geologic conditions beneath new orleans

7/30/2019 Geologic Conditions Beneath New Orleans

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Geologic ConditionsGeologic Conditions

Beneath theBeneath the

New OrleansNew OrleansAreaArea

J. David Rogers, Ph.D., P.E., P.G.J. David Rogers, Ph.D., P.E., P.G.Karl F.Karl F. HasselmannHasselmann Chair in Geological EngineeringChair in Geological Engineering

and Associate Directorand Associate DirectorNatural Hazards Mitigation InstituteNatural Hazards Mitigation Institute

University of MissouriUniversity of Missouri--RollaRolla


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Areal distribution of abandoned channels and distributaries of the

Mississippi River. The Metairie Ridge distributary channel(highlighted in red) lies between two different depositionalprovinces in the center of New Orleans.


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Major depositional lobes identified in lowerMajor depositional lobes identified in lowerMississippi Delta around New Orleans.Mississippi Delta around New Orleans.


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North-south geologic cross section through the central

Gulf of Mexico Coastal Plain, along the Mississippi RiverEmbayment.

Note the axis of the Gulf Coast Geosyncline beneath

Houma, LA, southwest of New Orleans. In this area theQuaternary age deposits reach a thickness of 3600 ft.


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Transverse cross section in a west to east line, acrossTransverse cross section in a west to east line, acrossthethe Mississippi River DeltaMississippi River Delta a few miles south of Newa few miles south of NewOrleans, cutting across the southern shore of LakeOrleans, cutting across the southern shore of LakeBorgneBorgne..

New OrleansNew Orleans is located on a relatively thin deltaic plainis located on a relatively thin deltaic plaintowards the eastern side of the deltatowards the eastern side of the deltas depositionals depositional

center, which underlies the Atchafalaya Basin, west ofcenter, which underlies the Atchafalaya Basin, west ofNew Orleans.New Orleans.


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Pleistocene geologic map of the New Orleans area. The yellowstippled bands are the principal distributory channels of the lowerMississippi during the late Pleistocene, while the present channel isshown in light blue. The Pine Island Beach Trend is shown in theochre dotted pattern. Depth contours on the upper Pleistocene agehorizons are also shown.


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Contours of theContours of theentrenched surfaceentrenched surface

of the Wisconsinof the Wisconsinglacial age depositsglacial age deposits

underlying Newunderlying New

OrleansOrleans.. Note the wellNote the well

developed channeldeveloped channel

leading southward,leading southward,towards what usedtowards what used

to be the oceanicto be the oceanic

shoreline. Thisshoreline. This

channel reaches achannel reaches amaximum depth ofmaximum depth of

150 feet below sea150 feet below sea

level.level.


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ArealAreal distribution and depth to top of formation isoplethsdistribution and depth to top of formation isoplethsfor thefor the Pine Island Beach TrendPine Island Beach Trend beneath lower Newbeneath lower NewOrleans.Orleans.


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Block diagram of the geology underlying New Orleans. The princiBlock diagram of the geology underlying New Orleans. The principalpalfeature dividing New Orleans is thefeature dividing New Orleans is the MetairieMetairie distributarydistributary channelchannel, shown, shown

here, which extends to a depth of 50 feet below MGL and separatehere, which extends to a depth of 50 feet below MGL and separatessgeologic regimes on either side. Note the underlying faults, begeologic regimes on either side. Note the underlying faults, beneath Lakeneath LakePontchartrain.Pontchartrain.


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Block diagram illustrating relationships betweenBlock diagram illustrating relationships between subaerialsubaerial andandsubaqueoussubaqueous deltaic environmentsdeltaic environments in relation to a singlein relation to a single

distributarydistributary lobe. The Lakeview andlobe. The Lakeview and GentillyGentilly neighborhoods ofneighborhoods ofNew Orleans are underlain byNew Orleans are underlain by interdistributaryinterdistributary sediments, overlainsediments, overlainby peaty soils lain down by fresh marshes and cypress swamps.by peaty soils lain down by fresh marshes and cypress swamps.


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Sedimentary sequenceSedimentary sequence caused by overlapping cycles of deltaic deposition,caused by overlapping cycles of deltaic deposition,along a trend normal to that portrayed in the previous figure.along a trend normal to that portrayed in the previous figure. As long asAs long asthethe distributarydistributary channel receives sediment the river mouthchannel receives sediment the river mouth progradesprogradesseaward.seaward.

