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Case 1:05-cv-01119-SGB Document 27-5 Filed 10/04/2006 Page 1 of 46

FINAl REPORT

THE DIRECT IMPACT OF
THE MRGO ON HURRICANE STORM SURGE

Preformed under:
Contract No. 2503-05-39
Hydrodynamic Modeling Effort for MRGO Study

Prepared for:

State of Louisiana
Department of Natural Resources

February 2006
URS Ale: 1922750.001

Prepared by:URURS Corporation

7389 Florida Blvd.. Suite 300
Baton Rouge, LA 70806
225.922.5700

In Association with:

,\""j

WORtDn7NDS

Case 1:05-cv-01119-SGB Document 27-5 Filed 10/04/2006 Page 2 of 46

FINAL REPORT

THE DIRECT IMPACT OF THE
MISSISSIPPI RIVER GULF OUTLET
ON HUCANE STORM SURGE

Performed under:

CONTRACT NO. 2503-05-39
Hydrodynamic Modeling Effort for MRGO Study

Preparedfor:

State of Louisiana

Departent of

Natural Resources

Februar 2006

UR7389 Florida Blvd.

Suite 300
Baton Rouge, Louisiana 70806
(225) 922-5700

Case 1:05-cv-01119-SGB Document 27-5 Filed 10/04/2006 Page 3 of 46

TABLE OF CONTENTS

Executive Summary ........................................,............................................................... ES-1

Section 1

Introduction and Background ................................................................1-1

1.
1.2
1.
1.4
1.5

objectives and organization ............................................................... I-I
The MRGO and Vicinity ................................................................... 1-2
Local Hurricane Storm Surge Threat.................................................1-3
1-5
Related Issues ....................................................................................1-8

Local Hurricane Storm Surge.............................

Previous studies of

Section 2

The 2003 Corps Study.....................................................................:........ 2-1

2.1
2.2

2003 ADCIRC Grid ....................................,...................................... 2-1
Summary of2003 Corps Study.......................................................... 2-2

Section 3

Factors for Further Study......................................................................... 3-1

Section 4

Modeling of 124-Knot Synthetic Storm ........................................................4-1

4.1
4.2
4.3
4.4

Simulation Models ............................................................................. 4-1
QA Check.................. .................. ................................ ....................... 4-1
Simulation Results ............................................................................. 4-2
Discussion of Results ..,.................. ... ........................... ...................... 4-3

Section 5

Modeling of Hurricane Betsy........................................................................... 5-1 .

5.1
5.2

Simulation Results ................................"........................................... 5-1
Discussion of Results........ .......................... ...................... ................. 5-1

Section 6

Modeling of Hurricane Katrina.................................................................... 6-1

6.1
6.2
6.3

Hurricane Katrina .............................................................................. 6-1
Simulation Results ............................................................................. 6-1
Discussion of Results ................. .......................... .................. ............ 6-2

Section 7

Modeling of Levee Alignment Sensitivity .......................................................... 7-1

7.1
7.2

Simulation Model.............................................................................. 7-1
Simulation Results .................".......................................................... 7-2

UR

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TABLE OF CONTENTS

7.3

Discussion of Results... ......... ........................................ ..................... 7-3

Section 8

Wave Run-Up Analysis ............................................................................ 8-1

8.1
8.2
8.3

Selection of

Location for Initial Calculation ..................................... 8-1
Wave Generation and Attenuation Calculation ................................. 8-2
Run-Up Calculation ........................................................................... 8-3

Section 9

Conclusions .................................... ............... ..................,...... ........... ...... 9-1

Section 10

Recommendations ..... ......... ............ ................... ................ ......... ...... ....... 10-1

Section 11

References.. ......... ............ .......................... .................... ...... .......... ........... 11-1

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TABLE OF CONTENTS

TABLES

Table I

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

FIGURES

Figure I

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10
Figure II
Figure 12

Figure 13

UR

Saffir-Simpson Scale for Tropical Cyclone Characteristics ..........................1-4
Overview of Storm Surge Models ................................................................. 1-6
Nine Synthetic Hurricanes ................................................ 2-2

Characteristics of

Difference in Maximum WSE (ft) Baseline MRGO versus Bayou La
Loutre Barrier ................................................................................................ 2-3
Summary ofURS Simulations....................................................................... 3-2

QA Check Comparison of Maximum Surge WSE URS/WorldWinds
ADCIRC Run versus 2003 Study Using the l24-Knot-Fast Storm............... 4-2
Difference in Maximum WSE for Baseline, MRGO Closure, and Bayou
La Loutre Barrier Scenarios, 124-Knot-Fast Storm....................................... 4-3
Difference in Maximum Surge WSE for Baseline and Closure Scenarios
Hurricane Betsy ............................................................................................. 5-1
Difference in Maximum Surge Water Surface Elevations for Hurricane
Katrina Simulation ofMRGO Baseline and Closure..................................... 6-2
Difference in Maximum Surge Water Sudace Elevations for Hurricane
Katrina Simulation ofMRGO Baseline and Modified Levees...................... 7-2

Project Area
Corps Estimate of Regional Inundation from Category 4 and Above Storm
Surge
L WR Estimate of Regional Inundation from Hurricane Georges (1998)
Prior to Track Turn
Full 2003 ACIRC Grid
Detail of2003 ADCIRC Grid for MRGO and Surrounding Area
3D Depiction of 2003 ADCIRC Terrain for MRGO and Surrounding Area
Comparison of Surveyed versus 2003 ADCIRC MRGO Channel Near Shell
Beach, Plan
Comparison of Surveyed versus 2003 ADCIRC MRGO Channel Near Shell
Beach, Cross Section
Tracks for Hurricane Simulations

3D Depiction of

Maximum WSE for I

MaximumWSE for I

2003 ADCIRC Terrain with Closed MRGO
24-Knot-Fast Storm, Baseline MRGO
24-Knot-Fast Storm, Closed MRGO

Difference in Maximum WSE for 124-Knot-Fast Storm,
Closed MRGO

Baseline versus

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TABLE OF CONTENTS

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21
Figure 22
Figure 23

Figure 24
Figure 25

Figure 26
Figure 27

Figure 28
Figure 29

Figure 30

Figure 31
Figure 32

Figure 33

Figure 34

Figure 35

Storm Surge Stage Hydrographs, 124-Knot-Fast Storm, Baseline versus
Closed MRGO
Maximum WSE for Hurricane Betsy, Baseline MRGO
Maximum WSE for Hurricane Betsy, Closed MRGO
Difference in Maximum WSE for Hurricane Betsy, Baseline versus Closed
MRGO
Storm Surge Stage Hydrographs, Hurricane Betsy, Baseline versus Closed
MRGO
Storm Surge Current Speed Hydrographs, Hurricane Betsy, Baseline versus
Closed MRGO
Hurricane Katrina Simulation
Maximum WSE for Hurricane Katrina, Baseline MRGO
Maximum WSE for Hurricane Katrina, Closed MRGO
Difference in Maximum WSE for Hurricane Katrina, Baseline versus Closed
MRGO
Storm Surge Hydrographs, Hurricane Katrina, Baseline versus Closed MRGO
Storm Surge Current Speed Hydrographs, Hurricane Katrina, Baseline versus
Closed MRGO
Baseline 2003 ADCIRC Levees
Modified Levees
Maximum WSE for Hurricane Katrina, Modified Levees and Closed MRGO
Difference in Maximum WSE for Hurricane Katrina, Baseline versus
Modified Levees
Storm Surge Hydrographs, Hurricane Katrina, Baseline versus Modified
Levees

Schematic of

Wave Set-up and Run-up

HPL Cross Section near Bayou Dupre, Station 673
Lake Borgne to HPL Levee § Bayou Dupre Transect for Wave Run-up
Analysis
Schematic of Wave Generation and Attenuation for Lake Borgne to HPL
Levee § Bayou Dupree
3D Depiction of Higher Resolution ADCIRC Terrain for MRGO and
Surrounding Area with Closed MRGO

ATTACHMENTS

Attchment I

Numerical Modeling of Storm Surge Effect ofMRGO Closure

UR

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Exec_lVe Summa..

In March 2005 URS Corporation (URS) was tasked by the Louisiana Departent of

Natural
Resources (LDNR) to evaluate the impact of the Mississippi River Gulf Outlet (MRGO) on
regional hurricane storm surge by examining the immediate and direct effects using a
hydrodynamic model of certain selected storms. In October 2005, LDNR requested that
Hurricane Katrina in this evaluation. The URS task follows a 2003
US Ary Corps of Engineers (Corps) report on the effect of blocking the MRGO at Bayou
La Loutre on hurricane storm surge.

URS include modeling of

The URS project team reviewed the 2003 Study results and identified
factors for further study:

seven additional

1.

2.

3.

4.

5.

6.

7.

The impact of complete closure (i.e., fillng in) ofMRGO;

The effect on surge across the entire study area;

The influence on surge scour velocity;

The impact on storm surge arrival and draining;

The impact of a severe storm;

The sensitivity of storm surge to levee alignment; and

The effect ofthe MRGO on levee wave run-up.

