Riprap and Scour Calculator in HEC-RAS (Part 1: Riprap)

hello my name is stanford gibson i am the sediment transport specialist at the hydrologic engineering center and today we're going to talk about a new feature in hc ras version 6.1 called the riprap and scour calculator the rip weapons scour calculator was funded by the regional sediment management program and it's kind of the headline feature of version 6.1 and so in order to get to the riprap and scour calculator you're going to have to go to the hydraulic designs function if you have never been in the hd tools you might be in for a surprise there are a number of very cool calculators that are essentially raz post processors raz computes hydraulic parameters and then we use the hd calculators or the hydraulic design features in order to kind of post-process those into some sort of hydraulic design or sediment transport tools and so to get to the hydraulic design features you're going to go either to this hd button right here or you're going to go to run hydraulic design functions and that's going to open up this editor now there are a number of different hydraulic design computations you can do in raz if you go to type you can see all of them bridge scour is the federal highway administration hcc 18 analysis and it is one of the most popular analyses you can do in raz in general let alone the hd editor but then there are a number of other cool tools here we've got a semi-transport capacity calculator which is you know a way you can look at sediment transport capacity without doing a full-blown sediment transport model some stable channel design tools which are really useful if you're say trying to you know design a restoration channel or something like that but the new tool is right here it's number two it's the rip wrap calculator and so we're gonna click on that and like razmapper we wrote this in net and so raz is going out of process so it just takes a second to load but this is the riprap and scour calculator now the first thing you'll notice is that it has a very different look and feel of the rest of raz and that's intentional um we actually use the rip wrap calculator to experiment a little bit with the raz 7-0 look and feel that we're developing the rest of the software in and you notice that you'll just have it's just a more modern software a cleaner look and we've got these kind of feature ribbons that you'd be used to from say any sort of microsoft suite project but this is the riprap calculator and the idea behind the riprap calculator is that the core of engineers has a very specific process that we use to size rip-rap it's called engineering manual 1601 and really it's all built around what we call the maynard equation which is the equation we use to compute a appropriate riprap size for a particular bend in the river or some sort of toe protection and so what we've done is we've taken that analysis and we've put it in raz because it uses results anyways and it that kind of standardizes the process for our agency and the other folks who use our software um in future versions we'll have other rip-rap approaches in including the maricopa county approach but for now this is um the riprap design approach from em1601 okay so what you'll see is that we immediately bring in some hydraulic parameters but the question is where should these hydraulic parameters come from which cross section should you define well what you'll see is if you go into the cross section is that we have a river bend here and now this is a completely fake model this is this is an idealized model that was developed just to simplify this analysis for educational purposes it comes with raz but it shouldn't be used for engineering or research purposes but in general if you're looking for you know where is the location of maximum scour where things most likely scour where would you need the biggest rock it tends to be on the downstream end of the outside of the bend if you think about how reverse meander they tend to move that way right and that's because that's the region of maximum shear and so the outside and downstream end of a river bend is where you're going to have the maximum share so this is going to be our design cross section right here 3100 but the maynard equations were not developed for the velocity at the cross-section itself um the velocity at riverbend cross-sections is fundamentally multi-dimensional there's lots of helical flow stuff going on it's just not a one-dimensional problem and these equations were developed for not only one-dimensional flows but idealized one-dimensional flows when steve maynard developed these equations he had a riprap testing facility and he keyed the stable rock not off the velocity in the bend but off the velocity upstream of the bend and so the riprap calculator and some of the bensco equations that we're going to handle in the next video don't actually use the velocity for the design cross section but for an upstream reference cross section a cross section upstream that isn't deformed by the bend but actually is a relatively symmetrical run or crossing cross section now this is an idealized cross section so all of these are trapezoidal but if you were to look at say a real model this is actually the sacramento river and you look at a real bend in a real model okay here is an actual bend in the sacramento river and so if we go look at what these cross sections look like let's just kind of walk downstream these are crossing cross sections they're relatively symmetrical kind of like you would expect that a one-dimensional model will do quite well here but as we step into the bend we end up with these highly aesthetical cross sections where multi-dimensional forces are really engaged so in this case in the case of the sacramento you would want to analyze your rip wrap at the outside of this bend but you would use the symmetrical crossing cross section upstream and so we call that the upstream reference cross section and so in the riprap calculator you're going to need to define two cross sections if you go to edit you have the upstream reference cross section which we decided was going to be 3500 and then you have the design cross section which is going to be 3 100. now nowhere in the riprap calculator as it currently stands does the riprap calculation the usac calculation use this design cross section it's all keyed on this upstream reference graph section when you move to the scour depth analysis then some of these will use the design cross section and the maricopa county methods that we're putting in also use the design cross section but for now if you change your design cross section it won't change your rip rap at all okay so what you see is that we went in and we got an average velocity a hydraulic depth and a width from this upstream