9.2: Weir Overflow Rate

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Another calculation is one that measures the flow of water over a structure called a “weir.” A weir is an obstruction used to raise the level of flow and evenly disperse it. It allows the measurement of flow to be easily calculated. In a treatment plant process weirs are commonly used to “skim off” the flow of water from the sedimentation process before the water travels to the filters. Weir overflow rates are typically expressed as gallons per minute per foot of weir. They can also be expressed as million gallons per day per foot of weir.

The important thing with this formula is that you determine the actual length of the weir and express it using the flow rate specified in the problem.

This formula is used:

\[\text { Weir Overflow Rate (gpm/ft) }=\dfrac{\text { Flow (gpm) }}{\text { Weir Length (ft) }} \nonumber \]

Exercise 9.2

What is the weir overflow rate of a 100 foot long weir with a flow of 3,450 gpm?
A weir over flow rate is 28 gpm/ft with a flow rate of 2,300 gpm. What is the length of the weir?
A 75 ft weir has a daily flow 1.25 MG. What is the weir overflow rate?
A circular weir with a diameter of 85 feet has a weir overflow rate of 17.5 gpm/ft. What is the flow rate in MGD? (Note: you will need the formula for circumference in this problem)
A 8.5 MGD treatment plant has a calculated weir overflow rate of 32 gpm/ft. How long is the weir?