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
| 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).
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).
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.
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). |
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:
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.
For Additional Information:John K. Lovelace,
U.S. Geological Survey,
3535 S. Sherwood Forest Blvd.,
Baton Rouge, LA 70816;
Benjamin F. McPherson,
URL for this document: http://water.usgs.gov/nwsum/WSP2425/andrew.html