Analysis of Seafloor Change around Dauphin Island, 

Alabama, 1987–2015

By James G. Flocks, Nancy T. DeWitt, and Chelsea A. Stalk

Open-File Report 2017–1112

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Acting Director

U.S. Geological Survey, Reston, Virginia: 2017 

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

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Although this information product, for the most part, is in the public domain, it also may 

contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items 

must be secured from the copyright owner.

Suggested citation: 

Flocks, J.G., DeWitt, N.T., and Stalk, C.A., 2017, Analysis of seafloor change around Dauphin Island, 

Alabama, 1987–2015: U.S. Geological Survey Open-File Report 2017–1112, 19 p., https://doi.org/10.3133/

ofr20171112.

ISSN 2331-1258 (online)
iii

Acknowledgments

This project is part of a collaborative effort between the U.S. Geological Survey (USGS), U.S. Army Corps of 

Engineers (USACE), and the State of Alabama, funded by the National Fish and Wildlife Foundation (NFWF) to 

investigate viable, sustainable restoration options that protect and restore the natural resources of Dauphin Island, 

Alabama. The authors would like to thank their collaborators at USACE-Mobile District, NFWF, State of Alabama, 

USGS Wetland and Aquatic Research Center, and the crew of the USACE S/V Irvington. We thank William 

Butler and the USACE Engineer Research and Development Center team that collected and processed the 2015 

multibeam data. We would also like to thank the USGS St. Petersburg Coastal and Marine Science Center Seafloor 

Mapping Group (Julie Bernier, Kyle Kelso, Jake Fredericks, and Max Tuten), and Jeff Collier, Mayor of the town of 

Dauphin Island for their assistance. Reviews by USGS scientists Soupy Dalyander and Kathryn Smith and edits by 

Betsy Boynton and Marilyn Billone greatly improved this report.
iv

Contents

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

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

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

Results and Discussion .............................................................................................................................................10

Long-Term Change (1987–2015)...........................................................................................................................10

Stormy Period Change (1987–2006).....................................................................................................................12

Non-stormy Period Change (2006–2015)..............................................................................................................12

Elevation Profiles ...................................................................................................................................................13

Temporal and Spatial Sediment Flux .....................................................................................................................17

Conclusion.................................................................................................................................................................18

References Cited.......................................................................................................................................................19

Figures

1. Maps showing the regional location of Dauphin Island, and key features discussed in this study................1

2. Extensive shoreline erosion, overwash, breaching, and damage to the infrastructure at Dauphin Island

during Hurricane Katrina in 2005...................................................................................................................2

3. Digital elevation model generated from 2015 bathymetric data ....................................................................3

4. Trackline map showing survey extent and coverage of single beam and multibeam systems collected in

July and September, 2015.............................................................................................................................6

5. Trackline map showing survey extent, coverage, and survey identification from 1987–1988.......................6

6. Trackline map showing survey extent, coverage, and survey identification from 2006.................................7

7. Digital elevation model generated from the National Oceanic and Atmospheric Administration 1987−1988

bathymetric data............................................................................................................................................7

8. Digital elevation model generated from the National Oceanic and Atmospheric Administration 2006

bathymetric data............................................................................................................................................8

9. Map showing multidecadal elevation change determined by calculating the difference in digital elevation

models between 1987 and 2015 ...................................................................................................................8

10. Map showing 19-year elevation change determined by calculating the difference in digital elevation models

between 1987 and 2006, referred to as the stormy period............................................................................9

11. Map showing 9-year elevation change determined by calculating the difference in digital elevation models

between 2006 and 2015, referred to as the non-stormy period ....................................................................9

12. Elevation profiles across the Petit Bois ebb-tidal delta for the three time periods.......................................14

13. Elevation profiles from Mississippi Sound, across the island breach and into the Gulf of Mexico, for

the three time periods..................................................................................................................................14

14. Elevation profiles along the gulf side of Pelican Island for the three time periods ......................................15

15. Elevation profiles across the Mobile ebb-tidal delta for the three time periods ...........................................16
v

16. Elevation profiles across the gulf side of Dauphin Island for the three time periods ...................................16

17. Elevation profiles across the sound side of Dauphin Island for the three time periods ...............................17

18. Rates of change for the two time periods and long-term for each reference subsection ............................18

Tables

1. Tropical storms passing within 185 km of Dauphin Island since 1987, with major impacts highlighted........5

2. Accretion and erosion volumes, net change, and rates of change for reference subsections of

morphological features/areas of submerged areas around Dauphin Island................................................ 11
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)

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88, GEOID12A), and converted to 

MLLW for analysis purposes. Units of all vertical measurements are in meters.

Horizontal coordinate information is referenced in the geographic coordinates World Geodetic System of 1984 (WGS 84); however, data 

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

Abbreviations

DEM digital elevation model

DGPS Differential Global Positioning System

GMT Generic Mapping Tools

GPS Global Positioning System

Lidar light and detection and ranging

MLLW mean lower low water

NAVD North American Vertical Datum

USGS U.S. Geological Survey

UTM Universal Transverse Mercator

WGS World Geodetic System
Analysis of seafloor change around Dauphin Island, 

Alabama, 1987–2015

By James G. Flocks, Nancy T. DeWitt, and Chelsea A. Stalk

Introduction

Dauphin Island is a 26 km-long barrier island located southwest of Mobile Bay, Alabama, in 

the north-central Gulf of Mexico (fig. 1). The island contains sandy beaches, dunes, maritime forests, 

freshwater ponds and intertidal wetlands, providing habitat for many endangered and threatened species. Dauphin Island also provides protection for and maintains estuarine conditions within Mississippi 

Sound, supporting oyster habitat and seagrasses. Wetland marshes along the Alabama mainland are 

protected by the island from wave-induced erosion during storms approaching from the Gulf of Mexico. 

Over the years, the island has been eroded by storms, most recently by Hurricane Ivan (2004) and Hurricane Katrina (2005) (Ivan/Katrina), which breached the island along its narrowest extent and caused 

damage to infrastructure (fig. 2). Along with storms producing significant episodic change, long-term 

beach erosion has exposed numerous pine tree stumps in the shoreface. The stumps are remnants of past 

maritime forests and reflect the consistent landward retreat of the island (Parker and others, 1997).

Figure 1. Maps showing the regional location of Dauphin Island (inset), and key features discussed in this study. The polygon (outlined in purple) represents the extent of the 2015 bathymetric survey. The background satellite image is from the 2014 

U.S. Geological Survey Landsat 8.

