Engineering Geology And Geotechnics - Lecture 11

we okay to start okay well when I left you part one we were talking about competency issues and I just want to finish this off we use this concept a lot when we go to do channel improvements and that can be for flood control works or we want to have a channel of a nice smooth you know nice smooth floor nice smooth sides you're going to get greater capacity in a given area and you're going to get greater confidence for moving sediment yeah in both counts now where this comes up a lot is on is on bridges on a bridge say a a box culvert system going underneath the interstate highway something like that you're actually going to have a natural channel with a very high roughness coefficient coming in and then you're going to put it through this this Lillis tunnel that's you know a couple hundred feet long going underneath the interstate and what's going to happen is you're going to get a whole bunch of water piling up and dumping sediment at the upper end of that and then as it flows through this nice smooth box the competence and capacity is gonna go way up and it's gonna clean it out and you're gonna dig a huge hole at the end of it and so what you find is and that kind of a situation if we use a universal cross section it's not going to be very hydraulically efficient so in hydraulics what kills you and this kind of stuff especially with bank repairs and with bridges is the change in Manning's coefficient and roughness coefficient that you engender when you put an improvement in I mean ideally what you'd like to do when you do a bank repair or something is put something in there that's the same coefficient as what's upstream and downstream and you're gonna have less problems on the transitions with back Eddy erosion and so that's tough to do unless you've got something really rough like gabions or something and then when you put the gabion wall in it's be really tight and match the profile of the bank upstream and downstream or it's not going to work real well most things take a lot of finesse construction wise and is design wise to make them happen so they don't happen very often channel gravels channel is a grade locally where the conditions are favorable to their deposition so you're going to see channel gravels being deposited if you do see them being deposited what you want to ask yourself is why why are they here and over here and and am I on a point bar in this case I'm not on a plate bar I actually have a splitting up here going on at some helical flow stuff going on here in a fairly shallow river system this is down on the Andes in Peru this is back up in in British Columbia heading up to the Yukon and here you can see I got huge heads of gravels from the Late Pleistocene glacial retreat melt off and so I go miles and miles and miles I got these clean leg gravels paralleling the river which is way over here on the side of the valley and so you have to ask yourself you know are these local bars like this or these broad deposits going along a large area due to some kind of global climatic shift or base level change so you want to be asking yourself those kinds of questions flood damage one of the best things you can do is look at before-and-after pictures to get an idea of the power of running water it's staggering it's just staggering this is a picture along the Santa Margarita river coming out of the mouth of Temecula Canyon down in San Diego County before the January 1916 flood this was a nice beautiful little Canyon it's tree-lined this is a steam engine and that's the steam going up from the steam engine this is the station down here at the bottom in Santa Margarita and water tower here and this is a whole train and the Train is up here on the terrace going up the canyon to head over towards Sekulow cross the mountains across the coast range and this but this is a confined bedrock Canyon so they had a big flood in January 1916 I thought they had 100-year flood in January 1914 and then two years later they had a bigger flood so that's Q inflation and so this bigger flood comes down January 1916 and you can see here it not only just wipes out the canyon but it takes all the roughness out of it it takes all this vegetation and trees and everything sweeps these things out and you can see the high water lines way up here on the bedrock and when you need right here is where you break out of the mountains for the first time and you know almost 20 miles 18 Mile Canyon so right here you break out and so now the water spreads and this high reflected stuff you see all the places the sand and there's pieces of railroad rolling stock boxcars and stuff just hither skitter all over the floodplain this railroad never got rebuilt after this flood it just completely wiped it out of the canyon and that's what one kind of event can do now nothing I want you to look at is here's those fringe deposits going up the sides of the candies are sands deposited around these few trees that are still left along here so you see how that stuff gets deposited up here so if you see sands up here and those kind of positions up on the side slopes you know they've had a major flood in the recent historic past not just the recent geologic past but the recent historic past and you want to find out more about that flood in fact if you trench it right in this area you can find out about this flood and about the previous floods to try and get a recurrence frequency on how often they get water getting up that high that's pretty amazing and you can see down here and these trees the bottom 20 feet just stripped off and the everything is just you know that buildings are blown away the water tanks got all kinds of damage and it's pretty amazing what one storm event can do when when you see one another great area that I to see in practice all the time was channel constrictions we