Create a Floodplain Boundary from a Surface

[andy7918]

I want to create a boundary on an existing conditions drawing showing the location of the 100 and 500 year flooplains. I have elevations for both of the floods. Is there any way for autocad to create a polyline thats defined by elevation on a surface? I know i can snap to countour lines and figure it out that way, but it would be nice if i could have the program do it for me......

[jmayo-EE · Accepted solution]

If you have a TIN surface for the conditions that represent the flood plain elevations then yes you can do this with User Contours. Go to the surface's properties dialog, Information tab and assign or edit a surface style so it displayes thhe User Contours. Go to the Analysis tab, se;ect User COntours from the Analysis type drop down list. Enter 2 into the Range number and click the magic arrow button to populate the Range Details list. Assign the desired elevations to the two ranges that were created and close the dialog. THe two contours at the flood plain elevations will automatically be drawn.

[doni49]

Here's a screen shot to show you what John's talking about.  I'd also make sure to include descriptions for those contours.

Also -- don't forget to make sure that User Contours is turned on in the Style Properties that your surface uses.

[annw2]

That will work for ponds or if the floodplain elevation doesn't change througout a project.  The HEC-RAS import export would be some help for our mountain streams.  See the posts in the Stormwater Forums.  I never had a project where I needed to try it so I don't know how well it works.

For Flatter areas, you would need to keep adjusting the elevation and extracting the contours, then clip them as you proceed up and down stream.

[Neilw_05]

One technique I've used is to create an alignment and profile of the thalweg. The profile will consist of the computed water surface elevations at your cross sections. Next generate a featureline from the alignment and profile and apply a grading that targets the EG at 0%. That will give you a flood boundary where the grading meets the EG.

I am assuming your floodplain is for a channel that you have modeled.

[fcernst]

He needs to make sure he gets around all the natural islands and LOMA-F's.

Create a surface from the floodplain profile that is wider than any possible tie-in to EG. Then execute the command  _MinimumDistBetweenSurfaces.