TECHNICAL ISSUES OF DECISION MAKING FRAMEWORK

The screening described above is integrated into the technical issues of each step of the decision making framework (Figure 4.1). The decision making framework is divided into technical issues and additional issues. The focus of this study was on the technical issues. However, resources and information related to the additional issues are also briefly provided in Section 4.5. The technical issue which were briefly introduced in Section 3.4 are described in detail in Sections 4.4.1 to 4.4.3.

4.4.1 COLLECTION AND END USE

Steps 1 to 6 of the decision making framework, as shown in Figure 4.1, are related to the components and associated issues of collection and end use. Issues include quality, quantity, general study site conditions and collection options. Issues of quality and quantity are for both collected water and end use demand.

4.4.1.1 GENERAL STUDY SITE CONDITIONS

Step 1 in the decision making framework is to determine the type(s) and size of the area to be examined. This step relates to determining general study site conditions. The general study site conditions provide an understanding of the area that is being examined. This step compiles all of the information relating to the area size, ground conditions and zonal type.

In order to determine general study site conditions, the location of the study site needs to be identified. It is assumed for this project that the site is known. A specific neighbourhood may have been chosen, or a total catchment area may be examined. There may be some simplistic ideas behind why a certain site was selected, such as requirements to reduce stormwater runoff from an area or a desire to reduce potable water supply use in a certain area. However, there are also a number of complex issues that are beyond the scope of this research project. These include governance boundaries crossing a catchment area, political issues influencing what type of site would be selected, public pressure, reasoning behind targeting specific householders, as well as geographical limitations constraining the type of area that can be selected. These larger scale issues of site selection either need to be resolved prior to the planning stage or would be resolved in conjunction with utilising the decision making framework in the planning stage.

Identifying the study site consists of determining both the stormwater collection area as well as the proposed end use area. These areas can be the same area, have common overlapping areas or be two completely separate areas. If the areas are separate, the different areas may be closely located or some distance apart. These factors impact on eventual cost and ease of implementation of the scheme.

The general aim when trying to identify end use and collection areas is to examine the water demands that exist within the study site, and the areas which have the largest capture potential. If the focus of the project is to capture as much stormwater as possible, then large catchment areas should be examined. These should be matched to areas which either have fairly constant large water demands or end use patterns similar to the rainfall and potential runoff patterns.

If the focus of the project is to meet specific end use demands, then the end use areas are already known. A catchment area should be chosen as close to the end use area as possible to minimise distribution costs while obtaining sufficient stormwater runoff and yield.

Once the study site is identified, the size of the collection and end use area(s) needs to be determined. This is done by first obtaining plans or aerial maps of the area(s) showing the layouts of housing, road and grassed areas. The size of the area is then measured and scaled off from these plans based on the map or plan scale.

The zonal types of the catchment and end use areas are then determined. This is necessary because of the different infrastructure requirements and issues that are appropriate for the different zonal types. This project identified three different zonal types, as follows:

• Greenfield, redevelopment and/or infill area;

• Existing urban area; or

• Combination of greenfield, redevelopment, infill and existing urban area.

The above zonal types are distinguished by the stage of the housing development and state of the existing infrastructure. The importance of the zonal type, in terms of the decision making framework, is that limitations and restrictions are placed on the study site. This is of particular importance in terms of where drainage infrastructure can be constructed for a stormwater use scheme.

Greenfield, redevelopment and infill zonal types either do not have any existing residential development or otherwise the housing development was removed and would be reconstructed. A greenfield area is a new housing development area which was previously not zoned as a residential area. Zoning would be specific to individual planning schemes. Examples of non-residential zonings are Business, Industrial, Public Use, Public Park and Recreation, Special Use or Mixed Use Zones (obtained from the Victorian Planning Schemes website http://www.dse.vic.gov.au/planningschemes). An infill area is similar to a greenfield area, except that the infill area is surrounded by

existing urban areas. A redevelopment area includes areas where the previous residential area has been demolished and a new housing development was planned. The importance of the greenfield, redevelopment and infill zonal type in terms of the decision making framework is the location of existing infrastructure such as roads and utilities. In these zones, appropriate infrastructure is either not yet constructed within the development site, or any pre-existing infrastructure needs to be demolished and rebuilt to fit the new development layout. Greenfield sites may also not have any infrastructure surrounding the area. The cost of the new development can include construction of large infrastructure systems to connect to the existing systems, either nearby or far away. The distance between the new development site and the existing infrastructure may provide additional motivation for implementing new development ideas. Alternative water supply sources managed closer to the development site may minimise infrastructure requirements. Redevelopment or infill areas may have existing infrastructure on or near the boundary of the development site. This may limit the new development layout possibilities for the infrastructure, but generally the layout within the development is more flexible than within an existing urban area.

An existing urban area is any area with housing developments, fields, parklands and general developments already constructed. An existing urban area is also any area that does not fit within the previous zonal types. The infrastructure is already in place within an existing urban area. However, some older existing urban areas may have minimal or no drainage infrastructure.

The combination of greenfield, redevelopment, infill and existing urban areas is as the name suggests, a combination of the above two zonal types. The challenge with this zonal type is that some areas, such as the existing urban areas, have greater limitations and restrictions on how a stormwater use scheme can be implemented.

The type of feasible end use layout options for an area of existing urban or combined zonal types is limited due to initial screening that was conducted. Initial screening tools described in Section 3.3.2 stated that the options which separate roofwater and stormwater (including roofwater overflow) would not be examined within the existing urban or combined zonal type. The reason behind this was that it was assumed that the

cost involved in converting all of the downpipes and housing drainage systems within an existing urban area would be too difficult and expensive to justify having separate collection systems.

If the decision maker (council employee, land developer or engineer) would like to examine separate collection systems as part of their study and the study area is a combined zonal type, the different zonal types need to be separated. The study could then examine separate roofwater and stormwater collection in any greenfield, redevelopment or infill areas. Any existing urban area in the study would not include examination of separate roofwater and stormwater use scheme options.

Once the zonal type of the study site is identified, the total collection and end use area(s) is divided into different sub-areas, namely residential, open space, industrial and commercial areas. The impervious and pervious areas within the total collection area are also identified and measured. Impervious areas are considered as roofs and pavements within residential, industrial and commercial areas. Pervious areas are any grass or vegetated areas. The division of areas and area measurements are used to identify different runoff conditions and demand patterns. These values are also inputted into modelling tools such as UVQ (Mitchell et al., 2003) and Aquacycle (Mitchell et al., 2001) to determine stormwater yield and demand. The use of this sub-area data in modelling tools is explained further in Section 4.4.2.2.

Residential areas are further divided into areas of similar characteristics. If these areas are only used for stormwater collection, then the similar characteristics are dependent on runoff characteristics. This includes areas with similar impervious and pervious area sizes including roof, pavement and garden areas. If these areas are also end use areas, then the characteristics include similar water demand patterns. If there is a wide variety in the block sizes, the residential area is grouped into separate sub-areas of similar block sizes. Within each residential sub-area, the average block size is estimated. Average block information is inputted into Table 4.1. If block sizes and characteristics are fairly similar, the entire residential area is represented by a single typical residential block.

Table 4.1 Collection Area Residential Block Data Total number of blocks Average block size (m2₎ Average roof