Channel Flow Calculations

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Flow Calculations



 Rectangle Weirs:

        Contracted:

                Flow, Q = 3.33 (L  -  0.2H) H1.5
 
 

        Suppressed:

                Flow, Q = 3.33 L(H1.5

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


Cipoletti Weirs:

                Flow, Q = 3.367 L(H1.5

                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 H2.50
 
 

        60' V-notch:

                Flow, Q = 1.443 H2.50
 
 

        45' V-notch:

                Flow, Q = 1.035 H2.50
 
 

        30' V-notch:

                Flow, Q = 0.685 H2.45
 

           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:



Calculation of Flow Rate:
The flow can be calculated in cubic feet per second (cfs) using the following equation:
                Flow, Q = 4 (W)(Ha)1.52W0.026

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

Example:

The upstream head (Ha) 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?

                Q = 4 (W) (Ha) 1.52W0.026

           First convert the head to feet


 


 

This flow can then be converted to million gallons per day or gallons per minute. 

Flow Calculations