URS conducted a total of seven simulations using three hurricanes-a 124-Knot-Fast
Synthetic Storm, Hurricane Betsy, and Hurricane Katrina-including comparisons ofMRGO
Baseline versus Closure Scenarios. The simulations were conducted using the ADCIRC
hydrodynamic model and the 2003 grid. MRGO closure was represented by filling in the
channel to an elevation equivalent to approximately I foot above mean sea leveL. A levee
alignment sensitivity simulation was conducted using the .levees along the south bank of the
GIWW and the MRGO, and the intermediate levee at the 40 Arpent CanaL. URS also
provided a wave run-up analysis.

storm surge is part of a broad interest in the full range of

URS understands that public concern for the MRGO's immediate and direct contribution to
the MRGO's long-term impacts on
the regional communities and environment. Especially given the effects of Hurricane
Katrina, there is a justified interest in all measures that might protect human life and aid in
restoring the economic, cultural, "and ecological resources of the area for generations to

come. This phase of work, however, only provides the findings and recommendations

UR

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Execute Summa",

associated with
conditions.

modeling the surge impact of the MRGO for hurricanes under present

Major conclusions of

this study are:

.

.

.

.

.

.

The MRGO channel does not contribute significantly to peak surge during
severe storms, when the conveyance of surge is dominated by flow across the
lakes and marsh. Nor does the channel contribute

entire surface of the coastal

significantly to wave run-up.

Complete fillng of the MRGO-or blockage or partial filling-will not
provide significant immediate, direct mitigation of severe storm surge.

For a few locations outside the Hurricane Protection System (HPS) closure of
the MRGO may reduce the peak surge for certin fast, low-to-moderate
storms, when the surge is not dominated by flow across the open lakes and
marsh, and may modestly delay the onset of surge.

For some storms and locations MRGO closure would slightly increase storm
surge peaks and impair draining of storm surge following the storm passage.

MRGO closure would significantly reduce surge scour velocities at some

channel

locations, which is important to soft swamp and marsh organic soils.

Natural and man-made landform alignents (passes, ridges, levees, etc.) can
create surge concentration under certain storm conditions. The effect of the
"funnel" formed by levees along the GIWW and MRGO in concentrating
surge was evident in Hurricane Betsy but not in Hurricane Katrina. However,
the MRGO did not significantly impact the surge at the "funnel" for
either storm. Widening the funnel in the sensitivity simulation actually
resulted in an increase in surge at the IHNC.

closure of

The above findings on the role of the MRGO on storm surge imply that the surge conveyance
of the MRGO is not an importnt factor in establishing near-term HPS requirements. Near-
term HPS requirements should be based on a thorough analysis of surge height recurrence
frequency-and those factors that can reasonably be expected to effect total surge heights-
and the costs and benefits of alternative degrees of protection.

UR

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Execute Summarv

URS recommends that LDNR conduct further evaluations to better understand the long-term
various closure scenarios of

the MRGO on storm surges and the future implications of

role of

the MRGO, including:

.

.

.

.

.

Develop an improved, high resolution ADCIRC grid of the MRGO and
surrounding area, with accurate representation of the channel, and regional
topography and bathymetry corrected to updated NA VD-88 benchmarks.

Conduct calibration studies using the improved grid for a range of tidal and
storm events.

Perform surge simulations using the improved grid to better resolve locations
of impact, and degrees of impact (positive and negative), for various MRGO
baseline and closure scenarios.

Evaluate the effect of various MRGO closure alternatives, subsidence,
erosion, and sea level rise, and restoration measures-such as controlling
saltwater intrusion and introducing freshwater from the Mississippi River-Qn

the long-term regional

landscape.

Develop ADCIRC grids to represent long-term landscape scenarios and use
them to study the future impact of natural processes, MRGO alternatives,
regional wetland restoration alternatives, and other landscape changes on
hurricane storm surge.

UR

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SECTIONONE

Introduction and Background

In March 2005 URS Corporation (URS) was tasked by the Louisiana Department of

Natural
Resources (LDNR) to evaluate the impact of the Mississippi River Gulf Outlet (MRGO) on
regional hurricane storm surge by examining the immediate and direct effects using a
hydrodynamic model of certain selected storms. In October 2005, LDNR requested that
URS include modeling of Hurricane Katrina in this evaluation. This study was always
intended as a preliminary evaluation of this specific issue, which-as this Introduction and
Background show-is part of a broader, more fundamental, suite of concerns requiring
careful examination. Nevertheless, a correct understanding of the MRGO's role in storm
surge conveyance is one importnt key to establishing a sound scientific and engineering
approach to protecting and restoring St. Bernard Parish and eastbank Orleans Parish.

1.1

OBJECTIVES AND ORGANIZATION

Due to the extensive media coverage and public attention directed at the impact of the
MRGO on storm surge, it is important to understand the objectives and limited scope of this
phase of work. The URS task is a follow-on to earlier modeling performed the US Army
Corps of Engineers (Corps). In 2003 the Corps-as part of an assessment of options to
reduce salinity intrusion caused by the MRGO-valuated the impact of a saltwater barrier at
Bayou LaLoutre on mitigating hurricane storm surge. The 2003 Study concluded that "the
MRGO has minimal influence upon storm surge propagation." (A copy ofthe 2003 Study is
included as Attachment I and is discussed in Section 2).

Since the 2003 Study only addressed blocking of the MRGO at Bayou La Loutre, the LDNR
asked URS to evaluate the surge mitigation effects of totally filling the channeL. The URS
task examines this question with the following limitations:

.

.

The URS simulations examine the impact of closing the MRGO channel-
that portion which extends southeast from the Gulf Intracoastal Waterway
(GIWW) to Breton Sound. The simulations do not look at possible impacts of
filling or blocking the GIWW (e.g., at Paris Road).

As an economical first effort the task was limited to the use of three
"diagnostic" storms (one synthetic storm, Hurricane Betsy, and Hurricane
Katrina), rather than a comprehensive suite of storms with widely varying
intensities, sizes, tracks, and forward speeds. URS would recommend further
simulations warranted by the results of this phase of work.

UR

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SECTION

ONE

Introduclion and Background

.

The past and future effects of the MRGO on regional landforms (e.g.,
wetlands) and the impact of regional landform changes on storm surge are
beyond the scope of this modeling task. This task examines only the
immediate and direct impacts of MRGO closure on hurricane storm surge
under current conditions.

The remainder of Section I provides background information for this report.
topics are addressed in the ensuing sections:

The following

Section 2.
Section 3.
Section 4.
Section 5.
Section 6.
Section 7.
Section 8.
Section 9.

Section 10.

The 2003 Corps Study
Factors for Further Study

Modeling of I

24-Knot Synthetic Storm

Modeling of Hurricane Betsy
Modeling of Hurricane Katrina
Modeling of Levee Alignment Sensitivity
Wave Run-Up Analysis
Conclusions
Recommendations

1.2

THE MRGO AND VICINITY

The MRGO is a 76 mile man-made navigation channel bisecting the wetlands of St. Bernard
Parish and connecting the GIWW in eastern New Orleans to Breton Sound and the Gulf of
Mexico (see Figure I). Construction of the 500-foot wide 36-foot deep MRGO was
completed in 1968 by the Corps at a cost of $92 million. The MRGO facilitated the
development of port facilities and expansion of commerce along the GIWW and Inner
Harbor Navigation Canal (IHNC). The Corps has routinely performed dredging (typically to

depths of 40 feet) to maintain the channeL.

The region surrounding the MRGO is dominated by low-lying coastal wetlands, including
cypress swamp, fresh-intermediate marsh, brackish marsh, and saline marsh. These wetlands
are tidally connected to the adjacent coastal waterbodies-Lake Borgne and Breton Sound.
Aside from minor remnant ridges, these wetlands are typically less than 2 feet above the

local mean sea level (MSL) of

the coastal

lakes and bays.

The New Orleans urban area developed into eastern Orleans Parish and into St. Bernard
Parish along the higher natural Mississippi River levee and swamp ridges, which commonly

UR

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SECTIONONE

Introduction and Background

lie several feet above MSL. Development through the 20th Century expanded into adjacent
swamps, which were drained by an extensive network of canals and pump stations.

Several small fishing vilages, including Reggio, Yscloskey, Shell Beach, Hopedale, and
Delacroix in St. Bernard Parish, arose near the coast. These communities, along with the
roadways leading to them, were established on low natural ridges of abandoned deltaic
distributaries at elevations a few feet above MSL.

1.3

LOCAL HURRICANE STORM SURGE THREAT

The more heavily developed portions of

the New Orleans area are surrounded by a Hurricane
Protections System (HS) of levees and floodwalls. Major upgrades to the regional HPS
were begun by the Corps in the 1960s in the wake of Hurricane Betsy. The HPS was
typically designed to handle the surge from a "standard project hurricane" (equivalent to a
fast-moving Category 3 hurricane). The HPS and floodgates are largely owned, operated,
and maintained by local Levee Districts (e.g., Lake Borgne Levee District in St. Bernard
Parish).

The coastal fishing villages lie outside the HPS. Given their low elevation, these
communities are susceptible to significant damage from even minor tropical storm surge
events. Moreover, evacuation in the face of a major hurricane is severely hampered by the

advancement of

high water and inundation of

routes to safety.

Regional storm surge risks are magnified by the subsidence of the drained swamplands
within the levee system. Such areas have subsided many feet below MSL, resulting in
topography that resembles a large "bowl". Gradual post-construction settlement of the
levees also exacerbates inundation risk. Communities and key evacuation roads outside of
the levee system are also subsiding. Ironically, it is only those healthy coastal wetlands
sustained by natural or simulated deltaic nourishment which maintain their elevation
(primarily through the accretion of natural detritus).

