Floodplain mapping & stair stepping results

Hi Chris,

I have been using the advanced water surface elevation with the Floodplain Mapping Module. At a point in the stream where there is a 90 degree bend I am getting the stair-step edge on the outside bank of the curve and at several cross sections nearby. The step size at 10 feet, the restrict contours to model results button off, and I have tried adding an additional cross section at the bend.

It manually looking at the computed water surface elevation and nearby topo, it seems that the floodplain should spread outside the banks further. Do you have more suggestions?

Thanks,
Nancy Chin
nchin@mwater.com

Dear Nancy Chin :

The software connects the ends of the cross-sections with each other, and this then becomes a bounding polygon. The floodplain mapping stays within these bounds so that the time required to develop a digital terrain model is limited. Therefore, from what you have told me, you will need to extend the cross-sections further out to make the bounding polygon larger.

If you have already done that, make certain that you select "Always Regenerate Topo Surface". This will cause the digital terrain data be generated to the new limits of the clipping polygon. Otherwise, it will continue to use the same digital terrain model based upon the original cross-sections.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

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