Stormwater Management for Residential Developments
Drainage Design
An application for a 'Legal point of Discharge' must be submitted to the Council as part of a planning application. Following the receipt of the application and payment of the fees, council will provide drainage information and applicable conditions. Commonly, only one nominated legal point of discharge can be used per site unless otherwise approved.
When a Legal Point of Stormwater Discharge Permit (Permit) is issued by the Council's, the Permit may contain specific conditions and/or requirements from Council, Melbourne Water and/or Vicroads that need to be included in the drainage design criteria. These will be confirmed at the stormwater drainage plans assessment process for compliance with the specified requirements set out in the Permit.
The application for drainage design assessments should be submitted with a drainage plan approval application form and will be accepted for processing when the application fee is paid. It is the responsibility of the Developer to ensure that drainage plans are approved by Council prior to commencement of onsite drainage construction work.
If approval of the submitted design is granted by the Council, it confirms and verifies that the proposed design solution is acceptable to Council at time of submission. The validation of the approved design will be necessary, and this must be in the form of a compliance statement from a civil engineer with current qualifications recognised by Engineers Australia and accompanied by as constructed drawings and computations.
It is the common requirement for all developments that the applicant must make provision for overland flows through the subject site in the event of the piped system blockage or a storm exceeding the piped system capacity. The design of the overland flow path must ensure that all storm flows in excess of 1 in 10 years intensity can discharge from the site without causing property damage and/or soil erosion.
All floor levels within the property adjacent to an overland flow path need to be a minimum of 300mm above the highest calculated water level for a 1 in 100 year ARI design flow.
On the sites where the land slopes to the front of the allotment, the stormwater shall discharge into the Council barrel drain/drainage pit within the road reserve. On allotments where the drain or a drainage pit do not extend past the front boundary of the site, it shall discharge to the kerb and channel via a kerb adapter approved by Council.
The minimum requirements include:
- The development has one outlet to the kerb and channel or barrel drain/drainage pit
- If more than one dwelling is proposed e.g. two dwellings on separate titles, with individual street frontages, than each of these buildings shall discharge to the kerb and channel or drain/drainage pit via individual outlets
- Minimum size of a stormwater drainage outlet for a single dwelling is to be no less than 100min diameter pipe, SN8 sewer quality UPVC, with an inspection opening placed at each change of direction
- The common stormwater drainage pipe from a two-dwelling or multi-unit development shall be no smaller than 150mm diameter
- AS/NZS 3500.3: 2015 Stormwater Drainage standard shall be applied to all internal drainage works
- Connections to the front of a property shall be located at least 6 metres from the side boundary to maintain clearance to future vehicle crossings
- If the property drains connect to a drain in an easement,than a junction pit must be provided at the point of connection
- All drainage pipes shall be laid on grades no flatter than 1 in 100 unless otherwise approved
Drainage Design Plans and Drafting Standards
All submitted plans of private drainage systems must comply with the following requirements:
- Plan to be drawn to scale at 1:100, 1:200 or 1:250.
- All drawings must be a minimum of A3 size with all text legible.
- Plans are to be submitted in hard copies (unless advised by Council otherwise).
- Plans to include the following information as a minimum:
- North point
- Layout Plan showing all dwelling and drainage alignments (existing as well as proposed)
- Pit numbers and pit schedule
- Pavement makeup (typically concrete or asphalt)
- Construction notes required by the Council
- Detail and notes for any detention systems, tanks, soakwell and pump systems
- Floor levels for all dwellings and garages that are consistent with approved development /architectural plans, where applicable
- Percentage of development site impervious area
- Engineers contact details
- Easements and council assets
- All other additional details as directed by Council
Determine Catchment Area, Imperviousness and Discharge Points
The area of all external impervious surfaces within the development site needs to be determined to enable the stormwater design to be developed.
This includes surfaces such as roofs, balconies, veranda's, pergolas, concreted and paved areas. Depending upon the type of roof construction, sections of the roofed areas may drain to different points of the development, and therefore may need to be separated into sub-roof areas. Sub-roof areas may be combined where the roof run-off will be diverted to a common rainwater tank.
If a swimming pool drains to sewer, it can be excluded from the site stormwater catchment area. If there is an impervious splash zone (up to 1 metre wide) around the pool perimeter - it can be excluded from the stormwater catchment area.
The legal point of discharge for the property should be established. If there are WSUD treatments within the development - all discharges from those will need to be conveyed to the legal point of discharge.
