Reuse of stormwater in cities reduces demand for outside water supplies.
Much of the rain in cities falls onto impermeable surfaces such as roads, is collected into a stormwater drainage system, and discharged into the sea or rivers along with rubbish and other pollutants it picks up along the way.
Some of this water can be diverted into settling ponds in wetlands, to trickle down into natural underground aquifers, as much of it would have if the entire city's surface was permeable. It can be reused later as borewater.
Litter traps can be used to remove some of the rubbish that is washed from roads into the stormwater drainage system.
Rainwater that falls on roofs can be collected in nearby tanks and used for drinking and cooking, to irrigate gardens, and to flush toilets.
Stormwater. Your Home Technical Manual. Australian Greenhouse Office
Stormwater re-use leads to water savings and reduced environmental impact.
Water Sensitive Urban Design
Water Sensitive Urban Design provides for temporary retention of stormwater on site. WSUD seeks to approximate the natural water balance on-site prior to the land being built on. It achieves this by slowing the water velocity of stormwater run-off, providing natural filtration, storage 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.
The objective is to minimise impervious surfaces so that the least amount of water flows off-site into the stormwater system. At the scale of the individual housing lot, WSUD uses permeable paving, infiltration trenches, soakwells, lawn, garden areas and swales to detain the water and allow it to percolate into the soil. The slope of the block and the depth and type of soil determine the application of each of these practices.
Water Sensitive Urban Design provides the improved aesthetics and comfort associated with more vegetation. Habitat for native wildlife is improved and the area is cooler in summer. It reduces the need for garden watering and decreases water bills. Erosion and the downstream effects of stormwater pollution on nearby rivers, lakes or ocean are reduced.
Stormwater - Problems
The Value of Water: Inquiry into Australias management of urban water. Australian Parliament. 2002.12
Executive Summary PDF
Existing stormwater systems were designed to prevent flooding and the prevailing view was that rainwater falling on cities should be transported as quickly as possible into the receiving waters of streams, rivers and the sea. The water generally moves at speed through our stormwater systems and brings with it high levels of pollution from urban activities. Pollutants include litter and general detritus, sewer overflows, vehicle emissions, animal faeces, garden fertilisers, silt and vegetation.
While natural ecosystems can absorb some pollutants, metropolitan centres produce waste streams that are too concentrated and which move too quickly via concrete drains and pipes to be assimilated by receiving waters. The results are algal blooms, fish kills, closed beaches and shrinking fisheries, all of which have direct effects on the health, prosperity and amenity of urban areas. River systems and enclosed waters such as Moreton Bay and the Great Barrier Reef are particularly vulnerable to effluent and stormwater pollution.
There are also hidden pollutants in stormwater derived from pharmaceutical products such as endocrine disruptors, as well as the chemicals and antibiotics in agricultural runoff.
Water Sensitive Urban Design
Techniques are now available that are vastly more efficient and which sustainably reintegrate stormwater flows into urban water cycles, making effective use of this water as a resource. Developments such as the Lynbrook Estate in Victoria demonstrate the techniques of water sensitive urban design and also show that the associated costs of construction and maintenance are comparable with conventional methods. At the same time, they offer surrounding communities increased levels of utility and aesthetics.
Unfortunately the application of water sensitive urban design principles remains the exception rather than the rule, even in new developments, where the implementation costs are much lower.
Some developers adopt features of WSUD that are ostensibly environmentally friendly and attractive to buyers, but which play no role in water efficient design. It is common for example, to see natural, functioning wetlands be drained in housing estates, and replaced with small ornamental lakes that have no filtering or purifying role in stormwater management.
However, much of Australias stormwater infrastructure will reach the end of its useful life over the coming twenty years and this provides Australia with a rare opportunity to replace this infrastructure with more ecologically sustainable systems.
Stormwater Management - Australia
Introduction to Urban Stormwater Management in Australia. Environment Australia. 2002
Stormwater - Pollution
Any rain that falls on roofs or collects on paved areas like driveways, roads or footpaths is called stormwater. The stormwater drainage system is separate from the sewage system.
As stormwater travels over land, it picks up all kinds of chemicals and sediments that are not naturally found in our waterways. Some of these are poisonous, even in small amounts. Others, such as nutrients, are not poisonous but may be produced in such great quantities that natural systems simply can’t cope. They can eventually cause toxic algal blooms and other pollution problems in our waterways.
Stormwater pollution includes:
- Organic material such as leaves, grass clippings and sediment
- Chemicals such as detergents, coolant, oil, grease, fertiliser and paint
- Litter such as plastic bags and cigarette butts.
Stormwater - Pollution Control
Stormwater. Roads. ACT.
Gross Pollutant Traps
Gross Pollutant Traps act as the first water pollution control point in the stormwater network. They prevent the flow of coarse sediment, trash and debris into engineered waterways, water quality control ponds, and urban lakes.
Minor GPTs are below-ground and are located within or at the downstream end of a pipe system. Major GPTs are above-ground and are generally located within or at the downstream end of an engineered waterway. GPTs are composed of a concrete lined 'wet basin' with a trash rack on the downstream side to intercept floating trash and debris. The concrete construction of the wet basin allows for easy trash removal and dredging of settled sediments.
Water Quality Control Ponds and Wetlands
Water quality control ponds and wetlands hold the water for a sufficient amount of time for various biological and physical processes to act to improve water quality.
Large aquatic weeds (macrophytes) with microbial root mats, filter the water and absorb nutrients and other pollutants from the water. Significantly the reeds also trap oil.
The stillness of pond waters allows fine particles to fall out of the water and settle on the bottom of the pond. Decomposition and grazing of organic matter by microbes, insects, fish and birds also occurs in this zone. Under the right conditions periodic algal growth will occur in ponds, this will trap dissolved excess nutrients and allow them to enter the food chain or settle on the bottom of the pond.
Lynbrook Estate. SaveWater
Water sensitive residential design more closely matches the natural water balance by slowing down the runoff and allowing more rainwater to be absorbed back into the soil. Drainage water is filtered through grass swales. Gravel filled trenches in the streets and is then treated in wetlands before entering the stormwater system and flowing into our natural environment.
Wetlands - Lynbrook Estate. Melbourne Water
Grass swales and underground gravel trench system (0.6m .deep) collects, infiltrates and conveys road/roof runoff via a 150mm diameter perforated PVC pipe from local streets to the main boulevard.
Main Boulevard acts as a bio-retention system with underground gravel filled trench and a 150mm perforated pipe which allows infiltration and conveyance.
Catchment runoff goes through wetland system then is discharged into an ornamental lake.