Culverts in Hec-GeoRAS

[JohnGalt]

Hi one and all,
Noob to the forum from Exeter, England.

Im working on a high level piece of flood analysis of a small catchment within the cities boundaries.
I have been using ArcGIS 10 to import lidar data, then using the hydro/spatial/3d analysis tools to get slope data, contours and the relevant tin surfaces.

The catchment in question contains two steam reaches which are connected by a culvert.
My question is regarding the digitisation of the stream and the culvert. i have crated the layres in the RAS Geometry and used the editor to sketch the course of the upper reach of the stream. When i get to the culvert should i......

a) stop, then start the new stream digitisation for the lower reach and then digitise the culvert seperately in the gap.

b) continue the stream from top to the very bottom including the culvert.

If i do b) do i then have to trace directly over the line where the culvert is situated?

I only ask as my thought is that if i draw the line from top to bottom including the culvert, when i set the stream attributes will it think that the culvert is actually lying on the surface of the tin?

Hopefully that makes sense as these things are always tricky to put into words....

The end result im hoping for, is to extract the data into Hec-RAS and complete some more in depth hydraulic analysis.

If anyones got any words of wisdom or guidance that would be great.
Many thanks

John

[MattMead]

The stream sections should be represented by a single stream centerline for a number of reasons.  1) In HEC-RAS your culverts are just placed as structures within the stream not connections between them.  2) When you bring this into HEC-RAS your culvert will be placed based on its upstream and downstream cross section.  If you break the stream centerline HEC-RAS will view that as two separate streams and not allow you to do so.  3) If you split the stream at the culvert HEC-RAS would view this as a junction.  You cannot have a junction with only 2 elements.

Also, dont use the bridge/culvert tool in Hec-GeoRAS.  The end result is almost never what you want it to be.  A better way to go about it is to draw an additional cross section along the bridge/culvert top of road(or high ground if not a road) and use this cross section river station for placement and geometry for your structure deck.