tag:blogger.com,1999:blog-1675184707067447729.post1343298996878211423..comments2017-06-22T09:30:24.125-07:00Comments on The RAS Solution: Downstream Boundary-Normal Depth?Chris G.http://www.blogger.com/profile/00354834185663924786noreply@blogger.comBlogger88125tag:blogger.com,1999:blog-1675184707067447729.post-10775638763174978302017-06-02T07:39:48.828-07:002017-06-02T07:39:48.828-07:00That would be my first thought as well. But 0.06,...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.Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-89833587570347316872017-06-01T13:41:27.878-07:002017-06-01T13:41:27.878-07:00Hi Chris,
I have a situation where the output of m...Hi Chris,<br />I have a situation where the output of my steady flow run results in the WS elevation remaining constant throughout the entire reach. Thus, after the first few cross-sections it shows a water column a hundred feet high. <br /><br />Here's an image of my output: http://imgur.com/a/0esAk<br /><br />Other's have suggested it's an issue of downstream boundary conditions, which I have set at 0.06. Any suggestions?Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-35733899906930143362017-05-22T08:46:28.025-07:002017-05-22T08:46:28.025-07:001. Yes.
2. That would be a good idea. But if yo...1. Yes.<br />2. That would be a good idea. But if your area of interest is far enough away from your boundary (and it should be), then it shouldn't matter.<br />3. That sounds like something else is the problem. You can check what the normal depth should be for a target water surface elevation by checking normal depth in the HD Uniform flow module in HEC-RAS. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-17645496918337393792017-05-21T03:50:15.408-07:002017-05-21T03:50:15.408-07:00Hello Chris,
I would like to ask you some questi...Hello Chris, <br /><br />I would like to ask you some question. I am simulate steady flow on a small study river with the length of 4 Kilometers. and my question are:<br /><br />1. there is no observed data of W.S at the downstream, so i decided to take the W.S from the previous study which has two cross section on my study area, as a boundary condition in the downstream (Known W.S). Is it acceptable for doing this ?<br /><br />2. As have mentioned above, I have used OBS W.S at the downstream as the Boundary Condition and the simulated results is the same as I entered at the Downstream. So If I want to calibrate the Manning's n, DO I need to Calibrate at the down stream? <br /><br />3. One more thing is that, I tried to use Normal Depth as BC but the result seem goes wrong unless I entered the (0.0000001) very small slope. Is there any problem with that ? Would you mind guiding me?<br /><br />I am looking forward for your repose.<br />Thanks you,<br /><br />Thaileng THOL<br />thol thailenghttp://www.blogger.com/profile/05688073262698187869noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-86511001183823247192017-05-03T08:44:43.260-07:002017-05-03T08:44:43.260-07:00Frank. Great question. First off, there's th...Frank. Great question. First off, there's the problem with measuring depth normal to the bed slope or vertically (parallel to gravity if you will). Under normal hydraulically shallow slopes, they are essentially the same. Once you exceed 10%, they begin to diverge significantly. The equations in RAS make no distinction between the two ways to measure depth. Also, when you get into steeper slopes, other phenomenon that RAS does not account for become much more prevalent, like vertical accelerations and air entrainment/bulking. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-59514080534927044592017-05-02T17:04:25.526-07:002017-05-02T17:04:25.526-07:00Chris-
Quick question regarding this old post. Ma...Chris-<br /><br />Quick question regarding this old post. May I know why HEC-RAS is not suitable for steep slopes?<br /><br />ThanksFrank Bhttp://www.blogger.com/profile/05412347470115363605noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-26767202244216432722017-05-01T11:08:33.921-07:002017-05-01T11:08:33.921-07:00At over 40%, that is too steep for HEC-RAS. 10% i...At over 40%, that is too steep for HEC-RAS. 10% is generally considered the maximum. You might consider either not modeling that part of the reach, or simulating it with an inline structure. You might also consider using the hydrologic routing option there. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-42267728752963510102017-04-28T15:18:25.078-07:002017-04-28T15:18:25.078-07:00Hi Chris, I´ve been modeling a mountain river, in...Hi Chris, I´ve been modeling a mountain river, in Colombia (https://drive.google.com/file/d/0B9PlUzaxo-3tREFBT2xpby1ySXM/view?usp=sharing) and I am not pretty sure about the upstream boundary because it´s so steep. I really appreciate your help. lunita_85http://www.blogger.com/profile/18226622832177178695noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-17449927217901558762017-02-22T13:34:39.391-08:002017-02-22T13:34:39.391-08:00Is this happening upstream of a bridge or culvert?...Is this happening upstream of a bridge or culvert? If so, you need to make sure you have consistency with your ineffective flow areas, and you may consider adjusting your coefficients. Criss-crossing profiles typically happens when the bridge or culvert switches from low flow to high flow equations. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-46666580637730079372017-02-16T21:58:20.235-08:002017-02-16T21:58:20.235-08:00Hi All,
