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Geomorphology and Depositional Subenvironments of Gulf Islands National Seashore, Mississippi

Prepared in cooperation with the National Park Service (NPS).

By Robert A. Morton¹ and Bryan E. Rogers²

¹ U.S. Geological Survey, St. Petersburg, FL 33703,
²Jacobs Technology, contracted to U.S. Geological Survey, New Orleans, LA 70128

Contents

• Project Description and Purpose
• Project Maps
• Project Data
• Data-Layer Classification Descriptions
• Project Metadata
• Further Reading

Project Description and Purpose

The U.S. Geological Survey (USGS) is studying coastal hazards and coastal change to improve our understanding of coastal ecosystems and to develop better capabilities of predicting future coastal change. One approach to understanding the dynamics of coastal systems is to monitor changes in barrier-island subenvironments through time. This involves examining morphological and topographic change at temporal scales ranging from millennia to years and spatial scales ranging from tens of kilometers to meters. Of particular interest are the processes that produce those changes and the determination of whether or not those processes are likely to persist into the future. In these analyses of hazards and change, both natural and anthropogenic influences are considered. Quantifying past magnitudes and rates of coastal change and knowing the principal factors that govern those changes are critical to predicting what changes are likely to occur under different scenarios, such as short-term impacts of extreme storms or long-term impacts of sea-level rise. Gulf Islands National Seashore was selected for detailed mapping of barrier-island morphology and topography because the islands offer a diversity of depositional subenvironments and the islands' areas and positions have changed substantially in historical time. The geomorphologic and subenvironmental maps emphasize the processes that formed the surficial features and also serve as a basis for documenting which subenvironments are relatively stable, such as the beach ridge complex, and those which are highly dynamic, such as the beach and active overwash zones.

The primary mapping procedures used supervised functions within a Geographic Information System (GIS) that classified depositional subenvironments and features (map units) and delineated boundaries of the features (shapefiles). The GIS classified units on the basis of tonal patterns of a feature in contrast to adjacent features observed on georeferenced aerial photographs. Land elevations from recent lidar surveys served as supplementary data to assist in delineating the map-unit boundaries.

Project Maps

The Geomorphology and Depositional Subenvironments of Gulf Islands National Seashore, Mississippi, was divided into four individual map tiles,one for each island, and are provided as is in PDF format. For users of GIS software, the datasets are also available.

Map Tiles in PDF format:

Click on the links below to download the PDF map tiles or on the index map to view the map tiles in JPEG format. You will need to have the free Adobe Acrobat® Reader installed on your computer to view and print the high resolution PDF map tiles.

Print Versions

• Tile 1 of 4: Cat Island (6.7 MB PDF)
• Tile 2 of 4: Ship Island (4.2 MB PDF)
• Tile 3 of 4: Horn Island (1.6 MB PDF)
• Tile 4 of 4: Petit Bois Island (5.2 MB PDF)

View On Screen Versions

• Tile 1 of 4: Cat Island (1.4 MB PDF)
• Tile 2 of 4: Ship Island (1.9 MB PDF)
• Tile 3 of 4: Horn Island (1.3 MB PDF)
• Tile 4 of 4: Petit Bois Island (1.0 MB PDF)

Project Data

ESRI ArcMap 9.3 Datasets

These datasets include shapefiles for each data-layer classification relevant to the individual island.

• Cat Island Map (174 MB) (9.8 MB zipped)
• Ship Island Map (115 MB) (1.4 MB zipped)
• Horn Island Map (137 MB) (3.1 MB zipped)
• Petit Bois Island Map (65.1 MB) (836 KB zipped)

Data-Layer Classification Descriptions

The barrier-island classification was referenced and mapped using September 2007 high-resolution orthorectified aerial photography from the National Agriculture Imagery Program (NAIP), October 2007 low-altitude color videos of the Mississippi barrier islands acquired from Great Divide Pictures under contract to the National Park Service (NPS), and June 2007 1-meter-pixel-resolution Experimental Advanced Airborne Research Lidar (EAARL) collected by National Aeronautics and Space Administration (NASA) and the USGS. Each geomorphic layer is stored in standard shapefile format viewable in any GIS software.

NATURAL FEATURES

Active Dune Complex

Barren to sparsely vegetated mounds or ridges of wind-blown sand that form hummocky topography landward of the beach.

Stabilized Dune Complex

Mounds or ridges of wind-blown sand that are typically densely vegetated with salt-tolerant grasses. The vegetated dunes form hummocky topography landward of the beach. The sand in these dunes is protected by the vegetation and is not moving.

Active Overwash Zone

An area that is frequently flooded by high water and ocean waves generated by storms. Typically low lying with sparse vegetation and composed of sand with patches of shell at the surface.

