Stormwater run-off can be reduced by:

• collecting and storing rainwater in storage tanks
• using permeable paving
• incorporating swales to slow the rate of surface water movement
• installing a green roof as part of a new building design.

Rainwater storage

Stormwater run-off can be reduced by collecting and storing rainwater. If rainwater is used by a household, other benefits will include:

• reduced water costs for users on a metered water supply
• provision of an emergency water supply
• reduction of demand on local authority water supplies
• reduction of demand on stormwater disposal systems
• provision of an independent supply for watering the garden in times of drought.

If household use is not an issue, rainwater can be stored during wet periods and may then be used for garden irrigation, cold water laundry washing and toilet flushing, or released slowly to a drainage route approved by the local authority.

Permeable paving

Stormwater run-off rate can be reduced by using permeable paving for driveways, footpaths and parking areas instead of hard, impervious paving such as asphalt or concrete. Read about a recent study.

Permeable or porous surfaces include:

• gravel
• concrete-grass paving
• porous concrete/asphalt
• open-jointed paving over gravel
• green roofs.

Permeable surfaces are best suited to:

• areas with a slope of 1:10 (5°) or less
• low traffic volumes
• low speed traffic.

Swales

Swales are wide, shallow drainage channels running across the slope of the ground that forms part of the landscape. They reduce run-off 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 should be grass covered and mown infrequently. On steeper slopes, they should include a check dam to slow the flow rate. If used for driveways over 30 metres 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.

Green roofs

A green roof has vegetation planted into a layer of growing/drainage medium laid over a waterproof membrane. Green roofs help to reduce the water run-off rate by retaining the water, which is then lost through slow drainage, transpiration and evaporation.

Pitched roofs up to 25º are suitable as green roofs, but lower pitches require less depth of growing medium. Steeper roofs require a deeper layer and measures to prevent erosion. Obtain specialist advice before deciding on the type of growing medium and plants to use.

Disadvantages of green roofs include:

• they cost more
• the roof is heavier than a conventional roof and will require additional structure for support
• they require specific design under the NZ Building Code
• the membrane, flashings and rainwater collection and retention system must be carefully designed and detailed to prevent deterioration from plant roots and damp conditions
• if the roof leaks, it may be difficult to find the cause, and the cost of repairing it will be expensive
• they are likely to require more maintenance than a conventional roof
• in arid regions, they require an irrigation system for the plants, which will increase the cost.

Advantages of green roofs include:

• protection of the waterproof membrane by the growing medium from ultraviolet light
• they reduce the rate of rainwater run-off
• good noise reduction
• a natural appearance that blends into the landscape.

More information

•    www.smarterhomes.org.nz/water/managing-stormwater
•    www.smartwatermark.org
•    Water>On-site wastewater treatment>Greywater recycling
•    Water>Rainwater systems>Water storage options