Prepared in cooperation with the U.S. Fish and Wildlife Service

Analysis of Seafloor Change at Breton Island, Gosier 

Shoals, and Surrounding Waters, 1869–2014, Breton 

National Wildlife Refuge, Louisiana

By James G. Flocks and Joseph F. Terrano

Open-File Report 2016–1069

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

For more information on the USGS—the Federal source for science about the Earth,

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must be secured from the copyright owner.

Suggested citation: 

Flocks, J.G., and Terrano, J.F., 2016, Analysis of seafloor change at Breton Island, Gosier Shoals, and surrounding 

waters, 1869–2014, Breton National Wildlife Refuge, Louisiana: U.S. Geological Survey Open-File Report 2016–1069, 

27 p., http://dx.doi.org/10.3133/ofr20161069.

ISSN 2331-1258 (online)
iii

Acknowledgments

This study builds on the reports of DeWitt and others (2016), Terrano and others (2016a), and Miner and others 

(2009); the authors would like to thank the field teams, processing teams, report writers, and reviewers of these 

prior efforts. The authors would also like to thank Noreen Buster and Julie Bernier of the U.S. Geological Survey, 

whose reviews of this report contributed greatly. This project included funding from the Louisiana Outer Coast 

Early Restoration Project, obtained by the natural resource trustees for the Deepwater Horizon Oil Spill, pursuant to the Framework for Early Restoration Addressing Injuries Resulting from the Deepwater Horizon Oil Spill, 

executed April 20, 2011.
iv

Contents

Acknowledgments ...........................................................................................................................................................................................iii

Abstract ........................................................................................................................................................................................................... 1

Introduction...................................................................................................................................................................................................... 1

Description of Study Area ........................................................................................................................................................................... 3

Breton Island Shoreline Change and Storm History ................................................................................................................................... 4

Methods........................................................................................................................................................................................................... 5

Historical Data............................................................................................................................................................................................. 6

2014 Data Collection and Processing......................................................................................................................................................... 6

Final 2014 DEM .......................................................................................................................................................................................... 8

Bathymetric-Change Analysis ..................................................................................................................................................................... 8

Sediment Composition................................................................................................................................................................................ 9

Results and Discussion ................................................................................................................................................................................... 9

Long-Term Change (1869–2014).............................................................................................................................................................. 15

Pre-MRGO (1869–1920)........................................................................................................................................................................... 17

MRGO and Hurricane Katrina (1920s–2014)............................................................................................................................................ 17

Post-Katrina Recovery (2007–2014)......................................................................................................................................................... 17

Elevation Profile Comparison.................................................................................................................................................................... 18

Bathymetry and Sediment Composition.................................................................................................................................................... 22

Conclusions................................................................................................................................................................................................... 24

References Cited........................................................................................................................................................................................... 25

Figures

1. Maps showing location of Breton Island within the Breton National Wildlife Refuge, Gosier Shoals, and the Mississippi RiverGulf Outlet shipping channel........................................................................................................................................................... 2

2. Map showing change in the surface area of Breton Island since 1869 .......................................................................................... 3

3. Photograph from the U.S. Geological Survey National High Altitude Photography program showing Breton Island in 1983, 

with the 2014 shoreline overlain as a black line ............................................................................................................................. 5

4. Graph showing change in island area between 1988 and 2014..................................................................................................... 6

5. Trackline map showing survey extent and coverage of single-beam and interferometric-swath bathymetric systems around 

Breton Island, Gosier Shoals, and offshore.................................................................................................................................... 7

6. Digital elevation model generated from 1869 hydrographic sheets.............................................................................................. 10

7. Digital elevation model generated from 1921–1922 hydrographic sheets.................................................................................... 10

8. Digital elevation model generated from 2007 bathymetric data.....................................................................................................11

9. Digital elevation model generated from 2014 bathymetric data.....................................................................................................11

10. Map showing long-term elevation change determined by calculating the difference in digital elevation models between 

 1869 and 2014, considered to be accretion and erosion over the time period............................................................................ 12

11. Map showing pre-Mississippi River-Gulf Outlet elevation change determined by calculating the difference in digital elevation 

models between 1869 and 1920, considered to be accretion and erosion over the time period.................................................. 13
v

12. Pre/post-Mississippi River-Gulf Outlet and Hurricane Katrina impact determined by calculating the difference in digital elevation 

models between 1922 and 2014, considered to be accretion and erosion over the time period.................................................. 14

13. Post-Hurricane Katrina recovery determined by calculating the difference in digital elevation models between 2007 and 2014, 

considered to be accretion and erosion over the time period....................................................................................................... 15

14. Seismic subbottom profiles collected in 2007 and reoccupied in 2014 across the Mississippi River-Gulf Outlet shipping channel 

reflect the infilling of the channel after its decommission and cease in maintenance dredging following Hurricane Katrina........ 19

 15. Elevation profiles derived from the digital elevation models for the four time periods across the Breton/Gosier platforms and for 

the Mississippi River-Gulf Outlet................................................................................................................................................... 19

 16. Elevation profiles derived from the digital elevation models for the four time periods across the southern Breton platform and 

offshore......................................................................................................................................................................................... 20

 17. Elevation profiles derived from the digital elevation models for the four time periods across Breton island and the 

offshore shoal ............................................................................................................................................................................... 20

 18. Elevation profiles derived from the digital elevation models for the four time periods across Breton Sound, the Gosier platform, 

the Mississippi River-Gulf Outlet, and the offshore shoal................................................................................................................. 21

 19. Elevation profiles derived from the digital elevation models for the four time periods across Breton Sound, Gosier Shoals, 

and offshore.................................................................................................................................................................................. 21

 20. Average grain size of sediment samples collected from the seafloor within the Breton National Wildlife Refuge study area, 

color-coded by size, with the accompanying Wentworth size classification ................................................................................. 23

 21. Sorting of sediment samples collected from the seafloor within the Breton National Wildlife Refuge study area, color-coded 

by the classification from Folk (1968) ........................................................................................................................................... 23

Tables

1. Sources of bathymetric and shoreline data used in the analysis of seafloor change at Breton Island, Gosier Shoals, and 

surrounding waters, Breton National Wildlife Refuge, Louisiana,1869–2014................................................................................. 3

2. Bathymetric dataset statistics by survey platform used to create the Breton 2014 DEM ............................................................... 8

3. Accretion and erosion volumes, changes, and rates of change during the four study time periods for the four target features 

and other areas within the Breton National Wildlife Refuge, Louisiana, study area ..................................................................... 16
vi

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

micron (um) 0.001 millimeter (mm) 

centimeter (cm) 0.394 inch (in.)

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

kilometer (km) 0.621 mile (mi)

kilometer (km) 0.540 mile, nautical (nmi) 

Area

square kilometer (km2

) 247.1 acre

square kilometer (km2

) 0.386 square mile (mi2)

Volume

cubic meter (m3

) 1.31 cubic yard (yd3)

cubic meter (m3

) 35.31 cubic foot (ft3)

Flow rate

cubic meter per year (m3

/yr) 0.000811 acre-foot per year (acre-ft/yr)

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88, GEOID09), and the units of all vertical measurements are in meters.

Horizontal coordinate information is referenced to the geographic coordinates North American Datum of 1983 (NAD 83, 2011); however, 

data were projected into the Universal Transverse Mercator (UTM) coordinate system for analysis purposes.

Supplemental Information

Data used in this report can be found in Miner and others (2009), and are available as an accompanying USGS Open-File Report Terrano 

and others, 2016a; (http://dx.doi.org/10.3133/ofr20161039). Additional accompanying data is available through a Data Series obtained 

from DeWitt and others, 2016; (http://dx.doi.org/10.3133/ds1005). Light and detection ranging (lidar) data from 2014 are available for 

download as a data release from Terrano and others, 2016b (http://coastal.er.usgs.gov/data-release/doi-F70G3H6G/), along with digitized 

shorelines and rates of change from Terrano and others, 2015 (http://coastal.er.usgs.gov/data-release/doi-F7XS5SGM/).

