tag:blogger.com,1999:blog-1675184707067447729.post1454243263174850062..comments2017-06-22T09:30:24.125-07:00Comments on The RAS Solution: Weir Equations in HEC-RASChris G.http://www.blogger.com/profile/00354834185663924786noreply@blogger.comBlogger11125tag:blogger.com,1999:blog-1675184707067447729.post-12995093945510626242017-06-12T13:34:35.900-07:002017-06-12T13:34:35.900-07:00It is as written in the post. The following artic...It is as written in the post. The following article does a good job deriving the weir equation if you want verification: http://www.codecogs.com/library/engineering/fluid_mechanics/weirs/discharge.php<br />Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-74406893077200318812017-06-12T13:18:42.420-07:002017-06-12T13:18:42.420-07:00As mentioned in the post, HEC mistakenly uses Cd i...As mentioned in the post, HEC mistakenly uses Cd in the software occasionally when they actually mean C. All weir coefficients you enter in RAS are the non-dimensionless C values (i.e. they are different for English versus SI units. So be careful with this. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-81340457031453145362017-06-12T10:51:50.663-07:002017-06-12T10:51:50.663-07:00My conundrum regards the Weir coefficient in the “...My conundrum regards the Weir coefficient in the “Storage Area Connection Weir data”<br />Q1: It is written, Weir coefficient (Cd). Now from your explanation Cd is the Discharge coefficient and C is the weir coefficient. This is very clear. Have the makers of HEC-RAS made a mistake? Did they mean C (Weir coefficient) or Cd (The dimensionless weir coefficient)? You wrote “In fact, you’ll see the coefficient Cd labeled occasionally in the HEC-RAS software and literature when discussing weir coefficient. “. Is this one of these occasions?<br />The HECRAS reference manual also sometimes refers to C as coefficient of discharge, further compounding the problem.<br />Q2: You also wrote that C=2/3*Cd-(2g)^(0.5) but some references have [C=2/3*Cd-(2/3*g)^(0.5)] ; This is confusing. <br />Emmanuel Jjunjuhttp://www.blogger.com/profile/16037830668397974731noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-73699204348229987082017-06-12T10:29:38.547-07:002017-06-12T10:29:38.547-07:00Hello,
if you look at the "Storage Area Conn...Hello,<br /><br />if you look at the "Storage Area Connection Weir Data", there is a field for input of "Standard Weir Equation parameters". Under this it is written, Weir coefficient (Cd). Now from the explanation Cd(discharge coefficient) whose relationship to C is C=2/3*Cd-(2g)^(0.5) [by the way some books have [C=2/3*Cd-(2/3*g)^(0.5)] which is confusing. have the makers of HEC-RAS made a mistake? did they mean C or Cd?Emmanuel Jjunjuhttp://www.blogger.com/profile/16037830668397974731noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-62282784794402513742016-12-12T13:34:15.065-08:002016-12-12T13:34:15.065-08:00Toby-Great questions! the quick answer is: "...Toby-Great questions! the quick answer is: "if it is acting like a weir, model it as a weir, otherwise model it with the 2D equations." However, as with many things in RAS, there's a lot more nuance involved. I like to think about the input data required and how confident I am with it. The weir equation requires a very subjective weir coefficient. The 2D equations do not. Although the 2D equations come with their own subjective assumptions and limitations: terrain quality, theta value, eddy viscosity, full momentum or diffusion wave, etc, etc. So for me, if it's an elevation connection, I'd lean towards the weir. If it is overland flow, I'd lean towards using the 2D equations. HEC does caution against using the 2D equations if the connection is elevated such that water going over it will experience "free fall" (i.e. a waterfall). The 2D equations can't handle that. This might just be another good example where you try them both as part of your sensitivity analysis. Also, keep in mind that the weir width is not used at all in the computations (weir or 2D equations). It is purely there for graphical purposes. As far as advice to reduce 1D/2D flow errors over lateral structures: Break up long lateral structures into multiple smaller ones. Be careful not to use too high of a weir coefficient. Do not have cells adjacent to the lateral structure that reside completely on the slope of the levee/berm/whatever you're modeling with the lateral structure. I typically like to take the cross sections right to the peak of the high ground feature, but start my 2D cells at the toe of the feature. Good luck!Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-31200805613391461182016-12-12T12:22:36.268-08:002016-12-12T12:22:36.268-08:00Hi Chris,
Do you have any insight into the new &q...Hi Chris,<br /><br />Do you have any insight into the new "Overflow Computational Method" for lateral structures into a 2d area (e.g. Normal 2D Equation vs. Use Weir Equation)? If one is trying to model a 1d/2d connection where there is overland flow and a non-elevated overbank, what might be the benefits or shortcomings of each method? What would you set your weir width to for an overland flow situation with a low weir coefficient chosen from the table above? Any other advice to reduce 1d/2d flow errors over lateral structures?<br /><br />Thanks!Tobynoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-75183613389867227822016-09-22T08:00:37.410-07:002016-09-22T08:00:37.410-07:00I don't know what the "with" paramet...I don't know what the "with" parameter is. Please clarify.Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-57716174905321435782016-09-21T19:48:50.697-07:002016-09-21T19:48:50.697-07:00Chris
In a ogee crest spilway what is the meaning...Chris<br /><br />In a ogee crest spilway what is the meaning of the with parameter?Rodrigohttp://www.blogger.com/profile/00260309680557414370noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-10971923044384543322016-09-16T07:50:21.121-07:002016-09-16T07:50:21.121-07:00Hi Chris..........My name is Rodrigo and i am from...Hi Chris..........My name is Rodrigo and i am from Chile.<br /><br />I have a question about modelling gated weirs with ogee shape.<br /><br />The downstream section of the weir is the natural stream section or its located at the end of the ogee profile??<br /><br />Best regardsRodrigohttp://www.blogger.com/profile/00260309680557414370noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-38591692440757537192016-07-15T07:34:24.628-07:002016-07-15T07:34:24.628-07:00HEC-RAS is not a suitable tool to simulate detaile...HEC-RAS is not a suitable tool to simulate detailed hydraulics down a stepped spillway. However, to the extent you can simulate the effect of a stepped spillway on the upstream energy head using the equations above, it can certainly be part of a HEC-RAS model. Chris Goodellhttp://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-35700008496838171112016-07-15T00:21:01.245-07:002016-07-15T00:21:01.245-07:00Can a stepped spillway simulation performed with H...Can a stepped spillway simulation performed with HEC-RAS?Unknownhttp://www.blogger.com/profile/11529639341314911423noreply@blogger.com