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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 from entering the underground drainage system and causing blockages.
- Site drainage must be designed in accordance with 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 a 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
- 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 childproof locking system for the surface grate where the depth of the below ground storage facility exceeds 1,200 mm
- 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 200 mm in areas where vehicles will be parked or otherwise no greater than 300 mm
- 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 tank's total volume can count toward the overall site storage when the tank is connected to the dwelling for reuse.
The tank's active storage volume 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 lot's boundaries, 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 an 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
| Parameter | Nomenclature (Unit) | Guidance |
| Orifice diameter | d (millimetres) | Minimum diameter is 50 mm |
| Height difference | H (metres) | Distance from the top water level to the centre of the 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 an approved equivalent must be used for outlet control when the required orifice diameter is less than 50 mm.
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 450 mm.
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 a 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
| Ref | Requirement | Comments |
| 1.1 | OSD storage volume equals or exceeds the required storage volume | Storage = m3 |
| 1.2 | Outlet orifice controls the rate of discharge to the permissible site discharge (PSD) when OSD storage is full (maximum system head) | Outlet Q = litres/sec |
| 1.3 | The top level of the 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 |
| 1.4 | Storage volume is apportioned between tanks and the below-ground system according to relative catchment sizes | |
| 1.5 | Below-ground storage system pipes are not located beneath a habitable building | |
| 1.6 | Orifice pit outlet invert level is above the invert level of the property connection to the legal point of discharge (LPD) | |
| 1.7 | 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) | |
| 1.8 | The finished floor level of dwelling(s) is an absolute minimum of 150mm above the surcharge water level | |
| 1.9 | 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
| Ref | Requirement |
| 2.1 | Pits are the appropriate size for depth to invert: • ≤ 600 mm 450 x 450 mm • ≤ 900 mm 600 x 600 mm • ≤ 1200 mm 600 x 900 mm • ≥ 1200 mm 900 x 900 mm |
| 2.2 | Pits deeper than 900mm are fitted with step irons |
| 2.3 | Pipes have an adequate grade: • ≤ 150 mm 1:100 min. • ≤ 225 mm 1:200 min. • ≤ 300 mm 1:250 min. • ≥ 375 mm 1:300 min. |
| 2.4 | Internal drainage connected to the legal point of stormwater discharge (LPD) via a 100 mm diameter pipe (residential) |
| 2.5 | Internal drainage connected to the legal point of stormwater discharge (LPD) via a 225 mm diameter pipe (commercial/industrial) |
3. Drainage within the driveway pavement
| Ref | Requirement |
| 3.1 | Pits in the driveway pavement are an appropriate class for the expected level of vehicle traffic. • Passenger vehicles: Class C • Heavy vehicles: Class D |
| 3.2 | Pipes beneath the driveway pavement must have a minimum of 100 mm cover from the bottom of the pavement to the top of the pipe. |
4. Plan details
| Ref | Requirement |
| 4.1 | Plans are legible and to a scale of generally no less than 1:200 at A3 or 1:100 at A1 |
| 4.2 | Site boundaries, natural surface levels and proposed finished surface levels (spot levels or contours) are shown on the plan |
| 4.3 | Location and size of all existing Council assets are shown on the plan |
| 4.4 | Location and finished floor levels of all existing and proposed buildings are shown on the plan |
| 4.5 | Pits are numbered and listed in a pit schedule |
| 4.6 | Pipes are labelled with diameter and gradient |
5. Proposed Council drainage assets
| Ref | Requirement |
| 5.1 | The proposed design includes future City of Casey drainage assets |
| 5.2 | All new Council assets reference the appropriate City of Casey or MPA Standard Drawing |
| 5.3 | New pits have a minimum 1 metre offset from vehicles' crossovers |
| 5.4 | Long section plan plotted at a scale no smaller than 1:500H and 1:50V |
| 5.5 | 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 |