Full Momentum: An HEC-RAS Vodcast (Ep.8). Manning's Roughness for 2D Models

[Music] all right welcome to full momentum in hec raz vodcast my name is ben carey and here as always with me today is chris goodell chris welcome to episode 8 of the full momentum podcast thanks ben can't believe we're already at episode eight it's going by really quick it is we've tackled a number of really cool stories and topics and um the cool and kind of crazy thing about heckraz is looking forward it almost seems like there's going to be an unlimited number of of these things just because there's so many topics to cover um even in the current version let alone when one comes out so yeah you know i was thinking about something recently that um there's probably a lot of folks out there who are tuning in who have questions and ideas and thoughts about hecraz and we ought to get them involved so if you've got a question if you've got some topic you're interested in or a specific problem with heck raz go ahead and leave it in the comments below or email us at hecraz kleinschmidtgroup.com that's h-e-c-r-a-s at kleinschmidtgroup.com and we'll try and get you guys involved too and and um see if we can maybe help some people out yeah it's a good point and i would i would say that that would go for any type of question whether it's you know just a really short a quick question or if it's maybe more of an in-depth topic that we need to tackle obviously if it's if it's a longer topic that'll have to be built into kind of our list down the road of topics that we handle especially if it's like a really quick question or something you're curious about or a specific feature that you have a question about add those two um you know like chris said add those to the comments or send us an email about those and i think those would be great to try to answer during these podcasts so we can make it feel a little bit more like there's some pers participation with the uh the audience so yeah make sure you give a little context if you send an email and you say my model's crashing tell me why uh we're probably not gonna be able to help you but uh give us a little detail a little but not too much we don't want to read a novel either so you know some keep it brief but um yeah we'd love to get you guys involved that's why you're used to chris is is me shooting you an email and saying chris my model's crashing why is it happening i wish i would know the answer to that every time you give me a little a little more information yeah no doubt no doubt hey before we get started today i just wanted to give a heads up to folks that are that are observing this and maybe are watching closely you'll notice that uh partway through this video there's a transition where chris and i change shirts my beard gets longer my hair gets longer and that's because we had a an issue with the recording last time and so we lost the front half of our presentation um and so we're recording the intro intro here and then we're going to merge that with the video from last time so don't be spooked if you see that sudden change in our appearance hey but you know what maybe uh the wardrobe change should be a thing you know halfway through we uh switch shirts and just mix it up a little bit yeah and i know my thing i know my wife was excited about me getting rid of the beard but if if you feel differently leave that in the comments let us know what you thought i like the beard but you look much younger ben without it so uh yeah yeah i'll let you decide cool well uh we got we got a really cool show today uh we're gonna cover uh we're gonna get in depth on a really cool topic and that's going to be manning's roughness uh in for 2d models and specifically 2d areas in the previous vodcast episode 7 we talked about manny's roughness in 1d models and kind of how that all works how to apply and edit 1d manage values to 1d cross sections different references today we're going to get into in depth into how to add and edit manning's roughness for a 2d model within your 2d areas but before we get into that we have a really cool topic for today and it's not really a news topic more of just a really interesting um kind of out of the box application of heck grabs that chris goodell um has worked on over i guess started on a number of years ago and kind of has periodically made updates to it and has had an opportunity to present on on this too it's it's a topic that's really cool specifically for those those folks that are um in the know when it comes to the north west the northwest and the geography around here because it was a really really important event that really changed a lot of the topography in the inland northwest particularly in the columbia gorge as well and that is the missoula floods um so without further ado chris i'll let you take it from there yeah thanks ben and uh this you're absolutely right if you live in the in this area in the portland area or along the columbia river or eastern washington you know all about the missoula floods it's a it's a very important geologic event or events that have really shaped a lot of what we see on an everyday basis out here but what's particularly interesting about it to folks like us is that it was a massive flood in fact one of the largest floods to ever occur on planet earth and it happened about between 10 and 40 000 years ago in the last ice age and it all started with a reservoir that was uh formed by an ice dam up in the panhandle of idaho and in western montana and this lake was enormous and eventually that ice dam failed and it uh it caused that flood that we're going to talk about here in a second but um back then in the last ice age most of the northern us was covered with glaciers and that includes the northwest of the u.s in fact a lot of the northern parts of washington state and idaho had glaciers that were hundreds and hundreds of feet thick that blanketed the area and whenever one of those things crossed a river it would block it and it would damn it if there was no other way for that river to re-route it would form a big lake and that's what a glacial lake is and so that's what this discussion is all about it's the glacial lake that formed behind this ice dam and was called glacial or is called glacial lake missoula and then the floods that resulted when that ice dam failed so uh i'm going to go ahead and turn it over to um past ben and passed chris when we recorded this a couple days ago uh so you'll see a little bit of a blip as i get kind of back into my discussion but this is about where i left it off so hope you all enjoy and and as always leave comments below let us know what you think about this but also about our discussion that's going to follow on 2dn values here's the ice dam right