Going, Going . . . Gone
Projecting Barrier Island Disappearance Dates
Maps, photographs and recent satellite images of barrier islands all depict significant changes in the barriers over the last 150 years. But what is disturbing to observers is that while some of these islands seem to be only shifting location, many are disappearing completely.
According to Randy McBride, assistant professor of geology at George Mason University in Virginia (formerly from Louisiana State University), Isles Dernieres, Timbalier Island and the Grand Terre islands are disappearing the fastest and are expected to erode entirely in 20 to 50 years. "They have short life expectancies because of both gulfside and bayside erosion. It's devastating," McBride said. "These islands are on the brink of catastrophe. Unless something large-scale is done now, they're history."
Long-term rates of change between 1855 and 1989 show both accretion and erosion for barrier island shorelines facing Louisiana’s coast. Between the late 1800s and the 1980s, deposits left from washovers caused some of these shorelines to migrate landward at rates up to 24.0 meters per year. Erosion, in contrast, has caused loss on other shorelines at rates from 1 to 5 meters per year.
Scientists like McBride determine long-term changes in Louisiana's barrier islands by first comparing more than 130 years of data that depict shoreline positions. The data detail shoreline change according to magnitude direction and rate.
McBride predicts shoreline positions 30 and 100 years in the future by calculating a rate of change based on the island’s yearly changes in shoreline positions. To determine this figure, he calculates how many meters of shore-line have eroded in the last 30 or 100 years and then divides that figure by the number of years being used. For example, if an island’s shoreline has eroded 210 meters over the last 30 years, the long-term rate of change equals 7 meters per year.
| 2000 | The Fast Track to Disappearance | 2110 |
The timeline above shows anticipated years of disappearance for several of Louisiana’s barrier islands through 2110. Although these barrier islands are in danger, other more stable barriers exist. For example, Grand Isle has a life expectancy about 900 years longer than that of Timbalier Island.
To estimate the change in the island’s shoreline 100 years from now, the long-term rate of change is multiplied by 100. This calculation predicts that an island losing 7 meters of shoreline a year will move landward another 700 meters in the next 100 years.
Predictions of how much an island will change in the future, and ultimately when the island will disappear, include rates of change for the total area of an island as well as shoreline change. Area change includes breakup within an island, such as inlet cutting or interior pond development, and provides more accurate data on the island’s total erosion. The final disappearance date calculation is then based on the long-term loss rate, which is calculated in the same manner as shoreline change (in this case, hectares lost over a certain number of years divided by the number of years).
To figure a disappearance date, scientists divide the island’s current area by the long-term loss rate. For example, if an island is currently 1,000 hectares (1 hectare equals 2.471 acres) and over the last 100 years has been losing 8 hectares a year, the island is projected to disappear 125 years from now.
Long-term rates of change between 1855 and 1989 show both accretion and erosion for barrier island shorelines facing the Gulf of Mexico are eroding at rates up to 23.1 meters per year. Grand Isle’s gulfside accretion at about 1 meter per year is overshadowed by the erosion on the remaining islands.
For more detailed information see McBride’s article:
McBride, R.A. and Byrnes, M.R., 1997. Regional variations in shore response
along barrier island systems of the Mississippi River Delta; historical change
and future prediction. Journal of Coastal Research, v. 13, n. 3, pp. 628-655.