Water
Designing homes to use water efficiently, reduce costs and cope with more frequent droughts or storms.
Stormwater Control
Stormwater is disposed of either into natural watercourses, water storage tanks, and soakpits, or through a local authority system of culverts and pipes.
In a natural landscape rainwater surface runoff averages about 27% but in urban areas, due to the increase in hard surfaces such as roads, driveways and reduced vegetation, average run off is in the region of 58%. Runoff can cause pollution by carrying soil, contaminations (such as fuel) from roads and vehicles, human and animal waste, and chemicals (e.g. fertilisers, pesticides, industrial chemicals and household cleaners) into waterways.
- Change in surface water runoff due to site development (figures approximate)
Rainwater surface runoff in urban areas is often more than double that of a natural landscape due to the increase in hard surfaces such as roads, paths and driveways and the reduction in vegetation.
Increased runoff can cause pollution by carrying contaminants (such as fuel) from roads and vehicles, and animal waste and chemicals (e.g. fertilisers, pesticides, industrial chemicals and household cleaners) into waterways.
Reducing stormwater run-off
Designers can help to reduce the rate of runoff from houses and land by:
- retaining rainwater (sometimes called stormwater detention) in storage tanks
- using permeable paving
- slowing the rate of surface water movement with swales
- specifying green roofs where appropriate
- selecting appropriate planting
- feeding water into natural or developed ponds or wetlands.
Rainwater storage
If rainwater is the only source of water for a household, the tank will usually be kept as full as possible at all times and the effect of runoff will be minimal. If household use is not an issue then rainwater can be retained during rainy periods and released slowly to a drainage route approved by the local authority, or used for garden irrigation, cold water laundry washing, and toilet flushing. See Rainwater systems: uses for more.
Permeable paving
Rainwater run-off rate can be reduced by using alternatives to hard, impervious paving such as asphalt or concrete for driveways, footpaths and parking areas. Runoff can be slowed by using porous surfaces such as:
- gravel
- open jointed paving with gravel underneath
- concrete-grass paving
- porous concrete or porous asphalt
Alternatively, runoff can be slowed by using surfaces where the material is impervious but water can infiltrate through gaps, such as concrete paving blocks installed with open joints over free draining material.
Porous and permeable surfaces are best suited to:
- areas with a slope of 1:10 (5°) or less
- low traffic volumes
- low speed traffic.
- Concrete/grass paving
Rainwater runoff can be slowed by using impervious surfaces where water can infiltrate through gaps, such as concrete paving blocks installed in grass.
Porous and permeable surfaces are best suited to low traffic volume and low speed areas with a slope of 1:10 or less.
- Permeable paving
Concrete blocks with sand base and joints can be used to slow rainwater runoff. They are best suited to low traffic volume and low speed areas with a slope of 1:10 or less and are not as permeable as concrete/grass paving options.
Swales
Swales are low-gradient, wide, shallow drainage channels that form part of the landscaping and reduce run-off rate by:
- retarding the flow rate of surface water
- providing some means of infiltration into the subsoil.
Swales should be wide and shallow, and have a fall rate of less than 1:25. They should have grass cover, ideally 150 mm long and mown infrequently. On steeper slopes they should have check dams to slow the flow rate. For driveways over 30 metres, incorporate an underdrain.
- Cross section of a swale
Swales are wide and shallow drainage channels that reduce rainwater run-off by slowing the flow rate of surface water and providing some means of infiltration into the subsoil. They should have grass cover, ideally 150 mm long and mown infrequently.
Swales should have a fall rate of less than 1:25. On steeper slopes they should have check dams to slow the flow rate.
- Cross section of a check dam in a swale to slow water flow
Runoff is slowed and absorption of rainwater into the ground is increased by use of a check dam on sloping ground.
Green roofs
On a green roof, vegetation is planted in a thin layer of growing/drainage medium on top of the waterproof membrane. Green roofs help to lower surface water flow rate by absorption. The retained moisture is then lost through slow drainage, transpiration and evaporation.
Pitched roofs up to 25º are suitable as green roofs, but lower pitches allow a thinner, lighter layer of growing medium. Steeper roofs require a deeper layer and measures to prevent erosion. Deciding on the type of growing medium and plants to use requires specialised advice from a suitably qualified and experienced person.
- Cross section of a green roof
Vegetation on a green roof is planted in a thin layer of growing/drainage medium on top of a waterproof membrane. Green roofs help to reduce rainwater runoff by absorption. The retained moisture is then lost through slow drainage, transpiration and evaporation.
Pitched roofs up to 25° are suitable as green roofs, but lower pitches allow a thinner, lighter layer of growing medium. Steeper roofs require a deeper layer and measures to prevent erosion. Choosing the growing medium and plants to use requires specialised advice.
A disadvantage of green roofs is that they impose higher than normal loads on the structure. This means that they (a) cost more and (b) require specific design under the NZ Building Code by a structural engineer. In arid regions they require an irrigation system for the plants, which increases the cost. Other disadvantages include:
- the weight of the roof and the additional structure for support
- the membrane, flashings and rainwater collection and detention system must be carefully designed and detailed to prevent deterioration from roots and damp conditions
- if the roof leaks it may be difficult to find the cause, and repair will be expensive
- they may require more maintenance than a conventional roof although this depends on the plants and roofing membrane selected.
Advantages of green roofs include:
- the growing medium protects the waterproof membrane from ultraviolet light
- they reduce the rate of rainwater run-off
- noise reduction
- natural appearance that blends into the landscape.
For your clients
To help your clients understand the impact of building and landscaping decisions on stormwater runoff, and how their actions affect contaminants in stormwater, refer them to www.smarterhomes.org.nz/water/managing-stormwater.