Shifting Sands on Petit Bois Island

Investigate natural and human impacts on barrier islands

NASA Earth Observatory image modified by SkyTruth

Shifting Sands on Petit Bois Island

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Shifting Sands on Petit Bois Island

Investigate natural and human impacts on barrier islands

NASA Earth Observatory image modified by SkyTruth

Step 1: Understand the Issues

Investigate Natural and Human Impacts on Petit Bois Island

OVERVIEW

Barrier islands have always changed shape and size through natural processes. But human activities have altered these processes.  Now, barrier islands like Petit Bois are shrinking. How fast is the island shrinking? What will happen if Petit Bois disappears? What solutions might slow or stop that process?

In this activity you will examine maps and satellite imagery to observe and document changes in Petit Bois island, and research options for addressing these changes.

BACKGROUND

For more than 160 miles, the barrier islands of the Gulf Islands National Seashore shelter the Gulf Coast shoreline.   Starting with Santa Rosa Island near Pensacola, Florida, and running west through Mississippi, ten narrow islands of shifting sand, dunes, and seagrass absorb the brunt of hurricanes, while nurturing sea turtles, shorebirds and juvenile fish. National Seashores are coastal areas managed by the National Park Service to protect natural habitat for the use and enjoyment of people.  Two of the islands in Mississippi – Petit Bois and Horn Island — are so wild, that Congress also designated them as wilderness in 1978. Federal law defines wilderness as

“an area where man is a visitor… who does not remain.”

Wilderness areas are places where nature is supposed to take its course, and people enjoy these places in their wild state.   But even in wilderness, human activities outside the area can unintentionally alter that course.

NPS Photo
NPS Photo
NPS Photo

Barrier islands constantly move

To geologists, the barrier islands of Gulf Islands National Seashore and Dauphin Island (which divides the Florida islands from the Mississippi Islands) are young – only a few thousand years old.  They formed after glaciers retreated from most of North America, sea levels changed, and shoals appeared along the Gulf Coast. Once formed, the shoals helped trap sand and sediment brought in by currents, creating islands. Even today, currents pick up sand in the east and carry it westward where some of it drops out of the waves on the western edge of each island. As this continues over time, the islands change shape. Many migrate west and towards the mainland, as the eastern and seaward ends of the islands erode and the western ends grow from deposited sand. Storms such as hurricanes can also change the size and shape of the islands by cutting through narrow or low-lying areas and accelerating erosion.

In fact, French explorers in the 18th century found that Petit Bois Island was connected to Alabama’s Dauphin Island to its east back then. A French mapmaker named Jean Baptiste Bourguignon d’Anville showed this connection in his 1732 map of Louisiana.

But by 1816, other maps show that Petit Bois had become its own island, probably because of hurricanes and other storms breaching the sand that connected it to Dauphin.

Since then, the barrier islands have continued to change shape. Information compiled by scientists at the U.S. Geological Survey from maps, aerial photographs, GPS, and lidar technology, reveals how Petit Bois has changed from 1848 through 2005 as sand continually erodes from its eastern end.   Look at the layered map below and examine where Petit Bois has lost land. Has it gained any?

Morphological changes in Petit Bois Island between 1848 and 2005. Robert A. Morton, “Historical Changes in the Mississippi-Alabama Barrier Islands and the Roles of Extreme Storms, Sea Level, and Human Activities“, USGS, 2007, p. 7.

Dredging disrupts the natural process

As the Gulf currents continue to run from east to west, they erode the eastern end of Petit Bois Island. In the past, sand carried by currents would deposit on the western end of the island, allowing it to replenish at least some of its losses. Today, that part of the natural process is halted. Ships access the Port of Pascagoula just west of Petit Bois Island through a channel called Horn Island Pass. In order to keep that navigation channel open, the U.S. Army Corps of Engineers dredges sand out of the channel, making it deeper than it would occur naturally, and deposits the sand in other parts of the Gulf. By removing new sand that deposits in the navigation channel west of Petit Bois, sand lost from the eastern end can’t be replenished. Over time, Petit Bois Island has been shrinking.

Army Corps of Engineers recently created a sediment budget to study how much sand is lost to the barrier islands each year.

Mark R. Byrnes, et al, “Littoral Sediment Budget for the Mississippi Sound Barrier Islands“, July 2012, p. 28.

Just as with a bank account, too many withdrawals (such as through dredging) and not enough deposits (through natural replenishment in currents) will eventually deplete the island “account.”

Climate change is expected to increase storm intensity

Strong storms such as hurricanes often change the shape of barrier islands by creating powerful waves with tremendous energy hitting island shores. This increased wave energy can accelerate erosion, making the islands narrower. Large waves can completely overwash islands, particularly in low-lying and narrow sections, breaking a single island into two (as they did with Petit Bois and Dauphin Island in the 18th century). By absorbing this increased wave energy, barrier islands protect mainland coastal areas from the full force of hurricane waves. For example……But in the process, they take a beating themselves.

