GeologicGeologic FactorsFactors PromotingPromoting GroundGround SubsidenceSubsidence andand CoastalCoastal LandLand LossLoss inin thethe MississippiMississippi DeltaDelta andand thethe GreatGreat DebateDebate AboutAbout WhatWhat toto DoDo AboutAbout ItIt

J. David Rogers, Ph.D., P.E., R.G. Karl F. Hasselmann Chair in Geological Engineering University of Missouri-Rolla for symposium on The Role of Geology in Planning and Mitigation of Natural Hazards Annual Meeting Geological Society of America Denver, Colorado October 31, 2007 • Between 1932-1990 an average of 35 square miles of land was being lost to the sea each year in the Mississippi Delta; and an average loss of 44 square miles/year of wetlands. •• Acute wind shear from Hurricane Katrina stripped off large tracts of floating marsh across the Mississippi Delta. Can we construct sustainable levees on these kinds of materials? •• During Hurricane Katrina, 115 square miles of land area was lost. This shows “land loss” in Breton Sound (in light blue) after the 2005 hurricane season. MECHANISMSMECHANISMS DRIVINGDRIVING GROUNDGROUND SETTLEMENTSETTLEMENT

•• TheThe causescauses ofof groundground settlementsettlement areare aa contentiouscontentious issueissue inin coastalcoastal LouisianaLouisiana •• ThereThere appearappear toto bebe manymany differentdifferent causes,causes, brieflybriefly summarizedsummarized inin thethe followingfollowing slidesslides •• Block diagram illustrating various types of subaqueous sediment instabilities in the Mississippi River Delta, taken from Coleman (1988). FLUIDFLUID EXTRACTIONEXTRACTION OFOF OIL,OIL, GAS,GAS, ANDAND WATERWATER

R.A. Morton of the USGS has blamed oil and gas extraction for the subsidence of the Mississippi Delta. Morton has constructed convincing correlations between petroleum withdrawal and settlement rates on the southern fringes of the delta, near the mouth of the Mississippi, but this theory fails to account for marked settlement further north, where little or no deep withdrawal of oil or gas has been carried out. GrowthGrowth FaultsFaults •• Geologic cross section through the Gulf Coast Salt Dome Basin, taken from Adams (1997). •• This shows the retrogressive character of lystric normal faults cutting coastal Louisiana, from north to south. •• The faults foot in a basement-salt- decollement surface of middle Cretaceous age. • This plot illustrates the en-echelon belts of growth faults forming more or less parallel to the depressed coastline. The Baton Rouge Fault Zone (shown in orange) has emerged over the past 50 years, north and west of Lake Pontchartrain (data from Woody Gagliano, Coastal Research). Saucier’s 1994 structural geology map of the Mississippi Delta, published by the Corps of Engineers. This shows salt basins, salt domes, and active growth faults that pervade the delta region. •• Draining lowlands and backswamps for agricultural or urban development increases the effective stress on underlying sediments and hastens rapid biochemical oxidation of organic materials, causing settlement of these surficial soils. • New Orleans was built on compressible peaty soils. This isopach map shows the areal distribution of the surficial peaty soils (contour interval = 4 feet). From Gould and Morgan (1962) and Kolb and Saucier (1982). CompactionCompaction ofof SurficialSurficial SedimentsSediments

The interdistributary sediment package covering the old back swamps around New Orleans are under consolidated, and they exhibit gross settlements. These examples are from the Lakeview area adjacent to the 17th St. Canal failure, where the ground appears to have settled 10 to 16 inches since 1956. Most residential structures in New Orleans are founded on wood pilings 6 to 8 inches in diameter, driven 30 feet deep. The ground beneath these post-1950 homes has settled 10 to 18 inches. The mechanisms driving this settlement are likely drainage of near-surface soils, by dewatering and interception of storm waterwater runoff,runoff, whichwhich isis collectedcollected and conveyed to the city’s pump stations. IndustrialIndustrial andand municipalmunicipal groundwatergroundwater withdrawalwithdrawal

As groundwater levels are depressed by withdrawal of groundwater, organic-rich peaty soils are oxidized, and the ground settles. This settlement creates depressed zones below the grade of adjacent watercourses, which creates challenges for providing flood control. • Record of historic settlement in the town of Kenner, which is underlain by 6.5 to 8 feet of surficial peaty soils. • The steps in these curves were triggered by groundwater withdrawal for industrial use and urban development. This area was covered by dense cypress swamps prior to development. Much of New Orleans lies below sea level, Lake Ponchartrain, and the Mississippi River, making it particularly vulnerable to flooding. The Mississippi levee crest is 24.5 feet, while the Pontchartrain levee crest is 13.5 feet, above mean gulf level. • Topographic map with one foot contours prepared under the direction of New Orleans City Engineer L.W. Brown in 1895. This map was prepared using the Cairo Datum, which is 21.26 feet above Mean Gulf Level •• Map showing relative elevation change between 1895 and 1999/2002, from URS (2006). •• The net subsidence was between 2 and 10+ feet, depending on location. STRUCTURALSTRUCTURAL SURCHARGINGSURCHARGING

