Creating a Floodplain Elevation Line or Contour in Civil 3D

August 29, 2018

Craig Batchelor | Applications Expert

Here is a simple exercise to create a flood line at a specific elevation in a Autodesk Civil 3D drawing. Starting with a topographic survey of the area set to 1-foot contours, the exercise is to create a line on the drawing at a specific elevation. Below are the key points on how this is accomplished.


Starting in Surface Properties, select the default Slope Banding (2D) style that ships with the program and with the edit (pad and paper icon) drop-down, Copy current selection.


Name the style “Flood” in the Information Tab, and on the Display tab, uncheck Slopes and set the Visible for User Contours to on.  Select OK


On the Analysis Tab change the Analysis type: and Legend to User-Defined Contours. Set the Range Number to 1 and Project the contour down with the arrow key.  Set the Elevation to the desired contour elevation and then select OK


The Surface should have the single contour at the specified Elevation. Next, with the surface selected Extract from Surface > Extract Objects.

Select the default and OK


The result will be a Polyline at the specified elevation


Select the Polyline and change the as needed.  Polyline Edit (PEDIT) the line to Smooth, then in Surface Properties change the Surface Style back to 1-foot contours.



The user defined contours have many uses if the interval between contours does not fall on an even foot. Here is one way to utilize the command and extract out one line at the desired elevation.

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