Investigation into the effects of stormwater attenuation within the lower third of a catchment on downstream waterways

Karaka, Jayden Manawa (2019) Investigation into the effects of stormwater attenuation within the lower third of a catchment on downstream waterways. [USQ Project]

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The planning and design guidelines of urban stormwater systems within Queensland is undertaken in accordance with The Queensland Urban Drainage Manual (QUDM). QUDM states that when designing detention systems to control downstream flooding, it is important to consider the issue of coincident flood peaks. This issue occurs when the peak runoff from a development is delayed or extended and causes this runoff to arrive at a critical location at the same time as flows arriving from the upper catchment. The simple ‘one third rule’ was developed in response to this issue. This rule stated that stormwater detention systems may not be appropriate within the lower third of a catchment because it could increase the peak runoff by aligning the peak discharge from different areas of the catchment.

This dissertation will examine the validity of the ‘one third rule’ by modelling a hypothetical undeveloped catchment planned for a residential subdivision that is located in Toowoomba, Queensland. The catchment was split into three equal sub-catchments and designed with an appropriate stormwater drainage system. Detention basins will then be designed for each sub-catchment to ensure that peak pre-developed stormwater discharge does not exceed the peak developed stormwater discharge. Detention basins will then be removed from each catchment in various configurations to assess the impact on the downstream waterway.

To examine the different detention basin scenarios throughout the catchment stormwater quantity and quality software packages were researched. This research into the software packages determined the most appropriate stormwater quantity package to be DRAINS and the most appropriate stormwater quality package to be MUSIC. DRAINS is arguably the industry leading hydrological and hydraulic software package and MUSIC is the industry leading stormwater quality software package.

Different scenarios were modelled in DRAINS and MUSIC. The scenarios including providing detention tanks with:

1. Pre-Development (no detention tanks);
2. Post-Development (no detention tanks);
3. All 3 thirds of the catchment;
4. Upper catchment only;
5. Upper and middle catchments;
6. Upper and lower catchments;
7. Middle catchment only; 8. Middle and lower catchments;
9. Lower catchment only;

The research demonstrated that the most appropriate detention scenario is to provide all sub catchments with detention basins. The research also confirmed that in some circumstance’s detention within the lower third of a catchment does not have to be provided to ensure a non-worsening a peak discharge to downstream waterways. The one third rule does not aggravate peak flows or coincide with upstream peak flows to a degree that creates a worsening effect.

There is little to no benefit in some circumstances to provide detention within the lower third of a catchment. This has the potential of saving developers time and money by reducing design costs and construction costs.

The research showed the MUSIC has the potential to conceptually model detention scenarios. Although further research needs to be undertaken to confirm if it is possible as this research was not able to definitively determine this aspect. MUSIC does not have the capability of designing drainage systems and therefore it is very limited as to what functions it is able to perform.

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Item Type: USQ Project
Item Status: Live Archive
Faculty/School / Institute/Centre: Historic - Faculty of Health, Engineering and Sciences - School of Civil Engineering and Surveying (1 Jul 2013 - 31 Dec 2021)
Supervisors: Gillies, Malcolm
Qualification: Bachelor of Engineering (Civil)
Date Deposited: 16 Aug 2021 23:29
Last Modified: 26 Jun 2023 22:38

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