# Irregular Weir Flow Calculations in a Composite Outlet Structure

 Product(s): PondPack, SewerGEMS, CivilStorm Version(s): CONNECT Edition Area: Calculations

# Problem

How is weir flow calculated when using the Irregular weir option in a composite outlet structure, in PondPack, SewerGEMS or CivilStorm?

# Solution

When using the Irregular Weir option (for example to model a trapezoidal weir, step-weir or some other irregular shape), the standard Weir equation is used:

Where

Q = Flow (cfs, m^3/s)
C = Weir coefficient (typically 3.0 ft^0.5/sec)
L = Length (ft, m)

However, the program breaks up the calculation into multiple segments and adds those flows together for the overall weir flow at a given headwater elevation.

Below is an example of a trapezoidal weir with 4:1 (H:V) side slopes. This is broken into three separate segments, one for each of the side slopes with a head (h) based on the average depth in that segment, and one segment for the middle, flat portion. For more complex, non-trapezoidal cross sections, additional segments would be computed.

Blue: Weir shape / Red: effective rectangular weir segments