here and this came down right in the panhandle of of the state of idaho and blocked the clark fork river which created the glacial lake missoula that you see right here now when this thing failed it sent a torrent i mean the amount of water that came through these channeled scab lands and actually created the scab lands were about 10 times the volume of all the rivers in the world today all at once through this small area in eastern washington including spokane then so um you guys were watching a basketball game at the time it would have been bad news for you and my my understanding is that is part of the the way that um i guess it contributed to the depth of lake coeur d'alene is that yeah correct because it it more or less scoured out a large chunk of that you probably went there a lot right that's where you ever go on the weekends and and uh have your parties right um there's another lake here right where the ice dam was called uh lake panda ray and lake pondere is also a very very deep lake and this was where the breach happened and uh those of you who do breach modeling know that at the breach location and just downstream you can get a really big scour hole right well this is an enormous scour hole on epic proportions right when you have 10 times the volume of all the rivers in the world coming through this small area at once you can imagine the the scour potential there right yeah and it even scoured d50 is not going to protect your channel no and it's funny you mentioned the d50 because there's a lot of of sediment movement through here and you have evidence of that all over the place through these scab lands you have these big piles of boulders that are pushed off to the side of the valleys and i'll get to this later but it even um a very interesting component to this is is how it transported giant boulders down as far as eugene oregon which is way down here and i'll show you those in a second but basically this is the path of the flood wave and here or the flood way and here's the columbia river gorge right here here's portland where ben and i are right now doing this vodcast and um so this is something that interests me from the very beginning when i first read about it being a water resources engineer and a hydraulic modeler i thought boy i wonder if you could even model this in hec-ras and this was well before 2d ever came out so i went on the process of trying to build a 1d hec-ras model to simulate this flood and it was not easy to say the least but here's a close-up view of the ice dam and what is now modern-day lake ponderay here's spokane right here and you can see how the flood would have just come right down here and just blown right through the location of spokane and uh there's some cool features if you uh if you're ever out in that area well first of all look at this sign right here this is the highest level of lake missoula and look at the town down there of saint ignatius montana and you can get a perspective of hi how deep this lake was in fact the lake was so big volume wise it was about half of lake michigan or about lake erie and lake ontario combined 950 feet deep at missoula and in fact if you ever get out to missoula montana and you look around the hillsides you can see the lakeshore remnants on the hillsides each of these lines represent a different lake level that paused periodically and just the wave action would would kind of erode these these lines in the hills and it was a mystery for the longest time people had no idea why these lines were here and how they could be so perfectly horizontal until the mystery was solved of the missoula floods which wasn't until just recently and this was led by a geologist named jay harlan bretts out of the university of chicago and he came out to eastern washington was fascinated by the scab lands out there and back in 1923 we didn't have aerial imagery to see these features so he was interpreting everything from the ground level i think he might have gotten in an airplane a few times but generally he was he was at ground level and he saw these features that were way larger than they look like they should be there were little tiny trickles of a stream going through but the feature was so large that it looked like there must have been a massive river there at one point and nobody really knew what caused that but he came up with this theory of the of the floods and he called them the spokane floods um which is another name for it we call them the missoula floods as well but he was immediately rejected from the geologic community and this is a really good lesson for everybody out there that um you know don't don't necessarily reject theories that sound unreasonable because you never know i mean it's you need to look at things closely and examine and um take a really unbiased look but back then back then everything was very biased they looked at geology through the lens of uniformitarianism which is a hard word to say but it was also a a belief that everything we see in geology happened over millions of years and and that is the case with a lot of it but there were some of these events that were very quick and brief and did a lot of change of geometry in a very short period of time and that was the missoula floods and um he proposed that theory i think well he first started viewing the scab lands in 20 1923 and um it wasn't until 1965 that the geologic community actually embraced j harlem brett's and and he actually won the the highest metal you can win in geology in 1979. what's the highest metal we can win in hydraulic engineering or hydraulic modeling band because uh i'd like to know i think it's i think it's being a guest on our podcast hey there you go all right cool um so just some examples of striking features around the the northwest this is um dry falls right here and it's um five times wider than niagara falls there's just a little relative trickle of water that spills in here when you look at it both in the aerial view but in in person it's hard to imagine what could create such a thing then if you do some more exploration around you you see these ripple patterns but these are ripple patterns that are on an enormous scale first of all you can see them in aerial imagery you guys everybody out there watching this you can get on google earth and just uh um dial into this area the west bar area of washington and look at the columbia river here and these ripple patterns you see on this point bar and yeah this is the feature i'm most familiar with when it comes to missoula floods because it it's one of those things where if you're driving through eastern washington you won't notice it just kind of looks like rolling hills there's nothing necessarily that stands out but when you get in an