A secondary outlet also known as overflow arrangement must be provided to cater the rarer storms than OSD facilities were designed for, or in the event of a blockage anywhere in the site drainage system.
The most commonly used arrangement for an above-ground storage is a broad-crested weir which can be designed to pass the entire overflow discharge with only a few centimeters depth of water over the weir. Example of OSD secondary outlet design as broad crested weir is used in Car park storage to minimise the potential water damage to parked vehicles.
The overflow weir must be from durable, non-erodible materials such as concrete, pavers or brickwork to ensure the discharge capacity of the overflow.
Access chamber or manhole is commonly design as secondary outlet for below ground storage. If this not practicable, an overflow pipe is provided at the top of the storage to discharge to a safe point down stream.
Editor :- Dilah – 13 Feb 14