Units for V-notch vs rectangular vs irregular weir

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


What is the difference between the weir coefficient with a v-notch versus other weir types?

What typical values are expected for a v-notch weir coefficient?

Problem ID#: 47628


A typical value when using a v-notch weir is around 0.6. This is a unitless discharge coefficient. The weir equation used in this case multiplies this value by the necessary constants.

A typical value when using rectangular and irregular weirs is around 2.6 - 3.0 in US units (ft^0.5/s). The weir equation used in this case basically expects that the coefficient has already been multiplied by the necessary constants, hence it has units.

To enter the value in SI units, right click, choose Units and Formatting and change to "m^0.5/s". Or, convert using the equation CUS = 1.811 * CSI

A note on SewerCAD

Version (V8i SELECTSeries 2) of SewerCAD had a known issue that incorrectly causes units to display for the weir coefficient field when using a v-notch weir. V-notch weirs use a discharge coefficient similar to orifices, which is different from the weir coefficient used for other weir types. It is unitless and is multiplied by (2g)^0.5 for use in the calculations.

However, due to this known issue, the value that you enter for the v-notch weir coefficient field will be converted to US units first, then used in the calculations. Therefore, if you're using metric units for that field, the coefficient used in the calculations will not be correct. In this case, there are three solutions:

1) Change the units on the weir coefficient to US, by right clicking the field, choosing "units and formatting", then choosing ft^(1/2)/s. Then, enter the correct value again (~0.6)

2) Upgrade to a newer version of SewerCAD, where the issue is resolved.

3) Download and apply the latest cumulative patch set for SewerCAD, for a partial fix. The field will then be unitless and correctly accept the normal coefficient of discharge value such as 0.6.

If you apply the latest cumulative patch set for SewerCAD, please note the following:

 a) The default value for this field will still remain 3.0. A better default value for future versions would be 0.58.

 b) If you are in metric units and you set the weir coefficient when the weir type is inline and change it to V-notch, the value presented will change. It will show the converted US value. This is because the underlying schema the field has a storage unit. Be sure to correct the value as needed.

 c) If you had a file with a V-Notch weir with units in Metric, when you open the file after installing the patch the coefficient presented will be different. Be sure to correct as needed.

See Also

Downloading OpenFlows | Hydraulics and Hydrology Software 

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