Description of Study Area

Low topographic relief characterizes southeastern Florida, with most areas lying less than 15 ft above NGVD 1929. Prior to development, Miami-Dade, Broward, and Palm Beach Counties included a broad array of aquatic, semiaquatic, and upland habitats. Many of these habitats now have been altered or destroyed as a result of agricultural and urban development. Populated urban areas continue to encroach upon inland agricultural lands and wetland areas.

A warm dry season (November-April) and a hot, humid wet season (May-October) characterize the climate of southeastern Florida. The tri-county area (Miami-Dade, Broward, and Palm Beach Counties) is subject to high rates of evapotranspiration, periodic floods and droughts, and infrequent but highly destructive hurricanes. Annual precipitation averages 50 to 62 in., with greatest rainfall amounts occurring in coastal areas (fig. 3A). Between 65 and 80 percent (or between 38 and 46 in.) of the average annual rainfall occurs from May to September due to a nearly daily pattern of thunderstorms and occasional tropical waves and hurricanes (fig. 3B). Rainfall during November to April ranges from 10 to 17 in., with greatest rainfall occurring near coastal Palm Beach County (fig. 3C). Rainfall patterns in southern Florida exhibit a minor bimodal wet-season peak and a 5- to 6-year cycle that parallels global climate patterns (Duever and others, 1994). Annual rainfall has ranged from a low of 37 in. to a high of 106 in. Periodic extreme climatic events (fig. 4) are generally associated with floods and droughts (South Florida Water Management District, 2000). Mean monthly temperature in degrees Fahrenheit, generally ranges from the 60s during the winter to the 80s during the summer. Evapotranspiration losses generally range from 70 to 90 percent of the annual precipitation (U.S. Army Corps of Engineers and the South Florida Water Management District, 1999).

Figure 3. Typical average (A) annual, (B) wet-season, and (C) dry-season rainfall patterns in southern Florida. From South Florida Water Management District (2000). [larger image]

Figure 4. Anticipated rainfall during an extreme 3-day rainfall event for a 100-year return period in southern Florida. From South Florida Water Management District (2000). [larger image]

Southeastern Florida consists of several physiographic provinces including the Atlantic Coastal Ridge, Big Cypress Swamp, coastal marshes and mangroves, the Everglades, and the Sandy Flatlands (fig. 5). The Atlantic Coastal Ridge comprises areas of greatest relief in the tri-county area and forms a natural barrier to drainage from interior areas. The ridge extends northward from southeastern Miami-Dade to Palm Beach County and locally is bisected by the natural sloughs and drainage canals. The ridge rises from 8 ft to as much as 24 ft in the north and ranges from 2 to 10 mi wide. The low-lying Sandy Flatlands area lies west of the Atlantic Coastal Ridge. Farther west, the Everglades forms a 30- to 40-mi-wide natural trough (lower than the Sandy Flatlands) that extends southward from Lake Okeechobee and is subject to seasonal flooding. A coastal marsh and mangrove region borders southernmost Miami-Dade County and some parts of Broward and Palm Beach Counties. The Big Cypress Swamp extends into limited areas of northwestern Miami-Dade and southwestern Broward Counties.

Figure 5. Physiographic provinces in Miami-Dade, Broward, and Palm Beach Counties. Modified from Davis (1943), and Parker and others (1955). [larger image]

The historic Everglades formed an approximate 3,860 mi² freshwater wetland marsh, extending from Lake Okeechobee to mangrove estuaries that border Florida Bay (fig. 5). Under natural predevelopment conditions, surface water exited the southern shoreline of Lake Okeechobee within eight rivers that traversed a dense custard apple tree swamp (Williams, 1883; Davis, 1943) and that transitioned southward into sawgrass plains. Surface water then moved southward into sloughs that formed the Everglades (Davis and others, 1994). The Atlantic Coastal Ridge restricted eastward movement of surface water from the Everglades, except in the transverse glades areas of lower elevation that effectively divide the Atlantic Coastal Ridge areas into a series of topographic "islands." The predevelopment Everglades physiographic area generally was covered by surface water over much of its extent, and the low-lying Sandy Flatlands was subject to seasonal flooding.