The Lewis River Hydroelectric Projects - PacifiCorp

Lewis River Interpretive & Education Plan
(stretching or bending forces break it much more easily than do compressing
forces). Concrete arch dams are designed to direct the force of the water outward
onto the walls of the canyon and downward to the dam’s foundation.
The central part of Merwin Dam is a concrete arch, chosen for this location
because of the narrowness of Shirt Tail Canyon, and the exposed rock on the
canyon wall and in the streambed. There is also a small concrete arch dam
included in the structure of Yale Dam.
Before a dam is built, the flow of the river must be diverted to allow for
construction on the riverbed. In the case of the Merwin Dam, a tunnel diverted
water around the dam site. Once the water was diverted, workers cleaned
the “overburden” (surface deposits) from the bedrock at all points of contact
with concrete. This meant washing the surface with high-powered hoses, then
“vacuuming” debris from low areas. The concrete arch was poured in several
stages, using a large wooden form that was moved across the span as each
section was completed. Imprints from the boards of the concrete forms are still
visible on the dam’s surface.
Earthen embankment dam
In some dam sites, rock abutments and exposed rock streambeds are not
available to support concrete arch dams. At these sites, earthen embankment
dams are more appropriate. Basically, embankment dams are giant piles of earth
and rock. Some are simple and homogeneous but the larger types such as Yale
and Swift are quite complex—they are “zoned” dams with layers of materials
within designed to impede or conduct water.
If you were to slice the top from Yale or Swift dam, you would see distinct
bands representing a series of different types of material within: large boulders,
coarse gravels, fine clay, and mixed dirt. These materials are arranged to provide
strength, drainage, and impermeability to the structure: the heavier, more
compact materials provide strength, more porous materials allow drainage, and
clays form impervious barriers.
Most earthen dams are built from materials on site. In the case of Yale Dam,
Saddle Dam, and Swift Dam, materials include riverbed boulders and gravels,
rocks and soil excavated for the bases of the dams, and rocks quarried from the
surrounding hillsides. The impervious layers in these dams are made of volcanic
clay taken from ancient mudflows within the valley. Materials are placed in
carefully engineered patterns: water is added or subtracted to create the correct
density and cohesion, and some layers are compacted with heavy machinery
such as sheeps-foot rollers and special weighted containers.
The base of a concrete arch dam such as Merwin interfaces directly with the
Sea Reach Ltd • 146 NE yamhill Street • Sheridan, OR draft 3 • November 2008 • page 70