This report describes the results of the hydraulic model tests for the
forebay structure of the Mayfield hydroelectric power plant conducted at the
St. Anthony Falls Hydraulic Laboratory during the period May 1973 through
There are three existing power generating units, each of which is fed
by an 18-ft-diameter penstock. Provisions have been made for an additional
unit to be installed in the near future. Water is taken from Mayfield Lake,
conveyed through a 37-ft-diameter tunnel, and discharged into a forebay.
There are four intake structures, one for each penstock, at the downstream
end of the forebay. Presently, the operation is hampered at high loading
conditions. It is generally believed that unstable hydraulic conditions in
the forebay are the cause of the present poor operating conditions. Strong
vortices at the intakes have been observed in the field. The flow in the
forebay is rough and turbulent. There are large shock waves at the pier
noses, causing the water surface to fluctuate and become higher at units 42
and 43 and lower at unit 44.
It is planned to add the fourth unit in the immediate future. Because
an additional unit would mean an increased flow requirement, it is probable
that the problems stated above would worsen. For this reason, a model study
has been conducted to assess the problems for the expanded operation and
find means of alleviating them.
A 1:36 scale model was constructed and tested. After calibration, the
model satisfactorily reproduced the existing flow conditions. The model
indicated an increase in flow distortion and in water surface instability
for the planned expansion. Air-entraining vortices were observed for all
flows tested without modification of the existing facilities. On the basis
of the experiments with various alternatives, the following schemes are
recommended to provide good flow conditions: (a) two 18-ft-wide submerged
guide vanes to be installed near the tunnel portal to redistribute the flow
and improve the water surface profile and (b) a V-shaped vortex suppressor
to be installed at each intake bay to eliminate the air-entraining vortices.
A documentary motion picture showing the highlights of the model tests has
been made as part of this test program.
Department of Public Utilities - City of Tacoma, Washington; Harza Engineering Company
Hydraulic Model Tests for Mayfield Power Plant.
St. Anthony Falls Hydraulic Laboratory.
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