A number of studies compare construction costs of WSD against conventional approaches to stormwater management.
These show that WSD capital costs are broadly equivalent to conventional approaches and can be markedly less in greenfield sites (Environmental Protection Authority Victoria, 2008; Shaver, 2010a; Lloyd et al., 2004).
There is a perception in the development industry that WSD requires more input from designers with an associated increase in up-front design costs. There is also a perception that obtaining regulatory approval can take longer, leading to longer timeframes before development rights can be secured, potentially affecting loan repayments or timeframes for due diligence.
From a medium to long-term perspective, the up-front time required to achieve a consensus within a design team reduces design iterations going forward, avoids 'fatal' design flaws, and potentially saves time and money. Consent processing timeframes will become more efficient as WSD aligns more closely with the evolving policies and objectives of the PAUP.
Generally the potential up-front costs associated with WSD tend to favour land development professionals with a longer term outlook and an appropriate level of financial equity. Alternatively a developer can approach the Council early in the design process to seek opportunities for collaboration if any opportunities exist for public-private partnerships.
In the instance of a greenfield development, a WSD approach minimises site disturbance, reducing the extent of bulk earthworks, stockpiling and disposal. A smaller construction footprint lowers the risks associated with an 'open site' and also reduces the need for sediment and erosion control measures and associated costs.
The majority of potential cost savings for WSD is through reduced infrastructure demand (Wise et al., 2010). A compact development form promoted by WSD requires less infrastructure for stormwater, roading, etc. Potential savings have been shown between 10-25% for clustered versus conventional development in the US (Conservation Research Institute [CRI], 2005).
Specific WSD approaches can further reduce infrastructure requirements. While pervious pavement can be more expensive than other surfaces, depending on the product, pervious surfaces can lower total development costs by as much as 30% through reducing stormwater runoff volumes (CRI, 2005).
There is also a potential direct cost saving in using WSD approaches. Swales and raingardens are generally less expensive than piped infrastructure, especially if the number of catchpits is reduced. The cost differential may be as much as 80% for swales and 30% for raingardens (Lloyd et al., 2002). Comparison costs are available through COSTnz, developed by Landcare Research, which compares the acquisition, maintenance, and decommission costs of stormwater management practices based on New Zealand data (Ira et al., 2008).
Operation and maintenance costs
WSD generally results in a reduced level of infrastructure in a catchment, with corresponding savings in operational budgets. It is important to note that maintenance frequency and responsibilities need to be considered. Devices on private land mitigating a single dwelling should be maintained by the property owner. Where devices on private land service multiple properties, a body corporate should be set up to carry out inspections and maintenance. If the device is located in land to be vested in Council, an operation and maintenance plan should be provided and costed to allow Council to plan for maintenance of the vested infrastructure. This arrangement must be approved in advance by Council.
WSD maintenance primarily involves the management of landscapes and key operational areas such as outlets. WSD devices such as raingardens and swales are relatively inexpensive to maintain, generally costing 3-4% of the total acquisition cost (Shaver, 2010a). Mowing costs can also be eliminated by planting native grasses in swales. Raingarden maintenance is estimated at 6-8% of the total acquisition cost, primarily to allow for replenishment of soil media every 20-25 years (Shaver, 2010a). However, contaminants in stormwater are generally captured in the upper 200 mm of the raingarden media and management of plants and mulch may be adequate to maintain these systems (Trowsdale & Simcock, 2008). Where raingardens have pre-treatment to capture gross sediment, the finer sediments that enter the raingarden can be assimilated and the system becomes self sustaining. In this instance, where WSD practices are properly designed and installed as part of a treatment train, they can perform for a much longer timeframe than conventional systems.
This is also the case for living roofs and pervious pavement. They can be installed with operability in mind, and require minimal maintenance. Living roofs in particular not only require minimal maintenance once established, they can also protect the roof membrane from the elements, extending the lifespan of the roof itself.