Applies To
	Product(s):	Bentley FlowMaster
	Version(s):	08.11.XX.XX
	Environment:	N/A
	Area:
	Original Author:	Mark Pachlhofer, Bentley Technical Support Group

Problem

What is the difference between crest breadth (Lr) vs. the Crest Length (L) for a broad crested weir in FlowMaster? 

Solution

In equation 5.42 in the FlowMaster help "Broad-Crested Weir", "L" is defined as the 'Length of Roadway Crest', which is the distance perpendicular to the flow direction.  This is the value that you'd enter as your "Crest Length" in the Broad Crested Weir worksheet.

Figure 5-9 located just below figure 5.42 in the help document (or shown just below this paragraph for clarification purposes) it shows the Breadth Of Crest, "Lr", which is the distance parallel to the flow direction (the thickness of the weir crest). This is the "Crest Breadth" entered in the FlowMaster worksheet and is used in the calculation of the adjusted Cd (equation 5.43) - see Figure 5-10.

 

Equation 5.43 and Figure 5-10 are shown below:

The figure below also helps to show the difference between the Crest Breadth (Lr) and Crest Length (L):