Shoreline Retreat

No, I’m not talking about a vacation. I’m talking erosion.

The natural character of sandy beaches is to change shape constantly, and the Texas Gulf Coast is among the most dynamic coastlines on Earth. The shape of our shorelines is influenced by the forces that move sand (namely wind, waves, and currents), by the supply of sand, and by the shoreline’s setting – whether or not it’s sheltered from waves, adjacent to a tidal or storm channel, or next to a jetty or seawall, for example. According to the Texas General Land Office, the average erosion rate along Texas’ 367-mile coastline is 4.1 feet per year. But 64% of the Texas coast is eroding at an average rate of 6 feet per year (some locations are losing 30 feet a year), while only about 7.9% of the coastline is accreting. On average, the state loses 235 acres of shoreline each year. The highest rate of erosion, 46.2 feet/year, was recorded along the 2.5-mile long section of Matagorda Island. Relative sea-level rise, local circulation patterns, high intensity storms, and lack of sediment supply, combined with human activities along developed areas of the barrier island, have all contributed to this chronic erosion, although the exact measurements of each contributing factor is still largely unquantified.

The numbers are derived from studies at the Bureau of Economic Geology (established in the 1970s) at the University of Texas. The 4.1 feet/year is actually an average rate for the upper Texas coast, which is the part that’s eroding most rapidly. For the Texas coast as a whole, the average rate is about 3.9 feet per year. And for the lower coast, it’s about 3.3 feet per year. That’s among the most rapid rates of shoreline retreat in the United States. Places such as Louisiana and Mississippi have higher (more rapid) rates, and places such as the West Gulf Coast, the Northeastern Gulf Coast, and Florida have lower rates.

Those rates are determined by comparing topographic charts from the U.S. Coast and Geodetic Survey in the late 1800s and aerial photographs of the coast beginning in 1930 with recent maps produced by modern techniques, such global positioning system satellites and receivers and airborne laser-based systems for remotely mapping swaths of the shoreline. By determining past shoreline positions relatively accurately and comparing those positions over time, scientists can calculate the rates of shoreline change.

Erosion can result in decreased property rates of homes and businesses, which can be significant considering almost 40% of the U.S. population lives near a coastal area. In addition, ports, roads, and other infrastructure can be at risk, and farming and fishing industries can be affected. Also, healthy beaches, dunes, and wetlands help protect the coast. Without them, hurricanes would be even more destructive. Even a one-foot rise over the next 80 years — which is a relatively low estimate — would be detrimental. That one foot could cause a 200-foot retreat in shoreline. From 2000-2019, surveys by the Bureau of Economic Geology show that some of the areas most affected by beach erosion include the coastline just west of Sabine Pass along the Texas Point and McFaddin National Wildlife Refuges; parts of Follet’s Island; the area around the Brazos River mouth; the thin barrier island lining the San Bernard National Wildlife Refuge and East Matagorda Bay, as well as parts of the Matagorda Peninsula; and Padre Island.

People often assume that the environment they live in is going to be stable over time. But anyone who’s been to a beach knows that the beach is a dynamic environment that changes from hour to hour and day to day with tidal cycles, wave energy, wind, storms, etc. To understand and predict the rate of change, scientists have to distinguish between long-term, short-term, and episodic changes, and to understand their causes. Long-term change occurs over tens to thousands of years. Short-term change refers to movement occurring over several seasons, up to a few decades. Episodic change occurs from a single storm.

Long-Term Change

Shorelines along the Texas coast have been retreating for most of the last 20,000 years (during the last glacial-interglacial cycle). The peak of the last glaciation was about 20,000 years ago. At that time, sea levels were approximately 325 to 400 feet lower than today, and the shorelines were out near the edge of the continental shelf. As that glaciation ended, the melting of all those glaciers and ice sheets increased the amount of water in the oceans, and the sea level rose rapidly until about 5,000 years ago. They’ve continued to rise at a slower rate during these last 5,000 years, so most of the shorelines along the Texas coast (and the Gulf of Mexico as a whole) have been eroding naturally for the last 20,000 years.

