How the Size of the Airmont Spill Was Calculated
The recent article in the Journal news reporting on the discrepancy of the amount of sewage spilled into the Cherry Brook needs to be further clarified so the residents are aware of the magnitude of the problem.
I am the engineer that calculated the estimated volume of the sewer spill for the residents, stated to be 2.5 million gallons at the meeting . I am also a concerned resident.
First, what needs to be understood, is that these spill numbers are estimates. There is no right or wrong answer. No one can "not tell the truth" by providing an estimate. An estimate, however, needs to be based on some reasonable information to be believable, useful, and accurate.
Here is the basis for my calculations. In light of not having all the facts, I have had to make some assumptions. I welcome any comments from other concerned individuals.
1. Duration of Overflow
The sewer overflow was first reported on Friday at 4:30 PM, and the spill was stopped on Monday at 1:00 PM.
No. of hours in period = 68.5 hours
How many hours was the spill actually occurring? How many hours was the spill occurring before it was noticed by the residents?
The residents say it was continuous. The sewer district says it was "intermittent." How many hours is intermittent?
2. Pipe diameter = 12"
My original calculations did indeed use the correct pipe diameter of 12", not 24" as was erroneously stated in the article.
3. Calculate cross sectional area of pipe:
Cross sectional area = 3.14 x radius squared = 3.14 x 0.5 x 0.5 = 0.785 square feet
4. What was the velocity in the pipe?
Engineers design a sewer system so that the MINIMUM velocity is 2.0 feet per second. This is done to prevent solids from dropping out if the velocity gets too low.
For a 12" diameter pipe, the minimum slope used to maintain a 2 feet per second minimum velocity is 0.22 percent (2.2 feet vertical drop per 1,000 feet of pipe length.)
The overflow occurred at manhole no. 12010. According to the as-built sewer construction drawings I examined at the RCSD office, the slope of the inlet line to this manhole is 1.82 percent (18.2 feet vertical drop per 1,000 feet of pipe length)
Also, the link directly upstream of this section has an even steeper slope of 6.85 percent (68.5 feet vertical drop per 1,000 feet of pipe length)
Considering the very steep slope of the inlet line, and that a 2 feet per second velocity would be developed at a much lower slope, the 2.0 feet per second velocity used for this estimate seems like a very conservative and reasonable figure.
5. Pipe flowing full?
Gravity sewers do not usually run full, but considering the steep inlet slope to the overflowing manhole, the flow obstruction, and the low velocity used in the calculation, it is fair to assume and likely that the pipe was indeed flowing full or nearly full in that pipe section at the time of the overflow to produce a discharge height of 2 - 4 feet above the top of the manhole as reported by the residents.
6. Calculate discharge
For a pipe flowing full:
Q = A x V
Flow rate (cubic feet per second) = pipe cross sectional area ( square feet) times velocity (feet per second)
Plug in the numbers.
Q = 0.785 square feet x 2.0 feet per second
Q = 1.57 cubic feet per second
Since one cubic foot = 7.48 gallons
The calculated flow rate is 1.57 x 7.48 = 11.74 gallons per second.
The sewer district estimated the spill to be 4,000 gallons.
At my estimated flow rate, 4,000 gallons would have leaked out of the manhole in about 5 1/2 minutes.
Using my estimated flow rate, if the leak occurred for the entire 68 1/2 hour time period, the volume of sewage spilled would have been approximately 2,895,084 gallons
The sewer district should now provide the basis for their official estimate of the overflow to help the public understand the magnitude of the problem. The public, who will ultimately foot the tax bill, has the right to know how to best solve the sewer overflow problems we have been having, and the cost involved, to ensure their continued health and safety.
Ronald A. Glisci, P.E.