South East Queensland has undergone unprecedented levels of population growth over the last five years placing high levels of pressure on land owners and developers to release lots suitable for housing. Within the western corridor between Brisbane and Ipswich, many vegetated pockets of land are being subdivided into
urban developments, some in excess of 100 lots. Developments such as these increase the stormwater runoff volumes and peak quantities due to the inclusion of
impervious surfaces such as roads and roofs. These new urban catchments, if unmitigated, also detrimentally affect water quality and the health of streams and rivers due to the higher volumes of pollutants entering the waterways.
Bio-retention basins, which are a relatively new stormwater management concept, have been a popular inclusion to new developments to mitigate stormwater pollution
and excess stormwater quantities. These basins are commonly modelled using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC) program
developed by the eWater CRC.
The functionality of these bio-retention basins are a key part of South East Queensland’s waterway health and designers, assessors and engineers need to be aware that there are five key parameters which, through the results of a sensitivity analysis have been shown to greatly affect bio-retention system performance.
Bio-retention systems are an effective stormwater treatment system and while the MUSIC program does have some shortfalls, as a whole the program is the most
practical conceptual design package to enable effective development progression within time and cost constraints for South East Queensland’s expansion needs.
The bio-retention design requires designers to understand the programs sensitivity to five key parameters and ensure that these parameters accurately reflect the site
constraints in order for the model to provide an accurate assessment of the likely stormwater treatment of such basins.
The five key design considerations outlined within this report are:
1. The bio-retention filter media hydraulic conductivity, k.
2. Soil Seepage into in-situ soils
3. The filter media depth
4. The basin surface area and proportion of filter area
5. The extended detention depth
The analysis of sensitivity testing concluded that the hydraulic conductivity (k) of the filter media can greatly influence the basin’s treatment efficiency and if poorly managed can lead to excessive sediment build-up, clogging and ultimately basin failure. Soil seepage plays a large role in removing nitrogen and phosphorous within
the model and can in circumstances with high seepage rates (360mm/hr +) can lead to a 94% increase in nitrogen removal. Seepage into the in-situ soils is difficult to
gauge without soil testing and it is recommended that wherever possible, tests be undertaken to assess soil seepage.
The basin area, filter and extended detention depths also greatly affect basin performance and there are legislative, safety and physical constraints which often dictate these parameters. Nevertheless careful design can incorporate these constraints to allow and achieve a basin that effectively treats stormwater.
The report also explores the effect of recent decreasing rainfall patterns and the effect the lack of rainfall has on the MUSIC modelling package. Most local authorities
require the MUSIC program to be modelled using ten years of data from 1980-1990. This process, due to the larger rainfall volumes generates larger, less concentrated
volumes of pollutants which can not be treated as effectively treated as the smaller, more concentrated volumes experienced in modern weather patterns. Modelling catchments using more recent rainfall data shows a great improvement in theoretical treatment performance but raises a more complicated dilemma, what will future rainfall patterns be like?