9Structural Controls - Department of Water - The Western Australian ...

Figure 9. Extended detention basin construction in
Stage 2. (Photograph: TME 2003.)
but where they were laid in low permeability or clayey soils then some modification to the flow patterns
may occur. In these instances, impermeable clay plugs were installed in the service trenches to prevent
longitudinal flow along the trenches. The spacing and location of these plugs were set depending on the soil
conditions encountered in each trench, and were assessed as the services were being laid on-site. The clay
plugs were required to extend to the depth of the low permeability material that was removed and replaced
with sand backfill for services installation.
Maintenance:
Ongoing maintenance is essential for most BMPs to continue to manage the quantity of flows and to
effectively remove contaminants from stormwater. The stormwater management system proposed in this
development will be readily maintainable by the Shire in conjunction with their routine maintenance
practices. This stormwater system will require cleaning out of the soakwells and sweeping of all roads at
least once per year. Annual inspection of the extended detention basin, flow dissipation structure, soakwells
and detention areas will also be required and any cleaning or repairs be undertaken as necessary.
Results/achievements
When Boronia Ridge was developed in 2002, it was unique in WA for pioneering the integration of
an at-source infiltration system with a piped drainage system to convey larger events. The stormwater
management system has performed well during its four years of operation. The experience gained in this
stage of development has been extended to the design of the final stages that are due to be constructed in
2007. The success of this system has led to greater acceptance of similar stormwater management systems
in subsequent developments by the Shire. Contingency measures implemented in the Boronia Ridge Stage
2 development, such as a piped system to accommodate overflow from greater than 5 year ARI events,
provided reassurance to the Shire regarding their concerns about systems that were fully reliant upon
infiltration on-site.
Good vegetation regrowth has been observed and there are no noticeable signs that the hydrology of the
area and its associated vegetation have been impacted.
Challenges and opportunities for improvement
Similar projects in the future could be improved by designing a below-ground infiltration device that is cost
efficient, but takes up less of the road verge than required for the installation of circular soakwells. Linear
infiltration trenches may be more suitable where land space is a constraint.
Overflows onto private property have also been identified as an issue. While this has not caused a major
problem or any property damage, residents who are not familiar with WSUD have voiced concerns about
36 Stormwater Management Manual for Western Australia: Structural Controls
Figure 10. Flow dissipation structure at outlet of
sub-catchment. (Photograph: TME 2003.)