HEC-RAS Boundary Condition Question for BFE Determination 2

[adaters]

I'm modeling a stream with HEC-RAS that ties into a studied river with established BFE's. I'm trying to establish a BFE on a residential lot adjacent to the stream. My area of interest along the stream is only about a 1/2 mile upstream from the junction with the river. Am I correct in assuming my known water surface elevation downstream is the BFE for the river at the junction, and then using that for my downstream boundary reach condition on my stream?

Or do I just need to ignore the BFE of the river and set my downstream boundary condition of the stream to normal depth since they won't both peak at the same time?

Thanks for any suggestions!

 

[francesca]

If you have a cross-section close enough, you want to set your downstream boundary condition to the established WSE. If not, then you want to make certain that your normal depth is reasonably above that elevation. I'd go with the former.

 

[adaters]

Thanks for the help. Yes, I can cut a section right where my stream hits the river with the established BFE's. I guess for clarification, do I want to use the 100 year BFE as my known downstream WSE when I run a 100 year flow thru my stream model, or do I need to find the normal WSE?

Thanks again.

 

[gbam]

Maybe, check with FEMA's requirements. I have studied a smaller tributary that has a significantly smaller watershed and typically use the concept of "Frequencies for coincidental occurrence", see FHWA's HEC-22. This approach typically sets the design frequencies based on drainage area ratios. For instance a 10,000:1 ratio suggests using a 10-Year trib with 100-year main and visa versa. It has been a while since I submitted a FEMA study for a tributary. I would check with FEMA first.

 

[francesca]

The only way to determine if the downstream main river has a backwater effect on your stream is to model it with the downstream BC as the BFE. While your tributary's peak flow won't coincide with the main river's peak flow, the highest flood may occur with a lower flow when the main river is peaking, due to backwater effects. In that situation, using the 100-year flow on your tributary with the BFE as downstream BC is the conservative approach. If the main river has no backwater effect, then as long as you have enough cross-sections between there and the limit of your area of interest, HEC-RAS will settle and you'll get the same results as using normal depth or critical depth as your BC.

 

[gbam]

francesca - good point that combination would be the worst case.

 

[adaters]

You guys got to it before I did. Both stream and river will most definitely NOT peak at the same time. My big hangup was trying to figure out what kind of backwater effect the main river was going to have on my stream. I'll be sure to cut enough cross sections and test a couple different scenarios and see what kind of results I get. Thanks again for helping out this HEC-RAS beginner!