4.3.17. Inverted Siphons
Siphons, as distinct from inverted siphons, are rarely used in stormwater infrastructure and are not acceptable.
Inverted siphon systems require specific approval by the council and shall only be considered when other alternatives have been exhausted.
Inverted siphons may sometimes be necessary to pass major obstacles such as large immoveable services or other underground structures. Generally inverted siphons shall be avoided, and other options shall be considered first. Any design of an inverted siphon is to be submitted to the council for consideration and approval. Inverted siphons are potentially high maintenance devices as they are points where debris and silt can build up and block the flow of stormwater. If an inverted siphon is approved for use, the maximum slopes on the downward and upward legs should be 45 degrees and 22.5 degrees respectively to help the movement of debris and solid materials.
For marginal situations, specific head-loss calculations may be required to ensure flow rates can be achieved and to ensure water does not backflow out of the siphon and discharge out of the inlet (e.g. catchpit).
Where a direct connection to a stormwater network is made by way of an inverted siphon system, a gravity chamber shall be installed. The chamber shall be completed prior to connection to the public system to prevent backflow into the private system.
Kerb discharge from inverted siphons will only be permitted as a last resort.