feasibility study by ben c. gerwick, inc. new orleans east ......thursday, december 20, 2012 1 east...

THURSDAY, DECEMBER 20, 2012

EAST LAND BRIDGE1

Feasibility study by Ben C. Gerwick, Inc.

New OrleansEast Land Bridge StudyLPV 111 to Chef Menteur, Chef Menteur to Rigolets

Presentation to the SLFPA-Eby Dale E. Berner, PhD PEand Jean O. Toilliez, PhD, PE

› Existing Studies

› Published research and body of results available

› Background information and key vulnerable locations

› Met ocean assessment and coastal state of the art

› Plan formulation

› Definition of goals, constraints, performance criteria and screening

› Preliminary levee design parameters

› Rationale for numerical model tests

› Hydrodynamic model test results

› Definition of test cases, goals of the modeling efforts

› Proposed plan and analysis

› Proposition for immediate implementation and concept design, draft cost analysis

› Conclusions and recommendations

› Action plan, recommendations for immediate implementations and future efforts

Outline


EAST LAND BRIDGE2

East Land Bridge feasibility study


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Chapter 2Existing studies

› Louisiana Draft Master Plan (2007 and 2012)

› General framework; use it to define objectives and constraints for plan formulation

› UNO Report on Hydrodynamic Modeling of the Tidal Prism in the Pontchartrain Basin

› Optimal pass opening widths study using FVCOM model

› Recommend wide openings at both passes to minimize disruption on tidal hydrodynamics (tidal prism and hydro-period)

› Louisiana Coastal Protection and Restoration (LACPR) Final Technical Report

› Very large database available directly applicable to current study, with information on:

› Storm surge

› Present day/future condition hazard

› Future projects

› Large collection of case studies directly connected to the East Land Bridge

› Major basis for future work; this study to draw on these results

› Framework for Environmental Assessment of Alternative Flood Control Structures on Chef Menteur and Rigolets Passes

› Rigorous assessment of pass impact on ecosystem; framework for future installations

› NRC Review of the LACPR Technical Report

› Others: see report for complete list of references

Background References


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Existing studies

ELB as Critical Feature and Location for Future Flood Protection System


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› Land bridges

› "These features are critical contributors to the long-term sustainability of a comprehensive risk reduction system for coastal communities. The coastal landscape, and the restoration and maintenance of that landscape, are important considerations in a comprehensive system for risk reduction"

› ELB as Strategic Location for Flood Defense System

› Several alignment directly on ELB path

› CPRA (2007) states:

› "levees, or some other form of flood control structure, are recommended for high risk areas that must be protected in order to avoid severe consequences for the state and nation"

LACPR Technical Report // CPRA


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› In LACPR Technical Report

› Two solutions were envisaged for the partial or complete hydraulic closure of Lake Pontchartrain.

› Both solutions dismissed based on cursory review of engineering feasibility issues

› This report to investigate this matter in greater depth and scope

› Barriers and levees considered along US 90 alignment in LACPR against various design storm surge cases

› Use this information when devising additional cases to evaluate

Levee at ELB – Overtopping vs. Non-overtopping and Consequences

LACPR Technical Report

Hurricane storm surge scenario

Full barrier elevation[ft-NAVD88 2004.65]

Weir barrier [ft-NAVD88 2004.65]

100-YRP 25 12.5

400-YRP 32 12.5

1000-YRP 36 12.5

Added value: design basis for flood control system to be investigated in this study

Present Day and Estimated Future Condition Storm Surge Hazard at ELB


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› In LACPR Technical Report

› Include high estimates for future sea-level rise conditions

› Estimates provide a good reference for the existing flood risk in the South Lake Pontchartrain region, in the absence of a flood control system

› Return periods: 100, 400 and 1000-YRP

› Double check with most recently (1984) FEMA FIRM stating 18ft-NGVD29 storm surge elevation

LACPR Technical Report // FEMA

Return period Present conditions (base case) [ft]

Future conditions [ft]

100-YRP 14.6 17.9

400-YRP 17.8 21.5

1000-YRP 19.4 23.8

Added value: design basis for flood control system to be investigated in this study


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› Alternative EA

› Full closure at US-90 with presence of closed flood control structures at the Chef Menteur and Rigolets passes. The levee is non-overtopping, with a design crest elevation of 27ft. This option is the most effective at reducing storm surge near Slidell.