airplane and you fly from seattle to spokane it looks like you know the tide flats of a of a sandy beach as far as the ripple patterns and and at that point you know it's very easy to understand that there was a lot of water moving over here at some point um and like you when you get the scale of the columbia river you understand that hey and you know the the example you're showing there the west bar that's relatively close to the columbia right so maybe you could infer that there was a the columbia was a lot larger at some point and maybe it flooded over there it's like okay well that's a little bit you can see these these ripple patterns in the middle of nowhere well away from the columbia river and that is a real sign of like hey there was something that happened here yeah that isn't necessarily associated with just the flood of the columbia so that's pretty cool that's right and look at this boat um down here in an image for scale and these are the ripples that you see here these are the same exact kind of ripples you see like you said then on a beach uh as the water recedes or in a in a sand bed stream and these are enormous 50 feet high wavelengths of up to 500 feet i mean no one's ever seen anything like that or or back then when jay harlan brett's was exploring the area and so all these little puzzle pieces work together here's one like you were mentioning out in the middle of nowhere right some ripple patterns and here you can see a truck driving through the middle of it on a hot on a highway so that's another example and you can see that um from aerial photography and then this was what i was getting at before these ice rafted erratics so as i mentioned the the the dam was an ice dam right and it was formed by a glacier and this glacier as it moved and advanced southwards it picked up rocks as it went giant rocks and then trained them inside of the glacier and when the ice dam broke it broke into all sorts of pieces little icebergs and they all floated downstream some of these icebergs actually had giant rocks embedded in them and they floated these rocks downstream and they would settle out in some backwater area and fall out and end up melt melting and then left behind was this rock that was sitting in the middle of a field it just was out of place there was no reason for it to be there and furthermore uh scientists start to figure out that hey the composition of these rocks the mineral makeup is not anything we have around this area how is it possible and then then they start figuring out well it's the same kind of rock you have in montana and um yeah talk about scratching your head and for a long time people thought these rocks were put here by aliens or they were meteorites or something like that and um it's just again another piece of the puzzle here's a really good example of an ice rafted erratic on the side of this highway and you can even see it from google earth right here you see a bunch of other erratics in this field and farmers don't even deal with them they just they just plant their crops around these erratics and just go around them they just they just deal with it if you look at um the uh here's a map of the portland greater portland area and all these triangles you see here are all identified erratics now they're harder to find in the portland area because we have a lot of vegetation so you've got trees and bushes and grass growing over some of these and they've been weathered some of them a lot so some don't even exist anymore this is a really good example of an erratic that you can go to these are my girls several years ago we did a hike up to this and um perfect example it's a great place to go to it's in the middle of wine country so you can go look at the erratic and then go have some wine too at a really good winery somewhere so here's how i set up the model this is just the uh the terrain that i used and when i first did this i did it in 1d because i mentioned earlier we didn't have 2d and hec-ras back then and computing power was a lot less than what we have on our typical computers today as well and so i had to really manage the size of this terrain and keep it um keep it small and keep it efficient and so i used 30 meter resolution or sorry 130 horizontal resolution this came from the one degree dem usgs aero site and converted that to an esri grid and then used that to cut my cross sections and um the hard part at least originally i thought the hard part was going to be how do i set this model up there are so many different flood paths and i can imagine i'm going to have a lot of junctions a lot of diversions a lot of lateral structures which was true but my original worry that it was going to be hard to figure out how to set this up was overcome pretty quickly when you look at the aerial imagery because you can see in the scavlands at least the flood path still everybody get on google earth zoom into eastern washington you can see these flood paths from the google or from the from the missoula floods okay and so these were where i drew my flow lines so it made it actually very easy to set up the model relatively i wouldn't say very easy that said well that's it easier than i thought it would be yeah that's true that's a good point ben and here's what it ended up looking like these are all my cross sections and my um my reaches and rivers and in total i had 2 346 cross sections when you count up all the interpolated ones these just show the uninterpreted cross sections and 68 reaches 34 junctions 36 external boundaries that was one of the harder parts was to figure out what what amount of flow was i going to put in each of these different external boundary conditions so i got onto usgs and looked at gauges and tried to get an idea of base flow amounts and that's what i put in there but a lot of it really ben was trying to stabilize this thing and this was the biggest time investment of putting this together was stabilizing this model and this wasn't something i was doing for work so this was on my own time which meant i was doing an hour or two in the evenings and some time on the weekends and all told it probably took a couple of months to get this model put together and stabilized um and again that was just you know an hour here hour there so usually i would come home from work and i would make a little change to try and get overcoming instability and i'd run the model and let it run overnight and then see how it turned out the next day so here's another look at the model with the interpolated cross sections you can see i've got some inline structures in here if you zoom in close or some lateral structures as well at a 30 second time step and i was running it for 21 days and with the size of this model you can imagine it takes