Abbreviations

BICM Barrier Island Comprehensive Monitoring

DEM Digital Elevation Model

DGPS Differential Global Positioning System

GMT Generic Mapping Tools

GPS Global Positioning System

H-sheet Hydrographic sheet

IFB Interferometric-Swath Bathymetry

Lidar Light and detection ranging

Micron Micrometer

MRGO Mississippi River-Gulf Outlet

NWR National Wildlife Refuge

SBB Single-Beam Bathymetry

T-sheet topographic sheet

USGS U.S. Geological Survey

UTM Universal Transverse Mercator

ybp Years before present
1

Analysis of Seafloor Change at Breton Island, Gosier 

Shoals, and Surrounding Waters, 1869–2014, Breton 

National Wildlife Refuge, Louisiana

By James G. Flocks and Joseph F. Terrano

Abstract

Characterizing bathymetric change in coastal environments is an important component in understanding shoreline evolution, especially along barrier island platforms. Bathymetric change is a function of the regional sediment budget, long-term wave and current patterns, and episodic impact from 

high-energy events such as storms. Human modifications may also cause changes in seafloor elevation. 

This study, conducted by the U.S. Geological Survey in collaboration with the U.S. Fish and Wildlife Service, evaluates bathymetric and volumetric change and sediment characteristics around Breton 

Island and Gosier Shoals located offshore of the Mississippi River Delta in Louisiana. This area has 

been affected by significant storm events such as Hurricane Katrina in 2005. Sedimentation patterns at 

Breton Island and offshore have also been modified by the excavation of a shipping channel north of the 

island. Four time periods are considered that encompass these episodes and include long-term change 

and short-term storm recovery: 1869–2014, 1869–1920, 1920–2014, and 2007–2014. Finally, sediment 

characteristics are reported in the context of seafloor elevation.

Introduction

Barrier island systems in the northern Gulf of Mexico are highly dynamic and undergo rapid 

changes in area, position, and elevation over century timescales (Morton, 2008). Over the past century 

in Louisiana, the barrier islands have experienced loss in area and landward migration because of storm 

impact, land subsidence, and sea-level rise (Penland and Boyd, 1981; Penland and Ramsey, 1990; Williams and others, 1992). Changes also occur along the island platform as sediments are moved through 

long-term wave-dominated transport, punctuated by rapid redistribution during storm impacts. Monitoring shoreline change at varying time scales can be accomplished by using repetitive sets of nautical 

charts and aerial and satellite imagery collected over the past century and by using recently collected 

light and detection ranging (lidar) data. Monitoring seafloor change is difficult, however, because of a 

paucity of repetitive bathymetric datasets, yet it is a critical component to understanding island evolution.

Breton Island is a small barrier island located east of the Mississippi River Delta (fig. 1). The 

island and the Gosier Shoals to the north are part of the Breton National Wildlife Refuge (NWR), which 

extends north to the Chandeleur Islands. Breton NWR is recognized as a globally important bird habitat because of the critical resources it provides and because it hosts one of Louisiana’s largest historical brown pelican nesting colonies. Breton Island has been severely impacted by long-term erosional 

processes and by a series of extreme events over the past century, including Hurricane Katrina in 2005, 

which completely submerged the island. Additionally, the Mississippi River-Gulf Outlet (MRGO) ship 

channel was dredged in 1968 between Breton Island and the Gosier Shoals, affecting sediment transport 
2

in the area. In response to these factors, island area has decreased by 87 percent since the late 1800s 

(Terrano and others, 2016a), directly affecting the availability of nesting habitat (fig. 2). Without proper 

management, Breton Island will continue to deteriorate (Lavoie and others, 2010) and will no longer be 

able to support critical habitat. A comprehensive geologic assessment of the island platform and surrounding area is necessary to adequately characterize the geologic framework and physical processes 

affecting the island and to provide refuge managers the information they need for restoration design, 

implementation, and future monitoring. This assessment includes high-resolution mapping of seafloor 

elevations with comparison to historical datasets to determine seafloor change. This effort builds on the 

study provided by the Louisiana Coastal Protection and Restoration Authority Barrier Island Comprehensive Monitoring (BICM) project (Miner and others, 2009; Kindinger and others, 2013), which evaluated seafloor change from 1869 to 2007 and relied on studies provided by a U.S. Geological Survey 

(USGS) erosion study using data from 1878 to 1989 (List and others, 1994).

This report is part of a series of USGS reports that present bathymetric data (DeWitt and others, 

2016), topographic/bathymetric lidar data (Terrano and others, 2016b), and shoreline change analysis 

(Terrano and others, 2016a) for Breton Island and the surrounding waters. The purpose of this study is 

to present a comparison of bathymetric elevation models from 1869, 1920, 2007, and 2014 (table 1) 

to evaluate and illustrate seafloor change over these time periods. Areas of physiographic significance 

(such as the Breton and Gosier platforms) are delineated to specify volumetric change across distinct 

morphologies, and two-dimensional profiles of the bathymetric datasets are provided along transects 

across areas that have shown significant change over the study period.

Figure 1. Maps showing location of Breton Island within the Breton National Wildlife Refuge, Gosier Shoals, and the Mississippi River-Gulf 

Outlet (MRGO) shipping channel. Background image is 2014 USGS Landsat 8 satellite imagery.
3

Table 1. Sources of bathymetric and shoreline data used in the analysis of seafloor change at Breton Island, Gosier Shoals, and surrounding 

waters, Breton National Wildlife Refuge, Louisiana,1869–2014.

[H-sheets, hydrographic sheets]

Figure 2. Map showing change in the surface area of Breton Island since 1869. From Terrano and others (2016a).

Year-bathymetry Year-shoreline1 Data Type Bathymetry source

1869 1869 H-sheets2 Louisiana Department of Natural Resources, 2013

1920–22 1922 H-sheets2 Louisiana Department of Natural Resources, 2013

2007 2007 Single/Swath bathymetry Louisiana Department of Natural Resources, 2013

2014 2014 Single/Swath bathymetry DeWitt and others, 2016

1

Shoreline polygons used in this study from Terrano and others (2015).

2

Digitized from U.S. Coast and Geodetic Survey Hydrographic Survey Smooth Sheets (Miner and others, 2009).

Description of Study Area 

Breton Island is the final remnant of a transgressive island arc that formed during abandonment 

of a subdelta lobe of the St. Bernard delta complex and is one of the oldest barrier islands in Louisiana 

(Penland and others, 1988). In the literature Breton Island is considered to be the southernmost extension of the Chandeleur Island chain that began developing about 2,500 years before present (ybp), fol-
4

lowing the avulsion of the La Loutre subdelta lobe (Frazier, 1967; Penland and others 1988; Rogers and 

others, 2009). However, its longitudinal position landward of the Chandeleur Island chain and its proximity to the terminal end of the Terre aux Boeufs subdelta lobe (Frazier, 1967; Rogers and others, 2009) 

suggest that the island may have formed earlier than the Chandeleur Islands, about 3,500 ybp. Following abandonment of the subdelta lobes, dewatering and compaction of the underlying sediments led to 

subsidence and flooding of the headland marsh, detaching and isolating the island from the mainland 

(Penland and others, 1988).

In the flooding stage of barrier island evolution, sandy sediments necessary to maintain island 

elevation and position are largely restricted to alongshore transport through littoral processes. In the 

southern Chandeleur Islands, the prevailing net longshore transport of sediment is from the north to 

the south (Ellis and Stone, 2006). Because of a similar wave climate and protection from the modern 

Mississippi River Delta to the south (fig. 1), it is believed that similar littoral processes exist at Breton 

Island. Sediment supply is limited, however, and as a result, Breton Island is losing elevation and island 

area. The island is also retreating landward through rollover produced by overwash of the sandy shoreface to the back barrier during storm impact. Presently, a limited area of marsh deposits located at the 

core of the island resists erosion from wave action and provides a locus for sand retention.