•• An interesting aspect of the URS-FEMA settlement study is the marked increase in settlement in the Central Business District, where tall structures are founded on deep piles. This area settled 5 inches in 100 years, but the settlement was much less away from the heavy structures •• Even the sandy river levees have settled 2 inches; likely due to surcharging by MR&T levee heightening between 1928-60 MechanismsMechanisms ofof GroundGround SettlementSettlement --11

•• 1)1) ElasticElastic deformationdeformation ofof MississippiMississippi DeltaDelta fromfrom siltsilt depositiondeposition ((isostasyisostasy)) •• 2)2) TectonicTectonic compactioncompaction causedcaused byby formationformation ofof pressurepressure ridgesridges andand foldingfolding •• 3)3) SubsidenceSubsidence onon seawardseaward sideside ofof lystriclystric growthgrowth faultsfaults •• 4)4) DrainageDrainage ofof oldold swampswamp andand marshmarsh depositsdeposits increasingincreasing stressstress onon underlyingunderlying claysclays •• 5)5) BiochemicalBiochemical oxidationoxidation ofof peatypeaty soilssoils MechanismsMechanisms ofof GroundGround SettlementSettlement --22 •• 6)6) ConsolidationConsolidation ofof compressiblecompressible soilssoils causedcaused byby surchargingsurcharging withwith fillfill •• 7)7) SurchargingSurcharging byby structuralstructural improvementsimprovements •• 8)8) ReducedReduced groundwatergroundwater rechargerecharge inin developeddeveloped areasareas becausebecause ofof impermeableimpermeable surfacessurfaces •• 9)9) ExtractionExtraction ofof oil,oil, gas,gas, andand waterwater causingcausing pressurepressure depletiondepletion •• 10)10) SolutioningSolutioning ofof saltsalt domesdomes andand seawardseaward migrationmigration ofof lowlow densitydensity materialsmaterials (salt(salt andand shale)shale) Coastal land loss in Louisiana is also exacerbated by sea level rise, which averaged about 1 foot during the last 100 years. This value could accelerate during the 21st Century, to as much as 3 feet. TheThe NationalNational DebateDebate thatthat hashas eruptederupted sincesince KatrinaKatrina…….. •• CanCan wewe buildbuild leveeslevees thatthat wonwon’’tt fail?fail? •• ShouldShould we,we, asas aa society,society, allowallow oror encourageencourage urbanurban developmentdevelopment ofof landslands thatthat areare either:either: 1)1) belowbelow seasea level;level; or,or, 2)2) barelybarely aboveabove seasea level?level? •• PeoplePeople whowho choosechoose toto livelive inin highhigh riskrisk areasareas shouldshould paypay greatergreater insuranceinsurance premiumspremiums forfor thethe privilegeprivilege ofof livingliving inin thosethose areasareas •• ShouldShould wewe botherbother tryingtrying toto savesave thethe MississippiMississippi Delta?Delta? Why?Why? NewNew OrleansOrleans shipsships thethe greatestgreatest volumevolume ofof exportedexported goodsgoods fromfrom thethe USA,USA, mostlymostly wheat,wheat, corn,corn, andand soy.soy. IsIs itit withinwithin thethe graspgrasp ofof ourour societysociety toto savesave thethe MississippiMississippi Delta?Delta? •• OneOne modernmodern analoganalog –– TheThe Netherlands,Netherlands, followinfollowingg thethe 19531953 floods;floods; butbut thisthis alsoalso involvedinvolved aa substantialsubstantial investment,investment, almostalmost 30%30% ofof theirtheir GNPGNP forfor 2020 years.years. •• CanCan wewe applyapply 2121st CenturyCentury technologytechnology toto helphelp solvesolve thesethese immenseimmense challenges?challenges? •• DivertingDiverting sedimentssediments ontoonto thethe landland willwill requirerequire aa muchmuch greatergreater investmentinvestment inin operationsoperations andand maintenance,maintenance, severalseveral ordersorders ofof magnitudemagnitude moremore thanthan wewe’’veve spentspent heretofore.heretofore. EarthenEarthen LeveesLevees areare oldold technologytechnology Levees are susceptible to erosion by overtopping, by edified flow, and by undercutting. Once flood waters overtop an embankment they quickly scour the land-side toe of the embankment, and deep scour holes develop on either side of the “hydraulic jump” that forms at the point of overflowage, enlarging the breach, as shown here. COULDCOULD SOILSOIL REINFORCEMENTREINFORCEMENT TECHNOLOGIESTECHNOLOGIES BEBE APPLIEDAPPLIED TOTO CREATECREATE MOREMORE ROBUSTROBUST PROTECTIONPROTECTION SYSTEMSSYSTEMS ?? •• GeogridGeogrid--wrappedwrapped hayhay balesbales oror gravelgravel filledfilled HDPEHDPE basketsbaskets cancan bebe usedused asas facingfacing elementselements forfor mechanicallymechanically--stabilizedstabilized embankmentsembankments.. •• TheseThese materialsmaterials dodo notnot corrodecorrode andand plantsplants andand treestrees cancan taketake rootroot inin them,them, soso theythey areare moremore ““environmentallyenvironmentally friendlyfriendly”” •• We can make embankments slopesslopes asas steepsteep asas wewe desire by using “false layers,” of geotextiles, between primary reinforcement layers of mechanically stabilized embankments (MSE). •• These false layers restrict slope raveling and erosion •• Example of a 45 degree fill face supporting aa shoppingshopping center complex adjacent to a line of mature trees. The embankment was constructed usingusing falsefalse layerslayers ofof geotextiles spaced 1 foot apart (from Rogers, 1992) •• SameSame 4545 degreedegree fillfill slopeslope afterafter hydroseedinghydroseeding andand sproutingsprouting ofof fescuefescue grassgrass mixmix (from(from Rogers,Rogers, 1992).1992). SegmentalSegmental LeveeLevee SystemsSystems

