Dam Safety
The New Exchequer Dam is an earthen embankment or
rock-fill dam as can be seen in the image above from Google Earth. While there is a thin concrete surface on the
reservoir side of the dam, the downstream side of the dam is composed of small
to medium sized rocks packed together.
Embankment dams are perfectly safe in normal operation. The problem is that embankment dams must NEVER be overtopped.
In current operation, there is 12 ft of headroom between the spillway height and the height of the dam. Raising the level of the reservoir by 10 feet would reducing this headroom to 2 ft which would seem to greatly increase the probability of overtopping of the dam. This must never be allowed to happen with an earthen or embankment dam such as the New Exchequer Dam as dam failure can follow quickly thereafter.
The following is from the Wikipedia entry for embankment dams:
Overtopping or
overflow of an embankment dam outside of its spillways will cause
disastrous flooding through the eventual failure of the dam. In the failure
process the sustained hydraulic force and pressure caused by an overtopping
surface runoff; immediately erodes the dam's material structure as it flows
over the top of the dam. Even a small, sustained overtopping surface flow can
remove thousands of tons of overburden soil from the mass of the dam within
hours. The removal of this mass unbalances the forces that stabilize the dam
against its impoundment. The mass of water still impounded behind the dam
presses against the lighter mass of the embankment, (made lighter by surface
erosion). As the mass of the dam gets lighter, the impoundment begins to move
the whole structure. The embankment, having almost no elastic strength, begins
to break into separate pieces, naturally allowing the impounded water to flow
between them eroding and removing more material as it goes. In the final stages
of failure the remaining pieces of the embankment offer almost no resistance to
the flow of the water; as they continue to fracture into smaller and smaller
sections of earth and/or rock. The overtopped earth embankment dam
disintegrates into a thick mud soup of earth, rocks and water.
Therefore safety
requirements for the spillway are high, requiring the spillway to be capable of
containing a maximum flood stage. Specifying a spillway able to contain a five
hundred year flood is common.
Introduction Page – HR869 & HR2578
MERG Current Thoughts on HR 869
& HR 2578
New Exchequer Dam and
Lake McClure
FERC Project Boundary Question
Is Raising
the Dam Crest Elevation Unlawful?
The
Effect on the Limestone Salamander
Other Options for Additional
Water Storage
