CockburnCoast - Western Australian Planning Commission

CockburnCoast
It is envisaged that the majority of stormwater from both roadways
and private property will be disposed via underground soaks. The
location of this infrastructure will be determined at subsequent
planning stages.
2.12.4.2 Water management recommendations
The following summarises the conclusions and management actions
recommended in the urban water management overview.
Groundwater
Groundwater on the site is generally deep and therefore is not a
constraint to development. However, the use of groundwater on the
site has its risks, primarily the risk of salt water incursion and the
historical contamination that has occurred from previous land uses
such as landfills and industrial sites. Therefore the use of
groundwater on the site is not recommended.
Stormwater
Due to the potential constraints on groundwater use in the structure
plan area, opportunities to utilise as much stormwater runoff as
possible should be encouraged.
Stormwater on the site is currently infiltrated through a series of
fenced sumps. These are unappealing and not considered
appropriate for high-density residential areas. Drainage swales and
basins can be incorporated into the east-west open space
corridors. The location of the corridors takes advantage of the
natural catchments, requiring minimal reworking over much of the
site to achieve a suitable gradient.
Based on the site geology much of the structure plan site is
considered suitable for on-site disposal of storm water by soakage
(soakwells and direct recharge via basins and passive community
open space and landscaped soakage areas). The east-west
corridors of green space can provide drainage opportunities as well
as flora and active open space interconnections.
The infiltration of stormwater on contaminated sites can cause
contaminants to enter the groundwater. This issue may be
addressed through site remediation and/or not infiltrating
stormwater on contaminated sites. This issue will need to be
addressed prior to the design of the new stormwater system.
Potable water use reduction
The development at Cockburn coast is of a medium to high density
residential type and most dwellings will have little or no garden. The
potable water use per house is likely to be significantly less than for
a conventional low density development.
Requiring home builders and developers to install water wise fittings
and appliances is likely to be the most cost-effective way to reduce
water use.
Potential sources for domestic ‘third pipe’ systems for toilet flushing
and washing machines include rainwater from roofs, the Port
Catherine injection bore, grey water and recycled wastewater.
Groundwater in the area may not be of an appropriate standard to
be used in the irrigation of public open space. If this is the case,
possible sources of water include the Port Catherine injection bore
and recycled wastewater.
2.12.4.3 Additional work required
The following is a list of some key work that requires consideration
following the conclusion of the structure plan. Irrespective of the
ultimate administrative vehicle that will manage the development of
the Cockburn coast, the following work is considered necessary
during the detailed planning phases for Cockburn coast:
• Given Department of Water requirements for
predevelopment groundwater monitoring, the multiple land
ownership and the probable complexity of groundwater
contamination issues, it is recommended that a districtlevel
groundwater monitoring program be initiated as soon
as is practical. While probably not eliminating the need
for lot-level groundwater investigations, the district
monitoring program will assist individual landowners
define their groundwater contamination issues and
therefore costs incurred may be recoverable by scheme
contributions;
• Review and survey of the existing drainage system;
• Seek the advice of the Department of Water and the
Department of Environment and Conservation with respect
to the infiltration of stormwater on and adjacent to
potentially contaminated sites in the area;
• Gain a broad spatial understanding of what sites will not
be remediated and/or do not meet other criteria for
infiltration and overlay the local structure plan to determine
areas suitable for infiltration and the stormwater system
design;
• Broad design work for water, wastewater and stormwater
infrastructure to service the new development, including
infrastructure locations;
• Undertake costing and feasibility studies for alternative
water supplies, including negotiations with regulators to
determine the preferred potable water management
strategies; and
• Conduct a water audit on existing and new commercial
premises to assess the options for alternative water
supplies on a case by case basis.
2.12.4.4 Manning Lake management
The location of Manning Lake within the structure plan area limits the
potential for development to have an adverse impact. The plan
proposes the alignment of Cockburn Coast Drive to be located on
the western side of the ridge, consistent with the MRS zoning for this
regional road. Other development proposed is also on the western
side of the ridge which has been identified as a surface water
divide. Direct impacts from development are therefore considered
to represent a minimal risk to Manning Lake. In the event that road
construction works to impact upon the eastern side of the ridge, it is
recommended that the road management plan includes the
consideration of sediment and runoff containment to limit the
potential risks to the Manning Lake environs.
Over-extraction of groundwater and the potential for saltwater
incursion represent the only apparent direct risk to Manning Lake
from the development of the Cockburn coast. Evidence exists that
suggests the salt water wedge extends as far inland as Manning
Lake. Impacts on the relationship between the salt water wedge
and lens of fresh water above the wedge could alter the salinity
levels in Manning Lake. Groundwater extraction is not
recommended in the Cockburn coast area without further
assessment and modelling of this issue.
district structure plan
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