McLeod, Julia (2017) Performance of water sensitive urban design bioretention installations on the Gold Coast. [USQ Project]
|
Text (Project)
McLeod_J_Trzcinski_Redacted.pdf Download (35MB) | Preview |
Abstract
Bioretention systems are a Water Sensitive Urban Design technique used to manage and treat stormwater runoff and protect urban infrastructure and aquatic ecosystems. They involve a basin where stormwater collects and is filtered through dense vegetation and a filtration layer, reducing sediments and contaminants and slowly releasing runoff into natural waterways. When used appropriately, bioretention systems provide effective treatment of stormwater quality. Poor outcomes can still result however due to design, construction and maintenance practices.
This research project investigated existing bioretention systems on the Gold Coast to discover trends in design, construction and maintenance that were affecting their performance. The study aimed to discover whether these practices were ensuring that design objectives and performance outcomes were being achieved.
Twenty-seven bioretention systems on the Gold Coast were identified from City of Gold Coast assets and included in the project. The sites were chosen to be spatially distributed and to range in size, type and characteristics. Design documentation was obtained where possible and catchment areas defined for each system. Field testing included taking measurements, inspecting inflow and outflow regimes, noting vegetation coverage, type and diversity and inspection and testing of the filter media. Hydraulic conductivity testing of the filter media was undertaken at eighteen of these systems. Field data was collected in GIS software and statistical analysis performed using Microsoft Excel. Several sites were also modelled using MUSIC software to determine whether water quality performance objectives were being met.
The general condition of the systems was fairly mixed, with roughly thirds split between good, average and poor condition. In terms of the filter media, problems at many sites included sediment layers, ponding and hydraulic conductivity outside of the recommended range. Vegetation cover was also found to be an issue, with only two sites having coverage greater than the 90% recommended for effective treatment and ten sites having less than 30% vegetation cover. Where local waterways were observed, they were found to be still in good condition at the majority of locations, despite the average or poor condition of many of the systems.
This research suggests that bioretention systems in the field may not be performing in the way they are intended and performance objectives and outcomes may not be adequately achieved throughout the design life of each system. Recommendations for design considerations included improved sediment and flow controls, allowing a contingency factor for losses in hydraulic conductivity, ensuring appropriate planting for site conditions, providing planting diversity and including trees and shrubs in filter media planting.
During construction works it was recommended that systems be protected with turf or geofabric capping and that planting be delayed until works are completed. The importance of erosion and sediment control plans and practices was also emphasised.
Examining the costs and benefits of maintenance and rectification of bioretention systems and prioritising maintenance budgets was recommended. A planting program was suggested for existing systems to introduce trees and shrubs into the filter media, providing benefits to the system and reducing maintenance costs. It was also suggested that a monitoring program be implemented in addition to the maintenance program to ensure that bioretention systems are meeting their performance objectives that maintenance is in accordance with the specification, to identify and address filter media and vegetation deterioration and to improve outcomes by identifying opportunities for improvements in design, construction and maintenance practices.
Finally, it was found that while bioretention systems may not be performing as designed, they are still providing a level of treatment to stormwater, localising issues and protecting waterways. If design, construction and maintenance practices can improve the performance of these systems, particularly in nutrient removal, then they will have greater potential to protect waterways.
|
Statistics for this ePrint Item |
Actions (login required)
 |
Archive Repository Staff Only |
