvement, it must provide a broad range of benefits and not cause conflict with the Heritage elements of the Parklands, as well as being acceptable to visitors. The Ponds Management Plan 2012 delivers all this and more. The strategic nature of the Plan is rooted in the principles and approaches adopted, including catchment analysis, stakeholder collaboration, system-wide and individual pond objectives that align. The actions that have been proposed as a result of this are targeted specifically at solving specific water quality problems and priority is assigned according to water quality benefit. There is a recognition that pond water quality is fundamental to their broader function in the Parklands. The Ponds Management Plan 2012 provides a balance of strategies to ensure long term success in management of the pond system, comprising: catchment stakeholder consultation, investigations to fill data gaps, capital works and life cycle maintenance activities. A comprehensive response has been developed which is cost effective, as it has been based on feasibility assessments to determine life cycle costs of various options. The alternative to the works and strategies is that pollution enters the ponds and moves through the system where it festers and is extremely difficult and expensive to remove and dispose of. To round off the success of this Plan, there is a clear commitment and ability to deliver on the various actions. Over the period major capital works and maintenance activities were completed in two of the most important ponds. Two GPTs were installed, accumulated sediment was removed, noxious aquatic weeds were removed, and banks were stabilised and revegetated. Some of these works were completed using Parklands volunteers and they have been a complete success. The development of the Ponds Management Plan 2012 has resulted in a comprehensive document that has incorporated management recommendations and concept designs on a pond-by-pond basis that has been based on a critical review of existing pressures and challenges facing the ponds and their continued maintenance. The Plan and subsequent and ongoing works represent an extraordinary effort and investment made by the Trust to maintain and improve one of their key assets.

13 6. SOURCES OF INFORMATION Brown MR (1997) Rehabilitation of Centennial Park Ponds. Proceedings of the 10 th International Urban Drainage Conference, Sydney. Centennial Park and Moore Park Trust (2006) Centennial Parklands Plan of Management, Storm Consulting (2012) Centennial Parklands Ponds Plan of Management 2012 Storm Consulting (2011) Kensington Ponds Design Drawings & Technical Specification, Project Nos & Storm Consulting (2010). Musgrave and Model Yacht Ponds Pollution Management Options, Project Report Water Resources Consulting Services (1997) Centennial Park Ponds Restoration Program: Strategy Plan.