reference cross section that's actually all the hydraulic parameters you need for the maynard equation and we pull them directly out of rest in this case we only have one flow if you press this drop down it'll give you all of the profiles from your steady flow analysis and the question is which flow should you use there's a section in the raz manual about how to choose your flow it's supposed to be the flow that gives you the maximum velocity in the banks but sometimes it's going to be a channel forming discharge or just you know some sort of recurrence interval that you've decided based on some sort of external risk insert analysis but in this case we only have one flow so we're going to choose it with that drop down and then we're going to put in our data now all the data here under intermediate computations if the box is gray you don't have to put it in some of these already come with defaults and so really there are only two things you have to put in here one is the radius of curvature and there is a section in the raz manual about how to compute the radius of curvature but generally you're going to fit a circle to the bend the actual radius of that circle is your radius of curvature now you have to be careful about bends that bend on multiple scales what we call fractal or compound bends there's a section in the rasmanian that talks about that but let's say here that the radius curvature is 300 feet and the side slope angle is 25 degrees in a lot of places in raz you put side slope in at like vertical versus horizontal here we're actually just going to put it in degrees and in the raz manual there's a conversion equation and then the safety factor well i'm going to use a safety factor of 1 because i actually want to see what the actual size of rock is without you know inflating it with a safety factor before i decide explicitly and externally when my safety factor is going to be but it's worth noting that em1601 does recommend a safety factor on maynard's equation of 1.14 right and then the unit weight of the of the material this is dry unit weight of the rock generally is between 150 and 165. the wordpress calculator has the ability to switch back and forth between size and weight and it will use this to do the conversion and then the angle of repose which is only used in one equation um in one non-default algorithm so we put a default in there for you you can change it if you want but it may or may not change your results based on what you've done then finally manners equation has several coefficients out in front of it these coefficients tend to be categorical they're categorical coefficients that have a certain value based on certain choices that the model is going to make and so what we've done is we've actually instead of having you put those coefficients in we've given you the categorical choices up in this ribbon up here and so you can choose is it natural trapezoidal is your rock angular around it where is it is it on the outside of the bend which is the analysis we're doing right now but or is it straight if it's straight then the bend is not going to be as important or these transitions downstream of concrete channel or at the end of the dike things are going to be more dramatic you're going to need larger rock and so there are compensating issues there and so the one that's pretty interesting is the rounded angular number because the rounded coefficient is about 25 percent higher than the angular coefficient and this is just really important just kind of as a heuristic to keep in mind is that a lot of times you'll be in a project and there'll be project sponsors or stakeholders that are really interested in rounded rock for the aesthetic value they don't really like the look of rap they'd rather go around it and you know that's fine but it's important to recognize that right off the bat rounded rock has to be 25 percent larger by diameter but because that size is cubed you know the 25 percent larger can be a hundred percent heavier and so you know you just have to make that decision explicitly and this tool gives you the option to do that and so you can see here that we have already calculated the d30 that's what the main equation does it calculates the d30 of stable rip wrap given the hydraulic parameters at the upstream reference cross section and so actually that's pretty straightforward right that's a pretty painless process compared to the way it used to be we already have a d30 we have a d30 in the bed and a d30 on the side slope we report this in inches and you'll notice that the side slope d30 is bigger because it has the the gravitational component and so it needs to resist the gravitational component and needs to be bigger alright but we're not actually done with the computation yet because all we have is d30 and if you go to a quarry and you you need to specify rock you can't just specify the d30 you have to actually specify the full range and you have to decide how thick should my rip-rap lift be and so you're going to need actually a gradation and so to associate a gradation with these d30s we've built an interactive tool over here this interactive gradation selection tool starts out by populating these default gradations from em em1601 um em1601 has some kind of like idealized default gradations you notice these aren't lines but areas because they give you a min and a max for like a d10 and a d50 and a d100 and so we've actually populated every other one of these 1601 gradations but you can see that our d30s are plotting here the brown d30 is the bed and then the green is the side slope and what you're going to do is you're just going to select one of these gradation curves to associate with those d30s but what i want to do is i want to get a little more resolution here so i'm going to go to gradations i'm going to go to these em 1601 gradations and i'm going to turn off the big ones because the big ones are too big and they're just not going to matter and i want to add more resolution at the low end and so now we have these 1601 gradations 1 through 5 and they're all there available for us and so what we can do now is we can either click bed up here or we can click this radio button and go select whichever one of these gradations should be associated with that d30 and it's going to be this one right this is the one the next one larger and so i'll click on that and what you see is it goes in and picks off the d100 from up here it also picks up the d50 but you can't see that and then it computes a thickness and em 1601 uses the equation um your rip wrap thickness should either be one and a half times the d50 or 1d 100 thick um and the idea there is that you don't want your riprap to be prone so your thickness should at least be the size of your biggest class so your biggest class doesn't