USGS Water Resources Information
National Water Summary on Wetland Resources
United States Geological Survey Water Supply Paper 2425


Restoration, Creation, and Recovery
Effects of Hurricane Andrew (1992) on Wetlands in Southern Florida and Louisiana

By
John K. Lovelace, U.S. Geological Survey

Benjamin F. McPherson, U.S. Geological Survey
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Hurricane Andrew was a small but powerful storm that caused massive destruction along a path through southern Florida and south-central Louisiana in late August 1992 (fig. 57). Rainfall associated with Andrew was light for a hurricane because of the small size and rapid forward movement of the storm. However, rainfall totals of more than 7 inches were recorded for the storm period in southeastern Florida and Louisiana; a high of 11.9 inches was recorded in Hammond, La. (Rappaport, 1992). Maximum sustained windspeeds of 141 mph (miles per hour), with gusts of 169 mph, were recorded on August 24, just before landfall in Florida (Rappaport, 1992). A storm surge of about 17 feet above sea level was recorded at Biscayne Bay, Fla. (fig. 58) and about 9 feet near Terrebonne Bay in south-central Louisiana (fig. 59).

Hurricane Andrew originated in the North Atlantic Ocean,moved westward over the Bahamas, and made landfall near the southern tip of Florida on the morning of August 24. After passing over the Florida Everglades, the storm proceeded in a northwesterly direction across the Gulf of Mexico and made landfall in south-central Louisiana at Point Chevreuil on the morning of August 26. Andrew deteriorated rapidly after landfall in Louisiana and was downgraded to a tropical depression on August 27. The remnants of Andrew proceeded on a northeasterly path, producing severe weather throughout the Southeastern States (Rappaport, 1992).

Hurricane Andrew moved across southern Florida at an average forward speed of 18 mph (National Oceanic and Atmospheric Administration, 1992). As it crossed southern Florida, Andrew left a path of destruction 25 miles wide and 60 miles long (Gore, 1993).


Andrew left a path of destruction 25 miles wide and 60 miles long
Figure 57

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Figure 57. Storm path and areal extent of tropical-storm- and hurricane-force winds produced by Hurricane Andrew, August 1992. (Source: Data from National Oceanic and Atmospheric Administration, National Weather Service. Landsat images (photographs) from U.S. Geological Survey, EROS Data Center.)
aug23.92 aug24 aug25 aug26
 
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Hurricane Andrew passed through the heart of the largest wetlands in the United States, the Florida Everglades. (See article "Florida Wetland Resources" in the State Summaries part of this volume.) Perhaps the most dramatic effect of the storm's passage through these wetlands was the major structural damage to trees caused by the strong winds. The storm passed directly over Biscayne National Park and Everglades National Park, knocking down or severely damaging mangrove trees on about 70,000 acres of wetlands in the two parks. Within the storm's path, virtually all large trees located in hammock areas (islands of dense, tropical undergrowth), typically hardwoods, were defoliated and about 25 percent of the trees were windthrown or badly broken. About one-fourth of the royal palms and one-third of the pine trees in Everglades National Park were broken or damaged by the winds (fig. 60). Damage to woody vegetation was most severe near the eye of the storm where winds were the strongest (Davis and others, 1994). However, within 20 days surviving trees and shrubs had sprouted new growth (Alper, 1992).

Figure 58

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Figure 58. Storm-surge elevations, in feet above sea level, at selected points along the coast of Florida; < indicates less than. (Source: Data from U.S. Geological Survey files.)

The storm appeared to have only minor effects on the interior freshwater wetlands of The Everglades, which are composed mainly of sawgrass. Nearly all post-storm (August 28 to September 17, 1992) water-quality properties sampled by the South Florida Water Management District were within the range of pre-storm values. These properties included turbidity, color, ammonia, and dissolved phosphate. Wind-heaped, vegetative marsh debris was evident along the edges of some forested islands, and the characteristic periphyton mat (group of stalkless micro-organisms that live attached to surfaces projecting from the bottom of freshwater bodies) was absent or altered in structure. However, much of The Everglades' marsh appeared undamaged by the storm. A significant effect of the wind in the freshwater wetlands was the destruction of, or damage to, about 80 percent of the hydrologic and meteorologic monitoring stations located along the storm's path (Davis and others, 1994).

Figure 59

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Figure 59. Storm-surge elevations, in feet above or below sea level, at selected points along the coast of Louisiana. Graphs indicate water levels at sites A, Vermilion Bay, near Cypremont Point; B, Wax Lake outlet, at Coleman; C, Houma Navigation Canal, at Dulac. (Source: Data from U.S. Geological Survey files.)