› Alternative EB

› Same as EA but with an overtopping levee, with top crest elevation of 12.5ft. Again, the two passes at ELB Are in a closed position.

› Alternative EC

› This alternative is identical to EB, however in this case the two passes are in an open position (i.e. no flood control structures).

Structures at ELB: Existing ResultsLACPR Technical Report


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Effect of Structures: Value of High-Crested Levee System


Alt. EA Alt. EB

Alt. EC


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Effect of Alternatives on MEOW and MWLD at Key Locations at and Near ELB


MEOW (left) and MWLD (right) bar charts show the impact of hard overtopping and non-overtopping structures at different sites near the ELB for various alternatives. The massively non-overtopping flood control structure (Alt. EA)

diverts some amount of storm surge toward neighboring areas, in exchange for a large reduction in storm surge near Slidell, LA.

Undeniable value of high-crested levee system to reduce storm surgeMust balance hydraulic burden created by that system!


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Chapter 3Background information

Timeline for study: 2060

NOAA DEM


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Critical location: direct line of defense against storm surge

Regional influence: St. Tammany, Orleans, St. Bernard parishes and State of Mississippi


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Federal Levee Lake Pontchartrain and Vicinity (LPV) 111 to Rigolets Pass. As delineated in the project, the area of

considered here covers approximately 40 square miles.


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Projections 20-YRP [ft] 50-YRP [ft] 100-YRP [ft] 500-YRP [ft]

Maximum water level

10 11.6 12.715.4

Minimum water level

-1.8 -2.0 -2.1-2.3

Wind speed 25-YRP [mph] 50-YRP [mph] 100-YRP [mph] 500-YRP [mph]

Bay Waveland, MS 74 77 79 82

Shell Beach, LA 82 88 94 107

WIS Station Water depth Significant wave height

Peak Period

73145 15 m 6.5 m (21 ft) 12.50 s

73143 20 m 7.3 m (24 ft) 12.50 s

73142 20 m 8.0 m (26 ft) 12.50 s

73141 21 m 8.8 m (29 ft) 12.50 s

Large waves near ChandeleurIslands capable of entering into Lake Borgne and vicinity

Estimated wind speeds: exposed location

Subject to frequent, severe floodingSource: Shell Beach, LA


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Local wave climateLACPR Technical Report

STWAVE maximum wave height spatial distribution for 150 storms evaluated for the 2007 (current) conditions near the ELB, per (USACE 2009)

6-8 ft significant wave height expected near ELB

Degraded Chandeleur Islands still provide significant reduction in wave height (breaking/friction)

Evidence of value of marhsland in wave attenuation


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Sea-level RiseLACPR Technical Report // IPCC // State guidance (Calif.) // Vermeer and Rahmstorf

Coastal feature SLR by 2060 (confidence level)

Low-lying shoreline protection where primary purpose is to slow down erosion/shoreline retreat where failure would be local and limited in severity.

0.47m or 1.55ft(calculated from the mid-range VR, akin to a 50% confidence level)

Critical feature (e.g. levee or floodwall) destined to protect public and private properties and where failure would be catastrophic.

0.7m or 2.3ft(upper limit of VR results, akin to a 90% confidence level)


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Impact of Static Relative Sea Level Rise on ELB

The majority of the ELB terrain, as delineated in this study, lies between 0 and 1ft.

A rise in the mean sea level would have significant consequences on the ELB

A relative area analysis reveals that only 13.7% of the ELB lies above 3ft


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Residual dry land at ELB assuming a +1ft RSLR

Residual dry land at ELB assuming a +3ft RSLR

Note US 90 and CSX railroad as assets

RSLR includes SLR and subsidence

› A vulnerable location

› Large waves

› Low-lying land, subject to frequent flooding (temporary) and inundation (permanent) even relatively small permanent variations in the relative MSL.

› Combined action of waves in conjunction with an appreciable rise in sea level could further accelerate marshland degradation and coastline recess.

› Establish relevant objectives

› If global accelerating trends in SLR continue complete disappearance of the ELB becomes a plausible scenario.