30°24’

30°12’

-88°24’ -88°12’ -88°00’

0 5 10 KILOMETERS

0 2.5 5 NAUTICAL MILES

Mississippi Sound

Gulf of Mexico

Petit Bois Island

Petit Bois

Pass

Mobile

Pass

Dauphin Island

Pelican

Island
2

Island change has prompted the State of Alabama to evaluate restoration alternatives to increase 

island resilience and sustainability by protecting and preserving the natural habitat, and by understanding the processes that influence shoreline change. Under a grant from the National Fish and Wildlife 

Foundation (NFWF), restoration alternatives are being developed that will allow the State to make 

decisions on engineering and ecological restoration designs based on scientific analysis of likely outcomes and tradeoffs between impacts to stakeholder interests. Science-based assessment of the coastal 

zone requires accurate and up-to-date baseline data to provide a valid image of present conditions and to 

support modeling of coastal processes. Bathymetric elevation measurements are essential to this requirement. In August 2015, the U.S. Army Corps of Engineers (USACE) and the U.S. Geological Survey 

(USGS) conducted single beam and multibeam bathymetric surveys around Dauphin Island using a 

variety of shallow draft vessels and equipment. More than 95 square kilometers (km2) of seafloor was 

imaged. The data were integrated into a seamless digital elevation model (DEM) to provide a highresolution bathymetric map of the seafloor (fig. 3) extending 9.5 km seaward from the island’s eastern 

end and approximately 2 km along the rest of the island on the gulf and sound sides. Water depths range 

from 0.3–15.0 meters (m), with depths greater than 10.0 m constrained to the Mobile ship channel on 

the extreme eastern flank of the coverage.

To measure seafloor change, two periods of historic hydrographic survey data were acquired 

from the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information data archive. The two timeframes (1987–1988 and 2005–2007) were selected for their 

Figure 2. Extensive shoreline erosion, overwash, breaching, and damage to the infrastructure at Dauphin Island during 

Hurricane Katrina in 2005. The view is of the central portion of the island looking west. The image was taken August 31, 2005, 

U.S. Geological Survey post-storm aerial oblique photography (https://coastal.er.usgs.gov/hurricanes/katrina/post-stormphotos/obliquephotos.html).

Overwash

Breach

Gulf of Mexico

Mississippi

Sound
3

completeness of spatial coverage and because they encompass a period of significant storm impacts to 

the island. These timeframes were compared to each other and with the 2015 dataset to monitor elevation gain (sediment accretion) and elevation loss (sediment erosion) over time. Sediment dynamics is by 

far the most significant driver of nearshore elevation change in this area. The Mississippi-Alabama inner 

shelf is a passive margin (Flocks and others, 2011), and other influences on elevation change (for example tectonic adjustment, Holocene subsidence, and eustatic sea-level rise) are neither significant nor 

variable enough over this time period to have an imprint.

Description of Study Area

Dauphin Island is typically characterized in east and west segments based on geomorphology 

(fig. 3). The eastern quarter of the island is up to 2 km wide with elevations that exceed 8 m. This part of 

the island rests on a core of hardened Pleistocene barrier ridge-deposits that became the locus of sediment deposition at the beginning of island evolution (Otvos and Giardino, 2004). In contrast, the western three quarters of the island is narrow (< 500 m) and consists of low-elevation (<4-m) sandy dunes 

that are subject to frequent overwash and breaching (fig. 2). In the submerged environment, seaward 

of the eastern end of the island is the highly dynamic Mobile Bay ebb-tidal delta, which extends approximately 10 km seaward of the island (fig. 3). On the western flank of the delta is a region of shoals 

and ephemeral islands that reflect the net westward transport of sand along the periphery of the ebbtidal delta shield, driven by a prevailing southeast wave approach and tidal-flow dynamics (Byrnes and 

others, 2008). Pelican Island (fig. 1), also referred to as Sand Island, is a shoal that has been migrating 

toward and appending to Dauphin Island over the past century. (It is presently appended to Dauphin 

Island, adjacent to transect T3 in fig. 3.) West of these features, along the length of the island, the shoreface and inner shelf are comparatively featureless, with a gentle (0.6 degree) seaward slope. This morphology is consistent with most of the Mississippi-Alabama inner shelf at water depths less than 20 m 

(Flocks and others, 2011). 

Figure 3. Digital elevation model (DEM) generated from 2015 bathymetric data. Overlain on the DEM are transect locations 

(T1–T6) used to represent vertical change over time.

30°15’

30°12’

-88°19’ -88°16’ -88°12’ -88°08’ -88°05’

0 2.5 5 KILOMETERS

0 1 2 NAUTICAL MILES

Elevation in MLLW. Shoreline derived from lidar

0

−2

−4

−6

−7

−9

−11

EXPLANATION

2015 depth (m)

2015 shoreline

T1

T2

T3

T4

T5

T6
4

The western end of the island terminates at the Petit Bois Pass (fig. 1) and its ebb-tidal delta. 

The pass formed as a breach in the island during an unnamed hurricane in the mid-18th century (Otvos 

and Carter, 2013), separating Petit Bois Island from Dauphin Island (fig. 1). Since this breach, Petit 

Bois Island has been migrating westward, widening the pass and expanding the ebb-tidal delta deposits 

seaward (Flocks and others, 2015). As with the inlet at the eastern end of island, tidal dynamics control 

sediment transport processes within the Petit Bois Pass.

The sound side of Dauphin Island is composed of a narrow (~ 0.5 km) island platform less than 

2 m deep that slopes into Mississippi Sound (fig. 3). Water depth within the sound ranges from 2.5 to 4.5 

m. The Intracoastal Waterway Ship Channel passes through the sound just beyond the 2015 bathymetric 

coverage and is not included in this investigation. Flood-tide delta and storm overwash deposits are visible in the bathymetry at the aforementioned breach in the central part of the island (fig. 3). The breach 

occurred during Hurricane Ivan (2004) and was significantly widened by Hurricane Katrina the following year (fig. 2). In 2011 a rock structure was added to close the breach and, subsequently, longshore 

transport has begun to build out a beach in front of the structure.