Run-off from balconies is typically more polluted compared to other roof run-off and as such should not be diverted to rainwater tanks. Balcony runoff should be treated prior to discharge from the site. Minimum perviousness objective is > 20% of the total site area (clause 55.03).
On-Site Stormwater Detention
When a site is developed it usually increases the percentage of impervious area adding more surfaces such as roofing and paving. These changes cause the increases in the flow rate and volume of storm water runoff and could cause local flash flooding downstream of the site.
In order to control the runoff discharge rates of individual development sites and mitigate the potential flooding issues the On-site Stormwater Detention can be utilised. Typical on-site stormwater detention system consists of the following components:
- a flow control device limiting the stormwater discharge to an acceptable rate and
- a storage system to detain the excess discharge on site until the downstream drainage system has the capacity to accept the inflows
The properly designed on-site stormwater detention systems shall be able to control the peak discharge rates to accommodate the capacity of the downstream drainage system. While the total volume of stormwater leaving the site is not reduced the temporal pattern is changes where this volume is released gradually and only when downstream drainage capacity becomes available.
The rate of stormwater discharge from a development site must be controlled to prevent any adverse impact on downstream properties for all storm events up to and including the 100-year ARI event. On the receipt of a planning application Council makes an assessment on the need for the on-site stormwater detention and any such requirements will be included as a condition on the planning permit issued by Council.
The property owner is to be responsible for the future operation, maintenance and replacement of the on-site stormwater detention system. Where on-site stormwater detention system is on a shared area for a multidwelling site then the Body Corporate/Property Manager shall be responsible for the future operation, maintenance and replacement of the on-site stormwater detention system.
The purpose of the Permissible Site Discharge (PSD) is to limit the site discharge to a pre-determined rate. The PSD is calculated as the runoff generated from the pre-developed site during a 20%2 AEP design storm event of 5 minute duration.
The Permissible Site Discharge can be calculated using the Rational Method Formula:
| PSD(ltr/s) | = | (C20 x I20⁵ x A)/3600 |
|---|---|---|
| where, | ||
| C20 | = | 20% AEP Runoff Coefficient |
| 0.95 x [fimp x 0.9 + (1-fimp) x 0.143] | ||
| fimp | = | fraction Impervious for Pre-Developed Site |
| I20⁵ | = | Rainfall Intensity for 20% AEP event of 5 minute duration (mm/hr) |
| A | = | Site Area (m²) |
Integrated Stormwater Management and WSUD
In relation to IWM and WSUD local planning policy clause 22.12 applies to applications for:
- New buildings
- Extensions to existing buildings where in the floor area is 50 square metres or greater
- A subdivision in a commercial zone
The policy states that all development applications must achieve the best practice water quality performance objectives set out in the Urban Stormwater Best Practice Environmental Management Guidelines. All new planning applications for development must include a Stormwater Management Assessment detailing the treatment of stormwater run-off from the proposed development site and addressing the objectives of Clause 22.12.
The SMA response submitted to Council must be a form of a report that includes the following information:
- Stormwater Summary Report with detail description of the stormwater treatment system proposed
- A site layout plan
- Sufficient detail for the stormwater treatment systems proposed
- Site management plan
| REQUIREMENT | DETAIL REQUIRED |
|---|---|
| A site layout plan showing the proposed stormwater treatment measures | Show location, area draining to a treatment measure, and the connection points, if any:
|
| A report outlining how the application achieves the objectives of the policy | Include an assessment from an industry accepted performance measurement tool such as STORM or MUSIC or equivalent |
| Design details, such as x-sections, to assess the technical effectivness of the proposed stormwater treatment measures | As appropriate |
| A site management plan detailing management process during construction | A statement outlining construction measures to prevent litter, sediments and pollution entering the stormwater system |
| A maintenance program that sets future operational and maintenance arrangements | A statement is required outlining operational and maintenance mesaures to check the effective operation of all systems |
Stormwater run-off WSUD treatment options
A range of WSUD systems can be used to treat stormwater run-off from urban developments. Selection of the WSUD treatment system/s shall be guided by the scale and layout of the proposed development.
Diverting roof run-off to a rainwater tank and using rainwater for toilet flushing and other internal uses represents one of the most effective options for achieving water quality treatment objectives for roof run-off.
Extracting water from rainwater tanks (e.g. for toilet flushing) diverts the rainwater and associated pollutants away from the downstream waterway.
