Onsite stormwater detention permit

When a permit is required

Development within Merri-bek which may increase stormwater runoff usually requires an onsite stormwater detention system.

An onsite stormwater detention system will be included as a condition on a legal point of discharge drainage report.

About onsite stormwater detention systems

Impacts on stormwater

Usually redevelopment of a site results in an increase in the area covered by hard impervious surfaces, such as roofing and paving.

The increase in hard impervious surfaces significantly increases the volume of stormwater flow, which reduces seepage into the ground, impacts stormwater drains and other assets, and increases erosion of creek areas. It can also cause local flash flooding downstream of the site and has negative effects on fish and fauna.

Benefits of onsite stormwater detention

Onsite stormwater detention is the management of stormwater at a site. It is designed to temporarily retain stormwater on site which enables the rainwater falling on the site to be controlled and released at a slower rate than it would normally do. This limits the flow rate of rainwater into the Council drainage system.

Moreland City Council has adopted the use of onsite stormwater detention systems to prevent local flooding and decrease the peak flow rate into drains and waterways so as to better manage stormwater runoff from high density developments.

Types of detention systems

There are different types of onsite stormwater detention systems but all have the following components:

• Discharge control pit – this is located at the lowest point on the site. All flows leave the site through this pit. The pit contains an orifice at the bottom of a baffle wall located centrally within this pit. Refer to Council’s SD128 for details of this pit. The orifice is sized to limit the discharge of water from the site to the maximum permissible rate. Some designs may have an orifice opening with a rainwater tank being used as the detention and the remainder of the water to be used for re-use within the development. Refer to Council’s SD128 for details of the dual purpose detention/storage.
• Storage – this can be located underground by the use of pipes, or above ground through rainwater tanks. The storage fills up due to the orifice restricting the flow and then empties through the pit once the rain eases.
• Collection network – consists of gutters, channels, pipes and hard surface areas which are all collected and discharged to a pit.

Fee

All onsite stormwater detention approval applications incur a fee.

The fee amount depends on the type and size of development and is reviewed annually.

How to apply for an onsite stormwater detention system (OSD)

What you need

Before submitting an application

1. Your development plans must first be endorsed by Council's Statutory Planning Unit, before you can submit onsite stormwater detention system design plans. You are asked to enter the date plans were endorsed in the application.
2. Complete the 'Checklist and Information for Submission of Engineering Civil Plans’, which is included in the:

• Drainage design criteria for developments (PDF 1Mb)
• Drainage design criteria for developments (DOC 2Mb)

In your application

• One copy of full set of the drainage design plans drawn to scale.
• One copy of onsite detention system calculations, including breakdown of how the system achieves the minimum detention storage and maximum permissible site discharge.
• One copy of pump system design calculations that complies with Section (8) of AS/NZS 3500.3:2018 and ‘Appendix L’.
• One copy of endorsed Storm Rating Report that reflects the details of WSUD elements within drainage plans.
• One copy of Compliance Certificate from suitably qualified Engineer stating that the drainage and pump design complies with AS/NZS 3500.3:2018 and any other relevant standards and codes.
• One copy of Melbourne Water consent letter, if Melbourne Water drainage asset was nominated as the legal point of discharge on the legal point of discharge drainage report.
• Payment of fee

Apply online

1. Register with Council Online Services. As a registered user, you don't need to re-enter your personal information and can keep track of requests and applications on any device.
2. Apply for an Onsite Stormwater Detention System permit. You need to sign in to Council Online Services to apply.
3. As part of the online application process, the fee is paid online by Visa or MasterCard.

Apply for an Onsite Stormwater Detention System permit

Apply by post or in person

1. Complete the Application for approval – Onsite Stormwater Detention (OSD) Drainage Plans for Developments form (DOC 738Kb).
2. Attach any relevant and required information to support your OSD design to your application with the fee.
3. Post the application and fee to Engineering Services, Merri-bek City Council, Locked Bag 10, Merri-bek VIC 3058, or deliver in person to the Merri-bek Civic Centre, 90 Bell Street, Coburg.

What happens after you apply

Once Council receives your complete application, including drainage and onsite stormwater detention system design plans, computations and approvals from other authorities, if required, and fee, Council sends you an acknowledgment letter or message confirming receipt of your application.

Council may take up to 30 days to approve, specify alterations or refuse the application under Section 15 of the Subdivision Act 1988 and Regulation 30 of the Subdivision (Procedures) Regulations 2011.

When approved, you need to refer to the Council Drainage Standard Drawings (PDF 2Mb), along with the approval letter and approved (stamped) plans from Council.

Once approved, the applicant is responsible for communicating and coordinating all proposed works with the applicable private property owner(s) located directly adjacent to the nature strip area or other Council road reserve area, so that compatible land uses are achieved by all parties, such as future driveway crossovers, ingress-egress, and proposed utility easements.