Table I summarizes
Simpson Scale:

the basic characteristics of tropical cyclones based on the Saffir-

UR

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SECTION

ONE

Introduction and Background

Saffr-Simpson' Scale for Tropical Cyclone Characteristics

Table 1

Tropical Depression

Tropical Storm

t
2

3
4
5

"=39
39-73
74-95
96-110
111-130
131-155

greater than 155

" 980
979-965
964-945
944-920
"=920

3 to 5

6 to 8

9to 12

13 to18

19+

The probability that any hurricane will pass within 75 miles of New Orleans in any given
year is about 12.5 percent, or about once every 8 years. The odds of a major hurricane
(Category 3 and above) passing within 75 miles are about 3.2 percent per year, or about once
every 30 years (see Sheets, Hurricane Watch, Appendix D, 2001).

Following close calls with Hurricanes Andrew (1994), Georges (1998), Isadore (2003), and
Lily (2003) there was increasing research attention, public awareness, and governmental
concern for the threat to life, public health, and the regional economy posed by severe
hurricane storm surge in southeast Louisiana. For example, the LSU Center for the Study of
Public Health Impact of Hurricanes began a five-year study, Assessment and Remediation of
Public Health Impacts Due to Hurricanes and Major Flooding Events. The ability of levees
to withstand storm surge, problems of evacuation, and the catastrophic consequences of
inundating the "bowl," became front page news and a priority for federal, state, and local
emergency planning and response offcials. In February 2001, in response to post-Hurricane
Georges assessments, the Corps drafted an"unwatering" plan for New Orleans. In June 2002
the New Orleans Times-Picayune ran a multi-day feature: Special Report: Washing Away.

In July 2004 the Louisiana Offce of Emergency Preparedness in conjunction with the
Federal Emergency Management Agency (FEMA) conducted a planning exercise, which
featured widespread inundation of the New Orleans area from a simulated Hurricane Pam, a
slow-moving, large Category 3 hurricane.. In October 2004 Category 4 Hurricane Ivan
threatened southeast Louisiana, precipitating a full-scale evacuation of hundreds of
thousands of people.

Hurricane Katrina, a large storm with Category 5 winds up until a few hours before landfall,
struck southeast Louisiana at daWn on August 29, 2005. The hurricane center passed due
north through the eastern flank of St. Bernard Parish. In the ensuing, hours, days and weeks

UR

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SECTION

ONE

Introduction and Background

the storm's impacts far exceeded the preparations of all governments, private organizations,
and individuals. As of this date the total storm related fatalities in Louisiana have reached
area homes

1,100-128 in St. Bernard Parish and 286 in the Lower 9th Ward. The number of

that may ultimately have to be demolished may reach the tens ofthousands.

1.4

PREVIOUS STUDIES OF LOCAL HURRICANE STORM SURGE

standard project hurricane-without the advantages of

Following Hurricane Betsy in 1965, the Corps calculated storm surge heights using a
to day's complex computer models. In
the 1970 Flood Insurance Study for the Louisiana Gulf Coast, the Corps estimated the 10,
50, 100, 200, and 500 years storm surge elevations in St. Bernard Parish south of Lake
Borgne at 7.0, 11., 12.2, 13.0, and 13.7 feet MSL. The Corps designed floodwalls and
levees in the MRGO-GIWW area to elevations of roughly 14 to 17.5 feet above NGVD-29.
Pre-Katrina surveys have shown that some portions of the HPS were more than two feet
below design grade (information provided by Lake Borgne Levee District)."

Beginning in the 1980s the FEMA and the National Oceanic and Atmospheric
Administration (NOAA) began to undertake more sophisticated numerical computer
modeling of storm surge (see Table 2). Throughout the 1990s models became more readily
used by planning agencies, such as the Southeast Louisiana Hurricane Preparedness Study
prepared by the Corps in 1994. Figure 2 shows Corps estimates of peak inundation for a
Category 4 storm surge for southeastern Louisiana developed with the SLOSH modeL. ¡The
inundation depths shown would not be produced from a single storm of a given track and
forward speed, but are the depths each location could experience from a severe surge
scenario for that area.)

- Today most fonnal references to elevation are given in the North American Vertical Datum of 1988, (NA VD-
88). In the New Orleans area references to the National Geodetic Vertical Datum of 1929 (NGVD-29) and
NA VD-88 are nearly equal. Elevations in either the NGVD-29 or NA VD-88 are not equivalent to local MSL.
An elevation of 0 feet in NGVD-29 or NA VD-88 is at about -1 foot local MSL. Thus, a reference to + i foot
NGVD-29 or NA VD-88 is roughly equal to local MSL. A levee height which is accurately surveyed to 15 feet
NA VD-88 is about 14 feet above local MSL. Accurate elevation detenninations in the New Orleans area must
also be based on benchmarks that are valid at the time of survey.

UR

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SECTIONONE

InlrodUClion and Background

Table 2

Overview or Storm Surge Models

Several different modeling techniques have been used to model storm surge in the coastal waters surrounding
MRGO. These models are briefly reviewed.

SLOSH Model - A numerical-dynamic, tropical storm surge model, Sea, Lake, and Overland Surge from
Hurricanes (SLOSH), was developed by Jelesnianski at the National Weather Service (NOAA) for real-time
forecasting of hurricane storm surges on continental shelves, across inland water bodies, along coastlines and
for inland routing of water. Overtopping of barriers such as levees, dunes, spoil banks, etc. is permitted. Also,

channel flow and flow through barier cuts are entertined. The model is two-dimensional, covering water
bodies and inundated terrain and is applied on a polar coordinate system. The SLOSH model does not address
wave set-up or wave run-up.

The SLOSH model is run to simulate the flooding caused by hurricanes. The model is designed for operational
forecasting, and the model's input parameters that describe the hurricane are relatively simple and predictable.
The hurricane's position, size and intensity all enter as input to the modeL. Verification runs of the SLOSH
model indicate that the accuracy of storm surges prediction is +/- 20 'Y.

FEMA Model - An overland flooding model has been developed by the Federal Emergency Management
Agency (FEMA) to predict hurricane flood elevations for the National Flood Insurance Program. The model
uses an explicit, two dimensional, staggered finite difference scheme to simulate the flow of water caused by
tides and wind systems. The inputs to the model include the bathymetr, coastline configuration, boundary
conditions, and bottom friction and other flow resistance coeffcients. Also required are the surface wind
velocity and atmospheric pressure distributions of the hurricane. The model predicts water level elevation and
water transport everyhere in the modeled region. The model uses a rectagular grd to discretize the
simulated region or the ocean and coast. The model grid expands during a simulation to predict the flooding of
low lying areas. Barriers and rivers which occur in the coastal zone have a controllng influence on flood

levels. Bariers can include roadways, levees and natural features such as cheniers. Rivers include channels,
canals and inlets. These features are typically much smaller in width than a typical grid cell, having widths that
are about i 00 to 1000 feet. These features can be included in the computations as sub-grid scale elements. The
FEMA model also does not address wave set-up or wave run-up.

ADCIRC Model - The ADvanced CIRCulation model was developed by Westerink and Luettich as a two-
dimensional depth integrated finite element hydrodynamic circulation code for ocean shelves, coasts, and
estuaries. The finite element approach allows for modeling very large domains, with a flexible mesh that can
provide coarse elements in open water far from areas of interest and fine resolution along areas of interest.
Finite element also allows more accurate representation of interior features and model boundaries. The model
incorporates recent developments in effcient finite element solution schemes and the code has been parallelized
to run on commodity computer clusters. It includes wetting and drying algorithms and can represent hydraulic
features such as levees, weirs, and culvert. The ADCIRC model does not address wave set-up or wave run-up.

UR

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SECTIONONE

Introduction and Background

Storm surge models were also used by university researchers. Figure 3 illustrates surge
inundation estimates, prepared by Dr. Joseph Suhayda using the FEMA model, of the
potential impact that the 1998 Hurricane Georges might have had on the New Orleans area
had the storm not veered eastward (Louisiana Water Resources Research Institute, Louisiana
State University).

These analyses fueled concern over hurricane storm surge risks and the need for further
studies into the adequacy of levee protection. In 2002, the Corps completed a Hurricane
Protection Reconnaissance Study to examine surge protection needs of the entire New
Orleans area. The Corps recommended conducting an $8.6 million Feasibilty Study to
examine alternatives for upgrading protection-including the option of providing Category 5
protection throughout New Orleans. As of August 29, 2005 the Feasibility Study was

awaiting funding. Following Hurricane Katrina, the Corps was authorized by Congress to
prepare a Category 5 hurricane protection technical report for south Louisiana.

As part of their ongoing pre-Katrina levee assessment, the Corps funded development of a
sophisticated computer storm surge model using ADCIRC (Westerink and Luettich). The
finite element model provides several advances over the traditional FEMA and SLOSH
storm surge models of southeast Louisiana. By facilitating large domains the model can
more accurately represent hurricane storm surge in the Gulf of Mexico. At the same time,
the model allows for high resolution of critical south Louisiana features, such as the HPS and
the MRGO. Figure 4 ilustrates the ADCIRC Grid developed for the Corps. The grid
includes 600,331 elements and 314,442 nodes, with node spacing ranging from 15.5 miles in
the mid-Atlantic to 330 feet in the New Orleans area. Simulations using this grid must be
done in I to 2 second time-steps-with one day of simulation requiring tens of bilions of
node-steps. Multi-day hurricane simulations can be completed in a matter of hours on super
computers utilizing parallel clusters of commodity processors (such as those typically
operated by the Corps and universities).