I have a question. I am using steady stat...Hi All,<br /><br />I have a question. I am using steady state model for generating river depth inundation maps for different RPs (return periods) such as 10RP, 20RP, 50RP and 100RP. The flow is gradually increasing in every RPs in higher order. All other parameters are constant. While looking over the maps, at few pixels, I am getting higher depth in 10RP map than 20RP. I am not undersatnding, why it is happening? Would you suggest me something??? Vishal Singhnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-75400032842037443702016-08-25T08:26:06.268-07:002016-08-25T08:26:06.268-07:00Jeremy-it is impossible to say. Every river is di...Jeremy-it is impossible to say. Every river is different, but the slope and flow rate plays a primary role in how far downstream you should place your downstream boundary. A very flat river (say the Lower Mississippi River) might project backwater effects upstream for many kilometers, while a steep mountain stream may only project backwater effects upstream several meters or less. <br /><br />To approximate how far downstream to get topographic data, sometimes I'll construct a basic model with approximate cross section shapes (without any topo data, but trying to be as close to reality as you can). You can easily approximate channel slope off of readily available topo maps, and you can approximate a channel shape/size based on aerial photography and/or a site visit. Simple trapezoidal channels should be good for this exercise. After you do a number of models, you'll be able to get pretty close just on gut feel. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-22603286759787173902016-08-24T18:17:59.450-07:002016-08-24T18:17:59.450-07:00Chris, thanks so much for your help! I have one mo...Chris, thanks so much for your help! I have one more question on boundary conditions. In your experience, if I'm using the normal depth boundary condition and I'm fairly confident with my friction slope, how far downstream will I need to extend my model from my study reach to be outside the introduced error? Is it normally 50', 100', 500', or more? I know I'll need to run a sensitivity analysis, but having a general guideline will help in knowing how much more field run geomorphic data I'll need to collect. <br /><br />Thanks,<br />Jeremy<br /> Jeremy Paynehttp://www.blogger.com/profile/14948536433814207926noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-3789464458029200322016-08-22T07:46:03.930-07:002016-08-22T07:46:03.930-07:00Critical depth would be appropriate as a downstrea...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. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-12214313596537508382016-08-21T16:28:59.806-07:002016-08-21T16:28:59.806-07:00Hi Chris,
I typically use normal depth as my down...Hi Chris,<br /><br />I typically use normal depth as my downstream and upstream boundary conditions following the guidance you provide in this post, but the company I work for has always used the critical depth boundary condition for every model they run. Before I try to change policy, I was wondering under what conditions would critical depth be a more appropriate boundary condition than normal depth, and would the boundary cross section for one method need to be placed further up/downstream from my study site than the other method to limit the error? Thanks so much for your help!<br /><br />Jeremy Jeremy Paynehttp://www.blogger.com/profile/14948536433814207926noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-57369261836340309472016-07-25T08:14:26.412-07:002016-07-25T08:14:26.412-07:00That's strange. It works fine for me. Perhap...That's strange. It works fine for me. Perhaps you need to set your Windows international settings to American English. And make sure that you are using periods (.), not commas (,) for decimal place separator. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-45708365230951502752016-07-24T08:33:35.012-07:002016-07-24T08:33:35.012-07:00Hi Chris, for steady flow in reach boundry conditi...Hi Chris, for steady flow in reach boundry condition while I putting number into dowstream and upstream value, it turns automatically exact number. For example when I put 0.03 and I press 'ok' it turns 3. or I put 0.0017 it turns 17 same way. Also I realize no difference between 0.3 0.03 or 0.043 the program turns all automatically 3. What is the problem? Thanks.vatan tosunhttp://www.blogger.com/profile/01891008761191173634noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-28340095789957419552016-03-30T07:55:04.386-07:002016-03-30T07:55:04.386-07:00You're welcome. Actually, from my understandi...You're welcome. Actually, from my understanding RAS does not compute differently depending on the slope. They are pointing out in the text that the errors due to the differences in how depth is conceptualized is significant when the slope is greater than 10%. What is not mentioned is that some of the other equations used in the computations (Manning's primarily, but also weir equation, orifice equation, etc.) were developed under the premise that normal depth and vertical depth are one in the same. Meaning once the two depths become significantly different, (i.e. > 10% slope), the equations don't work as intended. That being said, the errors, even with a slope greater than 10%, are small enough that with proper model setup (tighter cross section spacing for one), and appropriate calibration, you can still get reliable results. Bottom line is, don't blindly trust the results. Calibrate...and then validate. But you should do that with all models anyway, not just those with slopes greater than 10%. