Stream Restoration Making It Happen: The Restoring The Waters Project

Transcription

1 Stream Restoration Making It Happen: The Restoring The Waters Project STEPHEN FROST, Fairfield City Council ROD WIESE, Department of Land & Water Conservation BARBARA SCHAFFER, Schaffer Barnsley Landscape Architects DREW BEWSHER, Bewsher Consulting Pty Ltd. SUMMARY The Restoring The Waters project is located on Clear Paddock Creek within the Georges River Catchment in south-western Sydney, Australia. The project involves the removal of 2.5km of concrete-lined stormwater channel and the reinstatement of a natural creek system. The surrounding land uses are residential and there has been significant community interest and involvement in the project. Stage 2 of the project is nearing completion and involves preparation of construction plans for the restoration of the creek. These plans are being prepared by a multi-disciplinary design team. The Paper presents details of the multi-disciplinary team, the community consultation process, design issues, the impact of the design on flood behaviour, and a summary of the principal lessons learnt through the design process. 1. BACKGROUND 1.1 Project Overview The Australian Conservation Foundation s (ACF) Restoring the Waters Project in Sydney offers a strategy towards achieving ecologically sustainable management of stormwater. The project aims to remove 2.5km of an existing concrete lined stormwater channel and reinstate a natural creek system. The collection, transport and utilisation of stormwater will be managed to reduce pollution in the receiving waters whilst enhancing the physical environment. The aim is to create biologically diverse urban waterways, which both purify and control the movement of stormwater by incorporating well planned and appropriately located gross pollutant traps, litter traps, sediment ponds, constructed wetlands and innovative canal conversion methods. Benefits include improved stormwater quality, local biological diversity, habitat creation and the enhancement of aesthetics for the community. This innovative model for stormwater management in a highly developed catchment is a challenge of both international and national significance. Careful documentation will ensure the project can be appropriately translated to similar sites or situations across the country. 1.2 Site Selection The Southern Sydney Regional Environmental Plan identifies stormwater pollution as the second major environmental concern in the Georges River catchment. Within this catchment, Prospect and Cabramatta Creeks were identified as the two most polluted subcatchments. Councils in these two subcatchments were invited to nominate sites consistent with the objectives of the Restoring the Waters Project. Clear Paddock Creek in the Prospect Creek subcatchment within the Fairfield local government area was selected as the optimum site to set up this demonstration project. The site stretches from Edensor Road in St John s Park to King Road in Wakeley. Four road bridges and a number of pedestrian bridges cross the existing concrete channel over its 2.5km length from Edensor Road to King Road. In the lower portion of the site, there is an existing retarding basin immediately upstream of King Road. The project also involves the conversion of this basin into a major wetland whilst retaining its function as a retardation basin (see Figure 1). 1.3 Progress To Date The project has been divided into three stages, each funded by grants. Stage One of the project dealt with selection of the optimum site and the preparation of concept plans. The focus of Stage Two which is currently drawing to a close, is development of construction plans for the restoration of Clear Paddock Creek. Stage Three will involve construction, once funding becomes available. It is likely that this stage may be divided into various phases to suit funding cash flow. 1.4 Funding And Costs The environmental significance of the project is recognised by a large number of bodies and funding authorities. Many have contributed to the funding of Stages One and Two including the NSW Environment Protection Authority, The Restoration and Rehabilitation Trust, Sydney Water, The Department of Land and Water Conservation, Total Catchment Management Enhancement and the Australia Council. The project has been further supported by state government agencies, Fairfield City Council and numerous community groups.

