Jacob H. Douma
stored in the reservoir was being released to produce power, no water was released
through the outlet works tunnels.
But, if a large flood occurred and the reservoir water level increased so the spillway
operated, then the outlet tunnels would be operated to control the reservoir level and
spillway flow. The spillway and outlet tunnels are designed to be large enough so that
their combined maximum discharge will control the probable maximum flood at the dam.
This, also, determines how high the dam needs to be to prevent overtopping by the
probable maximum flood.
The Bureau, at that time, was involved in a lot of dams. It was working on Grand Coulee,
then, in the late
and had just finished Hoover, right?
Hoover was
in about 1936, and Grand Coulee a few years later.
Do you remember any of the dams that you were doing work on in your laboratory work?
A:
I remember one especially, Lahontan Dam in Nevada. It had already been constructed
with an unusual type of spillway, a step spillway. Instead of having a smooth concrete
spillway invert down to the river channel, it had a series of steps. The water flowed about
100 feet on a level floor and then dropped vertically about 10 feet. There were about eight
such steps before reaching the river channel.
The dam was operated a couple of times during large flows, which caused erosion at the
bottom of those steps. A model study was not made of the stepped spillway during the
design stage, so a model was designed and tested to determine what changes had to be
made to improve the operation. I did those model tests and found that rounding the top
of the steps improved the flow over them sufficiently to eliminate the erosion problem.
Tests also showed that some of the steps could be eliminated, but at greater cost.
The prototype spillway was modified in accordance with the model test results. A few
years later a large flood occurred, and the spillway operated satisfactorily without any
erosion. I haven't heard anything about it since. It's probably still operating
satisfactorily.
Q ..
When you were in Denver, did you do any additional work at either the University of
Colorado or Colorado State, both of which apparently offered some advanced work in
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