Top width problem/error

[pagesk]

I decided to model a box culvert as a bridge so that I could add shape to the channel through the box. In Washington, we frequently do whats called natural channel design, where the culvert is buried up to 50% of its depth in natural stream materials. I'm replacing a 50-inch and 30-inch CMP culvert set with a 6x20 box culvert that will have from 2 to 3 feet of material in the bottom.

I noticed from the viewing the profile there was very little head loss through the box, investigating further I found that the top width reported by RAS was nearly 40 ft where as the bridge opening is only 20 feet wide. Additionally, the flow area was reported as 1,500 sf rather than just under 80 sf. Which explains the decrease in velocity and lack of head loss. I found I got the same result if I added a lid to the cross section rather than identifying a bridge. I'm using RAS 4.0.

Any ideas? I searched the forum and didn't find any similar problems.

TIA

[damber]

Do you have a deck in your bridge unit? I'd think that by setting the deck to be super high and wide, you would stop any flow over the top and force all flow through the opening

[pagesk]

Yes, there is a bridge deck. No water is going over the bridge. Thanks for your suggestion.

I have been playing with this for about an hour and have found that if the bridge deck or lid is horizontal or uniformly sloped there is no problem. But if I add elements to the lid or bridge deck that do not coincide with the ground points it blows up, which is contrary to the users manual.


Page 6-35 of the users manual states that "The stationing of the deck does not have to equal the stations in the bounding cross section, but the must be based on the same origin"

It's not a problem to toss in a few extra ground points.

[pagesk]

Oh well, I'll keep experimenting

[jarvus lunt]

pagesk:

Are you aware that RAS can "fill" the bottom of a culvert with sediment? So you could, for instance, fill the bottom third of a circular culvert. This is the "Depth Blocked" field on the culvert editor. You can also give the sediment a different manning's n value with the Manning's n for Bottom field.

However, this option won't work if you need to vary the depth through the length of the culvert.

[pagesk]

Thanks Jarvis. I am aware that you can fill a culvert and as you noted it has to be uniform. I was using a bridge so I could vary the shape in the box. My problem arose when I tried to add the corner blocking in the lid of the box. Horizontal lid..no problem. Stepped down a the corners and the reported top width and cross sectional area blew up.

Thnaks again for having a look at my problem.