Stormwater control

Figure 1: Change in surface water runoff due to site development

Careful planning of stormwater management can reduce the pressure on council stormwater systems, reduce flooding risks, protect the environment from contamination and cut water use in a household. This article by BRANZ explains how.

Stormwater must be managed to minimise the risk of flooding. But allowing all stormwater from a property to run into drains not only wastes a potential source of garden water; it also means that contaminants such as oil, paint and animal droppings are carried into waterways.

Stormwater can instead be harnessed for irrigation or otherwise disposed of onsite, reducing demand on a property’s water supply. With an estimated 10-30% of household water used for gardening, this can have benefits in terms of water efficiency.

In most areas, there are council requirements around stormwater runoff that new construction must comply with. Under Auckland’s Unitary Plan, for example, rainwater runoff after a new development must not be any greater than the runoff that occurred before the development.

On the Kāpiti Coast, all new residential developments must be “hydraulically neutral” – new housing areas cannot discharge additional stormwater into the council’s system.

AS E1/AS2 for stormwater references AS/NZS 3500.3:2021 Plumbing and drainage Part 3: Stormwater drainage (with modifications) as an Acceptable Solution for the design and installation of surface water drainage systems.

CONTROLLING STORMWATER RUNOFF

Stormwater may be disposed of:

• Into a local authority stormwater system – either directly, or by slow release with the use of an on-site detention tank. Detention tanks are readily available in different sizes and can be installed above or below ground.

• Into a soak pit, following the guidance in New Zealand Building Code Verification Method E1/VM1. This may require a building consent.

• Into a retention tank, to store water for gardening, toilet flushing etc (but not drinking).

• Into a raingarden with suitable planting.

• Into a natural watercourse (with local authority consent).

Water from roofs can be harvested into a water storage tank for all water requirements.

In a natural landscape, rainwater surface runoff averages about 27%. In urban areas, due to hard surfaces such as roads, driveways and reduced vegetation, average runoff is in the region of 58%.

Runoff can cause pollution by carrying soil, contaminations (such as fuel) from roads and vehicles, animal waste, and chemicals (eg, fertilisers, pesticides, industrial chemicals and household cleaners) into waterways.

Stormwater runoff can be reduced by:

• Collecting and storing rainwater in storage tanks.

• Using permeable paving and concrete.

• Incorporating swales to slow the rate of surface water movement.

• Planting a rain garden below road level, filled with plants that love water

• Retaining or planting trees.

RAINWATER STORAGE

Stormwater runoff can be reduced by collecting and storing rainwater for gardening, toilet flushing or other uses. This also has other benefits, such as reducing water costs for properties on metered supply, and reducing demand on other stormwater disposal systems.

PERMEABLE PAVEMENTS

Stormwater runoff rate can be reduced by using permeable paving or concrete for driveways, footpaths and parking areas instead of traditional impermeable asphalt or concrete.

Permeable or porous surfaces include:

• Gravel

.• Concrete-grass paving.

• Porous concrete/asphalt

.• Open-jointed paving over gravel.

• Proprietary paving stones where water flows through the paver itself.

Permeable surfaces are best suited to:

• Areas with a slope of 1:10 (5°) or less.

• Low traffic volumes.

• Low speed traffic.

Paving blocks on a sand base and with open joints can be used to slow rainwater runoff but they are not as permeable as concrete/grass paving options.

SWALES

Swales are wide, shallow drainage channels running across the slope of the ground that forms part of the landscape. They reduce runoff rate by:

• Retarding the flow rate of surface water.

• Providing a means of infiltration into the subsoil.

Swales should be wide and shallow, with a gradient across the slope of less than 1:25. They can be grass-covered and mown infrequently, or filled with water-loving plants. There are many suitable native options, from small reeds to nikau palms. On steeper slopes, they should include a check dam to slow the flow rate. If used for driveways over 30m long, an underdrain should be incorporated.

Run-off is slowed and absorption of rainwater into the ground is increased by use of a check dam on sloping ground.

Figure 2: Cross section of a swale

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Figure 3: Cross section of a check dam in a swale to slow water flow

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RETAINING OR PLANTING TREES

Retaining mature trees on a site, or planting trees, can reduce the volume of rainwater hitting the ground in the first place. Trees catch rainwater, holding it in their canopies and allowing it to gradually evaporate from there.

This ‘canopy interception’ for large trees has been found to average around 500–650L per tree, per storm in American research. Approximately 20-40% of rainfall can be caught by a tree canopy, preventing it from reaching the ground and the stormwater system.

REDUCING GARDEN WATER USE

The amount of water needed for garden irrigation can be reduced significantly by selecting plants that require little water.

Observe plants that grow well with limited or no watering within the local area. This will give a good guide to plants that are largely self-maintaining and will not require extensive irrigation.

Garden water use can also be reduced by using an efficient irrigation system – that is, one that applies a smaller volume of water directly where it is needed, such as by drip irrigation rather than sprinklers.

Water use should be controlled by the use of timers (for example, to water in the evening to reduce the amount of water evaporation) and/or in-ground moisture meters.

Other ways to reduce garden water use include storing and using rainwater for irrigation, using greywater for irrigation, and diverting stormwater for irrigation instead of allowing it to run into drains.

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Originally published by BRANZ in its Level: Sustainable Building Series publication. Copy and figures reprinted with permission from BRANZ.www.branz.co.nz