a long time to run which it did it did for sure and i had to set up a hot start too that was the only way i could get this thing stable for initial conditions but um so anyway um fast forward to a few years ago when 2d came out in heck razz i decided hey we need to we need to set this up as a 2d model because obviously you look around here and all these different flow paths is very much a two dimensional example so let me show you what i put together recently relatively recently and here is the same location and i've got a giant actually two different 2d areas there's one downstream of the ice dam and one upstream which represents the lake missoula and once i had this 2d area drawn in here this thing ran almost immediately i had a few errors to work out but there were no instability problems because i have my current numbers correct yeah yeah yourself huge sales yes huge cells for sure uh which allowed me to have a bigger time step so this thing could run in a reasonable amount of time it took i think i want to say it takes about a couple hours to run um off the top of my head i can't remember exactly but it ran the first time it was so exciting because it took me i literally a couple months of running and re-running the 1d model and this thing went in in right away and you know of course i made some tweaks and improved it and everything but let me show you the animation of this because it's super cool so here we see the state of washington this is idaho montana over here where the lake is and then oregon down here here's portland where we're sitting then and if i go ahead and animate this this is our 2d model it's really cool look at the the hot spot there right at the breach and here it is uh heading its way down to gonzaga [Laughter] watch out that's right and it's going down the spokane river here and eventually we'll tie into the columbia at um lake roosevelt which is what's backed up behind cran coulee today well the interesting thing here is too is you can see that it's it's following the river but it's also bypassing the river just blowing through the palouse and that that whole um the scab lands there pretty pretty amazing yeah because the water got so deep here it actually spilled over the edge this is all the colombian the spokane this is all through a canyon but you've got some flat land here and and back before these floods it was it was all fertile silt there hundreds of feet thick of silt and that all got scoured out to leave behind what we have now which are the scab lanes there yeah but well you know it's interesting too because oregon benefited from this pretty mightily because we took a lot of that topsoil and it got posited in the willamette valley which is part of the reason why the willamette valley has some of the best soil for agricultural purposes um that's in the whole region so yeah in fact there's some really great wine um areas too uh viticultural areas you've got the willamette valley um ava here you've got columbia river avia walla walla right here has great wines look at look at walla walla right here this is the backwater area all of this silt got dumped right here yep got dumped right here up here in the yakima valley where they have uh great orchards of apples and pears and stuff right yeah and ultimately here you can see it's still moving down the columbia river gorge just about to get into portland i'm going to zoom in here do you want to fast forward a little bit chris because i think it's like 15 minutes or five minute time steps it's showing right now so yeah yeah let's do that but let me let me get it into portland here a little bit and here's here are the willamette valley vineyards right right the halem valley here so um and you can pause this and we can take a look at the particle tracing too and zoom in um notice where these funnels are right here these jets you can actually drive out their bend today and you you can see the carving that was done in these areas this is lake oswego which is actually was formed by this event and here's on the willamette river and you if you look around oregon city you can see these basalt cliffs around oregon city even over here there's some scab lands uh further over actually right here the tonkin scab lands you can see some features there as well so anyway this is it all told it took about 21 days for the flood to finally recede and drain out to the pacific ocean and you can see too went all the way as far south as eugene oregon and so you even see some erratics down there as well so yeah pretty pretty wild stuff what's up chris what was the final one of the cool features i've seen you show is the final inundation map compared to um what is basically mapped by geologists and how well that overlaid yeah let's let's take a look at that i'm going to turn on the max water surface right here yeah go ahead and turn on the water surface of the depth so it's easier to see because the velocity is kind of messy yeah you're right so here's the max water surface depth and let's see if i can go back to my uh powerpoint and we'll go up to this first slide here here is the flood that uh geologists uh think happened and here's how mine turned out so it's pretty close right yeah yeah it looks see a lot of these these islands here show up there as well um so yeah yeah it's that's a cool really cool application chris this is i think one of those things that shows that raz is really capable of modeling almost anything um it's just a matter of you know knowing how to stabilize your model um obviously one of the limitations of this model is you know i think you had like 500 foot cells or something like that so there's no way that you were going to be able to do any type of you know detailed uh site-specific analysis with this model but for the purpose of it which is simulating one of the largest dam breaches in the history of of the earth it works fine right i mean so this is this is one of those uh things to pay attention to it's really important to ask yourself what is what's the purpose of my model and is the cell size and time step that i've chosen appropriate for that in this case totally absolutely is um but probably wouldn't want to do a bridge scour analysis on the columbia river for this no but here's the cool thing is you can take this model and you can use it as boundary conditions for little small pieces like i can build my own i could build a separate 2d model just the portland area maybe carve out here and here and maybe right here or or a little further down make this its own 2d model with much smaller cells and then use the bigger model to inform the boundary conditions and i've actually done some of that already i want to do a lot more of it so you can get a really good picture of the flow