Historical wave climate measured from a buoy approximately 54 km southeast of Dauphin Island 

(NOAA National Data Buoy Center Station 42012) indicates a predominantly southeast wave approach 

(Flocks and others, 2015), directing sediment transport from east to west along the island shoreface. Over 

the past half century, Byrnes and others (2008) estimated that approximately 4.6x104 cubic meters per 

year (m3/yr) of sediment was transported from the Mobile ebb-tidal delta west to Dauphin Island. Over the 

same time period, 2.4x105 m3/yr of sediment was eroded from the middle and western portion of the island 

(Byrnes and others, 2008). This suggests that five times the amount of sediment is eroded from the island 

shoreface than is delivered each year. Steady longshore sediment-transport rates and volumes are punctuated by storm impacts which rapidly erode sand from the beach and shoreface. Since 1987, 14 named 

storms with tropical storm strength or greater passed within 185 km (100 nautical miles) of Dauphin 

Island (table 1). Storm surges up to 3 m (Hurricane Ivan) caused shoreface erosion, island overwash, and 

breaching. Since Hurricane Katrina in 2005, only two storms have passed within 185 km of the islands 

(table 1), in general reflecting a period of low storm activity in the northern Gulf of Mexico in general.

Methods

A bathymetric survey was conducted in 2015 using a suite of acoustic systems and platforms; 

tracklines of coverage are shown in figure 4. In July 2015, the USGS collected single-beam bathymetry 

in the shallow waters around the island. For a complete description of the methodology used to collect 

these data see DeWitt and others (2017). Shallow draft vessels, including personal watercraft equipped 

with single-beam acoustic systems and Digital Global Positioning System (DGPS), were used to access 

these areas. Positioning was corrected using DGPS base stations installed over geodetic benchmarks located on the island. Variable sound velocity within the water column was corrected using periodic casts 

of portable sound-velocity profilers. Boat heave, pitch, and roll were compensated using an internal motion reference system, whereas the personal water craft used a very high GPS sampling rate (0.1 second) 

and a narrow (4 degree) transducer beam angle to compensate for motion.

In September 2015, the USACE Engineer Research and Development Center completed multibeam surveys in deeper water (up to 6-m water depth) around the Petit Bois Pass and sound-side of the 

island using a similar methodology (William Butler, oral comm.), while a joint USACE/USGS survey 

occupied the deeper waters on the gulf side of the island using the USACE survey vessel Irvington

equipped with a multibeam system. These data, along with 2015 lidar elevations of the shoreline acquired by the USGS, were integrated to generate a DEM of the coastal zone (fig. 3).
5

All of the bathymetric data was processed in the World Geodetic System (WGS84) with elevation control in the North American Vertical Datum NAVD88 (GEOID12A), and converted to Universal 

Transverse Mercator (UTM) zone 16 and mean lower low water (MLLW) for comparison with legacy 

bathymetric datasets. The merged DEM extends landward to include the shoreline, a zero-meter elevation contour extracted from a 2015 USGS topographic-lidar survey of Dauphin Island. Once the various 

datasets were merged, a 5-m running mean was applied across the data to avoid aliasing short wavelengths. The blockmean function in the Generic Mapping Tools (GMT, ver. 5) suite of data manipulation tools was used for this process. The resulting data file was then gridded to 50-m grid cells 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 (fig. 4, extent of analysis). The root-mean-square (RMS) error for the grid relative to the soundings 

across acquisition platforms averaged 0.022 m. 

To monitor seafloor change over time, trackline data points from the 1987–1988 and 2005–2007 

time periods were downloaded from the NOAA National Centers for Environmental Information website (https://maps.ngdc.noaa.gov/viewers/bathymetry/). Data coverages and identification numbers 

for these time periods are shown in figures 5 and 6. The data were converted from WGS84 to UTM 

zone 16 (meters) for volume estimations. Dauphin Island shorelines for 1987 and 2006 were digitized 

from Landsat satellite imagery (Guy, 2015), assigned a zero elevation, and merged with the bathymetric data. As with the recent dataset, a 5-m running mean was applied to each data merge, which were 

then gridded using the GMT surface algorithm at the same range and grid spacing as the 2015 DEM. 

The resulting DEMs are shown in figures 7 and 8. The RMS error comparing the DEM to original data 

Table 1. Tropical storms passing within 185 km (100 nm) of Dauphin Island since 1987, with major impacts highlighted. 

Dashed line separates the 1987–2006 and 2006–2015 time periods. Storm data extracted from the National Oceanic and 

Atmospheric Administration Historical Hurricane Track Tools v. 4.0 (https://coast.noaa.gov/hurricanes/).

[KM: kilometers; MB: millibars; KTS: knots; M: meters]

Name Date

Closest 

distance

KM

Category

Central 

pressure

MB

Central

wind speed

KTS

Wind speed 

at Dauphin

KTS

Surge at 

Dauphin

M*

Ida Nov 2009 8 TS 998 45 30 0.8

Claudette Aug 2009 166 TS 1005 40 23 -

Katrina Aug 2005 136 H3 925 107 66 2.1

Dennis Jul 2005 135 H3 942 110 44 0.9

Cindy Jul 2005 26 TS 995 45 44 -

Arlene Jun 2005 74 TS 991 50 34 0.8

Ivan Sep 2004 34 H3 946 105 79 2.9

Hanna Sep 2002 35 TS 1003 50 36 1.1

Georges Sep 1998 57 H2 964 90 37 1.6

Earl Sep 1998 176 H2 988 85 45 0.3

Danny Jul 1997 15 H1 984 70 63 1.8

Opal Oct 1995 98 H3 940 105 53 0.8

Erin Aug 1995 110 H2 973 85 36 -

Alberto Jul 1994 154 TS 993 55 21 -

*Negligible or unknown surge marked as (-).
6

is 0.098 m and 0.080 m for the 1987 and 2006 grids, respectively. Areas where bathymetric data were 

missing from the legacy datasets were masked and assigned NULL values in their respective DEMs. 

The DEMs were clipped to the extent of the 2015 survey, and certain areas of the legacy grids were 

masked because of insufficient or no data (gray background in figs. 5 and 6). Once the three DEMs were 

sampled to the same geographic extents and vertical datum (MLLW), the bathymetric change between 

each period was determined by subtracting the older period from the more recent period using the 

30°12’

30°15’

-88°21’ -88°15’ -88°09’ -88°03’

0 2 4 KILOMETERS

0 1 2 NAUTICAL MILES

EXPLANATION

2015 bathymetric trackline

2015 shoreline (lidar)

Extent of analysis

Figure 4. Trackline map showing survey extent and coverage of single beam and multibeam systems collected in July and 

September, 2015.

