HEC-RAS Questions

I am trying to model flow through a culvert with an invert below the existing stream bed and have several questions.

1. Does HEC-RAS assume that flow is carried through the culvert to the invert elevation or to the low point of the adjacent cross-sections?

2. If flow is passed all the way down to the invert, can I use the Blocked Obstruction Option to account for lost flow area due to siltation?

3. Where can I find documentation in the manual to answer these questions, if at all?

Thank you in advance for your assistance.
Jason Wallace
DESINE, Inc.

Dear Jason Wallace :

Please see my responses below.

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

> I am trying to model flow through a culvert with an invert below the existing stream bed and have several questions.

Note that HEC-RAS has been known to crash when the culvert invert is below the channel bed elevation.

> 1. Does HEC-RAS assume that flow is carried through the culvert to the invert elevation or to the low point of the adjacent cross-sections?

It looks at the culvert barrel independent of the cross-section. Therefore, it will not see that the end of the culvert is blocked.

> 2. If flow is passed all the way down to the invert, can I use the Blocked Obstruction Option to account for lost flow area due to siltation?

The Blocked Obstruction option can only be used for normal flow cross-section equations. It cannot be used to block flow area withing a culvert.

> 3. Where can I find documentation in the manual to answer these questions, if at all?

There are not answers to the above questions, since these are quite technical in nature. However, you can ask me, since I am one of the product developers at BOSS International.