Coastal Louisiana’s 

changing lands

U.S.Geological Survey
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Chapter 9

Eff ects of Environmental 

and Engineering Changes

Avital part of the Hurricane Katrina story lies in nearly two centuries of natural and 

man-made changes to the Louisiana coastline. When New Orleans was settled 

in 1718, the primary fl ood threat was from the Mississippi River, not the Gulf of 

Mexico. An expansive coastal landscape separated the city from the Gulf and served as a 

buff er from any storms moving ashore.1

Th at protective landscape no longer exists. Th e ever-changing and disappearing coastline 

has left New Orleans more susceptible to hurricanes and contributed to the damage infl icted by Katrina. Should this trend continue, New Orleans and the rest of coastal Louisiana 

will become even more vulnerable to damage from future storms, and eff orts to protect the 

city with levees and fl oodwalls will be undermined. 

While a comprehensive analysis of coastal Louisiana’s environmental challenges and potential remedies is beyond the scope of this report, this chapter briefl y examines some of the 

potential impacts of Louisiana’s altered landscape on hurricane protection.

Louisiana’s Changing Coastal Landscape is Increasing Hurricane Vulnerability

Th e Louisiana coastline is changing more rapidly than any other part of the country and, as 

a result, is becoming more vulnerable to hurricanes. Over the last 70 years, Louisiana has 

lost more than 1,900 square miles of coastal land – an area roughly the size of Delaware.2

 At 

the peak of the trend in the 1960s and 1970s, Louisiana was losing 40 square miles of coastal 

land per year.3

 Th is loss has slowed in recent years, primarily because the most vulnerable 

lands have already disappeared, but Louisiana is still losing 10 square miles of coastal land 

per year.4

 As a civil-engineering magazine put it, “in southeastern Louisiana a football fi eld 

worth of wetlands sinks into the sea every 30 minutes.”5

Th ese coastal lands primarily consist of wetlands, including extensive cypress swamps and 

grass marshes. But Louisiana’s barrier islands (an elongated chain of islands running parallel to the coast and serving as a barrier against waves) and even many higher ridges, which 

were formed by large amounts of sediment piling up along past banks of the Mississippi 

River, are also disappearing. Th e U.S. Geological Survey (USGS) projects that an additional 

700 square miles could be lost by 2050 if no further actions are taken to halt or reverse current processes.6

Th e Mississippi River is the single most important factor in sustaining coastal Louisiana.7

Th e river brings water, sediments, and nutrients from 41 percent of the land area of the 

contiguous U.S. to the coast of Louisiana. Prior to the extensive building of levees and dams 

along the Mississippi, the river carried nearly 400 million tons of sediment to the Louisiana 

Coast every year – enough to cover 250 square miles of land a foot deep in sediment.8

 Th e 

growing wetlands fed by the accumulating sediments, nutrients, and fresh water of the Mississippi have added 9,600 square miles of land to the Louisiana coastline over the last 6,000 

years – a rate of 1.25 square miles per year.9

 At its peak, this land, known as the Mississippi 

deltaic plane, accounted for nearly 20 percent of the land area of present-day Louisiana, 

including New Orleans.

Major causes of land loss in Louisiana have been identifi ed.10 Dams and diversions along 

the Mississippi River and its tributaries have greatly reduced the amount of sediment that 

reaches coastal Louisiana, and levees force the remaining sediment so far off shore that it falls 
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122

directly onto the outer continental shelf and beyond, where it no longer contributes to sustaining or building coastal lands.11 By blocking natural fl ooding cycles, levees prevent fresh 

water and nutrients from the Mississippi River from nourishing and sustaining wetlands.12

Ten major navigation canals and more than 9,000 miles of pipelines servicing approximately 

50,000 oil-and-gas production facilities in coastal Louisiana result in a large direct loss of 

land and also contribute to wetland loss from saltwater intrusion and dredging.13

