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.