› Mitigation and resiliency plan to strike a balance between shoreline preservation and flood risk reduction

› Leverage pre-existing assets

› Here, it is the well-consolidated railroad that traces across the ELB and along GIWW, standing at over 6ft (1.83m) on average

SummaryMet-ocean Assessment and Relative Sea-level Rise


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Statistical analyses // Buoy data // NOAA-NGDC

Historical and Projected Shoreline Change

› Potential loss of an additional 5,200 square miles (13,468 square kilometers) of Louisiana coastal area by 2100 assuming a 3-ft (0.9-m) RSLR over that period.

› Non-conservative estimate of RSLR

› Historical loss of approximately 2,400 square miles of Louisiana coastal lands since that time

› Reference: Blum, Michael D., and Harry H. Roberts. 2009. “Drowning of the Mississippi Delta Due to Insufficient Sediment Supply and Global Sea-level Rise.” Nature Geoscience 2 (7) (June): 488–491. doi:10.1038/ngeo553.

Blum and Roberts

THURSDAY, DECEMBER 20, 201220 EAST LAND BRIDGE


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Photographic EvidenceCoastal degradation

Location Observed changes

Petites Coquilles Visible signs of shoreline erosion on the South side of US-90

East of Unknown Pass

New interior water bodies have replaced elevated vegetative cover.

Alligator Bend, interior area

Larger water bodies in lieu of vegetative cover.

Comparison of ELB aerial photographs, spanning approximately 30 years (1980-2011). Note that useful comparative visual data covering the West end of the ELB is not available. Source: Google Earth compiled from USGS and NOAA data.


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Shoreline Movement RatePenland et al.

Location of shore-normal transects for Lake Borgne. From (Penland et al. 2002)

Measured trends in shoreline rates for transects 27-38, as per . Bars indicate historical trends, as recorded between 1850 to 1995. Curves represent more recent trends.

Reference: Penland, S., A. Beall, D. Britsch, and S.J. Williams. 2002. Environmental Atlas of the Lake Pontchartrain Basin. New Orleans, LA: U.S. Geological Survey. http://coastal.er.usgs.gov/pontchartrain/.


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› Data provided by USGS reveals that at the ELB, the historical average shoreline movement rate is 5.7ft/year, with accelerating trends recorded in more recent epochs

› Between 1930 and 1995, the rate is approximately 9.4ft/year

› Slows down to 7.2ft/year when measured between 1960 to 1995: restoration projects

› While no data exists for the region of the ELB located at the Lake Pontchartrain side

› Assume from satellite imagery that this area is less likely to undergo coastal recess rates as severe as those recorded on Lake Borgne side

› Prone to severe retreat

› Alligator Bend has the fastest recorded rate of erosion with over 12ft per year;

› Justifies the on-going shoreline protection efforts


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Design Subsidence RateLACPR // USGS // Design values

Location Easting Northing Subsidence by 2060 [m (ft)]

[m] [m] Low Average High

Chef Menteur,

LA229904 3329253

0.15 (0.49)

0.35 (1.15)

0.6 (1.97)

Michoud, LA

217710 33255640.35

(1.15)0.75

(2.46)1.2 (3.94)

LocationSubsidence

rate [mm/year]

Subsidence over 50 years

[m (ft)]

Subsidence over 100

years [m (ft)]

ELB 3.0 0.15 (0.50) 0.30 (1.0)

LAPCR: 2.0 mm/year regional rate; or 0.3ft in 50 years.

USGS: 3.0 mm/year local subsidence rate; or 0.5ft in 50 years.

Design subsidence rate


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Key Vulnerable Locations within the ELB Project AreaAnticipating changes and implementing solutions

Key vulnerable locations within the ELB area: orange for marsh degradation; red for observed shoreline retreat.

In general, the ELB exhibits more vulnerable spots on the Lake Borgne (surge side) region than on the Lake Pontchartrain (protected side) region, with coastal erosion visible from satellite data near the Petites Coquilles area.


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On-going projects

Name Completion date Purpose Location Sponsors

PO-22 Dec. 2001 Shoreline protection against wave action on marsh

Bayou Chevée area

NRCS (federal) and LACPR (state)

PO-34 Sep. 2012 (anticipated) Shoreline protection and marsh restoration

South-west bend of ELB

USACE (federal) and LACPR (state)

Nearby projects that directly affect the ELB restoration and protection efforts can be found from the Louisiana Coastal Wetlands Conservation and Restoration Task Force database.