In 1968, the MRGO shipping channel was completed to reduce shipping distance between the 

Gulf of Mexico and the Port of New Orleans. This 152-meter (m)-wide, 11-m-deep shipping channel 

was dredged immediately to the north of Breton Island (fig. 1) adjacent to a natural tidal channel between the Breton and Gosier platforms. Over the past half-century, Breton Island began to experience 

rapid erosion rates (Terrano and others, 2016a), likely attributed to storm impact (for example, Hurricane Camille in 1969), the removal of sediment from the system through maintenance dredging and 

disposal, and overall reduction in littoral sediment transport across the MRGO (Lavoie, 2009). Shoreline 

change analysis from Terrano and others (2016a) measured a 51-percent reduction in island area before 

and after construction (1950–1978). In April 1996, Louisiana State University initiated the Beneficial 

Use Monitoring Program, which was designed to create a feeder berm using sand dredged from the 

MRGO to replenish the sand being lost on Breton Island. Between 1993 and 2006, four sand placements 

occurred adjacent to the island (Penland and others, 1997). Following Hurricane Katrina in 2005, the 

MRGO was decommissioned, and maintenance dredging of the channel has ceased. This study records 

the subsequent infilling of the channel through natural processes. The only other human development on 

the uninhabited island is oil industry infrastructure (bulkhead) on the center of the island and a rock weir 

extending along the Sound-side at the north end of the island.

Breton Island Shoreline Change and Storm History

A high-resolution shoreline change analysis was completed in Terrano and others (2016a), 

which will be summarized here. The study acquired 14 shoreline datasets that spanned from 1869 to 

2014 (Terrano and others, 2015). These shorelines were digitized from U.S. Coast and Geodetic Survey 

topographic sheets (T-Sheets) (1869–1950), USGS and U.S. Department of Agriculture aerial imagery 

(1983–2012), and USGS lidar surveys (2001–2014). For a complete description on how these data were 

processed, see Terrano and others (2016a). The island has segmented and lost all of its southern half, 

and it has retreated landward approximately 846 m over the past 145 years. Only a small part of the 

northern end of the island has remained, stabilized by mangrove and marsh vegetation and underlying 

marsh substrate (fig. 3). This area was the only remnant to remain following Hurricanes Ivan and Katrina, which completely submerged the island with storm surge and reduced island area 64 percent (from 
5

0.52 to 0.18 square kilometer [km2]). Following Hurricane Katrina, Breton Island showed a slow rate of 

land loss until 2008, after which the island began to recover, aided by the absence of severe storms since 

the storm. The unsubmerged part of the island has increased from its smallest area of 0.14 km2

 in 2007 

to 0.43 km2 in 2014.

Although Hurricane Katrina was the most significant storm to impact Breton Island in terms 

of storm surge and intensity, the island has suffered numerous significant erosional events (Hurricane 

Andrew, 1992; Hurricane Opal, 1995; Hurricane Georges, 1998; Hurricane Isidore, 2002; and Hurricane 

Ivan, 2004; fig. 4). The relative impacts of the storms have been scored by measurements of various 

physical parameters taken when the storms were at their closest approach to the island. Distance and 

position relative to island, heading, central pressure, wind speed, direction, and wave heights were measured at National Oceanic and Atmospheric Administration buoys near the island. Figure 4 also shows 

placement events, where sediment dredged from the MRGO shipping channel was placed adjacent to 

Breton Island to allow littoral transport of sediment to the island shoreface.

Methods

A summary of the legacy data is described below, followed by a description of the data collection, processing, and grid generation of the 2014 dataset.

Figure 3. Photograph from the U.S. Geological Survey National High Altitude Photography (NHAP) program showing Breton Island in 1983, 

with the 2014 shoreline (derived from lidar) overlain as a black line. The southern half of the island is presently submerged. Inset is an oblique 

aerial photograph of the northernmost part of the island, where the only vegetation exists. The black arrow points to the same location in the 

photograph and the 2014 shoreline. Oblique aerial photography from Morgan (2015) and shoreline from Terrano and others (2015).
6

Historical Data

This study builds upon the Louisiana Coastal Protection and Restoration Authority Barrier Island 

Comprehensive Monitoring (BICM) project, which compared bathymetry around Breton Island during 1869, 1920, and 2007 (Miner and others, 2009; Kindinger and others, 2013). The digital elevation 

models (DEMs) developed through the BICM project are used in this study; complete information on 

their development can be found in Miner and others (2009) and List and others (1994). Land elevations in the DEMs are assigned a uniform elevation of 0.5 m (Miner and others, 2009). The DEMs were 

reported in the Universal Transverse Mercator zone 15, North American Vertical Datum of 1988 (UTM 

zone 15, NAVD 88). For this study, the DEMs were converted to UTM zone 16, and their spatial extents 

were correlated to the 2014 DEM using QGIS, version 2, and Generic Mapping Tools (GMT), version 5, 

software packages.

2014 Data Collection and Processing

The 2014 bathymetric data collected around Breton Island and Gosier Shoals were acquired in 

July and August 2014. A complete description of acquisition and processing, including system specifications, can be found in DeWitt and others (2016) and will be summarized here. The survey area 

encompasses approximately 356 km2

 and extends from the shorelines of Breton Island and Gosier 

Shoals approximately 5 kilometers (km) west into Breton Sound and approximately 10 km east into 

the Gulf of Mexico (fig. 5). Slightly more than 1,558 line-km of single-beam bathymetry (SBB) and 

991 line-km of interferometric-swath bathymetry (IFB) were collected. Survey platforms included 9-m 

Figure 4. Graph showing change in island area (blue line) between 1988 and 2014 with blue diamonds denoting island area measurements 

from Terrano and others (2016a). Red lines mark dates of storm impacts to the island during the time period. Height of lines represent a ranking 

of relative storm impact during its closest 6-hour approach to the island based on the storm’s central pressure, wind speed, wave height, distance, 

heading, and bearing. Green squares annotate Mississippi River-Gulf Outlet dredge spoil placement events (with estimated amounts of sediment 

placed). Dredge spoil was placed in shallow water adjacent to the island to assist in island recovery through littoral transport during normal wave 

conditions. [m3

, cubic meters]
7

and 5-m outboard vessels, a personal watercraft for shallow water work, and the 29-m research vessel 

(RV) Tommy Munro for deep water offshore. Water depths ranged from 0.4 to 10.6 m. Navigation was 

recorded by using differential global positioning system (DGPS) receivers corrected to measurements 

from a global positioning system (GPS) base station installed over a USGS benchmark on Breton Island 

(fig. 5). Offshore navigation was corrected by using the Marinestar HP differential navigation service. Sound velocity through the water column was acquired by using sound velocity profilers (SVPs) 

mounted on the IFB system and through periodic SVP casts by all vessels. Internal motion referencing 

systems measured heave, pitch, and roll on the RV Tommy Munro and the two outboard vessels. The 

personal watercraft compensated for motion by using a very high GPS sampling rate (0.1 second, compared to 1.0 second for the larger vessels) and a narrow (4 degree) transducer beam angle. Single-beam 

bathymetry was collected at 50-millisecond intervals by using a 200-kilohertz (kHz) transducer. IFB 

was collected by using a 468-kHz system.

All bathymetric and navigation data were processed by using the CARIS geospatial software 

package. The final product was transformed to the North American Datum of 1983 (NAD 83) UTM 

zone 16N, and NAVD 88 (GEOID09) horizontal and vertical datums (DeWitt and others, 2016) prior to 

creating the 2014 DEM. 

