Skip to main content

What the UNDO tool will not do

It is important for users to understand the limitations of any model. Below is a list of exclusions. The UNDO tool:

  • does not set targets: While it is recognised that target setting exercises may be crucial in the land-use planning framework, target setting needs to be a process that operates independently of the development of the nutrient decision support tool. If nutrient targets have been set for a catchment, criteria for the UNDO tool can be set whereby the exports calculated by the tool are required to meet target loads.

  • is not a detailed design tool:The UNDO tool provides first-pass estimates for treatment device sizing for nutrient treatment, but the tool does not include hydraulic and rainfall-runoff processes that can be used to accurately design nutrient-reducing structures. It cannot be used to size conveyance, storage or other components of the stormwater management system or to undertake the modelling of any rainfall-runoff processes. The UNDO tool does not include all structural and non-structural BMPs available to treat nutrients or manage runoff quantity. Therefore, 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.

  • is strictly to assess nutrients:The tool does not assess other environmental or social factors which may be important to maintain in an urban setting (e.g. biological function, habitat diversity, ecological corridors, urban heat reduction, aesthetics). It does not assess other contaminants that may be relevant in an urban setting (e.g. heavy metals, pesticides/herbicides, hydrocarbons or coliforms). The UNDO tool does not address managing runoff quantity to achieve flood protection, prevent erosion, maintain the hydrologic regimes required for healthy waterways or achieve other quantity management objectives. The tool does not include managing stormwater as a potential water supply source or as part of a total water cycle management system.

  • provides a steady-state solution: Therefore it does not account for time-dependent processes including:

    • excessive nutrient discharge due to construction (e.g. groundwater de-watering discharge from sewer installation or sedimentation from earthworks and building construction)
    • catchment hydrological changes that occur through time (e.g. reduction in runoff due to tree growth)
    • changes in nutrient export as a result of climate change.

  • does not provide sub-annual solutions: Daily, seasonal or monthly breakdowns of nutrient exports are not available from the tool.

  • will not always provide accurate answers: UNDO is a conceptual tool so it is not expected to provide precise answers and some level of error is expected. It is recognised that many subtle variations will affect the export of nutrients (e.g. variations in soil type, soil redox potential, soil biology, vegetation type, surface topography). It is important to find a balance between a tool that is overly simple and lacks adequate precision with a tool that is overly complex and therefore too onerous to use. There is a constant drive in nutrient modelling to push towards a more complex solution but the UNDO tool was designed with usability as a topmost priority, and a simple conceptual model is the most pragmatic tool for the required use in land-use planning.

  • does not consider pre-development nutrients: The UNDO tool evaluates long-term planning and design for nutrient removal and so does not specifically consider pre-development nutrients (also known as ‘legacy nutrients’) which will generally be flushed from a subsoil system within 2–3 years on the Swan Coastal Plain (Barron et al. 2013). The issue with elevated pre-development nutrients should be considered before applying the UNDO tool, and elevated nutrient concentrations in groundwater may need to be managed concurrently.

  • does not replace existing Department of Water policy and procedure: Existing Department of Water policies and recommendations regarding best practice water sensitive urban design for urban developments are not compromised by using the UNDO tool in the urban planning process. Using the UNDO tool does not negate any existing procedures.