ABSTRACT

Data from two tide gauge networks established and maintained by the National Ocean Survey (NOS) and the U. S. Army Corps of Engineers (USACE) were analyzed to determine rates of sea-level rise and subsidence in Louisiana. Eighty tidal stations along coastal Louisiana were initially examined in this study. Only 20 of these stations had long-term records, records of 20 or more years of data, and quality data sufficient for analysis. The tidal data indicated that Louisiana is facing the highest rates of subsidence and sea-level rise in the United States. Within the Mississippi River delta plain, the Houma tide gauge documented a relative sea-level rise rate of 1.09 cm/yr (0.43 in./yr) from 1946 to 1988. On the coast, Eugene Island documented a slightly higher relative sea-level rise rate of 1.17 cm/yr (0.46 in./yr). Representative water-level histories from the Chenier plain, Teche basin, Terrebonne delta plain, Barataria basin, Balize delta plain, St. Bernard delta plain, and Pontchartrain basin indicate the regional rates of relative sea-level rise decreased to the east and the west from the Terrebonne coastal area.

In comparison with other National Ocean Survey tidal records throughout the U. S. Gulf Coast, Louisiana is experiencing the highest relative sea-level rise at 1.04 cm/yr (0.41 in./yr) for Grand Isle. The rates of sea-level rise decrease from 0.63 cm/yr (0.25 in./yr) at Galveston, Texas to 0.15 cm/yr (0.06 in./yr) at Biloxi, Mississippi. Mean relative sea-level rise in Louisiana is more than five times the Gulf of Mexico average, and 10 times faster than the rest of the globe.

Rapid rates of sea-level rise observed in Louisiana can be attributed to compactional subsidence in the Mississippi River delta plain. Louisiana directly overlies the entrenched Pleistocene valley of the Mississippi River, which is filled with Holocene deltaic sediments more than 150 m (490 ft) thick. Subsidence contributes up to 80% of the observed relative water-level rise rate at the tidal stations. In Texas, subsidence contributes up to only 60% of the water-level rise rate. Mississippi and Florida tidal stations show only the effects of eustasy.