In 2004 researchers at the LSU Hurricane Center (Kemp, Mashriqui, and' van Heerden, in
conjunction with Westerink) utilized the parallel version of ADCIRC and the 2003 Corps
Grid to simulate a very large, slow-moving Category 3 storm (referred to as Hurricane Pam)
on the LSU supercomputer "Super Mike" for a multi-agency emergency planning exercise.
The synthetic storm portrayed the catastrophic inundation of the New Orleans area resulting
from massive storm surge (see http://hurricane.lsu.edu/).

UR

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SECTIONONE

Introduclion and Background

In addition to raising levees, the Corps, Levee Districts, and others began discussing
potential alternatives for mitigating hurricane storm surge by restricting the penetration of
surge at key locations. Such locations included the GIWW at Paris Road, which transmits
surge westward into the IHNC, and the Rigolets and Chef Menteur Passes, through which
surge enters Lake Pontchartrain.

In the wake of Hurricane Georges and the Hurricane Pam exercise, one conveyance feature
which received significant attention, particularly by residents and offcials of St. Bernard and
Orleans Parishes, was the MRGO. The conventional opinion has been that the MRGO
facil itates the transmittal of storm surge from Breton Sound into St. Bernard Parish and
upward into the GIWW and IHNC, placing these areas under an increased threat.

1.5

RELATED ISSUES

By way of background, there are the several environmental and economic issues which are
related to concerns over the role of the MRGOin storm surge conveyance. While these

issues are not the subject of

this report, they are important to acknowledge.

.

.

The erosion of the MRGO banks. Bank erosion is estimated at up to 15
feet/year, widening the channel to as much as twice its original design in some
places, and over time significantly increasing conveyance. Erosion is
primarily due to ship wave action (Britsch and Ratcliff, 200 I). The Corps and
the LDNR are investigating methods to improve bank stabilization using
articulated concrete mattresses, rock dikes, and other armoring techniques
(e.g., CWPPRA Project PO-32).

The increased salinitv in the upper estuaries. Increased salinity is occurring in
part due to the introduction of saltwater via the MRGO channel, causing rapid
disappearance of surrounding fresh and brackish coastal wetlands. Some
locations have seen a 3 to 4 fold increase in salinity during the decades since
the MRGO was opened. An estimated 11,000 acres of fresh/intermediate
marsh and cypress swamps have converted to brackish marsh and 19,000
acres of brackish marsh have converted to saline marsh. Land losses include
fresh/intermediate marsh (3,400 acres), brackish marsh (10,300 acres), saline
marsh (4,200 acres), and freshwater swamps (1,500 acres). The Coast 2050
Plan and the Louisiana Coastal Area (LCA) Feasibility Study acknowledged
the role of the MRGO in contributing to major coastal degradation and

UR

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SECTIONONE

Introduction and Background

.

.

targeted reversing associated wetland loss in St. Bernard Parish as a critical
objective. Local offcials have asked if the MRGO's role in damaging
regional wetlands increases the long-term threat of hurricane storm surge.
Methods to control salinities in the MRGO, such as the installation of a
saltwater barrier (sil or gate) near Bayou La Loutre are being assessed by the
Corps in a MRGO Re-Evaluation Study and by the Corps and LDNR in a
MRGO Ecosystem Restoration Study.

The continued economic viability of a deep channel in the face of light cargo
traffic. An average of five cargo vessels per day utilize the MRGO (R.
Caffey, 2002). The Corps has been evaluating long-term alternatives
including modifying the MRGO to a shallow barge channel (MRGO
Reevaluation Study, in progress as of August 2005).

Regional rates of subsidence. sea level rise. and changing ocean climate.
Within St. Bernard Parish, benchmarks along the natural levee of the
Mississippi River are subsiding at a rate of 3 feet/century. Benchmarks along
Paris Road near the GIWW (across former wetlands) are subsiding at 6
feet/century (Shinkle and Dokka, 2004). Natural geologic processes
contribute significantly to subsidence in the St. Bernard Parish area. In some
locations-such as drained swamps-natural subsidence rates are exacerbated
by human intervention. Gulf Coast eustatic sea levels are estimated to be
rising at a rate of about I foot per century (R. Twilley, 2001). Moreover,
hurricane researchers theorize that the Atlantic Basin is experiencing
increasing frequency and intensity of hurricanes. Regional vulnerability to
storm surge may therefore be increasing.

UR

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SECTIONTWO

The 2003 Corps Studv

Concern over the contribution of MRGO conveyance to local hurricane storm surge
inundation led the Corps in 2003 to expand their study of a Bayou La Loutre Saltwater
Barrier concept to include an evaluation of whether such a Barrier might mitigate storm
surge. The Corps conducted this assessment using the ADCIRC storm surge model (a copy
of the 2003 Study is included as Attchment I).

2.1

2003 ADCIRC GRID

ADCIRC model terrain of

The 2003 Grid was prepared by Dr. Joannes Westerink of Notre Dame for the Corps (also
referred to by the Corps as Grid S08). Figure 5 shows the density of ADCIRC grid nodes
for the MRGO and surrounding area. In addition, the figure ilustrates the location of weirs
used to represent elevated levees and roads incorporated into the modeL. Figure 6 depicts the
the area, including the MRGO and levees, in a 3D oblique view.
The terrain model appears to be a reasonable coarse representation of regional topography
and bathymetry. URS did not perform a detailed check of terrain values during this phase of
work. We understand that several sources of topographic and bathymetric data were utilized
and that accurate reconciliation of datums was not always possible. Certin features (e.g.
portions of levees) are known to be off by a couple of feet. All ADCIRC simulations were
conducted with the starting stil water surface at model elevation O.O-which we refer to as
MSL. The surge results are not readily convertible to NGVD-29 or NA VD-88 due to the
variety of datums reflected in the grid topographic and bathymetric elevations.

It is interesting to note that the ADCIRC grid used in the 2003 Study represented the MRGO
channel cross-sectional area at roughly twice the Corps' most recent surveyed cross-sectional

ADCIRC representations of

the MRGO near Shell Beach. This comparison is typical of

area. Figures 7 and 8 present plan and cross-section comparisons of recent surveys versus
the
entire surveyed and modeled channeL. The larger representation of the MRGO in the 2003
grid results from using a number of minimum-sized elements in representing the channel
width, which in needed to control numerical stability. Thus, the 2003 Study ADCIRC grid
signifcantly over-represents the conveyance of the MRGO. As shown in Figure 5, the 2003
Grid also has the alignment of MRGO slightly off, to the south, but the discrepancy in

overall channel

length and orientation with the coast is very minor.

The Bayou La Loutre Barrier was represented in model simulations by raising several grid
nodes in the MRGO where it bisects the Bayou La Loutre ridge-in effect restoring the ridge

across the MRGO. The remainder of

the grid was not altered for the Barrier Scenario.

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SECTIONTWO

The 2003 Corps Studii

2.2

SUMMARY OF 2003 CORPS STUDY

In order to evaluate potential storm surge effects of the Barrier, the 2003 Study utilized nine
synthetic storms (see Table 3) and one historic storm, Hurricane Betsy (1965). The Betsy
simulation represented the storm with top winds of 135 knots and a forward speed of about
20 knots at landfall, and included tides. The storm tracks are shown on Figure 9. All ten
storms were simulated for the Baseline Scenario (without-Barrier) versus with-Barrier
Scenario.

Table 3

Characteristics of Nine Synthetic Hurricanes

'.':i,'#~~~ràspt~iI¡I'\

.",.',',.",
. .',.'. . . .' '.

c,: " CelltråiP~.,siire (iib)Y

."', To"Wi~~sd:

124-Knot

I 24-Knot

I 24-Knot

100-Knot

100- Knot

100-Knot

65-Knot

65-Knot

65-Knot

Fast, 20 Knots

Medium, 15 Knots

Slow, 5 Knots

Fast, 20 Knots

Medium, 15 Knots

Slow, 5 Knots

Fast, 20 Knots

Medium, 15 Knots

Slow, 5 Knots

934

934

934

955

955

955

989

989

989

I knot equals 1.55 miles per hour.

Hydrographs for the two
study for four locations:

scenarios (Baseline versus Barrier) were presented in the 2003

.
.
.
.

IHNC
GIWW at Paris Road

Bayou Dupre
Shell Beach

Comparison of

of the nine synthetic storms plus Betsy is re-produced as Table 4. (The hydro

the maximum water surface elevation (WSE) in MSL at each location for four
graphs are
included in the copy of the report in Attchment I). Table 4 shows that there were two
reductions in maximum storm surge over 0.5 feet. These reductions represented about a 27
percent reduction of Baseline peak surge (in the case of the 65-Knot-Fast storm at Bayou

UR

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SECTIONTWO

The 2003 Corps Studv

Dupre) and 16 percent reduction (for the 124-Knot-Fast storm at Shell Beach). Reductions
in the range of 0.3 to 0.5 feet were also seen at Shell Beach for the Betsy simulation and for
the other three locations for the 65-Knot-Fast storm. The reductions in peak surge for the
these the maximum surge

less than 0.3 feet. In six of

other 16 data points on Table 4 were all

was slightly increased by the Barrier.

