The following is the text of the cover story of
Scientific American
, Vol. LXVII. - No. 13, September 24, 1892
.
The article is titled "The Great Dam Across the Colorado River, at Austin, Texas." and features Austin.
The Austin, Texas, Dam.
There is being built across the Colorado River at Austin, the capital
of the State of Texas, a massive granite dam, the object of which
is to furnish the city with water works and electric light, and to
also furnish manufacturing enterprises with cheap water power. This
wonderful structure is being built by the citizens of Austin, who
at a public election voted to bond the city in the sum of $1,400,000
for this purpose. The dam is 1200 feet in length and 60 feet high.
It is 16 feet thick at the top, increasing downward and spreading
out in a broad toe or apron, making its extreme width at the bottom
50 feet. The body of the dam is of limestone rock. The upstream face,
downstream face, and toe are being made of granite. The capping will
also be made of granite in as large blocks a can be handled, worked
to regular shape. The entire work is being laid in hydraulic cement.
The structure is being built to allow a depth of 16 feet of water
on its crest, and the abutments on either side will go to that height.
At one end of the dam the natural rock goes far above this.
The other end is occupied by an artificial bulkhead, called a gatehouse,
containing the sluices for drawing off the water. The wheels will
be some two or three hundred yards from the dam, and the canal, which
is being excavated in the rock, will also be that length. The function
of the gatehouse at the entrance to the canal is to enable the water
to be shut out in case of repairs, and to prevent overflow in time
of floods. The water will be drawn from the penstocks through iron
pipes, pass the wheels, fall into the wheel pits, and be discharged
through underground races into the river. There will be three water
wheels, forty-five inches in diameter, each capable of giving 600
horsepower on a head of 60 feet. The dam is situated two miles above
the city and will create a lake twenty-five miles long and from one-half
to one-quarter of a mile in width. Mr. J.R. Frizzell is the chief
engineer of this great work and T.J. Fanning consulting engineer.
During the flood season the amount of water that will flow over this
dam, it is estimated, will be 200,000 to 250,000 cubic feet per second,
which is nearly equal to the volume Niagara Falls, to wit, 275,000
cubic feet per second.
Our illustrations were made from photographs, for which and the above
particulars we are indebted to Mr. W. W. Wilson.
In a report upon the work to the Austin board, made last year by Consulting
Engineer Fanning, he recommended a modification of the profile of
the dam, and remarks as follows"
"Not for its length alone, or its great area of flowage is the dam
remarkable, for in France we observe three longer masonry dams, at
Buzey, Cazilla, and Gros Bois, 1,545, 1,759, and 1,805 feet respectively,
and in Wales the Vyrnwy dam, 1350 feet long, the latter being the
storage reservoir of the Liverpool water supply. Not in height alone,
for in France there are three dams, in Spain two, in Belgium one,
and in the State of California one masonry dam exceeding 150 feet
in height. There are fourteen other notable masonry dams having heights
exceeding 100 feet.
"But none of these dams are upon great rivers, and very few of them
have any water pass over their crest. On the other hand, The Austin
Dam stands in the channel of the Colorado River, where it has 40,000
square miles of watershed, and will have floods of 200,000 to 250,000
cubic feet of water per second to pass from its crest to its toe.
Your citizens will appreciate your responsibility when they learn
that no other dam in existence has to pass a volume of water, in flood,
even approximating this, through so great a height. Limestone and
sandstone yield rapidly to the eroding force of falling water. The
evidences of this are abundant is the canyon of the Niagara River
below Niagara Falls; of the canyon of the Genesee River below Genesee
Falls; of the Mississippi River below St. Anthony Falls; and here,
of the Colorado River across Travis Co., as well as in the channels
of numerous streams that flow down each of the Rocky Mountain slopes.
Such evidences admonish us that this great flood must not be permitted
to have sheer fall through so great a height and act with a destructive
force such as has heretofore created canyons, but it must be made
to glide down the slope of the dam and not be permitted to exert the
force due to its velocity except at such distance below the dam that
the foundation will not be endangered.
"The profile of the dam shown to me seemed not to fulfill the required
conditions for passing floods, because the slightly rounded or nearly
angular form at the front of its crest. The diagram accompanying shows
an advised modification to the profile of the upper part of the dam,
which is better adapted to pass the flood in a gliding sheet down
the face of the dam, and to deliver it to the lower level without
a direct blow, and so that its velocity will be expended chiefly in
a horizontal direction in the backwater below the dam. The lower part
of the downstream face of the dam has a curve of 31 feet radius, to
which low water surface is tangent. The central part of the dam has
a batter of 4-5 inches per foot. The new profile at the top part,
as suggested, completes the downstream face and crest of the dam with
a curve of 20 feet radius, to which both the front batter and the
surface of the pond at a level of the crest are both tangent, this
curve ending on the crest at 5 feet from the upper angle of the crest.
The upper angle of the entire front of the dam becomes a reverse curve
of ogee form; the form of dam adapted best of all to pass a large
volume of water through so great a height. The top curve conforms
nearly to the theoretical form of a medium flood stream. At higher
flood stages the will be a tendency to vacuum under the curved stream
immediately after it has passed the crest, which, together with the
pressure of the atmosphere on the top of the stream, will keep the
full flood stream in close contact with the curved face of the dam,
and cause even the highest flood to glide down the fall without shock
upon the face of the dam or the soft rock foundation."