The long-term rise in relative sea level has moved the shoreline along the upper Texas coast by simply inundating it – the shifting action of waves and currents slowing moving the shoreline landward. Relative sea-level rise has also limited sand supply to the coast by drowning ancient river valleys and forming the coastal bays, such as Galveston and Matagorda Bays. Rivers that used to supply sand to the beaches now dump their sand at the heads of these bays where it is kept from reaching the open coast, and there’s not much sand left offshore to resupply eroding beaches due to the natural geologic setting of the Texas coast. Generally, the sand turns to mud within 2.5 miles of the shoreline. Thus, nature has created a shoreline that is low in sand supply and that is undergoing long-term relative sea-level rise. For these reasons, the shoreline will continue to undergo long-term retreat unless human intervention prevails. Global average rates of sea level rise for the last century are from about 1 to 3 millimeters per year. On the Texas coast, that causes submergence of low-lying marshes and tidal flats. It also brings wave energy up higher on the beach, increasing the erosion potential.

Shoreline retreat also serves as a ‘canary in the mine’ for monitoring global climate change. Retreat rates are influenced by sea-level rise, which of course is related to global warming – regardless of the cause, whether natural or human-induced. It’s certainly a fact that sea level is rising. That rise affects shoreline change rates along the Texas coast, and scientists get clues about climate change and sea-level rise by monitoring shoreline positions.

Short-Term Change

Shoreline change that occurs over a few decades or less can be quite variable and contrary to the long-term trend, making it difficult to predict. One portion of the coast may be experiencing retreat while just a few miles away, stable or advancing conditions may prevail. A shoreline that has retreated over the last 100 years may experience periods of advance, as is the case for several locations along Matagorda Peninsula since the 1930s. However, though a particular beach may have been advancing or stable over the last several years, if it has been retreating over the long term, then the retreat will eventually resume over the short term, without human intervention.

In addition to global sea level rise, there is a contributing factor on the Texas coast called subsidence, a natural compaction of unconsolidated sediments. The sediments’ own weight causes the ground surface to sink. Much of the Texas coastal plain is composed of unconsolidated sediments – sand, silts, and clays that have not been hardened into a rock. These loose sediments can be dug up by hand or pulled apart with a shovel or a rake. They’re not lithified like granite, sandstone, or shale, though they might eventually compact into those forms. Usually, unconsolidated sediments on the shore are pushed and pulled by the wind-generated waves, augmented by the tidal cycle, causing normal, natural shoreline erosion. But subsidence can be accelerated by removal of fluids, such as ground water withdrawal or oil and gas production. In ground water withdrawal, water moves from clay particles into sandy reservoirs, and then the clay compacts, causing the strata to thin in the area above where the water is being pumped from. In production from relatively shallow oil and gas reservoirs, those reservoirs are under pressure when they’re being produced and that pressure declines over time. Some of that pressure may have helped hold up the sediments above the reservoirs, so when it declines, the land above the reservoirs sinks. Land subsidence effectively leads to a higher rate of relative sea level rise – i.e., sea levels rise relative to the local land surface, and those artificially inflated levels can be higher than the globally averaged rates of sea level rise.

Episodic Change

Shoreline retreat is not always a continuous and steady process. Tropical storms and hurricanes along the upper Texas coast can move the shoreline more than 100 feet in a day. A particular shoreline with a retreat rate of 6.5 feet/year would normally take 60 years to move 400 feet landward. A single storm, however, could cause most of this movement in one go. Dramatic shoreline recovery often lasts months, or years, following such a storm, but it’s usually incomplete – the shoreline remaining significantly altered from its pre-storm position. There are also storms that affect the coast only by creating elevated tides and strong wind-driven waves. Storm surges move a tremendous amount of sediment around. Some of that sediment leaves the system completely and isn’t available for the beach to recoup once the storm has passed. These sediments are hauled off into deeper water or deposited further ashore as overwash deposits.

Texas has a variety of shoreline types that are constantly shifting and mostly retreating landward, resulting in loss of public and private property and important natural habitats such as beaches, dunes, and marshes. To address this dilemma, the Texas Legislature passed the Coastal Erosion Planning and Response Act (CEPRA) in 1999, authorizing the Texas General Land Office (GLO) to conduct a coastal erosion response program. In support of the program, the Bureau of Economic Geology (BEG) coastal researchers identify and study eroding areas along the Gulf of Mexico and coastal bay shorelines of Texas, analyze shoreline movement rates and causes, and update a comprehensive, digital database of historical shoreline positions and average annual rates of shoreline movement – available to the public online.