patterns and depths and velocities shear stresses that um that were going on through here it's pretty good very cool thanks for sharing that chris of course yeah and and one quick thing too is um i give a presentation on this too and i've done probably you know a dozen or more of these so always happy to share more detail about this event and the model and what you can get out of the model if anybody's interested in uh seeing a presentation let me know great very very cool all right well before we move into our technical hec-ras topic for today i just wanted to take the time to thank our sponsor for this episode and our sponsor as always is our engineering firm kleinschmidt associates who is known throughout the industry as a firm that provides practical solutions to complex problems affecting energy water and the environment you can learn more at kleinschmidtgroup.com and at kleinschmidtgroup.com there's also a uh an advertisement for our fall hec graz class which will be taking place for six weeks from october 7th to november 11th it's one four-hour webinar session a week plus workshops in between the lectures uh we had we just finished our first class uh a couple weeks ago it went very well we got a lot of great feedback chris and i felt good about how the in-person curriculum transferred to online curriculum in some ways there was actually some benefits there was a number number of folks that commented that they really felt like they were able to retain the information better because they were taught they were diving into the material for six weeks as opposed to three days um and so uh definitely encourage everybody who enjoys these vodkas who wants to learn more who wants to take their hec grass game to the next level um this is kind of a fun project and something fun for for across users to pay attention to and to get a thing or two here there um to up their hec-ras game but you really want to take your your modeling expertise to the next level um i think chris island both states it's really important to to take a class like this yeah you'll get a similar format it's chris and i just you know going back and forth talking about it it's more structured but uh talking about hec-ras topics answering questions going through workshops it's a really great yeah it's a deeper dive right ben i mean we go into more um uh more topics but we go deeper into it we get into theory and we get into and and you get to do workshops too where we're there to help you through it and guide you and talk about them afterwards so it's a it's a great opportunity and we have a lot of fun too uh so i highly if you didn't make the first one um there's plenty of time to sign up for the next one so i hope to see you guys there yep and again you guys can find the ad and the pre-sign up on our linkedin pages ben kerry and chris goodell or at kleinschmidtgroup.com yeah all right well let's get into the technical topic for today and we're going to talk about manning's roughness values in 2d models all right heck grass i'm going to go ahead and share my screen here as i kind of give a little bit of introduction so for those of you guys who are able to see our previous episode we talked about manning's roughness in one for 1d models and in some ways there's a little bit more customization and it can be a little bit more complicated working with mannings in 1d obviously you can assign cross sections to or um individual main values to interval individual cross sections the bigger the model you have the more remaining adjustments you might have to do you might have to adjust the location of your leftover bank and your right over bank and your channel there's a lot of of customization that can potentially need to go on with 2d uh it's it's pretty easy it's very quick to set up um it's it's nice because you get you can have a very spatially varied um and layer that's associated with your 2d uh mesh and we're going to talk about kind of how that works right now so for the this is razzmapper everyone should be familiar with this tool by now uh obviously one of the things chris and i was always saying when you start a new project you want to add a projection file to your project you want to bring in aerial imagery and then you're going to add your terrain so this project already has the terrain in here um the next thing that chris and i always suggest people do is to bring in a mannings and layer and the way that you do that is you go to tools um new land cover okay and then you're gonna get an option to pop up here that looks like this and you're gonna have the option to add a manning's n value layer okay and this is going to be most of the time this the format for this is going to be a shape file but you can also add a tiff or an image file that represents your your manning's n values for your 2d project area and there's really two ways to add a manual layer one would be to add in a general lan classification database something that's publicly available something like the nlcd database let me pull up a presentation that chris and i give is actually part of our class here so this is an example of the nlcd database you can see that it's a it's a nationwide database of land cover in the united states and it's often what heck rise users use to bring in as a mayonnaise and uh layer the problem with this is you can see it's it's pretty coarse and so you're not going to have a lot of detail when it comes to around your your channel itself so you can see here the blue is the channel in this case and you can actually see that there's some gaps in the channel and so you you would transition from one mains layer to another in the same channel and so this is going to give you you know this is going to create some issues especially if you're going to be doing some near field study or some design for a specific location so if you're doing that what chris and i would recommend instead of using like a national database like this would be to create your own mannings and layer and this is something you can do in gis and then bring in as a shapefile into mannings and again the way you would do that is either go under tools new land cover or you can come into map layers right click on map layers and add new manny and layer it's a little bit confusing because this option and new land cover are both they both are the same thing they just have two different names a little tricky tricky thing there to understand but just to confuse you guys yeah exactly exactly so uh again i i would say that more often than not what chris and i do is we actually would just create our own mannings and layer for our project so you know if we're doing a project that's just in this area here that's easy enough to spend a few hours and discretize