Most notably, the Bayou Chevée Shoreline Protection (PO-22) and the Alligator Bend Marsh Restoration and Shoreline Protection (PO-34) projects are the immediate vicinity of the ELB


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Chapter 4Plan Formulation


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Risk reduction and shoreline stabilizationPlan FormulationRisk framework: USACE ER 1105-2-100 (USACE 2000)

› Objective 1: To reduce flood risk at the LPB by placing a physical flood barrier against storm surge.

› Objective 2: To preserve the integrity and encourage efforts to expand and strengthen the shoreline at the ELB.


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› Absolute constraints

› Do not contradict or otherwise negatively interfere with Master Plan requirements and guidelines.

› Do not jeopardize existing or future flood reduction system near study area.

› Non-absolute constraints

› Further loss of existing marshes, tidal flats in the study area and reduction in the quality of existing marshes.

› Negative impact on salinity, tidal fluxes and sediment quality within the study area.

› Negative impacts to native species; reduction of total habitat value.

› Negative impacts to existing infrastructure function within the study area.

› Features that reduce or limit future ability to adapt to increased flood risk doe to changing climatic conditions in excess of those considered during the evaluation period.

ConstraintsPlan Formulation


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› Completeness

› Completeness is a determination whether or not the measure includes all elements necessary to achieve the objectives of the plan. It indicates the inter-dependence of the outputs of the plan upon those of other plans.

› Effectiveness

› Weighs the ability of a given measure to achieve the planning objectives. Those that do not contribute, in an unambiguous manner, to achieving these objectives, will be dropped altogether.

› Efficiency

› Efficiency measures cost effectiveness and is expressed in estimating the net impact to be obtained from the measure. Benefits can be both monetary and non-monetary. Measure with a very high cost and little or no foreseeable benefits will not be retained; measures that provide moderate benefits but outstanding value per dollar spent will be considered.

› Acceptability

› Acceptability is all-encompassing metric that seeks to define whether the measure or plan is sound technically, environmentally, economically and socially. Unpractical or downright unfeasible plans and measures will be dropped altogether.

Planning CriteriaPlan Formulation


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Multiple Lines of Defense PrincipleMeasure selection

Barrier Island

Marsh Restoration

Rock Dike

Augmented Armored Berm

Elevated and Augmented Highway

Earthen Levee, Floodwalls and Flood Gates

Limit realm of potential measures to well-defined and recognized framework

Lopez, J., and M. Davis. Framework for Environmental Assessment of Alternative Flood Control Structures on Chef Menteur and RigoletsPasses Within the Lake Pontchartrain Estuary, Southeast Louisiana. New Orleans, LA: Lake Pontchartrain Basin Foundation, April 2011.Lopez, J., N. Snider, C. Dufrechou, M. Hester, and P. Keddy. Comprehensive Recommendations Supporting the Use of the Multiple Lines of Defense Strategy to Sustain Coastal Louisiana 2008 Report (Version I). New Orleans, LA: Lake Pontchartrain Basin Foundation, 2009.


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Barrier Island Marsh restoration Shoreline

protection

Elevated highway Levee alignment Full closure

Summarized

net benefit for

flood reduction

Potential for regional-scale

surge reduction.

Promotion of low-energy

climate.

Increases natural

wildlife habitat.

May reduce incoming

wave energy during

storm surge

Limit coastline recess

and provides benefits

for marsh restoration

projects.

Strengthens ELB's

role as flood defense

line by maintaining

current marsh area.

Leverages US90

alignment and

provides direct

protection against

moderate storm

surge with limited

impact on local

system

Physical, high-

crested line of

defense against

moderate to severe

storm surge

Enhanced potency of

flood risk reduction

system with ability to

hydraulically lock LP

Positive

acceptability

factors

● Planning opportunity to

define new protection status

to large areas; enlargement

of natural habitats and

wildlife refuge.

Remote location and low

social impact.

● Provides long-term

benefits with low

disruption to existing

landscape.

● Expands natural

habitat and

promotes favorable

environmental

conditions.

● Opportunity to use

as building block for

replanting and

restoration efforts.

● Inert building

material.

Reduced footprint

Opportunities for

including small-scale

restoration projects

along US 90

alignment.