The BEG Coastal Studies Group also runs the Texas Shoreline Change Project, which oversees the aforementioned online resource containing maps depicting historical Gulf and selected bay shoreline change rates along segments of the Texas shoreline. This data is published and maintained by the BEG through funding assistance from the GLO as required by the CEPRA statute (Texas Natural Resources Code §33.607(b)-(d)), which requires the Texas Land Commissioner, in consultation with the BEG, to monitor historical erosion rates on an ongoing basis and publish historical erosion rate data for public consumption.

Shoreline retreat is a threat to people only because coastal populations and infrastructure have increased exponentially in the last few decades. Once someone builds a home or a business on the low land along the Texas coast, they’re at risk from storm surge and shoreline retreat. Many homes have been built in aesthetically desirable locations, right on the beach edge, only to find themselves on the beach itself within a few years as a result of shoreline retreat and storm erosion. In Texas, the Open Beaches Act protects public access to the beach, so when structures end up on the beach due to erosion, they are restricting the public access to that beach. (Unique among most states, Texas maintains a “rolling easement” on the Gulf shores to protect public access to the state’s beaches.) Unfortunately, there aren’t many practical options for mitigating the effects of shoreline erosion. In some areas, such as the city of Galveston, the construction of engineered structures like the seawall slow shoreline retreat. Breakwater structures can be placed along canal shorelines to reduce wave action from barge traffic. Beach nourishment is another means used to combat erosion. Other tactics reduce the coasts’ susceptibility to erosion include proper vegetation strategies (vegetated terraces or mounds built in eroded marshes and wetlands can reduce wave energy, improve water clarity, and limit further loss of shoreline since plant roots are capable of binding sand grains and providing increased stability) and boosting naturally occurring microbes that can strengthen and stabilize beach sands.

Of course, the barrier islands on the Texas coast are the first line of defense for inland areas against erosion from tropical storms and hurricanes, largely because they’re big sandy features that have well-developed dunes behind the beach. In some cases, those dunes are the highest point for miles around – a natural sea wall. However, the dunes aren’t well-developed along the entire coast. Areas like the upper Texas coast don’t necessarily have well-developed dunes, owing to a lack of sediment supply, long-term erosion, and the impacts of recent storms in those areas. Dunes are always damaged by major storms, and they take a while to recover. Hurricane Ike struck the upper Texas coast in 2008, and though it was only a Category 2 storm, it caused a tremendous amount of beach and dune damage along the upper Texas coast. That part of the shoreline is still recovering from Ike, and some areas will never fully recover.

The Texas coast, and really the whole Northern Gulf of Mexico, is a naturally dynamic environment, and has been for as far as we can look into the past. It’s a constantly changing zone that, even though we live here, most of us aren’t completely accustomed to. We have our piece of land surveyed and expect it’s going to stay there for our lifetime, and many generations after – and that’s just not the case on the Texas coast, as many have learned the hard way. You can plant your flag on a slice of land, but waves and sea levels and storms don’t pay much attention to survey markers.

Where I learned about shoreline retreat, and you can too!

Bureau of Economic Geology
www.beg.utexas.edu/research/programs/coastal/the-texas-shoreline-change-project
coastal.beg.utexas.edu/shorelinechange2012/

Texas General Land Office
glo.texas.gov/coast/coastal-management/coastal-erosion/index.html
glo.texas.gov/coast/coastal-management/forms/files/coastwide-erosion-response-plan.pdf

TPWD
tpwd.texas.gov/huntwild/wild/wetlands/central-coast/erosion.phtml

University of Houston
uh.edu/~jbutler/physical/coastal/shorelinechange.html

EarthSky
earthsky.org/earth/jeffrey-paine-retreating-shoreline-along-texas-gulf-coast

USGS
www.usgs.gov/media/before-after/shoreline-retreat-corpus-christi-texas

Texas A&M AgriLife Extension
coastalresilience.tamu.edu/home/wetland-protection/policy-framework/bay-and-ocean-side-submerged-lands-some-fundamental-differences-in-law-and-management/the-texas-open-beaches-act-an-exceptional-example-of-a-rolling-easement/

Texas A&M Today
today.tamu.edu/2020/10/27/texas-am-expert-storms-worsening-states-beach-erosion-problem/

Beachapedia
www.beachapedia.org/State_of_the_Beach/State_Reports/TX/Beach_Erosion

FindLaw
codes.findlaw.com/tx/natural-resources-code/nat-res-sect-33-607.html

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