do this to save money we're trained as civil engineers to save money save money save money do it the cheapest you can possibly do it if you have you know a seemingly small most of the time dry channel then encroach on the channel with approach fills put in a concrete abutment wall and then put a much smaller bridge across here and that'll that'll carry your you know your 10 year flow and everything's smaller than the 10 year flow but when you get to something like the March 2nd 1938 flood which was probably a hundred year close to a hundred year event in Los Angeles San Bernardino area here's what happens that water squeezes through this little opening you've left here it digs a really deep hole because you know Q equals V times a and it back Eddy's and removes the approach Vil pronto scant o approach Phil's gone you get a major event you got an approach Vil going halfway across the channel two-thirds the way across the channel you're encroaching the hundred-year channel is going to be gone if you have any kind of duration of this was not a long-duration storm by any stretch it's a relatively short duration storm just a day and a half but they had every bridge in Los Angeles of San Bernardino County was either destroyed or severely damaged the only bridges that survived that storm were some of the bridges on the Los Angeles River not the railroad bridges they got taken out but most of it was this reason most those because they had used approach fills because they're in a semi-arid environment the channels don't have much water in them perennial E speaking year-round and so you can do this kind of thing but what's going to happen you get a 75 or a hundred year event you're gonna lose it because these things will weren't even armored now the most vulnerable side of the embankment the down gradient side not the upstream side the down gradient sides where you get the back Eddy scouring after you go through the narrow under the bridge back Eddie scours so when you go out in one of these things you're watching the flood your tendency as an engineer is always to be looking up streams see what's coming downstream be careful check your six all the time look behind you because the side you're gonna get killed on that it happens every year out west is the downstream side that's where it gets taken out from it gets taken out by back Eddy scour motion on the downstream side okay Fleiss floods are what we get in arid and semi-arid areas desert areas this is Palm Springs and this is what happens when you have a short term event and you get lots and lots of turbidity it's very very turbid it's gonna pick up anything it's cohesionless silt and sand and it's gonna keep that in in the in the channel and move it through here very very rapidly and so the best way to combat this is with an Arizona dip you see there right here they have a concrete apron that dips down goes underneath the channel and then back up on the other side this is a golf course down your white water wash Palm Desert area this is how the same thing they deal with in Arizona it's why they call it an Arizona dip it's it's infrequent enough these type of flows are infrequent enough that you just come in here and clean it all out later and you just accept the aggradation in the channel and you have to live with it and work with it because the cost of diking this thing and putting clear span bridges is colossal compared to how often you might use them you might only use them once every 10 years something like that and so people don't feel like it's a cost-effective thing to do and so this is very very typical of the kinds of damage you get in the southwest we had I had one in the Coachella Valley Water District where the channel like this 8 down 17 vertical feet and one event coming on the transition coming out of the mountains coming into an alluvial fan and boy you can get you know a lot of erosion in 24 hours in that kind of environment there's no cohesion in the material okay that ends care Durov channels let's move on now with the next section and that's going to be classic floodplains what do I want you to know about classic floodplains most of you have seen these kind of diagrams in your physical geology Tech's classic floodplain is the bank-to-bank between these terraces this whole area can flood and then we can get terraces in set terraces within the floodplain those are actually very very common and 75% of the sediment deposited on the continents on all six of the non-frozen continence is silt flood plain silt on major flood plains like the Mississippi River you can't can't compare anything with it I mean just in terms of volume in one event like the 1927 flood the Lower Mississippi River you had inundated areas or almost a hundred miles wide and then you're you're putting in there you know five six seven feet is silt over that large of an area that's a colossal amount of sediment so it's mostly silt it looks like mud everybody calls it mud everybody calls it clay but if you get a little bit put on your tongue it's a little gritty at silt floodplains silt is what's comes out of the river now the further away you get from the river the finer grained typically the material is because you lower energy environment in the slack water deposits you get slack water deposits over here on the edges of the floodplain here's what's going on when you have a river in flood you have something looks like this if it's an ideal Hill straight section it looks something like this and when the water comes up it reaches its greatest efficiency right there that's where the hydraulic radius number it is the most favorable the ratio between the volume cross sectional area flow and the wetted perimeter it's most efficient for transporting sediment right to there now when it comes up like this it also deepens itself at the same time but as soon as it gets up