Transformation of an urban lake at Mt Annan, NSW

Transcription

1 Transformation of an urban lake at Mt Annan, NSW Mal Brown*, Dov Ben Avraham*, Sean Harris^, Sandra Kubecka # *Northrop Consulting Engineers, Sydney ^Harris Environmental/Naturaft, Jamberoo, NSW # Camden Council, NSW Key points: Urban lake restoration Green infrastructure to improve community amenity Resilience in pollutant assimilation Abstract Like many Councils in Australia, Camden Council strives to maintain green assets in its LGA. This is challenging in the case of Lake Annan which experiences severe algal blooms with associated odour and visual amenity decline. The community voiced their concerns about the Lake s amenity over the last decade. They wanted a picturesque and robust ecosystem as the backdrop for their views and community recreation activities. In response, Council assessed a wide range of options to transform Lake Annan. The key was to create a resilient and self sustaining system. A key constraint was the presence of an existing wetsump GPT the removal of which would have had a huge cost impact. The resulting system adopted re purposed the GPT as an attractive, landscaped inlet. Floating Treatment Wetlands at the Lake s inlet zone provide rich habitat and ongoing pollutant removal. The transformation of the Lake is stunning and very popular with the local community. Key words: Urban lake, Green infrastructure, Asset repurposing, Floating Treatment Wetlands Introduction Lake Annan was constructed in the 1990s as a centrepiece of a residential subdivision. Its main inflow stems from a 270Ha catchment which has continued to become developed since that time. As a result, the catchment is now contributing large quantities of stormwater pollution which the lake was never designed to assimilate. This leads to significant water quality issues (e.g. algal blooms) resulting in amenity issues for the local community. An on line, concrete, wet sump Gross Pollutant Trap with trash rack was constructed on the major inlet. This device was ugly, odorous, and difficult to clean out. Consequently a large amount of pollution was being flushed from this device directly into the lake. In addition, the lake s inlet channel and island banks were eroding and unsightly. The solution Camden Council investigated a wide range of options to optimise water quality and to improve amenity in and around the lake. The final design selected by Council in consultation with the community struck a balance of amenity and habitat improvement, water quality treatment, and available funds. The result is a stunning transformation of the lake inlet that will also serve to improve lake water quality over the long term. The key elements of the inlet transformation are listed and described as follows:

2 A new Gross Pollutant Trap a large CDS Retention and re purposing of the existing concrete GPT landscaped to enhance its amenity and appearance as the GPT would no longer perform any pollution reduction function A stabilised inflow channel to the lake use of rock armoured low flow channel with grassed high flow overbanks Floating Treatment Wetlands placed along the flow path in the lake, adjacent to the inlet NaturaftTM FTWs. The NaturaftTM FTW s have special bird protection systems to keep the existing Ibis and Swamp Hen populations at bay. Stabilisation of eroding island banks using NaturaftTM Streambank EdgeTM The result The inlet works were completed in late 2016 and they have been subjected to very high inflows over the wet March April period of They are in perfect condition with all areas stable, plants growing and healthy, and otherwise being enjoyed by the local community. Figure 1: Aerial view of Lake Annan, before and after

3 Figure 2: Revitalised former GPT at inlet Figure 3: Inlet showing low flow rock channel and high flow grassed waterway

4 Figure 4: NaturaftTM Floating Treatment Wetlands (Note: bird protection system) Figure 5: NaturaftTM Streambank EdgeTM stabilising the island banks Conclusion This project represents an excellent case study for the restoration of urban lakes in Australia. Existing stormwater assets can be re purposed at low cost. Floating Treatment Wetlands and Streambank Edge represent excellent and innovative products for transforming urban lakes.

5 Presenter: Dov Ben-Avraham Title: Mr Position: Group Manager, Senior Engineer Organisation: Northrop Consulting Engineers Ph: e mail: dov.benavraham@sydney.northrop.com.au Title: provided above abstract Stream/s: The role of stormwater assets in the creation of liveable and resilient cities