Energy For Bulgaria 7th Grade.rar
Rosaura Woolf
Regardless of the method you use to get an energy slope, there will be some error associated with what you use. Besides the fact that you have to more-or-less guess at an energy slope, natural rivers and streams rarely-if ever exhibit uniform flow conditions. But, if we can manage the error associated with using normal depth, it can serve as a very convenient tool for estimating the downstream boundary condition.
To manage the error, I do a couple of things:
Here, you can see with the sensitivity study, the three profiles (with three different downstream boundary energy slopes) converge at the bridge. It might be wise to move the downstream boundary downstream a little further. Comments Eric on March 4, 2011
Is calculating the energy slope via average bed slope of the last two XS's preferred over taking the average bed slope of the entire reach (using GIS, lowest and highest elevation on reach divided by the channel length)?
Hi Juan. Glad you enjoy RASModel.com. Normal Depth is a very easy and convenient way to set your downstream boundary. It's probably the most commonly used (at the downstream end). Keep in mind though, that there will always be some error with the assumption of normal depth and your assumption of the energy slope. This is why it is good to have your downstream boundary far enough away from your area of interest such that the errors associated with the boundary do not show up at your area of interest.
With regard to split flow optimization, if the results upstream of the split are subcritical, the upstream boundary condition does not matter. RAS only uses the flow just upstream of the split, and an energy balance on the two downstream reaches to figure out the flow distribution at the split.
For critical depth to be that much higher than mean high high tide, something is not correct. You may need to push your downstream boundary further into the estuary. It's best to have your downstream boundary well into the receiving body of water, where depths are deep, velocities are slow (near zero) and water surface and energy grade elevations are almost the same. That makes for a good boundary location if you're using MHHW. Normal depth can be applied on an adverse slope, just use either the average bed slope in your reach, or even more accurately, use the average slope just downstream of your downstream-most cross section.
Hi Chris. I am modelling dam breach and flood inundation mapping. I used the model BREACH of NWS to model the breaching process and got an outflow hydrograph with a tremendous peak discharge, i.e. 92,000m3/s. I used this hydrograph as input for HEC-RAS to simulate unsteady flow. I extracted elevation data from google earth and set 26 cross-sections for my 28km reach length.bed slope of the last cross-sections is used for normal depth boundary condition.downstream release of the dam ranging 3-5m3/s is used as minimum flow and initial flow condition. Having set all these input, I tried to run the model by varying the computation time step, adjusting the minimum flow and d/s slope a bit. However, I got unrealistic results such as, very undulated water surface profile, profile with highly exagerated depth (up to 100m), no water surface profile view excepth energy grade lines.
Assuming you are talking about using the energy slope for a normal depth boundary condition. In that case, what you have set as the downstream end of your model may not be a good place to set your boundary. You want to pick a place that exhibits fairly uniform flow conditions. However, if you are limited to using this as your downstream boundary, then I would measure the bed slope downstream of your downstream-most cross section (use a topo map if you have one). Just make sure that your area of interest is well upstream (and out of the influence of) your downstream boundary.
Does this apply in all cases where you have steep sloped channel sections? In my specific case I have a subcritical condition (1% slope) that breaks over a 20% slope for a distance of about 100 feet then back to a subcritical tailwater condition. Using mixed flow regime, the energy slope reaches .28 ft/ft during the supercritical section. Must one take the depth computed by RAS in the steep sloped region and apply the correction factor? I understand the influence is generally small.
Thanks so much for your time!
Thanks for your quick response and helping me to come to an understanding on this (learning hydraulics). So for slopes < 1:10, RAS computes the vertical depth as normal depth (negligible error acknowledged), but for slopes > 1:10, it is computed as d x cosine of slope angle. I guess I was just a bit confused by the language on Page 2-21. I didn't understand why they would need to acknowledge "the error" on slopes > 1:10 if the software correctly (generally speaking) computes the vertical depth on the steep sloped sections and then later in the text suggest to obtain the "correct depth" instead of using the term "normal depth" or "perpendicular depth". Working through the energy/momentum equations by hand helps me to understand my misinterpretation of the text. Thanks again!
Critical depth would be appropriate as a downstream boundary placed on a natural drop (waterfall) or a severe grade break where you would expect critical depth to occur. That being said, if the boundary condition is far enough downstream such that the introduced error works itself out before your area of interest, than it will work fine. Normal depth is not perfect either, but typically provides a much closer approximation of the depth in rivers and streams. When running a sensitivity study on the downstream boundary, a common approach is to bracket the possible boundary conditions by using critical depth to get your lowest possible starting water surface elevation and normal depth with a really low energy slope to get your highest possible starting water surface elevation.
That would be my first thought as well. But 0.06, while steep, should not cause the issue on it's own. Are you getting any errors or warnings in the summary of errors, warnings, and notes. Perhaps the steep d/s boundary condition creates a low stage which then overestimates the stage at the next cross section. Closer spacing of cross sections should help in that case. I'd also suggest trying some different energy slopes for your downstream boundary to see if that fixes the problem.
I have a project that ends approximately 300 feet upstream from a culvert. We used HY-8 to develop a rating curve from the culvert to use as a downstream boundary condition of our HEC-RAS model. How would I do a sensitivity analysis to determine how far downstream I need my last cross section using a rating curve? I understand how to do it using normal depth by varying the energy slopes, but how does that work with a rating curve?
When modelling a channel (dredging) with no slope along 7 km long what boundary condition should be used as a normal depth? I mean since HECRAS does not accept zero value as normal depth condition what would be the ideal approach for it? For instance if I use 0.0001 as ND BC it gives a water depth of 5 m in the channel but if I changed it 0.00001 then it is a different story and the depth of water in the channel becomes 10 m. I have tried sensitivity analysis you have suggested in your post and it took me no where, cos every time I have lowered the energy slope the depth of water in the channel ends up with some dramatic changes. I am not sure which answer is correct in the case when channel line has no slope.
In your case, I would focus in on the energy grade slope for the downstream part of your reach and use that instead. It will require you to do some trial and error with different normal depth slopes to get a feel for what energy slope to use.
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