Siphons

Many older dams were not constructed with a means of draining the reservoir. Lowering the reservoir may be needed for temporary construction, for emergency response, or when regular releases aren't needed. Siphons can be a low-cost means of providing a reservoir outlet if one does not exist. The key operational parameters are: (1) the required hydraulic lift cannot exceed the effective local atmospheric pressure adjusted for vapor pressure and frictional losses; (2) the discharge point of the siphon must be lower in elevation than the body of water to be siphoned; (3) the pipe or hose used for the siphon must be designed to operate at less than atmospheric pressures; (4) siphon outlets should only be used when there are no other feasible options; and (5) siphon outlets are difficult to prime.

Siphon Stories

• Siphons when regular releases aren't needed
• Siphoning May Not Always Be Possible
• Low Pressure During Siphoning Can Collapse Pipe
• Pump... or Siphon?
• Siphons can pose Reservoir Level Control Challenges.

Siphon as an Outlet Replacement

4 Key Questions that must have a Yes answer:

1. Is the dam small enough and at a low enough elevation so that the use of a siphon is physically possible?
2. Will abandonment of the original outlet works not impact the reservoir's ability to pass the required inflow design storm (dictated by regulatory agencies)?
3. Is there an alternate means to passively control reservoir level to prevent overfilling?
4. Can the siphon reasonably be used for emergency drawdown if needed (in combination with pumps in some cases)?

References

• Simple Steps to Siphoning
• Guidelines for Use of Pumps and Siphons for Emergency Reservoir Drawdown
• Drawdown System Selection Flow Chart