St. Charles Herald-Guide

217 square miles of coastline still underwater after storms - REPORT

Staff Report - December 1, 2006

The latest analysis of land change data from the U.S. Geological Survey’s National Wetlands Research Center indicates that 217 square miles of Louisiana’s coastal lands were transformed to water after Hurricanes Katrina and Rita.

Some recovery is expected in the short term, but exactly how much of this transformation of land to water will be permanent can only be determined after continued field studies as well as aerial photography and analysis of satellite imagery after each growing season.

The number of square miles of land that changed to water in southwestern Louisiana were: in the Calcasieu/Sabine basin, 22; Mermentau basin, 62; Teche/Vermilion, 5; and Atchafalaya, 9. The 62 square miles of land in the Mermentau basin included significant flooded marshes primarily between Calcasieu Lake and White Lake.

In southeastern Louisiana, basin-by-basin land losses were: Terrebonne basin, 19; Barataria, 18; Mississippi River Delta, 18; Breton Sound, 41; Pontchartrain, 19; and Pearl River, 4.

Land transformed to water along the coast and on barrier islands further reduced Louisiana’s natural protection from future storms. Louisiana had already lost 1,900 square miles of coastal lands, primarily marshes, between the years 1932 and 2000. The 217 square miles of land loss from the 2005 hurricanes represent 42 percent of what scientists had predicted would take place over a 50-year period from 2000 to 2050, even though they had factored storms into their model.

Some transformations of land to water will be permanent since they were caused by direct removal of land by storm surge. However, some areas that held temporarily impounded storm water may recover fairly quickly.

Many new open-water areas will undoubtedly become permanent lakes. For example, as of Sept. 16, flights indicated that some of the large marsh shears or rips in the particularly hard-hit Breton Sound area have remained open water.

Continued monitoring will show whether some areas recover. Some areas that are now open water may actually be temporary ponding of marsh, temporary removal of floating and submerged aquatic plants, or even normal water-level variations.

There were even a few new land gains calculated in the latest USGS work. These land gains are probably also temporary, caused by winds depositing piles of marsh vegetation or mats of aquatic vegetation.

Marsh losses varied in different areas along the coast but followed similar patterns. Shears were often located in marshes that fringed areas where land had already decreased from 1956 to 2000.

But shears also occurred in some historically stable areas such as in the upper Breton Sound basin, the lower Pearl River basin, the marshes bordering the east bank of Freshwater Bayou in the southwestern Teche/Vermilion basin and the marsh just north of Johnsons Bayou and south of the Sabine National Wildlife Refuge in the Calcasieu/Sabine basin.

Hurricane Rita’s surge removed remnant marsh from areas with historical land loss caused by a rapid collapse during the mid-1960s to the mid-1970s in western Barataria basin and central and eastern Terrebonne basin. This pattern was also seen on the west bank of Freshwater Bayou, east of Pecan Island in the southwestern Teche/Vermilion basin; south of Sweet Lake in the Mermentau basin; east of Deep Lake in the Mermentau basin. Story provided by LSU SeaGrant.

217 square miles of coastline still underwater after storms - REPORT