 
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The hurricane had little effect on wildlife in The Everglades. Thirty-two deer wearing radio collars for a National Park Service study survived the hurricane, and the releafing of vegetation provided them with food and cover. Adult alligators appeared unaffected, but nests and young may have been adversely affected. Most wading birds survived; estimates of pre- and post-storm populations were similar, and about normal for the late-summer wet season (Davis and others, 1994).

In the marine environment, the major effects of the hurricane were changes in nearshore water quality, patches of intense bottom scouring, and beach overwash. Dramatically increased turbidity persisted in some areas for at least 30 days, particularly in western Biscayne Bay where mangrove peat soils continued to break down and enter the water. In northeastern Florida Bay, at the southern edge of the affected area, concentrations of ammonia, dissolved phosphate, and dissolved organic carbon increased dramatically. Phytoplankton (microscopic drifting aquatic plants) blooms added to the increased turbidity and, combined with low dissolved-oxygen concentrations, could have had severe effects on fish and invertebrate populations. In addition, fuel from hundreds of damaged boats and marina fuel tanks in Biscayne Bay continued to discharge into the water for at least 27 days after the hurricane had passed (Davis and others, 1994).

In Louisiana, the storm surge produced significant flooding in a few populated areas in the southern part of the State. However, there was no major flooding of inland rivers. The greatest surge was east of the point of landfall, where the counterclockwise rotation of winds, combined with forward motion of the hurricane, pushed water northward (fig. 59). Andrew also produced a negative surge of as much as 3 feet below sea level along the coast from about 10 miles west of landfall to the Texas State line, as the counterclockwise winds west of the hurricane's eye pushed water away from the shore. Because the hurricane was moving in a northwesterly direction at the time of landfall, areas near landfall experienced a negative surge as the hurricane was to the southeast, then a positive surge as the hurricane moved past and was to the west.

After making landfall in Louisiana, Hurricane Andrew curved back towards the northeast, passing over the Atchafalaya River Basin, which contains the largest hardwood swamp (1.5 million acres) in the United States, and Louisiana's largest palustrine wetland. (See article "Louisiana Wetland Resources" in this volume.)

In parts of the basin, the storm severely damaged trees, primarily willows and some cypress. Near the coast, about 80 percent of the trees were knocked down; about 20 miles inland, the estimates were about 30 percent. With the loss of trees, an estimated 50 to 75 percent of the young squirrels in the area, those produced during the second litter of the year, died. The storm had little effect on deer (David Morrison, Louisiana Department of Wildlife and Fisheries, oral commun., 1993).


Hurricane Andrew passed through the heart of the largest wetlands in the United States.

 
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Figure 60a

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Figure 60b

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Figure 60. Hammock (top) and pine forest (bottom) in Everglades National Park, Fla., after Hurricane Andrew, September 1992. (Photographs by Benjamin F. McPherson, U.S. Geological Survey.)

Figure 61

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Figure 61. Dead fish in the Atchafalaya River Basin, La., September 2, 1992. (Photograph by Charles R. Demas, U.S. Geological Survey.)

In the Atchafalaya River Basin, an estimated 182 million freshwater fish perished because of the resuspension of anaerobic bottom materials in the water column (fig. 61). Most of the fish probably died during the first 24 hours after the storm as toxic hydrogen sulfide was released from bottom sediments, and decaying organic matter consumed dissolved oxygen, causing fish to asphyxiate (Gary Tilyou, Louisiana Department of Wildlife and Fisheries, oral commun., 1993). After the storm, U.S. Geological Survey personnel measured dissolved-oxygen concentrations of less than 1 mg/L throughout most of the basin, in an area extending northward more than 60 miles from the coast (Charles Demas, U.S. Geological Survey, oral commun., 1993). Dissolved-oxygen concentrations in the upper water column of larger water bodies in the Atchafalaya River Basin generally range from 3 to 6 mg/L during summer months (Dennis K. Demecheck, U.S. Geological Survey, oral commun., 1994). During the 2 weeks following the hurricane, fishkills were caused primarily by the movement of water containing low concentrations of dissolved oxygen into previously unaffected water (Gary Tilyou, Louisiana Department of Wildlife and Fisheries, oral commun., 1993). The value of freshwater fish killed was about $160 million, most of which was attributed to the estimated 29,000 paddlefish that died. (The paddlefish is an endangered species and its valuation is based on the $2,500 per-fish fine for killing paddlefish.) Estimates of the number of other species killed (in millions) include shad, 100; bream, 23; crappie, 7; largemouth bass, 5; fresh-water drum, 11; buffalo, 12; catfish, 11; and carp, 1 (Harry Blanchet, Louisiana Department of Wildlife and Fisheries, oral commun., 1993).