● Located in mostly uninhabited areas

● Ample opportunities for mitigating

ecological impact during/after construction.

● Earthen levees may be used as support

blocks for replanting/marsh revitalization

efforts.

● Recreational value: structures may be

integrated into state park or recreation

areas with bike/walk paths, beaches, etc.

●● Project brings focus to restoration and

revaluation of nearby natural landscape

Negative

acceptability

factors

● Large-scale disruption of

existing landscape.

● Limited ability to provide

direct, measurable reduction

in storm surge in densely

populated areas.

● May be overwhelmed by

change in sea-level.

●Short-term

environmental

disruption during

marsh nourishment

phase.

● Sediment for

restoration subject

to limited quantity

and availability.

● Non-native

building material

and potential

disruption to natural

habitat.

● Construction may

be conducted in

sensitive areas.

● Elevated structure

would require

upgrades to existing

infra-structures that

would affect local

ecosystem.

● Right-of-way

issues; inhabited

areas may be left

unprotected

● Large-scale project significantly alters

landscape and local ecosystem.

● Heavy pre-construction soil preparation;

construction may incur environmental

impact; mitigating measures should be

enforced.

● Hydrodynamic consequences of repeated

and/or prolonged pass closures

Measuring acceptability


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Objectives Task description Costa

1 Background study Literature/Data collection $10,000

2 Hydrodynamics Grid generation, model setup, model calibration, simulations $40,000

3 Salinity Model setup, calibration and simulations $10,000

4 Flushing time Calculation of Lagrangian currents $5,000

5 Temperature Model setup, simulations $10,000

6 Dissolved oxygen Determination of dominant bio-chemical processes and

corresponding parameters, model setup and simulations$10,000

7 Nutrient supply Determination of dominant bio-chemical processes and

corresponding parameters, model setup and simulations$10,000

SUBTOTAL (estimated) $95,000

8 Geomorphology Determination of bottom sediment properties and sediment

intake, model setup and simulations$10,000

9 Other water quality

parameters

Additional modeling efforts for specific parameters $5k to 10k

TOTAL (estimated) $115,000

a: cost for estimating purposes only; may be subject to revision; cost estimates valid as of July, 2012.

Supplemental Environmental Impact StudyPost-feasibility study: establish performance of gates

Determine the fate of water quality during closures spanning from one week to a month

Estimate the time needed for the local ecosystem to recover from the closure and restore itself to pre-closure conditions after the lake is re-connected to Lake Borgne

Performance of non-traditional/composite gate structures aimed at minimizing hydrologic disruption


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Measure Screening and DiscussionPerformance criteria

Measure Planning objective

addressed

Retained Screening rationale

1 2

Levee ● ● Yes Selected for next screening round

Armored berm ● Yes Selected for next screening round

Augmented

berm

● ● No This measure was eliminated from

further consideration because of

potential cost and acceptability issues.

The measure would necessarily interfere

on existing infrastructure (Alligator

Bend)

Barrier Island ● No This measure does not properly address

storm surge reduction and is incomplete:

the concept spans outside the realm of

authority of the SLFPA-E.

Elevated

highway

● No This measure leaves out a large area of

ELB vulnerable to shoreline erosion and

marshland loss. Alignment does not line

up with interests of stakeholders.

Objective 1 To reduce flood risk at the LPB by placing a physical flood barrier against storm surge.

Objective 2 To preserve the integrity and encourage efforts to expand and strengthen the

shoreline at the ELB.


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Discussion and Path Forward

› Armored berm

› Shoreline/foreshore protection alone is limited

› Small-scale protection measures can only limit dame sustained from minor events.

› Very potent flood risk reduction measure

› No direct answer to shoreline stabilization

› Cost and real estate constraints

› Trade-off conceded

› Higher level of flood protection at the expense of a more focused foreshore and shoreline stabilization project (FSSP).

› Trade-off is modulated favorably by the presence of on-going foreshore stabilization (Alligator Bend Marsh Restoration and Shoreline Protection)

› CLEAR assessment strongly indicates that these efforts should have tangible impact on the marshland. Consequently, the armored berm option is given a lower priority.

Plan formulation: conclusion

Priority-level Name Description

1 High-crested levee Feasibility study and conceptual design of high-crested levee

protection at ELB.