•• InIn areasareas exposedexposed toto stormstorm surgesurge somesome thoughtthought mightmight bebe givengiven toto equippingequipping leveeslevees toto bebe moremore efficientefficient energyenergy dissipationdissipation systemssystems;; andand notnot justjust simplesimple barriers.barriers. ThoughThough moremore complicated,complicated, theythey mightmight notnot havehave toto bebe asas high.high. SoilSoil ReinforcementReinforcement

•• SoilSoil reinforcementreinforcement couldcould bebe appliedapplied toto engenderengender greatergreater shearshear strength,strength, allowingallowing muchmuch steepersteeper crosscross sectionssections •• SteeperSteeper sideside slopesslopes formform moremore efficientefficient barriersbarriers toto dissipatedissipate forwardforward wavewave energyenergy •• NoteNote curvedcurved troughstroughs betweenbetween thethe embankments.embankments. •• HDPEHDPE geogridgeogrid soilsoil reinforcementreinforcement couldcould bebe alsoalso bebe employedemployed toto engenderengender greatergreater flexibilityflexibility inin leveelevee profiles,profiles, asas suggestedsuggested here.here. •• LeveesLevees couldcould bebe moldedmolded intointo moremore efficientefficient barriersbarriers toto retardretard andand survivesurvive stormstorm surgesurge •• TheThe lowerlower sketchsketch showsshows howhow bearingbearing loads could be reducedreduced byby employingemploying soilsoil reinforcement,reinforcement, allowingallowing aa greatlygreatly reducedreduced crosscross sectionssections.. CypressCypress SwampSwamp diedie--offoff

The entire delta is slowly subsiding. If new sources of sediment do not replenish the swamp, the young cypress shoots cannot germinate in water > 2 feet deep; and Cypress forests die off all at once, becoming a treeless, grassy marsh, with a forest of dead tree trunks. MatureMature CypressCypress swampsswamps retardretard stormstorm surgessurges byby mechanicallymechanically dissipatingdissipating incomingincoming wavewave energyenergy Much propaganda has been circulated concerning the potential mollifying effects of cypress forest dispelling wave energy, when hurricanes make landfall along the Louisiana Coast. The most common-cited figure is 1 foot of storm surge reduction for every 2.4 miles of mature cypress swamp. This value is not valid for the entire coastline. The actual impact depends on a number of physical factors. GreenGreen Levees:Levees: CreateCreate troughstroughs toto promotepromote growthgrowth ofof cypresscypress treestrees asas mechanicalmechanical barriersbarriers