In addition to the acoustic bathymetric data, topo/bathymetric data were collected at Breton 

Island and Gosier Shoals by using airborne lidar. The data were collected by the Quantum Spatial 

Figure 5. Trackline map showing survey extent and coverage of single-beam and interferometric-swath bathymetric systems around Breton 

Island, Gosier Shoals, and offshore. The historical digital elevation models (DEMs) were clipped to this survey extent. Figure modified from DeWitt 

and others (2016).
8

Company in December 2013, using a Riegl VQ-280-G airborne laser scanner acquiring at a target point 

density of 4 points per square meter or greater. Survey altitude was 600 m with a laser pulse rate of 284 

kHz and a laser spot diameter of 60 centimeters (cm). The same DGPS base station used for the acoustic 

bathymetry surveys was deployed on the island during flight, and ground control data to verify accuracy were acquired by using GPS rapid static measurements. The differences between ground control 

elevations and lidar elevations ranged from 0.009 to 0.167 m, with an average of 0.071 m at 15 bare 

earth, vegetated, and submerged sites. The lidar data were reported in NAD 83 UTM Zone 16, NAVD 

88 (GEOID12A) and were converted to GEOID09 for this study. For complete information on lidar data 

acquisition and processing, see Terrano and others (2016b).

Final 2014 DEM

The 2014 DEM was created by combining the bathymetric soundings and lidar elevations from 

the individual platforms. A running mean of 2 m was applied to each dataset by using the GMT blockmean filter to avoid aliasing short wavelengths. Because of the dense data volume within the shallowwater data collected by the personal watercraft and overlapping topo/bathymetric lidar survey, the two 

data files were merged and a blockmean filter was applied to the merged data. The merged data were 

first sorted by geographic position so that the blockmean filter would prioritize by position rather than 

by acquisition platform. The resulting data file was then gridded to 50-m grid cells by using the GMT 

surface algorithm with a search radius of 200 m to initialize the grid and a tension filter of 0.03 to suppress spurious oscillations. A grid mask generated from a polygon digitized around the survey area was 

applied to the resulting grid to exclude areas of no data using the GMT grdmask and grdmath functions. 

Statistics for the datasets listed by platform, along with the root-mean-square error for the resulting grid, 

are reported in table 2.

Bathymetric-Change Analysis

The mathematical difference between two DEMs provides the change in elevation values between the two time periods. The DEMs of the historical datasets (1869, 1920, and 2007) were clipped 

to the extents of the 2014 DEM. The 2007 survey did not extend as far offshore as the 2014 survey, 

and that portion of the 2007 grid was assigned null values. Portions of the 1920 DEM within the 2014 

survey area did not contain sufficient soundings, and those areas were assigned null values. All of the 

DEMs were sampled to same grid interval of the historical DEMs (100 m). It should be noted that the 

Platform Data Number of X/Y—Extents Z—Measured depth RMSE 

type data points NAD 83 UTM 16N Min. (m) Max. (m) m

RV Sallenger IFB 671,459 283665/293290/3256000/3263570 –8.35 –2.44 0.03

RV Jabba Jaw SBB 466,235 277310/304240/3253770/3275640 –10.65 –1.38 0.06

Personal watercraft SBB 436,813 281715/302838/3258501/3275640 –10.44 –0.62 0.06

RV Tommy Munro IFB 1,013,072 285000/299610/3251855/3262865 –10.34 –3.69 0.03

Topographic/

bathymetric lidar TB 321,301 285382/301321/3260594/3271639 –2.57 2.08 0.06

Table 2. Bathymetric dataset statistics by survey platform used to create the Breton 2014 DEM. 

[Root-mean-square error (RMSE) represents how well the 2014 DEM represents the data. Table modified from DeWitt and others (2016). Depths are relative to North American Vertical Datum of 1988. Abbreviations: NAD 83, North American Datum of 1983; UTM, Universal Transverse Mercator; m, meter; 

RV, research vessel; IFB, interferometric bathymetry; SBB, single-beam bathymetry; TB, topographic/bathymetric light detection and ranging (lidar)]
9

sampling densities (for example, trackline spacing) between surveys are highly variable and inherently 

increase error in this type of analysis. See Miner and others (2009) for maps showing trackline spacing 

of the pre-2014 surveys. 

Once the four DEMs were sampled to the same geographic extents and vertical datums, the 

bathymetric change between each period was determined by subtracting the older period from the more 

recent period by using the grdmath function in GMT to provide an isopach grid of erosion (negative 

values) and accretion (positive values). To better delineate areas of erosion and accretion, isopach differences between –0.25 m and +0.25 m were considered beyond the error of analysis and set to zero. The 

time periods chosen for analysis were 1869–2014, 1869–1920, 1920–2014, and 2007–2014. Bathymetric change between time periods can also be directly compared along 2-dimensional transects across the 

DEMs with stacked results in profile to show relative elevations. These profiles were extracted from the 

DEMs by using the transit plug-in in QGIS. 

Spatial and volume change between the time periods was determined by using the grdvolume 

function in GMT, which calculates area, volume, and volume per unit area for a provided polygon 

within a DEM. The polygon delineates areas of interest within the study area such as geomorphic features. For this analysis, the Breton Island platform, the Gosier Shoals platform, the MRGO corridor, and 

an offshore shoal feature were selected as important geomorphic features that have experienced significant change over the study period. The areas of interest were outlined from the 2014 DEM and applied 

to the remaining years.

Sediment Composition

Grain size and sorting of sediments collected from within the study area are briefly introduced in 

this report with respect to bathymetry. Island, shoal, and seafloor sediments were collected by using 1-m 

hand augers, and ponar grabs, and by vibracoring. Sample locations were determined by using DGPS. 

Subsamples of the ponar grabs and the top intervals of the augers and vibracores were selected to represent the seafloor sediment characteristics. Grain size and sorting statistics of these subsamples were 

analyzed by using a Coulter LS200 particle-size analyzer.

Results and Discussion

The DEMs for 1869, 1920s, 2007, and 2014 are shown in figures 6–9, respectively. Shorelines 

for Breton Island and Gosier Shoals (the shoals are only emergent in 2014) have been overlain on the 

maps. The DEMs were clipped to the extent of the 2014 survey, and certain areas of the 1920s and 2007 

grids were clipped because of insufficient or no data (gray background). The 1869 (fig. 6) and 1920s 

(fig. 7) maps pre-date the MRGO and show two distinct tidal channels between the Breton and Gosier 

platforms. Evident in all years is shoaling offshore of the inlets. The most recent coverage (fig. 9) shows 

a shoreline recovery of Breton Island and emergence of Gosier Shoals in the years following Hurricane 

Katrina.

The erosion/accretion isopach maps for the time periods 1869–2014, 1869–1920s, 1920s–2014, 

and 2007–2014 are shown in figures 10–13. These time periods represent long-term change, pre-MRGO, 

dredging of the MRGO/Hurricane Katrina impact, and post-Hurricane Katrina recovery, respectively, 

and will be discussed in the context of these periods and events.
10

Figure 6. Digital elevation model (DEM) generated from 1869 hydrographic sheets (H-sheets) (see table 1 for source). Overlain onto the DEM 

are transect locations (dashed lines, T0–T4) used to represent vertical change profiles between time periods.

Figure 7. Digital elevation model (DEM) generated from 1921–1922 hydrographic sheets (H-sheets) (see table 1 for source). Overlain onto the 

DEM are transect locations (dashed lines, T0–T4) used to represent vertical change profiles between time periods. Areas where no trackline data 

exist were excluded from the DEM.
11

Figure 8. Digital elevation model (DEM) generated from 2007 bathymetric data (see table 1 for source). Overlain onto the DEM are transect locations (dashed lines, T0–T4) used to represent vertical change profiles between time periods. Areas where no trackline data exist were excluded 

from the DEM.

