Align Cells with Flow Direction: HEC-RAS 2D Mesh

hello my name is Stanford Gibson I am the sediment transport specialist at HC and this is a regional sediment management tools and training video the objective of this video series is to help sediment modelers and analysts develop best practices to help them support Regional sediment management decisions with high quality engineering and science so in this video we're going to talk about how to edit an orthogonal 2D mesh in HC Raz to make it align with the direction of flow and I'm going to use the terrain from a model that we're actually using using for an active RSM study so when you make a mesh in HC Raz you get a structured grid and so if I turn on the mesh associated with this this is the default mesh it's got cells of 100 by 100 which is way too big for this system and they're orthogonal you know they're Square in the north south east west Direction so the first thing we'll do here is we'll just let's just edit this mesh and then click on the perimeter I always forget where this is frankly and uh click the edit 2D Area Properties and we get this properties for the debris Basin and let's just say hey we you know we want spacing of 10 x 10 and so we'll generate our computation points and it's going to overwrite the ones that you have and we'll say okay and there we go now we have a much more appropriate mesh but it's still orthogonal this ends up being a mostly structured grid you know there's some unstructured elements on the edge but most of it is just these squares now HC res can use this grid um we can do hydraulics or sediment transport with a grid that is not aligned to the flow but you're going to get better performance both for stability and accuracy if your grid is aligned with your direction of flow and so in this video I'm just going to show you two different ways to do that the first way is the way that we normally do that I'm going to focus on kind of this main Channel up here um I would do this in three different parts and we're going to start with a brake line um now the first thing that I want to do though is I want to measure kind of the distance of this Channel and so if I go in here and measure kind of from Toe to Toe of this channel you'll see down in the bottom right hand corner this is like a 75t bottom width and then the top width if I go kind of bank to bank you know that's uh 100 and 25 here it's 100 farther upstream and so you know we're going to have a number of cells you know between you know 7 to 10 cells across this and so what I'm going to do is I'm going to go and I'm going to create a break line and so if I go up here and try to edit I can't because I have one of these other tools selected this is pretty common if you have like the pan tool selected and then you go to edit you're like oh no I can't edit um all you have to do is click on this Arrow so if you ever can't edit this happens to me all the the time um click on the arrow and that'll give you the editing tools this is the editing feature tool but you we can't use that yet because we don't have a feature so I'm going to add a new brake line this is going to be my first brake line and I'm going to press shift now in order to move around since I'm in the middle of adding a feature and I'm going to just zoom in I'm using the the roll bar to to zoom in and I'm just going to do my best to bict this channel if this was an operational model uh I'd be a lot more careful to do it actually precisely but I want to be careful of YouTube runtime so we're going to do it a little bit quickly all right so I have a brake line and so I'm going to call that channel Center Line okay and you'll see that already this brake line is kind of molding the orthogonal grid to its well it wants these cell faces to line up with the brake line as much as possible and I haven't even enforced it yet and so in some cases they're already snapping to it but I don't want to enforce this yet I actually want to go in and Define some characteristics of this brake line before I enforce it and so I'm going to go right click on the brake line and the general rule of thumb and mapper is if you ever want to do anything to anything make sure it's editable and then right click on it if you ever want to find anything in Ras mapper right click on the tree and if you don't find it there right click one level up or below that's how I do it too and we're going to go to edit brake line properties and here you'll see our Channel Center Line and all of our brake line properties and so what we're going to do is we're going to do repeat cells if we were to just go in and enforce this brake line we would get one cell on either side of the brake line which is cool you know that you know the center line channel is now aligned with flow but the rest of the channels orthogonal and so that's not exactly what we want what we want to do is we want to do repeats if we repeat these cells along this brake line what we'll get is a whole swath of the model that is aligned in this direction of flow and so I'm going to go to the brake lines I'm going to say edit brake line properties and I'm going to say I want my near spacing that's the spacing near the brake line to be 10 and then I want to do let's do actually eight near repeats and what that's going to do is for 80 ft on either side of this brake line it's going to repeat these 10-ft cells now this far spacing is interesting as well because what that does is that can actually create more aligned cells farther out but at a larger spacing um we don't want to do that right now and so I'm just going to make eight repeats of a near spacing of 10 and if I say okay and then I go to enforce my brake lines or I can right click here and say enforce my brake line or I can right click on my brake line and enforce my brake line now what you'll see is that this whole area is aligned with flow and if I turn off the terrain you can see that we've got all of these cells repeated outside of the center line