Great Feats of Engineering: The Hudson River Siphon
Getting Water to NYCThe Hudson River siphon is a good example of how monstrous the NY Water Supply System really is even though the average New Yorker hasn't a clue about how the system works. One of the marvels of the system is a tunnel 1,100 feet beneath the surface of the Hudson between Storm King Mountain and Breakneck Ridge. All it takes is one trip down the Storm King Highway and you will realize why George Washington considered West Point the Gibraltar of America. The fortifications he caused to be built are a wonder of military engineering.
What carries water down from the Catskills to New York City is the Catskill Aqueduct system that has to cross the Hudson River. It was not practical to bridge the river because the depth of the sediment at the bottom was too deep to place the necessary footings, so it was decided to use an inverted siphon instead.
The inverted siphon is an old technology dating back to the days of the Romans based on the simple fact that water always seeks its own level, and for the thing to work all one has to do is have one end where the water runs out to be lower then the end where water runs in. This was the sort of device they chose to cross the Hudson.
The first problem facing the engineers was to find out the depth of the sediments at the bottom of the river. The first method they tried was to drill a series of holes using a diamond drill mounted on a barge. This was slow and time-consuming work that created all sorts of problems with ships going up and down the Hudson River. They abandoned this approach when they discovered it wasn't practical. The next method they tried was drilling holes from the opposite shores that were 1400 feet deep. The two drill holes stayed in solid rock, so the engineers tried raising the depth at which the drills still remained in solid rock. From this work it was determined that the inverted siphon would have to cross the river at 1100 feet below its surface, or one-fifth of a mile deep. The contract for the work was rewarded to the T.A. Gillespie Company who already had bids from the city and state worth millions. They were not the lowest bidder with a bid $210,000 higher then the lowest bidder. This later became the subject of a special grand jury investigation. The Hudson River siphon job was also known as contract Number 80. The investigation centered on the alleged graft charges relative to the contract for the job of constructing the Hudson River Siphon. This was not unheard of in either city or state cases of alleged bid-rigging.
The Hudson River siphon was built in the early 20th century as the world's largest inverted siphon. This amazing structure was built to a depth of 1100 feet below the surface of the river. The bore was 13 feet in diameter that was lined with concrete. Once this monster was filled with water and no one has entered it for over a century.
The Hudson River siphon is part of an even larger construction project called the Catskill aqueduct where construction began in 1906 and was finally concluded in 1924. The whole system is operated by gravity; water flows throughout the system at a rate of about 4 feet per second, or about 550 million gallons per day to the north of Kensico Reservoir in Valhalla, New York. The aqueduct system works well below its capacity and the daily averages of water delivered amounts to about 350 - 400 million gallons per day. It is estimated that about 40% of the total water supply for New York City comes from the Catskill aqueduct. The rest is from two other reservoir systems in the Catskills, and other reservoirs east of the Hudson River.