Riprap and Scour Calculator: Part 2 Computing Toe Scour

hello my name is stanford gibson i'm the sediment transport specialist at hcc and this is part two of the riprap and scour calculator introductory video the rip prep and scour calculator is a new tool in htc raz version 6.1 it was funded by the regional sediment management program and the interfaces you'll see today were coded by zac morris so in part one of the video we showed you the riprap calculator and how to size rip rap with raz results but it isn't just called the rip wrap calculator it's called the rip-rap and scour calculator we've embedded these two tools together because of pretty important reason because it which is fundamental to the core of engineers analysis you see when you place riprap you don't actually just get to size the stone when you size the stone you aren't actually done because the primary failure mode for riprap is not like individual class removal but it's actually toe scour the river can scour the toe particularly on the outside of the bank which is generally what we're protecting and if the river scours the toe then your riprap can slough into the toe scour hole leave the bank unprotected and then the bank will continue will fail and your work wrap will actually do be of little value so there are two strategies for what we call toe protection for avoiding this kind of toe skill scour failure the first is keying the toe and the idea here is that you're just going to go and you're going to kind of pre-excavate what you expect the scour to be and then you're just going to rip wrap all the way down into that keyed toe so that when the river does scour it only scours down to the level of protection it doesn't get beneath the level of protection or undermine your protection the second kind of easier and more common and kind of smaller systems approach is what we call the launchable toe in this case you just kind of overload the toe so that when the river does scour the grip wrap launches into that's where the term comes from it launches into that scour hole and continues to protect it's a kind of self-healing method so that as the river excavates the toe you have sacrificial rock there to go and fill that hole and so the toe doesn't get undermined and so both of these methods both the keto and the launchable toe they both require a calculation they require you to calculate the actual depth of scour and a lot of times this is the depth of the scour on the outside of a bank but you know sometimes if you're kind of looking to protect say a pipeline you're interested in just you know how much is the channel going to scour in general and so those are very empirical equations you know coming up with a ben scour that's actually a very complicated three-dimensional problem so whenever there's a complicated three-dimensional problem you've got a couple options you can run a complicated three-dimensional or physical model and then still have a lot of uncertainty or you can run kind of a suite of simplified empirical models and that's what this reprap calculator offers we paired the riprap calculator with a scour calculator that runs a suite of kind of older simpler empirical equations they're pretty simple equations most of them are collected in a document written by pemberton laura we have links to that in our users documentation and so the tool will actually apply not one scour method but a suite of scour methods some of which may be appropriate some not and then you can look at what are the scours that are computed by the different methods which methods are appropriate and then look at some sort of ensemble representation of what the scour could be and then use engineering judgment and kind of local expertise to determine what you think the scour depth is going to be and protect either by keying the toe or adding a launchable toe so this is the riprap editor if you're interested in how to use this you can go back to the first video in this series but the idea here is that we've computed a d30 for our stable riprap and then went in and found gradations and thicknesses for that okay so we have we've sized our rock with the riprap calculator it was a pretty straightforward calculation scour depth is less straightforward and so what you'll see is we just have two tabs up here rip rap and scour depth so now we're going to click on the scour depth tab and rads will import all of the hydraulic information that you need both from the upstream reference cross section and from the design cross section now for riprap we only needed the upstream reference cross section because the manner equation only uses that upstream reference cross section but different scour equations use different equations most of the benz scour equations they use the hydraulic data from the upstream reference cross section for the same reason that the manner equation does is because you're computing scour on the outside of the bend that's a very multi-dimensional process and so in order to kind of apply a pretty simple 1d empirical equation you're actually going to look at the upstream reference cross section where hydraulics are pretty well behaved if you're looking at scour in like a general like across the channel itself well then you're actually going to use the design cross section you're going to use the section that you're computing scour at and so you'll notice that we bring in hydraulic data both at the local cross section and at the upstream reference cross-section we actually bring in a lot more hydraulic data from raz than we do for the ripprap calculator because we're not just solving one equation that has a couple variables we're actually solving a suite of equations and so the different equations require different variables and so you really only need to put in two values to get most of the results here again you need a radius of curvature well we already had that for the riprap calculator so we're going to put in our arrays of curvature and then you need a d50 and so let's just say that the d50 is one millimeter then you're also