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3 Funding for Stage Three (ie implementation) is currently being sought through the Natural Heritage Trust Grants. Preliminary cost estimates for construction are $9 million. 2.0 MULTI-DISCIPLINARY DESIGN TEAM 2.1 Composition Of The Team A multi-disciplinary team comprised of hydrologists, river engineers, fluvial geomorphologist, habitat ecologist, landscape architects and artists were selected to participate in Stage Two of the project ensuring a holistic approach to the design of the new creek. 2.2 Designing As A Team The design objectives for the new creek were identified and then ranked into, aquatic habitat, recreation and aesthetics, biodiversity Figure 1and abundance, and education. Ranking allowed controversial decisions to be made in the light of an agreed set of priorities. In addition, improving water quality was seen as a key to achieving many of the above objectives. It is easy for each team member to work in isolation but the challenge lies in each one being taken beyond their immediate response to a problem. Proper coordination of all the activities by the project coordinator is a key to ensuring an efficient, comprehensive and competent design outcome. 2.3 Intellectual Property Issues The project aims to demonstrate innovative stormwater practice. The objective is for intellectual property to become communal property for ecological benefit. 2.4 Multi-Objective Design Standards and Liability Issues Standards were available within individual disciplines (e.g. engineering, landscaping), however, multi-objective design standards were not available. For example, in the case of rock armouring, engineering standards might dictate that the size of rock required needed to be at least 900mm diameter. From an aquatic habitat point of view however, a predominance of such large rock sizes was seen as detrimental to future habitat values. It was therefore impossible to select a rock size when each standard was viewed in isolation. The team decided to compromise using a range of rock sizes varying from pebbles up to the occasional rock of 900mm diameter. The team recognised that there was a joint liability, should the rocks be eroded in a major flood event. 3. INTERACTING WITH THE COMMUNITY 3.1 The Importance Of Community Consultation It was recognised early that if the community and its elected representatives were not involved in meaningful ways in the development of the project, then it was doomed to failure. Community consultation invariably heralds change to which people respond in different ways. Figure 2 below provides a `bathtub analogy in which the response to change can often be downhill in the beginning. If you don't have any strategies to assist people then they may even disappear down the drain, never to begin the climb up the other side. 3.2 The Consultation Process The formal strategy consisted of community meetings, field trips, on site open days, surveys, media coverage, an information brochure, public exhibition of the plans, presentations to the Councillors and a community art project called "The Memory Line". In addition, there were numerous discussions between Fairfield City Council and community groups. The project has received considerable publicity through coverage on Quantum, Earthbeat and the Sydney Morning Herald, and some other national and international exposure. 3.3 The Community's Concerns Council's survey of the community showed that residents supported restoration of the Creek. However there was real concern that the area could become a breeding ground for vermin, that Council would not maintain restoration works and it could become a rubbish dump. As there are two public schools along the creek, many people were concerned that dense vegetation would encourage anti-social behaviour. Some people were particularly concerned about the possibility of more properties being flood liable if flood levels were raised. 3.4 The Council's Concerns Fairfield City Council owns the Creek and is responsible for its maintenance. This is a huge commitment for the future and although Council has supported the project to-date, it is yet to implement Stage Three. Council will need to give a firm undertaking and financial commitment to the maintenance issue prior to giving approval for commencement of construction. Given that the project is being totally funded from grants, a major concern for Council is the risk of a budget blow out after construction has commenced. As Council will be the owner and project manager, and has only limited financial resources, it would be unable to meet any substantial cost overruns. 3

4 3.5 The Role Of The Community Artists The team s artists reacted with the community through "The Memory Line" community art project. This was an innovative idea which involved marking the original location of Clear Paddock Creek by planting a sterile crop of ryecorn grass to remind people that there was once a creek there instead of a concrete channel. Figure Planform The designed planform (see Figure 1) consists of a sequence of pool and riffle sections that mimic local natural streams. A bankfull flow generally represents an average recurrence interval of 1-2 years. Using Manning s equation, the required cross sections for the design planform were estimated. Typical sections for pools and riffles were then generated from local geomorphological characteristics, including channel widths and depths. Pool and riffle spacings for stable creeks were taken as 5-7 times the channel width. The radius of curvature of the meander bends in natural systems were adopted as times the bankfull width, with the median at 2-3 times the width. The meanders were estimated using a width to depth ratio of 10:1 with pools generally 2m deep. When commissioned, the stream will function similar to natural systems and the pools will fill with sediment to a depth so that a stable creek profile will develop without potential erosion problems. The planform slope will be nearly constant. Using data from the existing concrete drain, the static water level for the design channel approximates the invert of the concrete drain at the same chainage. This will minimise sediment problems in areas not designated for sedimentation. It will also limit the potential for exacerbating existing flooding problems. The artists worked with a number of community groups and local schools to undertake conceptually related projects that focussed on environmental themes about water and ecology and people's relationship to water. The Memory Line project culminated in the Festival of the Waters, where all the community art projects were displayed on site. For example, school children cut out and decorated cardboard shapes of creek creatures. They were then strung across the channel and displayed during the festival. By connecting the community with the past, the Memory Line Project reached a much larger audience and proved to be a very effective community consultation tool. 4. DESIGNING A NATURAL CHANNEL. 4.1 Learning From The Local Geomorphology The principle objective of Stage Two was to prepare designs and construction documentation for turning the concrete drain into a natural, functioning stream. The team inspected natural streams in the area to gauge the characteristics of each stream element. Channel sections, pools and riffles as well as riverine vegetation were all examined. A number of site constraints influenced the planform. The first being cadastral boundaries. Reserve width is very narrow in some areas (50m) which restricts sinuosity and planform options. It is critical that property boundaries are not undermined over time. Even though the planform is designed to be quite stable, the property holders should be given some guarantee. This necessitated reinforcement of some critical sections with geosynthetics. The design team considered that bridges would not be altered due to cost constraints. The designed planform conforms to each bridge section, defining planform location and channel slope. In view of the high cost of relocating sewers, the planform has been modified in some locations to avoid vertical conflicts with the sewer. Riffles have been placed at these locations to stabilise the planform and provide protection to the sewer. 4.3 Riffles And Chokes Riffles and chokes are characteristics of streams such as Clear Paddock Creek. A `riffle has shallow turbulent water flowing over it and is generally located immediately downstream of a pool (see Figure 3). A `choke is similar to a riffle in that it defines the static water level of the upstream pool and also has shallow turbulent water flowing over it. However, chokes look quite different having a very narrow width of often only 0.3 to 0.5m. 4

