Jacob
Douma
concrete-lined spillway, consideration be given to constructing a separate emergency
spillway to carry part of the spillway discharge at maximum flows.
Fortunately, conditions were favorable for constructing a
spillway about 1,000
feet from the left abutment of the dam. A
wide earth channel about 1,200 feet
long, with its invert at about the same elevation as the concrete spillway crest, could be
excavated. The
is located near the end of the earth channel. The
is a
low gravel on rock embankment with its base on the bottom of the earth channel. The top
of the
would be to 10 feet below the top of the dam.
When a large flood occurs the concrete spillway is in operation. As the flood increases
in size and exceeds the capacity of the concrete spillway, the
in the earth channel
is overtopped and washes out. For the maximum probable flood, both the concrete-lined
and
spillways will discharge at the design discharges.
The Canadians hadn't any experience with
spillways. I told them about Professor
Kenny, who was at Washington State University. He was a consultant for a Pacific
Northwest public utility district on the design of a dam in the State of Washington. He
recommended that a
spillway be considered.
He tested the
spillway in a
scale model in the university laboratory. His
main concern was how to construct the fuseplug, so when it's overtopped, it all won't
wash out suddenly to greatly increase downstream discharges. The small scale model tests
indicated good performance, but he wanted to test it full scale. He found a small stream
where he tested a short length at full height, full base width and full cross-section. It was
constructed of what he considered to be the correct mixture of materials. By releasing
water upstream, he experimented with the mixture of materials. It was concluded that the
correct mixture should withstand 2 to 3 feet of water of the top of the
before it
started washing out, and it should continue to wash out at a slow rate until it is completely
washed out.
The board and Saskatchewan Power agreed that the emergency
spillway should
be considered for Rafferty Dam. Since a small-scale model had already been constructed
of the dam, concrete spillway and outlet works, the proposed spillway was added to that
model. It operated well, and it was adopted as part of the project.
When the Corps reviewed the plans, they raised questions about the
spillway.
"Where has it been used before? Has it been model or prototype tested? Will FEMA
approve its use?"
By the way, the
Kenny developed and tested was constructed as part of the dam
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