Siphon weirs are fixed weirs. In the past, they are used as spillways in dams. They have a high specific discharge capacity.
When the water level of the reservoir rises to a certain level, the siphon starts. This leads to a discharge pressure in the pipe with full flow. This discharge pressure has a high discharge capacity which is larger than the discharge capacity for a free overfall. When the water level has fallen again, air is sucked into the siphon. This abruptly stops the flow of water.
The transparent siphon weir HM 163.36 has an air vent to allow a comparison of the function and discharge capacity of the siphon weir with and without venting. Additionally, the discharge of the activated siphon weir can be interrupted anytime by using the air vent.