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Detention basins temporarily store flood water run-off during flood events and release it through an outlet structure (drainage pipes) in a controlled manner to reduce peak flows of water downstream.
The detention basins at Garnet Lehmann and Ballin Drive parks are designed to a one in 100-year flood event (a flood event with a 1% chance of occurring in any given year). They also provide flood mitigation benefits for flood events exceeding one in a 100-year events.
The detention basins on East Creek will help to reduce major flood peaks downstream around the inner city and where East Creek joins West Creek. The combined detention basins at Ballin Drive and Garnet Lehmann parks provide several key benefits:
The project has included earthworks, culvert installation and landscaping for the 40ML detention basin, construction of a new pedestrian bridge and extensive landscaping in the project area.
While the detention basin, in conjunction with other East Creek projects, will provide significant flood mitigation benefits for the city, it is the enhanced landscape and improved parkland space that is the highlight of the project.
The project has also included associated waterway rehabilitation works, riparian habitat areas and the large-scale landscaping which will support long-term biodiversity.
The site has remained fenced since works were completed to allow the plantings and turf to become established.
More than 310 mature trees have been planted along with more than 20,000 shrubs and more than 53,000 ground covers. Additional unsheltered tree ferns will be planted at the downstream end of the site near the construction fence in September. The tree ferns have been nurtured at Council’s plant nursery and will be planted when conditions are favourable. A swampy area on the Amanda Drive side of the park has also been enhanced as part of the works. The area, populated with tree ferns and long grass, is a frog habitat. A coloured concrete garden edge and mulching will help tidy up that section.
Council would like to thank nearby residents and the wider community for their patience and co-operation during the construction phase.
The detention basin was constructed as part of the largest flood recovery program the region has seen. The project was funded through the Natural Disaster Relief and Recovery Arrangements, which includes a joint State and Commonwealth contribution of $20 million with Council contributing $5 million.
Work to upgrade the Spring and Ramsay streets detention basin in Toowoomba has been completed. The upgrade, which included improved landscaping, is part of our flood mitigation works.
The project involved installation of a large box culvert under Spring Street to improve the detention basin function as well as the construction of new headwalls and a low-flow pipe in the basin area to improve the drainage function. The appearance of the basin area has been significantly improved with upgraded landscaping including the planting of additional native trees and shrubs to complement the existing exotic species. A new shelter has been installed in the park area as well as a water bubbler. Additional indented car parks have been installed on Ramsay Street and Spring Street.
The landscaping also included extensive topsoil and turf added to the site.
The project was funded through the Natural Disaster Relief and Recovery Arrangements (NDRRA), which includes a joint State and Commonwealth contribution of $20 million and $5 million from Council for a number of flood mitigation projects in the Gowrie Creek Catchment.
Council will manage tree removal sensitively and with minimal impact to the community. However, the basins cannot be constructed without removing a number of existing mature trees.
Council has listened to community feedback and changed the designs to allow a significant number of trees to beretained. The number of mature trees removed has been reduced to 289 in Garnet Lehmann Park and 87 trees in Ballin Drive Park.
Council has committed to replanting a minimum of two trees in the East Creek corridor for each tree removed as part of the project.
The replanting will be a mix of mature trees, shade tree species and shrubs to attract birds and other wildlife. Trees that cannot be replanted in the immediate basin area will be replanted in nearby catchment locations.
Toowoomba Regional Council's position on the priority use of topsoil removed for the construction of the detention basins has not changed.
The top soil removed during construction will be stockpiled on site for re-use where suitable in the parks. Surplus spoil excavated from the site will be taken to another location which is yet to be determined. However, the Bridge Street quarry is one of the locations being considered.
If additional quality top soil is required for either project, it will be imported in as necessary.
Council engaged independent specialists, Red Leaf Projects, to assess the existing fauna and flora at Garnet Lehmann Park and Ballin Drive Park and nearby areas and to identify any possible impacts from construction.
Red Leaf Projects was also asked to provide recommendations about minimising impacts during construction works and has prepared a Species Management Plan to mitigate any construction impacts.
What will the basins look like when completed?
When completed, the basins will look and operate as a park and can be used for normal recreational activities such as dog off-leash walking, picnics and bicycle riding. Detention basin areas will be turfed and fully landscaped.
A sediment forebay is proposed for the upstream end of the Ballin Drive detention basin. Large quantities of sediment are not anticipated in the proposed Garnet Lehmann detention basin following a major flood event.
Maintenance would normally be required in parks along East Creek and for most infrastructure such as road culverts and stormwater outfalls following a major flood event.
Safety is Council's number one priority. Feedback from meetings with emergency services included increasing the size of the pipes at the basin outlets, as well as signage in the park and education programs addressing the dangers of flood waters. There will also be safety rails at the top of the culverts.
Council has prepared a Failure Impact Assessment in accordance with the Department of Energy and Water Supply (DEWS) "Guidelines for Failure Impact Assessment for Water Dams" which confirms that there is no population at risk at the completion of the Garnet Lehmann Park and Ballin Drive detention basin projects. The Failure Impact Assessment was lodged with DEWS in September 2014 for review and comment. DEWS has now completed its review and a summary of the Failure Impact Assessment can be downloaded from below.
Notice accepting failure impact assessment for East Creek Detention Basins
East Creek detention basins failure impact assessment
Drawings below for Garnet Lehmann Park and Ballin Drive Park detention basins are associated with the Waterway Barrier Works (Fisheries) Approvals issued by State Government Department of State Development, Infrastructure & Planning (DSDIP).
Read the Garnet Lehmann Park Waterway Barrier Fisheries Decision Notice DSDIP
Garnet Lehman Park Basin - layout plan
Garnet Lehmann Park Basin - typical sections (sheet 1 of 3)
Garnet Lehmann Park Basin - typical sections (sheet 2 of 3)
Garnet Lehmann Park Basin - typical sections (sheet 3 of 3)
Read the Ballin Drive Park Waterway Barrier Fisheries Decision Notice DSDIP
Ballin Drive Park detention basin - layout plan (sheet 1 of 3)
Ballin Drive Park detention basin - layout plan (sheet 2 of 3)
Ballin Drive Park detention basin - layout plan (sheet 3 of 3)
Ballin Drive Park detention basin - typical sections (sheet 4 of 6)