-88°18’ -88°12’ -88°06’

0 2 4 KILOMETERS

0 1 2 NAUTICAL MILES

EXPLANATION

1987 bathymetric trackline

1987 Shoreline (landsat)

No 1987 data (masked)

Extent used in analysis

H10226

(1988)

D00078

(1987)

H20247

(1987)

30°12’

30°15’

Figure 5. Trackline map showing survey extent, coverage, and survey identification from 1987–1988. National Oceanic 

and Atmospheric Administration hydrographic survey data (https://maps.ngdc.noaa.gov/viewers/bathymetry/).
7

grdmath function in GMT. This calculation provides an isopach grid of erosion (negative values) and accretion (positive values). To account for measurement uncertainty, isopach differences between –0.25 m 

and +0.25 m were considered within the error of analysis and set to zero (no change). The resulting 

time periods (2015–1987, 2006–1987, and 2015–2006) with erosion/accretion isopach maps are shown 

in figures 9–11. Bathymetric change between time periods can also be directly compared along twodimensional transects (transects T1–T6 shown in figs. 3, 7, and 8) across the DEMs with stacked results 

in profile to show relative elevations. These profiles were restricted to the areas of common coverage 

across time periods and were extracted from the DEMs using the transit plug-in included with the QGIS 

GIS (ver. 2.18) software.

Figure 6. Trackline map showing survey extent, coverage, and survey identification from 2006. National Oceanic and Atmospheric Administration hydrographic survey data (https://maps.ngdc.noaa.gov/viewers/bathymetry/).

Figure 7. Digital elevation model (DEM) generated from the National Oceanic and Atmospheric Administration 1987−1988 

bathymetric data. Overlain on the DEM are transect locations (T1–T6) used to represent vertical change over time.

30°15’

30°12’

-88°19’ -88°12’ -88°05’

0

-2

-4

-6

-7

-9

-11

No data

EXPLANATION

1987 depth (m)

1987 shoreline

0 2.5 5 KILOMETERS

0 1 2 NAUTICAL MILES

Elevation in MLLW.

Shoreline derived from Landsat imagery

T2

T1

T3

T4

T5

T6
8

Area and volume change between the time periods was determined using the grdvolume function in GMT, which calculates area, volume, and volume per unit area for a provided polygon within a 

DEM. The polygons delineate areas of interest within the study area such as geomorphic features. For 

this analysis, the Mobile Bay ebb-tidal shoal (A), the Pelican Island shoreface (B), the Dauphin Island 

gulf shoreface (C), the Petit Bois Pass tidal shoal (D), and the Dauphin Island/Mississippi Sound (E) 

(figs. 9–11) were selected as important geomorphic features to compare for relative change around the 

0 2.5 5 KILOMETERS

0 1 2 NAUTICAL MILES

Elevation in MLLW. Shoreline derived from Landsat imagery

30°15’

30°12’

-88°19’ -88°12’ -88°05’

0

-2

-4

-6

-7

-9

-11

No data

EXPLANATION

2006 depth (m)

2006 shoreline

T1

T2

T3

T4

T5

T6

Figure 8. Digital elevation model (DEM) generated from the National Oceanic and Atmospheric Administration 2006 

bathymetric data. Overlain on the DEM are transect locations (T1–T6) used to represent vertical change over time.

Figure 9. Map showing multidecadal elevation change determined by calculating the difference in digital elevation models 

(DEMs) between 1987 and 2015. The change is considered to represent accretion (positive change) and erosion (negative 

change) over the time period. Elevation differences within ±0.25 meter (m) are considered no change. Overlain onto the DEM 

are polygons (labeled A–E) that represent morphological cells from which volume change statistics are calculated (table 2).

30°17’

30°14’

30°12’

-88°17’ -88°10’ -88°04’

0 2 4 KILOMETERS

0 1 2 NAUTICAL MILES

A

B

C

D

E

1987 shoreline

2015 shoreline

No 1987 data

1987 – 2015 Change (m)

–4.0

–2.0

2.0

No change

4.0

EXPLANATION
9

island. For effective volumetric change comparisons, only areas that contain bathymetric coverage for 

all three time periods were considered. The areas encompassed by the polygons are referred to as reference subsections of their respective feature.

Figure 11. Map showing 9-year elevation change determined by calculating the difference in digital elevation models 

(DEMs) between 2006 and 2015, referred to as the non-stormy period. The change is considered to represent accretion (positive change) and erosion (negative change) over the time period. Elevation differences within ±0.25 meter (m) are considered 

no change. Overlain onto the DEM are polygons (labeled A–E) that represent morphological cells from which volume change 

statistics are calculated (table 2).

Figure 10. Map showing 19-year elevation change determined by calculating the difference in digital elevation models 

(DEMs) between 1987 and 2006, referred to as the stormy period. The change is considered to represent accretion (positive 

change) and erosion (negative change) over the time period. Elevation differences within ±0.25 meter (m) are considered no 

change. Overlain onto the DEM are polygons (labeled A–E) that represent morphological cells from which volume change 

statistics are calculated (table 2).

-88°20’ -88°13’ -88°07’

0 2 4 KILOMETERS

0 1 2 NAUTICAL MILES

30°17’

30°14’

30°12’

1987 shoreline

2006 shoreline

No data

1987 – 2006 Change (m)

–4.0

–2.0

2.0

No change

4.0

EXPLANATION

A

B

C

D

E

30°17’

30°12’

–88°19’ –88°12’ –88°05’

0 2 4 KILOMETERS

0 1 2 NAUTICAL MILES

2006 shoreline

2015 shoreline

No 2006 data

2006 – 2015 change (m)

–4.0

–2.0

2.0

No change

4.0

EXPLANATION

A

B

C

D

E
10

Results and Discussion

Morphologic features seen in the three time periods and referenced in this study include the 

highly dynamic Mobile and Petit Bois Pass ebb-tidal deltas and Pelican Island. In contrast, the Gulf of 

Mexico and Mississippi Sound sides of Dauphin Island contain few morphologic features, with the gulf 

side seafloor sloping gently seaward and the sound side being relatively flat. Other features include the 

breach in the island and associated overwash deposits that are visible in the 2006 DEM (fig. 7). Storm 

breaching has occurred in the same general vicinity in the past (Byrnes and others, 2010), reflecting the 

vulnerability of narrow island width and low elevations to storm inundation.

 The three time periods (1987–2015; 1987–2006; and 2006–2015) represent multi-decadal 

change, impact of Hurricanes Ivan/Katrina, and recovery following Hurricanes Ivan/Katrina, respectively, and will be discussed in the context of these periods and events. It should be noted that other significant storms impacted the island during the early (1987–2006) time period, such as Hurricane Danny 

(1997), and Hurricane Georges (1998); however, since 2006, there has been relatively few significant 

storm impacts to the island (table 1). Thus, the early (1987–2006) and late (2006–2015) timeframes can 

be considered “stormy” and “non-stormy”, respectively. In the 20 years before 1987, only four tropical 

storms passed within 185 km (100 nm) of Dauphin Island (one of which, Hurricane Frederic, passed 

directly over the island as a category 4 storm). Relative to this prior two-decade period of 4 storms, the 

1987–2006 timeframe (12 storms in two decades) can be considered exceptionally stormy.