Table 4

Difference in Maximum WSE (ft)

Baseline MRGO versus Bayou La Loutre Barrier

,

,

Hu¡'ncåne

,', ,

Surge',
"Range'
'1ft MSl,

IUC

"

,, " i,

,',

"", GIWW ~

,', ','
'".,MRGO~

Parislload " Baypu Dupre

MRGO ~
Shell'Beach

-0.53

-0.26

-0.19

-0.16

"'5

"'10

-0.16

-0.11

124-Knot-Fast
124-Knot-Slow
65-Knot-Fast
65-Knot-Slow
Betsy
A positive value is an increase in surge associated with the Ban-ier and a negative value is a decrease in surge.

-0.14
-0.54

-0.37

-0.13

"'3

"'4

0.02

0.02

0.03

0.03

-0.33

0.02

-0.3

0.02

-0.01

-0.3

"'12

Examination of

the hydrographs for the 124-Knot-Fast and Betsy storms for Shell Beach and
Bayou Dupre reveal that the Barrier Scenario had a noticeable negative impact by impeding

draining of storm surge.

The 2003 Study showed that, at most, blocking the MRGO would only slightly reduced peak
surge, with the most reduction occurring for a fast storm. For the higher surge simulations,
conveyance across the entire marsh appears to dominate the propagation of surge. The 2003
Study concluded that "the MRGO has minimal influence upon storm surge propagation."
This inference is buttressed by the very conservative representation of the channel cross
section in the 2003 model grid. Representing the MRGO channel in the 2003 grid at almost
twice the cross-sectional area causes the model to over-estimate the role of MRGO
conveyance. Thus, the reductions noted in Table 4 with the Bayou La Loutre Barrier
simulations are probably over-predicted.

UR

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SECTIONTHREE

Faclors for Funher Siudii

The URS project team reviewed the 2003 Study results and identified seven factors that
should be evaluated in order to establish a more complete and definitive set of findings with

regard to the immediate and direct impact of

the MRGO on storm surge:

8.

9.

10.

11.

Complete closure of MRGO. Significant conveyance from Lake Borgne to
the GIWW, upstream of Bayou La Loutre via the MRGO, might stil be
occurring in the 2003 Barrier Scenario. To better assess the role of the
MRGO in contributing to storm surge, modeling of a complete closure
the MRGO should be performed. (This factor was also noted by

(fillng-in) of

several

local St. Bernard offcials in their review of

the 2003 Study.).

The potential for surge reductions throughout the area. The potential for
storm surge reduction should be assessed more systematically across the
surrounding area, and not be limited to the four locations.

Possible impacts to surge scour velocity. Closing the MRGO may impact
surge velocities within the MRGO footprint and these should be examined.

The timing of storm surge arrivaL. Impacts to the timing of surge can be
critical for the evacuation of areas. The evaluation of MRGO contribution to
surge threats needed to also address the timing of storm surge onset.

12. Wave run-up analvsis. In addition to modeling these surge issues a wave run-
up analysis for the St. Bernard HPS should be performed to determine if
regeneration of waves in the Baseline versus Closed MRGO has a significant
impact. (ADCIRC only models the mean sea level during surge.)

These first five factors were incorporated into the URS evaluation of the Baseline versus
Closure scenarios for the hurricane simulations discussed in Sections 4 and 5..

13.

Assessment of impacts with a severe storm. None of the surges for the nine
synthetic storms used in the 2003 Study reached elevations that would
threaten to overtop the HPS. Therefore, URS recommended that a simulation
should be conducted with a storm that generated a surge approaching the
height of the MRGO levee. Hurricane Katrina struck just after the review of
the initial simulation results and it was chosen for this assessment.

UR

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SECTIONTHREE

Factors lor Funher StudY

14.

Evaluation of the sensitivity of storm surge to levee alignent. While this
was not initially a focus of the URS effort, results of initial simulations and
concerns over the role of levee al ignents for Hurricane Katrina suggested
that a simulation to examine this issue was needed.

URS conducted a total of seven simulations and Table 5 presents a summary of the grids and
time-steps utilized. A one second time-step was used for the Hurricane Katrina runs to

provide added assurance of

model stability given the higher surge levels.

Table 5

Summary ofURS Simulatious

,

' ,

HU,rric8De ", S .
,', ',',

SÌlniiation

'"

. . cena,nos.

'

'" ,

Gnd

124,Knot-Fast
I (Svnthetic)

Baseline MRGO

2003 ADCIRC Grid

Closed MRGO

2003 ADCIRC Grid with MRGO Filed In

Betsy

Baseline MRGO

2003 ADCIRC Grid

Closed MRGO

2003 ADCIRC Grid with MRGO Filed In

Katrina

Baseline MRGO

2003 ADCIRC Grid

Closed MRGO

2003 ADCIRC Grid with MRGO Filled In

Modified Levees

2003 ADCIRC Grid with MRGO Filed In

Height of Levee on South Bank ofMRGO and

GIWW Reduced

Height of Interior Levee Increased

Time Step

2 seconds

2 seconds

2 seconds

2 seconds

1 second

1 second

i second

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SECTIONFOUR

Modeling of 124-Knol S!lnlhelic Siorm

The first simulation pair was performed using the 124-Knot Fast synthetic hurricane from the
2003 Study. This storm was chosen as an initial test for five reasons:

.

.

.

.

.

Use of one of the previous storm scenarios would allow direct comparison of
a Closed MRGO and a Bayou La Loutre Barrier simulation.

The 2003 Study showed that this storm produced one of the highest (albeit

stil modest) reductions of surge of any of the Barrier simulations-i.e., 0.53
feet at Shell Beach. This reduction was greater than that seen for Betsy.

The MRGO is probably a more important factor in surge conveyance for fast
storms than for slow storms (due to the greater head differential created in the
MRGO by fast storms).

A strong, fast storm seemed most likely to create the greatest head gradient
along the MRGO.

The fast storm scenario was relatively quick to run.

4.1

SIMULATION MODELS

URS utilized the same parallelized version of ADCIRC and the same 314,442 node grid used
in the 2003 Study. Runs were performed on an 8-node cluster owned by WorldWinds, Inc. at
Stennis, Mississippi. (WorldWinds had previously run parallel ADCIRC to assess storm

surge scenarios on the Mississippi Gulf

Coast in 2004.)

To compare the Baseline MRGO scenario URS undertook two pairs of simulations-with the
MRGO channel represented as it had been in the 2003 Study-to a fully Closed MRGO
scenario-in which the grid nodes within the footprint of the MRGO were changed to the
the surrounding marsh (approximately I foot above MSL). Figure 10 ilustrates

elevation of

the 2003 ADCIRC terrain configuration with the Closed MRGO.

4.2

QA CHECK

URS first re-ran the Baseline I

24-Knot-Fast storm and compared results to the 2003 Study in

order to confirm that the model and grid were performing correctly on the W orldWinds
cluster. Results of the "QA Check" run are provided in Table 6 using a comparison of

UR

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SECTIONFOUR

Modeling 01

124-Knot Synthetic Storm

maximum surge WSE and
performing properly.

show that the WorldWinds parallel version of ADCIRC was

Table 6

URSlWorldWinds ADCIRC Run versus 2003 Study Using the 124-Knot-Fast Storm

QA Check Comparison of Maximum Surge WSE

,

UR

, , Maxium WSE

, , (ft MSL)

4.18
4.41
3.85
3.31

2003 Corps

Maxium WSE

(ft MSL)

.

"

" ',,

'

4.1
4.3
3.9
3.5

,

, Location

'

,' ,

IHNC
Paris Road
Bayou Dupre
Shell Beach

4.3

SIMULATION RESULTS

The I 24-Knot-Fast storm was then run with the Closed MRGO grid. Maximum WSE within
the area surrounding the MRGO for the Baseline and Closed MRGO simulations for the 124-
Knot Fast storm are provided on Figures II and 12. Figure 13 depicts the difference between
the two simulations.

Table 7 compares the maximum inundation for the Closure Scenario at the six locations-the
four locations from the 2003 Study, plus Caernarvon and the MRGO Mouth-with
maximum inundations for the Baseline and Bayou La Loutre Barrier Scenarios. (The latter
graphs for both simulations at the six

data is taken from the 2003 Study.) Stage hydro

locations are included on Figure 14.

The Caernarvon location was added in order to examine a lag in storm surge peak reported
by local St. Bernard Parish offcials for this area. The lag is clearly seen when comparing the
hydrographs on Figure 14 and is attributable to the westerly driven emptying of the surge
from the Lake PontchartrainlBorgne waterbodies, combined with the southwestern
movement in the Gulf below New Orleans in the post-storm period.

UR

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SECTIONFOUR

Modeling of 124-Knot Synthetic Storm

Difference in Maximum WSE for Baseline, MRGO Closure, and Bayou La Loutre Barrier Scenarios,

Table 7

124-Knot-Fast Storm

I,

'.

MaximumWSE

I," Difference in Max WSE

Location

I,,

,

"
(ftl\fSI, "
" ,'.'MRGO

MRGO

, ,Baseline

Closhre ,',," I, ""Çl~sure

,',

", ' , '

(ft) "

Barrier'

Difference in Max WSE
(percent), ,,", ,

MRGO'
Closure

,

Barrier ','

IHNC
Paris Road
Bayou Dupre
Shell Beach
Caernarvan
MRGO Mouth
A positive value is an increase in surge associated with the Barer or Closure and a negative value is a decrease
in surge.

