Siphoning May Not Always Be Possible

Message: Site constraints may eliminate siphons as a viable option

The outlet works of this Montana dam was in disrepair. The outlet works consist of two cylinder gates. One cylinder gate had experienced failure of both operating/lift stems and had slammed shut earlier during operational discharges. It was damaged and no longer operable. The mandatory instream flow of 21 cubic feet per second in the downstream creek needed to be maintained during the dewatering, inspection, and repair of the outlet works gates. The elevation difference between the dam crest and the lowest reservoir level ruled out siphoning.
Take Aways from the project: Site constraints would not allow a siphon to function and left the operator with pumping as their only option. Two different pump rental companies offered different pumping plans and related costs. Due to in‐stream flow requirements, the pumping plan was chosed that ensured continuous flow without requiring shut‐down to move suction lines due to fluctuating water levels. It was also the less expensive option. Six 8-inch submersible pumps, two diesel generators, HDPE piping, valves and fittings, and all mobilization, assembly, and demobilization cost $70,000 per week.

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

Reservoir elevation = 2,867 feet; Dam crest = 2,875 feet
Discharge pipes running down dam embankment
Submersible pumps and floatation
Diesel Generators
Pump discharge line

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: 4744
Revision Date: 12/02/2022