2FSSP evaluation and monitoring

program

Track success through quantifiable metrics such as a

regrowth index, organic top-layer measurements; aerial

photo and on-site collection campaign.

3Complement/augment existing

FSSP with ad-hoc measures

wherever required.

Conditioned to efficacy of high-crested levee in limiting storm

surge, augment and/or complement existing FSSP at critical

areas where shoreline stabilization is critical and participates

in improving the efficacy of the flood control system.


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Chapter 6HydrodynamicModeling


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Hydrodynamic Evaluation& General Objectives

ADCIRC/SWAN modeling

Objective A

•To assess the effect of subsidence and marshland within the project area and their impact on hurricane storm surge hazard;

Objective B

•To assess the effects of gates at Chef Menteur and Rigolets passes

Objective C

•To assess the impact of a new structure on the time and spatial redistribution of storm surge near the ELB.

Define case studies


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Configuration Surge

elevation

[ft-NAVD88]

(future

conditions)

RSLR [ft]

(already

included in

estimates)

Wave height

(significant)

[ft]

Approximate

levee crest

height

[ft-NAVD88]

Overtopping barrier-weir considered for testing in the LACPR

(shown for reference; see Section 2.2.6)

12.5

100-YRP level 18 2.6 6 22

400-YRP level 21.5 2.6 8 26

Non-overtopping barrier considered for testing in the LACPR

(shown for reference; see Section 2.2.6)

100-YRP level 25

400-YRP level 32

Levee Design ParametersNumerical investigation: preliminary design parameters to be tested for efficacy

Project alignment for tentative levee concept (orange line). The proposed path follows a simpler alignment than a similar alignment considered in the LACPR Technical Report.

Establishment of crest elevation for levees based on storm surge elevation assessment from LACPR Technical Report. Future conditions for surge elevations are those reported for the Or-leans area, and correspond to Future Case 2, featuring a more conservative SLR projection.

Case studies to integrate a high-crested levee, with a crest elevation of 22 ft-NAVD88

Computational Setup


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› General

› 40 simulations

› 100- and 400-year storm surge, focused on ELB using the coupled ADCIRC and SWAN modeling systems

› All runs include 2.8 ft. RSLR

› Meshes

› Five sets of mesh structures

› 300 x 600 ft. mesh size at ELB

› Less than 300 ft. at channels and openings

› Future Slidell levee implemented

› Runs executed on super computer; no instabilities detected

› Modular value: may be tuned to support detailed environmental studies


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ADCIRC/SWAN modelingStorm SelectionADCIRC/SWAN modeling

› Storms

› Select storms from recently completed FEMA FIS and LACPR studies

› Each show distinct features, including max wind speed, forward speed and minimum pressure

› Selected based on ability to replicate statistical surge level within the ELB area

› Not necessarily valid outside of that area


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› BASE CASE

› FWOA-ELB-intact

› Future without Action (FWOA) scenario

› ELB remains intact and vegetated.

› Basis for comparison with most scenarios

› SCENARIO 1

› FWOA-ELB-degraded

› where the ELB has been allowed to erode and disappear.

› Estimate the role of the ELB topography and vegetation in suppressing storm surge in Lake Pontchartrain.

› SCENARIO 2

› Levee-Gate-Closed-ELB-intact

› Proposed levee across the ELB with no openings at the Chef Menteur and Rigolets passes.

› Hydraulically isolates Lake Pontchartrain from Lake Borne and the Gulf of Mexico.

› Approximate the redistribution of storm surge at nearby locations

› SCENARIO 3

› Levee-Gate-Open-ELB-intact aka "Open-pass-intact"

› Openings at the Chef Menteur and Rigolets passes.

› Estimate the change in surge response within Pontchartrain and at nearby locations including the Mississippi coast.

› SCENARIO 4

› Levee-Gate-Open-ELB-degraded aka "Open-pass-degraded"

› Open gates and anticipates a degraded ELB

› Measure the impact of coastline degradation on the storm surge time/space distribution in Lake Pontchartrain and vicinity.

Test CasesADCIRC/SWAN modeling


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Degradation ScenarioSCENARIO 1 vs. BASE CASE

Coastline recess rate

Adopt “road maintained” scenario so that the highways are assumed to be maintained at current elevation (through mitigating measures)

Anticipated profile evaluation

Evaluate impact of degradation on efficacy of ELB as line of defense, with and without flood control system

Smart shift of ELB terrain along delineated area: focused assessment


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ELB Degradation and Storm Surge

SCENARIO 1 vs. BASE CASE

› Test case measures impact of fate of ELB on storm surge distribution

› New water depth approx. 2.5 ft.

