GOOD RIDDANCE TO RUN-OFF

Uncontrolled run-off from building sites that may contain chemicals or toxins can enter and clog waterways, drains and the stormwater system, killing plants and wildlife. It may also be carried onto nearby properties, upsetting neighbours. There are many practical ways to stop it

Bare soil and loose materials on site can be swept up by heavy rain (or water from any other source) to become run-off. Where this runs down stormwater drains, it will flow straight into local streams or the sea. This can be seriously harmful, especially if the run-off carries concrete waste, paints or chemicals.

It’s not just bad news for nature – under the Resource Management Act, those responsible could be fined up to $600,000 or face two years in prison.

Ways to prevent run-off include:

• Scheduling earthworks for dry weather.
• Using tarpaulins or shotcrete to protect exposed soil.
• Removing the minimum amount of vegetation possible.
• Having just one stabilised accessway onto the site and putting down gap 65 aggregate to stop drivers tracking dirt onto the road.
• Diverting water (other than rain) away from earthworks and onto stable ground (grass, gravel or permeable paving).
• Keeping as much existing grass coverage on site as possible and stabilising bare soil as soon as possible with mulch, gravel,
• planting or new grass.
• Controlling the flow of water with channels or contour drains.

Sediment control

Control sediment run-off with:

• Vegetation.
• Silt fences on the downhill side of sloping sites to capture sediment run-off (figure 1).
• Hay or straw bales to trap sediment.
• Sediment ponds.

Keep silt control devices clear and ensure that cleared material cannot run into waterways or drains.

Concrete, asphalt and bitumen waste

Concrete wash water, cement slurry, asphalt and bitumen can join run-off when the materials are laid, equipment is washed or concrete is waterblasted to expose aggregate. Concrete and cement products can change the pH levels of streams, killing wildlife. One bucket of concrete wastewater in a stormwater drain can kill hundreds of fish and eels. It takes 100,000 litres of freshwater to dilute just 1 litre of concrete slurry to safe levels.

Contain run-off from concrete, asphalt and bitumen on site by:

• Dewatering pile holes and footing trenches before pouring concrete to stop slurry spilling out and into drains.
• Diverting it onto on-site grass or soil for later clean-up.
• Containing it in an appropriate storage tank if possible.

Talk to subbies and delivery drivers to make sure they wash concrete equipment on unsealed ground on site, such as gravel or grass, and they don’t let washing water or concrete slurry run into stormwater drains.

This article has been reproduced from Builder’s Mate 95 with permission from BRANZ