Home Introduction Physical Setting Urbanization Natural Hazards Mineral Resources Water System Environmental Quality Outdoor Recreation Everglades N.P. Coastal Zone Fish & Wildlife Miccosukee Indians Conclusion References

The principal natural hazards in south Dade County are flooding from storm runoff and hurricane tides, destruction by hurricane winds, and foundation problems arising from unstable rock conditions.

Earthquakes, a significant hazard in nearby South Carolina and Cuba, are a minimal risk in Florida.

Because south Florida is flat, drainage of storm water was extremely slow and large areas were flooded until the network of drainage canals was constructed. This network has significantly reduced the storm flood hazard, but some low areas are still flooded often. In places, it is necessary to raise the land surface by filling to avoid flooding.

The Dade County Public Works Department has established minimum elevations for lots on which buildings can be constructed and on streets that serve them to prevent or minimize seasonal flooding, and to provide for proper operation of septic tank where they are sole method of wastewater disposal. Land below these established minimum elevations cannot be used for streets or building unless filled to the required level. As much as 5 feet of fill is required in low parts of south Dade County. This has important environmental implications.

Much of the land in the area requires several feet of fill to raise it to the flood-criteria level established by the Dade County Water Control Division. Lack of fill material may inhibit development. 1/ [larger image]

Zoning, as required under the National Flood Insurance Act of 1968, could significantly reduce future development in the low-lying areas of south Dade County affected by the 100-year flood.

Hurricanes cause loss of life and destruction of property as a result of high winds, torrential rains, and extraordinarily high tides.

The figure in each of the segments of the diagram is the number of hurricanes whose paths were in that segment of the diagram. Most hurricanes approach the area from the eastsoutheast. A second major trend is from the southwest. [larger image]

Hurricane tides can inundate much of the coastal area. Shown here are areas prone to such flooding as defined by the National Hurricane Center of the National Oceanic and Atmospheric Administration from historical records of hurricane tide elevations and topographic data. 2/ [larger image]

September and October are months of anxiety in southeastern Florida because of hurricanes. Coastal areas such as south Dade County are particularly sensitive because the point of hurricane entry from the ocean is generally the area of greatest damage. More than half of the 17 hurricanes since 1886 whose path crossed within 50 miles of the center of south Dade County approached from the ocean. The tidal surge and wave action developed by the intense winds and low barometric pressure may produce the greatest damage. A tidal surge of about 14 feet has been recorded in the south Dade area. Such a surge would inundate a large part of south Dade County. Metropolitan Dade County building codes incorporate design features which protect homes and businesses from destruction under hurricane-force winds. However, design features that would offer full protection under Great Hurricane force are not economically feasible. Heavy rainfall associated with hurricanes floods low-lying areas for several days. It is difficult to estimate the maximum total damage of a great hurricane; however, some estimates place the economic losses near the billion dollar mark. ^3/

Most of Dade County is underlain by limestone having solution cavities. A few general localities underlain by exceptionally large cavities are known but have not been mapped. The cavities significantly reduce bearing strength so that high buildings generally require special foundations. Normal concrete footings, however, are usually sufficient to support two- or three-story buildings.

The cost of special foundations required in areas of unusually porous limestone is high. Nevertheless, poor foundation conditions have not inhibited construction.

Distribution of cavities is shown schematically at two sites in the south Dade area. [larger image]

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