Gulf Islands National Seashore Is Losing Sand: Keeping a Beach's Sediment Budget

The shoreline at the Perdido Key and Fort Pickens Units of Gulf Islands National Seashore retreated about 16.5 feet a year between 2018 and 2022. The U.S. Geological Survey measured a retreat of almost two and a half feet a year for the same area between 1965 and 2001. Jeff Bracewell, a geographic information system specialist with the Gulf Coast Inventory and Monitoring Network, explains how the National Park Service tracks that loss and what it does about it, in an article from Park Science (Volume 36, Number 2, Winter 2022).

Surveying the shoreline. Image credit: NPS / Jeff Bracewell.

A budget of sand

Beaches and dunes depend on a sediment "budget." Like money in a checking account, sand comes and goes with currents along the shore, changes in water level and prevailing winds. At Gulf Islands, those forces move the islands steadily north and west: wind and water take sand from the south side and deposit it on the north, and currents move it from east to west. Plants and animals have adapted to these thin, shifting strips of land. Over 300 bird species visit the seashore, including the snowy plover, which weighs a little over an ounce and nests on the open beach. Loggerhead, green and Kemp's ridley sea turtles nest there too. Since park biologists began a systematic nest survey at the Florida units in 1994, they have found an average of around forty nests a year. The federally endangered Perdido Key beach mouse feeds at night near the toe of the dunes, spreading seeds that help the vegetation anchor the sand.

A loggerhead hatchling crawls past the shell hash left by the previous high tide toward the Gulf of Mexico at Perdido Key. Image credit: NPS / a Scientist in Parks intern, Gulf Islands National Seashore.

People add pressure. Gulf Islands was the ninth most visited of more than 400 National Park units in 2021, with over five million visitors. Driving on the beach or clearing sand from roads changes how sand moves naturally. Harsher changes begin far from the park: dams, jetties and shipping channels alter sediment patterns, trapping or removing beach-building sand before it reaches the islands. Casual observers may notice sand where there was open water, but sound decisions call for measuring accurately, reproducibly and over time.

How the beach is measured

The Inventory and Monitoring Program tracks an ecosystem's "vital signs." It splits the work among 32 networks, and the Gulf Coast network covers two seashore parks, Padre Island and Gulf Islands, where coastal geomorphology is one of its vital signs. In 2017 the network laid out equally spaced transects across the shore at Gulf Islands. Researchers record latitude, longitude and elevation where the slope changes, and software turns the points into profile graphs showing dune height, the dune's distance from a fixed point and the sand in the dune. Comparing graphs over time shows what changed. The network also records the shoreline's position by walking the wrack line, the line of shells and debris left by the last high tide, and plots it over time to calculate the rate of advance or retreat.

A profile graph of beach and dune elevations. Distances to the dune crest and shoreline, and dune heights, can be read off and compared over time. Image credit: NPS / Jeff Bracewell.

What the numbers show

The retreat is fastest next to shipping channels, and the dune profiles show the islands retreating too. Part of the recent rate comes from the 2020 Atlantic hurricane season, the most active on record. Seven hurricanes affected the northern Gulf of Mexico that year, and Hurricane Sally breached Perdido Key in several places and undermined park roads.

Shoreline change near Pensacola Pass: the solid line is the 2022 shoreline and the dotted line is 2001. Red lines on the east side of Perdido Key mark some of the highest rates of retreat. Image credit: NPS / Jeff Bracewell.

Scientists think warming oceans will bring Atlantic hurricanes with more rain and stronger winds, and projected sea level rise means more coastal flooding in storms. They agree less on whether hurricanes will become more frequent. Sea level is rising at Pensacola by about one tenth of an inch a year, and at nearly twice that rate at nearby Panama City Beach, Florida, and Dauphin Island, Alabama.

Hurricane Sally in 2020 eroded beaches and damaged roads. Image credit: NPS / Jennifer Manis.

Putting sand back

NPS marine ecologist Monique LaFrance says the Service has moved toward nature-based solutions rather than hardened structures such as seawalls and jetties, which can interrupt the along-shore movement of sand so that some areas lose it while others gain. In practice that means adding sand. Dredge boats take sand from shipping channels, which need routine dredging, and place it on the shore where it would have arrived without the obstruction. The Army Corps of Engineers does this under agreements with agencies such as the Park Service.

Beach replenishment on South Padre Island in 2010. Image credit: U.S. Army Corps of Engineers.

It is complicated and costly. Projects need close coordination with local governments, nonprofits and private contractors, and satisfying state and federal requirements, designing the work and mobilizing equipment often takes years. Costs can quickly reach millions of dollars. The results can be large. After nourishment, the southernmost beach at Padre Island extended seaward by up to 554 feet between spring 2019 and spring 2021. At Gulf Islands, a project added about 181,000 cubic yards of sand near the eastern end of Perdido Key, in the area Sally breached.

Monitoring continues afterward. Bruce Leutscher, who leads Science and Resource Stewardship at Gulf Islands, expects it to show how well the Perdido Key placement works over the long term and to help plan future ones. Shelly Todd, Padre Island's Science and Resource Management lead, says it informs everything from beach recreation to sea turtle nesting and also helps visitors understand how man-made structures affect sediment and sea level. Bracewell calls nourishment temporary but worthwhile for protecting habitats, cultural sites and wildlife while harder, permanent solutions are worked out.

Sources

• National Park Service, Park Science, Volume 36, Number 2, Winter 2022, "Helping Islands Stay on a 'Budget,'" by Jeff Bracewell, GIS specialist with the Gulf Coast Inventory and Monitoring Network (https://www.nps.gov/articles/000/helping-islands-stay-on-a-budget.htm); rewritten in hubnx's own words. The original writes "Panama Beach"; Panama City Beach is the writer's correction. A sea level projection for that site and a soccer-field comparison are left out.