Figure 9. Digital elevation model (DEM) generated from 2014 bathymetric data. Overlain onto the DEM are transect locations (dashed lines, 

T0–T4) used to represent vertical change profiles between time periods.
12

Figure 10. Map showing long-term elevation change determined by calculating the difference in digital elevation models (DEMs) between 1869 

and 2014, considered to be accretion (positive change) and erosion (negative change) over the time period. Elevation differences within +/-0.25 

meter (m) are assumed to have no change. Overlain onto the DEM are locations of key features (labeled polygons A–D) identified in the 2014 

DEM and used to generate volume change statistics. Remaining areas are divided as follows (see inset map for locations): E, Breton Gulf; F, 

Breton Sound; G, Offshore gulf; H, Gosier Gulf; and I, Gosier Sound.
13

Figure 11. Map showing pre-Mississippi River-Gulf Outlet (MRGO) elevation change determined by calculating the difference in digital elevation 

models (DEMs) between 1869 and 1920, considered to be accretion (positive change) and erosion (negative change) over the time period. Elevation differences within +/-0.25 meter (m) are assumed to have no change. Overlain onto the DEM are locations of key features (labeled polygons 

A–D) identified in the 2014 DEM and used to generate volume change statistics. Although the time period predates MRGO construction, the polygon is included for reference. Remaining areas are divided as follows (see inset map for locations): E, Breton Gulf; F, Breton Sound; G, Offshore 

gulf; H, Gosier Gulf; and I, Gosier Sound. The 1922 survey contains areas with no data, which reduces the accretion/erosion volume estimates for 

those areas and cannot be compared to other time periods.
14

Figure 12. Pre/post-Mississippi River-Gulf Outlet (MRGO) and Hurricane Katrina impact determined by calculating the difference in digital 

elevation models (DEMs) between 1922 and 2014, considered to be accretion (positive change) and erosion (negative change) over the time period. Elevation differences within +/-0.25 meter (m) are assumed to have no change. Overlain onto the DEM are locations of key features (labeled 

polygons A–D) identified in the 2014 DEM and used to generate volume change statistics. Remaining areas are divided as follows (see inset map 

for locations): E, Breton Gulf; F, Breton Sound; G, Offshore gulf; H, Gosier Gulf; and I, Gosier Sound. The 1922 survey contains areas with no 

data, which reduces the accretion/erosion volume estimates for those areas and cannot be compared to other time periods.
15

Long-Term Change (1869–2014)

The long-term change (fig. 10) shows dramatic rollover of Breton Island, with more than 1 km of 

landward migration in some places, and 4–8 m of vertical loss to the foreshore. Erosion occurred across 

the length of the platform, with accretion along the platform margins. Approximately 42.01 x 106

 cubic 

meters (m3

) of sediment was removed from the Breton platform during this time period (table 3). The 

former southern tip of Breton Island submerged and migrated offshore, raising the elevation there by 

3 m (fig. 10). On the Gosier platform, up to 4 m of accretion occurred on the terminal end, and the shoals 

themselves gained elevation and became emergent. Unlike the Breton platform, the Gosier platform 

gained 33.84 x 106

 m3 of sediment (table 3). Excavation of the MRGO is clearly evident with a sediment 

deficit of 12.95 x 106

 m3. The offshore shoal deflated over the past century, losing 17.13 x 106 m3 of sediment, migrating to the flanks of the shoal up to 3 m thick. The entire study area experienced a net loss of 

sediment, although about 40 percent of sediment loss was through the excavation of the MRGO (table 3).

Figure 13. Post-Hurricane Katrina recovery determined by calculating the difference in digital elevation models (DEMs) between 2007 and 

2014, considered to be accretion (positive change) and erosion (negative change) over the time period. Elevation differences within +/-0.25 meter 

(m) are assumed to have no change. Overlain onto the DEM are locations of key features (labeled polygons A–C) identified in the 2014 DEM and 

used to generate volume change statistics. Remaining areas are divided as follows (see inset map for locations): E, Breton Gulf; F, Breton Sound; 

G, Offshore gulf; H, Gosier Gulf; and I, Gosier Sound. The 2007 survey contains no data offshore, so volume estimate for the offshore shoal 

feature for this time period was not calculated.
16

Table 3. Accretion and erosion volumes, changes, and rates of change during the four study time periods for the four target features (Breton 

platform, Gosier platform, MRGO, and offshore shoal) and other areas within the Breton National Wildlife Refuge, Louisiana, study area. 

[MRGO, Mississippi River-Gulf Outlet; m3, cubic meters; m3/yr, cubic meters per year]

Time period Interval

years

Feature/ Area Accretion

(106 m3)

Erosion

(106 m3)

Change

(106 m3)

Rate of change

(106 m3/yr)

1869–2014 144 Breton platform 11.04 53.05 -42.01 -0.29

Gosier platform 44.77 10.93 33.84 0.23

MRGO 0.41 13.35 -12.95 -0.09

Offshore shoal 14.56 31.69 -17.13 -0.12

Breton Gulf 37.89 8.57 29.32 0.20

Breton Sound 17.47 18.97 -1.51 -0.01

Offshore Gulf 0.50 27.34 -26.85 -0.19

Gosier Gulf 5.71 10.31 -4.60 -0.03

Gosier Sound 25.48 14.07 11.41 0.08

1869–1920 50 Breton platform 21.43 17.48 3.94 0.08

Gosier platform1 17.30 5.13 12.17 0.24

MRGO 0.17 2.14 -1.97 -0.04

Offshore shoal 23.49 7.50 15.98 0.32

Breton Gulf 35.86 1.06 34.80 0.70

Breton Sound1 10.70 3.43 7.28 0.15

Offshore Gulf 8.41 0.13 8.28 0.17

Gosier Gulf 1.03 7.20 -6.17 -0.12

Gosier Sound 7.71 13.94 -6.23 -0.12

1920–2014 94 Breton platform 4.35 50.18 -45.83 -0.49

Gosier platform1 28.01 12.82 15.19 0.16

MRGO 0.63 11.42 -10.78 -0.11

Offshore shoal 3.39 36.73 -33.35 -0.35

Breton Gulf 10.51 17.08 -6.57 -0.07

Breton Sound1 9.58 16.63 -7.05 -0.07

Offshore Gulf 0.01 34.95 -34.94 -0.37

Gosier Gulf 6.28 2.50 3.79 0.04

Gosier Sound 25.51 9.61 15.90 0.17

2007–2014 7 Breton platform 2.31 14.04 -11.73 -1.68

Gosier platform 5.42 13.94 -8.52 -1.22

MRGO 8.31 0.39 7.92 1.13

Breton Gulf 1.48 17.72 -16.24 -2.32

Breton Sound 0.42 19.18 -18.76 -2.68

Gosier Gulf 0.55 12.15 -11.60 -1.66

Gosier Sound 0.18 14.44 -14.26 -2.04

1

Data gaps occur in the 1920s coverage for these areas. Volumes cannot be compared to other time periods.
17

Pre-MRGO (1869–1920)

The period prior to the construction of the MRGO was the only period of analysis where the 

net sediment volume for the study area increased. During this time period, the northern half of Breton 

Island rotated landward approximately 600 m while the southern half remained stationary (fig. 11). The 

former platform has eroded more than 2 m, yet the Breton platform experienced a net gain of sediment 

(3.94 x 106 m3; table 3). The sediment eroded from the shoreface appears to have migrated both landward, contributing to the island platform west of the 1920s island position, and downdrift, offshore of 

the southern half of the island. Within the inlet between the Breton and Gosier platforms, approximately 

2.14 x 106 m3 of sediment was eroded along the pre-MRGO tidal channel (table 3). The Gosier platform 

experienced a net gain of 12.17 x 106

 m3, with up to 4 m of sediment accumulating at the southern terminal end of the Gosier platform (fig. 11). The deposition rate on the Gosier platform over this time period (0.24 x 106

 cubic meters per year [m3/yr]) is consistent with the long-term (1869–2014) deposition 

rate (table 3), while the deposition rate on the Breton platform decreased from a surplus (0.08 m3

/yr) 

to a deficit over the long term (–0.29 m3

/yr for 1870–2014). This reversal suggests a detrimental influence from the excavation of the MRGO shipping channel in the 1960s and an inability of the island to 

recover from more intense storm impacts over the past half century (Terrano and others, 2016a). The 

offshore shoal experienced erosion along its northern flank and deposition along its western and southern flanks, with a net increase in sediment (table 3).