The Lakeview andThe Lakeview and GentillyGentilly neighborhoods lie on aneighborhoods lie on a deltaic plaindeltaic plain with marshwith marshand swamp deposits delta front deposits closer to Metairieand swamp deposits delta front deposits closer to Metairie--GentillyGentilly Ridge,Ridge,the nearestthe nearest distributarydistributary channel.channel.


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Portion of the 1849 flood map showing the mapped demarcationPortion of the 1849 flood map showing the mapped demarcationbetweenbetween brackish and fresh water marshesbrackish and fresh water marshes along Lakealong Lake

Pontchartrain. This delineation is shown on many of the historiPontchartrain. This delineation is shown on many of the historiccmaps, dating back to 1749.maps, dating back to 1749.


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1816 flood map of New Orleans showing areal distribution ofcypress swamps north of the old French Quarter. Theseextended most of the distance to the Lake Pontchartrainshore.


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Distribution and apparent thickness of surficial peat deposits invicinity of New Orleans.


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Geologic map of the greater New Orleans area. The sandy

materials shown in yellow are natural levees, green areas denoteold cypress swamps and brown areas are historic marshlands. Thestippled zone indicates the urbanized portions of New Orleans.


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Geologic cross section along south shore of Lake Pontchartrain inthe Lakeside, Gentilly, and Ninth Ward neighborhoods, where the17th Street, London Avenue, and IHNC levees failed duringHurricane Katrina on Aug 29, 2005. Notice the apparent

settlement that has occurred since the city survey of 1895 (blueline), and the correlation between settlement and non-beachsediment thickness.


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Wood and other organic debris was commonly sampled inexploratory borings carried out after Hurricane Katrina throughoutthe city.

This core contains wood from the old cypress marsh that was

recovered near the 17th Street Canal breach. Organic materialsare decaying throughout the city wherever the water table hasbeen lowered, causing the land surface to subside.


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Overlay of 1872 map by Valery Sulakowski on the WPA-LA (1937)map, showing the 1872 shoreline and sloughs (in blue) along LakePontchartrain. Although subdivided, only a limited number of

structures had been built in this area prior to 1946. The position ofthe 2005 breach along the east side of the 17th Street Canal isindicated by the red arrow.


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Aerial photo of the 17Aerial photo of the 17thth Street Canal breach site beforeStreet Canal breach site before

the failure of August 29, 2005. The red lines indicatethe failure of August 29, 2005. The red lines indicatethe positions of the geologic sections.the positions of the geologic sections.


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WestWest--toto--east geologic cross section through theeast geologic cross section through the 1717thth Street CanalStreet Canalfailurefailure approximately 60 feet north of the northern curb of Spencerapproximately 60 feet north of the northern curb of SpencerAvenue, close to the yellow school bus. A detailed sketch of tAvenue, close to the yellow school bus. A detailed sketch of thehebasal rupture surface is sketched above right.basal rupture surface is sketched above right.

The slip surface was about one inch thick with a high moistureThe slip surface was about one inch thick with a high moisturecontent (watery ooze). A zone ofcontent (watery ooze). A zone ofbrecciationbrecciation 3 to 4 inches thick3 to 4 inches thickwas above this. Numerous pieces of cypress wood, up to 2 incheswas above this. Numerous pieces of cypress wood, up to 2 inchesdiameter, were sheared off along the basal rupture surface.diameter, were sheared off along the basal rupture surface.


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WestWest--toto--east geologic cross section through the 17east geologic cross section through the 17thth Street CanalStreet Canalfailure approximately 140 feet north of the northern curb offailure approximately 140 feet north of the northern curb ofSpencer Avenue.Spencer Avenue.

Large quantities of bivalve shells were extruded by high waterLarge quantities of bivalve shells were extruded by high waterpressure along the advancing toe thrusts. Note the slight backpressure along the advancing toe thrusts. Note the slight backrotation of the distal thrust sheet.rotation of the distal thrust sheet.


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Bivalve shells ejected by high pore pressures emanating from toeBivalve shells ejected by high pore pressures emanating from toethrusts on landside of failed levee at the 17 Street Canal (detathrusts on landside of failed levee at the 17 Street Canal (detailil

view at upper left). These came from a distinctive horizon at aview at upper left). These came from a distinctive horizon at adepth of 2 to 5 feet below the predepth of 2 to 5 feet below the pre--failure grade.failure grade.


