Costs and Benefits of On-Site Stormwater Devices

The Auckland Plan and the Proposed Auckland Unitary Plan (PAUP) place significant emphasis on green growth and water sensitive design. Freshwater policies seek a water sensitive design approach for new developments such as Special Housing Areas (SHAs) and for comprehensive brownfields development. Additionally, stormwater discharge, flow and quality rules in the PAUP apply to both new development and redevelopment in Auckland.

This paper summarises the Auckland Council Technical Publication TR2013/043, which compares the life cycle costs of a range of stormwater management devices that can be used to meet controlled activity requirements in the PAUP stormwater management flow and quality rules. This includes the reduction of runoff volumes through retention of small, frequent events in SMAF areas and the treatment of high-contaminant generating areas at source. The flow requirements in particular cannot be met using solely catchment-scale management devices such as wetlands. This, along with policy supporting a Water Sensitive Design (WSD) approach shifts the focus from end-of-pipe treatment to on-site management of stormwater quality and quantity (for flows less than the 2 year ARI).

This paper compares the cost implications of traditional wetland treatment with on-site stormwater management for different development scenarios. Construction, average annualised maintenance and total present cost of the various options have been presented to inform decision making and transparency of costs in implementing the stormwater provisions.

Since limitations associated with quantifying non-market benefits make it difficult to conduct a solely quantitative cost-benefit analysis, TR2013/043 does not attempt to provide Auckland specific monetary values for the wider benefits provided by stormwater management options. The benefits are broadly assessed in terms of detention, retention and water quality outcomes. Case studies have been presented in the publication to show examples of cost-benefit analysis that have been undertaken locally and internationally.