The Methodology for Developing Stormwater Management Areas for Flow Control for the Auckland Unitary Plan

In preparing input into the Auckland’s Unitary Plan it became clear early on that to better protect good quality urban streams in the Auckland region from the effects of impervious development there needed to be a set of policies and rules to manage stormwater flows from both infill and new development areas. Traditional rules have proven to be inadequate to protect the quality of these streams. Increasing evidence is showing that to maintain stream health you have to reduce both volumes and peak flows from those frequent small events up to the 1 in 2yr recurrence interval. The paper describes the development of a set of Unitary Plan overlay maps defining specific Stormwater Management Areas for Flow control (SMAF1 and SMAF2 areas) which require stormwater management for stream health. Due to the need to cover all the urban streams in the Auckland region, the challenge was to have a process that could use the efficiencies of Council’s GIS mapping data base while including catchment specific characteristics. The development of the overlay maps included an initial GIS mapping of three ‘primary criteria’ (stream slope, cumulative impervious and Macroinvertebrate Community Index), followed up by a set of workshops with catchment planning and freshwater council staff and further catchment specific ‘moderating factors’. The paper explains the process and gives examples of how to meet these stream health controls and indicative costs. This new approach is building on similar approaches in recent plan changes in the Auckland Region that have gone through very extensive technical work and legal challenges and are now operative.