Improving water quality in the Derwent Estuary

Situated at the heart of the Hobart metropolitan area, the Derwent estuary is a waterway of great natural beauty and an important and productive ecosystem supporting a wide range of habitats and species.

An integral part of Tasmania’s cultural, economic and natural heritage, the Derwent estuary has some 200,000 people - 40% of Tasmania’s population - living on its margins. The Derwent River supplies the majority of the region’s drinking water and is a major source of hydroelectric power.

While there have been significant improvements in the treatment of sewage and industrial wastes over the past decade, the Derwent still faces a number of environmental challenges, in particular:

• Heavy metal contamination
• Introduced marine pests
• Loss of estuarine habitat and species
• Intermittent faecal contamination of recreational waters
• Depressed oxygen levels and organically enriched sediments
• Elevated nutrient concentrations
• Environmental flows and barriers

All parties recognise that a strategic and coordinated planning approach across all levels of government, industry and the community is the best hope for restoring the estuary to health.

To address the issues, the Derwent Estuary Program (DEP) in Tasmania was established in 1999 to restore and promote the Derwent Estuary. The partnership between state government, six local councils, industry and the community employs six people with scientific and technical backgrounds to work with the DEP.

Stormwater is delivered to the Derwent estuary via 13 major rivulets and over 270 pipes. The State of the Derwent Report Card (2007) identified stormwater run-off as the main source of pollution to the Derwent estuary. Local councils surrounding the Derwent estuary, in cooperation with the Derwent Estuary Program, have monitored stormwater quality. The program has a dedicated stormwater project aimed at improving water quality by developing and implementing resources and programs for local government, business and industry, schools and the community and has the following initiatives:

• Encouraging water sensitive urban design (WSUD)
• Soil and Water Management for building and construction sites
• Regional stormwater resources for local government
• Community- and school-based education on how to prevent stormwater pollution

The DEP has funding from the Australian Government’s Caring for our Country Community Coastcare grants program for a project titled Improving Biodiversity and Water Quality in the Derwent Estuary. One of the project’s objectives was to increase the application of WSUD through the implementation and promotion of WSUD at five high-profile sites around the Derwent estuary working in collaboration with industries, roads and transport and education partners, including:

• University of Tasmania
• Southern Water
• Cadbury Schweppes
• Centro Shopping Centre
• Department Energy, Infrastructure & Roads/Lyell Highway

A variety of WSUD projects will be scoped and designed at these sites, focusing primarily on car park and road run-off. A consortium of project proponents and WSUD specialists will be assembled to develop, share and promote these projects.

The overall objectives are to achieve water-quality improvements, broaden interest and awareness of WSUD, enhance regional expertise and implement projects at high-visibility sites.

Steep challenges

The first WSUD project, completed in April 2011, is a large bioretention system at University of Tasmania (UTAS) at Sandy Bay in Hobart.
Two engineering consulting firms, Urban Initiatives and Aquatic Systems Management, applied eWater’s music (model for urban stormwater improvement conceptualisation) to develop the WSUD.

The high-profile site, located next to the student facilities, experiences high traffic with a lot of car movements. The drainage grade is very steep and is unsuited to standard vegetated swales systems. Moreover, the kerb must be maintained.

To further complicate matters, a rivulet passes under the site through a 1200 mm diameter pipe; and given that the upstream catchment is large, overland flow is an important issue.

The pavement area around the buildings - including the forecourt of the Student Café - drains to the proposed WSUD treatment area. While anthropogenic litter isn’t much of an issue, leaf litter is and requires consideration. The entry to the drainage chamber upstream had three small restricted openings which needed to be increased with the cut in of a raised grill structure. The road geometry, near Churchill Ave, was not conducive to WSUD at the lowest point of the car park and design consideration was required for handling the insertion of WSUD.

The consultants recommended an alternate cobbled swale be established behind the kerb with a number of leaf litter trap points with sedge bands. This cobble channel could then discharge along with the final cut from the kerb to an elevated bioremediation filter cell built within a rock or crib wall retaining structure. Under this design, discharge goes to an existing drainage pit.

To establish an effective WSUD retrofit to service the area, the consultants proposed provision of two broad planted swales along the car park to reduce leaf litter from the surrounding vegetation and the insertion of three kerb drain entries to divert the run-off into the treatment systems. They also recommended provision of cobbled swales with sedged margins between the elements.

The design allows the student and wider community will see cobbled drainage lines and broad planted swales while being able to view the planted bioretention filter area. An education display board explains how the system works. This system will complement an existing WSUD design in an adjacent university site. This new project will enable a number of devices and treatments to be viewed in one precinct.

“There were some challenges with the project at UTAS,” says Stormwater Management Officer John Chrispijn, who provides advice on WSUD projects that are incorporated in developments around the estuary.

“The planned WSUD project was to capture the stormwater run-off from the car park on the campus. The university car park had to be in use whilst constructing the WSUD. The site and car park is also at a steep location. It was important to get accurate details of existing infrastructure at the university so nothing was disrupted.

“We looked to achieve the stormwater pollutants reduction targets of 80% total suspended solids (TSS) and 45% for nitrogen (TN) and phosphorus (TP)” Chrispijn says. “When modelling music for the site, the steepness had to be considered. The optimal WSUD includes cobbled stone drainage lines and broad planted swales.”

Engineers and landscape architects worked together on the project. University of Tasmania implemented the WSUD and provided funding.

“With the help of music, targets were achieved and beyond, 91% TSS, 64% TP and 49% TN,” Chrispijn says.

He points out that people think the stormwater bioretention system looks like a landscape feature rather than WSUD.

“music gives quality reassurance and confidence that targets can be met with different design options. music let us look at different water sensitive urban designs and see what best suited this particular site and conditions,” Chrispijn says. “By applying music, organisations can demonstrate that stormwater pollutant reduction targets can be met in their WSUD.”

“The WSUD project at UTAS has been a successful project. We had a good chance to do background research of the site before applying for funding and investigate the site for the viability of stormwater treatment. We could make an informed decision of the optimal WSUD,” Chrispijn says.

Award winner

In November, the project won the ‘Tasmania WSUD’ award from the Australian Institute of Landscape Architects.

An initiative of the Derwent Estuary Program, the pilot project involved work to encourage the adoption of WSUD techniques to improve the overall health and water quality of the estuary ecosystem.

A number of sites from different locations around the Derwent estuary were selected and analysed for their suitability for retrofitting and establishment of WSUD initiatives. Comparative modelling of alternative design proposals for each site was carried out to determine their relative potential to contribute to improved stormwater quality outcomes, with design interventions ranked for future construction according to their assessed level of impact and achievability within that particular context.

While relatively modest in terms of overall size and budget, the jury commended the project for its analytical approach and design methodology, which provides valuable insights and support for broader-scale application and which highlights the value of evidence-based landscape design processes for building community understanding and support for WSUD initiatives.