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OSD design guidelines and checklist

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OSD design system elements

The key elements of an On-Site Stormwater Detention (OSD) system are:

  • a runoff collection system consisting of gutters, downpipes, pits and pipes
  • a runoff storage system
  • an outlet structure to control the rate of stormwater discharge from the runoff storage system to the Council drainage network

Each element is discussed further below.

Runoff collection system

The runoff collection system must collect runoff from all pervious and impervious areas and direct it to the runoff storage system.

Surface inlet pits must be fitted with a suitable grate to prevent litter entering the underground drainage system and causing blockages.

Site drainage must be designed in accordance with Australian Standard AS3500.3:2003 National Plumbing and Drainage Code – Stormwater Drainage.

The Relevant Building Surveyor for the development is responsible for approving the design of the site drainage system in accordance with Regulation 610 of the Building Regulations 2006.

Runoff storage system

The runoff storage system is required to temporarily store stormwater runoff during a storm. The runoff storage system must:

  • provide the storage volume stated in the planning permit
  • drain within 72 hours to ensure the storage volume is available for a subsequent storm event The runoff storage system may consist of the following structures (or combination of structures):
  • below ground tank(s) or pipe(s)
  • above ground storage in a grassed or landscaped area where the storage volume can be provided with minimal adjustment to ground levels
  • above ground tank(s) connected to the internal reuse system of a building on the development site
  • above ground tank(s) fitted with a controlled discharge orifice to maintain an active storage volume within the tank

Each storage system is discussed further below.

Below ground storage system

Below ground storage systems must:

  • be structurally designed to withstand all service loads and provide a service life of at least 50 years
  • be located outside the tree protection zone (TPZ) of any trees to be retained
  • have a minimum soil cover of 300mm in landscaped areas
  • be fitted with a child proof locking system for the surface grate where the depth of the below ground storage facility exceeds 1,200mm

Above ground storage system

Above ground storage systems must:

  • not be unsightly or hazardous
  • be totally impermeable except for infiltration into the ground where soil conditions allow and the system is located at least five metres from any building foundations
  • have a ponding depth no greater than 200mm in areas where vehicles will be parked or otherwise no greater than 300mm

Rainwater tanks

A rainwater storage tank can be credited toward the total volume of the runoff storage system when the tank is:

  • connected to the dwelling for regular uses such as toilet flushing and clothes washing, or
  • fitted with a controlled discharge orifice to maintain an active storage volume within the tank

One-third of the total volume of the tank can count toward overall site storage when the tank is connected to the dwelling for reuse.

The active storage volume of the tank can also count toward the overall site storage when the tank is fitted with a controlled discharge orifice.

Council may reduce the storage volume credited to the rainwater tank in proportion to the amount of roof area that drains to the tank.

Flow control outlet

The flow control outlet must be located within the boundaries of the lot between the storage system and the Council drainage system. The flow control outlet must not be located within a Council drainage easement.

The outlet must limit the rate of flow to the Council drainage system to the permissible site discharge (PSD) when the OSD storage is full (maximum head).

The outlet must be resistant to blockage and must include a trash grate located upstream of the outlet or sufficient screening on all inlets.

The flow control outlet must use either the SVC MC2 Multicell (or approved equivalent) or an orifice and baffle wall arrangement.

The orifice sizing calculation is detailed below.

Outlet control orifice sizing

The sizing of the outlet control orifice is based on the following parameters:

Table 1 – Outlet Control Orifice Sizing Parameters



Nomenclature (Unit)



Orifice diameter


d (millimetres)


Minimum diameter is 50mm


Height difference


H (metres)


Distance from top water level to centre of orifice

Permissible Site Discharge


PSD (litres/sec)


Specified by Council

The required orifice diameter is calculated as follows: d = 21.9 * √[ PSD / √(H) ] mm

The minimum orifice diameter is 50mm. An SVC MC2 Multicell unit or approved equivalent must be used for outlet control when the required orifice diameter is found to be less than 50mm.

Example orifice sizing calculation

The permissible site discharge for a unit development is 8 litres per second.

A 450mm diameter pipe is proposed beneath the driveway to provide runoff storage.

When the runoff storage is full the height difference between the top water level and the centre of the orifice will be 450mm.

Calculate the required orifice diameter to control the rate of discharge: d = 21.9 * √[ PSD / √(H) ] mm

d = 21.9 * √[ 8 / √(0.45) ]

d = 75.63 mm Adopt 75 mm orifice

Overland flow

The runoff storage system and flow control outlet structure must be designed to safely convey overflows to an adequate Council overland flow path or drainage system. This requirement may be waived if Council deems that no suitable overland flow path or drainage system is available and the OSD system has been designed to detain the 100 year ARI storm.

OSD design checklist

1. OSD system








OSD storage volume equals or exceeds required storage volume

Storage =m3



Outlet orifice controls rate of discharge to permissible site discharge (PSD) when OSD storage is full (maximum system head)

Outlet Q =litres/sec



Top level of orifice pit baffle wall is set at or above the maximum (design) water level needed to achieve the required storage volume

Maximum water level =m AHD Top of baffle wall level =m AHD



Storage volume is apportioned between tanks and below ground system according to relative catchment sizes



Below ground storage system pipes are not located beneath a habitable building



Orifice pit outlet invert level is above the invert level of the property connection to the legal point of discharge (LPD)



Safe surcharge location provided if orifice outlet blocked or OSD storage capacity exceeded (desirable but not mandatory if detaining 100 year ARI storm on-site)



Finished floor level of dwelling(s) is an absolute minimum of 150mm above surcharge water level



Finished surface levels of grated pits upstream of the orifice pit are above the top level of the baffle wall or the finished surface level of the nominated surcharge pit


2. General drainage requirements








Pits are appropriate size for depth to invert:

≤600mm 450x450mm

≤900mm 600x600mm

≤1200mm 600x900mm

>1200mm 900x900mm



Pits deeper than 900mm are fitted with step irons



Pipes have adequate grade:

≤150mm 1:100 min.

≤225mm 1:200 min.

≤300mm 1:250 min.

≥375mm 1:300 min.



Internal drainage connected to the legal point of stormwater discharge (LPD) via 100mm diameter pipe (residential)



Internal drainage connected to the legal point of stormwater discharge (LPD) via 225mm diameter pipe (commercial/industrial)


3. Drainage within driveway pavement








Pits in the driveway pavement are appropriate class for the expected level of vehicle traffic

Passenger vehicles: Class C Heavy vehicles: Class D



Pipes beneath driveway pavement have absolute minimum 100mm cover from bottom of pavement to top of pipe


4. Plan details








Plans are legible and to a scale of generally no less than 1:200 at A3 or 1:100 at A1



Site boundaries, natural surface levels and proposed finished surface levels (spot levels or contours) are shown on the plan



Location and size of all existing Council assets shown on the plan



Location and finished floor levels of all existing and proposed buildings are shown on the plan



Pits are numbered and listed in a pit schedule



Pipes are labelled with diameter and gradient


5. Proposed Council drainage assets








The proposed design includes future City of Casey drainage assets



All new Council assets reference the appropriate City of Casey or MPA Standard Drawing



New pits have a minimum 1 metre offset from vehicles crossovers



Long section plan plotted at scale no smaller than 1:500H and 1:50V



Long section includes:

Pipe diameter, material and class Pipe gradients

Actual discharge and pipe capacity Flow velocity

Pipe length Invert levels Depth to invert

Finished surface levels

Existing surface levels


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