HEC-RAS cross sections

Dear fellow HEC-Ras users:

I am attempting to create a model of a river with many 90 degree bends in it. When cross sections are made for this river they cross. To handle this problem I have divided the river into multiple reaches so that cross sections within a reach do not cross. The problem is that several points in space will be represented by multiple cross sections and the elevations of these points might not be identical for each cross section, thus introducing errors in the flood plain boundary. My question is what is the preferred method for creating cross sections for windy rivers? In some of the examples, cross sections with bends in them were used, can this be done with HEC- RAS 2.0?

Thanks,
Paul Harmsen
W&H Pacific

This is a common problem as people begin to use models such as HEC-2 or HEC-RAS. It is important to remember that a cross-section does not have to be perfectly straight; they can be "dog-legged", as shown on the attached bitmap graphics files.

Whenever two cross-sections approach one another, don't let them intersect. Instead, turn one or both of them so that they have same compass heading before they intersect. Then they can parallel one another indefinitely as they head toward "higher ground".

The important principle to keep in mind is that each portion of the cross-section should be approximately perpendicular to the direction of flow in that part of the cross-section. In a sharp bend in a stream, the flow within the channel itself is probably following a path that closely approximates the sharp bend in the channel. However, if there are overbank flows, some of them are probably "cutting the corner" and following a shorter path through the reach that contains the bend. Therefore, it is very reasonable to arrange the cross-sections on the inside of the bend so that they radiate from a small area on the inside of the bend and become farther apart on the outside of the bend.

I hope that this brief explanation helps. If you need further information, you might be interested in downloading the draft text of the "McGraw-Hill Handbook of Flood Plain Management", which I am writing. This is currently about 7 months from being finished, so it is still very much in draft form. I use it as a text for my course on flood plain management and the National Flood Insurance Program that I teach for ASCE. Chapter 4 of the text contains the two figures that I have attached to this message, along with a lot of other information. You can download it from our company website at http://www.dodson-hydro.com/Download...oads_FEMA.htm. (Remember, it's in draft form; major revisions are underway and will be posted at this location over the next several months!)

Good luck!!