Gravity Overflow Blow Off Develops into Siphon
Description of Unusual Occurrence in Croton Aqueduct System—Water, in Flowing Over Stop-planks, Entrains Air in Drop into Intake Bay Causing a Vacuum
THE development of a gravity overflow blow-off on the new Croton aqueduct into an effective siphon presents interesting features that warrant a description of what happened, and the causes thereof.
The new Croton aqueduct between Croton Lake and the City Line is built largely as a grade tunnel with a small amount of cut and cover section. Just above the Jerome Park reservoir, which is an equalizing and distributingreservoir near the City Line, the aqueduct drops about one-hundred-and-twenty feet and continues at this low level for some seven miles to the 135th St. gatehouse, which is a combined gatehouse for the old and new Croton aqueducts. From this gatehouse some William W. Brush twelve pipe lines, mainly 48 inches in diameter, lead to the distribution system, pumping station and Central Park distribution reservoirs. The water level in the Jerome Park reservoir is usually at an elevation of about 132 feet, while at 135th St. gatehouse the level is about 124 feet, this level varying under different draft conditions, and also under orders to control it at such level as may he desired.
When the aqueduct crosses the Harlem River the tunnel is some 300 feet deeper, this depth being needed to secure sound rock for construction purposes. On the west or down stream side of the river crossing is located shaft No. 25, which is the river tunnel uptake shaft, and the ground level is here below the hydraulic gradient. About 500 feet further west is shaft No. 26, where the ground rises above the hydraulic gradient. At this latter shaft is provision for a blow-off to the Harlem river, as shown in the illustrations.
The two 48-inch cast iron blow-off pipes are laid to the vicinity of shaft No. 25 a distance of 560 feet, where they join in one 48-inch pipe 150 feet long, which is laid down the sharp slope to the river, where the blow-off water escapes. The grade of these pipes is such that the rate of discharge would be over 200 million gallons for 24 hours.
Description of Blow-Off Chamber
The blow-off chamber is a brick lined structure of the dimensions given in the illustrations, with stop plank grooves into which planks may be placed, and the level in the aqueduct at this point controlled at any desired elevation. The tunnel shaft gives a free large waterway front the tunnel to the overfall weir, which is eighteen feet wide. The stop planks in place were a double set about nine feet high with earth rammed between them to form a tight coffer dam. The level to which the planks were carried was 129 feet, the water in the aqueduct at this point being usually about one foot higher than at the 135th St. gatehouse, which is two miles away.
In the roof of the chamber there are two five foot manholes carried to the ground surface, and covered by the usual cast iron manhole plates resting on a stone head. The manhole plates were in two parts.
How the Siphon Was Developed
On May 13, at about 4 P. M., four of the 48-inch mains leading from 135th St. gatehouse to Central Park reservoir were closed for the purpose of lowering the level in the reservoir. At the same time the gates controlling the flow into the aqueduct from above Jerome Park reservoir and from that reservoir, were operated to maintain about the flow required, which normally is around 200 million gallons per day. The water level at 135th St. gatehouse rose, as was to he expected, and reached an elevation of over 129 feet. The blow-off at shaft 26 started to operate in the usual manner and the excess flow went through the 48-inch blow-off pipes to the river. About a half an hour later there was a sudden drop at 135th St. gatehouse of some seven feet in the water level, and evidently this was the time when the siphon action started. The siphon was created by the water flowing over the stop planks and entraining air in its drop to the intake bay for the 48 inch blow-off pipes the bottom of which is 23 feet below the blow-off spillway level. The air was exhausted at such a rate that it could not be replenished through the small openings in and around the manhole covers, although the air whistled from the velocity with which it was drawn in. The suction at the cover over the manhole was such that when one of the men placed his foot over the small hole used for raising the cover plates he found some difficulty in withdrawing it.
REPRESENTATIVES OF THE METER DEPT., WORTHINGTON PUMP AND MACHINERY CORPORATION
How the Siphon was Broken
At first when the siphon developed it was thought that the bulkhead made up of the stop planks had given away and that the unusual rush of water was occasioned thereby. Steps were taken to reduce the head of water on the aqueduct tunnel, and the level at 135th St. gatehouse was lowered to about elevation 118 feet by opening the 48-inch gates which had been closed. This lowering of the water level was sufficient to overcome the vacuum in the chamber, break the siphon and stop the flow. The water level in the shaft immediately fell to about ten feet below the overflow level and normal flow conditions were thereby immediately restored.
An examination of the interior of the chamber at shaft 26 showed that none of the stop planks had been even disturbed, but that the earth filling between the double row of planks had been washed and drawn out. The general level to which the water had risen in the chamber could not be determined, but there was evidence that the water striking the wall opposite the overflow had dashed up several feet above the chamber floor level, which is at elevation136 feet.
The available evidence would indicate that in this entire chamber a vacuum of several feet of water greater than ten feet existed, while the action continued, and that if the covers had been air tight substantially a perfect vacuum of 34 feet of water might have been created. Such a condition was evidently never considered by the designers of the aqueduct system, and had this siphon not actually developed few would have thought it likely or even possible that such a siphon action would take place in a gravity blow-off structure such as has been described. The water works manager must always be ready to meet unusual developments which general practice does not record as likely to happen.