to the top here and spills over this lip right here and spills over what's going to happen the spillover is you have a lot higher roughness and you only got like one inch of water when this thing starts to come over so your ratio between Q and roughness is suddenly you know order or two orders of magnitude greater than it is out here and the slicker than snot on a doorknob you know Channel and so what happens is the sediment suspended load starts dropping out because velocity drops dramatically going in this direction as compared to going straight down the channel so what happens is you build up natural levees like this and this is what you see on all alluvial systems lluvia systems are channels with hydraulic grades less than 1.5 percent so they drop 1.5 feet per 100 feet or less you're an alluvial environment and so something like a Mississippi is the lluvia limit virtually all the way way up into Minnesota so what you end up with are natural levees that look like this that are the high ground and a floodplain with floodplain silts out here on lower ground below the natural levees now this is what general AAA Humphreys observe in his landmark study for the Corps of Engineers back in the early 1850s and he was a West Point trained mathematician and his mistake was he looked at it like this like a class and said well if this is what the channel looks like if I put levees right here and heighten this I take some of this silt right here drag line it up and make the levees higher now when I have a flood I can bring the flood way up to here and this will dig down deeper and that is the most inexpensive way I can train this thing and keep it from flooding everybody out who lives over here on the floodplain places like Memphis huh now what's wrong with that well what's wrong with that and why it didn't work the levees only pull up a program 1877 to 1931 it didn't work they because it was a two-dimensional representation of a three-dimensional situation in other words the Mississippi River doesn't look like that anywhere except in geology textbooks and channel hydraulics checkbooks what does it look like it's an asymmetric channel so it never looks like this what he's doing here is he's ignoring curvature and asymmetry and if you fly over the Mississippi River the one thing you'll come away with if nothing else is one it's muddy and two it's serpentine it's moving around like a bunch of intestines and it's got all these ox bows and cut offs all over the place testifying to how dynamic a natural system it is the larger the river the more dynamic the system you look at something like the Mississippi or the Danube River and Eastern Europe Wow those systems are incredible for how dynamic they are how much the channel can move around one storm the channel moves ten miles across the floodplain Wow that's scary if you're the engineer you know trying to save everybody so this levees only policy didn't work because of the curvature and one side actually ends up being a lot steeper than the other side and so that side fails the bank undercutting you lose the levee here and well off the floods all over one side of the floodplain pretty quickly and because the river is so serpentine it gets around to the other side as well so didn't work now what you could do is you could put training dikes right here and then you'd have a setback levee back here somewhere that would protect your high-value property or developments over here that way you don't put your you don't put the one that really counts the high the high-value setback levee you don't want to put that thing right next to the channel because you're put in a point of greater vulnerability I just use common sense on that so setback levees real good idea and then you use training levees dikes out here near the active channel which you can you you sacrifice the training levees during the big flows and you realize you're gonna lose them but you're not going to inundate everybody's neighborhood if you have the setback levee for the safety so here's a picture here's what we're looking at along the Mississippi River 1913 one of the first big comprehensive reports in the 20th century we look at the Mississippi River at Bell point Louisiana and what do we see across the we see the river up here as the high ground and then we see everything else trailing off below it you know you look at a map of Missouri and you look over here in the southeastern corner of Missouri and there's the Mississippi River leaving the state and everybody assumes that's the lowest plain in the state and you'd be wrong you'd be wrong by 30 feet that River is 30 feet higher actually then the st. Francis River where it leaves the state of Missouri on the other side of the Bootheel 30 miles to the west because it's a bigger River bigger rivers more competence more q more silt more silt they big up a higher Mound and now you see the problem for flood control here's your natural levees pokin out right there but if the water comes out here what happens it's got a real nice healthy hydraulic grade which means it flows downhill and this slope is much steeper than the slope of the river so when it breaks through here it breaks fast and it comes down with a vengeance and cuts a hole and it gets out here to the swamp area lickety-split ski dip ski doesn't take any time at all foosh because you got drop hydraulic drop in a case Mississippi River it's about a 30-foot drop across the floodplain that's plenty even over a couple miles that's way higher than the gradient of the river I mean this river you know drops very very low gradient as you get down below the where the mouth of the the Ohio River is and so if we look at the types of sediments we would expect to see now in this environment here's what we're going to see we're going to see point Bar sands and gravels historically back here where the active