The five areas of analysis (reference subsections A–E), where all three time periods contain 

seafloor measurements, are shown as polygons for each time period in figures 9–11. From these subsections sediment volumes were calculated and compared to assess volumetric change over time (table 2). 

Finally, six two-dimensional shore-perpendicular transects at various locations are compared over the 

three time periods to evaluate vertical change (locations shown in figs. 3, 7, and 8). The profiles will be 

discussed following the results of the volumetric change assessment.

Long-Term Change (1987–2015)

Over the three decades from 1987 to the present, barrier island retreat and breaching is evident in 

the long-term (1987–2015) comparison, as well as the occurrence of the breach in the middle of the island (fig. 9). In general, seafloor elevations around Dauphin Island have declined, with only the Mobile 

Bay and Petit Bois ebb-tidal delta regions producing net accretions of sediment (table 2). Mobile Bay 

ebb-tidal delta experienced only a slight net accretion. Sediment volumes increased 2 percent within 

the reference subsection (A) at a rate of 3.7x103 m3/yr. This rate is relatively negligible when compared 

to change rates in other areas, suggesting the Mobile ebb-tidal delta cell is in equilibrium. Sediment 

transported westward to Pelican Island and beyond is replaced by sediment migrating from the eastern 

lobe of the Mobile ebb-tidal delta, which is separated from the western lobe by the Mobile Outer Bar 

ship channel. The ebb-tidal delta retains equilibrium despite large volumes of sediment being dredged 

from the ship channel and removed offshore. Byrnes and others (2008) estimate that between 1990 and 

2006, 10.8x106 m3 of sediment had been removed from the ship channel. Some of the historical offshore 

disposal areas are immediately offshore of the ebb-tidal delta and appear to be supplying sediment back 

to the western side of the Mobile ebb-tidal delta system (Byrnes and others, 2008).

The gulf-facing shoreface of Pelican Island has experienced the most change in elevation over the 

long-term time period as the island rapidly migrated landward and appended to Dauphin Island (fig. 9). 

The reference subsection (B) has lost 49 percent in sediment volume (table 2) as sediment moved out of 

the zone through shoreface erosion and sediment transport, both along the shoreline of Dauphin Island 
11

and rollover into the Mobile ebb-tidal delta. As a result, it has experienced the largest loss in volume per 

unit area (–0.38 m) of any of the cells. The adjacent Dauphin Island cell (C) received sediment transported 

from Pelican Island, resulting in accretion over the long term on the east edge of the cell (fig. 9). This 

positive budget is offset by shoreface erosion that increases westward as the long, narrow portion of the 

island migrated landward from 77 m on the east end to 164 m on the west end between 1987 and 2015. 

At the position of the 1987 shoreline, 3–4 m of vertical elevation has been lost. The gulf-facing shoreline 

reference subsection (C) lost about 530x103 m3/yr (table 2), the highest rate of any cell. Most of this loss 

occurred along the immediate shoreline, but erosion also occurred offshore across the western half of the 

island (fig. 9). Much of the sediment eroded from the gulf shoreface of Dauphin Island was transported 

through littoral transport westward to the island’s terminal spit and Petit Bois Pass ebb-tidal delta, which 

is reflected both by the extension of the island and accretion offshore (fig. 9). From 1987–2015, the volume in the Petit Bois Pass reference subsection (D) increased by 79 percent, or +28.3x103

 m3/yr, building 

shoreface shoals up to 5 m high. This influx of sediment has greatly expanded the Petit Bois Pass ebb-tidal 

delta over the past century through tidal-driven sediment accretion (Flocks and others, 2015).

In Mississippi Sound, the only appreciable gain of sediment in the reference subsection (E) occurred at the breach through storm overwash (fig. 9). This excludes deposition at the immediate shoreline through island migration because the reference subsection, in general, begins 200 m offshore (figs. 

9–11; no 2006 data available at the shoreline). The rest of the area (E) uniformly lost 0.4 to 1.0 m in 

elevation at a rate of –394x103 m3/yr. There appears to be some shoreface erosion and subsequent deposition further into the sound, which will be explored further by comparing elevation profiles later in this 

discussion.

Table 2. Accretion and erosion volumes, net change, and rates of change for reference subsections of morphological 

features/areas of submerged areas around Dauphin Island. (Areas are shown for each time period as lettered polygons in 

figures 9–11.)

[m3 cubic meter; m3/yr, cubic meter per year; GOM, Gulf of Mexico; MS, Mississippi Sound; ETD, ebb-tidal delta]

Time period Interval

years

Feature/area Accretion

(105 m3)

Erosion

(105 m3)

Change

(103 m3)

Rate of change

(103 m3/yr)

1987–2015 28 Mobile ETD (A) 50.10 −49.06 103 3.69

Pelican Is. (B) 21.53 −44.19 −2,266 −80.92

Dauphin GOM (C) 2.06 −150.53 −14,847 −530.24

Petit Bois (D) 18.08 −10.16 791 28.26

Dauphin MS (E) 5.32 −115.64 −11,031 −393.97

1987–2006 19 Mobile ETD (A) 20.05 −23.49 −344 −18.11

Pelican Is. (B) 14.00 −38.73 −2,473 −130.17

Dauphin GOM (C) 0.77 −99.31 −9,854 −518.62

Petit Bois (D) 18.40 −6.78 1,162 61.15

Dauphin MS (E) 4.92 −71.13 −6,621 −348.47

2006–2015 9 Mobile ETD (A) 37.20 −28.69 851 94.56

Pelican Is. (B) 14.41 −11.25 315 35.05

Dauphin GOM (C) 3.92 −31.32 −2,740 −304.45

Petit Bois (D) 4.53 −7.40 −287 −31.86

Dauphin MS (E) 8.15 −31.15 −2,300 −255.55
12

Stormy Period Change (1987–2006)

During the 19-year time period ending in 2006, Dauphin Island was impacted by 12 storms, 4 of 

which had significant impact on the island (table 1), including the breach. Although the Mobile Bay ebbtidal delta experienced net accretion over the long term, during this time period a net loss of sediment 

occurred. Over twice as much sediment was removed from the reference subsection (A) during this time 

period than was gained long-term (table 2), but at a relatively low rate (−18x103 m3/yr) when compared 

to other areas around the island. Although removal of sediment from the system during storm impact 

is a large driver of the loss, approximately 7.0x105 m3/yr of sediment was dredged from the Mobile 

ship channel during this time period and placed in offshore sites (USACE, 2016), likely reducing the 

net sediment available to migrate westward into the study area. Accretion of sediment on the western 

flank of the Mobile cell occurred during rollover of Pelican Island (fig. 10), noting that coverage of this 

change is likely incomplete given that some of this area was not captured during the 2006 survey.

