As an incident in the work of building the Industrial Canal, it was
necessary to create a disappearing river.
This is the famous siphon--the quadruple passage of concrete that will
carry the city's drainage underneath the shipway. It is one of the
largest structures of its kind in the country.
A word about New Orleans' drainage problem. The city is the bowl of a
dish, of which the levees against river and lake are the rim. There is
no natural drainage. The rainfall is nearly five feet a year,
concentrated at times, upon the thousand miles of streets, into
cloudbursts of four inches an hour and ten inches in a day. In the
boyhood of men now in their early thirties it was a regular thing for
the city to be flooded after a heavy rain.
To meet the situation, New Orleans has constructed the greatest
drainage system in the world. There are six pumping stations on the
east side of the river, connected with each other by canals, and with a
discharge capacity of more than 10,000 cubic feet a second. The seven
billion gallons of water that these pumps can move a day would fill a
lake one mile square and thirty-five feet deep.
Three of the canals empty into Lake Pontchartrain, the fourth, the
Florida Walk Canal, into Bayou Bienvenu, which leads into Lake Borgne,
an arm of Pontchartrain.
Because of this drainage contamination, the lake shore front of New
Orleans has been held back in its development. Yet it is an ideal site
for a suburb--on a beautiful body of water, and just half a dozen miles
from the business district.
So the Sewerage and Water Board has been planning ultimately to turn
the city's entire drainage into Bayou Bienvenu, a stream with swamps on
both sides, running into a lake surrounded by marsh.
The Industrial Canal crosses the Florida Walk drainage canal. This made
it necessary to build the inverted siphon.
A siphon, in the ordinary sense, is a bent tube, one section of which
is longer than the other, through which a liquid flows by its own
weight over an elevation to a lower level. But siphon here is an
engineering term to describe a channel that goes under an
obstruction--the canal--and returns the water to its former level.
Like the famous rivers that drop into the earth and appear again miles
further on, the Florida drainage canal approaches to within a hundred
or so feet of the Industrial Canal, then dives forty feet underground,
passes beneath the shipway, and comes to the surface on the other side,
in front of the pumping station, which lifts it into Bayou Bienvenu.
At first it was planned to build a comparatively small siphon, but
while the plans were being drawn, New Orleans entered upon its
tremendous development. The engineers threw away their blueprints and
began over again. They designed one that is capable of handling the
entire drainage of the city. And in April, 1920, it was finished--a
work of steel and concrete and machinery, costing nearly three-quarters
of a million dollars, and with a capacity of 2,000 cubic feet of water
a second, 7,200,000 an hour, 172,800,000 a day.
It was a work that presented many difficulties. First the Florida Walk
canal had to be closed by two cofferdams. The space between was pumped
out, the excavation was made, and the driving of foundation piling
begun. Quicksands gave much trouble. They flowed into the cut, until
they were stopped with sheet piling. The piles were from 30 to 60 feet
in length and from three to five feet apart on centers.
Forty-six feet below the ground surface (-26 Cairo datum) was laid the
concrete floor of the siphon.
The siphon is divided into four compartments. There are two storm
chambers, measuring 10 by 13 feet each, one normal weather chamber
measuring 4 by 10 feet, and one public utilities duct, measuring 6 by
10 feet. These are inside dimensions. The floor of the siphon is two
feet thick; the roof, one foot nine inches. The whole structure is a
solid piece of concrete and capable of standing a pressure of more than
2,000 pounds to the square foot. Its total length is 378 feet; the
shipway passing over it is 105 feet wide and 30 feet deep.
In the public utilities duct are carried the city's water pipes,
cables, telephone and telegraph wires, and gas mains.
The storm chambers will handle the rainfall of cloudbursts. In ordinary
weather the water will be concentrated through the smaller chamber, in
order to produce a strong flow and reduce the settlement of sediment to
a minimum.
Eight sluice gates, each 6 by 10 feet, open or close the water
chambers. They are operated by hydraulic cylinders of the most approved
type.
For sending workmen inside the siphon to make repairs or clearing away
an obstruction there are eight manholes. Four measure 6 by 13 feet, two
6 by 6 feet, and two 6 by 4 feet.
As soon as the Florida Walk canal can be deepened and a few link-ups in
the drainage system can be made, the entire drainage of New Orleans, in
normal weather and during light storms, will, according to announcement
by the Sewerage and Water Board, be sent through this outlet. During
the occasional cloudbursts it will be necessary to send some of the
drainage into the lake, but this will be rapidly flowing water and will
sweep offshore. It means a great deal to the suburban development of
the city.
A year and a half the siphon was in the making. Preparations for the
structure cost more than $250,000--excavation foundation, etc. The
concrete alone cost $170,000. Machinery and the work of housing and
installing it cost $60,000 more.
Four bascule steel bridges now cross the Industrial Canal. They are the
largest in the city. Three of them--at Florida Walk, for the Southern
and Public Belt Railways; Gentilly, for the Louisville & Nashville; and
on the lake front, for the Southern, weigh 1,600,000 pounds
each--superstructure only. The fourth--at the lock--weighs 1,000,000
pounds. They are balanced by 800-ton concrete blocks and concrete
adjustment blocks. Their extreme length is 160 feet; the moving leaf
has a span of 117 feet.
With a 30-foot right of way for railroad tracks, 11 feet for vehicles
and trolley cars and four feet for pedestrians, they are designed to
meet traffic conditions of a great and growing city. They will support
50-ton street cars or 15-ton road rollers--New Orleans has nothing as
heavy as that now--and trains a great deal heavier than are now coming
to the city. No bridge in the South will support as heavy loads.
The tensile strength of the steel of which the bridges are constructed
is from 55,000 to 85,000 pounds to the square inch, and they will bear
a wind load of 20 pounds to the square inch of exposed surface.
They are operated by two 75-horse power electric motors, 440 volts,
60-cycle, 3-phase current, which is stepped down from 2,200 volts by
means of transformers. In addition, there is a 36-horse power gasoline
engine, to be used if the electrical equipment is out of order. To open
or close the bridges will require a minute and a half.