MRGO and Hurricane Katrina (1920s–2014)

Over this 94-year time span from 1920 to 2014, eight major (category 3 or greater) hurricanes 

passed within 150 km of Breton Island (Terrano and others, 2016a), and the study area as a whole 

experienced a larger net loss of sediment than during any other time period discussed in this report. 

The sediment volume deposited on the Breton platform over the previous half-century was removed 

10 times over, at a rate of –0.49 m3

/yr (table 3). In contrast, the Gosier platform increased in volume 

by 12.82 x 106

 m3 during this time period, again suggesting that the maintenance of the MRGO ship 

channel was inhibiting sediment deposition to Breton Island. Island rollover continued to occur at rates 

similar to those of the previous time period (fig. 12), although the southern part of the island has completely submerged. Aerial photo surveys of the island show that the final removal of the southern half 

of Breton Island occurred following Hurricanes Ivan and Katrina (Westphal, 2009). During the rollover 

process, sediment was eroded from the former shoreface and deposited at the present island location and 

offshore to the southwest. On the Gosier platform, more than 4 m of sediment was deposited on the terminal end of the platform (fig. 12). Most recently, the Gosier Shoals have emerged. The offshore shoal 

lost 33.35 x 106 m3 of sediment over this time period, probably during large storm events when wave 

base directly impacted the shoal. This sediment appears to have been completely removed from the 

shoal system because there is not a similar amount of sedimentation around the flanks of the shoal. The 

opposite is observed during the long-term (1869–2014) time period when sediment migrated from the 

shoal directly to the flanks (fig. 10). This time series captures the excavation of the MRGO, represented 

by a net loss of 10.78 x 106

 m3.

Post-Katrina Recovery (2007–2014)

From 2007 to 2014, all areas of analysis experienced sediment loss, except for the MRGO 

polygon. Because the offshore shoal and offshore Gulf have no coverage from the 2007 DEM, these 
18

areas are not included in this analysis. Overall, more sediment was lost from the Breton platform than 

deposited, but Breton Island shows some recovery of its southern shoreline (fig. 13). It is clear in the 

change map that sediment for this recovery is derived from immediately offshore and to the north of the 

island, where about 2 m of erosion occurred. Although island area increased by 300 percent since the 

devastation of Hurricane Katrina, the entire Breton platform lost 11.73 x 106 m3 since the 2007 survey 

(table 3). When comparing the subaerial areas of the DEMs, there are some discrepancies. In the earlier 

grids, land was assigned a uniform elevation of +0.50 m (Miner and others, 2009). In the 2014 grid, land 

elevations were derived from lidar. Breton Island elevations in the 2014 DEM average 0.52 m ±0.20 m. 

The uniform elevations of the land areas in the older grids compared to the actual elevations in the 

2014 grid affect volumetric comparisons; however, both the difference in averaged versus actual elevation and the number of grid nodes containing land values are very small. The departures in volumetric 

comparisons should be minimal when considering the order of magnitude in volume.

Like Breton Island, the Gosier Shoals show significant increase in elevation, derived from sediment eroded immediately offshore (fig. 13). Active migration of the shoals to the southwest was occurring, yet the Gosier platform experienced a net decrease (–8.52 x 106

 m3) in sediment over this time 

period. After Hurricane Katrina, the MRGO ship channel was decommissioned and was no longer maintained; infilling of the channel can be clearly seen in figures 13 and 14. The channel is infilling at an average rate of 1.13 x 106

 m3/yr (table 3). If the MRGO 1920–2014 volume change is an approximation of the 

material excavated to create the shipping channel, then the net deposition since 2007 suggests that within 

the study area, the channel is 73 percent infilled. Adding this volume to the 1920–2014 volume produces 

a net sediment volume within the channel of 20 x 106

 m3. The MRGO was authorized as an 11-m-deep 

shipping channel that was 152 m wide at the base (U.S. Army Corps of Engineers, 2007). Within the 

study area, the channel is 12.6 km long. These dimensions provide an excavation volume of 21 x 106

 m3, 

only a 5-percent difference from the volume measured through the seafloor change analysis.

Elevation Profile Comparison

A series of transects (figs. 15–19) parallel and perpendicular to the Breton/Gosier platforms have 

been constructed to directly represent the change in bathymetry over time (see figs. 6–9 for locations 

of transects for each time period). Transect locations were chosen from the bathymetric change models 

(figs. 10–13) to sample areas that show significant change over time.

Transects across the DEMs provide a profile comparison of the relative elevations between 

time periods. The transect (T0) along the axis of the island platform (see figs. 6–9 for location) shows 

the massive loss of island elevation across Breton’s southern half between the 1920 and 2007 periods 

(fig. 15, location A). The profiles also demonstrate the consistent landward retreat of Breton Island over 

the years (fig. 15, location B). Observation of aerial photographs taken after storms (Westphal, 2009) 

verifies that comparison between the 1920 and 2007 profiles largely represents the disintegration of the 

island following Hurricanes Ivan and Katrina, with significant erosion of the island’s flanks. Since then, 

some rollover and recovery have occurred (fig. 15, location B). In the area of the MRGO, the 2007 profile captures the designed channel, followed by infilling in 2014 (fig. 15, location C). Along the Gosier 

platform, the accretion of sediment on the terminal end of the platform (figs. 11, 12) is evident following 

1869 (fig. 15, location D), and the emergence and southward migration of Gosier Shoals are evident in 

the 2007 and 2014 profiles (fig. 15, location E). 

Onshore-offshore transect T1 (fig. 16) crosses the southern part of the Breton platform (see 

figs. 6–9 for location) and shows a consistent increase in elevation in the early years (fig. 16, loca-
19

Figure 15. Elevation profiles derived from the digital elevation models for the four time periods across the Breton/Gosier platforms and for the 

Mississippi River-Gulf Outlet (transect T0, see figs. 6–9 for location during each time period). Vertical exaggeration is 1300x.

Figure 14. Seismic subbottom profiles collected in 2007 (top) and reoccupied in 2014 (bottom) across the Mississippi River-Gulf Outlet (MRGO) 

ship channel (see fig. 13 for location) reflect the infilling of the channel after its decommission and cease in maintenance dredging following Hurricane Katrina.
20

Figure 16. Elevation profiles derived from the digital elevation models for the four time periods across the southern Breton platform and offshore (transect T1, see figs. 6–9 for location during each time period). Vertical exaggeration is 790x.

Figure 17. Elevation profiles derived from the digital elevation models for the four time periods across Breton island and the offshore shoal 

(transect T2, see figs. 6–9 for location during each time period). Vertical exaggeration is 975x.
21

Figure 18. Elevation profiles derived from the digital elevation models for the four time periods across Breton Sound, the Gosier platform, the 

Mississippi River-Gulf Outlet, and the offshore shoal (transect T3, see figs. 6–9 for location during each time period). Vertical exaggeration is 635x.

Figure 19. Elevation profiles derived from the digital elevation models for the four time periods across Breton Sound, Gosier Shoals, and 

offshore (transect T4, see figs. 6–9 for location during each time period). Vertical exaggeration is 720x.
22

tion A). Following Hurricanes Ivan and Katrina, the island platform was severely eroded, and sediments 

were transported offshore (fig. 16, location B) and, to a lesser extent, to the sound-side of the island 

platform. The high resolution of the 2014 survey captures sandwaves represented by the high-frequency 

oscillations in the profile. Comparison of the 2007 and 2014 profiles shows continued loss of elevation 

in this area, with some slight migration offshore. 

