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Slowing the flow – part 2

Onsite stormwater management for new developments is now a requirement in many jurisdictions. In the second of this two-part series, we look at detention and retention tanks

New developments place additional demands on existing stormwater systems, which increasingly do not have the capacity to handle further loads. When permeable surfaces like grass are replaced by houses, driveways, roads and decks, rainfall that used to soak through the soil or slowly drain over land runs off the land much faster.

Move to hydraulic neutrality

Both the volume of water and the peak flow are increased as a direct result of development. Many authorities now impose a requirement of hydraulic neutrality on new housing developments. In other words, stormwater must be managed on site to ensure the peak flow rate is the same or less than what it was prior to development.

Common management techniques include detention and retention tanks, which we look at in more detail here. 

Detention tanks

Detention (sometimes known as attenuation) devices store stormwater temporarily, releasing it gradually to manage peak flows in the stormwater network. Options for large areas may include wetlands or ponds, but for a residential site, the detention device is usually a tank.

Stormwater detention tanks are designed to store rainwater run-off from roofs, driveways, paths and other impervious areas. The water discharges from the tank to the stormwater system through a small-diameter pipe at a controlled rate that the stormwater system can cope with (see Figure 1).

Common systems include below-ground tanks (typical storage capacity 2,000–5,000L) and interlinked modules with smaller individual capacity that can be installed beneath floor slabs, decks, paving, driveways or even steps. The collection area of modular tanks may be limited – for example, 350m2 in Wellington.

Installation and flow

The inlet is from downpipes with a leaf diverter and silt trap to reduce sediment build-up within the tank.

The outlet is a small-diameter orifice – outlet pipe – at a specific height above flood level, regulating discharge to the stormwater network.

The overflow pipe capacity must equal or exceed the inflow capacity from downpipes and must discharge where flow is visible and does not cause a problem.

It is recommended that a mesh screen is installed over the silt trap and outlet pipes to prevent debris from entering and a first-flush diverter installed to reduce contaminants in the stormwater system.

Detention tanks should be installed by approved installers in accordance with the manufacturer’s specifications.

Detention tanks:

  • Reduce the peak flow of stormwater leaving the site.

  • Are useful in urban areas with reticulated stormwater.

  • Ideally discharge by gravity but some sites may require a pump.

  • Do not require seismic and wind restraints for in-ground tanks.

  • Are not for storage – additional tanks are required for emergency water.

Retention tanks

Retention devices reduce the volume of run-off through disposal or reuse on site. Typical devices include tanks (with or without detention capability), soak pits, pervious paving and bioretention systems.

Choosing the right size

Establishing the appropriate device size is a function of several factors including the catchment area and rainfall intensity.

Rainfall intensity curves are available for most areas from the building consent authority (BCA), tables in E1/AS1 or online from NIWA.

Where differing design rainfall intensities are provided for a particular location, the most conservative rainfall intensity should be used. Design guidelines are available on some BCA websites, but the calculations are not simple and are best completed by someone with experience.

A resource consent will usually be required, and all tank designs and soak pits must comply with the requirements of the Building Code. Some installations also require a building consent – check with the BCA.

Installation and flow

Retention tanks are installed above ground in accordance with the manufacturer’s specifications, including seismic and wind restraints:

  • Inlet – from downpipes with a leaf diverter.

  • Outlet – specific size and height above ground, connected to the stormwater network.

  • Overflow pipe – capacity must equal or exceed the inflow capacity from downpipes and must discharge where flow is visible and does not cause nuisance.

  • A first-flush diverter and mesh screen over the inlet and outlet pipes is recommended.

Retention tanks key points:

  • Water is collected and must be used on site, often within a specified timeframe so the entire tank volume is available for the next rainfall event.

  • Water is stored for use in the house for toilet flushing and in the garden, and some councils allow its use in the laundry. It can be used as an emergency water supply and must be boiled first.

  • Useful to reduce demand on water supply and in areas without reticulated stormwater.

  • The collection area may be limited – it is 400m2 in Wellington, for example.

  • If used in the house, a pump is required. Backflow prevention may also be required.

Detention and retention tanks

Detention and retention tanks combine the benefits of both, reducing the peak flow of stormwater leaving the site and providing water storage for on-site use.

They are only suitable for water from the roof due to contaminants from areas such as driveways and are usually installed above ground with seismic and wind restraints.

They have a small-diameter orifice part way up the side of the tank, allowing the slow release of roof run-off during and after rainfall. The volume below the orifice stores rainfall collected from roof areas for non-potable use within the building or garden.

These tanks require:

  • A dead storage zone – typically 150mm – at the bottom of the tank to allow sediment to settle.

  • Installation by approved installers in accordance with the manufacturer’s specifications.

Minimum tank maintenance

The minimum maintenance schedule for tanks is:

  • 3 monthly – wash out leaf litter/debris diverters and first-flush diverters.

  • 6 monthly – inspect the roof gutters and clean off leaf litter, animal droppings, pollen, ash etc.

  • Annually – inspect and maintain mesh screens, orifice outlets, filters, seals, pipes and valves and silt traps for modular in-ground tanks.

  • 2–3 years – drain above-ground tank and remove any sediment and debris.  

Original article by Ann Galloway, FNZIA, Ann Galloway Architect Ltd, Napier.

The full article was first published in Issue 183 of BRANZ Build Magazine.


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