Rainwater tanks
Run-off from roof areas can be diverted to rainwater tanks and used for toilet flushing, hot water service and laundry connections as well as for irrigating gardens and lawns, and car washing.
There are various types of rainwater tanks that be located either above or below ground. Above ground tanks are generally cheaper than below ground tanks. Run-off from roof areas can be discharged directly from down pipes into above ground rainwater tanks or conveyed via charged underground pipes. Where the majority of the roof drains to a single tank the charged pipes are recommended. Rainwater tanks will require a pump and mains water switch system, which switches to mains water supply when the water level in the rainwater tank is low.
Rainwater tanks can assist to reduce peak flow rates from a site, however complying with Clause 22.12 will not guarantee that the development will meet on-site detention requirements specified by Council's engineering department when applying for legal point of discharge. This must be considered separately and is typically applied for once a planning permit has been issued. Refer to Council's website for further details on Stormwater Discharge (e.g. stormwater discharge).
All rainwater tank's overflows are typically connected to the legal point of discharge. Tank overflows can also be discharged to If the development has other WSUD treatments, such as raingardens than the tanks overflows can be discharged to those devices before the overflow connection reaches the legal point of discharge.
The minimum acceptable rainwater tank size for water quality treatment is typically 2,000 L, provided the tank is connected to the internal toilets and harvests at least roof area of 50m². It may be beneficial to adopt a larger rainwater tank than the minimum size needed to meet stormwater quality standards to ensure maximum water harvesting. This can be achieved using Tankulator, an online rain harvesting calculator that enables the size of rainwater tanks to be matched to the roof catchment area and water demands.
In ground raingardens
In ground raingardens are specialised garden beds that treat stormwater runoff by infiltrating the water through a filter media. Stormwater that enters a raingarden is temporarily stored on the surface of the infiltration bed before passing through the filter media. In ground raingardens comprise of several media layers, including an upper filter media layer (loamy sand), a middle transition layer (coarse sand) and a lower drainage layer (gravel).
The infiltrated stormwater is collected at the base of the raingarden by a perforated pipe (underdrain) and the water is conveyed to the site's legal point of discharge. An overflow pipe protruding above the surface of the filter media conveys flows to the stormwater drainage system when the storage capacity above the raingarden is full.
In ground raingardens are extremely effective at removing suspended solids and nutrients from stormwater run-off, and provide onsite retention of stormwater run-off. They can be readily integrated with garden beds and planted with a wide range of plant species.
Note: The minimum acceptable size for an in ground raingarden is 1m² with a minimum width of 350mm.
Planter box raingardens
Planter box raingardens are 'mini' raingardens constructed in elevated garden beds. Planter box raingardens are structured in the same way as an in ground raingarden, with several filter layers and an underdrain connected to the stormwater drainage system.
Planter box raingardens are generally used where there is no garden space available to construct an in ground raingarden (i.e. paved areas or balconies), on flat sites to allow the faltered flows to gravity feed to the legal point of discharge or for a desired landscaping outcome.
Note: The minimum acceptable size for a planter box raingarden is 1m² with a minimum width of 350mm.
Porous and permeable pavements
Porous and permeable pavements allow stormwater run-off to infiltrate to the underlying soils rather than running off impervious surfaces into the stormwater drainage system. Porous pavements comprise of traditional masonry tiles that have a porous jointing material between the tiles. The jointing material generally comprises of sand or gravel, and allows the stormwater to pass through to the ground below.
Permeable pavements comprise of either tiles or solid pavements that have been designed to allow stormwater to pass through the tile or pavement surface to the ground below. Porous and permeable pavements are often used in lieu of concreted surfaces for paths and courtyard areas. The infiltration of stormwater into the underlying soils provides passive irrigation and helps to increase the volume of water infiltrated to the local groundwater table.
Note: Areas to be covered with permeable paving are assumed to be pervious and can be excluded from STORM calculations. Areas to be covered with permeable paving must be clearly indicated on the plans.
Compliance and tools
The size of the rainwater tank and/or WSUD treatment system required to meet best practice standards may be determined using the STORM Calculator developed by Melbourne Water.
This tool allows the designer to change the size of WSUD treatment systems via a number of iterations in order to develop a solution with a STORM score of 100%. The results of the STORM assessment must be submitted to Council as part of the WSUD Response to demonstrate compliance with the Clause 22.12 objectives. STORM is appropriate to use for development sites that are 1000m² or less.