Engineering 101: Manning’s Expression

A couple of weeks ago, we discussed the Manning’s equation as it relates to the Hydraflow Extensions.    Have you ever wanted to label your pipe with the flow and pipe velocity?  Join me for a little expression…no not, Impression.

Expressions.   They provide wonderful design power, when done correctly.   Civil 3d 2010 provides additional pipe values, like Q, Recurrence Interval, HGL elevation and EGL elevation.   Pipe Velocity will still need an expression.

I started with the basics.

AreaRound:  SQR({Inner Pipe Diameter}/2)*pi

HydRad: ((AreaRound)/(2*({Inner Pipe Diameter}/4)*pi))

HydRad is the Hydraulic Radius, or Area over wetted perimeter.  Watch the location of PI in the expression – for some reason, location matters.

Manning’s Flowing Full: (((ABS({Pipe Slope}))^0.5)*(POW(HydRad,0.67))*AreaRound)*(1.486/{Manning Coefficient})

Velocity:  {Mannings Flowing Full}/AreaRound

This works well for round pipe.  Change the 1.486 constant to 1.0 for metric drawings.   If you mix round and rectangular, you may need to look at the All Signs Positive post to create a scaling factor for IsRound or IsRectangular to either scale your Manning’s label parts.

Too bad, this doesn’t work:

If (({Inner Pipe Diameter}) ! null, SQR({Inner Pipe Diameter}/2)*pi, {Inner Pipe Width}*{Inner Pipe Height})

I guess if the pipe is rectangular, the diameter does not contain a value, and hence the if-then-else can not be evaluated.

Keys:  Be sure the Manning Coefficient is correct in your part list.  10.0000 is pretty far off,  a square root of a negative pipe slope will have some issues, and again, watch pi. 


  1. Fred Mitchell says:

    Something I found out just today, which will cause us to NOT use Hydroflow. You can’t change your Kj for your hydraulic jumps. To my knowledge, these can differ between municipalities.

  2. Fred – not sure I’ve seen a municipality specify a coefficient for a jump. Hydraflow uses the momentum principle to determine depth and location, and assumes a length of 5 times the sequent depth, which is a conservative guess. Jumps can be very complex – engineering judgment is needed. See