Onshore-offshore transect T2 (fig. 17) crosses the northern part of the Breton Island and platform 

(see figs. 6–9 for location). The profile shows the steady landward retreat over time and loss of elevation and eroded shorefaces following Hurricanes Ivan and Katrina (fig. 17, location A). The 2014 profile 

records the post-Katrina rollover and island emergence at this location. The sound-side of the island 

has been experiencing erosion since 1869, and comparing the 1920 profile to later years shows that 

elevation did not recover following Katrina (fig. 17, location B). The offshore shoal shows a landward 

shift over the past century and deflation in elevation since 1920 (fig. 17, location C). The area between 

the offshore shoal and island platform (Breton Gulf; table 3) accumulated 29.32 x 106 m3 of sediment 

(table 3) over the long-term (1870–2014), either from sediments derived from the offshore shoal and 

(or) erosion of the Breton platform. 

Onshore-offshore transect T3 (fig. 18) extends across the southern end of the Gosier platform 

(see figs. 6–9 for location) where extensive deposition is evident at the terminal end of the platform 

(fig. 18, location A). Although the 1920 profile does not reflect significant accretion at this location, 

examination of the long-term accretion-erosion maps (figs. 10–12) shows this deposition has been occurring since 1869 and is not due to rapid deposition during the most recent extreme storms. The profile 

crosses the MRGO ship channel, and infilling of the channel since its decommission is evident (fig. 18, 

location B). 

Onshore-offshore transect T4 (fig. 19) reflects the highly dynamic nature of the Gosier platform 

(see figs. 6–9 for location). Between the 1869 and 1920 surveys there was a decrease in elevation of the 

platform at this location with deposition along the margins (fig. 19, location A), but sediment loss was 

mainly due to transport of sediment to the terminal end of the platform (see previous discussion about 

transect T3). The 2007 profile reflects the significant transport of sediment offshore during Hurricanes 

Ivan and Katrina (fig. 19, location B) and some shedding of sediment to the sound-side of the platform. 

The 2014 profile shows dynamic storm recovery and the emergence of Gosier Shoals above sea level by 

moving sediment back onto the former platform (fig. 19, location B), ostensibly through normal wave 

action because no major storm events impacted the area during this time period. On the sound-side, adjacent to the Gosier platform, approximately 11.41 x 106

 m3 of sediment has accumulated over the long 

term (table 3).

Bathymetry and Sediment Composition

Sediment samples collected in 2014 from the seafloor throughout the study area were analyzed 

for textural characteristics (for example, size and sorting). These characteristics are dependent in part on 

elevation. Figures 20 and 21 show the 2014 bathymetry overlain with mean grain size of the sediment 

samples and sediment sorting, respectively, plotted at each sample location. Sediment grain size is represented according to the Wentworth size class (Folk, 1968), which separates the sediment sizes into sand 

and silt fractions. The results show that the Breton and Gosier platforms and the offshore shoal contain 

the largest median grain size, typically as fine sand. Very fine sand occurs mainly along the periphery of 

the Breton platform and offshore shoal, and silt samples are found predominantly in the deeper waters 

off platform and within the MRGO. Presently the MRGO is infilling primarily with fluidized mud, sedi-
23

Figure 20. Average grain size of sediment samples collected from the seafloor within the Breton National Wildlife Refuge study area, colorcoded by size (in microns), with the accompanying Wentworth size classification (medium silt through medium sand). Sample locations plotted on 

the 2014 bathymetry.

Figure 21. Sorting of sediment samples collected from the seafloor within the Breton National Wildlife Refuge study area, color-coded by the 

classification from Folk (1968). Sample locations plotted on the 2014 bathymetry.
24

ment samples acquired from within the former channel are composed of silt and clay. Two sites within 

the natural tidal inlet adjacent to the MRGO contain medium-sized sand and represent preferential sorting of the coarser sized sand by tidal energy. Figure 21 shows sorting, or the standard deviation about 

the mean grain size, for 90 percent of the distribution (Folk, 1968). The results are divided into sorting 

classifications from well sorted (includes two very well sorted samples), to very poorly sorted. Sediment 

samples from Breton and Gosier platforms are predominantly well sorted, and sediment samples from 

the offshore shoal are predominantly well to moderately well sorted. The two samples that show medium sand within the tidal inlet (fig. 20) are moderately well sorted. Samples collected in deeper waters 

range in sorting but are predominantly poorly sorted. In summary, sediments within shallower water 

depths that are affected by wave action and areas of high tidal currents have higher median grain size 

(with naturally higher sand content) and are better sorted than the deeper, muddier sediments.

Conclusions

Breton Island, the surrounding platform, and the neighboring Gosier platform to the north are 

highly dynamic systems with respect to elevation change over the past century. Two other morphologic 

features within the study area also reflect change over the study period: a shoal located 5–11 kilometers 

(km) offshore of Breton Island and the Mississippi River-Gulf Outlet (MRGO) shipping channel that 

bisects the Breton and Gosier platforms through a natural tidal channel (fig. 10). Seafloor change within 

the study area has primarily been caused by the erosion and deposition of sediments over time. The effects of land subsidence and eustatic sea-level rise are small compared to sediment dynamics over the 

145-year observation period of this study. Seafloor change occurs relative to long-term wave processes, 

short-duration/high-energy storm impacts, and anthropogenic modifications. The influence of these parameters is observed by comparing bathymetric surveys from 1869, 1920, 2007, and 2014. 

The long-term observation (1869–2014) represents all of these influences (fig. 10). The dramatic 

loss of island area, shoreface erosion, and landward rollover in island position are evident (figs. 2, 17, 

location A). This movement is accompanied by erosion and redistribution of sediment from the island 

platform to the platform margins. The Gosier platform, although experiencing some erosion in the foreshore due to rollover, is dissimilar to the Breton platform in that accretion is dominant, occurring mainly 

on the southern terminal end of the platform. Most recently, the shoals on the Gosier platform have 

emerged (fig. 15). The offshore shoal has lost elevation during the study period, with sediment moving 

to the shoal flanks (fig. 10). It does not appear that the offshore shoal has contributed sediment to the 

Breton platform over this time period, although the area between the shoal and platform appears to be 

infilling (Breton Gulf, table 3). 

The period from 1869 to 1920 represents natural morphologic change prior to the construction of 

the MRGO, and this is the only time period where the study area experienced net accretion in sediment. 

Over this half-century most of Breton Island remained relatively stationary, with only the northern half 

experiencing rollover (fig. 11). Erosion of the shoreface occurred at this location, yet the island platform prior to construction of the MRGO acquired a net gain of sediment (table 3). The Gosier platform 

accreted along its southern terminal end at roughly the same rate as during 1869–2014. The natural 

inlet prior to the MRGO experienced net loss of sediment through tidal scour, likely through increasing 

currents as the terminal end of the Gosier platform accreted into the tidal pass (fig. 11). This accretion 

continued until present while, following construction of the MRGO, the Breton platform underwent 

significant erosion (fig. 12). Similarly, the offshore shoal experienced erosion (figs. 12, 17, location C), 

losing twice as much as it gained during the previous half-century (table 3).
25

In the short-term assessments, there is clear correlation between storm impact and loss of island 

area (fig. 4), particularly the dramatic loss of area during major storms Opal, Isidore, Ivan, and Katrina. 

Sand placement adjacent to the island did increase island area following Hurricane Georges, which 

significantly impacted the Chandeleur Island chain, yet was ephemeral and completely removed by the 

next major event (Hurricane Isidore). The bathymetric change analysis indicates that during these events 

sediment is transported offshore (fig. 19, location B). In the recovery years following storm impacts, 

some of this sediment is available to replenish the shoreface through sand-bar welding to the shoreface (fig. 17, location A, and fig. 19, location B). This is observed in the final years of the study when 

land accretion occurred on Breton Island (fig. 4) and the Gosier Shoals became emergent, derived from 

sediments eroded immediately offshore (fig. 13). Maintenance dredging of the MRGO ship channel has 

ceased following decommission and has infilled since Hurricane Katrina (figs. 13, 14, 15, location C, 

and 18, location B). This restoration of the seafloor elevation to natural conditions may strongly affect 

the sediment transport processes between the Gosier and Breton platforms and offshore, and warrants 

continued monitoring of seafloor change within this dynamic system.