our own land use shapefile maybe some you'd have a value to represent your channel maybe some areas that represent like if we turn on the aerial imagery here and zoom in to this area you can see looks like we have like a reservoir area and some river channel and then we have some overbank areas that represent some heavy heavily forested areas as well as some open space and some rural residential so you could discretize all of this with it with a a simple shape file we usually don't go to the level where you're actually discretizing individual homes but you can do that as well depending on again the level of your analysis that you're doing um but when you when you when you discretize that and you have your shapefile that represents your meanings and values again you do that by coming in here adding new mannings and layer you choose a specific layer okay so in this case what i'll do is i'll navigate to a sample here uh let's go to gis let's try this okay and then you'll get these different options so you'll see here in this case i don't have any i don't have there's not multiple manis and uh shape shapes in this shape file and so i only have one option but you can see you can select the naming fields you can customize which field you want to be represented for your management layer and then you can actually customize the individual mani values for each type of shape file and so if you had multiple shape files those would all show up here you could manually add in what you want your mannings and layer to be um the other thing the other option that chris is going to talk about here in a second is using override regions and so i'll let chris dive into that in uh in a second but once you have the mannings layer brought in to your headquarters project you have to associate that main zen layer just like you have to associate a terrain layer with a specific geometry you can do that by right clicking on the terrain manage terrain associations and then if we look at this dialog box you can see that each one of my geometries has a terrain that i associate with it as well as a manning's end layer okay and if you don't have your main zen layer associated with the geometry if you don't have a main zen layer brought into your project period you will not be able to use override regions which is what chris is going to talk about right now yeah um so ben mentioned the shapefiles that you can bring in uh to be your land cover uh there's also a really simple way to do n values too and that's to apply a single n value for the entire 2d area and that's done right here in the 2d area editor but the third option has been introduced here is the refinement regions which i'm going to talk about now but the only way you can use a refinement region is if you already have a land cover layer established and associated with this geometry file so you can double check on that by going into raz mapper and let me get into raz mapper here really quick it's thinking okay so here you can see my land cover um layer and i've got it highlighted right here oops highlighted over here which means since it's highlighted if i hover over i can see the different end values associated with it so that's really cool but to check that it's associated just right click on the geometries label go to manage geometry associations and then make sure you've got probably by a different screen chris yeah you're right it is there we go okay so just make sure that you've got your land classification right here established for each of the geometries you want to use it now once you have that that unlocks the ability to use these refinement regions and the reason the refinement regions are here override overhead regions right chris yeah same thing um oh no you're right override regions yes sorry um don't confuse that with the refinement regions yes override regions um okay the reason we have these override regions are well they were initially put in so that you would have a way to calibrate so you run your model and you see oh my my depths are a little bit too high over here but they're a little low on this side well what i could do is i can make a region over here and another one over here and those regions then i can adjust the base level and values to something different to try to calibrate my results and so if you go back to the geometry window you can see i've got some regions in here they're very small let me zoom in now in this case i'm using regions to now you can't even see them because of the mesh here but i'm using regions in these little uh refinement regions i've got some override regions as well i'm using those to simulate a what's called an um a large wood structure in the river it's a restoration element that's commonly used here in the northwest and around uh probably around the world too but great way to provide some habitat and some great control as well but this this is going to have a much higher end value right and so instead of going back and making a new land cover i can just simply click this 2d area regions mannings and regions override regions and i can draw in an override region where i'd like to have one okay just give it a name or six okay and there's another override region now what i with that override region these other ones in here the ability to make end value changes is very easy all you have to do is go into the tables menu item go down to your mannings and by land cover and here you can see your land covered table now the way this table works let me scrunch it up a little bit here is you've got your base and values or your default end values right here and these are what come out of your land cover layer okay we've got an end value of 10 for buildings uh 0.12 for medium density residential etc all the way down okay now for these override regions each override region you you've drawn in your geometry you've got a column for that override region here are the five that were already in there at the beginning this is the one i just created and let's say that you know in that particular region i'm doing some work there it's going to be uh pretty maybe graded out and just a bunch of of dirt okay so i might want to just force an n value anywhere that that region exists of 0.04 okay and i can copy that paste it all the way down okay so now anytime anywhere where that override region exists the n value will automatically be 0.04 okay no matter what it was over here it's going to override that with 0.04 so that's a that's another example of how to use these override regions is just to provide a specific end value maybe for a proposed condition like in this case this is a restoration site and i've got my existing conditions set with my land cover layer but my proposed conditions i'm going to define with