channel has been in the The Late Pleistocene early Holocene then we see the holocene channel sands right here and the levees and the bed of the river are very sandy but as we get away a short distance few hundred meters that starts getting finer so you get finer sand then you get into silt finally you get to Clay's and out here in the marsh yes it's like a sewage treatment plant what does that mean Peet's clay and something they call organic booze you don't want to know what organic goos is but if you ever had a baby that had diarrhea that's kind of what it's like it's a it's a very soft it doesn't have shear strength and doesn't have bearing capacity what it has is it's a brown and it's liquid and it doesn't smell real good this is not real great stuff for doing much on at all so that's why it took 150 years till after we built the Panama Canal and we had drag lines and large kind of Earthmen equipment they came along and made drainage districts and they dug huge ditches out in these areas so they could drain them so they could do agriculture in them so that agriculture didn't even start till after the first world war so post 1918 and between 1918 and 1940 the little River drainage district down in southeastern Missouri just based out of Cape Girardeau moved more earth than the Panama Canal project no Panama Canal they move 96 million cubic yards of material they moved a hundred and 16 million cubic yards of material just putting in drainage ditches for the Little River drainage district well could you got million acres so we reclaimed these floodplain areas that previously had not been good for much of anything because they flooded every year every year in the spring flood Mississippi would break somewhere and store these huge floods out here and it has a big huge loss of irrigable land so man conquered nature until the next big flood came along and then nature kurz man all right well here's what I'm talking about here we are going along Lower Mississippi and Bayman we got this serpentine channel it's not straight anywhere not for very long anyway it's sneaking around and it's the high ground and you have these back swamp areas that have very poor drainage sometimes you get a Yazoo tributary that eventually coalesce into a channel and way downstream somewhere like at Vicksburg Yazoo River comes back in to the Mississippi but when this thing floods and that's the high ground the channel is a high ground here's what happens the whole darn floodplain becomes a swimming pool not part of it not the right part the whole thing goes underwater this is what happened along lower Missouri River and middle Mississippi and then Nathan 93 floods you just saw the whole entire floodplain cliff to cliff miles across goes underwater and it was underwater the whole summer so that's the problem you're battling with when you try to type these things off and protect these developments that have been put in here here and there selectively so it's a big big challenge geologically geologically it's that's a huge challenge I can't trivialize it at all this is a bigger challenge as the Dutch trying to take the mouth of the Rhine you know and pulled her it off and they had to spend 35 percent of their gross national product of that country for 30 years to do that so you have to have the political will to do that that usually comes after you have a major flood like they had in 1953 and it has to be a major flood that wipes everybody out and kills a lot of children then you get action you get political resolve to do something here's what I was just talking about here's a Earths picture earth resources technology satellite picture this is the Lower Mississippi River this is the Illinois River coming along this actually used to be the main stem of the Mississippi River prior to the Illinoisan glacial break and then it moved over here about two hundred thousand year ago and so this is a very youthful channel it comes down they join back together here and then they come down here towards st. Louis this is st. Charles and this is the little muddy what's the little money that the Big Muddy that's the Missouri River there's what it looks like most time not a whole lot of water and there's the junction and there's st. Louis so there is the confluence look what this thing looked like in the 93 flow to the summer of 93 what's going on here is you have the Missouri River in full flood that entire summer lake spring and summer and then you get down to places like this near st. Peter's and st. Charles and you got dikes protecting the development st. Charles predates st. Louis and so you got dykes here you're narrowing the water and so as you join this other River here you have a backwater effect and that's what you see damming these other things up there back watering and washing out all the Army Corps of Engineers lock and dams going up stream here for quite a ways so this system comes around and then it hits the improvements around st. Louis st. Louis is protected look at the back water in the Merrimack River right here very significant coming down Merrimack River actually used to be a pour-over point at grey summit here this used to be a big ice dam right here in your chain of rocks so during the Ice Ages the Missouri River used to actually flow over here and it cut this big huge channel of the lower Merrimack River that looks so out of place because it doesn't have the watershed area who have cut that big of a channel you have a 150 foot deep alluvium in Eureka Valley and that's because of the Glacial pour-over that occurred in here comes down like that so Wow that's yeah you see something like this and you realize whoa baby what are we dealing with here we're dealing with a tremendous amount of water from the normal everyday - what happens when you have a large event all right so the big thing I want to leave you with as