Soakage and on-site stormwater disposal in Auckland
Not every Auckland property drains to a public stormwater pipe. On many sites the water is meant to go into the ground instead, and Auckland Council publishes a homeowner guide describing ground soakage as a property hidden helper. It is a good description when the ground cooperates. When it does not, soakage becomes the most disappointing item on a drainage plan.
What soakage is
Soakage disposes of stormwater by letting it infiltrate the ground rather than piping it away. In practice that means a soak pit, soakage trench or infiltration system sized to take roof and paved-area runoff and let it disperse into the surrounding soil.
Where it works, it is the cleanest answer available. There is no connection to negotiate, no downstream network to protect, and the water returns to the ground close to where it fell.
Auckland ground is not one thing
This is the whole issue. Auckland sits on wildly varied ground, and infiltration performance varies with it. Free-draining volcanic and sandy soils can take water readily. Heavy clay, which covers a great deal of the region including much of the south, drains slowly and can be close to useless for soakage without engineering.
Two neighbouring properties can behave very differently, which is why soakage is never assumed. It is tested.
A soak pit in the wrong ground is not a drainage system. It is a hole that fills up and stays full.
Soakage testing comes first
A soakage or infiltration test establishes how quickly the ground actually accepts water at the depth the system will sit. That figure drives the sizing of the system, and sometimes it establishes that soakage is not viable at all, which is equally useful information before rather than after the design is committed.
Groundwater matters as much as soil type. A system installed into ground that is saturated for part of the year has nowhere to discharge when it is needed most.
When soakage is not the answer
Heavy clay with very low infiltration rates
High or seasonally high groundwater
Sloping ground where discharged water may resurface downhill or affect a neighbouring property
Close proximity to foundations, retaining walls or unstable ground
Contaminated ground where infiltration is undesirable
Sites where the required system is larger than the available space
Where soakage cannot carry the load, the answer is usually a piped connection, a detention system, or a combination. That decision belongs at design stage with the engineer.
How it fits with everything else
Soakage rarely stands alone. It interacts with any overland flow path across the site, with any detention requirement, and with subsoil drainage where the ground is holding water. On wet sections the underlying problem is often groundwater rather than surface water, which is covered in subsoil and land drainage.
Questions we get asked
Can I put a soak pit anywhere on my section?
No. Position is constrained by ground conditions, groundwater, slope, and distance from foundations, retaining and boundaries. It is a design decision, not a convenience one.
How do I know if my ground will take water?
Through a soakage or infiltration test at the depth the system will sit. Soil type varies enormously across Auckland, so neighbouring properties are not a reliable guide.
Does soakage work in clay?
Generally poorly. Heavy clay has low infiltration rates, and systems sized on optimism rather than testing tend to fill and stay full.
Do I need consent for a soakage system?
Requirements depend on the site and the scale of work. Confirm with Auckland Council before proceeding.
Can soakage and detention be combined?
Yes, and on constrained sites that is often the practical answer. The engineer sets how the two work together.
This page is general in nature. Confirm stormwater disposal requirements for your property 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.
