For more info on research and model projects that have tips and ideas for your Water Sensitive Urban Design principles, visit CRC WSC website.
Water sensitive development involves simple design and management practices that take advantage of natural site features and minimize negative impacts on the water cycle. It is part of the movement towards more sustainable solutions that protect the environment and maximize use of natural resources.
Urbanization has resulted in a significant increase in paved and impervious surfaces. Paved surfaces have significant adverse impacts on the water cycle. They contribute to increased stormwater run off and the need for expensive drainage infrastructure. They also reduce infiltration to the subsoil preventing water from re-entering the system
to recharge subterranean aquifers or from being re-used on site by vegetation.
These impacts can be reduced by:
- Limiting the area of paved surfaces.
- Directing stormwater run off from paved surfaces to landscaped areas, garden and lawns rather than the street
- Using porous paving/alternative surface systems.
- Employing swales and berms.
The potential for stormwater harvesting
According to the National Water Commission:
Whilst rainwater tanks offer a supplementary water supply option for a growing number of households around Australia, larger-scale stormwater harvesting and reuse can supplement the normal water supply for urban communities.
The volume of stormwater runoff from a city is often greater than its entire combined household water use, and has the potential to provide water for irrigating parks, gardens and ovals, as well as replenishing groundwater supplies. Use of stormwater for these purposes can help to take pressure off the potable (drinking) water supply.
Many projects around Australia are demonstrating the value of regional-scale stormwater harvesting and reuse. For
example, the Waterproofing Northern Adelaide project is collecting and cleansing stormwater in urban wetlands and aquifers, and in Western Australia stormwater is replenishing the Cottesloe aquifer.
Porous paving / alternative surfaces
Porous pavement is an alternative to conventional impermeable pavements with many stormwater management benefits.
These surfaces allow stormwater to be filtered by a coarse sub-base and allow infiltration to the underlying soil.
A number of products/systems are available including:
- Special permeable asphalts or stabilized gravels/aggregates.
- Concrete grid pavements.
- Modular paving units.
- A combination of rigid plastic cells and gravel.
- Widely spaced impermeable pavers interspersed with planting, pebbles or gravel.
These products can be used in conjunction with underground stormwater tanks which allow for stormwater infiltration or re-use. Modules are designed for maximum stability under high weight bearing load situations.
Studies show that oils and greases are subject to microbial decontamination in porous paving. The addition of sand filters and retention trenches with geotextile fabric lining further aids in decontaminating stormwater.
Infiltration capacity of porous paving in heavier soils can be improved by:
- Providing a retention trench below the sub-base.
- Providing an overflow to the street drainage system or other stormwater management measure.
- Limiting the run off area contributing stormwater to the porous paving surface.?
Porous paving must be carefully designed in areas with:
- High water table levels.
- Soil salinity.
- Clay soils that collapse in contact with water.
- Rock with little or no permeability.
Note: porous paving should not be constructed on slopes greater than 5% unless an engineering design is completed to
assess the impact of the paving system on downstream environments and the stability of surrounding areas.
Swales and berms
Swales are shallow, low depressions in the ground designed to encourage the accumulation of rain and let it infiltrate into the soil. Swales ideally are tree lined or vegetated and store water for the immediate landscape as well as help cleanse the water as it percolates down. Swales can be installed separately or as part of a larger water rain catchment system.
Berms are raised beds or mounds that can be used to direct water to swales. Ideally berms and swales should be designed into the landscape where there is any noticeable slope to slow and capture run off.
Swales can be grassed, gravel or rocked and designed to slow and retain the flow of run off. Soil in the swale should not be compacted or sealed but should be loose to encourage absorption.
Swales lined with native plants are an extremely cost-effective, and often overlooked low-tech water conservation technique.
Urban design incorporating ‘rain gardens’ in more formal contexts than swales and berms is useful for aiding rainwater infiltration and purifying stormwater run off in cities.
The simple principle of installing garden beds below street and footpath levels is an effective means of entrapping water flow and useful for heavy rainfall events, particularly where space is limited. There are issues with safety with sudden drops so designers should be careful to minimise any risk to the public.
Excerpt from an article by landscape Architect Matt Huxtable on Permeable Surfaces, published in the Summer/Autumn edition of LIAWA’s shape magazine:
‘The use of stabilised gravel is becoming increasingly popular in Western Australia, with its use in the eastern states particularly established. A good example is the Australian Garden in Cranbourne Victoria, designed by Taylor Cullity Lethlean Landscape Architects.
Stabilised gravels often have less maintenance compared to other pathway materials such as compacted saw dust and
crushed limestone paths, as they are more resilient to traffic and erosion.
There are a number of processes labelled as stabilised gravel.
One process involves mixing a fine dust of crushed stone (fines) with a binding agent and water. The product mix is then applied over a prepared sub base, screeded and compacted. The result is a hard stand that allows water penetration and accommodates light vehicle and pedestrian traffic. Cement is often used as an additive but other options are coming onto the market such as an organic material with the base ingredient a by product of the processing of timber into pulp. As a cement substitute it provides an alternative that is environmentally friendly
and allows significantly more water permeability. A number of gravel fines have been used in Perth including split pea gravel, summerstone & blue metal which provide a variety of design options.
Another process involves the installation of lightweight plastic cells that interconnect and combine to form a rigid structure. Loose gravel around 5-7mm is then filled in to the matrix. The result is a reinforced gravel system that allows for both vehicular and pedestrian traffic as well as full water penetration.
The importance of directing, cleaning and harvesting stormwater as well as substituting water demanding ground covers, is making the design of our landscapes more challenging but provides opportunities for innovation and creativity whilst adhering to best practice in sustainable design. I think it’s an exciting time for the landscape
professional to demonstrate how technology and beauty can be integrated effectively in both residential and urban landscapes.’
‘Rainwater Harvesting for Drylands and Beyond’ by Brad Landcaster is a useful reference for WSUD. Part of a three volume series, the second volume presents detailed information on ‘water harvesting earthworks’.
The Grove Precinct
The Grove Precinct Library project makes extensive use of stabilized gravel fines in its landscaping. An advantage of
this material is that planting can be incorporated directly into these areas.
This landmark project is a great example of WSUD principles including stormwater and rainwater harvesting
and wastewater re-use. For more
information, visit www.thegroveprecinct.com.