•• SegmentalSegmental leveelevee systemssystems couldcould bebe craftedcrafted intointo ““greengreen”” systems,systems, whichwhich wouldwould employemploy swampswamp troughstroughs betweenbetween thethe embankmentsembankments •• AsAs thethe cypresscypress treestrees mature,mature, theirtheir impactimpact onon stormstorm surgesurge dissipationdissipation wouldwould increase,increase, helpinghelping toto offsetoffset freeboardfreeboard losseslosses causedcaused byby regionalregional settlementsettlement •• EachEach flowflow obstructionobstruction willwill draindrain energyenergy headhead fromfrom thethe advancingadvancing stormstorm surge.surge. •• TroughsTroughs withwith steepsteep facesfaces areare somesome ofof thethe mostmost efficientefficient energyenergy dissipaters,dissipaters, becausebecause thethe deflectdeflect forwardforward energyenergy upward.upward. •• AsAs thethe obstructionsobstructions getget larger,larger, moremore andand moremore wavewave energyenergy isis bledbled offoff andand expendedexpended •• ThisThis mightmight allowallow forfor aa LOWERLOWER crestcrest elevationelevation forfor thethe highesthighest embankmentembankment •• TheThe systemsystem shouldshould bebe designeddesigned toto survivesurvive shortshort--termterm overtoppingovertopping (e.g.(e.g. 1212 hours)hours) MaximumMaximum Flexibility,Flexibility, withwith InnumerableInnumerable DesignDesign OptionsOptions

•• GreenGreen segmentedsegmented leveelevee protectionprotection systemssystems couldcould bebe tweakedtweaked forfor anyany numbernumber ofof variables;variables; includingincluding aa designdesign overtoppingovertopping depthdepth toto accommodateaccommodate lowlow frequencyfrequency events;events; withoutwithout usingusing asas muchmuch earthearth fillfill asas conventionalconventional embankments.embankments. CANCAN WEWE DODO AA BETTERBETTER JOBJOB OFOF RETARDINGRETARDING LANDLAND LOSSLOSS FOSTEREDFOSTERED BYBY DIMINSIHEDDIMINSIHED SILTATIONSILTATION ININ THETHE DELTA?DELTA? •• The Mississippi River drains approximately 41% of the continental United States, discharging about 580 km3 of water each year (420 billion gpd). •• Almost 50% of the water comes from the Ohio River, which has 1/6th of the watershed. •• The Missouri River encompasses 43% of total watershed area, but only contributes 12% of the discharge. •• Prior to 1700, the average sediment discharge was about 440 million tons/year, mostly from the Missouri River watershed. •• This figure has decreased about 50% since 1950, because of dams constructed on Missouri and Arkansas Rivers. •• This loss somewhat compensated by 5 to 10 fold increase in sediment load carried down the Ohio River, because of row farming and deforestation. Cocodrie

St. Bernard

Plaquemines Sole-Cypremont Teche

Lafourche Balize

•• TheThe riverriver hashas depositeddeposited enormousenormous deltaicdeltaic lobeslobes duringduring thethe latelate Holocene:Holocene: BalizeBalize (550(550 yrs);yrs); PlaqueminesPlaquemines (750(750--500);500); LafourcheLafourche (1000(1000-- 3000);3000); St.St. BernardBernard (2500(2500--1000);1000); TecheTeche (3500(3500-- 2800);2800); CocodrieCocodrie (4600(4600--3500);3500); andand SoleSole-- CypremontCypremont (>4600).(>4600). DynamicDynamic Environment:Environment: ChandelierChandelier Islands;Islands; beforebefore andand afterafter KatrinaKatrina

•• Wave erosion stripped off sandy cohensionless deposits during Hurricane Katrina •• 4000 people were killed in this area in the 1915 hurricane •• Coastal Land Loss since 1932 (red) and land gain (green). We are losing about 50X more land mass than we are saving at present, even with the 2050 plan. (data from Ray Dokka at LSU). TheThe physicalphysical situation:situation: thethe riverriver runsruns downdown inin aa deepdeep holehole…… TheThe designdesign problem:problem: howhow cancan wewe extractextract itsits sediment?sediment? •• Typical cross section through the sandy bank levees of the Mississippi River, illustrating how the river’s main channel lies above the surrounding flood plain, which were poorly drained swamp lands prior to reclamation. •• There is significant hydraulic sorting of materials deposited on either side of these levees, as sketched below. HahnvilleHahnville isis justjust upstreamupstream ofof NewNew OrleansOrleans •• NoteNote classicclassic birdfootbirdfoot patternpattern ofof sandsand--filledfilled distributarydistributary channels,channels, shownshown inin yellowyellow •• NoteNote developmentdevelopment ProfileProfile ofof thethe LowerLower MississippiMississippi

