7. CASE STUDIES
7.2 Development Type: - Proposed Unit Development
24 Catchment Avenue, Parramatta
A multi-storey unit development is proposed on an existing site containing 3 single residential dwellings. The existing site grades from the northern boundary to the southern boundary and has a total site area of 2,580 m2.
The proposed structure comprises of 15 two bedroom units spread over 3 levels and has a basement which is below the natural ground level containing 15 garages and three visitor parking spaces. The building has approximately been centralised on the property ensuring compliance with Council’s setback requirements. Due to the gradient of the existing site the floor level at the rear building line will be lower than the existing ground level thereby requiring retaining structures along the northern and eastern boundaries.
The overall site layout is given in Figure 7.6. The site and development data is:
Site area = 0.258 ha
Total roof area = 0.135 ha
Area of site draining to OSD Storage = 0.239 ha No. of dwellings on the site = 15
Consideration was also given to installing a rainwater tank for each dwelling with rainwater to be used for outdoor, toilet flushing and laundry in each 3 person household.
The data on the rainwater tanks includes:
% of roof draining to each rainwater tank = 65%
Volume of each rainwater tank = 3 kL Volume of dedicated airspace = 0.6 kL Daily demand on each rainwater tank = 0.56 kL/day A Solution
The existence of a large landscaped setback area located at the south-eastern frontage of the site and a check of the required detention volume reveals that it is possible to provide an above ground detention basin with relatively minor excavation. As such the construction of a water tight retaining wall along the southern and eastern boundary at the required levels will permit water to pond in the landscape area which will not be utilised significantly by the future owners of the property. Maintenance of the detention area will also be straight forward and minor.
All downpipes and pipes from surface inlet pits will be drained directly to the extended detention storage and primary outlet. This will cause water to begin ponding within the above ground basin. Upon the volume of storage of the extended detention being achieved at RL 36.75 m, water will cascade into the adjoining secondary outlet pit. Finally at RL 36.90 m water will cascade over the weir located along the southern detention basin wall. The outlet pipes of each outlet pit are connected to the existing Council grated gully pit within Catchment Avenue.
The cross section details of the On-site Detention tank are given in Figures 7.7 and 7.8.
A volume calculation check is given in Figure 7.9.
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Other Considerations
Upstream Overland Flows
As the property grades from the northern boundary to the southern boundary it is likely that the same grade continues beyond the northern boundary and hence surface run-off from the upstream properties also currently flows through the site.
A check from Council’s catchment plans and site visit reveals that only the area of the properties located north of the site are generating the overland flows that drain through the site.
As such the necessary calculations are carried out to determine the amount of overland flows and from these a channel size is able to be determined. The channel being provided along the northern boundary of the property conveys these overland flows through the property.
The overland flow channel is drained to a pit that drains directly to the kerb and gutter, thereby bypassing the On Site Detention system provided on the site.
Basement Drainage
The area of the driveway entrance will be draining directly to the basement level of the proposed development and will require a pump out drainage system to be designed in accordance with the requirements of the local Council and to the relevant Australian standards. Water pumped from the basement will be drained directly to the kerb and gutter and the area contributing to the basement pump out system is considered as bypassing the On Site Detention system.
Results from On Site Detention calculations
The site and development data was entered into the HED On-Site Detention Calculation Sheet (see Figure 7.10). The HED calculation sheet gave the following results:
Extended Detention Required = 77.4 m3 Orifice diameter for extended detention = 64 mm Maximum Water level of Extended Detention = R.L. 36.75 Extended Detention Volume Provided = 80.3 m3 Overall Detention Required = 117.4 m3 Orifice diameter for detention = 117 mm Maximum Water level of Detention = R.L. 36.90 Detention Volume Provided = 129.5 m3
The reduction in storage requirements that would be achieved by installing 3 kL rainwater tanks on each dwelling was also calculated (see Figure 7.11).
The HED calculation sheet gave the following results:
Extended Detention Required = 66.3 m3 Orifice diameter for extended detention = 64 mm Maximum Water level of Extended Detention = R.L. 36.75 Overall Detention Required = 101.7 m3 Orifice diameter for detention = 117 mm Maximum Water level of Detention = R.L. 36.90
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Figure 7.6 Example 2 – Overall Site Layout
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Figure 7.7 Example 2 – Concept Outlet Detail (Section A)
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Figure 7.8 Example 2 – Concept Outlet Detail (Sections B and C)
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Figure 7.9 Example 2 – Concept On-Site Detention Volume Check
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Figure 7.10 Example 2 - OSD Calculation Sheet without Rainwater Tanks Figure 7.10 Example 2 - OSD Calculation Sheet without Rainwater Tanks
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Figure 7.11 Example 2 - OSD Calculation Sheet with Rainwater Tanks Figure 7.11 Example 2 - OSD Calculation Sheet with Rainwater Tanks
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