Troubleshooting a 2D HEC-RAS Model: Evaluating Hydraulic Connectivity

so another common problem with 2d models is we often get flow in places where flow shouldn't be um we'll get wet patches and cells and not really know how the water got there or if it should be there and so this is a model that i'm working on right now this is a big gravel mining pit and so we've got the main channel here but water is flowing into the mining pit from above and from the side how do i know that i'll show you in just a minute um but then i've got these flow sections over here um we've got flooding over here and then flooding in these little sub-watershed areas here and the question is is that real should that water be there is that a feature or a bug um now just because the water is disconnected doesn't mean that it's necessarily wrong that's just kind of part of the way that we map in raz but um i'm suspicious i'm not sure the water should be there and so the question is how do you know and uh for this um i almost always will turn on what we call the connectivity tools um in ras these were developed you know by the rest team anton and alex and cam were all involved in these and i just use them all the time when i'm troubleshooting a model and so the question is where is this water coming from and does that make sense and so the connectivity tools you can find by right clicking on your output and going to layer properties and there is just a lot of value buried in these additional options right here now the first one is plot2d hydraulic connectivity and so what that shows you is it just uses these blue lines to show you what cells are connected to what cells hydraulically now that is a lot of information um you know basically the only thing we know here is that this cell is not connected to this cell and so that's maybe not the most useful um output it has its place but i actually like the two other ones if we go to layer properties and turn that off and then we say plot 2d water surface gradient this is the one that i always turn on this shows you the kind of flow between the cells the magnitude and then also like the depth um the magnitude is the the size of the arrow and the depth is the color of the arrow and so like a red arrow means that the depth is actually quite shallow like approaching critical depth whereas the blue arrow means the depth is quite deep um and you can see now that we have a pretty detailed map of how water is flowing through our model and so the question is how is this water getting there well it's coming through here we have quite a bit of flow from this cell into this cell and then because this cell face doesn't follow the high ground the flow isn't going over this ridge the flow only has to go over you know this low area right here and so the flow is coming into this cell and filling this cell and then putting water into our basin and then it's going from that cell into here and it's just kind of working its way down again this cell the the cell face here isn't following high ground and so it's kind of jumping that ridge into the sub basin and then coming here flooding down this way and then backwatering into the the 2d area and so you can see that actually this this water here is probably illicit that probably shouldn't exist and so how do i fix that well i would go in and i would either use brake lines or move my points so that these cell faces follow the high ground and that way if your cell face follows the high ground well the subgrid technology knows that the water can't go over the ridge but if your cell face kind of follows this valley then the in order to move water between cells the stage only has to get high enough to push water past the lowest point there is another tool here that is comparable but more quantitative um if you go to draw perpendicular face velocities now this actually shows you not only the uh the vector and not only kind of the the magnitude but it also shows you the velocity of the water that's passing between the cells um and so you know this tells the same story that flow is you know getting it flow is getting over this ridge and kind of you know hopping hopping ridges to these little sub-watersheds until it gets down into this area which it just shouldn't be this water shouldn't be there uh the other thing that is really interesting in this data set with these connectivity tools is you know how is the water getting into the gravel basin um and so you can see that we actually have flow coming over this ridge as it ponds behind here and we also have flow these horizontal yellow arrows going laterally from the main channel and at this point i want to know you know am i following the high ground here this brake line should follow the high ground otherwise we could put water into this basin even though it looks it looks dry right that's just the way that we map the subgrid cells um this is telling us that looks dry but we're actually putting water into there and so in order to you know evaluate is this brake line following the high ground because that is the best practice you really want this brake the cell faces to follow the high ground and to do that you need the brake line to follow the high ground i'm going to go to my terrain and i'm going to turn on my contours and then i also almost always turn on this update with legend i already have it on so you can see but if i turn that off the colors really only go from the max to the min of the whole map but if i go to if i go in and i say update to my terrain i say update legend with view now we're going to use the whole visual scale for the train at whatever view you're at and so as you zoom in you get more color differentiation locally and it helps you track the high ground and so if i look at this um you know the the brake line you know it hits the high ground here but i could be doing a better job tracking this high ground and so i'll probably go in and do that so that i know that if my flow is actually making it from this channel into the debris basin that it's doing so because it's exceeding the high ground and not some artificial low point in one of my cell phases because of the way i drew this line my name is stanford gibson i'm the sediment transport specialist at hcc and this video is funded by hnc set funding

Channel: Stanford Gibson

Views: 4,371

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Id: jtaTHXJ1nQc

Length: 6min 45sec (405 seconds)

Published: Thu May 26 2022