HEC RAS backwater downstream cross section

[segeraj]

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!

 

[francesca]

You need to place a junction. Take careful notice of the Junction Modeling options in both the manual and the Hydraulic Reference Manual. Things like Junction width will dictate.

 

[segeraj]

I don't have sufficient data on the Main River to place a junction, and this application isn't worth the extra time and money to survey or obtain that information. Is there a 'more correct' method between placing the most downstream cross section at the confluence versus placing it at the edge of the studied backwater?

 

[francesca]

You can still determine the junction width and place the downstream XS immediately upstream, even if you don't model the junction itself. There is no quick answer, you actually have to do the work in order to have a reasonable model. Being conservative (WSE-wise) you would have the downstream cross-section at the confluence point. However, backwater effects will propagate further upstream than the confluence. How are you accounting for those?

 

[segeraj]

First, I was accounting for the backwater conditions by placing the most downstream cross section in my model at the upstream end of the known backwater and using that backwater elevation as a downstream boundary condition (The subject property is upstream of any backwater effects).

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)

 

[francesca]

Think of the physics of it. Depending on the angle of confluence, the water flowing down the tributary will be hitting a wall of water at high velocity going in a different direction. It will be at least partially ineffective (stagnant/low conveyance). That effect dissipates upstream. Setting a downstream boundary condition as a known water surface elevation assumes a fully-effective cross-section. Using the junction tool lets you model the energy losses at the confluence using the momentum equation.

... 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.

 

[gbam]

segeraj - Have you considered the times to peak for the main watercourse and the tributary. This is referred to as Frequency for coincidental occurrence. If the ratio of drainage areas are large (1,000:1), it is recommended to evaluate the tailwater condition for a 10-Year event while evaluating the tributary for the 100-Year event and visa versa. The probability of the two peaks hitting at the same time reduces with an increased area ratio.

However, one can assume worse case scenario and use 100-Yr - 100-Yr.

 

[segeraj]

gbam - Yes, i considered both basins and they are similar in size. That is why i feel comfortable using the 100-yr to 100-yr. One, it is a conservative approach, and two, it isn't that unreasonable.

francesca - Thank you for all your help, i really appreciate it!