so that this whole channel is now aligned with Flo all right so that channel is actually kind of ready to go except we want to make sure we get these inflection points right and so we're not totally guaranteed that that is going to work out to actually get our cell faces to transition right at the toe and the bank and so we actually do want to go in and draw in our toe and our bank so let's go ahead and do that I'm going to create new break lines and these new Brak lines which I'm again going to do kind of carelessly because we're going to try to keep this runtime down what you'll see is I've gone in here to my terrain and I've asked for plot Contours at a pretty tight level because you know if your plot Contours are at 40 then you can't really tell where things are happening but if your clock Contours are at one or or even less than one then you can actually see where the high ground is and I'm going to update my Legend with view so that the color goes with it and so now you can see basically the Contours point in the direction of minimum and maximum and so if I come in here and say I'm actually going to do a brake line along here and I'm actually going to use one single brake line and it just it turns out because of the spacing we chose the current cell spacing is actually pretty good for this toe and so I'm going to call this the right toe flow is in the upper right direction and so the uh the right bank is always looking Downstream and so now I have a right toe brake line and now let's go and do the same thing for the bank because you always want break lines along your high ground and so I'm going to start I'm actually gonna start my bank here and then my bank's G to get a little wonky here it's going to go up to this High Ground here and then it's going to follow this this engineered Bank pretty closely you can see I'm trying to hit every time the contour line points Downstream because that means that that is The High Ground all right and I'm going to call this right Bank all right and then I'm just going we'll go in and now look at the the brake line properties and now we have three brake lines and I'm going to just keep that one near spacing of one of 10 um and without repeats on both of them so now if I just enforce these brake lines we have our cells aligned in the downstream Direction and we have brake lines on our high ground and our toe to actually get those reflection points right so if you look at my final here in my final version I actually did this in three different reaches I did that Downstream reach that I just showed you but then we did another reach with more repeats so that we could cover this wider version as we go into the debris Basin that has this curved reach and then I did yet another one for or the Basin itself um that has more repeats and then I went in and I added these toes and Banks everywhere on both sides so that the channel and the banks and the near over banks are aligned but we're still capturing these The High Ground and the inflection point of the of the toe with the sort of detail that we'll want to get this right now what you'll notice is that where these come together these three different reaches come together it gets a little messy as good as the RAS mapper tools are it's never really possible to just you know use them to create your train without having to do any you know detailed editing you're going to have to go in and do some detailed editing and so in this case things get pretty messy here in the transition so what I'll do here is I'll come in and I will start editing and I'll choose my edit tool if I click on that and delete I'm just going to get rid of of these messy transition nodes things are going to fall right into place now it's not always that nice but if these Center Line stations align then it actually usually is going to turn out that well and if I turn off the the liar you can uh you can see a little bit better about what's happening I'm just clicking on these and deleting them and the essentially stitching comes out well okay and so you'll want to do some of that detailed work I like to go in and get rid of some of these smaller cells because they're going to drive your runtime if you use the Adaptive time step anything like this um this zigzag triangular pattern if you go in and get rid of some of those they'll snap into uh kind of near rectangular cells and uh th those are some of the the editing tricks that we could talk about in another video but that's how you use a brake line as your Center Line station and create repeats in order to align your Grid in the direction of flow and that's really the best way to do it it's not always this tidy if you're doing a natural system if a natural system that meanders you're going to have a lot more tight cells on the inside of the Meander and widely spaced cells on the outside of the Meander you're going to have to go in and do a little bit more manual editing at these meanders but this is still kind of the best practice for getting your cells aligned with your channel there is another way to do it however so let's actually go through that together so what I'm going to do is you know this is my this is my original orthogonal cells um if I turn that on you can see those are my 10- foot cells I'm just going to make a copy of this I'm going to save the geometry As and we're going to call this refine method and we'll press okay and so now instead of creating a Center Line Station what I'm going to do is I'm going to create a refinement region for my channel and so I'm going to zoom back over to my channel and now instead of a break line I'm going to click on refinement regions and I'm going to start editing and so now again I would do this with more care in the actual study but if you create a refinement region that includes your Channel and we'll call this Downstream [Music] Channel now you can refine the cells inside of it now if I was to just go in and say hey enforce my refinement region you know it's going