If these curves are too coarse, you can get weird results at bridges and culverts. This will help to refine the family of curves that RAS uses at this culvert during the computations.
![hec ras culvert tutorial hec ras culvert tutorial](https://www.hec.usace.army.mil/confluence/rasdocs/rasum/files/latest/39297889/39297888/1/1605725473784/worddavd3804f894312a7a3a99daea25f2f649d.png)
Also, provide a max elevation and max flow. I'll usually try 80/60/40 for my number of points/curves. Try to refine the parameters as much as possible. However, since it is an unsteady flow model, check the Htab parameters for your culvert first (its easy to do). This needs to be modeled with ineffective flow areas. It needs some distance for that water to contract.
#Hec ras culvert tutorial full#
Imagine a full flowing channel that has to squeeze through a culvert. But, I think my comment on the ineffective flow areas still applies, especially now for an unsteady flow model. I should mention that the model is a dam breach model, so it is unsteady, with a very steep hydrograph. Give that a try, if it's still causing problems, let me know.Īctually, it is the two structure cross sections created by HEC-RAS that have the jumps.
![hec ras culvert tutorial hec ras culvert tutorial](https://www.kleinschmidtgroup.com/wp-content/uploads/attachments/attachment-6668-1.jpg)
You might try interpolating some cross sections and see if that helps. I've also seen this phenomenon where the cross section spacing is too coarse around the crossing.
![hec ras culvert tutorial hec ras culvert tutorial](https://www.surfacewater.biz/wp-content/uploads/2017/05/08-culvert-vs-custom-bridge-sub.jpg)
Make sure you are properly defining the contraction and the expansion to and from the culvert. Usually when I see a jump in wselev in those cross sections, it is due to non-existent, or poorly-defined ineffective flow areas in those cross sections. I'm assuming that by the culvert cross sections, you are talking about the bounding cross sections (cross sections 2 and 3, in HEC-RAS "lingo"). I have tried changing the modeling approach and all of the other variables I can think of. I have a situation where the wselev jumps up a couple of feet in the culvert cross sections compared to the upstream and downstream cross sections. Ineffective flow areas upstream and downstream of the crossing. Coefficients (bridge and culvert coefficients). Things you can tweak that may provide more sensible results and a better set of Htab curves are: Not a bad technique, but obviously didn’t completely solve the problem. It appears that the creater of this model tried to lessen the impact of the ineffective flow areas instantaneously turning effective by significantly raising up the n-values in the overbank. Also, notice that the ineffective flow triggers turn off in this range. It’s very obvious from looking at the plot that this is the range at which the flow transitions from low flow to pressure flow and then on to pressure and weir flow. This example shows a significant discontinuity at around 10,000 cfs (you can click on the figure above to get a better view). First of all, I always like to open up the bridge plot along side its htab plot (make sure the vertical axis is consistent) so that I can graphically explain any discontinuities in the htab curves. Take the following example, in the figure below. Also, keep in mind that the equations for culverts are very different from those used for briges in HEC-RAS. going from low flow to pressure flow, or pressure flow to pressure and weir flow), or when ineffective flow triggers turn off/on. Typically you see problem areas where RAS changes equations (i.e. Here you’ll want to examine the curves and look for any abrupt changes, or discontinuities, particularly in the range of flows/depths where you are seeing the discrepancy. Click Type…Internal Boundaries, and you’ll see the family of rating curves for your crossing.
![hec ras culvert tutorial hec ras culvert tutorial](https://i.ytimg.com/vi/j8S0O4hulLM/maxresdefault.jpg)
Once that’s done, on the main RAS window, go to View…Hydraulic Property Plots. If you are running a steady flow model, you can create an unsteady flow plan and just run the geometry pre-processing task (you don’t need to run the computations or the post-processing. If you are running unsteady flow, this is done for you during the geometry preprocessing task. Create and evaluate the hydraulic property plots for the crossing. For the first case, “bad geometry”, here’s a technique that can be used to help spot sources of problems.