Development usually increases the amount of hard surface on a property. Roofs, driveways, roads and paved areas prevent rainfall from soaking naturally into the ground, which can increase the volume and speed of stormwater runoff.
On-site stormwater systems are designed to manage some of this water within the development before it reaches the public network or surrounding environment.
Depending on the site, this may involve storing water for later use, allowing it to soak into the ground or temporarily holding runoff and releasing it at a controlled rate.
The existing stormwater network does not always have unlimited capacity. As development increases the amount of runoff leaving a property, additional measures may be needed to manage how much water enters the network and how quickly it gets there.
Requirements can also arise from council standards, resource consent conditions or the particular constraints of the site.
The right solution depends on factors such as soil conditions, available space, site levels and the amount of hard surface being created. This means the stormwater approach should ideally be considered while the development layout is still flexible.
Rain tanks capture runoff from roof areas and temporarily store it rather than allowing all of the water to discharge immediately.
Depending on the system, stored water may be reused for purposes such as irrigation or gradually released after a rainfall event. Some tanks are designed specifically for detention, while others combine detention with water reuse.
A rain tank design needs to consider more than tank volume alone. Roof catchment size, available storage, outlet configuration and the downstream drainage system can all affect how well the tank performs.
The location also matters. Tanks need to fit around buildings, access and other services while remaining practical to maintain.
A soak pit allows stormwater to drain gradually into the surrounding ground rather than being discharged entirely through pipes.
Runoff is directed into an underground chamber or aggregate-filled area where it can infiltrate into the soil over time.
A soak pit design can be an effective option where ground conditions provide sufficient soakage, but it is not suitable for every property.
Soil permeability, groundwater levels, nearby structures and available land all need to be considered. If water cannot soak away quickly enough, the system may not provide the required performance.
Detention systems temporarily store stormwater during rainfall and release it gradually afterwards.
Unlike retention, where water may be captured for reuse or infiltration, detention is primarily intended to control the rate at which runoff leaves a site.
This can help reduce sudden increases in flow entering downstream pipes and drainage infrastructure.
On constrained developments, underground detention and treatment can provide storage beneath areas such as car parks, accessways or landscaped spaces without taking up valuable surface area.
There is no single system that is suitable for every site. In some cases, one solution may be enough, while other developments use a combination of storage, detention and soakage.
When assessing the options, important considerations can include:
These factors need to be considered together. A soak pit may appear simple, for example, but it will only work where the ground can reliably accept the required volume of water.
Most stormwater systems rely heavily on gravity.
Water needs to reach the tank, soak pit or detention system and then either infiltrate or discharge at an appropriate level. Small changes in site elevation can therefore have a significant effect on the design.
This is particularly important where a development involves substantial earthworks, retaining or changes to building platforms.
Considering stormwater requirements alongside proposed levels can reduce the likelihood of discovering later that a preferred system cannot be connected or discharged as intended.
Yes. A development does not necessarily need to rely on a single method.
Roof runoff might be captured in a rain tank, for example, while paved areas are managed through detention or another stormwater treatment system. On suitable sites, soakage may also form part of the overall strategy.
Combining systems can allow different areas of the development to be managed according to their runoff characteristics rather than forcing all stormwater through one solution.
The important part is understanding how the systems interact and ensuring that the overall design achieves the required outcome.
Stormwater infrastructure needs to keep working long after the development has been completed.
Rain tanks may require filters or outlets to be checked, soak pits can gradually become less effective if sediment builds up, and underground detention systems need suitable access for inspection and cleaning.
Maintenance requirements should therefore be considered during design rather than after installation.
Locating a system where it cannot be inspected or serviced easily can create unnecessary problems later, even if the original hydraulic design is sound.
On-site stormwater systems require physical space and appropriate levels, which means they can influence the wider development layout.
Identifying the likely approach early gives designers more flexibility to allocate space, establish suitable levels and coordinate the system with buildings, access and underground infrastructure.
It can also provide a clearer understanding of stormwater requirements before the design progresses into detailed engineering and approval.
Civix provides stormwater design for subdivision, land development and infrastructure projects.
The most suitable approach depends on the characteristics of the site and what the development needs to achieve. By considering storage, soakage, discharge and long-term maintenance alongside the wider civil design, stormwater infrastructure can be incorporated into the project in a practical and coordinated way.
Earthworks are often one of the first major stages of a subdivision or land development project. They involve reshaping the existing ground so the site can support buildings, accessways and other future construction.
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Engineering approval is the stage where detailed civil plans are reviewed before construction begins. It is commonly required for subdivisions and developments involving earthworks, roads, accessways, drainage or connections to public infrastructure.
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READ MORE Stormwater treatment is an important part of many subdivision, land development and infrastructure projects. As sites are developed, buildings, roads, car parks and other hard surfaces can increase the volume and speed of runoff while also changing the types of contaminants entering the stormwater system.
Development usually increases the amount of hard surface on a property. Roofs, driveways, roads and paved areas prevent rainfall from soaking naturally into the ground, which can increase the volume and speed of stormwater runoff.
Earthworks can produce sediment-laden runoff when exposed soil is disturbed by rainfall. Standard erosion and sediment controls are often used to slow this water and allow suspended soil particles to settle before discharge.
For private land developments, Civix Town Planners, Chartered Engineers and Licensed Cadastral Surveyors have got you covered from the initial site visit through to council sign off.
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