|
Stormwater Ideas for the Homebuilder
– Diana Leufkens
There are so many things to think of when building a home – design, materials, budget – and while perhaps not the most exciting action item, managing stormwater is an unavoidable consideration.
Part of the design and consent process for the homebuilder includes providing for the collection and treatment or disposal of stormwater in a sustainable way. Stormwater is water from rainfall (or melting snow) that does not soak into the ground. It flows from roofs, over paved areas and bare soil, and through the sloped lawns. As it flows, the runoff collects and transports sediment, pesticides, salt, fertilisers, oil and grease, leaves, litter, pathogens from bird and animal droppings, and other potential pollutants. More so, the transport of these pollutants does not need a heavy rainfall – small amounts of rain can supply enough water to send pollutants rushing towards streams, lakes, or other sensitive water bodies. Whether you hire a contractor, or work on the house yourself, this article will help you understand how stormwater affects the environmental quality of your property and the receiving environment. It also suggests some ways to reduce pollution risk.
Conventional approaches to residential land development in New Zealand substantially contribute to the problems. In particular: (i) New subdivisions alter the land surface, significantly increasing impervious surfaces and compacting hard ground to the extent that there is a near-total loss of permeability after development; (ii) Topsoil is commonly compacted or destroyed, washed away in storms, discarded into landfills, or sold, which increases the need for irrigation of gardens and green spaces and the cost of planting and restoration strategies; (iii) During infill housing, retrofitting and new development, impervious surfaces proliferate across whole districts, resulting in increased stormwater runoff and catchment-scale impacts.
There are a number of environmental problems directly associated with water pollution from building sites.
Pollutants such as soil and soil nutrients, vehicle-derived contaminants such as metals and hydrocarbons, and building materials are washed off building sites and enter stormwater drains, ultimately polluting natural waterways.
Changes to natural land surfaces alter the site drainage; this may lead to serious erosion. A single bare lot can lose up to four truckloads of soil in one storm. Although a single lot is a small part of the river catchment, the cumulative effect of polluted runoff from a number of building sites can have a dramatic impact on local waterways.
Turbid water, which is particularly a problem where 'dispersive' soils are present, restricts sunlight filtration. This in turn reduces photosynthesis and lowers the productivity of the aquatic ecosystem.
Deposition of sediment in watercourses reduces their ability to convey storm flows, leading to more frequent and serious flooding.
Dr Surya Pandey, Leader of the Urban Ecosystem Sustainability team at Landcare Research, says that before you buy or build a house, it is a good idea to visit the property during a heavy downpour to see how efficiently the property drains. Some considerations for ideal water drainage include: an unobstructed route for stormwater runoff to drain off the property; the property sloping away from the house in all directions; presence of gutters; and downpipes directed away from the foundation.
Dr Pandey says homeowners can take an environmentally responsible approach to the collection, treatment and disposal of stormwater from a site by:
using control measures such as fences or bunds to minimise sediment discharges offsite from disturbed areas.
avoiding the use of unpainted galvanised roofing and guttering and copper guttering.
diverting clean water, e.g., from roofs or from upstream of the site, away from disturbed areas. This reduces the amount of sediment carried by runoff water.
minimising the area of bare ground, and the length of time it is bare, to stop the soil being washed away into stormwater gutters or nearby streams.
eliminating paved surfaces. Concrete and asphalt roads, driveways and paths prevent rainwater from soaking into the ground. Consider alternative materials such as gravel or porous pavement made from interlocking cement blocks or rubber mats that allows the rainwater to seep into the ground.
creating a stable driveway entrance, and a generous spread of gravel aggregate to prevent soil being transferred off site by vehicles
changing landscape and layout. An important part of stormwater management should be keeping water from leaving the property, or slowing its flow down as much as possible. Provide low areas landscaped with shrubs and flowers (rain gardens) to encourage the water to soak into the ground. Use of storage tanks for roof-water collection will also reduce the overall volume of stormwater leaving the site and the tank water can be reused for toilet and laundry and garden watering.
providing a ‘naturalised’, vegetated corridor. If your property is next to a lake or stream, one of the best ways to slow and filter runoff is to leave a strip of thick vegetation along the waterfront.
According to Dr Pandey, the following techniques should also be considered for improving/building the home without contributing to water pollution:
Collect garden clippings – don't sweep them into streets or gutters. Many cities and landfills have composting programs for green waste. Do not rinse paintbrushes into gutters or streams – when using water-based paints, wipe the excess onto scrap material, then rinse the brushes into a sink that flows to the municipal sewer. Oil-based paints should never be poured into the sink. Thinners and solvents used to mix or clean up oil-based paint should be filtered and reused. If possible, don't mix up more paint than you need to avoid having a surplus. Excess oil-based paint and thinners should be disposed of through your local household hazardous waste disposal program. Allow wash water from vehicles or equipment to soak into the ground. If wash water from waterblasting is suspected to contain chemical additives, it should not be discharged to stormwater drains and watercourses.
During the rainy season, use plastic tarps to cover excavated materials, stockpiles of asphalt, sand, and garden clippings. A good cover will prevent wind from blowing contaminants into gutters and rainfall from washing such materials into the stormwater collection systems. If possible, schedule grading and excavating projects for dry weather.
For more information contact:
Dr Surya Pandey, Landcare Research, Hamilton
(07) 858 3700 PandeyS@LandcareResearch.co.nz
|