In addition, the Louisiana deltaic plane is essentially sinking, in a process known as subsidence, which occurs naturally as sediments deposited by the Mississippi are compacted over 

time.14 Oil and gas production further contribute to subsidence, potentially causing local 

subsidence three times greater than the highest natural subsidence rates.15 Finally, sea level 

is rising, primarily as a result of global warming.16

Th e deterioration of Louisiana’s coastal landscape of barrier islands, wetlands, and higher 

ridges, and the eff ects of subsidence have made coastal communities more vulnerable to 

hurricane fl ooding.17 New Orleans, in particular, is widely considered to be more vulnerable 

to hurricanes both because land in the city has subsided and because much of the barrier 

islands and wetlands that once surrounded the city has disappeared.18

Many of the mechanisms by which barrier islands, shoals, marshes, forested wetlands, and 

other features of the coastal landscape protect against hurricanes are well known. Geologic 

features such as barrier islands or the land mass associated with wetlands can block or channel fl ow, slow water velocities, and reduce the speed at which storm surge propagates. Th ese 

eff ects can signifi cantly restrict the volume of water available to inundate the mainland.19

Forested wetlands can greatly diminish wind penetration, reducing surface waves and storm 

surge. Shallow water depths weaken waves via bottom friction and breaking, while vegetation provides additional frictional drag and further limits wave buildup. Where wetlands 

and shallow waters lie in front of levees, they absorb wave energy and reduce the destructiveness of storm waves on the levees.20

Depending on the rate of relative sea-level rise, healthy coastal wetlands can maintain a 

near-sea-level landscape by trapping sediments or accumulating organic material, thus 

helping to counter subsidence and global sea-level rise. In contrast, when Louisiana’s coastal 

wetlands deteriorate and disappear, the land held in place by the wetlands undergoes wave 

erosion, eventually washing away and leaving behind open water 10 to 12 feet deep.21

On the other hand, the quantitative impact of wetlands and other coastal features on hurricane protection is poorly known. Anecdotal data accumulated aft er Hurricane Andrew 

suggests a storm-surge reduction along the Louisiana coast of about three inches per mile 

of marsh.22 During Hurricane Katrina, bottom friction and breaking reduced the average 

height of the highest one-third of waves from 55 feet in deep water (with peak waves above 

80 feet), to 18 feet in shallower water outside of the barrier island east of New Orleans,23 to a 

fraction of that height in protected areas. 

Researchers at the Louisiana State University (LSU) Hurricane Center found that, during Hurricane Katrina, levees protected by wetlands had a much higher survival rate than 

those bordering open water. For example, large sections of the Mississippi River Gulf Outlet 

(MRGO) levees that had little or no wetland separating them from Lake Borgne disintegrated, while the nearby 20-Arpent Canal levee, protected by a buff er of marsh and wooded 

wetlands, remained standing. According to LSU researchers, an area about the size of a 

football fi eld with the tree density equal to that found in most Louisiana swamps would 

reduce wave energy in a storm by 90 percent. Th ese researchers further found that friction 
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Emergency Management: Mississippi

from marsh grasses and shrubs reduced water speed from Hurricane Katrina in some places 

from seven feet per second to three feet per second.24

Subsidence is also contributing substantially to hurricane vulnerability. Subsidence occurs 

across the entire region, and therefore impacts not only natural features such as wetlands 

and barrier islands, but also man-made structures such as buildings and levees. According 

to a recent report by the U.S. Army Corps of Engineers (Corps) Interagency Performance 

Evaluation Task Force (IPET), which examines the hurricane-protection levee system, the 

average rate of subsidence across the area is 0.6 feet over a decade.25

Th e rate of subsidence is frequently greater under cities and towns than under natural features: when areas are drained in order to prepare them for buildings, organic material in the 

soil decomposes and leads to further subsidence. In addition, the levees themselves further 

subside due to their own weight pressing down on the unstable soils of the New Orleans 

area. As a result, the eff ectiveness of the levee system deteriorates over time as both the levees and the region subside. Th e IPET report concluded that some portions of the hurricane 

protection system around New Orleans are almost two feet below their original elevations,26

further increasing their own vulnerability, and that of the areas they are designed to protect, 

to the power of hurricanes. 

