Floodway mapping

Chris

I imported into RCAD a HEC-RAS file containing 3 profiles. Profile 1 is an un-ecroached run, Profiles 2 and 3 were created using an encroachment analysis. I plotted the floodplain for Profile 3 in RCAD, but now want to plot the floodway. How do I do this? I cannot find the procedure in the user manual.

Chris T. Long
northwest hydraulic consultants

Dear Chris :

To plot the floodway, you need to use the "Simple Water Surface" method. RiverCAD / RMS will then draw in a straight line representation of the defined / computed floodway limits. You can then edit, adjust, or redraw the floodway line so that it follows the river.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com

[Justin]

Dear Chris,

I too was wondering if there was a simple way to have RMS plot the floodway and found your recent response to Mr. Long's question.

However, I don't believe the method you suggest will provide the means for mapping the floodway.

This is because the Simple Water Surface plot will draw a straight line, from cross section to cross section, along the outer edge of the floodway water surface. In most cases, this is outside of, and one foot higher than, the 100-yr floodplain limits.

By definition, the floodway is not the edge of the encroached flood plain. Instead, it is the two encroachment lines that result in a theoretical one-foot rise in water surface.

I believe Mr. Long wants to know how to map the encroachment limits given by Method 1 after multiple iterations. I now measure out from the end of the cross section until reaching the Left and Right Encroachment Stations listed in summary table 110.

Is there an easier way to do this....perhaps similar to how RMS shows bank stations ?


Justin Turner

[Chris Maeder]

Dear Justin:

The "Simple Water Surface" option will display where the edge of water has been reported by either HEC-RAS or HEC-2. Hence, if you select this option for a water surface profile that corresponds to an Encroachment Method #1 analysis, RMS / RiverCAD will show you where these locations are on the topo map. Note, however, that RMS / RiverCAD can only show you straight lines between cross-sections, since it is "simply" displaying where HEC-RAS (or HEC-2) reported the encroached edge of water to be.

The engineer can then create a new layer, and draw a "refined" edge of water line between cross-sections show where he wants to place the actual floodplain encroachments. However, if the user has created cross-sections fairly close together (such as in our example Floodplain Encroachment exercise lesson), then the task is fairly simple to do (taking just a couple of minutes).

I hope that this response has answered your questions and concerns. If not, you know where I live :).

Sincerely,

Chris E. Maeder
Senior Technical Engineer
chris.maeder@bossintl.com

[Justin]

Chris,

Thank you for your quick reply !!

I tried your suggestion and used the Simple Water Surface plotting option for profile 2 and it worked just fine. The Left and Right Encroachment Stations were correctly identified.

This is wonderful...thank you.

Justin Turner.

[Chris Maeder]

To better illustrate this, I have attached a screen capture from our floodplain encroachment lesson file that comes with the RMS / RiverCAD.

Profile 1 shows the unencroached water surface. Profile 2 shows the encroached edge of water with a 1 foot rise of the computed floodway. Profile 1 used the Advanced Edge of Water Option, and Profile 2 used the Simple Edge of Water Option. You can see that the Simple Edge of Water Option shows where the encroachment stations are.

Also, you can see that the engineer will have to smooth the encroached edge of water line between cross-sections so that it follows the general curvature of the river.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com