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Urban drainage

The Perth metropolitan area lies on the Swan Coastal Plain, a relatively flat sedimentary zone located between the Darling Scarp to the east and the Indian Ocean to the west. This zone superficially consists of mostly infertile sandy soils with limited nutrient holding capacity and a shallow groundwater table expressed as ephemeral wetlands.

Many wetlands have been cleared and drained for urban and agricultural purposes, with fertilisers and other chemicals applied to increase soil productivity. These pollutants, along with those contributed by other catchment sources, readily leach through the soil into groundwater, entering the drainage system through groundwater discharge and overland flow where they are transported into the river system.

This is compounded by the construction of Perth’s drainage network to convey floodwater and lower the shallow groundwater table through open drainage channels, piping and compensation basins. Without natural buffers in place, this drainage system readily conveys excessive nutrients and other pollutants to the rivers.

Under natural conditions most rain falling on a catchment soaks into the soil, tops up aquifers, is used by vegetation, or travels over or under the ground towards rivers, lakes, reservoirs or the ocean. Urban development creates impervious surfaces such as roofs, roads and pavements. This means rainfall no longer infiltrates where it falls. Large volumes of water must be artificially drained away.

Many urban drainage lines and sumps were once natural streams and wetlands, but these have been highly modified to cope with increased runoff caused by urban development. 

In rural areas of the Swan Canning Catchment, modified creek lines often still exist, however surrounding land and fringing vegetation is often degraded causing increased runoff, nutrients and sediment loads to be delivered to the rivers.

The urban drainage system includes:

  • open drainage channels - mostly unlined, linear, trapezoidal shaped and excavated into the groundwater table so they flow all year round and are very efficient at conveying stormwater runoff to the receiving environment
  • closed pipes and drainage structures - usually constructed of concrete, also very efficient at conveying stormwater runoff to the receiving environment with little water quality improvement other than gross pollutant removal
  • compensating basins - sumps in a drainage system that allow high stormwater flows to be partially retained and infiltrated in the catchment ensuring that open drains and pipes do not flood
  • Best Management Practice stormwater treatment measures - including biofilters, wetlands, swales and living streams which improve water quality, habitat and amenity

More recently, a greater focus on Water Sensitive Urban Design (WSUD) has incorporated water cycle management in the urban environment so environmental and social values are enhanced. This approach allows smaller annual rainfall events to be retained in the catchment and uses vegetation and soil to filter stormwater, improving water quality but also enhancing habitat and amenity.

The Department of Water’s Stormwater Management Manual for Western Australia, developed in partnership with the Swan River Trust (now Parks and Wildlife) and other stakeholders, advocates the use of WSUD where appropriate, providing high-level policies, planning principles and practical on-ground best practice advice to manage stormwater.

Parks and Wildlife, through its Drainage and Nutrient Intervention Program, is working with partner organisations to remove nutrients from drains and tributaries in catchments that contribute high nutrient loads to the Swan Canning river system.

Acid sulphate soils

Acid sulphate soils occur naturally in Western Australia, including the Swan Canning Catchment, and are harmless when left in a waterlogged, undisturbed environment.

However, when exposed to air through drainage or excavation, iron sulphides in the soil react with oxygen and water to produce iron compounds and sulfuric acid. This acid can release other substances, including heavy metals, from the soil and into the surrounding environment, groundwater and waterways. Once mobilised, they can include kill vegetation and fish, damage fisheries and aquaculture industries, harm tourist resources such as fishing grounds, swimming areas and recreational waterways, and damage infrastructure including corroding concrete and steel pipes, building foundations and bridges

Activities with the potential to disturb acid sulphate soils must be managed carefully to avoid serious environmental harm. Proposed developments often require acid sulphate soils testing, and where appropriate management plans to prevent potential damage to the rivers or surrounding environment.

The Department of Environment Regulation has produced a series of fact sheets and guidelines to assist with the identification, investigation and management of acid sulphate soils in Western Australia.