Stormwater is the water draining off a site from the rain that falls on the roof and land, and everything it carries with it. The soil, organic matter, litter, fertilisers from gardens and oil residues from driveways it carries can pollute downstream waterways.
Rainwater refers only to the rain that falls on the roof, which is usually cleaner. However, stormwater can be a valuable resource. Reusing stormwater can save potable water and reduce downstream environmental impacts.
In urban areas stormwater is generated by rain runoff from roofs, roads, driveways, footpaths and other impervious or hard surfaces. In Australia the stormwater system is separate from the sewer system. Unlike sewage, stormwater is generally not treated before being discharged to waterways and the sea.
Poorly managed stormwater can cause problems on and off site through erosion and the transportation of nutrients, chemical pollutants, litter and sediments to waterways. Well-managed stormwater can replace imported water for uses where high quality water is not required, such as garden watering.
A homeowner can take simple steps to manage stormwater and reduce its environmental impact.
Take some simple steps to better manage stormwater and reduce the environmental impact of your home.
- Avoid cut and fill on your block when preparing the building foundations. Attempt to maintain the existing topography and drainage pattern.
- Retain vegetation, particularly deep-rooted trees. They lower the watertable, bind the soil, filter nutrients, decrease runoff velocities, capture sediment and reduce the potential for dryland salinity.
- Reduce erosion potential on site during building works by minimising the time that land is left in an exposed, unstable condition. Employ sediment traps and divert ‘clean’ stormwater around the disturbed site (see Sediment control).
- Minimise the area of impervious surfaces such as paved areas, roofs and concrete driveways.
- Grade impervious surfaces, such as driveways, during construction to drain to vegetated areas.
A stormwater site plan can help reduce stormwater runoff from the site.
- Detain stormwater on your block where practicable through use of permeable paving, pebble paths, infiltration trenches, soakwells, lawns, garden areas and swales.
- Harvest and store roof water for use (see Rainwater).
- Take care with the substances used on your land as they can end up in the stormwater. Don’t overuse fertilisers, herbicides and pesticides; follow the manufacturer’s instructions on the amount and frequency of application. Look for organic alternatives.
Water sensitive urban design slows stormwater runoff and improves filtration and infiltration.
- Avoid using solvent-based paints. After using water-based paints, clean brushes and equipment on a lawn area to trap contaminants before they reach waterways. Plant-based paints are the most environmentally benign.
- Visit a car wash that recycles wash water. If this is not an option wash your car on the lawn or on an area that drains to lawn. The nutrients (mostly phosphates and nitrates) in the detergent fertilise the lawn instead of degrading waterways. Note that many native plants do not tolerate detergents.
- Do not build on floodplains as the land may be periodically subject to inundation and may have a high watertable. Councils can advise on the 1 in 100 year flood level.
The traditional approach
The traditional stormwater management response relied on conveyancing. Water was conveyed by a pipe or channel from a collection area (e.g. house and street) to a discharge point (e.g. the nearest ocean, creek, river or lake). The conveyancing system sought to remove the most water (high quantity) from a site in the shortest time possible (high velocity). Large, impervious paved areas and big pipes are typical of conveyancing.
The traditional system of conveyancing is highly effective in reducing stormwater nuisance and flooding on site, unless the pipes get blocked. But it merely transfers the problem to the other end of the pipe and ultimately upsets local water balance. Stormwater is carried rapidly with its suspended litter, oil, sediment and nutrients, and dumped into a receiving waterbody that then becomes flooded and temporarily polluted because all the stormwater arrives at one time.
Water sensitive urban design
Water sensitive urban design (WSUD) seeks to imitate the natural water balance on site before the land is built on. It slows stormwater runoff to gain natural filtration, on-site detention and infiltration. The water eventually reaches the river, lake or ocean but has been cleaned and filtered by the soil and used by plants before it gets there.
Water sensitive urban design slows stormwater runoff to gain natural filtration, on-site detention and infiltration.
The objective is to minimise impervious surfaces so that the least water flows off-site into the stormwater system. At the scale of the individual household, options such as permeable paving on driveways and footpaths, garden beds designed for infiltration (raingardens), lawns and vegetation, swales and soakwells can detain stormwater and increase percolation into the soil.
In some cases it may be advisable to place perforated pipes beneath infiltration areas to direct excess stormwater to the stormwater system. See ‘References and additional reading’ for publications on options and possible designs.
The improved aesthetics and comfort associated with more vegetation also improve habitat for native wildlife and make the area cooler in summer. It reduces the need for garden watering and decreases water bills. Also reduced are erosion and the downstream effects of stormwater pollution on nearby rivers, lakes or the ocean.
Stormwater Management in the Indian Context
Every year during the monsoons, developed cities like Chennai, Delhi and Mumbai are crippled by continuous floods. While changing the city infrastructure as a whole would be a complicated and expensive concept, stormwater management practices can reduce the burden to a great extent. This is the only sustainable way to take care of the flooding, along with the pollution.
India faces a long list of problems as a result of uncontrolled urbanization but stormwater and the associated pollution is one of the first steps towards a truly developed nation.