Low Pressure During Siphoning Can Collapse Pipe

Message: Make sure the siphon pipe's pressure rating is adequate

Through water storage in Jackson Lake, Jackson Lake Dam supplies irrigation water to a local ranch. During the summer of 1999, the gate stem sheared off leaving the gate in the closed position. The spillway did have capacity to pass storm events so danger to the dam embankment due to overtopping was not a concern. The damages to the gate stem, however, jeopardized delivery of irrigation water. In order to maintain irrigation releases, a siphon was installed over the top of the dam using 80 psi PVC pipe. Soon after the siphon was primed, the siphon collapsed likely due to pipe material that could not withstand the vacuum pressures in the pipe. Another siphon was installed using stronger pipe material (120 psi PVC pipe). A vertical slide gate was installed at the downstream end of the siphon in the stilling basin. During the season, vandals opened the slide gate and the siphon was lost. The owner worked to reprime the siphon, but it collapsed midway down the downstream face of the dam.

Take Aways from the project: Reviewing the elevation of the crest of the dam at 5,181, and considering that 1’ of lift is lost for every 1,000 feet of elevation above mean sea level, this siphon could only theoretically lift water a total of 13.6 feet. The result is that the siphon would not work below a reservoir surface elevation of 5166.2 in best case conditions without risk of water vaporization. As the siphon drew the water surface level down, the vacuum pressures became greater and greater. It appears that the 80 psi PVC pipe was not strong enough to withstand the initial vacuum pressures and that the 120 psi PVC pipe only could withstand vacuum pressures to a certain point. It is speculated that the water surface elevation was too low when the owner tried to restart the siphon and vacuum pressures collapsed the pipe.

From Guidelines for Use of Pumps and Siphons for Emergency Reservoir Drawdown, Morrison Maierle, Inc. 2012

Reservoir elevation = 5,170 feet; Dam crest = 5,181 feet; Dam Height = 33 feet; Storage Capacity = 490 ac-ft
Dam Embankment
Collapsed PVC Pipe

Note: The content on this page was originally created as part of DamOutletWorks.org (DOWL, 2018). It has subsequently been updated and reformatted as part of the Dam Safety Toolbox.
Revision ID: 3200
Revision Date: 09/15/2022