Th e changes to Louisiana’s coastline have serious implications for the long-term sustainability of the region. Land subsidence and predicted global sea-level rise during the next 

100 years mean that areas of New Orleans and vicinity now 5 to 10 feet below mean sea 

level will likely be 8 to 13 feet or more below mean sea level by 2100.27 At the same time, the 

loss of wetlands, barrier islands, and other natural features could eliminate protection from 

waves and allow for higher and faster moving storm surges.28 According to the National 

Academy of Sciences, these trends will make much of Louisiana’s southern delta uninhabitable without substantial new engineering projects.29

In the long term, New Orleans and other regions of the Louisiana deltaic plane cannot be 

protected without taking proper account of the tremendous change that is continuing to 

occur to Louisiana’s coastal landscape. 

The Mississippi River Gulf Outlet’s Contribution to Damage from Hurricane Katrina

Congress authorized construction of the Mississippi River Gulf Outlet (MRGO) in 1956 to 

facilitate commercial shipping access to the Port of New Orleans from the Gulf of Mexico. 

Upon its completion in 1965, the MRGO provided a route 40 miles shorter than the alternative up the Mississippi River. Th e MRGO also provides a connection from the Gulf of 

Mexico to the Gulf Intracoastal Waterway (GIWW), which is a recreational and commercial waterway running east-west from Texas to Florida. Th ough the MRGO produced commercial benefi ts, those benefi ts came at a cost to the environment. Th e Corps estimates that 

the construction of the channel led to substantial loss of wetlands, which, as noted above, 

help slow and decrease the power of storms before they hit populated areas. 

Th e MRGO also contributed to a potential “funnel” for storm surges emerging from Lake 

Borgne and the Gulf into the New Orleans area.30 Th e “funnel” was created by the intersection of the MRGO from the southeast and the GIWW from the northeast into the confi ned 

channel, referred to as the GIWW/MRGO that separates New Orleans East and the Ninth 

Ward/St. Bernard Parish. Th e levees on the south side of the MRGO and the levees on the 

north side of the GIWW converge from being about 10 miles apart where they straddle 

Lake Borgne to a few hundred yards apart where the MRGO merges into the GIWW.31 Th e 

western part of the “funnel” is a six-mile-long section of the combined GIWW/MRGO, 

which was enlarged by a factor of three when the MRGO was built in order to expand it 

from a barge channel to accommodate ocean-going vessels.32
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Chapter 9

Prior to Hurricane Katrina, many warned that the potential funnel would accelerate and 

intensify storm surges emerging from Lake Borgne and the Gulf into the downtown New 

Orleans area. Th e funnel had been described as a “superhighway” for storm surges or the 

“Crescent City’s Trojan Horse” that had the potential to “amplify storm surges by 20 to 40 

percent,” according to some storm modeling.33 Researchers at LSU believed that in creating this funnel, “the US Army Corps of Engineers had inadvertently designed an excellent storm surge delivery system – nothing less – to bring this mass of water with simply 

tremendous ‘load’ – potential energy – right into the middle of New Orleans.” 34

Th e extent to which MRGO, and the funnel it helped create actually contributed to the hurricane’s damage is still being investigated, but there have been some preliminary fi ndings. A 

recent report issued by the Corps’ IPET concluded that the portion of MRGO running from 

the GIWW to the Gulf (called “Reach 2”) did not signifi cantly impact the height of Katrina’s 

storm surge, not because the “funnel” eff ect was nonexistent, but because the storm was so 

great it nullifi ed the impact of either the wetlands or the intersection of the MRGO and the 

