OSD Approvals at a Discounted Rate
On-site Stormwater Detention systems are used as part of the stormwater drainage system to reduce the impacts of site development on receiving drains and waterways. At the onset of a storm, stormwater will commence to discharge from the site. The earliest flow will be from areas nearest the discharge point but will increase significantly as water from the furthest points of the property reaches the discharge point. As the intensity of the storm approaches its peak, the discharge rate will increase relatively sharply to a maximum. The discharge rate will subside after the peak has passed. If the highest discharge rate for the catchment’s critical storm event will exceed the permissible site discharge (PSD) rate, as indicated by the Responsible Authority, an on-site stormwater detention (OSD) system will have to be installed. The system will temporarily store any excess flow and release it at a controlled lower rate over a longer period of time.
A detention system has two components – a device to control the flow rate of the discharge from the site and storage for the excess stormwater. The storage can be provided via underground or aboveground pipes or tanks. Ground level storage is provided by allowing water to pond in a broad but shallow depression in a paved area such as a driveway.
Background
Changes in land use for urban development generally increase stormwater run-off. Building hard surfaces such as pavements, roofs and site drainage increase the volume and speed of stormwater run-off. They also reduce the capacity of stormwater to soak into the ground. As the capacity of existing stormwater drainage systems is limited, you may need to provide on-site stormwater detention (OSD) systems to ensure stormwater run-off doesn’t increase flooding downstream of your development. On-site stormwater detention temporarily stores stormwater run-off. This means the run-off rate and volume can be controlled to ensure the receiving system is not overloaded. The required size of storage and rate of discharge vary depending on the location, size and subsequent impact of your development.
Designs of the OSD system must be prepared by an appropriately qualified professional. The design must follow and meet:
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the requirements in the guideline
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the requirements in the Onsite stormwater detention policy
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current best practice/principles outlined in the Australian rainfall and run-off guideline.
The design must also be certified by a chartered civil engineer stating that it has been prepared accordingly.
Design specifications
The total flow from the site from OSD storage and free run-off cannot exceed the calculated PSD. The development’s drainage system must be able to transport all run-off from a 100-year ARI storm event to the OSD storage. Surface inlets must have suitable grates to stop debris from entering the OSD system. This minimises blockages that may slow the steady discharge of run-off and potentially cause flooding.
The outlet shall be designed such that the flow going into the Council drainage system is limited to the Permissible Site Discharge when the storage is at the OSD storage volume (Vs). Acceptable flow control outlet types are either Orifice System or Multi Cell.
Service Provided by Northwater Consultants
Our company has been legally authorized by the State Government to attend and liaise with the Council and to receive necessary approvals from the Council on behalf of our client. We have well competent professional staff to attend to any issues related to the OSD approval process and are pleased to offer you a discounted rate (15-20 %) until we overcome the present situation prevailing in the country.