going to choose a bend severity now we recommend one for you based on the radius of curvature the ratio of the radius and curvature to the width actually puts you in one of three different categories moderate severe or straight and there are criteria for that and so we tell you right underneath what we recommend and then you can decide to use it or go your own way and so then there are a few other blanks here you'll notice that almost all of our equations solve these are the results you have the bend scale results and the general scour results neil does not and that's because neil requires bankful information now in future version we're going to let you choose a separate profile that would be your brainful profile but for now let's just go in and put in some uh manual bank full information that kind of scales to the the data that we have there and you can go in and look at this exponent what that all means in the user's manual but what you see now is that now we have all the information we need and we've computed you know four types of ben scour and four types of general scour so let's look at the general scour first the general scour plots as essentially just a raised or lowered cross section if it's raised well then you computed a negative scour and it's not appropriate for the application but you'll notice that we've computed a pretty good range here we have two threes and two sevens that's actually not a lot of spread you're pretty constrained if these equations are all appropriate well then it's in a range from three to seven but one question you want to ask is are these appropriate now you can go to the user's manual and look at you know what are the appropriate parameters for these different equations you know what were the conditions under which they were developed did you end up doing this kind of work you should know that stuff you should go and learn you know when are the different equations appropriate but we also give you these little buttons these little caution buttons if you get a caution button it means that you know something's gone wrong or you have the you've used the equation outside of the recommended parameters so for example lacy here you have this caution button it says hey you know what lacy was developed for silk bed rivers you know that is a d50 of less than .0625 is rd50 less than 0.0625 no it's not so maybe we don't want to use laci and so you can just turn that off and now your ensemble looks different all right and so that's the general scour the general scalar just kind of lowers the bed but what we're often interested in with riprap is the bend scour so in that case we're just going to turn all these off because we're not interested in them and what we're really interested is in is the ben scourer but you'll notice that the ben scour it plots in an unrealistic place you know it's all the way to the left bank well that's because we don't actually know what the toe of your bank is that's not something that we can kind of sense automatically and so you need to tell us if you wanted to visualize in the right place hey where's the toe of your bank so i'm going to go in here and i'm going to see that hey my bank is somewhere around 165 and so i'm going to go type in the tow station 165 and then if i zoom in my toe scour is going to essentially plot below whatever the closest node is to the line that i selected and that's just going to connect the left and right nodes of it and fit a spline to that so you can kind of visualize the different toast colors and you can see that one of these is plotting a lot lower than the other that's going to be the zeller bend equation but most of these are in the you know six and a half to ten range and so that's kind of the range in which you would be looking for an answer do you average these no i mean we will report these to uh three sig figs but they're are they reliable to three six figs no they're not reliable to one sig fig and so what the reason we provide you multiple different equations is so you can kind of look at what is the uncertainty and the spread and evaluate them from kind of an ensemble approach and then make a kind of a qualitative assessment based on the history of the system and your knowledge of these equations for what you think a appropriate scour depth that you want to protect to is also based on you know the risk associated with the project and also the consequence of failure and so for example um if we look at this we might say you know zeller does look like a low outlier i'm not sure that i trust that when the other three are grouping pretty well but what about maynard and we get this this air button for maynard and so what what's the problem with vayner well says hey manor was developed for slopes less than point zero zero two well what's our slope point zero zero two one well am i going to kick maynard out i don't think so you know maynard is essentially providing evidence that that six and a half to ten range is appropriate and you know the energy slope that we're looking at is point zero zero two one we're actually kind of close to the limit and so i think that that's kind of part of the ensemble evidence that you would use to estimate a toe scour at this location and so then you'll use the the riprap calculator provide your size and then the toe scour to determine you know what is the keying depth or the depth the extra rock that you would have to add in order to have a launchable toe that would fill the scourge and so those two editors work together that's why we added them together and this is the process that we use in the core we you know hcc was guided through this process by our partners at the coastal hydraulics lab david may chris herring and david biedenharn and again this work was funded by the regional sediment management program and the interface was coded by the excellent software developer zack morris

Channel: Stanford Gibson

Views: 4,740

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Id: USqcACyNpGw

Length: 11min 31sec (691 seconds)

Published: Sat Oct 30 2021