Water Resources: Hydraulics and Hydrology
The board was told to determine what needed to be done to the dam so it would safely
handle the maximum probable flood. Calculations showed that if a probable maximum
flood should occur, the water would flow about 6 feet over the top of the
high
concrete arch dam and about 11 feet over the top of an adjoining low earth embankment.
The earth embankment would very likely wash out, and water going over the top of the
concrete dam might severely erode its abutment, causing failure of the dam.
The board next looked at the possibility of enlarging the spillway. If the spillway was
enlarged sufficiently, the dam would not be overtopped nor would there be any question
regarding its safety. The spillway is concrete-lined with eight ungated bays and a
high ogee crest at each bay. Downstream of the crest, the spillway was lined with a
inch concrete slab placed on a 1 on 4 slope.
One method of enlarging the spillway was to increase the crest length by adding more
bays. It was decided that this was not feasible because of space limitations. The next
method which was considered was to extend the spillway bay piers upstream of the
spillway crest about 150 feet and connecting every other one at their upstream ends. The
top of the pier extensions was placed at an elevation, so when the spillway operated, the
pier extension served as the spillway crest. This elevation was several feet higher than the
existing ogee crest. The existing crest would then be cut down about 10 feet to enable
more water to flow through the spillway bays. The longer extended pier crest would result
in larger spillway discharges for high reservoir levels. The board concluded that this
method was feasible. It would not require raising the dam.
The third alternative would be to raise the dam about 12 feet, and make minor
modifications of the existing spillway to pass the maximum probable flood without
overtopping the raised dam. This method was the most costly of all, so it was abandoned.
The board decided that the second method was the most economical way to handle the
probable maximum flood without raising the dam. That's extending the spillway bay piers
out some distance in front of the existing spillway, and having those piers act as a spillway
crest. Then cutting out the existing spillway crest concrete between the existing bays, so
that more water could be handled by the spillway.
The board was concerned with a problem regarding the
concrete slab downstream
of the existing spillway crest. It was not anchored to the underlying rock. During
spillway flows, high velocities could produce uplift pressures through defective joints that
might cause complete failure of the concrete slab.
The board considered replacing the slab with a properly designed slab or adding three feet
of concrete on top of the old slab with a large number of anchors into the underlying rock.
The latter method would be very costly. The board concluded that the slab should be
Previous Page
