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The requirement for a nutrient model

Stormwater runoff is a significant contributor to nutrient export into estuaries, wetlands and lakes on the Swan Coastal Plain. These water bodies are not naturally adapted to high-nutrient conditions and are prone to excessive algal growth as a result of increased nutrient runoff. In many cases, this has resulted in toxic algal blooms, fish deaths, deoxygenation and release of noxious gases and odours, and water bodies unsuitable for boating and recreation. There are major environmental issues (fish deaths, eutrophication, toxic algae), social issues (unsightly water bodies, water bodies not fit for swimming, boating or fishing) and economic issues (millions of dollars per year spent by state and local governments treating these symptoms) associated with elevated nutrients in our receiving water bodies.

It is difficult to prevent the pollution of waterways by stormwater, as the runoff is mostly from diffuse sources. Interdisciplinary catchment-wide approaches are required for successful nutrient reduction. Such approaches involve implementing many best management practices (BMPs) (both structural and non-structural), with the capacity to reduce nutrient export to receiving water bodies. To assess and quantify the effectiveness of various combinations of BMPs in an urban setting, a tool to bring all this information together is required.

Currently, no tool is available to assess the nutrient impacts of development on the Swan Coastal Plain in a consistent and scientifically rigorous manner. The urban development industry and state and local governments have recognised the need to prioritise the development of a tool that can achieve the following capabilities:

  • Assess the nutrient impacts of an urban development in a consistent manner supported by local, national and international literature.
  • Allow land development proponents to enter data about their development and assess its nutrient impacts in a logical and easy-to-use system.
  • Allow land development proponents to make decisions about various management practices that would reduce the nutrient impacts (and adhere to targets if targets are outlined for the development area).
  • Report and summarise the design, proposed nutrient management practices and the nutrient exports clearly and concisely.
  • Allow assessment by state and local governments to streamline the nutrient-reduction assessment process in a consistent and logical manner.

The development of such a tool will help provide certainty for land development proponents and assessment officers, expediency in nutrient-reduction assessment processes, greater scientific understanding relating to nutrient issues, and efficiencies in investment to manage nutrients.

The UNDO tool brings together some best practice nutrient management options for Western Australia, and can estimate the nutrient export from a development based on a selection of these practices. The tool can be used at the planning stage of urban development so a developer can make planning decisions that will reach target nutrient export rates, or it can be used in an existing urban development to try to quantify the nutrient export reduction after retrofitting the development with various management practices.

The UNDO tool will be used to support nutrient-reduction strategies outlined in local water management strategies and/or urban water management plans prepared under the Better urban water management (WAPC 2008) planning framework. This will help develop and deliver local planning scheme amendments, local structure plans and subdivision proposals. The Better urban water management framework outlines the requirement of a ‘nutrient management plan’, which involves ‘strategies to minimise the use and application of nutrients on public and private open space’. These plans address nutrient issues associated with urban development and are often required to adhere to nutrient targets. The UNDO tool should be used in conjunction with overarching water sensitive urban design approaches, such as those outlined in the Stormwater management manual for Western Australia (DoW 2004–2007) and the Decision process for stormwater management in WA: Draft for consultation (DoW 2016), to design stormwater management systems that manage stormwater quantity and quality.