Soakage Testing and Soak Pit Design: Managing Stormwater On-Site
- Aug 8, 2024
- 4 min read
Updated: 6 days ago
If you are developing in the Wellington region, there is a good chance stormwater will need to be managed on-site rather than simply piped away. That starts with a question about the ground: how fast will water soak into the soil?
Get that answer wrong, or skip it, and you can end up with a drainage system that does not perform in heavy rain, a consent that stalls, or a redesign partway through the project. Soakage testing is how you get the answer before those problems appear.
Why on-site stormwater management matters now
Many councils across New Zealand are placing greater emphasis on managing stormwater on-site, including reducing runoff volumes, slowing peak flows and, where required, treating runoff before it reaches the public network or nearby waterways.
In the Wellington region, Greater Wellington Regional Council has moved in this direction through changes to its Natural Resources Plan. The reasoning is consistent wherever these requirements apply: reducing flood risk, protecting streams and harbours from contaminated runoff, and encouraging rainfall to soak into the ground where site conditions allow, rather than overloading pipes and downstream networks.
For anyone developing land, the practical effect is that stormwater is no longer a detail to sort out at the end. It can affect whether a site is feasible, consentable and cost-effective in the first place, which means it needs to be considered early.
What soakage testing is, and why it comes first
Soak pits, also called soakage pits or infiltration systems, work by allowing stormwater to soak into the ground rather than running straight off the site.
Whether that is viable, and how large the system needs to be, depends largely on how well the ground absorbs water. That can vary enormously from site to site. Free-draining sands and gravels may take water readily, while clays and silts can drain slowly or barely at all. Often you cannot tell reliably by looking, which is why the design needs to start with testing, not assumption.
Soakage testing, sometimes called percolation or infiltration testing, measures how quickly water infiltrates the soil at selected locations across the site. It usually begins with a site assessment, looking at the topography, soil conditions and natural drainage patterns to identify constraints and opportunities.
The infiltration testing then measures how fast water soaks away at representative locations. The results are used to establish an infiltration rate and drainage capacity, which become key inputs for the stormwater design.
Done properly, this testing tells you whether soakage is likely to work on your site and how the system needs to be sized to manage the rainfall events required by council.
Designing a soak pit that performs
The specific rules, and how strictly on-site management or hydrological control is required, vary by council and can change over time. It is worth confirming the current requirements for your site rather than assuming. With measured infiltration data in hand, a soak pit can be designed for the site rather than guessed at. We can help with that.
The main factors are:
Capacity: the system needs to be sized for the stormwater volumes and rainfall events the council requires it to manage.
Location: it needs to be placed where infiltration is suitable, with appropriate setbacks from buildings, boundaries, slopes and other constraints, so soakage does not create new problems.
Construction type: the design may use a rock-filled pit, modular infiltration system or another suitable arrangement to store water temporarily and release it into the ground.
Maintenance: soakage systems can clog with sediment over time, so access for inspection and cleaning is part of keeping the system working over the life of the development.
A soak pit designed without testing, or without proper allowance for these factors, is one of the more common causes of drainage problems after a development is finished. That is exactly when those problems are most expensive to fix.
What it means for the people involved
Getting stormwater right early helps everyone with a stake in the project.
For a developer or owner, it is part of confirming whether a site is feasible and consentable before committing significant money. It also helps avoid the redesigns and delays that can come from discovering a drainage constraint late.
For a contractor, a soak pit designed using measured ground data is one less thing likely to fail or need rework on site. For a council, testing-based design provides evidence that a development can manage its runoff as required and is less likely to add to flooding or degrade downstream waterways.
For the wider community and environment, on-site stormwater management can help reduce flood risk, protect streams and harbours, and allow rainfall to recharge the ground where conditions are suitable, rather than overwhelming the network.
There is also a limit worth being honest about: soakage is not viable everywhere. On tight clay soils, sites with high groundwater, or constrained sites with limited room for setbacks and overflow paths, infiltration alone may not be enough. The stormwater strategy may need to combine soakage with detention, attenuation, treatment or a piped connection. Part of the value of testing early is finding that out before it shapes, delays or derails the design.
Where we come in
Cook Costello’s Wellington team has carried out soakage testing and soak pit design across infill housing and larger subdivisions, so we understand both the local ground conditions and the stormwater requirements that commonly apply.
Because our geotechnical and civil engineers work together, the soakage testing and stormwater design are handled as one connected process. The ground data feeds directly into a drainage design that suits the site and supports the relevant council requirements. By testing rather than assuming, the system can be sized around measured infiltration conditions, not a conservative guess or an optimistic assumption.
If you are planning a development and need to understand your stormwater obligations, or whether soakage will work on your site, tell us about it and we will advise on testing and design.
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