Between 1987 to 2006, Pelican Island has rotated counterclockwise so that the southern tip has 

migrated landward approximately 350 m, and the northern tip has moved seaward about 60 m. The 

island had also accreted approximately 650 m towards Dauphin Island (fig. 10) at the expense of the 

southern end. This movement has resulted in up to 3.8 m vertical loss in the southern portion of the 

Pelican Island reference subsection (B) and up to 3 m vertical gain adjacent to Dauphin Island. Net 

change in the Pelican Island reference subsection over this time period has been a loss of 2.5x106 m3, or 

−130x103 m3/yr (table 2), as sediment has been removed from the system both through littoral transport 

and storm impact. Westward, along Dauphin Island, the most dramatic change was the formation of the 

breach, while the rest of the island experienced 1–3.25-m elevation loss at the shoreface through erosion and landward migration (50 m east end to 95 m west end). The rate of loss (5.2x105

 m3/yr) within 

the reference subsection (C) over this time period is the highest of any time period in any subsection, 

and is a result of littoral transport westward and wave erosion during storms. In contrast, the Petit Bois 

Pass ebb-tidal delta reference subsection (D) experienced a sizable gain of 6.1x104 m3/yr over this time 

period. The amount of accretion and rate of gain, although by far the largest of any cell during any time 

period (table 2), is only 12 percent of the rate of loss along the adjacent gulf-facing Dauphin shoreline, 

suggesting episodic storm processes dominated nonstorm littoral transport along the island shoreface.

The Mississippi Sound side of Dauphin Island maintained a very high net loss rate that is very 

consistent with the long-term loss rate (table 2). Vertical erosion is fairly uniform throughout the subsection, the highest elevation loss (~1 m) occurs 1+ km from the breach (fig. 10). Since the 2006 bathymetric 

dataset does not capture the shoreface on the sound side, the elevation gain of only a portion of overwash 

deposit at the breach can be measured and ranges from +0.3 to +1.0 in elevation between 1987 and 2006.

Non-stormy Period Change (2006–2015)

Since 2006, only two tropical storms passed within 184 km of Dauphin Island (table 1), thus 

normal (for example, non-storm) littoral processes are expected to be the dominant mode of sediment 

transport over the past decade. The Mobile ebb-tidal delta reference subsection (A) experienced a large 

amount of accretion during this period and the highest rate of gain of any subsection over any time 

period. Deposition more than doubled what had been lost over the previous period (table 2). Most of 

the elevation gain occurred within the northwest part of the subsection and is likely a result of littoral 

sediment trapping due to the welding of Pelican Island to Dauphin Island (fig. 11). Some rollover of 

the southern end of Pelican Island, although reduced from the previous (stormy) time period, may also 

contribute to the sediment surplus.
13

The Pelican island reference subsection (B), which had significant erosion in the stormy time 

period, reversed loss and experienced almost an equal amount of accretion at a rate of 

+35.1x103 m3/yr (table 2). The submergence of the southern portion of Pelican Island (fig. 11) likely 

contributed to sediment deposition within the reference subsection, and storm-induced rollover into the 

Mobile ebb-tidal delta cell has been reduced. West of where Pelican and Dauphin Islands merged, the 

remainder of Dauphin Island continued to thin through shoreface erosion, up to 100 m on the western 

end. Offshore sandbars migrated landward and welded to the shoreface, reducing seafloor elevations by 

up to 2 m approximately 200 m offshore, and increasing elevations by 1 m at the shoreline. The storm 

breach at the center of the island was closed by rock during this time period (2011), and there is approximately 2 m of accretion seaward of the structure. As throughout all time periods, the gulf side of 

Dauphin Island continued to lose sediment, although at almost half the rate of the previous time period 

(table 2), reflecting reduced storm-related erosion processes acting on the system. Within the Petit Bois 

Pass ebb-tidal delta, linear areas of erosion adjacent to areas of deposition are consistent with westward 

shoal migration (fig. 11). The shoals are 100–500 m in length and up to 2 m in height (fig. 3). The terminal spit has accreted 680 m westward from its 2006 position and littoral transport has contributed up 

to 3.5 m in elevation along the shoreface at the westernmost tip. Although the Petit Bois Pass reference 

subsection (D) had accreted substantially in the previous time period, during this period the net change 

was negative, accumulating 25 percent less sediment than was deposited during the prior decade. This 

suggests that littoral transport of sediment from Dauphin Island to Petit Bois Pass during non-stormy 

conditions is substantially less than what is liberated and transported during stormy conditions, and not 

enough to maintain equilibrium at this location without episodic deposition.

Like the gulf side of Dauphin Island, the sound side experienced erosion in all periods of analysis, although substantially less loss occurred during the 2006−2015 period than during the prior period. 

Rate of erosion in the reference subsection (E), although negative, decreased 65 percent from the stormy 

period (table 2). Elevation loss across the sound side was generally a uniform 0.2–0.3 m. Prior to the 

breach being closed in 2011, floodtide deposits through the former inlet increased elevations by 1 m 

above overwash elevations approximately 300–400 m north of the structure. This is the only area in 

this cell that shows net accretion (fig. 11). Much of the shoreface on the sound side was not surveyed in 

2006, but elevation change analysis from an area at the western tip of the island that was covered suggests erosion occurred all the way to the shoreline.

Elevation Profiles

Six transects (figs. 3, 7, and 8)—four shore perpendicular and two shore parallel—were chosen to compare elevation change across the 1987, 2006, and 2015 time periods. Transect T1 (fig. 12) is 

shore perpendicular and extends across the eastern margin of the Petit Bois Pass ebb-tidal delta. Sand 

ridges are visible as peaks in the elevation profiles, and comparison of the profiles over time show the 

sand ridges growing and migrating landward (fig. 12, location A). Shoreface accretion between 1987 

and 2006 is reflected by the seaward migration of the profile, but this elevation gain was lost by 2015 

(fig. 12, location B). Offshore, a divergence in the profiles over time represents the growth and movement of the ebb-tidal delta (fig. 12, location C).

