HEC-RAS Basics Part 5 of 8: Inflow, outflow, and rain-on-grid boundary conditions

so now let's have a look at the flow file I'm going to open up my unsteady flow data and I've gotten my boundary lines my boundary condition lines in here right now one handy thing that you could do in the geometry viewer that's not quite as handy in R as mapper is to measure the slope of your line now you're supposed to grab the energy gradient but at this point I'm just going to estimate what the energy gradient will be by going into my geometry and measuring using control on my keyboard measuring what the alignment is if I cross the channel here and up here in a couple of places and just double click I'll get a menu item here that allows me to cut from the terrain profile and if I cross that channel and roughly follow the channel and measure again holding ctrl down from this channel to this channel right here I should get a decent representation of the slope in this case it's about 1% so that's what I'm going to use I'll enter that 1 percent as the outflow boundary condition for my normal depth I'll also need to enter a slope for the inflow and I'll measure that the same way here we can get more accurate with this later on but as I cut this profile I went from the channel up over a hill and essentially that slope from low point to low points is just over 1% so I'm going to bring it in at one and a half percent on the inflow bring it out at 1% on the outflow so I'll exit back out of the geometry viewer and go ahead and start entering my boundary conditions here so on the downstream end my BC line here which I'll have to remember to rename to the downstream outlet is going to get a normal death slope and again the downstream I think was going to be 0.01 or 1% and for the inflow at the top I'm just going to give it a hydrograph with zero flow at the moment and the reason I'm going to give it zero flow is because at first I'm going to run a rain on grid model over the top of this thing so at the upper end just so I'm ready for this one when we run it I'm going to give it a one and a half percent slope and it's only has one hour duration and it's a zero flow now if I'm going to run this as a rain on grid model I'm going to need to take my 2d flow area and make a boundary condition out of it that's going to be a precip now at this point I'm going to show you a handy little tool that I've made for developing a what's called a frequency storm if you're in Australia the Bureau of Meteorology publishes IFD data and you can see that right here I've gone in and put in the coordinates for this project and now I've got all of my intensities frequencies and duration I'm going to copy these numbers and then I'm going to go ahead and paste them into a little spreadsheet that I've made that allows me to develop what's called the frequency storm again it has every worst-case duration frequency and intensity so I'm going to paste these numbers in and once I've got these numbers pasted in it's going to develop for me a spreadsheet that shows the intensity storm and now when I go into the spreadsheet it will give me this frequency storm here's a one hour storm that's going to have the worst case of everything into it so here's my one hour storm I'll go ahead and copy those values and paste them into my unsteady flow file these were at one minute duration or time interval and when I paste these in I can plot the data to make sure I've got the frequency storm represented and this is the worst 1 minute you'd ever see here the worst 5 minutes the worst 10 minutes the worst half hour and the worst hour so that's the storm that I'm going to rain on this catchment so the next step is to make a plan file here's my unsteady plan as always we 1 2 & 4 we have no sediment data for my time frame I'm going to choose a date well off in the future so that I know it's a simulation and not calibration to choose the same day for the end 8 and make it one hour of a small for my time step I'll go ahead and pick 5 seconds just to make it initial guess we'll check that with Courant number criteria later on the remaining ones I'm going to call one minute each and with that we should be just about there and ready to run our rain on grid model over the top of this and I save this with a new name and I'll call this gold Creek direct precipitation remember the short idea is very important it becomes the labels on your charts and it also becomes the folder name for your results so gold creek director precip and i realize i forgot to save my unsteady flow file with a name so i'm going to give this one a name first this is going to be gold creek direct precip as well with that sage i should be able to find both of those save plan and we're good to go now when i hit compute just while it's computing remember always always always put something in the plan description i've left that blank i'd never like to leave those fields blank because you think you might remember later on what you were doing but it's going to be hard to remember once you've run sometimes dozens sometimes hundreds of plans in a given model so it looks like this one ran through in a fairly short amount of time this one got through in 25 seconds any reasonable