Stormwater Management for Regeneration of the Three Kings Quarry

Fletcher Residential Ltd (Fletcher) own the former Three Kings quarry in central Auckland and are turning the quarry into a large scale residential development – a place of 1200 homes. The development is directed by a comprehensive masterplan which includes recognising the quarrying heritage, the former pit filled by about 70%, cultural recognition, sports fields and a range of different housing typologies.

The Three Kings quarry sits within a unique environment. The geology is complex, with a 1km diameter surrounding tuff ring (with a breach to the north), a series of scoria cones and basalt in dykes and lakes. Much of the geology is disturbed, interbedded and overlain by subsequent eruptions. The quarry sits at the top of the Meola Catchment with little separated stormwater infrastructure and only an overloaded combined sewer nearby. The planned development retains part of the former quarry pit, which means the development floor sits 15 to 17m below adjacent land with no way of providing piped gravity drainage or overland flow paths. And finally, groundwater pumping is to cease and the groundwater levels will return to the pre-pumping levels of RL 56.5m (about 21-23m below Mt Eden Rd).

Fletcher therefore faced the unique challenge of how to develop the land comprehensively, drain the development via soakage into the scoria and fractured basalt with groundwater close to the surface and manage long term flood risk. In short, groundwater provides a lower bound constraint: floor levels and freeboard provide an upper bound constraint, and the stormwater must be carefully directed to soakage in between these levels.

As well as the global upper and lower bound level constraints, development of the stormwater management concept had to deal with a range of design questions. The upper and lower bounds meant that the solution involved storage (above and below ground) and the soakage of stormwater at multiple sites. Design challenges included; managing flood risk, confirming aquifer responses to rainfall, assessing local groundwater mounding, how to keep sediment out of the soakage during construction and long term, the factor of safety to be adopted in soakage systems, identifying the variation in rock permeability, sports fields drainage and operation and the porosity of rock to be used in underground storage facilities. These constraints however provide a unique opportunity for the adoption of soakage at a large scale and provide an excellent example of implementing water sensitive design.

The stormwater management concept has been through a Plan Change process and has recently been updated as a result of an Environment Court mediation agreement and the Unitary Plan Three Kings precinct provisions.

This paper sets out the development of the overall stormwater management approach and how the challenges were addressed.

A companion paper to this paper by Messrs Strayton, Namjou and Matheson of PDP presented to this conference describes the hydrogeological assessments, groundwater effects and soakage investigations and construction in more detail.