Transect T2 (fig. 13) extends from Mississippi Sound south across the breach in the island and into 

the Gulf of Mexico (see figs. 3, 7, and 8 for location). Persistent erosion is seen in the loss of elevation 

across both the sound and the gulf (fig. 13, locations A and B), along with shoreface retreat (fig. 13, location B). The offshore and shoreface appear to have stabilized between 2006 and 2015. The island breaching 

between 1987 and 2006 is clearly evident, as well as the rock structure addition in 2011 (fig. 13, location C).
14

Figure 12. Elevation profiles across the Petit Bois ebb-tidal delta for the three time periods (transect T1, see figs. 3, 7, 

and 8 for locations). A, B, and C refer to locations discussed in the text. The vertical exaggeration is 250x.

-7

-6

-5

-4

-3

-2

-1

0

0 500 1,000 1,500 2,000 2,500

1987

2006

2016

EXPLANATION

Year

Distance (meters)

Elevation (meters)

North Transect T1 South

B

A

C

-12

-10

-8

-6

-4

-2

0

0 1,000 2,000 3,000 4,000

1987

2006

2016

EXPLANATION

Year

North Transect T2 South

B

A

C

T6 T5

Distance (meters)

Elevation (meters)

Figure 13. Elevation profiles from Mississippi Sound, across the island breach and into the Gulf of Mexico, for the three 

time periods (transect T2, see figs. 3, 7, and 8 for locations). Position of crossing transects (T5 and T6) are shown. A, B, and 

C refer to locations discussed in the text. The vertical exaggeration is 260x.
15

Transect T3 (fig. 14) extends seaward from the Pelican/Dauphin accretionary wedge offshore 

(see figs. 3, 7, and 8 for location). Sediment accretion as Pelican Island welded onto Dauphin Island is 

shown by up to a 3-m increase in elevation since 1987 (fig. 14, location A) and over 3 m accretion up to 

1 km offshore. Beyond 1 km, the seafloor has been stable throughout the time periods.

Transect T4 (fig. 15) is shore perpendicular across the Mobile Bay ebb-tidal delta (see figs. 3, 7, 

and 8 for location) and reflects the dynamic morphology of this deposit. At the Dauphin Island shoreline, shoreface retreat is evident by the landward migrating profiles and loss in elevation, between 1987 

and 2015 (fig. 15, location A). Immediately offshore, up to 1 m of sediment has infilled a former tidal 

channel present in the 1987 profile (fig. 15, location B). Erosion along the transect has occurred from 

1 km to 2.7 km, with approximately 0.5-m elevation loss occurring throughout (fig. 15, location C). At 

the seaward extent of the transect, along the edge of the ebb-tidal delta, landward shoal migration occurred between 1987 and 2006 as Pelican Island moved northward. Since 2006, the shoal has migrated 

landward of the transect and has been replaced by a small tidal inlet (fig. 15, location D).

Transect T5 (fig. 16) runs shore parallel along the gulf shoreline of Dauphin Island, from the 

Petit Bois Pass to Pelican Island (see figs. 3, 7, and 8 for locations). High-frequency nearshore sand 

waves are evident in all three time periods, some over 1 m in height (fig. 16, location A). A significant 

amount of erosion due to landward migration of the island is evident across four-fifths of the profile, as 

much as 4 m (fig. 16, location A). At the Petit Bois ebb-tidal delta, sand ridge development since 1987 

has occurred, along with the infilling of a tidal channel (fig. 16, location B).

Transect T6 (fig. 17) is a shore-parallel profile across Mississippi Sound (see figs. 3, 7, and 8 for 

location). In general, the profile records the consistent loss in elevation across the sound since 1987—up 

to 1.5 m of loss—and the flanks appear to be more stable than the central portion of the profile (fig. 17). 

No sense of sediment-transport direction can be discerned from the profiles due to lack of data in the 

earlier datasets at the western end of Dauphin Island.

Figure 14. Elevation profiles along the gulf side of Pelican Island for the three time periods (transect T3, see figs. 3, 7, 

and 8 for locations). A refers to a location discussed in the text. The vertical exaggeration is 240x.

-10

-8

-6

-4

-2

0

0 1,000 2,000 3,000

1987

2006

2016

EXPLANATION

Year

North Transect T3 South

A

Distance (meters)

Elevation (meters)
16

-6

-5

-4

-3

-2

-1

0

0 1,000 2,000 3,000 4,000

1987

2006

2016

EXPLANATION

Year

North Transect T4 South

A

B

C

D

Distance (meters)

Elevation (meters) -7

-6

-5

-4

-3

-2

-1

0

0 4,000 8,000 12,000 16,000 20,000

1987

2006

2016

EXPLANATION

Year

Southwest Transect T5 Northeast

B A

T2

Distance (meters)

Elevation (meters)

Figure 15. Elevation profiles across the Mobile ebb-tidal delta for the three time periods (transect T4, see figs. 3, 7, and 8 

for locations). A, B, C, and D refer to locations discussed in the text. The vertical exaggeration is 460x.

Figure 16. Elevation profiles across the gulf side of Dauphin Island for the three time periods (transect T5, see figs. 3, 7, 

and 8 for locations). Position of a crossing transect (T2) is shown. A and B refer to locations discussed in the text. The vertical 

exaggeration is 1980x.
17

Temporal and Spatial Sediment Flux

Comparing changes in erosion rates over time (fig. 18), over the long term (1987–2015), the ebbtidal deltas were the only features to either remain in equilibrium (Mobile ebb-tidal delta, A) or accrete 

(Petit Bois Pass, D). The Gulf of Mexico (C) and Mississippi Sound (E) seafloor lost sediment at relatively high rates, and Pelican Island lost sediment at a relatively low rate. Since 1987, Pelican Island has 

been migrating landward and rolling over through overwash into the Mobile ebb-tidal delta, producing 

net loss within the reference subsection (B).

During the stormy period (1987–2006), only the Petit Bois Pass reference subsection (D) experienced a positive sedimentation rate, while all other areas experienced erosion. Loss at the Mobile 

ebb-tidal delta (A) was relatively small and could be in part due to dredging of the Mobile Outer Bar 

ship channel and removal of sediment offshore. During the non-stormy period (2006–2015), the gulf (C) 

and sound (E) sides of Dauphin Island continued to experience overall erosion of the shoreface, at rates 

41 percent and 27 percent less, respectively, than during the stormy period (fig. 18). The ebb-tidal deltas 

flipped their response relative to the earlier period, with the Mobile ebb-tidal delta (A) gaining sediment 

at a rate higher than its prior loss, and Petit Bois Pass (D) losing sediment at a relatively small rate. The 

Pelican Island reference subsection (B) response flipped as well, gaining sediment during the nonstormy period. In addition, Pelican Island migrated toward and welded to Dauphin Island. The southern 

end submerged below water level, releasing sediment to the system. This change, and the reduction 

in significant storm-wave events that remove sediment from the system or cause the shoal to rollover, 

could explain the net gain to the Pelican Island cell. By welding onto Dauphin Island, Pelican Island 

trapped sediment within the Mobile ebb-tidal delta cell, resulting in accretion in the western part of the 

system during this time period (fig. 11).

