Wetlands are permanently or intermittently wet areas and land-water margins that support a natural ecosystem of plants and animals that are adapted to wet conditions (RMA, 1991, s2.1).
Wetlands provide significant detention of stormwater and very efficient removal of nutrients, hydrocarbons, metals and sediment. Engineered treatment wetlands in Auckland are normally associated with a larger catchment size. However, multiple smaller wetlands are also an option where spring seepages or seasonal high water tables can support wetland vegetation.
One of WSD's principles relates to the treatment of stormwater as close to source as possible. However, where there are large floodplain areas which have low functional values and are unsuitable for development, there may be value in rehabilitating wetlands in these environments for engineered stormwater detention and treatment. This circumstance assumes treatment of gross contaminants prior to stormwater entering wetland environments.