stand out but it's going to be the maximum of either 1.5 times the d50 or the d100 and so we'll compute that for you and then report the thickness okay now i want to associate a gradation with my side slope d30 and so i'm either going to come up here and choose side slope or come down here and choose this radio button and now this is active and so i'm going to come over here and choose this one because this gradation would be the most appropriate for that computed d30 and it goes off and picks the d100 and then looks at the max of the d100 then d50 which is 18 inches and we're done we actually have computed the d30 the d100 an associated gradation and a thickness for both our bed and our side slope now a couple of things that are interesting here one is how do you know you choose the right upstream reference cross section well you don't and it's actually sensitive to that result and so one of the things you can do is you can flip through these and look at the sensitivity and so you can flip from 3500 to 3600 and look at how that changes the results and or if maybe that all changes too quickly for you we have this tool that's similar to the persistent cross section tool in the cross cross-section editor which is one of my favorite tools in the cross-section editor because you can look at how things are changing upstream and downstream and so you can say hey keep the previous d30s on the plot and now i'm going to switch my upstream reference cross section and you'll see oh wow it's actually very sensitive to which of these cross sections i choose and so that can help you look at you know what is actually the maximum case what is your range of uncertainty based on which upstream reference cross-section you key to the uh the gray are calculated from previous and the brown dot uh brown and green dots are the dot are the calculations for the cross section you're on right now so we added that capability to look at the sensitivity of your result to the upstream reference cross section all right the chances that you actually have access to these like idealized platonic 1601 gradations out there in the world is very unlikely it's very low and so we've given you the ability and actually recommend that you define your own gradations and so that's what this button does if you press this gradation button this actually helps you manage a bunch of different gradations as well but we can turn off the default gradations and add our own and so we're going to add a gradation here and i'll just call this bob's quarry is and grad one this is gradation one from bob's quarry down the street and you have to define a d30 and a d100 and you should define a d50 these are the things we need for the calculation so those are compulsory but let's say that bob he also likes to define a d1 and a d85 and so we're going to put in those percentiles and then you can define it by weight which you know those of us who come to this from a sediment transport perspective we're used to our rocks having size right but people who are coming from the design point of view often define riprap in terms of weight i'm going to switch this to size but you can define it in either and it computes back and forth based on this unit weight and so let's give this a very simple gradation 4 5 6 7 9. okay and then let's actually define a couple more you know bob has a couple of different types of material in his quarry and so we're going to add an 85 here and we'll go five six 7 8 9 and then let's do just one more this is bob's super course gradation and we'll call that 8 9 10 11 13. okay and so now i have these three customized gradations and i'll turn those on and apply all and now when i close out it's we get these three curves and now i put in curves instead of regions you can go in and put in a region if you want but we've got these curves associated with the actual gradations that are available to us at a local quarry and so now what i'm going to do is i'm just going to go here and say hey this is the d30 i'm computing there this is the rock i have available to me that's most appropriate i'm going to click on that and now it's going to recompute the d100 and the thickness based on the rock that's actually available to me and then i'll go down to the side slope and i'll click on this one because that's the most appropriate one for that and it recomputes the d100 and the thickness now of course i could go in and define a region so let's go and define one more except instead of curve i'm going to do max min zone where you can go in and put a min or in a max weight and a min and a max size and that'll give you a result that's similar to one of these defaults where you get a region not just a single line all right then finally we have another cool tool up here that just kind of switches between weight and size i've done everything in size because i'm a 7 transport engineer and i think about rocks in terms of size but now i'm going to go across the hall and talk to my design engineer who thinks about things in terms of weight so i'm just going to convert everything over to weight i'm going to switch over to the em-1601 creations again so you can see how it works with that so i'm just going to push this button and you'll see that everything gets switched to weight the uh the stone weight is now in pounds um it's percent finer by weight and the the d30s are now defined in terms of weight and the d100s also in terms of weight the thicknesses are still converted back to inches and you'll notice we haven't actually changed any of the results and so we still have the same thicknesses because it doesn't really make sense to define a thickness and weight all right so that is the first tool now we've defined our riprap size but you can't go in and protect a bank without protecting the toe one of the biggest failure modes of you know bank protection is toe scour and so you always need to put in some sort of toe protection whether it's keying or a launchable toe and to figure out how much of that you need you need to compute the scour depth and so that's what the second tab is about and that's what the second video is going to be about how to go in and compute the scour depth required for your launchable toe but for now that's a introduction to the riprap calculator the new feature in ajc raz 6.1 this tool was coded by zack morris one of the developers on the raz team i was supported very heavily by the corps of engineers smes on riprap and scour design that's david may and david bienhardt and chris herring and this whole effort was funded by the regional sediment management program of the corps of engineer known as rsm

Channel: Stanford Gibson

Views: 12,054

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Id: M3AVtQIpcAo

Length: 20min 49sec (1249 seconds)

Published: Wed Sep 22 2021