these refinement regions okay and so that's just that's the third way of doing n values and the cool thing is you can do this right in the geometry window you don't have to create a separate layer or anything that's done for you but you can also set up regions refinement regions in the in raz mapper let me pull up brad's mapper so we're back in raz mapper if i go up to geometries and notice under this particular geometry the eljs i've got a manning's n group if i expand that then you'll see override regions so just like any other geometric layer input layer i can right click and i can edit that and that will allow me to draw some more readings in here here you can see the regions i already have i can just draw some more if i'd like to save that and they'll show up in the geometry window you can also right click on that override regions chris right now and go to open attribute table and once you have those drawn and this is where you can actually add those end values as well right yeah yeah good point so there's a lot of overlap between doing these in razmap or the geometry so for now do whichever one makes sense to you whatever you're more comfortable with i think ultimately you're probably going to be doing this in uh raz mapper or something similar to raz mapper in the future yeah and chris had a good point um that i've neglected to touch on and that was before you even bring in a maintenance layer uh your 2d area does have a default mannings and value and the what what that is is based on when you create a 2d layer so in this case under 2d flow areas perimeters i have a 2d layer for instance here you right click on this and this is i'm in raz mapper edit the 2d flow area properties and you'll see you you define your your cell sizes but you define your default manning's end values this will be the default meanings and value for every single cell face within your 2d mesh unless you specify or bring in a mannings and layer and use override regions so a lot of times when you're just initially setting up a model you don't actually need to go through and bring in a main zen layer maybe you just want to use just the default mannings you just want to get the model running making sure everything's looking okay and then you can add more detail again that's one of our secrets for success is start simple add more detail where it's needed yeah and let me let me make a quick point here ben before you continue on because this is a question i get all the time and that is how are the n values applied on the mesh and the way it works is you get one n value per cell face okay and the n value that gets applied to the entire cell face is the n value that the center of that cell falls within so if you look at the center of the cell face sorry the cell face if you look at the center of the cell face yeah right there you can see that it's it's close to the blue or the green it's hard to tell it's probably the blue in this case yeah and so that blue n value now is going to be applied to the entire cell face even though half of it is green whatever the number is i can't quite see the number there but so that's something that's very important for you to know and if that's going to have an effect on your results you're going to want to put a break line maybe right down there so that you have a separate face on either side of that um of that n value classification boundary yeah that's that's really good chris so the same line on this cell face here the center of the cell face is green so this entire cell face would get the manning's n value of 0.075 versus this cell would get the manning's end value of 0.035 so this is a this is one a disadvantage of having larger cells right because you're not going to have you're not going to capture the variance as well but like chris said if it's going to cause you big issues you can add a brake line along here so that your cell face ends right at that main zen layer and each cell face is more representative of the actual area um that's why some people that's that's why some people will will put brake lines along their banks of a river yep is yeah that's that's one reason to do that yeah perfect yeah so the last thing i was going to show was just kind of how this works in practice so this is a project that i was working on recently and i wanted to have my own customized meanings and layer and so these were the steps that i took so i obviously i brought in my terrain i added a projection file about an aerial imagery and then the first thing i did is i just went to features here so i didn't even use gis to do any of this this was all in raspberry i drew a uh like what i i went to features create new layer polygon layer and i called it base land cover base lc and all it is is just a just a simple rectangle here that's across my entire project area okay once i created that i right clicked exported the layer to a shape file okay and then i was able to bring that in as a manning's layer because now it's a shape file so i came into tools add new layer and i added that shape file in and that this is what it ended up looking like here so if i sorry um it's turned off there we go so this is what ended up coming in it was for my for my manning zen layers this this rectangle that represents a single manning's value of 0.075 okay so right now this is no better than my default value of 0.06 just a single value for my entire area but what i did then is i came in and i uh i associated that mains and layer with my geometry and then i drew in multiple override regions that represented my reservoir area my overbank areas some heavily forested areas some open areas some rural residential areas and i drew those all in okay and then um i assigned each of those override regions a manis and value and then when you go ahead and run the pre-processing of your 2d area what you what how you can see what your final maintenance n values are is under again my geometry the name of my geometry manning zen this shows my location of my overhead regions this shows what my final end values are for this model so you can see i got my 0.075 which is that large rectangle and then i have my override regions which represent kind of some of the more fine detail um in these different different areas here so this is a good example of a super quick way to do this it didn't take a ton of time or effort if i was going to use this model for a detailed hydraulic design in a particular area i'd probably want some some even more refinement maybe i would use something like gis to do some of that processing but it can be done really quick and what you end up with is a spatially varied mannings and layer for 2d model which is pretty slick yeah and just to reiterate to unlock the ability to use the override