References Cited

DeWitt, N.T., Fredericks, J.J, Flocks, J.G., Miselis, J. L., Locker, S. D., Kindinger, J. G, Bernier, J.C., 

Kelso, K. W., Reynolds, B.J., Wiese, D. S., and Browning, T.N., 2016, Archive of bathymetry and 

backscatter data collected in 2014 nearshore Breton and Gosier Islands, Breton National Wildlife Refuge, Louisiana: U.S. Geological Survey Data Series 1005, accessed June 30, 2016, at 

http://dx.doi.org/10.3133/ds1005.

Ellis, J., and Stone, G., 2006, Numerical simulation of net longshore sediment transport and granulometry 

of surficial sediments along Chandeleur Island, Louisiana, USA: Marine Geology, v. 232, p. 115–129.

Folk, R., 1968, Petrology of sedimentary rocks: Austin, Tex., Hemphill Publishing Company, 170 p.

Frazier, D., 1967, Recent deltaic deposits of the Mississippi River, their development and chronology: 

Transactions—Gulf Coast Association of Geological Societies, v. 17, p. 287–315.

Kindinger, J.L., Buster, N.A., Flocks, J.G., Bernier, J.C., and Kulp, M.A., 2013, Louisiana Barrier Island 

Comprehensive Monitoring (BICM) program summary report—Data and analyses 2006 through 

2010: U.S. Geological Survey Open-File Report 2013–1083, 86 p. 

Lavoie, Dawn, 2009, Executive summary, in Lavoie, Dawn, ed., Sand resources, regional geology, and 

coastal processes of the Chandeleur Islands coastal system—An evaluation of the Breton National 

Wildlife Refuge: U.S. Geological Survey Scientific Investigations Report 2009–5252, p. 1–6. 

Lavoie, Dawn, Flocks, J.G., Kindinger, J.L., Sallenger, A.H., Jr., and Twichell, D.C., 2010, Effects of 

building a sand barrier berm to mitigate the effects of the Deepwater Horizon oil spill on Louisiana 

marshes: U.S. Geological Survey Open-File Report 2010–1108, 7 p.

List, J.H., Jaffe, B.E., Sallenger, A.H., Jr., Williams, S.J., McBride, R.A., and Penland, Shea, 1994, 

Louisiana Barrier Island Erosion Study: Atlas of sea-floor changes from 1878 to 1989: U.S. Geological Survey and Louisiana State University, Miscellaneous Investigations Series I–2150–B, 81 p. 

Louisiana Department of Natural Resources, 2013, FTP directory /bicm//Vol3—Bathymetry, accessed 

April 26, 2016, at ftp://ftp.dnr.state.la.us/bicm/Vol3--Bathymetry.
26

Miner, M., Kulp, M.A., Penland, S., Weathers, D., Motti, J., McCarty, P., Brown, M., Martinez, L., 

Torres, J., Flocks, J., Dewitt, N., Ferina, N., Reynolds, B.J., Twichell, D., Baldwin, W., Danforth, 

B., Worley, C., and Bergeron, E., 2009, Louisiana barrier-island comprehensive monitoring program 

(BICM)—Volume 3, Bathymetry and historical seafloor change 1869–2007—Part 1, Bathymetry 

methods and error analysis: Prepared for the Louisiana Department of Natural Resources, Coastal 

Restoration Division, by the University of New Orleans, Pontchartrain Institute for Environmental 

Sciences, New Orleans, Louisiana, 45 p. 

Morgan, K.L.M., 2015, Baseline coastal oblique aerial photographs collected from Navarre Beach, 

Florida, to Breton Island, Louisiana, September 1, 2014: U.S. Geological Survey Data Series 952, 

accessed March 31, 2016, at http://dx.doi.org/10.3133/ds952.

Morton, R., 2008, Historical changes in the Mississippi-Alabama barrier-island chain and the roles of extreme storms, sea level, and human activities: Journal of Coastal Research, v. 24, no. 6, p. 1587–1600.

Penland, S., and Boyd, R., 1981, Shoreline changes on the Louisiana barrier coast: Institute of Electrical 

and Electronics Engineers, p. 209–219.

Penland, S., Boyd, R., and Suter, J., 1988, Transgressive depositional systems of the Mississippi Delta 

Plain—A model for barrier shoreline and shelf sand development: Journal of Sedimentary Petrology, 

v. 58, no. 6, p. 932–949.

Penland, S., and Ramsey, K.E., 1990, Relative sea-level rise in Louisiana and the Gulf of Mexico—1908–1988: Journal of Coastal Research, v. 6, no. 2, p. 323–342. 

Penland, S., Westphal, K.A., Zganjar, C., Connor, P., Tao, Q., Mathies, L., Nord, B., and Gunn, R., 1997, 

Part 4: Results of monitoring the beneficial use of dredged material at MRGO – Breton Island—Base 

year 1985 thru fiscal year 1996: Beneficial Use Monitoring Program, U.S. Army Corps of Engineers, 

New Orleans District, 18 p.

Rogers, B., Kulp, M., and Miner, M., 2009, Late Holocene chronology, origin, and evolution of the St. 

Bernard Shoals, Northern Gulf of Mexico, USA: Geo-Marine Letters, v. 29, p. 379–394.

Terrano, J.F., Flocks, J.G., and Smith, K.E.L., 2015, A GIS Compilation of Vector Shorelines and Associated Shoreline Change Data for Breton Island, Louisiana: 1869–2014: U.S. Geological Survey data 

release, accessed April 22, 2016, at http://dx.doi.org/10.5066/F7XS5SGM.

Terrano, J.F., Flocks, J.G., and Smith, K.E.L., 2016a, Analysis of shoreline and geomorphic change for 

Breton Island, Louisiana, from 1869 to 2014: U.S. Geological Survey Open-File Report 2016–1039, 

34 p., accessed April 22, 2016, at http://dx.doi.org/10.3133/ofr20161039.

Terrano, J.F., Flocks, J.G., and Smith, K.E.L., 2016b, Topobathymetric lidar survey of Breton and Gosier Islands, Louisiana, January 16 and 18, 2014: U.S. Geological Survey data release, accessed April 

22, 2016, at http://dx.doi.org/10.5066/F70G3H6G.

U.S. Army Corps of Engineers, 2007, Integrated final report to Congress and legislative environmental impact statement for the Mississippi River-Gulf Outlet Deep-Draft De-Authorization Study: U.S. 

Army Corps of Engineers, New Orleans District, 147 p.
27

Westphal, K., 2009, Louisiana barrier-island comprehensive monitoring program (BICM)—Volume 1, 

Barrier-shoreline post-storm assessment—Part 4, Historic photo-pairs/time series: Prepared for the 

Louisiana Department of Natural Resources, Coastal Restoration Division, by the University of New 

Orleans, Pontchartrain Institute for Environmental Sciences, New Orleans, Louisiana, 38 p. 

Williams, S.J., Penland, S., and Sallenger, A.H., Jr., eds., 1992, Louisiana Barrier Island Erosion 

Study—Atlas of shoreline changes from 1853 to 1989: U.S. Geological Survey Miscellaneous Investigation Series I-2150-A, 103 p. 
Flocks and Terrano—Analysis of Seafloor Change at Breton Island, Gosier Shoals, and Surrounding Waters, 1869–2014, Breton National Wildlife Refuge, Louisiana—

Open-File Report 2016–1069

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20161069