# What is the purpose of the "weir end coefficient" when using a trapezoidal weir for a control structure in SewerGEMS/Civilstorm?

 Product(s): CivilStorm, SewerGEMS Version(s): 08.11.XX.XX and 10.00.XX.XX Area: Layout and Data Input

# Problem

What is the purpose of the "weir end coefficient" when using a trapezoidal weir for a control structure in SewerGEMS/Civilstorm?

# Solution

The "Weir End Coefficient" is the discharge coefficient for flow through the triangular ends of the trapezoidal weir (green portion in the image below).

Note:

• Typical values are 2.4 - 2.8 (US) and 1.35-1.55 (SI).
• This field is only available for trapezoidal weirs.

The "Weir Coefficient" is the discharge coefficient for flow through the central portion of the weir (blue portion in the image below). It is also known as the coefficient of flow. This value is used to account for variables that are not otherwise directly accounted for in the weir equation (eg. vena contracta).

These coefficients are used to calculate the flow in the weir with the following equation.

Where:

Q = flow
L = crest length