direct precip model should probably take hours if not days to run to get any good resolution out of it you probably want to run it at your light or a resolution accuracy if at all possible because again these grids are defining sheet flow paths that are coming down fairly steep slopes it's going to be something that can give you pretty erroneous results when your grid sizes get way too large so now i'm going to close out of this and go back into razz mapper to have a look at our results and i can turn these on now and watch my rainfall deaths or sorry my runoff deaths it's i bring this around i'm going to animate this and have a look at the flow paths let me turn off the geometry so you can see what's going on underneath it and you can watch the flow accumulates and recede at the end of the hyetograph so now let's go in and see what happens here if we can extract a hydrograph from the profile line that we had defined and you can see the line right here as we click on this one I plot the time series I'll get the flow and you can see a nice smooth hydrograph my honey to craft peaked right here my hydrograph is peaking ten minutes later which makes me think that we've probably underestimated our roughness or we have some other issues I don't think this thing would have responded in ten minutes flat in that any case for this purpose we'll go ahead and go to the table and grab that hydrograph value which we are now going to insert into our unsteady flow file so I'll copy and paste this straight from the table into my unsteady flow file as the boundary condition I'll close out of that go back sorry to my unsteady flow file and on this flow hydrograph where before I had zeros I'm now going to go in and paste in the values that I've just extracted told them in has one-minute hydrographs or time steps and say okay sorry I always mean to check the data to make sure that we don't have any typos in there and I think we're good to go so with that I'm actually now going to delete my rainfall of the precipitation on this and just bring it in as internal inflow so I'm going to click on this one and delete my boundary condition take out my storage area now I only have two boundary conditions the inside and the outside and I'll go back now into my unsteady flow plan and we'll rerun this one I'll hit compute again and now with this computation it should run a little bit quicker because not all the cells are wet so again what I've just done is let it rain on the whole catchment and now I'm extracting a hydrograph from that rainfall runoff model essentially and using that as my inflow boundary condition it runs fairly quickly again but once this is over I'll go ahead and click on rasz mapper and have a look at our results so I'll exit out of this go into rasz mapper and let's have a look at our results you'll notice that before I had rainfall coming everywhere from the side slopes and from the sheep flow areas now I've only got rainfall or runoff in the channel itself but again one thing to watch out for if you turn on the profile line you can see what's going on here you're going to be able to have the flow going both directions and the flow is going to travel upstream and downstream so I've got some minor instabilities in there up in the upper end just let's zoom out and have a look at this model again what I've got now if I turn on my geometry is inflow that has been introduced in the middle of the model which you weren't able to do before if I wanted to do this in version 503 I would have had to take this external boundary and slice it to coincide with this inflow location but before signing off let me just have a look here again at what it looks like in your animation floods coming and going and when I go back to the unsteady flow options this is something I forgot to highlight earlier you have a few additional options that we didn't have before and that's on boundary conditions you have a tailwater check and you also have a couple of places where you can enter some additional criteria about the distribution of that flow now a few things to check that you can't do in razz mapper as easily as in the geometry viewer one of those is something you always want to do when you're entering in boundary conditions and that's to plot the terrain profile underneath that bc line and you can make sure that it's a good cross-section and doesn't dip off to one side another thing over here when I plot this one you'll see that I had a split flow right here which is why it's being brought in in a couple of different places I should bring it in here or here separately but bring it in together resulted in some of those instabilities as the flow came back together so with that I hope you've enjoyed this little tutorial on internal boundary conditions in heck rats 5 point 0.4 I'll probably try and clean this up a little better later on but let me just get this out there and in the meantime I hope you've enjoyed it tune in for another one and give me some feedback on what you'd like to see covered in future videos thanks

Channel: The RAS Solution

Views: 74,517

Keywords: HEC, HEC-RAS, 5.0.4, RAS, HEC-RAS 5.0.4

Id: c2rL8zvLvuE

Length: 10min 50sec (650 seconds)

Published: Tue May 08 2018