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-79139427435624851932016-03-30T06:35:44.697-07:002016-03-30T06:35:44.697-07:00Thanks for your quick response and helping me to c...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!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-62310296937062209132016-03-28T16:35:47.950-07:002016-03-28T16:35:47.950-07:00No, you don't have to apply a correction facto...No, you don't have to apply a correction factor. HEC is just pointing out that the difference between depth measured normal to the bed and depth measured vertically (i.e. parallel to gravity) becomes significantly different when your slopes exceed 10%. If you would like to know the difference then you can compute it with the cosine correction. However, I would be careful with relying on results in long stretches of slope greater than 10%. Other things start to come into play like air entertainment and bulking, cavitation, cross waves/standing waves. None of these things are considered in HEC-RAS but can become very important when modeling steep slopes. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-36389576900092367922016-03-28T10:37:23.396-07:002016-03-28T10:37:23.396-07:00Hi Chris,
In reference to Steady Flow limitations,...Hi Chris,<br />In reference to Steady Flow limitations, Chapter 2 of the RAS hydraulic reference manual (page 2-20 to 2-21) states that for slopes greater than 1:10, an error in the depth computation reported in the software exists due to the magnitude of the slope. The depth must be divided by the cosine of the slope angle to retrieve the actual depth of water. <br /><br />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.<br />Thanks so much for your time!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-76234364524336762302015-12-10T21:59:47.487-08:002015-12-10T21:59:47.487-08:00Sounds like you haven't added in your low chor...Sounds like you haven't added in your low chord for the upstream and downstream sides of your bridge. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-53354238612911948312015-12-10T19:50:39.344-08:002015-12-10T19:50:39.344-08:00Good night:
I'm doing a project for my hydrol...Good night:<br /><br />I'm doing a project for my hydrology course using HEC RAS.<br />When I run the program, it gives me the next error: <br />-Bridge does not contain an opening on the upstream side.<br />-Bridge does not contain an opening on the downstream side. Ramon Lopezhttp://www.blogger.com/profile/04057294857992453556noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-36154524768221142482015-05-15T07:56:52.317-07:002015-05-15T07:56:52.317-07:00Its impossible for me to say without knowing more ...Its impossible for me to say without knowing more about your project. The computation interval can vary quite a bit for a given project. Really, it's up to you to find an interval that creates a stable model and accurate results. Adherance to the Courant condition is not required, but generally gives you a good estimate for the computation interval. In that case, your time step would be a function of wave velocity and cross section spacing. I'm wondering why you assume increasing your time interval will reduce the fluctuation? And a 1 meter change in water surface elevation doesn't seem outrageous to me for an increase of 700 cumecs. I guess it depends on the size of your stream and how steep its bed slope is. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-70301805011367523672015-05-15T01:25:34.909-07:002015-05-15T01:25:34.909-07:00Hi Chris... recently I am doing one analysis for i...Hi Chris... recently I am doing one analysis for impact on a downstream irrigation project due to operation of Hydro Power plant. the hydro power plant will release about 700 cumec into the for 3 hours and for next 21 hours in the day the power plant remains closed. so no discharge downstream. An Irrigation scheme with side channel exists about 150 km downstream of this power plant. in this stretch of 150 KM, 2 major rivers also joins.<br />I did the analysis to see the fluctuation in the water level and used 3 minute as computational interval. however the result shows a fluctuation more than 1 m at the location which is hard to understand and accept. is it possible to increase the computational time interval and justify this increased computational time interval.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-61514687546841888962015-05-14T11:28:37.228-07:002015-05-14T11:28:37.228-07:00In theory, you should select a reach that is flowi...In theory, you should select a reach that is flowing under uniform (normal) conditions, in which case the bed slope and friction slope would be equal. However, that is rarely the case in natural streams. The bed slope can still be a decent approximation, but the friction slope is the theoretically correct term to use to satisfy Manning's equation.Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.com