› Use degradation approximation as described earlier

› Results show that, regionally, the influence of the land bridge is small but measurable

› However, numerical simulations do underscore significant negative consequences for New Orleans metro area if ELB is degraded

› +2-3 ft. increase in surge for the 400-year case

› Highlight the very intertwined and interdependent nature of the system of flood protection in that area


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Efficacy of gates at passes


› Dramatic reduction in surge levels at Lake Pontchartrain: up to 3-5ft for 100 and 400-year level

› Hydraulic cost:

› 3-4 ft. increase in surge at IHNC

› 1-2 ft. over large expanses extending well over MS and Caernarvon

› Note: for the areas away from ELB, the responses tend to be storm dependent and do not represent 100 and 400-year surge elevations well

› Results show trade off between large reduction in flood risk for LP and hydraulic burden for LB areas, and beyond.


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Open-pass Flood Protection System


› Limited efficacy in reducing storm surge levels, when compared to closed-gate counterpart

› 1-2 ft. reduction in storm surge elevation in Lake Pontchartrain

› Cost for adjacent areas

› 0-2 ft. increase near GIWW/IHNC floodwall complex

› Narrow foot print

› 100-year storm: No influence on the Mississippi coast

› 400-year storm: Minor to no influence on MS coast

› Best protection system with minimal impacts on the Mississippi coast


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Degradation and flood protection system efficacy

SCENARIO 4 vs. SCENARIO 3

› Open gate only

› Test case shows difference between Open-gate intact and open-gate degraded

› Increase of hydraulic connectivity thus reduced surge level in front of defense line: 0.5 ft.

› Conversely increased surge level at Lake Pontchartrain: 0.5 ft.


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Impact of RSLRADCIRC/SWAN modeling

› Not formally evaluated but results can be inferred

› Increased storm surge level

› Reduce bottom roughness, varies bottom geometry

› For larger values, RSLR:

› Influences storm flux, rather than storm parameters

› Would contribute to more expansive flooding area

› Reduce influence of ELB as line of defense


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Wave ClimateADCIRC/SWAN modeling

› Refined analysis focused on ELB

› Use SCENARIO 3 as base case for wave climate

› In general:

› Levee has minimal impact on wave climate outside of Lake Pontchartrain

› Reduces wave penetration in the lake

› Fetch-limited conditions

› Significant wave heights in front of levee

› 100-year conditions: 6 ft. on average

› 400-year conditions: 8 ft. on average

› Relatively unchanged design conditions

› Large waves present; potentially larger if ELB is further degraded


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Open-pass &Current Velocities


› Potentially very large currents developing due to strong hydrostatic gradient

› Recorded velocities up to 30-40 ft/s in some instances

› Will require robust scour control measures

› However

› Results to be interpreted cautiously due to inherent limitations of model (2D)

› Refined study needed

› Trade-off

› Reduced foot print

› More transparent design implies higher flux and larger current in constricted area


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Chapter 7Recommendations

Path Forward


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Recommendations

› Step 1: High-crested levee without gates*

› Adaptive design for a high-crested levee at the ELB

› ELB as another line of defense and integral component of a multi-layered complex

› Provides supplemental surge reduction for already existing projects, such as PCCP

› Step 2: FSSP monitoring program.

› Track success through quantifiable metrics such as a regrowth index, organic top-layer measurements; aerial photo and on-site collection campaign.

› Focus on the on-going Alligator Bend restoration project.

› Step 3: Complement/augment existing FSSP with ad-hoc measures wherever required.

› Additional efforts conditioned to efficacy of high-crested levee in limiting storm surge

› Augment and/or complement existing FSSP at critical areas where shoreline stabilization is critical and participates in improving the efficacy of the flood control system.

› * Discussed thereafter


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High-crested Levee &100-Year Level of Protection: an Adaptive Approach

STEP 1

Timeline for adaptive construction sequence, starting with the construction of a base design.

Base design

Bring protection to the 100-year level within a 20-year horizon.