GIWW – the funnel – at the height of the surge.35

While the IPET report concluded that the Reach 2 portion of MRGO had little impact on 

Katrina’s storm surge, it did fi nd that the six-mile combined section of the GIWW/MRGO 

(called “Reach 1”) carried the storm surge from Lake Borgne into New Orleans. Th e 

combined GIWW/MRGO served as a link between Lake Borgne and Lake Pontchartrain, 

enabling the storm surge in one lake to aff ect the storm surge in the other. During Katrina, 

a 14 to 17-foot surge coming from Lake Borgne into the funnel between MRGO and the 

GIWW was as much as 10 feet above water levels in Lake Pontchartrain.36 Th is large diff erence in the water levels between the two lakes increased the fl ow of water in the direction of 

the city and eventually into Lake Pontchartrain.

To address this problem, the IPET report recommended that fl ow through the combined 

channels “must be dramatically reduced or eliminated,” either by a permanent closure or a 

structure that can be selectively used to block storm surges fl owing between Lakes Pontchartrain and Borgne along the combined GIWW/MRGO.37

Researchers at the LSU Hurricane Center who have looked at models of Katrina have concluded that it is not just the volume of water that is important, but also the velocity. Th ese 

researchers found that the funnel accelerated the speed of the water when the larger volume 

in the funnel, and especially the water in the MRGO, was forced into the single merged 

GIWW/MRGO channel.38 Th e increased velocity of the water as it made its way through 

the channel pounded on the fl oodwalls lining the sides,39 weakening them and making them 

more vulnerable to the overtopping and scouring that occurred during the storm. Maximum current velocities in the combined GIWW/MRGO channel were greater than eight 

feet per second, which is nearly three times the velocity necessary to cause serious potential 

for erosion in the soils of the adjacent levee.40

Investigations continue into MRGO’s contribution to damage caused by Katrina, but there 

is general agreement that the presence of the MRGO destroyed wetlands that otherwise 

would have provided additional defenses. Th is happened because the MRGO served as 

a conduit for saltwater from the Gulf of Mexico to intrude into the freshwater wetlands. 

Th e saltwater damaged and destroyed wetlands, which resulted in the loss of land that had 

served as part of the city’s defenses against hurricanes and other storms.41 According to the 

National Academy of Sciences, MRGO has resulted in “tremendous environmental damage, 

including saltwater intrusion, land loss, and worsening the eff ects of wave damage during 

hurricanes and storms.”42
Effects of Environmental and Engineering Changes

125

Over the past 40 years, the erosion from the saltwater has contributed to the widening of the 

MRGO from 600 feet to 2,000 feet, an average of 35 feet per year, and the loss of more than 

19,000 acres of land.43 Had there been no wetlands at all east of the MRGO and the GIWW, 

preliminary storm modeling has shown, the Katrina storm surge may have been anywhere 

from three to six feet higher along St. Bernard Parish/Ninth Ward and New Orleans East.44

Continued wetland loss will increase the vulnerability of the city, making overtopping by 

storm surges even more likely in the future.45

Th e building of MRGO and the combined GIWW/MRGO resulted in substantial environmental damage, including a signifi cant loss of wetlands that had once formed a natural 

barrier against hurricanes threatening New Orleans from the east. MRGO and the GIWW/

MRGO provided a connection between Lake Borgne and Lake Pontchartrain that allowed 

the much greater surge from Lake Borgne to fl ow into both New Orleans and Lake Pontchartrain. Th ese channels further increased the speed and fl ow of the Katrina surge into 

New Orleans East and the Ninth Ward/St. Bernard Parishes, increasing the destructive 

force against adjacent levees and contributing to their failure. As a result, MRGO and the 

combined GIWW/MRGO resulted in increased fl ooding and greater damage from Hurricane Katrina. 