Figure 17. Elevation profiles across the sound side of Dauphin Island for the three time periods (transect T6, see figs. 3, 7, 

and 8 for locations). The vertical exaggeration is 1830x.

-5

-4

-3

-2

-1

0

0 4,000 8,000 12,000

T2

1987

2006

2016

EXPLANATION

Year

Southwest Transect T6 Northeast

Distance (meters)

Elevation (meters)
18

Conclusion

The seafloor around Dauphin Island is spatially and temporally dynamic, with specific areas 

changing elevation at different rates in response to morphology and oceanographic conditions. In general, the submerged environment can be divided into the following five geomorphologic features: two 

ebb-tidal deltas (Mobile Pass and Petit Bois Pass) at the inlets on either end of Dauphin Island, Pelican 

Island/shoal on the western flank of the Mobile ebb-tidal delta, the shoreface of Dauphin Island facing 

the Gulf of Mexico, and the shoreface of Dauphin Island facing Mississippi Sound. Bathymetric change 

within these areas was analyzed over two time periods (1987–2006 and 2006–2015) and compared to 

the long term (1987–2015). The first time interval (1987–2006) corresponds to a period of frequent and 

intense storm impacts with 12 tropical storms passing near the island, 4 of them severe (table 1). During 

this time, episodic erosion and rapid transport of the seafloor sediments is expected to be the dominant 

process affecting elevation. In contrast, only two tropical storms passed by Dauphin Island during the 

second time interval (2006–2015). During this period, normal east-to-west littoral sediment transport, 

driven by a prevailing southeast wave climate, is the main process of seafloor change.

The geomorphologic features identified in the study do respond differently over the stormy and 

non-stormy time periods, which can be quantified through variations in erosion and accretion rates 

(fig. 18). By far the most erosion, both in volume and persistence, occurs along the central and western shoreface of Dauphin Island, both on the gulf and sound sides, with reduced net erosion occurring 

during the nonstorm period. The ebb-tidal deltas at either end appear to be in equilibrium relative to the 

rest of the island. The Mobile Pass ebb-tidal delta (A) appears to be “recovering” from Hurricanes Ivan/

Katrina through a net accumulation of sediment since 2006. Some of this accretion can be attributed to 

the welding of Pelican Island onto Dauphin Island and trapping sediment within the ebb-tidal delta that 

would otherwise migrate along the shoreface of Dauphin Island though a prevailing westward sediment 

transport. This prevailing transport supplies sediment to the Petit Bois Pass ebb-tidal delta from the 

Dauphin Island shoreface. Based on rates and volumes of erosion and accretion at the island shoreface 

Figure 18. Rates of change (erosion/accretion over time) for the two time periods and long-term (1987–2015) for each reference subsection. See figures 9–11 for locations.

-600

-500

-400

-300

-200

-100

0

100

200

Dauphin

Gulf of Mexico

(C)

Dauphin

Mississippi Sound

(E)

Mobile

Ebb-tidal delta

(A)

Petit Bois

Pass

(D)

Pelican

Island

(B)

1987 – 2015

1987 – 2006

2006 – 2015

Rate of change (x10 m /yr)

EXPLANATION

3 3
19

and at Petit Bois Pass, westward sediment transport appears to be more significant during storm activity 

than during normal littoral transport. Petit Bois Pass experienced net accretion during the stormy period 

and net erosion during the non-stormy period, correlating with higher erosion rates at the Dauphin Island 

shoreface during the stormy versus non-stormy period (fig. 18). This suggests that sediment delivery 

during normal littoral transport, while occurring, does not maintain (or barely maintains) equilibrium at 

the Petit Bois Pass. The sound side of Dauphin Island mimics the trend of the gulf side and appears to 

also be supplying sediment to the Petit Bois Pass ebb-tidal delta through similar processes.

References Cited

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and adjacent to Mobile Pass, Alabama: Report prepared for the U.S. Army Corps of Engineers, Mobile 

District, 199 p., accessed March 17, 2017, at http://www.sam.usace.army.mil/Portals/46/docs/planning_

environmental/acf/docs/072108-A-AQ963-014.pdf.

Byrnes, M.R., Griffee, S.F., and Osler, M., 2010, Channel dredging and geomorphic response at and adjacent to Mobile Pass, Alabama: Report prepared for the U.S. Army Corps of Engineers, Mobile District, 

ERDC/CHL TR-10-8, 311 p.

DeWitt, N.T., Stalk, C.A., Flocks, J.G., Bernier, J.C., Kelso, K.W., Fredericks, J.J., and Tuten, T., 2017, 

Single-beam bathymetry data collected in 2015 nearshore Dauphin Island, Alabama: U.S. Geological 

Survey data release, accessed May 30, 2017, at https://doi.org/10.5066/F7BZ648W.

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Mississippi-Alabama shelf, northern Gulf of Mexico, in Buster, N., Holmes, C., eds., Gulf of Mexico; 

Origins, waters and biota, 3. Geology; Texas A&M University Press, College Station, Tex., p. 475. 

Flocks, J.G., Kindinger, J.L., and Kelso, K.W., 2015, Geologic control on the evolution of the inner shelf 

morphology offshore of the Mississippi barrier islands, northern Gulf of Mexico, USA: Continental 

Shelf Research, v. 101, p. 59–70.

Guy, K.K., 2015, Barrier island shorelines extracted from Landsat imagery: U.S. Geological Survey OpenFile Report 2015–1179, 3 p., accessed March 17, 2017, at https://doi.org/10.3133/ofr20151179.

Otvos, E.G., and Giardino, M.P., 2004, Interlinked barrier chain and delta lobe development, northern Gulf 

of Mexico: Sedimentary Geology, v. 169, p. 47–73.

Parker, S.J., Davies, D.J., and Smith, W.E., 1997, Geological, economic, and environmental characterization of selected near-term leasable offshore sand deposits and competing onshore sources for beach 

nourishment: Geological Survey of Alabama Circular 190, 173 p.

U.S. Army Corps of Engineers (USACE), 2016, Dauphin Island Restoration Organization Memorandum 

for Record, accessed March 17, 2017, at http://www.dauphinislandrestoration.org/gvt/corps/2016-

08-23-Corps-mtg-memo-final.pdf.
Flocks and others—Analysis of Seafloor Change around Dauphin Island, Alabama, 1987–2015—Open-File Report 2017–1112

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20171112