which represents approximately 17 mi 

 (27 km) of the 18 mi (29 km) of bridges 

 in the Overseas Highway. Of the 37 

 bridges proposed for replacement, 27 are 

 the spandrel arch type, one consists of 

 spandrel arch and pier sections, and 

 the remaining 9 are composite pile type 

 (Figure 49). The proposed bridge re- 

 placement will also involve the recon- 

 struction of approximately 21 mi(34 km) 

 of bridge approach. About 11 to 33 

 acres (5 to 13 ha) of submerged land 

 will be filled. 



The Florida Keys are composed of 

 flat limestone formations with elevations 

 ranging up to 15 ft (4.6 m ) above mean 

 sea level. About 95% of the land is less 

 than 5 ft (1.5 m) above mean sea level. 

 Shoal water commonly ranges up to 0.5 

 mi (1.3 km) offshore. Shoals are gener- 

 ally composed of the mangrove swamps, 

 submerged turtle grass beds, and ex- 

 posed limestone with little or no soil. 



The islands lie just north of the 

 Tropic of Cancer, with Key West being 

 the southermost city of the contiguous 

 United States. Key West is closer to 

 Cuba (90 mi or 145 km) than to Miami 

 (154 mi or 248 km). Hurricanes, which 

 occur frequently in the Florida Keys, 

 are probably the most significant clima- 

 tological feature of the area. 



The chain of 97 islands separates 

 Florida Bay on the Gulf of Mexico side 

 from Florida Straits on the Atlantic 

 Ocean side. Relatively deep channels 

 between the keys transport water be- 

 tween the Gulf and Atlantic Ocean. It 

 was estimated that the construction of 

 the original railroad system reduced the 

 cross-sectional water area between is- 

 lands by more than 50% (Bailey 1977), 

 which reduced water exchange between 

 Florida and the Atlantic Ocean. Salini- 

 ties in the upper portion of Florida Bay 

 are greater than 50 ppt for 9 to 11 mo 

 of the year, as compared to 34 to 37 

 ppt in the Atlantic Ocean (Davis 1977). 

 There are no historical records, but 

 reduced flow between Florida Bay and 

 the Atlantic Ocean may be a factor con- 

 tributing to the salinity difference 

 (Bailey 1977). Florida Bay system is 

 shallow as compared to the contiguous 

 Atlantic Ocean and experiences diurnal 



temperature changes of 10° to 15°F 

 (5.6° to 8.3°C) during part of the year 

 (Davis 1977). This large diurnal tem- 

 perature fluctuation does not occur in 

 the ocean. The difference in solar 

 energetics in the Bay as compared to 

 the ocean is probably also a factor con- 

 tributing to the salinity differences. 



The Florida Keys contain more 

 endangered, threatened, and rare plant 

 and animal species than any other re- 

 gion of the State. Thirteen major parks 

 and wildlife refuges lie partially or 

 wholly within the Florida Keys. 



The extensive emergent mangrove 

 forest and submerged turtle grass beds 

 are vital habitat for the propagation of 

 commercially and recreationally impor- 

 tant species of fishes, shellfishes, and 

 wildlife. Availability of habitat is the 

 limiting factor for these populations. 

 Protection of habitats is paramount to 

 protection of plant and animal species. 



After a lengthy series of public 

 hearings, advisory committee meetings 

 with concerned residents and Federal, 

 State, and local agencies, FD0T and 

 FHA issued a "Negative Declaration 

 State Road 5 (U.S. 1) Bridge Replace- 

 ments" (H. W. Lockner, Inc., Consult- 

 ing Engineer 1975). 



The negative declaration evaluated 

 each bridge site separately, considering 

 the following alternatives: 



Continue to maintain existing 



bridge; 



Remove existing bridge and 



construct new bridge on 



near existing alignment; 



Composite causeway struc 



ture- 



or 



C. 

 D. 



ture; 



Construct new bridge on Gulf 



or Atlantic side of old bridge. 



Alternative A was easily eliminated 

 based on economics and safety. Alter- 

 native C was carried to the final eval- 

 uation stage on nine bridges, but was 

 eliminated based on possible adverse 

 impact on natural and human environ- 

 ments. Alternative B or D was chosen 

 for each bridge on a site-specific basis. 



118 



