Jacob H. Douma
the upstream set of gates discharges into the expansion chamber which produces 225 feet
of back pressure, and the downstream set of gates discharges into the open diversion
tunnel. This way, the maximum head that either set of gates need to discharge under is
225 feet, which is about the upper limit for slide gates.
To our knowledge, that had never been done before. Hunter Rouse and I were on that
board, and Hunter, who was a theoretical expert, said, "It will work." I said, "Hunter,
I think we've got something here. Let's model test it."
A large 1: 10 scale model was constructed in a Canadian laboratory in Vancouver. The
model was constructed with clear plastic so flow conditions in the expansion chamber and
downstream open diversion could be observed. Pressures were obtained at critical
locations. The model tests verified that this plan would be satisfactory.
A City of Vancouver water supply pipe was near the laboratory. It had considerably
higher pressure than was used for the 1: 10 scale model tests. I said, "Let's hook that up
to the model and see what happens. The City of Vancouver was reluctant to give its
approval because of possible damage to their water supply. But the laboratory finally got
approval and the hook-up was made. The model operated very well under the higher
head.
That plan was constructed at Mica Dam. I don't know of any other place in the world
where a large outlet tunnel with an expansion chamber in it has ever been constructed.
It has operated satisfactorily under high heads several times since construction was
completed 15 or so years ago.
In reaching a solution like that, is that purely you sitting there and saying, "This might
work; here's the problem and let's look at this"?
A:
Exactly. The thing I had to go on was that I knew that these slide gates would operate
under 250 feet maximum head. Slide gates like these are installed at Pine Flat Dam,
where the Corps has operated them under 250 feet of head.
Initially, there was one question that brought forth a lot of discussion concerning flow in
the expansion chamber. The question was whether excessive surging back pressures in
the expansion chamber just downstream of the upstream set of slide gates would cause
excessive vibration and difficult operation of the slide gates. Fortunately, the model tests
showed that those pressures did not surge excessively, and that good flow occurred in the
expansion chamber.
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