-0.03
-0.08
0.17
-0.62
-0.03
0.03

-0.7
-1.8
4.5
-18.6
-0.9
0.7

-0.16
-0.19
-0.16
-0.53
NA
NA

-3.8
-4.3
-4.2
-16.0
NA
NA

4.15
4.33
4.02
2.70
2.76
3.99

4.18
4.41
3.85
3.31
2.78
3.96

4.4

DISCUSSION OF RESULTS

Figures II and 12 show similar patterns of maximum inundation for the Baseline and
Closure Scenarios. Figure 13 highlights a few small areas for which the Closure Scenario
has a lower surge peak:

.

.

Along the MRGO north of Hopedale, upstream of the Bayou La Loutre ridge,
and
Sporadic, isolated pockets of marsh.

There are two areas for which the Closure Scenario exhibited higher surge peak:

.
.

Near Chef

Menteur Pass, and

Along the MRGO from the GlWW to just above Shell Beach.

Table 7 shows that at three of the four locations (IHNC, Paris Road, and Bayou Dupre),
simulation of full MRGO Closure produced less reduction in peak storm surge (one was
actually an increase) than the Bayou La Loutre Barrier. The one location that showed a
greater reduction of peak storm surge with full closure, Shell Beach, saw a slight increase in
the peak reduction from 16 percent to 19 percent.

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SECTIONFOUR

Modeling of 124-Knol S!lnlhelic Siorm

The Baseline versus Closure hydrographs for the 124-Knot-Fast storm (Figure 14) provide
further evidence of little difference in storm surge for these locations except Shell Beach.

The onset of surge at Shell Beach under the Closure Scenario is delayed-with the arrival of
a I foot surge lagging by 5 hours. However, the draining leg of the storm surge at Shell
Beach is negatively impacted. Fifteen hours after the peak, the surge remains I foot higher
under the Closure compared to the Baseline Scenario. (Note: the 2003 grid does not include
Bayou Yscloskey. An improved grid which includes Bayous Yscloskey and Bayou La
Loutre might show different results.)

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SECTIONFIVE

Modeling of Hurricane Beisy

To further assess the, effect of full MRGO closure, URS modeled the Baseline and Closure
Scenarios using the Hurricane Betsy simulation, but without tides. Not including tides
shortened the model time by several days (and reduced simulation cost) and was not
expected to affect the relative comparison of scenarios. Hurricane Betsy was chosen because
it produced the highest surge peaks of any of the 2003 Study storms. The same grids that

were used in the I

24-Knot-Fast Storm were used for Betsy.

5.1

SIMULATION RESULTS

Maximum inundations within the area for the Baseline and Closed MRGO simulations for
Betsy are provided on Figures 15 and 16. Figure 17 depicts the difference between the two
simulations. Table 8 compares the maximum inundation for the Baseline and Closure
Scenarios for Betsy. Stage and current speed hydrographs are presented on Figures 18 and
19.

Difference in Maximum Surge WSE for Baseline and Closure Scenarios

Hurricane Betsy

Table 8

:':1':.

':: , '-':" :~: '¡,~/.: . '-':' : :" :::;1,;:
. ' ,DirrereD1è(;:¡iö-:M~x"
,,::WS~!(ll) . .'.,

',:"'. ":',' "~i"~,'
'ijifferenc~,inN,åx "
WSE (Pér~él1tr.
. " ',' ", ":J,

IHNC
Paris Road
Bayou Dupre
Shell Beach
Caernarvan
MRGO Mouth
A positive value is an increase in surge associated with Closure and a negative value is a decrease in surge.

10.13
10.35
7.47
5.58
8.24
6.47

3.1
4.3
1.
-1.4
0.4
0.6

10.44
10.80
7.54
5.50
8.28
6.51

0.31
0.45
0.08
-0.08
0.04
0.04

5.2

DISCUSSION OF RESULTS

Figures 15 and 16 again show similar patterns of peak surge for the Baseline and Closure
Scenarios. Figure 17 shows that the Closure Scenario produced no areas in which surge
reduction reached one foot. However, three areas of higher surge peaks occurred under the

Çlosure Scenario:

.
.

UR

Hopedale,

East of

the MRGO at the confluence with the GIWW,

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SECTIONFIVE

Modeling of Hurricane Beisy

.

Along the MRGO near Proctor Point. area for the Closure Scenario;

Five of the six locations on Table 8 indicate increases,' albeit slight, in surge peak for
Hurricane Betsy Closure versus Baseline Scenarios. Only Shell Beach saw a decrease in the
the i 24-Knot-Fast storm.

peak, 1.4 percent, a much smaller reduction than the 19 percent for

The Hurricane Betsy stage hydro

graphs (Figure 18) for Baseline versus Closure Scenarios
again are nearly identical at each location, with the exception of Shell Beach. Similar to the
124-Knot-Fast storm, surge onset is slightly delayed as Shell Beach under the Closure
Scenario, and the draining leg is negatively impacted.

hydro

for overland flow. The speed hydro

Figure 19 provides a comparison of Betsy Baseline versus Closure current speed
graphs for the six locations. In the case of the MRGO closure scenarios this current is
graphs are similar for four locations (IHNC, Paris Road,
MRGO Mouth, and Caemarvon), but are significantly different for Bayou Dupre and Shell
Beach. At Bayou Dupre the maximum current for Closure is lower by about 0.5 fps (on a
Baseline peak flow of about 2.2 fps) and the duration over which the current exceeds I fps is
cut by over half. At Shell Beach the maximum current for Closure is halved, from 7 to 3.5
fps. The Baseline Scenario depicted Shell Beach currents above 3.5 fps for 6 hours. This is
the higher currents on the adjacent

a significant difference considering the scouring impact of

marsh. Moreover, actual velocities are higher due to added energy of wave action.

UR

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SECTIONSIX

Modeling of Hurricane Katrina

The Hurricane Betsy simulation surge elevations did not approach the current HPS
elevations, reaching 10.35 feet above MSL in the GIWW at Paris Road. URS therefore
recommended performing a simulation with a truly severe storm to confirm the preliminary
conclusion. Following the surge impact of Hurricane Katrina on the study area on August
29, 2005, URS, at LDNR's direction, undertook to use Katrina as a diagnostic storm to
further assess the role of MRGO channel conveyance on peak surge. URS conducted
Hurricane Katrina MRGO Baseline and Closure simulations using the same ADCIRC

Baseline and MRGO Closure grids used for the I

24-Knot Fast Storm.

6.1

HURRICANE KATRINA

The track of Hurricane Katrina is shown in Figure 9. Katrina's forward speed was about
17.4 miles per hour as it was crossing St. Bernard Parish. The URS simulations used a
synthetic wind-field file prepared by Dr. Pat Fitzpatrick of WorldWinds, Inc.. Top winds
and central pressure on which the simulations were based are shown in Figure 20. The
simulated top winds following landfall at Buras, Louisiana are more than 10 knots higher
than the revised values provided by the National Hurricane Center in their December 20,
2005 report. The W orldWinds wind model does not take into account structural changes that
the hurricane was undergoing at the time of landfall-including degradation of the southern
eyewall. As a result the modeled Katrina is a more powerful storm than the actual one. The
model does not include tides, and as with the earlier runs, uses a reference datum of MSL.

The URS ADCIRC Katrina simulations allow HPS overtopping but do not include HPS
breaches and failures. These simulations examine the relative impact of MRGO closure for a
severe storm and are not meant as a detailed reproduction of actual Katrina events and
conditions.

6.2

SIMULATION RESULTS

Table 9 compares the maximum inundation for the Hurricane Katrina Baseline versus
Closure scenarios. Figures 21 and 22 ilustrate the maximum water surface elevation (WSE)
for the MRGO Baseline and Closed scenarios. Figure 23 depicts the difference in peak surge
between the two simulations. Stage and current speed hydrographs for selected locations are
provided in Figures 24 and 25.

UR

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SECTIONS

ix

Modeling 01 Hurricane Kalrina

Difference in Maximum Surge Water Surface Elevations

for Hurricane Katrina Simulation of MRGO Baseline and Closure

Table 9

,

Maximum WSE

-c
,,',

Location

IHC

Paris Road

(ft. MSL)

" '

Baseline

16.1

19.8

Closure

16.1

19.2

'

DilTe,renee iii Max

' WSE (ft)

Differe'nce in Max
WSE (perce~t)

,

-0.05

-0.57

-0.31
-2.86

22.3

20.6

Bayou Dupre
Shell Beach
Caemaron
MRGO Mouth
A positive value is an increase in surge associated with Closure and a negative value is a decrease in surge.

-1.24

-0.20

22.2

20.3

19.5

19.8

19.3

19.9

-1.04

0.36

-0.12

-0.03

-0.26

0.07

6.3

DISCUSSION OF RESULTS

As shown in Table 9 and in Figures 21, 22, 23, and 24, the Katrina simulations demonstrated
nearly identical patterns in surge WSE for the Baseline and Ciosure scenarios. The figures
clearly show that a massive wall of surge advanced entirely across the surrounding lakes and
marsh. There were only very slight reductions in peak WSE-amounting to less than 3
percent-with the closure scenario at Paris Road, Shell Beach, and Caernarvon (0.6, 0.3 and
0.2 ft, respectively). The peak surge was unaffected at the IHNC and Bayou Dupre by

closure, and was slightly increased at the Mouth of

the MRGO.

Figure 23 shows that the difference in peak surge for the two scenarios was less than 0.5 ft
over the vast majority ofthe region. The 0.6 ft reduction at Paris Road is indicated on Figure
23 by the yellow area, which extends into portions of New Orleans East. The reduction in
New Orleans East under the closure scenario is attributable to the slightly lower water levels
in the GIWW at Paris Road, which resulted in slightly reduced overtopping of the GIWW
levee.