1 Working Group for Post-Hurricane Planning for the Louisiana Coast, “A New Framework for Planning the Future 

of Coastal Louisiana aft er the Hurricanes of 2005,” Jan. 26, 2006, p. 8 [hereinaft er Working Group for Post-Hurricane 

Planning, “A New Framework”].

2 Working Group for Post-Hurricane Planning, “A New Framework,” p. 8.

3 Working Group for Post-Hurricane Planning, “A New Framework,” p. 8.

4 Working Group for Post-Hurricane Planning, “A New Framework,” p. 9.

5 Greg Brouwer, “Th e Creeping Storm,” Civil Engineering Magazine, June 2003, p. 10.

6 A study by the United States Geological Survey “forecasts that an additional 448,000 acres could be lost by 2050 if no 

further actions are taken to halt or reverse current processes.” 448,000 acres is the equivalent of 700 square miles. Source: 

U.S. Library of Congress, Congressional Research Service, Coastal Louisiana: Attempting to Restore an Ecosystem, by Jeffrey Zinn, Oct. 25, 2004, p. 5 [hereinaft er CRS, Coastal Louisiana: Attempting to Restore an Ecosystem].

7 National Academy of Sciences, Drawing Louisiana’s New Map: Addressing Land Loss in Coastal Louisiana. Washington: National Academies Press, 2006, p. 11 [hereinaft er NAS, Drawing Louisiana’s New Map].

8 NAS, Drawing Louisiana’s New Map, pp. 22-27. Tons converted to square miles using 1.5 tons per cubic yard. Source:

Sherwood M. Gagliano, “Th e Life and Times of Mississippi River Sediment,” presented at the Workshop on River Resources Management in the 21st-century, Feb. 20, 2003.

9 Working Group for Post-Hurricane Planning, “A New Framework,” p. 1.

10 CRS, Coastal Louisiana: Attempting to Restore an Ecosystem, p. 5.

11 CRS, Coastal Louisiana: Attempting to Restore an Ecosystem, p. 5. See also: NAS, Drawing Louisiana’s New Map, p. 

31; Virginia Burkett, David B. Zilkoski, and David A. Hart, “Sea-Level Rise and Subsidence: Implications for Flooding 

in New Orleans, Louisiana,” Subsidence Interest Group Conference, Proceedings of the Technical Meeting, Nov. 27-29, 

2001, p. 64.

12 John W. Day, Jr., et al., “Pattern and Process of Land Loss in the Louisiana Coastal Zone: An Analysis of Spatial and 

Temporal Patterns of Wetland Habitat Change,” Estuarine Research Federation, 1999, pp. 193-200.

13 NAS, Drawing Louisiana’s New Map, p. 30.

14 CRS, Coastal Louisiana: Attempting to Restore an Ecosystem, p. 5.

15 Robert A. Morton and Noreen A. Purcell, U.S. Geological Survey, “Wetland Subsidence, Fault Reactivation, and 

Hydrocarbon Production in the U.S. Gulf Coast Region,” Sept. 2001. 

16 James G. Tituse and Vijay K. Narayanan, U.S. Environmental Protection Agency, “Th e Probability of Sea Level Rise,” 

Sept. 1995, p. iii. According to the Environmental Protection Agency, “[t]here is new and stronger evidence that most of 

the warming over the last 50 years is attributable to human activities” and that the “warmer temperatures are expected to 

raise sea level by expanding ocean water, melting mountain glaciers, and melting parts of the Greenland Ice Sheet.” U.S. 

Environmental Protection Agency, “Global Warming – Climate, Future Climate,” Jan. 7, 2000. http://yosemite.epa.gov/
Chapter 9

126

OAR/globalwarming.nsf/content/ClimateFutreClimate.html. Accessed on Apr. 17, 2006; U.S. Environmental Protection 

Agency, Global Warming – Climate, Climate, Jan. 7, 2000. http://Yosemite.epa.gov/OAR/globalwarming.nsf/content/climate.html. Accessed on Apr. 17, 2006.

