Estadísticas de Recuento de Personas

Until 2001 the four local councils and the Trust have always refused to allow rainwater tanks to be considered as part of an OSD facility because of the probability that the tank would be full at the start of a major rainfall event. In recent years it has been argued, however, that a rainwater tank will not always be full at the start of a storm if its water is used inside and outside the dwelling for non-potable purposes – toilet flushing, laundry, hot water and garden watering.

As part of the detailed analyses of the cumulative impacts on peak discharges undertaken in recent years the interaction of rainwater tanks and OSD tanks was investigated. Analyses were undertaken of both rainwater tanks with dedicated airspace and dynamic airspace ie. the airspace in a rainwater tank that varies in response to rainfall and water demands (internal and/or external). This has led to a revision of the proportion of a rainwater tank which can be counted as part of the OSD storage. This

“credit” reduces the SSRL and SSRT for an OSD system.

Dedicated Airspace

Based on the analysis of the results reported by Cardno Willing, 2004 the following reductions in the SSR values may be allowed subject to Council approval⁴:

• 50% of the dedicated airspace can be credited against the required extended detention volume (SSRL);

• 100% of the dedicated airspace can be credited against the required overall detention volume (SSRT);

subject to:

• a maximum dedicated airspace credit no greater than the ratio of the area of roof discharging to the rainwater tank to the lot area times the overall site storage volume that is required;

4 It should be noted that Baulkham Hills Shire Council, Blacktown City Council, Holroyd City Council and Parramatta City Council currently do not permit dedicated airspace. Under exceptional circumstances consult with the relevant Council.

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• the rainwater tank has a dedicated outlet to ensure that the dedicated airspace is recovered after a storm event and the maintenance schedule specifically requires checking and cleaning of the outlet;

• the PSD for the dedicated rainwater tank outlet is no greater than 40 L/s/ha;

• all outflows from the rainwater tank (outflows from the dedicated outlet and overflows from the rainwater tank) are discharged to the OSD storage.

Dynamic Airspace

Based on the analyses of the results of various rainwater tank simulations undertaken in late 2004 the following procedure is used to calculate the reduced SSR values that are allowed:

The rainwater tank dynamic airspace at the start of a storm is calculated using (refer Figure 5.2):

Dynamic Airspace (kL) = 8.7 x Nett Tank Vol (kL) ^1.05 x Roof Area (m²) ^-0.5 x Demand (kL/d) ^0.35 where Nett Tank Volume = Total Tank Volume - Dedicated Airspace - Top-Up Volume Daily demands can be estimated using the following average daily demands for Western Sydney, as reported by Coombes and Kuczera (2003) (refer pp 240).

Outdoor Indoor (Total) (kL/d)

(kL/d) No. of Occupants

1 2 3 4 5+

0.260 0.231 0.448 0.665 0.882 1.099

In the absence of detailed information on the proportion of various indoor uses for rainwater the following data can be used:

% of Indoor Demand

Toilets 20%

Laundry 25%

Hot Water 25%

The reduced SSR values due to dynamic rainwater tank airspace is calculated using:

SSRL = 300 - (1,950 x Dynamic Airspace (kL) ^2.10 x Roof Area (m²) ^-1.50) SSRT = 455 - (1,650 x Dynamic Airspace (kL) ^2.30 x Roof Area (m²) ^-1.50) subject to:

• the development being residential, or its water usage can be considered to approximate that of a residence;

• the design is in accordance with Sydney water requirements (visit the Sydney Water website for the current requirements); and

• all overflows from the rainwater tanks are directed to the OSD storage.

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4.2.9 Areas not directed to the OSD storages

Where possible, the drainage system should be designed to direct runoff from the entire site to the OSD system. Sometimes, because of ground levels, the receiving drainage system or because of other circumstances eg. retention of major trees, this will not be feasible.

In these cases up to 30% of the residual site area may be permitted to bypass the OSD systems, provided that as much as possible of the runoff from impervious site areas is drained to the OSD system and specific Council approval is obtained.

The storage volume is still calculated on the entire site area, while the SRD is adjusted downwards in accordance with the values given in the table below (see also Section 3.4.3 where the development affects only part of a site).

Residual Lot

Note: 100% of Roof Area assumed to be directed to the OSD storage

4.2.10 Overflow

Provision needs to be made when designing a storage for storms more severe than the design storm or for blockages in the system. With most storages it is relatively easy to provide a weir capable of passing the entire discharge from a very large storm event with only a few centimetres depth of water over the weir.

The following design method should be used to check the adequacy of overflow structures, such as weirs or spillways, and freeboards to finished floor levels. Overflow and weir flow calculations must be included in the detailed design information submitted to the consent authority:

• Assume the outlet is blocked and the storage full.

• Calculate the approximate maximum 100-year ARI discharge to the storage (a Rational Method estimate will be adequate);

• Calculate the maximum depth over the spillway/weir assuming the entire 100-year ARI discharge passes over the spillway/weir;

• Check the floor levels of any buildings upstream of the storage to ensure that these buildings are not inundated. (refer Section 4.2.6 for suitable freeboards); and

• Overflows should be directed to a flowpath through the development so that buildings are not inundated nor are flows concentrated on an adjoining property.

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