 
 
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In the coastal waters, an estimated 9.4 million saltwater fish valued at $7.8 million were killed by the storm. The exact causes of death are uncertain, but popular theories include suffocation, caused by clogging of gills by sediment, and gas-bubble disease, caused by the formation of nitrogen bubbles in the fish's bloodstream due to increased pressure. Most of the fish were found along a band of coastline about 5 miles long, just southeast of the point of landfall. Species killed (in millions) include menhaden, 5.7; mullet, 0.9; croaker, 0.9; spotted sea trout, 0.2; sea catfish, 0.4; black drum, 0.03; and red drum, 0.02 (Harry Blanchet, Louisiana Department of Wildlife and Fisheries, oral commun., 1993).

Large segments of Louisiana's coastal marsh, primarily in Terrebonne and St. Mary Parishes, were damaged. About 40 percent of the Nation's tidal wetlands are located on Louisiana's gulf coast (S.J. Williams, 1993). A substantial part of these wetlands is composed of fresh and intermediate marsh (Chabreck and Linscombe, 1978). Much of this marsh is "floatant" (a floating type of marsh). The marsh is said to float because partially decomposed organic matter and intertwining plant roots form a dense mat that rises and falls with the water level. The roots of the plants that make up the mat are unattached, or only partly attached, to the bottom (Lee Foot, U.S. Fish and Wildlife Service, oral commun., 1993).

The marsh suffered substantial damage caused by wind, tide, and wave action. Three specific kinds of damage were identified:

  • Compressed marsh, where a net decrease in surface area results from the marsh being pushed together, somewhat like an accordion closing

  • Marsh balls, which are created by the marsh being piled, rolled, or otherwise deformed to create large mounds (resulting in decreased surface area)

  • Sediment deposition in thicknesses of as much as 10 inches, but averaging less than 1 inch, which killed vegetation and sank part of the floating marsh.

Other damage was attributed to vegetative scour, which resulted from large areas of attached plants having their roots torn from the bottom, and salt burn, which occurred when saline (salty) water from the Gulf of Mexico was pushed into freshwater areas, killing and damaging salt-sensitive plants (Lee Foot, U.S. Fish and Wildlife Service, oral commun., 1993).

About 25 square miles of coastal wetlands in Louisiana are being lost every year due to coastal erosion and wetland deterioration. The marsh suffered substantial immediate loss of coastal wetlands and possibly has hastened the erosion and deterioration processes already at work (Dunbar and others, 1992).


An estimated 9.4 million saltwater fish valued at $7.8 million were killed by the storm.
 
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References Cited

Alper, Joe, 1992,
Everglades rebound from Andrew: Science, v. 257, p. 1,852-1,854.

Chabreck, R.H., and Linscombe, Greg, 1978,
Vegetative type map of the Louisiana coastal marshes: Louisiana Department of Wildlife and Fisheries, New Orleans, La., 1 sheet.

Davis, G.E., Loope, L.L., Roman, C.T., Smith, G., and Tilmont, J.T., compilers, 1994,
Assessment of Hurricane Andrew impacts on natural and archeological resources of Big Cypress National Preserve, Biscayne National Park, and Everglades National Park, 15-24 September 1992: National Park Service, 158 p.

Dunbar, J.B., Britsch, L.D., and Kemp, E.B., III, 1992,
Land loss rates, report 3, Louisianna coastal plain: U.S. Army Corps of Engineers Technical Report GL-90-2, p. 27.

Gore, Rick, 1993,
Andrew aftermath: National Geographic, v. 183, no. 4, p. 2-37.

National Oceanic and Atmospheric Administration, 1992,
Special climate summary, Hurricane Andrew: National Oceanic and Atmospheric Administration, Southeast Regional Climate Center, Columbia, S.C., 7 p.
Rappaport, Edward, 1992,
Preliminary report, Hurricane Andrew, 16-28 August 1992: National Oceanic and Atmospheric Administration, National Weather Service, National Hurricane Center, Coral Gables, Fla., 28 p.

Williams, S.J., Penland, Shea, and Roberts, H.H., 1993,
Processes affecting coastal wetland loss in the Louisiana deltaic plain, in Magoon, O.T., Wilson, W.S., Converse, Hugh, and Tobin, L.T., eds., Coastal Zone '93-Proceedings of the Eighth Symposium on Coastal and Ocean Management, July 19-23, 1993, New Orleans, La.: New York, American Society of Civil Engineers, v. 1, p. 211-219.


For Additional Information:

John K. Lovelace,
U.S. Geological Survey,
Louisiana District,
3535 S. Sherwood Forest Blvd.,
Suite 120,
Baton Rouge, LA 70816;

Benjamin F. McPherson,
U.S. Geological Survey,
Tampa Subdistrict,
4710 Eisenhower Blvd.,
Suite B-5,
Tampa, FL 33634



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