a geological engineer is channels are asymmetric asymmetric you show me a symmetric channel you're showing me a textbook for freshmen okay channels are not symmetric hardly ever that's all once in a blue moon type thing the channels are asymmetric and because of their asymmetry they tend to have retro aggressive sliver failures over here on this over steep in banks and especially and they get high flow and they start having problems they scour out and and that's enough of a problem just for undercutting you've got to get some stuff that falls in but where you get this retrogressive slumping like you see right here this is from this is the abducted down there New Orleans this is from the flood stage dropping down fairly rapidly and when it drops down rapidly you get a rapid drawdown condition and then boom you're off to the races you get one sliver then another one then another one and we call that progressive failures or retrogressive failure I Bank undercutting is the colloquial term given to it but you can see Bank undercutting is gonna get the dike that's right next to the river so you don't want to depend on a dike right here on the outside of a turn that's a training dike that should not be your last line of defense unless you got a boat factory over here and you'd like to swim if you want to do is put a training dike in here but then you want to have a setback levee back here in a safer position because this thing is gonna fail at some point down the road all right low gradient channels love to meander and when they meander they have a symmetry there's steep on the outside of the turn and then they switch back the other side then they're over steepening over here so there they got bank losses going on here got Bank losses going on right there and as this thing continues to have Bank losses and migrate and move it moves outboard and downstream so it moves out and down down and eventually you're going to get a neck in someplace like this and when you have a neck here during some large event you're going to get a break through there and when that breakthrough occurs you're then going to have a very very high gradient that's very short-lived this is going to break through here very rapidly and then it's going to actually move around some in relationship to that this gets cut off and becomes an oxbow it plugs with silk and it sits there for a long long time as an oxbow lake and the only thing that's going to fill up an oxbow lake is then repeated flooding of the entire floodplain and gradually this thing is gonna fill in with jello it's gonna fill in with organic silt that's gonna take a long time then you'll put artificial fill over it and you'll wonder why half the buildings settling the other half isn't because you're you're on an old ox bow so ox bows are a huge pain in terms of geotechnical hazards you know if you have part of an interchange on an oxbow and part of it's not an oxbow obviously you're going to have very very contrasting conditions or foundation support so low gradient streams are the most problematic because they're the most sinuous so they tend to do pinch offs and cut offs much much more regularly because of their low gradient so ox bows and cut-offs are common features and they're very treacherous because of these changing foundation conditions that you have across there you can see there's an old channel right there that's gotten filled in and then the more recent channel over here so if we look at a map this is the Corps of Engineers engineering geologic map of Mississippi River right where the Ohio River flows into the Mississippi River near Cairo now the active River is shown in white that is the active stabilized River that the Corps took ownership by congressional mandate back in the Flood Control Act of 1928 so the court started look at this thing and trying to keep it in one place for flood control and navigation purposes and what you're seeing in blue are the historic wanderings of the Mississippi River just going back to the time of Mark Twain last 200 years Lew these things this thing is all over the place then you go out here further on the floodplain and what is all this hot stuff it looks like flames those are braid bar gravels from the Pleistocene Late Pleistocene melting of the glaciers when sea level was 360 feet lower the river had a much steeper gradient steeper gradient means more competence you can transport larger material so all these braids bar gravels are from when the Mississippi was flowing well 300 percent of the queue you see today it's much much higher queue on a steeper gradient and you'd never get this stuff today this stuff is underlying the whole city of New Orleans anywhere from 75 to 150 feet deep you got lag gravels gorgeous lag gravels ain't no gravels anywhere in the state of Louisiana that are exposed they're all buried under the Holocene so you have to I have deep piles going down into them so we can use these for aggregate sources of course they're not great for agriculture because they they're not fine-grained enough they lose water pretty quickly but you know what a dramatically different material here then these floodplains silts in the chocolate color and you can see how these channels are just pinched off and truncated and very very complex patterns very three-dimensional okay that's the one thing I want you to remember very three-dimensional so be careful when you do all your analysis using two dimensional cross-sections those are greatly oversimplifying the actual situation okay that closes part 6 part 7 is channel form and patterns alright one of the concepts we talked about in geomorphology or the study of landforms is antecedents and superposition so what are those things mean let's look at the definitions an antecedent stream is one whose path of flow within a valley was established before the mountainous structure