The stage hydrographs in Figure 24 further demonstrate that the Hurricane Katrina
simulations did not produce any significant instances of surge increase or delay with the
MRGO closure scenario. Nor do the stage hydrographs show any evidence of impeded
drainage with the closure scenario.

The vertical scales on the individual current speed hydrographs in Figure 25 show that the
rate of surge flow throughout the system is highly variable. Maximum velocities at the

UR

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SECTIONS

ix

Modeling of Hurricane Karrina

MRGO Mouth, Caernaron, Shell Beach, and Bayou Dupre are all in the range of 3.5 to 6
ftsec. The maximum velocity in the IHNC is about 1.5 ftsec under both scenarios (the
simulations do not include breaching of the floodwalls). The highest velocity occurred at
Paris Road-12.5 ftsec, which is reduced slightly to 11.6 ftsec under the closure scenario.
The largest impact to velocity from closure occurs at Shell Beach, with an increase from 3.8
to 5.9 ftsec. Closure of the MRGO channel does reduce the duration of flow above I ftsec
at Bayou Dupre.

The modeled current speeds discussed here represent basic longitudinal flows (up and down
the channel), and do not include the additional energy of wave action. High surge
longitudinal flow will cause scour along soft, unarmored channel bank and bottom soils. The
HPS scouring observed as a result of Katrina was reportedly due to lateral flow, which
occurred when water levels exceeded the height of HPS structures and they were overtopped.
URS understands that there have been no major reports of longitudinal scouring of HPS
structures to date. This is likely attributable to the fact that most of the HPS structures are
typically located hundreds of feet from the channel bank. High longitudinal currents do have
the potential to cause scour damage at structures supported by the channel bank and
bottom-such as bank revetments and piers for docks and bridges.

UR

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SECTIONSEVEN

Modeling 01 Levee Alignment SensitivitY

Since Hurricane Katrina, there has been significant attention given to the potential
concentration of surge (and aggravation of surge peaks) by the location and orientation of
regional landforms. Landforms with the potential to concentrate surge include both natural
passes and shoreline bends, and man-made constrictions formed by road embankents and
levees. One such feature of concern has been the "funnel" created by levees along the south

bank of

the MRGO and the north bank of

the GIWW, (see Figure I).

Figures 15 and 16 show that this constriction caused the Hurricane Betsy surge to pile up at
the funnel vertex near the Paris Road bridge over the GIWW. It is important to note that in
the Hurricane Betsy simulations,jìllng in the MRGO had no impact on this ''fnnel'' effect.

For Hurricane Katrina, the shape of

the WSE contours on Figures 21 and 22 shows that surge
piles up against the MRGO levee to the south in a nearly identical fashion for both the
Baseline and Closed scenarios. Interestingly, there is no exaggerated surge gradient at the
throat of the "funnel" as there was with the Hurricane Betsy scenarios. This is likely due to
Katrrna's track, and the shift in winds as the eye passed to the east.

At LDNR's request, URS conducted a single simulation of a modified levee alignment in
order to assess the effect of levee alignments on surge elevations in the MRGO, GIWW, and
IHNC. This single simulation was performed as an initial "sensitivity evaluation" and was
not intended as a comprehensive evaluation of the local levee alignments and designs. A
study of levee improvements would require a complete range of storm surge simulations and
levee alignment/design scenarios

7.1

SIMULATION MODEL

URS conducted the Hurricane Katrina Modified Levee simulation using the ADCIRC
MRGO Closure grid described in Section 4, with the MRGO filled-in to approximately one,
foot above MSL. The alignment and height of levees, which are simulated as weirs in
ADCIRC, were the same for all previous scenarios and are shown on Figure 26. The levee
alignment and height for the Modified Levees Scenario are shown on Figure 27. The major
changes included:

.

UR

Reducing the levee height along the south bank of the GIWW and MRGO to
the general height of the land grids behind the levee. (Note: for the purposes
of this sensitivity assessment URS used the existing land elevations in the
ADCIRC modeL. No adjustments to the grid topography were made.) As

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SECTIONSEVEN

Modeling of Levee Alignment SensilivII

illustrated in Figure 27, this resulted in reducing the levees along the MRGO
from 17.5 ft to mostly 6.5 ft. Levees along the GIWW (east of Paris Road)
were reduced from 14 ft to mostly 6.5 ft, with one stretch at 2.5 ft.

.

Increasing the height of the intermediate levee (north of the 40 Arpent Canal)
to 17.5 ft.

The effect ofthis modification was to widen the "funnel" and shift the vertex from the Paris
Road bridge westward to the IHNC. Given the roughly 20 ft surge in this area associated
with the Hurricane Katrina MRGO Closure Scenario, these simple grid changes were thought
to be one way to initially gauge the sensitivity of surge to levee alignment.

7.2

SIMULATION RESULTS

Table 10 compares the maximum inundation for the Hurricane Katrina Baseline versus
Modified Levee scenarios. Figure 28 illustrates the maximum water sudace elevation (WSE)
for the Modified Levees scenario. Figure 29 depicts the difference in peak surge between the
Baseline (no MRGO closure) and Modified Levees simulations. Stage hydrographs
comparing the Baseline and Modified Levee results for selected locations is provided in

Figure 30.

Difference in Maximum Surge Water Sunace Elevations

for Hurricane Katrina Simulation of MRGO Baseline and Modifed Levees

Table 10

Location

,

lHC

Paris Road

Maxium WSE

(ft. MSL)

Baseline

16.1

19.8

Modified
Levees
18.9

19.1

22.3

in Max'

Difference

WSE (percent)

,

17.4%
-3.4%

Difference in Max

',. ,

" WSE (ft)

2.80
-0.67
-1.4
-0.87

20.7

19.7

20.6

Bayou Dupre
Shell Beach
Caemaaon
MRGO Mouth
A positive value is an increase in surge associated with Modified Levees and a negative value is a decrease in
surge.

0.4%
0.3%

-6.9%
-4.2%

0.07

0.06

19.6

19.9

19.5

19.8

UR

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SECTION

S EVEN

Modeling 01 Levee Alignment SensitivilV

7.3

DISCUSSION OF RESULTS

As shown in Table 10 and in Figures 28, 29, and 30 the Modified Levees simulation resulted
in a significant increase in surge in the IHNC (17.4 percent), and modest increases along the
MRGO and GIWW, compared to the Baseline Scenario (no MRGO closure). Figures 28, 29,
and 30 show a marked change in the peak surge distribution, with levels increased across the
area south of the MRGO and the GIWW. Surge levels to the north of the MRGO were
reduced. Higher surge in the MRGO, GIWW and especially IHNC also resulted in greater

interior flooding of

the three major eastbank areas protected by levees.

Given the intensity and particular track of Katrina, this modification clearly facilitated
greater westward conveyance of surge. The increased volume of surge then resulted in a
higher setup at the IHNC. This result may seem counter-intuitive because one might expect
that widening the funnel should give the surge more room to spread out. However, this
simulation clearly illustrates the need to carefully consider the sensitivity of peak surge to
changes in alignments. '

UR

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SECTIONEIGHT

Wave Run-Up Analvsis

One of the processes that may contribute to the flooding of coastal areas protected by levees
is wave run-up (see Figure 31). As storm waves approach a levee they undergo breaking and
the broken wave runs up the face of the levee to an elevation well above the storm surge. If
the levee is at a lower height than the maximum run-up, then the levee wil be overtopped,
even though the mean storm surge elevation may actually be below the levee crest.

The MRGO-as a wide, long water body with moderate fetch-has the potential to create or
regenerate storm waves which can reach the St. Bernard Parish levees, and thus to impact the
magnitude of the wave run-up. Therefore, URS undertook an analysis of the effect of
MRGO on storm wave run-up. This section presents results of wave generation and wave
run-up calculations based upon preliminary hypothetical wind and surge data. The
methodology used in the analysis is taken from the Corps' Shore Protection Manual (SPM)
and ilustrates the influence of the various factors controlling the magnitude of the wave run,-
up.

8.1

SELECTION OF LOCATION FOR INITIAL CALCULATION

For these preliminary calculations a single levee location was selected as an example. The
levee location selected is near Bayou Dupre, as shown in Figure I. This location is at a mid-
way point along the St. Bernard Parish HPS and is where the amount of marsh separating
Lake Borgne and MRGO is minimaL.

Data giving the height and shape of the St. Bernard Levees were obtained from the Lake
Borgne Levee District. The levee cross section used in the calculations was taken at Station
673+00 and is shown in Figure 32. At this station the levee crest is about 350 ft from
MRGO. The levee has a crest elevation of 17.0 feet NGVD-29 and foreslope of about 1:4.
In front of the levee is a berm having a width of about 60 foot and elevation of about 8 feet
NGVD-29, with a foreslope of about I :8. The ground between the toe of the berm and
MRGO has an elevation is about 3 to 4 feet NGVD-29, and at the bank of MRGO there is a
, rubble mound structure with a height of about 8 feet NGVD-29. Levee cross sections at
other locations along the St. Bernard HPS have different characteristics than those of Station
673+00 and can be evaluated further if warranted.

For the Bayou Dupre transect, a fetch trending northeast across Lake Borgne and Mississippi
Sound was selected as having the greatest length and deepest water, as shown in Figure 33.
The fetch length is 150,000 ft long. The trend of the fetch is roughly perpendicular to the
route of MRGO. The marsh segment between Lake Borgne and MRGO has a width of about

UR

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SECTIONEIGHT

Wave Run-Up Analvsis

2,000 feet along the fetch, and MRGO has a width of about 1,500 feet. In the case of a
length of marsh of 1,500

closure of MRGO, the channel would be replaced by an additional

feet.