The bed of the Mississippi River is below sea level during the last 450 miles of its course, up to Greenville, Mississippi. We can only extract meaningful amounts of sediment during short-lived periods of high flow. CanCan wewe comecome upup withwith aa viableviable schemescheme forfor divertingdiverting siltsilt--ladenladen waterswaters duringduring highhigh flowflow periods?periods? OneOne ideaidea isis toto divertdivert Taps (shown in red) would be made at thethe flowflow ofof channel bends thethe riverriver duringduring highhigh flowsflows ThisThis mapmap showsshows ArtonishArtonish,, LouisianaLouisiana

•• ArtonishArtonish isis typicaltypical ofof thethe MississippiMississippi RiverRiver’’ss floodflood plainplain upstreamupstream ofof BatonBaton RougeRouge •• NoteNote multiplemultiple meandermeander beltsbelts •• ThisThis sectionsection showsshows ““tunneltunnel tapstaps”” 11 andand 1.81.8 milesmiles longlong thatthat couldcould bebe jackedjacked throughthrough thethe overbankoverbank sedimentssediments,, beneathbeneath thethe riverriver’’ss levee.levee. CouldCould wewe employemploy jackingjacking ofof precastprecast concreteconcrete tunneltunnel boxes?boxes?

•• During the Boston Big Dig, the world’s largest precast concrete box tunnel section, for Interstate 90, was hydraulically jacked into position, beneath existing rail lines and bridges •• TunnelTunnel tapstaps couldcould alsoalso bebe combinedcombined withwith siphons,siphons, inin conventionalconventional oror invertedinverted form.form. Distributors AbilityAbility toto Distributors shiftshift lineslines •• DistributorDistributor alignmentsalignments wouldwould needneed toto bebe flexible,flexible, toto accommodaccommod ateate eveneven distributiondistribution ofof sedimentsediment •• FlexibilityFlexibility willwill bebe keykey aspectaspect So,So, whatwhat portionportion ofof thethe lowerlower MississippiMississippi RiverRiver couldcould bebe targetedtargeted forfor massivemassive flowflow diversionsdiversions andand bebe aa receptaclereceptacle forfor newnew sedimentsediment ?? AtchafalayaAtchafalaya RiverRiver The Atchafalaya River is both steeper than the Mississippi (3:1 ratio in bed slope) and shorter (225 kilometers to the Gulf of Mexico from the Red River entrance versus 480 kilometers for the Mississippi). •• Under natural conditions, the Mississippi River would probably have switched its course to the Gulf of Mexico via the Atchafalaya distributary between 1965 and 1975, if not for the levees. •• The Atchafalaya now drains about 30% of the combined flows of the Mississippi and Red rivers to the Gulf of Mexico. •• ProfilesProfiles ofof thethe AtchafalayaAtchafalaya River.River. ItsIts steepersteeper gradientgradient wouldwould bebe muchmuch moremore efficientefficient forfor sedimentsediment diversion,diversion, transport,transport, andand distrbutiondistrbution thanthan thethe lowerlower MississippiMississippi River.River. SedimentSediment AccretionAccretion atat GrandGrand LakeLake 19171917--19501950

•• TheThe AtchafalayaAtchafalaya BasinBasin isis ourour bestbest analoganalog modelmodel forfor sedimentsediment accretionaccretion inin thethe deltadelta •• HistoricHistoric datadata onon sedimentsediment accretionaccretion herehere needsneeds toto bebe analyzedanalyzed andand thoroughlythoroughly understoodunderstood TheThe LikelyLikely TargetTarget

•• EveryEvery physicalphysical factorfactor savesave one,one, wouldwould seemseem toto favorfavor thethe AtchafalayaAtchafalaya RiverRiver forfor aa prototypeprototype sedimentsediment depositiondeposition schemescheme involvinginvolving thethe lowerlower MississippiMississippi RiverRiver •• TheThe sedimentsediment hashas bebe diverteddiverted fromfrom thethe MississippiMississippi channel.channel. •• WeWe shouldshould probablyprobably beginbegin byby evaluatingevaluating enhancements;enhancements; optionsoptions thatthat wouldwould useuse existingexisting facilities,facilities, likelike thethe OldOld RiverRiver DiversionDiversion StructuresStructures •• ThisThis presentationpresentation willwill bebe postedposted inin pdfpdf formatformat forfor easyeasy downloadingdownloading at:at: www.umr.edu/~rogersda/leveeswww.umr.edu/~rogersda/levees