Figure 11: Flow capacity expression
This expression assumes a Manning’s “n” value of 0.013, which is the standard for concrete pipe. In fact, pipes have a named component for Manning’s “n” value, which you can set. However, adding that component to your expression yields an undesired result, so you should not use it for this procedure.
Figure 12: Use the text component editor to display flow capacity (Q)
Display the flow capacity for design purposes. When you want to produce your construction plans, assign a style more suitable for production drawings. As pipes are created and labeled, the label style DESIGN FLOW will display the flow capacity (Q) of the pipe.
Figure 13: Flow capacity label
The flow capacity label is especially effective in profile view where you can adjust the invert grips of each pipe until the desired pipe capacity is reached. You may need to follow each iteration of invert raising or lowering with a REGEN command to regenerate the drawing.
These expressions demonstrate the importance of fully exploring the capability of mathematical expressions with AutoCAD Civil 3D. Making a couple of assumptions, it is easy to see that any user with a simple understanding of Manning’s equation can create an expression for flow capacity and compare that value to the expected flow calculated by the design storm. Then the user can use this comparison to help set the diameter and slope of a pipe.
For more AutoCAD Civil 3D tips and tricks, see Jeffrey Old’s technology bulletin postings at the Hagerman & Company, Inc. website: http://newsletters.hagerman.com/newsletters.