A final upgrade, scheduled to be completed 30 to 50 years from present day, is aimed at bringing the level of protection to the 400-year level by means of a hard structure (I, L or T-wall or batter-pile).

100-YRP levee

400-YRP levee

Detailed assessment of settlement needed

2-3 ft. settlement over 50 years anticipated for rock wall by Geoengineering


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Special Provisions for Current Velocity at Pass Openings


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STEP 1

› The hydrodynamic study reveals that current velocities of on the order of 30-40 ft/s may develop at deepest sections of passes during 100- and 400-year storms

› Design basis

› Hydraulic Design Criteria HandbookUSACE. 1987. Hydraulic Design Criteria. US Army Corps of Engineers.

› USACE. 2009. Louisiana Coastal Protection and Restoration (LACPR) Final Technical Report. New Orleans, LA: US Army Corps of Engineers. New Orleans District Mississippi Valley Division.

› Temple, W.H. Louisiana Standard Specifications for Roads and Bridges. State of Louisiana Department of Transportation and Development (LADOTD), 2006.


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ACU or heavy riprap

Heavy overtopping and complete submergence

Conservative design, robust structure capable of withstanding the most critical situations

› Preliminary cost estimates reveal that

› Bulk of the cost incurred from material hauling

› 100-YRP levee and 50-YRP levee similar in cost

› $1Bn approximately

› May not be able to achieve full base width of 50-YRP levee in one step

› Multiple steps necessary

› Crest heightening

› Base widening

› Power of a flexible, adaptive design

› Gate options not included in cost estimate

› Too many variables

› Must involve USACE to determine best route forward

› Environmental mitigation necessary, pending refined assessment

› Flap gates

› Adaptive crest height

› Semi-transparent structures (Jarlan-type, slits, etc.)

Cost Estimates


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Recommendations

› FSSP monitoring program.

› Primary objectives

› Track the efficacy of all on-going FSSPs near the ELB through quantifiable metrics

› Special attention should be given to Alligator Bend restoration project, which covers a large expanse of the ELB shoreline and may serve as a bellwether for future FSSP implementations near the ELB.

› Learn and optimize the efficacy of tentative future projects.

› Implementation

› On-site monitoring programs

› Other agencies and institutions to collect, organize and analyze data.

FSSP Monitoring Program


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STEP 2

Supplemental FSSP


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› Locally mitigate shoreline retreat due to environmental pressure from storm surge and waves action

› Be capable of withstanding complete submergence, without incurring significant damage

› Mostly near passes and on Lake Borgne side

› Area near Lake St. Catherine sheltered from most damaging effects

› Additional efforts, beyond the scope of this work, may be performed to determine the value of such FSSB on factors that extend past the strict realm of flood protection

STEP 3

Concept berm


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› Schematic of armored berm concept featuring a quarry run rock core, toe on both sides and a launch apron, intended to provide adaptability in the face of an evolving flood side shoreline.

› A 2.05 ft RSLR estimate was calculated from the "mid-range" scenario, as explained in Section 3.5.6, to which the design subsidence value of 0.5 ft, was added

› Expect 2-3 ft. settlement for type of structure (see Geoengineering rock wall)

STEP 3

Summary


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› The ELB as a critical component of a multi-tiered flood protection system

› Comprehensive assessment of existing assets, projects and anticipated changes

› Serious platform for high-grade levee system

› Promising restoration projects are anticipated to deliver

› Impact of high-crested system exists but is manageable

› Several ways to mitigate

› Recommended immediate action

› No-gate levee, adaptive steps leading to 100-YRP level of protection within 20-30 years

› FSSP monitoring program

Conclusion


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OpeningsConclusion

› Examine a combination of floodgate/flood-barrier

› Cost efficient

› Compliant design: bottom-hinged gate leaves could be designed to be overtopped as a variable crest height gate in order to limit the effect of increasing storm surge

› Minimize disruption of the existing tidal prism passing through the Rigolets.

› Evaluate usefulness of gate at Chef Menteur

› If warranted by navigational and environmental needs.

› Rigolets are #1-priority pass

› Refine the design of the recommended earthen levee

› Interface with the possible new surge gates/barriers across The Rigolets and Chef Menteur.


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feasibility study by ben c. gerwick, inc. new orleans east ......thursday, december 20, 2012 1 east...

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