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6 In the natural environments, chokes and riffles are formed from root masses, stiff clays and/or fallen trees but these are difficult to replicate in design. To achieve habitat objectives and to remain stable, riffles were designed from a mixture of rock sizes up to 900mm. Within the design, riffles have been utilised when crossing the existing concrete drain, crossing sewer lines or when head drops are greater than 300mm. Generally, chokes are more characteristic of streams in the immediate area. They will be constructed of coir logs, backfilled and planted. Plant roots will grow into the coir log providing reinforcement when the log eventually decomposes (approx. 10 years). Ballast will be placed in between the coir logs on a non woven geofabric to enhance oxygenation and improve habitat. Within the planform design, chokes have been utilised when the head drop is less than 300mm. 5. FLOODING AND ITS INTERACTION WITH EARTHWORKS AND VEGETATION 5.1 Community Concerns The community has expressed concerns about flood behaviour, and possible aggravation of existing flooding problems through restoration of the creek channel and the introduction of additional vegetation. 5.2 The Impact Of The Project On Flood Behaviour The existing concrete channel efficiently conveys flood flows from one end of the system to the other. During major floods, flows for the most part are supercritical with velocities up to 6m/s. When flood flows inundate the existing bridge decks (e.g. in a 100 year event), a number of hydraulic jumps occur upstream. Associated with the supercritical flow regime and the presence of hydraulic jumps, gravity waves and very dangerous flow conditions occur. Two people have been drowned in the creek system after attempting to surf on the gravity waves. After creek restoration, flood flows will return to a subcritical regime. Velocities will be lower, approximately one half of those at present. As a consequence, flood depths will be significantly increased. 5.3 Reducing Flood Levels By Lowering Ground Levels Community concerns about flooding, and Council s own liability for flood damage, makes it necessary to ensure creek restoration design does not aggravate flooding problems. One method was to substantially lower creek and overbank areas to maintain flood levels. This option was discounted early in the design process because of costs. It was determined that by local lowering invert levels under each bridge structure, significant flood level reduction benefits could be achieved during major floods (e.g. 100 year event). Flood conditions along the length of the creek system would not be exacerbated and velocities would be much slower and safety would be improved. 5.4 Managing Vegetation Density Flood behaviour after restoration of the creek system would be strongly depend on the density and type of vegetation introduced, given the increased hydraulic roughness associated with such vegetation. The floodplain would become rougher as vegetation developed and that the roughness was also a function of the manner in which the vegetation was maintained. In order to specify appropriate vegetation densities, the flood specialist and landscape architects inspected a number of existing vegetation stands and developed appropriate vegetation density clases by using photographs and diagrams. Ongoing control of vegetation density through regular maintenance will be necessary to ensure flood behaviour is not exacerbated. Part of this process will involve monitoring future flood levels and adjusting vegetation density as it becomes established. 6. LESSONS LEARNT 6.1 Change Doesn t Happen Quickly Stream restoration in urban environments is a new concept and development of such projects challenges people s attitudes. These are shared by the general community and professionals in the public and private sectors. Such attitudinal change does not happen quickly. 6.2 New Technologies And Design Standards Required Stream restoration of the type proposed at Clear Paddock Creek is relatively new in Australia. There is an absence of available technology and design standards to guide designers in restoration. 6.3 Working As A Team The ultimate success of projects such as Restoring The Waters obviously depends on the support of local, state and federal government. To date there has been a high level of support and enthusiasm from these bodies. However the standard of the final project is strongly dependent on the quality of the individuals within the multi-disciplinary team, and the way they can work together. Strong project coordination is required and team members must be committed to the overall objectives of the project and have a sound appreciation of professional disciplines beyond their own. Most importantly, they must be prepared to challenge their own professional standards and method of working to allow the objectives of the project to be achieved. 6

7 ABSTRACT The Restoring The Waters project is located on Clear Paddock Creek within the Georges River Catchment in south-western Sydney, Australia. The project involves the removal of 2.5km of concrete-lined stormwater channel and the reinstatement of a natural creek system. The surrounding land uses are residential and there has been significant community interest and involvement in the project. Stage 2 of the project is nearing completion and involves preparation of construction plans for the restoration of the creek. These plans are being prepared by a multi-disciplinary design team. The Paper presents details of the multi-disciplinary team, the community consultation process, design issues, the impact of the design on flood behaviour, and a summary of the principal lessons learnt through the design process. KEY WORDS Stream restoration, multi-disciplinary, community consultation, urban stormwater. 7