HEC RAS backwater downstream cross section
HEC RAS backwater downstream cross section
(OP)
I am modeling a average size stream (Model Stream) (14 sq miles) into another studied stream (Main River). I am using the backwater or BFE of the Main River as my downstream boundary condition. I want to know where to place my most downstream cross section on the model stream. Do I place it as close to the confluence with the Main River as i can get? Or do I place it at the extents of the Main River backwater (appx. 2000 feet upstream)? Thanks!





RE: HEC RAS backwater downstream cross section
RE: HEC RAS backwater downstream cross section
RE: HEC RAS backwater downstream cross section
RE: HEC RAS backwater downstream cross section
Then i placed the most downstream cross section near the confluence of the Main River and gave the same BFE for the downstream boundary condition. I found that it produced similar results. (by the way, this study is not to establish a new BFE, but to compare pre vs. post conditions on a subject property)
I'm not exactly an expert in HEC, but i know enough to formulate a model. Is setting the known backwater elevation (BFE of Main River) as your downstream boundary condition not the proper method? Thanks for all your help!
(I will try the junction option as well)
RE: HEC RAS backwater downstream cross section
... However, if your area of interest is sufficiently upstream of the confluence to be outside the influence of the backwater during flood conditions on the main channel, you may not need to model down to the confluence. You might be better off assuming normal depth as a downstream boundary condition and only modeling one or two additional cross-sections downstream. This is logical because the peak flood on a tributary rarely coincides with a peak flood on a main branch. (Times of concentration on the two need to be compared to verify this is the case.)
Using the higher known water surface elevation of the downstream confluence might mask the backwater effects of the change you're trying to model on your site. Using normal depth could potentially show a larger difference as a result of the development. You can test this by creating two flow scenarios with two different downstream boundary conditions.
RE: HEC RAS backwater downstream cross section
However, one can assume worse case scenario and use 100-Yr - 100-Yr.
RE: HEC RAS backwater downstream cross section
francesca - Thank you for all your help, i really appreciate it!