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StratigraphicStratigraphic interpretations and crossinterpretations and cross--canal correlations in vicinity of thecanal correlations in vicinity of the1717thth Street Canal breach on August 29, 2005. The swamp much appeareStreet Canal breach on August 29, 2005. The swamp much appeareddto be thinning northerly, as does the underlying Pine Island Beato be thinning northerly, as does the underlying Pine Island Beach Trend.ch Trend.TheThe lacustrinelacustrine clays appear to thicken southward, as shown.clays appear to thicken southward, as shown.

The approximate positions of the flood walls (light blue) and caThe approximate positions of the flood walls (light blue) and canal bottomnal bottom(dashed green) are indicated, based on information provided by t(dashed green) are indicated, based on information provided by the Corpshe Corpsof Engineers (IPET, 2006).of Engineers (IPET, 2006).


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SOURCES OFSOURCES OFGROUNDGROUND

SETTLEMENTSETTLEMENT

The causes of historic settlement are aThe causes of historic settlement are acontentious issue in coastal Louisianacontentious issue in coastal Louisiana

There appear to be many differentThere appear to be many differentcauses, summarized in the followingcauses, summarized in the following

slidesslides


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Topographic map with one foot contours prepared under thedirection of New Orleans City Engineer L.W. Brown in 1895.This map was prepared using the Cairo Datum, which is 21.26feet above Mean Gulf Level

MapMaph ih i


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showingshowingrelativerelative

elevationelevationchangechangebetweenbetween1895 and1895 and

1999/20021999/2002,,taken fromtaken fromURSURS(2006).(2006).

The netThe netsubsidencesubsidencewaswas

between 2between 2and 10+and 10+feet,feet,dependingdependingon location.on location.


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Block diagram illustrating various types ofBlock diagram illustrating various types ofsubaqueoussubaqueoussediment instabilities in the Mississippi River Deltasediment instabilities in the Mississippi River Delta, taken, takenfrom Coleman (1988).from Coleman (1988).


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Geologic cross section through the Gulf Coast Salt Dome Basin, taken from Adams

(1997). This shows the retrogressive character of young lystric normal faults cuttingcoastal Louisiana, from north to south. The faults foot in a basement-salt-decollement surface of middle Cretaceous age (> 100 Ma).


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This plot illustrates the enThis plot illustrates the en--echelon belts ofechelon belts ofgrowth faultsgrowth faults forming more orforming more orless parallel to the depressed coastline. The Baton Rouge Faultless parallel to the depressed coastline. The Baton Rouge Fault ZoneZone(shown in orange) has emerged over the past 50 years, north and(shown in orange) has emerged over the past 50 years, north and west ofwest ofLake Pontchartrain.Lake Pontchartrain.


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Lowering the water tableLowering the water table increases the effective stressincreases the effective stresson underlying sediments and hastens rapid biochemicalon underlying sediments and hastens rapid biochemicaloxidation of organic materials, causing settlement ofoxidation of organic materials, causing settlement of

thesethese surficialsurficial soilssoils.

The upper photo showsThe upper photo shows


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pp pgross neargross near--surfacesurfacesettlement of homes insettlement of homes in

the Lakeviewthe Lakeviewneighborhood, close toneighborhood, close tothe 17the 17thth Street CanalStreet Canalbreach. Most of thebreach. Most of the

homes were constructedhomes were constructedfrom 1956from 1956--75 and are75 and arefounded on wood pilesfounded on wood pilesabout 30 feet deep.about 30 feet deep.

The lower photo showsThe lower photo showsprotrusion of a brickprotrusion of a brick--lined manhole onlined manhole onSpencer Avenue,Spencer Avenue,suggestive of at least 12suggestive of at least 12inches of near surfaceinches of near surfacesettlement during thesettlement during thesame interim.same interim.


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Record ofRecord ofhistoric settlementhistoric settlement in the town of Kenner, which is characterizedin the town of Kenner, which is characterizedby 6.5 to 8 feet ofby 6.5 to 8 feet ofsurficialsurficial peaty soils.peaty soils.

The steps in these curves were triggered byThe steps in these curves were triggered by groundwater withdrawalgroundwater withdrawal forforindustrial use and urban development. This area was covered byindustrial use and urban development. This area was covered by densedensecypress swamps prior to development.cypress swamps prior to development.


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Structural geologic framework of the lower Mississippi River DelStructural geologic framework of the lower Mississippi River Deltata..Growth faults (solid black lines) perturb the coastal deltaic plGrowth faults (solid black lines) perturb the coastal deltaic plain,ain,as do salt domes (shown as dots). This study did not uncoveras do salt domes (shown as dots). This study did not uncoverevidence of growth faults materially affecting any of the leveeevidence of growth faults materially affecting any of the leveefailures from Hurricane Katrina, although such possibility existfailures from Hurricane Katrina, although such possibility exists.s.