was uplifted and we see that kind of stuff like here we have this you know sinuous kind of looking like a mini Mississippi and that was developed on a shale unit on a very low gradient then the land lifted up like the Grand Canyon and the river just eats straight down and so you end up getting this low gradient channel form on top of a rejuvenated landscape so that's antecedents superposition superimposed streams are one whose valley and direction of flow were developed later much later than the underlying structure and the river possess sufficient stream power to cut through these underlying structures and this is a lot of what we have back in the Appalachian Allegheny Great Smoky Mountains uplift in fact we even look at the French Broad River which runs through Knoxville Tennessee it goes all the way through the heart of the Great Smokies which go up to 8,000 feet all over to Asheville North Carolina so that water is coming from over the Carolinas coming across that mountain range and heading over to the Ohio River and thence into the Mississippi River so that's super position and that's why you get things like wind gaps sometimes where the river couldn't keep up so we're gonna see okay so what are some of the common drainage patterns that are structurally controlled this is geologic structurally control not like structural engineer to control but similar concept if you had a homogeneous kind of material rock or soil material you tend to get a dendritic drainage pattern and as you get it's you know smaller and smaller watershed hot you know lower lower order channels so you starting out third order fourth third second first order this is going to be dendrite dendritic like and that's the most common one that we see worldwide where we get away from that is if we have carbonate rocks like in a karst environment like around here we'll actually see rivers do right turns and left turns all over the place because they're following systemic regional joint patterns so we have a semi rectilinear pattern around here we also have a superposed older river system here but we see we get further up in the watersheds we actually see the rectangular controls here and there we can actually see some of these right turns and left terms even in some of the major rivers around here aside not talking about them Missouri River but rivers like the Gasconade and the Merrimack now when we're structurally controlled trellis drainage is where we have a combination of folds and bedding and structure that's actually exerting control on the channel and that's what you have in most of Pennsylvania places like the the Allegheny uplift it's definitely a trellis system of synclines and anticlines the big full belt and that impacts the channels here's a channel coming down a syncline system and then meeting the trunk screen the trunk stream here is superposed it was here first and it's pushed through these hard gaps as the land's lifted up so it's pushed through those gaps and you have these resistant ridges from the folding these are the resistant Rock units typically Dolomites s-- court sites things that sometimes sandstones if they're well cement it radial is what you get around volcanoes it's a very very unique to volcanic situations we do have those out in the Pacific Northwest the Cascadia volcanic field which starts out way up at Mount Baker at the Canadian border and goes down to Mount lassen in California all right what's a Water Gap we hear that term Water Gap gets used water gaps are where you have a existing drainage system developed on flat-lying soft sediments let's say it's a dendritic system like you see here and now the land uplifts we have the Allegheny uplift and here we have sufficient stream power like the New River Gorge over in West Virginia coming out of West Virginia into West Virginia that's going to play on words there flows north and it goes through these incredible water gaps these canyons that's because they've had enough stream power to eat through this resistant like a dam it actually eats through that on either side and comes through and so you have perturbance of all the feeding tributaries by this later structural control down here but a lot of the older main trunk channels are going to continue going where they went because you had enough water going through it so that's called a Water Gap it becomes a wind gap when it gets pinched off and there's insufficient stream power to keep pace with the uplift of that resistant unit and so here we get stream piracy another channel comes over and Pirates this watershed and it leaves you a wind gap the Cumberland Gap which was the the great thoroughfare between eastern seaboard and the Ohio Valley is one such wind gap on the upper end of the answer Potomac River drainage so see here you have a water gap here water gap here but when you got up here this water gap later got stolen away through piracy and that becomes a wind gap and the river turns and goes another direction so actually these things have been mapped out pretty well all through Pennsylvania Tennessee the Carolinas and the to Virginia's okay see if I can get the next one done here for you part eight alluvial deltas and fans now there's a lot of things I want to tell you about alluvial fans one is they're usually not alluvial usually they're deposited by debris flows debris events then they're reworked by the water that comes down and reworks the debris fans but a classic alluvial fan looks like a fan when you fly over it in the air and it develops where you have confined bedrock channels coming down and breaking into a broad sloping region like a valley so you get this sudden dispersion because this flow which is confined now can spread as it spreads just like that water coming up out of the Mississippi River as it spreads you now have a much shallower water depth and very high roughness