17 Working Group for Post-Hurricane Planning, “A New Framework,” p. 9; U.S. Library of Congress, Congressional 

Research Service, Protecting New Orleans: From Hurricane Barriers to Floodwalls, by Nicole T. Carter, Jan. 26, 2006, p. 10 

[hereinaft er CRS, Protecting New Orleans: From Hurricane Barriers to Floodwalls].

18 CRS, Protecting New Orleans: From Hurricane Barriers to Floodwalls, p. 10.

19 Working Group for Post-Hurricane Planning, “A New Framework,” p. 15.

20 Working Group for Post-Hurricane Planning, “A New Framework,” p. 15.

21 Working Group for Post-Hurricane Planning, “A New Framework,” p. 16.

22 Working Group for Post-Hurricane Planning, “A New Framework,” p. 15.

23 Abby Sallenger, e-mail to David Hunter, Senate Committee staff member, Mar. 14, 2006, 3:51 p.m.

24 Bob Marshall, “Studies Abound on Why the Levees Failed. But Researchers Point out that Some Levees Held Fast 

Because Wetlands Worked as Buff ers During Katrina’s Storm Surge,” New Orleans Times-Picayune, Mar. 23, 2006, p 1.

25 U.S. Army Corps of Engineers, Interagency Performance Evaluation Task Force, Performance Evaluation Plan and 

Interim Status, Report 2 of a Series, Mar. 10, 2006, p. III–6 [hereinaft er IPET, Report 2 of a Series].

26 IPET, Report 2 of a Series, p. I–2.

27 Virginia Burkett, David B. Zilkoski, and David A. Hart, “Sea-Level Rise and Subsidence: Implications for Flooding in 

New Orleans, Louisiana,” Subsidence Interest Group Conference, Nov. 2001, p. 63.

28 Working Group for Post-Hurricane Planning, “A New Framework,” pp. 15-18.

29 NAS, Drawing Louisiana’s New Map, p. 38.

30 IPET, Report 2 of a Series, p. E–7.

31 Ivor L. van Heerden, G. Paul Kemp, Wes Shrum, Ezra Boyd and Hassan Mashriqui, Louisiana State University, Center for the Study of Public Health Impacts of Hurricanes, Initial Assessment of the New Orleans’ Flooding Event during 

the Passage of Hurricane Katrina, p. 4 [hereinaft er LSU, Initial Assessment of New Orleans Flooding].

32 LSU, Initial Assessment of New Orleans Flooding, p. 4.

33 Michael Grunwald, “Canal May Have Worsened City’s Flooding; Disputed Project Was a ‘Funnel’ for Surge, Some 

Say,” Th e Washington Post, Sept. 14, 2005, p. A21.

34 LSU, Initial Assessment of New Orleans Flooding, p. 4.

35 IPET, Report 2 of a Series, p. E–7.

36 LSU, Initial Assessment of New Orleans Flooding, p. 4.

37 IPET, Report 2 of a Series, p. E–7.

38 Hassan S. Mashriqui et al., “Experimental Storm Surge Simulations For Hurricane Katrina,” Coastal Hydrology and 

Processes (publication pending) [hereinaft er Mashriqui, “Experimental Storm Surge”].

39 Mashriqui, “Experimental Storm Surge”; Matthew Brown, “MRGO’ing, going, gone? Most everyone wants to close 

the shipping channel. But how closed is close?” New Orleans Times-Picayune, Nov. 27, 2005, p. 1.

40 Mashriqui, “Experimental Storm Surge.”

41 NAS, Drawing Louisiana’s New Map, p. 30.

42 NAS, Drawing Louisiana’s New Map, p. 38.

43 NAS, Drawing Louisiana’s New Map, pp. 38-39.

44 Working Group for Post-Hurricane Planning, “A New Framework,” p. 16.

45 Working Group for Post-Hurricane Planning, “A New Framework,” p. 16.
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