Mechanisms of Ground SettlementMechanisms of Ground Settlement 11


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Mechanisms of Ground SettlementMechanisms of Ground Settlement --11

Elastic deformationElastic deformation of Mississippi Deltaof Mississippi Deltafrom silt deposition (from silt deposition (isostasyisostasy))

Tectonic compactionTectonic compaction caused bycaused byformation of pressure ridges and foldingformation of pressure ridges and folding

Subsidence on seaward side ofSubsidence on seaward side oflystriclystricgrowth faultsgrowth faults DrainageDrainage of old swamp and marshof old swamp and marsh

deposits increasing stress on underlyingdeposits increasing stress on underlyingclaysclays

Biochemical oxidationBiochemical oxidation of peaty soilsof peaty soils

Mechanisms of Ground SettlementMechanisms of Ground Settlement 22


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Mechanisms of Ground SettlementMechanisms of Ground Settlement --22

Consolidation of compressible soilsConsolidation of compressible soilscaused by surcharging with fillcaused by surcharging with fill

SurchargingSurcharging by structural improvementsby structural improvements Reduced groundwater rechargeReduced groundwater recharge becausebecauseof increase in impermeable surfacesof increase in impermeable surfaces

Extraction of oil, gas, and waterExtraction of oil, gas, and water casingcasingpressure depletionpressure depletion

SolutioningSolutioning of salt domes andof salt domes and seawardseawardmigrationmigration of low density materials (saltof low density materials (salt

and shale)and shale)


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Coastal land loss in LouisianaCoastal land loss in Louisiana is also exacerbated byis also exacerbated by seasea

level riselevel rise, which is currently averaging about 12 inches, which is currently averaging about 12 inches

per century.per century.

CONCLUSIONSCONCLUSIONS--SETTLEMENTSETTLEMENT


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CONCLUSIONS SETTLEMENT

and LAND LOSSand LAND LOSS Coastal Louisiana is losing between 25 and 118Coastal Louisiana is losing between 25 and 118square miles of land per yearsquare miles of land per year

The approximate rate of subsidence is about 4.2The approximate rate of subsidence is about 4.2ft per century, while sea level rise is about 1 ftft per century, while sea level rise is about 1 ftper centuryper century

The soft, compressible materials underlying muchThe soft, compressible materials underlying muchof New Orleans provide poor foundations, whichof New Orleans provide poor foundations, whichare sensitive to drainage and loading.are sensitive to drainage and loading.

Ongoing ground settlement and sea level riseOngoing ground settlement and sea level risehave combined to create a tedious situation,have combined to create a tedious situation,whereby flood control infrastructure mustwhereby flood control infrastructure mustcontinually be improved and elevated to provide acontinually be improved and elevated to provide a

consistent level of protection.consistent level of protection.

CONCLUSIONSCONCLUSIONS--SITESITE


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CONCLUSIONSCONCLUSIONS SITESITE

CHARACTERIZATIONCHARACTERIZATION Most of greater New Orleans is underlain byMost of greater New Orleans is underlain by

several layers of cypress swamp depositsseveral layers of cypress swamp deposits

These materials are highly permeable andThese materials are highly permeable andcompressible, with a dry density lower thancompressible, with a dry density lower thanwaterwater

These materials commonly contain numerousThese materials commonly contain numerousweak horizons, often containing thin lenses ofweak horizons, often containing thin lenses offlocculated clay; all of which were found toflocculated clay; all of which were found to

exhibit strain softeningexhibit strain softening This strain softening causes a significant lossThis strain softening causes a significant lossof shear strength; sometimes diminishing toof shear strength; sometimes diminishing tonear zero.near zero.

CONCLUSIONSCONCLUSIONS COSTCOST


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CONCLUSIONSCONCLUSIONS COSTCOST

BENEFIT RATIOSBENEFIT RATIOS If the Corps of Engineers had authorizedIf the Corps of Engineers had authorized

about $10 million more for detailedabout $10 million more for detailedsubsurface exploration andsubsurface exploration and insituinsitu soilsoil

testing beneath the flood wall additionstesting beneath the flood wall additions

erected in the last 25 years;erected in the last 25 years;

They might have prevented $100 billionThey might have prevented $100 billion

in flood damagesin flood damages This would have netted a costThis would have netted a cost--benefitbenefit

ratio of 100,000 to 1ratio of 100,000 to 1

geologic conditions beneath new orleans

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