The Changing Shores of Lake Borgne

Lake Borgne is a brackish lagoon in Southeast Louisiana. It is part of the Ponchartrain Basin, separated from Lake Ponchartrain and the Chandeleur Sound by land bridges. These narrow strips of wetland are subject to some of the highest rates of sea level rise in the world (Dangendorf et al., 2023). Shorelines are dynamic places, with deposition and erosion continuously changing their features.

Coasts around the world face the threat of sea level rise and coastal erosion. The National Oceanic and Atmospheric Administration predicted that Louisiana will see sea level rise of 4’ by 2100 (National Oceanic and Atmospheric Administration, 2022). Coastal erosion is an issue intertwined with sea level rise. As seas get higher, they will batter the shoreline and erode features along it.

The United States Geological Survey provides a free tool ArcMap called the Digital Shoreline Analysis System (DSAS). With this add-on, measuring changes in position through time and offering a 10 and 20-year forecast is possible. Here, I have used it to investigate how the shores of Lake Borgne, LA have changed between 1977 and 2022, as well as the rate of change per year.

A comparison of the four shorelines showed that in the 45 years of the study, Lake Borgne has consistently lost land. Somewhat of a surprise was that less than 1% of transects were positive, or depositional. The average annual rate of movement was -5.86 (uncertainty of +/- 1.94). Overall, the average transects lost 272 meters over the studied years. As the maps show, the lake's southwest to the northeast side has suffered the greatest land loss.

I have concerns about predictions based only on past positions of the shoreline. Other factors such as sea level rise and more intense hurricanes could skew the modeling. In a study, Dangendorf et al., (2019) find that the rate of sea level rise is accelerating, largely due to “thermal expansion” since the 1960s. In a new paper, Dangendorf et al. (2023) present research indicating that the Southeast coast of the United States, from Cape Hatteras to the Western Gulf of Mexico accelerating at rates of 10mm per year. These rates will likely go back to the baseline rates in a few years. With these varying rates of sea level rise, this tool might be more suited for measuring historical shoreline movement but may not be equipped for modeling future coastlines.

The state of Louisiana has embarked on projects to restore lost shorelines around Lake Borgne (Kemp et al., 2021) among other restoration schemes around the state’s coast. Marsh creation, shoreline, and barrier island restoration have been ongoing around the state funded by various agencies and organizations. This study shows the highest rates of land loss are in two areas not covered by current projects. Proctor’s Point and the shore between the Capes Pointe aux Marchettes and Malheureux Point have the highest rates of net shoreline movement around Lake Borgne. These two areas are on the list of future projects of the Louisiana State Government’s Coastal Master Plan to restore marshland or shoreline restoration (Coastal Protection and Restoration Authority of Louisiana, 2017). The results of this study show that land loss is ongoing and still needs attention.

References:

Coastal Protection and Restoration Authority of Louisiana. (2017). Louisiana’s Comprehensive Master Plan for a Sustainable Coast. In coastal.la.gov. http://coastal.la.gov/wp-content/uploads/2017/04/2017-Coastal-Master-Plan_Web-Book_CFinal-with-Effective-Date-06092017.pdf

Dangendorf, S., Hay, C. C., Calafat, F. M., Marcos, M., Piecuch, C. G., Berk, K., & Jensen, J. (2019). Persistent acceleration in global sea-level rise since the 1960s. Nature Climate Change, 9(9), 705–710. https://doi.org/10.1038/s41558-019-0531-8

Dangendorf, S., Hendricks, N., Sun, Q., Klinck, J., Ezar, T., Frederikse, T., Calafat, F. M., Wahl, T., & Törnqvist, T. E. (2023). Acceleration of U.S. Southeast and Gulf coast sea-level rise amplified by internal climate variability. Nature, 1935. https://doi.org/10.1038/s41467-023-37649-9

Esri, Maxar, & Earthstar Graphics. (2023, January 12). World Imagery Basemap. arcgisonline.com.

https://services.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer

Kemp, G. P., McDade, E. C., Day, J. W., Lane, R. R., Dawers, N. H., & Day, J. N. (2021). Recovery and Restoration of Biloxi Marsh in the Mississippi River Delta. Water, 13(22), 3179. https://doi.org/10.3390/w13223179

Landsat-2 1977 image courtesy of the U.S. Geological Survey

Landsat-5 1992 image courtesy of the U.S. Geological Survey

Landsat-7 2007 image courtesy of the U.S. Geological Survey

Landsat-8 2022 image courtesy of the U.S. Geological Survey

National Oceanic and Atmospheric Administration. (2022). 2022 Sea Level Rise Technical Report. https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html