The Schoharie Reservoir

Stream Control
Under Construction

Content and photos to come from GHS Quarterly winter 2012m v. 14.4 

Coffer Dams

The Board of Water Supply used 2 coffer dams to keep the construction site free of water. One was upstream of the construction site and the other was below the construction site.

The two coffer dams were connected by two 9-foot-diameter pipes. The water flowing downstream would be dammed above the construction site, could only flow downstream through the two pipes, and then would be let out below the second coffer dam. 

Coffer dams in practice

• The up-stream coffer dam was begun at end of 1920 spring
• this is one-half of the upper cofferdam and shows its timber crib made of 12 to 30 inch diameter timbers
• once this half was concreted, the other portion could be built and concreted
• a 20’ gap was left in the middle
• the lower cofferdam would then be constructed, and
• steel pipes placed into position and concreted in
• Finally the center gap on both bridges could be filled with a step-down opening.
• This system was operational by the fall of 1920
• This picture from 1921 shows how the coffer dam arrangement was integrated into the entire construction site, with pipes in place taking water from the upper cofferdam to the lower through the dam’s foundation
• However, everything doesn’t necessarily go according to plan. Floods overtopped the coffer-dams and flooded the work area 6 times in the 2.5 years of operation.
• These incidents caused a total loss of 104 hours, an equivalent of 4.25 days out of 912 days workedH