Home Introduction Study Area Methods & Procedures Estimation of Nutrient Loads & Water-Quality Analyses Summary & Conclusions References PDF Version

Description of Study Area

Miami-Dade County, located on the southeastern tip of peninsular Florida (fig. 1), encompasses an area of about 2,000 mi² (square miles). The county is bounded by the Atlantic Ocean on the east, Broward County on the north, Collier and Monroe Counties on the west, and the Florida Keys (Monroe County) on the south. The area is characterized as a subtropical, marine environment with long, hot, wet summers and mild, dry winters. Seasonal variation in rainfall is pronounced; about 75 percent of areal rainfall occurs during the 5-month wet season from June through October. Long-term records (1966-95) indicate that average annual rainfall in Miami is about 59 in. (inches), ranging from as low as 39 in. in 1975 to as high as 83 in. in 1968 (fig. 2).

A total of 15 east coast canal sites in Miami-Dade County (fig. 1) were selected for the study; 13 sites are gated spillways and 2 sites are gated culverts. All east coast canal sites that have automatic gate operation contain an overriding mechanism that closes the gates to prevent saltwater intrusion when the headwater and tailwater differential reaches a certain level. The operational description of the 15 east coast canal sites in table 1 is condensed from recently published reports by the U.S. Army Corps of Engineers (1995) and by Swain and others (1997).

Figure 3. Physiographic features of Miami-Dade County. [larger image]

Physiographic features (fig. 3) have significantly controlled the environment, drainage, and ultimately the land use in Miami-Dade County (Fish and Stewart, 1991, p. 4). The Atlantic Coastal Ridge, 2 to 10 mi in width, forms the highest ground in the county. Elevations along the ridge range from about 8 to 15 ft (feet) above sea level, but are 20 ft above sea level or greater in some places. West of Homestead, elevations of the Atlantic Coastal ridge are from 5 to 8 ft above sea level. The Atlantic Coastal Ridge is a natural barrier to drainage of the interior, except where it is breached by shallow sloughs or rivers.

The Sandy Flatlands in northeastern Miami-Dade County is lower in elevation (6-18 ft above sea level) than the Atlantic Coastal Ridge, and prior to development was poorly drained. The Everglades, by far the largest feature, is slightly lower than the Sandy Flatlands, and before development, was wet in most years and subject to seasonal flooding. Drainage was slow and generally to the south and southwest, channeled behind the higher Atlantic Coastal Ridge. The Everglades forms a natural trough in north-central, central, and southwestern Miami-Dade County. Elevations range from about 9 ft above sea level in the northwestern corner to about 3 ft above sea level in southwestern Miami-Dade County, except for tree islands or hammocks which may be a few feet higher than the surrounding land. Most of the eastern part of the Everglades within the county is used for agriculture, rock quarrying, or urban development.

Figure 4. Population of Miami-Dade County, 1960-94. [larger image]

A small part of the Big Cypress Swamp occupies northwestern Miami-Dade County where elevations range from 7 to 10 ft above sea level. Coastward from the Everglades and the Atlantic Coastal Ridge lie coastal marshes and mangrove swamps at elevations that generally range from 0 to 3 ft above sea level. This description of the physiographic features in Miami-Dade County was taken from a published report by Fish and Stewart (1991, p. 4).

One of the principal factors influencing the development of southern Florida water control is the rapid population growth along the lower east coast. According to the 1990 census, the population in Miami-Dade County doubled every 10 years between 1910 and 1960 (Metropolitan Dade County, 1995, p. I-79). Although the rate of population slowed over the next two decades, there still was a significant growth in population, with about 1.3 and 1.7 million people in 1970 and 1980, respectively (fig. 4). Immigration accounted for 78 percent of the population growth from 1960 to 1990; as of 1994, Miami-Dade County had attained a population of about 2.0 million people.

A rapidly expanding population in the urban and agricultural areas of southern Florida has resulted in an extensive water-management system that has evolved over the years. This system of canals, levees, pump stations, and gated water-control structures was constructed over the last century, initially for the purpose of draining the wetlands and for flood control. The earliest attempt at water management occurred in 1905 with the Florida Legislature’s creation of the Everglades Drainage District. This resulted in the construction of North River Canal and Miami Canal by 1913, and the construction of Hillsboro Canal and West Palm Beach Canals (north of the study area) shortly thereafter. By 1921, all four canals had hurricane gates at the Lake Okeechobee end. Many of the canals along the east coast of southern Florida were completed by the 1930’s for the purpose of flood protection, but were uncontrolled and did not impede saltwater intrusion.

During 1943-45, saltwater intrusion became a serious threat to the water supplies of southern Florida because of severe drought conditions. The need for a system of gated control structures on the primary east coast canals was realized and came to fruition with the creation in 1948 of the Central and Southern Florida Flood Control Project, designed by the U.S. Army Corps of Engineers (USACE). Operational control of this project came under the direction of the Central and Southern Flood Control District, now known as the SFWMD, which was created in 1949 by the Florida Legislature. An extensive series of levees was constructed, eventually resulting in the formation of major water-conservation areas used to supply water for ENP and to store water for recharging the Biscayne aquifer. Most of these water-conservation areas were completed by the early 1960’s. As a result of this highly managed system, the hydrology of southern Florida has been altered. Recent plans have focused on "replumbing" the water-management system to restore more natural flow to ENP.

The water-management system provides flood protection during the wet season (June-October) by storing excess water in the water-conservation areas and by discharging water through the east coast canals when flood events occur. During the dry season (November-May), replenishment of ground-water supplies along the east coast is accomplished by conveying water through the primary canals from the water-conservation areas. All of the major tributary canals along the east coast contain gated control structures that are opened during flood situations to permit discharge of excess water, and are closed during dry periods to maintain high freshwater heads, recharge ground water, and retard saltwater intrusion.

A key element in the operation of the water-management system is the communications and control system, which is a sophisticated electronic system that relies on remote acquisition and control units to operate control structures and to record hydrologic and meteorological data. These data are transmitted by way of telemetry to the operations control center in West Palm Beach, Fla. (north of Broward County), where decisions are made regarding water-management needs.

The close hydraulic connection that exists between ground and surface water in southern Florida is due to the highly transmissive nature of the Biscayne aquifer, the sole source of drinking water for residents of Miami-Dade County. Depending on the relation of the canal stages to the surrounding water table, water is exchanged from surface water to ground water or vice versa, and canals can be classified as either "gaining" or "losing" (fig. 6). At the east coast canal sites, the gates are opened during the wet season to discharge excess water to the Atlantic Ocean and are closed during the dry season to prevent saltwater intrusion. When the gates are opened during the wet season, canal stages generally become lower than the surrounding water table, inducing ground-water flow to the canals (gaining) and eventual discharge to tide. During the dry season, the gated control structures are closed to prevent saltwater intrusion; however, the ground-water hydraulic gradient is generally seaward, and the inland reaches of the coastal canals continue to collect ground water and transport it downstream to the coastal controls. The stages at the gated control structures are generally higher than the surrounding ground-water levels, and the canals (losing) recharge the aquifer and retard saltwater intrusion.

Canal-aquifer relations can also affect water quality. During dry periods when there is no flow, the canals tend to be chemically and physically stratified, resembling long lakes. During periods of flow, mixing occurs in the canals and ground-water seepage contributes highly mineralized water devoid of oxygen to the canals. Recharge to the tributary canals can be the result of rainfall, surface runoff, inflow from secondary canals, seepage through levees, and in some cases, releases from water-conservation areas or from Lake Okeechobee (by way of Miami Canal), all of which provide water of varying quality. The canals also provide a permanent surface for evaporation.