8.2 WAVE GENERATION AND ATTENUATION CALCULATION

Severe storm waves affecting the St. Bernard HPS could potentially occur with strong winds
coming from the east and northeast. The height of the waves depends primarily upon four
factors:

.
.
.
.

wind speed,
wind duration,
water depth, and
bottom roughness.

Wave properties along the selected fetch depend upon the processes of generation and
attenuation. Wave generation wil occur along the fetch until the marsh segment is
encountered, and then wave attenuation will occur as they cross over the marsh segment. The
storm waves wil re-generate slightly as the waves cross the MRGO before reaching the west

bank of

the MRGO. The effect of

these processes on wave height is ilustrated in Figure 34.

The MRGO will have an effect on wave run-up because there wil be a wave height
difference at the west bank of MRGO between the case with MRGO open and with it closed.
The deep open water associated with MRGO allows for some regeneration of the wave
heights during the storm. A closed MRGO channel would be replaced by marsh which
would further attenuate the storm waves.

In order to determine the magnitude of the storm wave height difference, wave forecasts
were made for Baseline and Closed MRGO Scenarios. The first step was to forecast a storm
wave height at the location of the eastern bank of MRGO using a hypothetical storm. This
forecast was based upon an average storm wind speed of 60 mph and a storm surge of ioft.
The fetch consisted of 130,000 feet of water having an average depth of 10 ft and a marsh
segment 2,000 feet long. This calculation resulted in significant wave heights of 5.1 feet
(above stil water) at the end of the open water fetch and 4.0 feet at the east bank of the

MRGO (following attenuation in the 2,000 feet of

marsh).

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SECTIONEIGHT

Wave Run-Up Analvsis

The next step was to calculate the re-growth of the storm waves as they pass across the
Baseline versus closed MRGO. Using the SPM methodology, the wave height is forecast to
increase under the Baseline Scenario to 4.1 feet at the west bank of MRGO. Under the
Closed MRGO Scenario the wave height is forecast to be 3.8 feet. The results of the
calculations indicate that the significant wave height at the western bank of MRGO would be
about 0.3 feet higher in the Baseline versus the Closed MRGO Scenario.

8.3

RUN-UP CALCULATION

In order to asses the effect of the wave height increase associated with the Baseline versus
Closed MRGO Scenarios, wave run-up calculations were made for the two wave height
cases. The run-up calculations were made using the SPM methodology. Run-up depends on:

.
.
.
.

shape and roughness of the levee,
water depth at the toe of the levee,
bottom slope in front of the levee, and
incident wave characteristics.

Because of the variety of levee designs and storm conditions, a complete theoretical
description of all of run-up may not be available for a particular levee and storm. Numerous
laboratory investigations have been conducted of run-up on smooth, impermeable slopes.
Because these studies are at a different physical scale than the prototype structures, the
laboratory results need to be adjusted for scale effects.

The wave run-up calculation indicates that the run-up on the selected location of St. Bernard
HPS is about 1.5 times the incoming wave height. This produces run-up values of 6.2 feet
(above stil water) for the Baseline Scenario and 5.7 feet for the Closed MRGO Scenario.
These results indicate that the run-up on the St. Bernard HPS wil be about 0.5 feet higher for
the Baseline versus the Closed MRGO Scenario.

UR

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SECTIONNINE

Conclusions

The following findings can be drawn regarding the immediate and direct impact of the
MRGO on storm surge from the combined results of the 2003 Study and this study's
ADCIRC simulations and wave run-up analysis.

.

.

.

.

.

.

.

The MRGO channel does not appear to contribute significantly to peak surge
during severe storms, when the conveyance of surge is dominated by flow
across the entire surface of the coastal lakes and marsh. This conclusion is
reinforced by the very conservative representation of the MRGO channel
cross section in the Baseline model grid.

Closure of

the MRGO channel by complete filling wil not provide significant
immediate, direct mitigation of severe hurricane storm surge. Likewise, other
closure scenarios-such as blockage or partial filling-wil not directly
mitigate storm surge.

The contribution of the MRGO to surge peaks is likely to be most significant
when the peak is low-to-moderate and flow across the marsh does not
overwhelm the fraction of flow conveyed through the MRGO. Preliminary
results suggest that closure of the MRGO may reduce the peak surge on the
order of 15 to 30 percent in a few locations for certin fast, low-to-moderate
storms.

In a few locations, for particular storms, closure of MRGO may modestly
delay the onset of surge, but would probably not reduce the peak. No delay is
expected for severe surges.

MRGO closure would slightly increase storm surge peaks in some areas for
some storms.

Closure of the MRGO may impair draining of storm surge for some storms
following the storm passage in some areas (e.g., Shell Beach, depending on
the effect of Bayou Y scloskey). This effect is probably not as significant for
severe storms due the size of surge.

Closure of the MRGO would significantly reduce storm surge scour velocities
at some locations. This is particularly importnt to soft swamp and marsh
organic soils.

UR

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SECTIONNINE

Conclusions

.

The circulation of surges is highly complex and is affected by many features,
including the natural topography and waterbodies, and man-made
embankments, walls, and channels. Curving and intersecting landform
alignments can create surge concentration at vertices under certin storm
conditions. Modifications of natural and man-made landforms and
waterbodies can cause significant changes in peak surges, which may only be
evident for particular storms.

UR

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SECTIONTEN

Recommendations

The above findings on the role of

the MRGO on storm surge have many implications:

.

.

.

.

.

.

.

The surge conveyance of the MRGO is not an important factor in establishing
near-term HPS requirements. Near-term HPS requirements should be based
on a thorough analysis of surge height recurrence frequency-and those
factors that can reasonably be expected to effect surge-and the costs and
benefits of alternative degrees of protection.

Wave run-up, as well as wave set-up, should be included in any estimate of
total surge height and specifically in evaluating the occurrence frequency of
surge heights.

Long-term HPS requirements need to consider the impact of wetland damage
on storm surge. Certin closure scenarios for the MRGO may help preserve
the surrounding swamps and marsh, and thus, indirectly mitigate surges in the

decades ahead. This issue was not addressed in this phase of

work.

Appropriate surge models-taking into account future broad topographic and
bathymetric conditions-should be used to estimate the future recurrence
frequency of various total surge heights.

The impact of the MRGO's role in increasing low-to-moderate surge,
particularly on low lying communities and wetlands outside the HPS, should

be addressed. This is especially importnt considering that low-to-moderate
surge events occur much more frequently than severe surges and may have a
significant cumulative impact. The localized increases in WSE associated
with low-to-moderate hurricane surges also occur during other wind driven
high WSE events.

Scouring of the soft swamp and marsh banks during surge events should be

addressed, including the additional effect of

wave energy.

Modifications to HPS and regional

landforms should be carefully considered
to avoid exacerbating surge. Efforts to relax or smooth-out landform
constrictions (or funnels) can potentially raise surge heights at points of surge
concentration (vertices) given particular storm tracks.

UR

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SECTIONTEN

Recommendalions

URS offers the following recommendations for further evaluation of the long-term role ofthe
MRGO on storm surges and the future implications of various closure scenarios of the
MRGO:

.

.

.

.

Develop an improved ADCIRC grid for the MRGO and surrounding area that
provides a more accurate representation of the channel and surrounding
topography and bathymetry corrected to updated NA VD-88 benchmarks.
Figure 35 ilustrates a recent update of the grid prepared by Dr. Westerink of
the University of Notre Dame. A study of the regional terrain should be
conducted to identify key areas for improved grid resolution.

Conduct calibration studies using the improved grid with a range of tidal and
storm events. The starting mean water level for the simulations should be the
actual sea level in NA VD-88. Calibrations should examine the full stage and

velocity hydro

graphs of the simulation period at representative locations.

Perform surge simulations using the improved grid to better resolve locations
of impact, and degrees of impact (positive and negative), for various MRGO
baseline and closure scenarios. Among the diagnostic storms that should be
considered is a fast, moderate storm which proceeds along a northwest track
west of the MRGO. This storm would likely maximize the head difference
between the GIWW and Breton Sound, and thus the conveyance role of the
MRGO.

Evaluate the role of the swamps and marshes surrounding Lake Borgne and
Breton Sound on reducing local hurricane storm surge and the value of
various measures-such as controlling saltwater intrusion and introducing
freshwater from the Mississippi River-to preserve these features.
Geomorphic and ecological models (incorporating subsidence, erosion, and
eustatic sea-level rise) should be used to predict future landforms given
various MRGO closure and other restoration scenarios. Future landform
(terrain) grids can then be used to simulate the impact of future landscapes on
hurricane storm surge.

UR

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SECTIONELEVEN

References

Bunpapong, et. aI., An Investigation of Hurricane-Induced Forerunner Surge in the Gulf of

Mexico, (prepared for the US Army Corps of

Engineers), 1985.

C. Jelesnianski, et. aI., SLOSH: Sea, Lake, and Overland Surges from Hurricanes, NOAA
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SECTIONELEVEN

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FIGURES

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Case 1:05-cv-01119-SGB Document 27-5 Filed 10/04/2006 Page 46 of 46

ATTACHMNT 1

NUMERICAL MODELING OF STORM SURGE EFFECT OF MRGO CLOSURE

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