to just give me the same cell spacing but it will go in and enforce you know soft break lines on the edges and so at this point you can do the exact same thing except instead of moving from the inside out with a break line Center Line what you're going to be doing is you're going to be moving from the outside in which isn't as clean but gets the job done particularly if you want more detailed spacing so what I'm going to do is I'm going to go to my refinement region I'm going to edit the refinement region properties and I'm going to keep my cell spacing 10 and 10 obviously if you wanted something something smaller you could but then I'm going to go in and ask for near repeats and I'm going to say you know this is about a 100 foot channel so I want five near repeats so that they'll meet in the middle and I'll say okay and now again you can right click on the refinement region to enforce you can press this button or you can right click here to enforce the region um these both enforce all the regions just like the the brake line if you want to just do enforce this region click right clicking on it so see it this says enforce region instead of enforce all regions I'm going to enforce that region and you'll see that you know this went pretty well but at the center um things didn't work quite the way I'd want to so I'm just going to go in and I'm going to edit that region again and let's actually say we want six okay right click enforce the region so actually this went worse because I'm enforcing cell spacing from both sides and they're overlapping and now we've actually overlapped more so let's actually go in here and say instead of five I actually want four and now I'm going to enforce this region and now things are looking better um we've got some of these kind of big cells in the middle um that are unstructured but mainly we've got structured cells both inside and outside of the channel it's not as good but it does the job and when you're dealing with natural systems that Meander and you're going to have to go do a lot of manual editing anyways this will actually get you a better start to your Meandering or uh you know irregularly shaped channel that you can then go in and move the cells around to make them more aligned the other thing that you can do with a refinement region is make one cell wide tributaries let me show you an example of that so this is another model that I'm working on and you can see that in this model this is a kind of a high gradient aluvial fan and in this model you know I left most of the flood plane as the structured grid but the channel itself I wanted to align with flow and so what I did is I made it a refinement region with with a single replicate similar to the spacing of the surrounding region but then I went in and edited it so that essentially the channel snaps to single cell except for where it gets wider and then it goes to you know one cell on each side sometimes I need an additional cell in there for a particularly wide spot and you'll notice I don't always get to snap right to the edge you know the the snapping of the uh the refinement region Edge or the brake lines in Raz does take a little bit getting used to it is a little bit artistic um but it does give you the channel that you want that is aligned actually with the direction of flow and that gives you the most importantly the cell edges that follow The High Ground of the channel Banks and so that is actually one of the ways that you can use this tool let me show you one other model where we use this so this is a model of the chipa river flowing into the Mississippi River in one of the Upstream pools and so what you have here is you know here's the Mississippi River and here's the chipa flowing South into it and this is actually a sediment model that we've developed for RSM because there's a lot of dredging right at this Confluence you'll see again we started with an unstructured grid and a lot of the overbank uses the unstructured grid but in both the Mississippi and the Chipawa we've used these refinement regions in order to to align the grid with the direction of flow and so you can just by looking at this you can tell already what we did obviously you know we went in and we put in some brake lines at all The High Ground because that's just best practice that's the first thing you do you go in and you put in a brake line any place where you have a ridge or a road where you want to make sure that your cell faces are aligned with that wear but then we went and we created this refinement region and we got three or four replicates on either side and ended ended up with this this Mississippi actually aligned with flow now a lot of this needed manual editing this didn't come out as clean as this we moved a lot of these points around but then we did the same thing with the chipa the chipa also we went in and we created a refinement region and aligned it with flow and created replicates both on the outside and inside did some manual editing along it and then this little tributary we did the same thing but this is more of the like one cell approach that we took on the uial fan and so these are the two ways in which you can align your orthagonal cells in Raz with flow you'll get better stability you'll get better Precision particularly for sediment transport and non- nutonian flow this is a best practice that you want to start implementing and I think you'll find it'll make your models better so this has been a regional sediment management tools and practices video the purpose of this series is to help spread best practices in sediment transport modeling and decision making so that the models we use can help us with our regional sediment management decisions this is funded by the RSM program of the AR core of engineers and look for more of these videos to come

Channel: Stanford Gibson

Views: 14,198

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Id: oTfFdSmXbYQ

Length: 20min 44sec (1244 seconds)

Published: Mon Apr 12 2021