Inactive Overwash Zone

An area that was historically overwashed and flooded by storm surge, such as during Hurricane Katrina in 2005. These areas are not frequently flooded by high water or ocean waves but are still vulnerable to flooding from extreme storms. The former overwash sand is commonly reworked into low, hummocky dunes that can be densely vegetated with salt-tolerant grasses.

Beach

A mostly unvegetated strip of sand parallel to the shore that extends from the water to the seaward edge of the dunes or crest of a washover terrace. The seaward part of the beach is regularly inundated by wave run-up during high-water phases of the tidal cycle.

Beach Ridge Complex

Sets of long, continuous ridges formed parallel to the ocean shore by sand that is deposited by a combination of wave run-up and the wind. Typically vegetated with salt-tolerant grasses.

Beach Ridge Swale

Topographic depressions within a beach-ridge complex. May be dry or may intermittently pond freshwater after heavy rain.

Marsh

Low vegetated wetlands that support plant assemblages tolerant of saltwater. Typically found along the sides of barrier islands protected from ocean waves or along the margins of tidal creeks.

Shoal

Prominent subtidal platforms surrounding the upland core of a barrier island. Composed of mobile sand and typically covered with large bedforms constructed by ocean waves and currents. These features are important components of the littoral sediment-transport system. Delineated from 2007 USGS bathymetric data.

Spit

A narrow, mostly unvegetated strip of sand at the end of a barrier island that extends the island alongshore. Spits form because of recent deposition by waves and currents.

Vegetaded Barrier Flats

A low, relatively flat interior part of the barrier island that is densely vegetated in some places and at the same time is sparsely vegetated in other places, both by salt-tolerant grasses.

Water

Unclassified water data surrounding a barrier island or small bodies of permanent water within the boundaries of an island. Island exterior boundary referenced to June 2007 Experimental Advanced Airborne Research Lidar (EAARL) elevation.

MAN MADE FEATURES

Structure

Miscellaneous man made features, including buildings, docking piers, roads, and walkways, and shoreline-protection structures.

Reclaimed Land

Former land areas that were eroded and rebuilt by emplacement of dredged material.

Dredge Material

Material dredged to construct artificial channels and placed along the margins of the channels.

Project Metadata

The following links will open general metadata text files associated with each class shapefile for Gulf Islands National Seashore. View the more detailed Extensible Markup Language (XML) files, accessible in metadata readers such as ESRI ArcCatalog, from their respective folders in the Project Data sections listed above.

• Active Dune Complex
• Stabilized Dune Complex
• Active Overwash Zone
• Inactive Overwash Zone
• Beach
• Beach-Ridge Complex
• Beach-Ridge Swale
• Marsh
• Shoal
• Spit
• Vegetagted Barrier Flat
• Water
• Structure
• Reclaimed Land
• Dredge Material

Further Reading

  Chaney, P.L., and Stone, G.W., 1996, Soundside erosion of a nourished beach and implications for winter cold front forcing: 

    West Ship Island, Mississippi: Shore and Beach, v. 64, p. 27-33.

 
  Fritz, H.M., Blount, C., Sokoloski, R., Singleton, N.J., Fuggle, A., McAdoo, B.G., Moore, A., Grass, C., and Tate, B., 2007,

    Hurricane Katrina storm surge distribution and field observations on the Mississippi barrier islands: Estuarine, Coastal and 

    Shelf Science, v. 74, p. 12-20.

  
  Morton, R.A., 2007, Historical changes in the Mississippi-Alabama barrier islands and the roles of extreme storms, sea level,

     and human activities: U.S. Geological Survey Open-File Report 2007-1161, 38 p. (Available online at: https://pubs.usgs.gov/of/2007/1161/)

  
  Morton, R.A., Guy, K., and Whitko, A., 2007, Morphological impacts of Hurricane Camille (1969) on barrier islands of Mississippi

     and western Alabama: U.S. Geological Survey website at: http://coastal.er.usgs.gov/hurricanes/historical-storms/camille/.

     
  Otvos, E.G., 1979, Barrier-island evolution and history of migration, north central Gulf Coast, in Leatherman, S.P. ed., Barrier islands

     from the Gulf of St. Lawrence to the Gulf of Mexico: New York, Academic Press, p. 291-319.

     
  Penfound, W.T., and O'Neill, M.E., 1934, The vegetation of Cat Island, Mississippi: Ecology, v. 15, p. 1-16.

      
  Pessin, L.J., and Burleigh, T.D., 1941, Notes on the forest biology of Horn Island: Ecology, v. 22, p. 70-78.

       
  Rucker, J.B., and Snowden, J.O., 1988, Recent morphologic changes at Dog Key Pass, Mississippi; The formation and disappearance

     of the Isle of Caprice: Gulf Coast Association of Geologoical Societies Transactions, v. 38, p. 343-349.

     
  Waller, T.H., and Malbrough, L.P., 1976, Temporal changes in the offshore islands of Mississippi: Mississippi State University 
 
     Water Resources Research Institute, 109 p.