Broad Crested Weir Calculator

The weir flow rate calculator helps to quickly solve the broad crested weir equation by using the inputted values. All the values are required in SI units. 

What is broad crested weir?

Weirs are installed for many purposes on streams and rivers and they have different types.

A broad crested weir is a strong weir made of ceramic or hard material as an obstruction to water or other liquid flow.  It is a flat weir with a net double drop (i.e two head measurements).

It is called broadly crested because of its wide edge over the entire stream. But the accuracy of such weirs is obtained through precise calibration.

This weir is usually used over dams in forests and similar places, where the other weirs like V-notch, suffer maintenance problems.  

Broad crested weir equation

Equations to calculate flow rates are different for different weirs. For broad crested weir, the equation used is:

q = C_d × b × h₂ × (2g (h₁ - h₂)) ^1/2

 In this equation:

• C_d is discharge coefficient 
• b is the breadth of the weir
• h₁ and h₂ are first and second head drops 
• g is the gravitational acceleration 

How to calculate the flow rate of a broad weir?

Example:

For the following set of values for a broad weir, calculate the flow rate. 

h₁ = 45m

h₂ = 42m

C_d = 1

b = 1 m

Solution:

Step 1: Put values in the formula:

q = 1 × 1 × 42 × (2(9.8) (45 - 43)) ^1/2

Step 2: Solve:                                    

q = 42 x (2(9.8) (2)) ^1/2

q = 42 x 6.261

q = 262.96 m³s^-1