Distance Learning Math Index
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Flow Calculations

 Rectangle Weirs:

        Contracted:

                Flow, Q = 3.33 (L  -  0.2H) H^1.5
 
 

        Suppressed:

                Flow, Q = 3.33 L(H^1.5) 

                Q = Flow, cubic feet per second 
                L = Length of crest, feet 
                H = Upstream head, feet 

Cipoletti Weirs:

                Flow, Q = 3.367 L(H^1.5) 

                Q = Flow, cubic feet per second 
                L = Length of crest, feet 
                H = Upstream head, feet 

V-Notch Weirs:

        90' V-notch:

                Flow, Q = 2.50 H^2.50
 
 

        60' V-notch:

                Flow, Q = 1.443 H^2.50
 
 

        45' V-notch:

                Flow, Q = 1.035 H^2.50
 
 

        30' V-notch:

                Flow, Q = 0.685 H^2.45
 

           Q = Flow, cubic feet/second
            H = Upstream head, feet
 
 

Example:

The upstream head for a 90* V-notch weir is 3 inches.  The flow in cubic feet per second is:

Convert head to feet:

Determine flow rate:

This can then be converted to other units of measure i.e. Million gallons per day or gallons per minute 
 

Example:

The flow can be calculated in cubic feet per second (cfs) using the following equation:

                Q = flow cfs 
                W = Width of the throat, ft 
                H_a = Upstream head, ft 
 

Example:

The upstream head (H_a) in a 2.5 ft. throat flume is 9 inches.  There is no submergence to be corrected for.  What is the flow rate in cubic feet/second?

           First convert the head to feet

Flow Calculations