so you deposit your sediment your sediment comes out and D cants out of the mixture when we look at Delta's it's a similar kind of phenomena but here we have controlling base level and so this sediment Layton water is coming out here on a very very low gradient and pushing it has to push against the ocean and the closer gets the ocean the more pushing it does and the only reason it's even able to win the battle is because the sediments makes that fluid heavier than the seawater see waters about 1.03 times density of clear water but this supercharged sediment coming down here is coming down at like 1.35 and so it's able to win the tug-of-war and push this sediment into the Gulf and what you get is this Pro grading cross beds coming down about thirty degrees that just keeps pushing out pushing out pushing out you get these foreset beds with the bottom set beds out here in front which are the turbidity channel deposits and so you're laying this thing out there very very rapidly now the the bird foot dike that we see in New Orleans you know below downstream in New Orleans and in Mississippi River that thing is really young I mean that that whole thing's been thrown in there just in the last two thousand years I mean most of it a thousand years ago so that wasn't even there 2,000 years ago if we look at the Mississippi and chaff Alaia rivers and we look at these Delta's we they've actually gone back and dated the various deltas here and you can see Delta number seven is the oldest so Delta number seven is the one that's way out here oh excuse me Delta seven is the youngest this is the one that just came out here where the modern Delft is and number one is the oldest nest the chaff Alaia and number one actually has much much thicker sediment accumulation than over here New Orleans is actually sitting way over on the distal east end of the Delta right there middle of Lake Pontchartrain the distal east end of it so it's it's way over the the Depot Center this Delta's over here in this area so very very fascinating and also the water that comes down the chaff Alaia has one-third the distance to go that the regular Mississippi has to go so if the river ever breaks out up here and comes down the chaff Alaia it's going to cut one of a canyon in a couple hours 24 hours all gonna be over and all the water is gonna come out of there that's called the old river control structure and it did fail in 1973 but they didn't have a catastrophic failure they they had undercutting of the hydraulic control structure but they didn't lose it cuz I was on battered piles and so they built a new one there next to it they more than doubled the capacity but the river would cut this thing in a flash because that's a shorter distance to the Gulf right down here boom it's 1/3 the distance of taking the current route which is very serpentine all the way out here now we look at the abandoned distributaries in the Gulf here's the modern bird foot Delta where EADS put his jetties starting in the late 1870s here is New Orleans right there between Lake Pontchartrain and the river and what I have highlighted in red there is the Metairie Gentilly Ridge which is a very prominent discontinuity or a distributary channel it's about five six feet above the floodplain around it so if you're lucky enough to live on the Metairie Ridge when they have big flooding like Hurricane Katrina and Rita you didn't get flooded out if you were along that Ridge you're up higher so all of these ridges these are ridges you see here this is all ground that's slightly higher because of sedimentation then the swamps between it so the channels are the high ground the swamps are the low ground and that's why when you go down here and Terrebonne Parish and you try to build an 18-foot high you know a hundred year flood dike you got a it's got to be about you know 1,500 feet wide with one on 16 slopes and even then it's just sinking into the pd soils of the swamp and just have to keep adding and adding and adding and it keeps sinking and sinking and sinking and it's a good deal if you're getting paid by the yard I mean gonna have a lot of work there you're just going to keep feeding the things feeding the monster now until we had the construction of the jetties by Jay in the late 1870s when you had low flow of the river you would have insufficient stream power to break down the bar sands that would form with the same province San Francisco Bay to toe came in and did some engineering to keep it cleaned out because you're gonna get bars forming down here during the dry season and you can't go upstream and that was three months at a time every year usually in the late fall and winter months okay now what I want to tell you is we have this you have two weeks you have so you've had one week so far to do the chapter eight homework you're going to get another week to finish that homework and I am posting your midterm on blackboard for you to take and you have a week to do the midterm so both assignments are going to be due same time which is going to be Monday November the 8th so Monday November the 8th I'd like to see your MIT take home midterm and I'd like to see your questions on chapter 8 now if you can't finish the questions on chapter 8 send me a promissory note tell me why and when you do promise to get them to me I'm a reasonable man I'm just like the boss at the office what I want to hear is honest and truthful communication okay that's all I have for you today and I'll look forward to seeing all of you again next week thank you

Channel: S&T CAFE

Views: 12,230

Rating: 4.9560437 out of 5

Keywords: engineering, engineer, geology, geotechnics, geologic, factors, site, selection, design, engineered, structures, missouri, sandt, s&t, university, science, technology, educational

Id: ZSLK9NANvwE

Length: 48min 23sec (2903 seconds)

Published: Thu Feb 17 2011