Large areas of the Auckland region are underlain by Waitemata complex soils in which fine clays are dominant. Many of these soils are highly vulnerable to degradation (compaction and loss of soil structure), difficult to rehabilitate over large areas, and take many years to recover naturally.
These fine clays are susceptible to erosion and can be carried in stormwater runoff until they reach estuarine and wetland environments, where deposition of sediment frequently occurs.
Small pockets (generally <1 ha) of soil with higher permeability and moisture storage potential can be found within the areas of fine clays. These pockets are primarily found on gentle slopes, broad ridges and terraces. Much of the Auckland region is underlain by granular soils and basalt rock, which can also have good infiltration properties.
The key to managing soil resources for stormwater management in Auckland is to:
- Identify soil properties and their extents through soil surveys
- Limit the development footprint and extent of soil disturbance and erosion where practical
- Provide for a development layout that considers vulnerable soils, steep slopes, overland flow paths, riparian margins, spring seepages and aquifer recharge
- Minimise the degradation and compaction of site soils, and apply appropriate sediment and erosion controls during construction to reduce sediment runoff
- Remediate soils where practical to optimise their beneficial properties
- Consider alternative land development approaches, other than the traditional 'cut-to-fill' operations, for site levelling.
Management of healthy soils for stormwater management is discussed in more detail in the Auckland Regional Council Technical Report TR2009/083 Landscape and Ecology Values within Stormwater Management (Lewis et al., 2010). The Auckland Regional Council Technical Publication TP90 Erosion and Sediment Control Guidelines for Land Disturbing Activities in the Auckland Region (1999) (which will be replaced by the Auckland Council Guideline Document GD2016/005) also sets out principles