regions you have to draw in you know even just a very simple rectangular uh land cover layer like you did there ben so that's a really cool application um and this would be great if you have maybe your 2d area is the majority a single n value but you just have a few little areas here and there then you could quickly just put some refinement regions on there and so really cool really cool technique yeah we just have a few more minutes here chris but i wanted to just briefly touch on some of the challenges that are associated with manning's n values for 2d models um obviously with 1d models and 1d cross sections you have some tools that you're uh that you're available to you that allow you to account for maybe depth varied manning's n or horizontally varied many zen and you don't have that same option for for banning zen so what do you think some of the big limitations are um for 2d mini zen values um one thing that stands out to me immediately is when you're doing any type of brain on grid modeling right you can't do rain on great 1d we do we we talk about rain on grid in 2d in our class and one of the limitations there is many many of your cells are going to have very very shallow depths and so having a manning zen value um of 0.075 for forested areas might be totally reasonable for for a flood event and a flood plain but when you're talking about depths that are less than an inch that seems like it's way too small yeah yeah you picked up on it that's exactly right and that vertical variation and values especially when you have really really shallow depths and depths that are on the same order magnitude as the roughness elements themselves so like a blade of grass for example if the water is flowing through the blades of grass that roughness is very high and if you were to ever look at a relationship between manning's end roughness and depth you'll see as the depth decreases you get a um a a really steep increase in n values at some point and so um it looks usually looks something like this so like if i got my chart here and this is n value no sorry this is depth and this is my n value here usually you get something like this a pretty constant n value at higher depths and then as it gets lower and lower it does something like this right and this is the zone where you're you're in sheet flow right and you've got water about the same order magnitude and depth as the roughness elements unfortunately in the current version of raz we can't simulate this with our n value you only get one n value for the entire range of depths right and so we can't do that in the current version uh future versions will allow for that so what you have to do is you have to identify these areas is it going to be primarily sheet flow especially if you're doing rain on grid and if it is then you need to elevate those n values otherwise you're going to have velocities that are too high and depths that are too low water is going to move through that area faster than it's supposed to and so calibrating is a very powerful way to figure this out yeah that can really not only can that jack up the results as far as the velocities and depths but i can throw off your timing yeah stimulating a large basin uh the time that it takes for rain to get from this portion to a river or a town is going to be uh overly uh conservative or too fast if you're using kind of just a default manning zen layer for normal channel flow yeah like chris i think this is more definitely more of an issue for rain on grid modeling in which case you need to identify those areas and maybe use artificially higher remaining zen value for the areas that you know are getting mostly sheep flow but even some of the areas um where you have really shallow backwater or really shallow depths in your floodplain you might have to take into account maybe i need to use the slightly higher meanings and value for those yeah and just keep in mind the end values that are in the publications that we mentioned in the last uh vodcast publications like barnes or chow or hicks and mason those are all created for flood conditions or higher depths and so those end values are more appropriate for rivers and flood plains where you've got appreciable water moving over them they're not necessarily going to apply for really shallow depths in a 2d sense okay the other thing to keep in mind too the 2d equations actually take into account some of the features that are lumped into the n values that you see in these books like turbulence losses okay that's something that's not explicitly computed when when they're defining the n values from those resources but we have that built into the equation so in theory our 2d mannings and values should be a little bit less than what you're getting out of those publications now in practice what you're going to do is you're going to use those n values if you've got normal depths of flow anyway and you might back them off a little bit based on your engineering judgment but in reality the calibration effort is going to cover that that range of uncertainty um so that's just another um plug for doing a calibration effort on your model because there's a lot more uncertainty with the 2d and values than with 1d for sure yep very very good yeah all right chris well before we take off for the day anything else that you want to add no i think we did a good job a really quick overview of 2d end values again we take a much deeper dive in the in the uh online training course and and the in-person training course when we get through this covet thing we'll we'll be able to do these in person again but uh but until then yeah check out the course we'll do a much deeper dive into end values and all the other features in building your hec-ras models very good yeah hope uh many many folks can join us i think we had almost 40 people last time we'd love to get more than that this time i think we will um but if you're interested uh you know feel free to ask a question on the comment page of this this youtube video uh but go do you know some research look at our linkedin uh posts for for some background on that class and uh and sign up yeah keep the conversation going in the comments too ben and i check these uh periodically and and uh you know if this generated any questions for you we'd be happy to entertain those awesome all right awesome all right well uh chris i hope you have a great rest your weekend everybody thank you attending this podcast stay safe stay healthy and uh until next time this has been full momentum in hec raz podcast

Channel: The RAS Solution

Views: 6,591

Keywords: HEC-RAS, 2D, Manning's, N Values, Land Classification, Missoula Flood, Ice Age Flood

Id: V_2VDL4RxnQ

Length: 59min 27sec (3567 seconds)

Published: Wed Jul 15 2020