Scientists predict that as global temperatures rise, storms such as hurricanes will increase in severity. Already, according to the U.S. Geological Survey, the large number of storms affecting the Mississippi coast since 1995 has accelerated land loss on the barrier islands.   This includes Hurricanes Danny (1997), Georges (1998), Ivan (2004), and Katrina (2005).

Climate change raises sea level

Rising carbon dioxide and global temperatures are causing sea levels around the world to rise. This occurs for three reasons: (1) as water warms, it expands in volume (2) as global temperatures rise, ice sheets in the Arctic melt, releasing water into the ocean and (3) ice sheets in the Antarctic disconnect from the land and, like ice cubes in a glass of water, raise ocean levels when they enter the water.

If sea levels change slowly over time, barrier islands might simply move landward, as water washes over the island eroding its ocean side and depositing sediment on the sound side of the island. Rapid sea level rise, however, could inundate barrier islands and coastal areas, leaving them completely covered.

Wilderness impacted by human activities

Petit Bois is an officially designated wilderness area. The National Park Service manages Petit Bois as part of the Gulf Islands National Seashore to retain its “primeval” character – a term identified in the Wilderness Act – and permits no permanent human structures and no motorized vehicles as part of this wilderness – mandate. But Park Rangers can’t control the human activities that happen outside park boundaries. Human activities that alter the island’s natural processes – including dredging and climate change — involve other government agencies, people who use the barrier islands and local ports, and everyone around the world contributing to greenhouse gases.

Step 2: Exercises

Use satellite imagery to track changes on Petit Bois Island

Satellite images can document changes in the size and shape of barrier islands from space. Using Google Earth Pro to view the four satellite images of Petit Bois Island in the KMZ file, document how the island has changed since 1984.

  1. First, download the Guide to Using Google Earth and Calculations Worksheet (under Resources in the left column).
  2. Then, open up the KMZ file in Google Earth Pro. Use the Guide to Using Google Earth to draw polygons around each of the satellite images.
  3. Measure the size of the island and record your measurements in the Calculations Worksheet.
  4. Analyze your findings:  what do you observe about the island’s size and shape?  At the current rate of change, how long will it take for Petit Bois Island to disappear?
  5. What other changes do the satellite images show? How might that affect the island’s trajectory?

Step 3: Group exercises and discussion

Explore options for management

The class will be divided into three groups, (A, B, and C) with each group responsible for researching and reporting on their assigned questions to the rest of the class. Based on the information presented by each group, the class will make a recommendation about how to manage changes on Petit Bois Island.

1: What are the implications of Petit Bois shrinking or disappearing?

  • Group A will consider the implications for fish and wildlife.
  • Group B will consider the implications for coastal communities.
    • To learn about how coastal communities depend on Petit Bois and the other barrier islands for recreation and a tourist economy, see the Visitor Use and Experience section and the Social and Economic Environment section of Chapter 3 of The Gulf Islands National Seashore Final General Management Plan.
    • Read about a Stanford University study on the value of barrier islands to coastal communities here and here.
    • Report to the class how coastal communities in Mississippi might be affected if Petit Bois Island disappears.
  • Group C will consider the implications for the nation.
    • To learn more about the economic value of the Port of Pascagoula review its website.
    • To learn more about the benefits of wilderness values see the Wilderness.net discussion here.
    • Share with the class the different benefits provided by each of these uses of the land and water.
  1. What can we do to reduce island loss? Each group will address one of three approaches, and report back to the class after exploring possible solutions. Consider scale (what do we have control over?), options, and the potential impacts of each proposed solution (positive and negative).
  • Group A will explore solutions for reducing climate change, sea level rise, and storm intensity. Will addressing climate change save Petit Bois Island?
  • Group B will examine dredging options. Are there ways to dredge in an environmentally sustainable way?   Consider the discussion by the Army Corps of Engineers on Beneficial Uses of dredged material and Land Creation and the Landsat image of dredge locations included in “Appendix B – Littoral Sediment Budget Report” (PDF page 17 of 185, page 4 of document). What does this mean for Petit Bois’ wilderness values?
  • Group C will examine alternatives to dredging the pass. What would happen if the Army Corps didn’t dredge the Horn Island Pass? Based on your earlier research, consider the impacts on businesses, communities, and wilderness.   Examine the Landsat image, included in “Mississippi Coastal Improvements Program (MsCIP) Comprehensive Barrier Island Restoration Hancock, Harrison, and Jackson Counties, Mississippi: Draft Supplemental Environmental Impact Statement”, (PDF page 27 of 282, document page 1-4). Are there alternative routes that would maintain benefits but reduce harmful impacts?
  1. Class discussion: make a recommendation. Should we let Petit Bois Island shrink and possibly disappear? If not, what steps should we take to slow or stop this process?

Resources

To complete this exercise, students will need:

For teachers, there is a guide for teachers and a teacher version of the Petit Bois Island KMZ file, in addition to the above resources.

Background Materials

Discussion Resources

Fish and Wildlife:

Coastal Communities Impacts:

National Implications:

  • To learn more about the economic value of the Port of Pascagoula review its website
  • To learn more about the benefits of wilderness values see the Wilderness.net discussion here

Climate change:

Dredging:

Prescribed BurnsUGVR Video