Appendix A : A guide to configuring subregions in the UNDO tool
The basic building blocks of the UNDO tool are called Subregions. The sum of all subregions in an UNDO project will be equivalent to the total development area. A subregion is simply a portion of land within the development area – subregions can represent a mixture of land categories (e.g. public open space, road reserve and urban residential) or they can represent a single land category. There is no limit to the number or size of subregions that can be entered into an UNDO project.
Some recommended basic rules and guidance for dividing a development into subregions for an UNDO project:
Rule 1: Minimise the number of subregions.
A development may be represented by a single subregion but most UNDO projects will require multiple subregions. There is no limit to the number that a user can enter into the UNDO tool but a maximum of around 12 subregions is recommended. The tool becomes cumbersome using more than this: UNDO projects are simpler to handle and the computation time is fastest and reporting and assessment simpler when the smallest possible number of subregions is used. That is, the principle of parsimony is recommended when dividing an UNDO project into subregions –start with as simple a solution as possible and gradually build in complexity. The following rules provide appropriate guidance for dividing a development into subregions.
Rule 2: Divide subregions based on drainage and soil type.
Each subregion is required to have the same drainage type and soil type (including fill properties). If a development, or part of a development, has different drainage and or soil characteristics, the UNDO tool requires that the user splits the development into multiple subregions. This is very common; for example, if a development uses subsoil drains to control groundwater levels then this is the pathway for urban lots. However, if a traditional pit-and-pipe system without any infiltration systems is used to manage road runoff, the urban residential land and the road reserves will need to be split into two subregions. A detailed guide for dividing subregions based on drainage type in the UNDO tool is provided in Appendix B. Note that when splitting a development into multiple subregions based on soil and drainage type, users must always consider the 5% principle – this is explained in Rule 3.
Rule 3: Use the 5% principle.
In considering the drainage and soil type, it is impractical to be overly precise when dividing a development area into subregions. For example, a change in soil type may occur on a very small portion of a proposed subarea, or a small structural treatment in an urban development may not be controlled by subsoil drainage in the manner of the remainder of the development. While, according to Rule 2, this would require an extra subregion, building subregions with a very high level of precision can quickly lead to overly complex and cumbersome UNDO projects. For the sake of simplicity, it is recommended that a 5% principle, as stated below, is followed:
If the drainage type or soil type in a portion of a subregion is different to the remainder of the subregion but constitutes 5% or less of the subregion’s total area, this difference should be ignored, and the anomalous portion be considered to be part of the larger subregion.
This principle makes using UNDO much simpler. For example, a development may have many small biofilters used to treat impervious flow. Biofilters will generally have a different drainage and soil type to the residential or road reserve portion of the development. However, the sum of all of these biofilters may equate to only 2% of the subregion’s area. Rather than create a separate subregion to represent the portions of land used to input each of these biofilters, the 5% principle allows users to lump the biofilters with the urban residential subregion. So there will only be one subregion in the UNDO tool, rather than many to represent all biofilter areas.
Conversely, if there is public open space in the development that supports a living stream, and it is greater than 5% of the urban residential subregion, and has a different drainage typology, the user will need to split the area into two subregions: one to represent the public open space and the other to represent the urban residential land use.
Rule 4: Consider ‘catchments’ for structural treatments and treatment trains.
The UNDO tool can treat nutrients using a ‘treatment train’ approach. Examples are shown in Figure A-1.
(a)
(b)
Figure A-1: Examples of treatment trains in UNDO
The UNDO tool requires that at least one subregion completely drains to a treatment train, i.e. a single subregion can drain to a treatment train (Figure A-1a) or multiple subregions can be combined to drain to a treatment train (Figure A-1b). It is not possible for a proportion of a subregion to drain to a treatment train – if this is the case for a development area, the catchment of the treatment train must be split into an extra subregion in the UNDO tool.
It is important to note that this issue only occurs when a treatment train is entered; if a development has a series of disconnected treatments, these can be ‘embedded’ into a larger development in the UNDO tool (see Section 6-2 for more detail on how to embed treatments). Embedded treatments allow the user to enter the treated area (i.e. the catchment area for the structural treatment), with the condition that the treated area is equal to or smaller than the subregion area. Any number of disconnected embedded treatments can be added to a subarea. Figure A-2 shows an example of two embedded treatments in the UNDO tool (a biofilter and a swale), with the biofilter opened in ‘editing mode’. Then, if disconnected treatments are used in a development, the UNDO tool does not require that catchments of each of the treatments are divided into extra subregions.
Figure A-2: Example of embedded treatments in the UNDO tool. These treatments do not require that the entire subregion drains to the treatment, and the ‘treated size’ is entered by the user
Rule 5: Revisit your UNDO tool project and build in complexity if required.
Undertaking an UNDO project is generally an iterative process. As a general rule, a first pass subregion design is entered initially into an UNDO tool project, and is based on soil and drainage properties of the development (See Appendix B). However, the development will generally be assessed against targets, and if the first pass of the development does not comply with targets, further nutrient treatment will be required. This may include implementing structural treatment trains, soil amendment, or possibly even a re-design of drainage. This will require the user to revisit the assumptions associated with the subregions and, in some cases, further divide the subregions based on the above rules.