River length problem in GeoRAS

[muellz]

Hi,

I'm using HEC-RAS 3.1.3 and HEC-GeoRAS 4.1.1 for ArcGIS 9.1 to delineate flood areas. I have the original HEC-2 cross-sections for the river and I'm using GeoRAS and a LIDAR DEM of the area to determine the floodplain in more detail. So, the channel data comes from the original HEC-2 cross-sections and the floodplain data comes from the LIDAR DEM with GeoRAS. Unfortunately, the distances between the cross-sections in HEC-2 do not match with the distances in the LIDAR DEM. So, when I bring in the HEC-2 cross-sections into GeoRAS they are not in the place they
should be according to the DEM (a bridge cross-section is 500 ft away
from the bridge in the DEM).

Has anyone else run into this problem or has any idea why this problem
is occurring? I'll appreciate any help anyone can give me.

Thanks!
Zach

[TimW]

The way I usually handle this is to import the HEC-2 sections into one geometry and your cross sections cut from the LiDAR into another. Make a list that details which HEC-2 cross section corresponds to each of your new cross sections. Add the channel data into the new cross sections using the "Graphical Cross Section Editor" tool (check the boxes for "Compare Geometry Files" and "Merge Cross Sections," and select your HEC-2 import geometry as the comparison geometry).

If you are submitting to FEMA you may want to re-name your new cross sections to have the same stations as the current effective model. In any case, FEMA will want to know how the elevations compare, which is where your list comes in handy.

I would be interested to know if anyone else has a better approach.

[muellz]

Thanks Tim for your advice, it has saved me a lot of time. I didn't know about the comparison tool and have been using Excel to manipulate the cross-sections. Your a lifesaver!