Effects of on-site detention systems on urban drainage catchments

Cuell, Jonathan (2016) Effects of on-site detention systems on urban drainage catchments. [USQ Project]

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Abstract

Urban development can have a detrimental effect on the natural water cycle. This can primarily be attributed to the changes urban development can have on stormwater drainage catchment features. These changes result in an increase in rainfall runoff volume and discharge rate, and stormwater management controls are used to try to decrease these impacts.

On-site detention (OSD) is a stormwater drainage control technique used to mitigate the impact of development or re-development on individual sites (Phillips, et al., 2015) (DEWS, 2013). Within parts of south east Queensland local governments require OSD systems to be designed using a site based analysis, without consideration of the entire drainage catchment. The purpose of this project is to investigate the catchment wide effects of implementing OSD systems, that have been designed using a site based approach, using a Direct Rainfall Method (DRM) two-dimensional (2D) hydraulic model.

The research was completed within an urban drainage catchment of Coomera, QLD in the City of Gold Coast (CoGC) region. Within this region OSD systems are typically developed using a site based one-dimensional (1D) modelling approach. Existing research and studies have shown that OSD systems need to be implemented throughout at least 20% of a catchment to have any positive impact, and that these systems have the potential to increase flooding problems if they are not designed properly particularly during events outside of the critical duration.

The catchment selected contained 6 OSD systems, which have all been built within the last 10 years. Each of these systems were designed using a site based 1D analysis, without consideration of any external or downstream catchments, as per the requirements of the CoGC. As part of this study, 2D DRM modelling was completed for pre-developed, post-developed and post-developed with OSD scenarios using TUFLOW. This was compared to a traditional lumped hydrograph TUFLOW method and a 1D XP-RAFTS model.

The results show that the OSD systems did not mitigate post development flows to within the pre-developed level for any of the modelled events, this was due to the existing storage within the pre-developed catchment attenuating flows. Peak flows were slightly reduced for most events, although some of the low average recurrence interval (ARI) events saw an increase in peak discharges as a result of the various OSD systems and the timing of their discharges.

Although peak flows were only slightly reduced, the OSD systems were effective at reducing local flood depths, with the 100-year ARI critical design storm event achieving a reduction of 50 mm across roads within the catchment.

As a result of implementing OSD systems throughout the catchment stormwater discharges and velocities were locally increased around the OSD outlets, which also resulted in significant increase in hazard. At other locations throughout the catchment, depths, velocity and hazard were reduced. It is important to note that it is common for OSD systems within the CoGC region to be designed using a site based analysis, and they are typically positioned at the lowest point of a site, which is often adjacent a road (unlike the subject catchment). The modelling showed increases of flooding hazard (depth velocity product) of up to 0.05 during the 100 year ARI critical event. Slight increases in depth velocity product have a large impact on the categorisation of hazard, and with large increases in hazard around the OSD outlets, particularly in large ARI events, it should be considered by local authorities and designers how best to manage this.

This research has highlighted that different modelling approaches (1D and 2D) produce vastly differing results when analysing or designing detention systems. This confirms the need for engineers and practitioners to remain aware of the constraints, strengths and weaknesses of each approach, and additionally confirms the need for further research into the differences between these techniques.

This research has shown that implementing OSD using a site based analysis is not always an effective approach. It has been shown that the catchment wide effects of multiple OSD systems can be unpredictable and hard to calculate and that a site based approach to stormwater management can cause varying (sometimes negative) impacts throughout an entire catchment. This has further highlighted the need for local government policy reform to allow for more accurate engineering techniques and to move away from these site based approaches.

Engineers and practitioners have multiple tools and techniques at their disposal, to constrain them to a site based approach has been shown to be ineffective, especially when so much data is readily available to accurately and cheaply analyse an entire drainage catchment.


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Item Type: USQ Project
Item Status: Live Archive
Additional Information: Bachelor of Engineering (Honours) Major Civil Engineering project
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Civil Engineering and Surveying
Supervisors: Brodie, Ian
Date Deposited: 20 Jul 2017 01:22
Last Modified: 20 Jul 2017 01:22
Uncontrolled Keywords: urban development; on-site detention systems; drainage catchments
Fields of Research : 09 Engineering > 0905 Civil Engineering > 090509 Water Resources Engineering
URI: http://eprints.usq.edu.au/id/eprint/31391

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