Stormwater detention tanks in Auckland: when you need one and what goes wrong
Twenty years ago most Auckland sites piped stormwater to the street and that was the end of it. That approach is largely finished. On a great many development sites today the water has to be held back and released slowly, and the system that does it is a significant line in the civil budget.
This page explains what detention is for, what typically triggers the requirement, and where installations go wrong.
What detention actually does
A detention system does not dispose of stormwater. It delays it. Runoff enters the tank or chamber during a storm, is held, and is released downstream through a restricted outlet at a controlled rate. The volume leaving the site is much the same. The peak rate is not.
The reason is the network. Auckland stormwater pipes were sized for the catchment as it was, and every roof, driveway and paved area added since sends water to them faster than the ground it replaced. Detention is how a site is allowed to intensify without pushing its share of the peak onto everyone downstream.
Detention is not about how much water leaves your site. It is about how fast. That distinction explains almost every design decision in the system.
What triggers the requirement
The requirement comes through the consenting process rather than from a rule of thumb, and it varies by catchment. In practice the triggers tend to be increases in impervious area, subdivision, and location within a catchment where the network or receiving environment is already under pressure.
The practical point for anyone developing: the size and type of system is determined by the design engineer against council requirements for that specific site. It is not something to estimate from what a neighbour installed, and it is not a line to guess at when pricing a job.
Tanks, chambers and the space they take
Detention takes several physical forms, and the choice is usually driven by available space and ground conditions:
Above-ground tanks, sometimes combined with rainwater reuse, which are cheapest to install and hardest to hide
Buried tanks, which free the surface but bring excavation, buoyancy and access considerations
Modular chamber and crate systems wrapped in geotextile, which spread storage across a shallower footprint and suit sites where depth is limited
Oversized pipe storage, where the detention volume is held in large-diameter pipe rather than a discrete structure
On constrained urban sites the modular chamber approach is common because it can sit under a driveway or yard. That is also where the installation risk concentrates, because it has to carry whatever drives over it.
Where installations go wrong
Buoyancy. An empty buried tank in wet ground wants to float. Anchoring and backfill sequence are not optional details.
Loading. A system under a driveway has to be rated for the traffic and bedded to suit. Getting this wrong is discovered slowly and expensively.
The outlet. The restricted outlet is the part that performs the function, and it is small. If it is not accessible for maintenance it will eventually block and the system stops working.
Levels. Detention only works if water gets in and out by gravity as designed. On a flat site there is very little margin.
Sequencing. These structures are large and go in early. Building the rest of the site around them without damaging or displacing them takes planning.
Detention almost always interacts with the rest of the site water design, including any overland flow path and whatever soakage the ground allows.
Who installs it
Detention systems sit in the civil package rather than with the plumber. The work involves bulk excavation, structural bedding, large-unit handling and connection into the site drainage, and on a subdivision it forms part of the wider reticulation covered in subdivision drainage.
Questions we get asked
Do I need a detention tank?
That is determined through the consent process for your specific site and catchment, and designed by your engineer. It commonly applies where impervious area is increasing or a site is being subdivided.
How big does it need to be?
Sized by the design engineer against council requirements for the catchment. Volumes vary widely between sites, which is why estimating from another property is unreliable.
Can it go under the driveway?
Frequently yes, using modular chamber systems, provided the system is rated and bedded for the loading it will carry.
Does a detention tank stop my property flooding?
It is designed to protect the downstream network from peak flows, not to solve on-site flooding. Site flooding is usually a levels and overland flow question.
Does it need maintenance?
Yes. The restricted outlet is small by design and needs to stay clear, which means it needs to remain accessible.
This page is general in nature. Detention requirements are set through the consent process for each site. Confirm with Auckland Council and your design engineer.
Draeinail Construction has been putting drainage in the ground across Auckland and the Franklin district since 2006. Request a quote or call Darryn Muir on 021 271 0035.
