NEW ORLEANS AND ITS DRAINAGE SYSTEM.
NEW ORLEANS is having complete drainage and sewerage systems installed, at a cost of $12,500,000. The work has presented grave engineering difficulties, which, combined with the lack of funds, have delayed the beginning of the undertaking. A bond issue has now insured the necessary supply of money, while three epidemics of yellow fever have hastened operations.
The amount of pumping machinery required is greater than in any other city, just as the amount of drainage called for is equaled only by some of the East Indian cities. The latest style of pumps, therefore, is being installed, just as the most up-to-date principles of drainage are being followed. The engineering difficulties are presented by the fact that the six miles of alluvial plain on which the city is built, lying for six miles betweeu the Mississippi river and lake Pontchartrain, slope from the river back of about fifteen feet, all of which is easily used up in the first three miles, and the greater part in the first half mile. Then follow a slight rise and a level swamp extending tojlake Pontchartrain At high water the Mississippi is seventeen or eighteen feet above the level of the city, and, as it was too costly a task to lift the mass of water to such a height, the capacity of discharge for the drainage canal ran to size rather than slope. Some of the discharge-ends will be twenty-live feet wide and twelve feet deep, and will have a cross-section of 175 square feet, which will secure the called-for discharge of 1,200 feet persecond. These will be kept pumped free of water, so that, when a rain storm takes place, they will act both as reservoirs and aqueducts to the pumps. This is a very necessary precaution, since the annual rainfall is from five to six feet, the monthly fall, fifteen or twenty inches; and the daily fall, four or five inches—fora few minutes at the rate of six inches an hour. The daily sewage volume is only about 20,000 gallons, or thirty cubic feet the second To avoid the risk incidental to this small amount of sewage barely dribbling along and in very hot weather drying up and festering in the draiuage canals, and at the same time not to pollute lake Pontchartrain by discharging the filth into its waters, the sewerage system will be entirely separate and will be pumped into the river below the city.
The main canal of the drainage system will be collected by laterals on each side at Broad street, the lowest part of the city and running parallel with the Mississippi Both laterals and canal will be lined and covered, and will be under the city streets, which will be supported on massive brick and concrete walls and steel arches Branches to the laterals and at right angles with them will intercept the flow of the gutters which run back from the river. The drainage to be thus collected into the main canal is at present drained into lake Pontchartrain. When the system is completed, it will be carried below the city and pumped into Bayou Bienveuu, a wide and deep artificial stream, whence it will flow into lake Borgne, in reality a big and comparatively unfrequented arm of the gulf of Mexico, into which lake Pontchartrain (a practically landlocked body of water) discharges through its one solitary narrow outlet. Sometimes the volume of drainage is too great to be totally disposed of in that direction, except at great cost. It is, therefore, mtended to discharge the surplus not into lake Borgne, but into lake Pontchartrain at three different points. To operate the drainage system will be found a central electric powerhouse, now completed, with a capacity of 10,500-horsepower, and nine pumping stations, of which three have been erected. The last, by pumping from the drains into the open canals leading to lake Ponchartrain, relieve the city directly; the other six stations will be built along the main canal to Bayou Bienvenu, as it would be impossible to send the water in one flood to the bayou and then lift it over the levees of the bayou. (The lift would be too great and the excavations to give the canal an adequate flow too deep.) The water will, therefore, flow to one station and be lifted to flow on to the next, and so on to the end. One of these small pumping stations, with other cuts, is shown by the courtesy of the Scientific American.
The old drainage system is a series of gutters close to the sidewalks These have so little slope that during heavy rains the streets are flooded. The water which is not soaked up by the earth, but gets into the open canals in the streets at the back of the city is swept over the levees of three navigation canals by a couple of oldfashioned paddle pumps into lake Pontchartrain. These pumps, however, drain only overflow waters, and do not relieve the city. What water remains after the overflow has been pumped up by one of these small pumping stations becomes stagnant and dries up.
The plans for the sewerage system have not yet been completed; tests and experiments are being made; and the probabilities are that the work will be begun next year.