elevation relative to water level. Such 

 changes can affect marsh plant ecology. 

 An example is the different elevation 

 requirements of Spartina alter niflora 

 and Spartina patens in northeastern 

 marshes. 



Causeways way possibly result in 

 disruption of fish and whale migration. 

 According to Brisby (1977), whale mi- 

 gration was slightly disrupted by the 

 causeway leading to Rincon Island, 

 California. 



Cumulative effects . The cumulative 

 effects of bridges and causeways are 

 referenced in the literature about the 

 Florida Keys. The case study of this 

 area should be consulted. 



Structural and Nonstructural 

 Alternatives 



Bridges and causeways can be de- 

 signed to respond to the physical and 

 environmental surroundings in which 

 they are built. Bridges can be placed 

 on piling or piers shaped and spaced to 

 provide minimum interruption of altera- 

 tion of water flow. Bridge spans can 

 be designed with longer lengths to re- 

 duce the number of piers or support 

 structures in the water; however, a 

 long span length may make the struc- 

 ture more costly to build. Causeways 

 can be designed with culverts or open 

 channels through the structure to allow 

 water circulation. Causeways can be 

 replaced by open pile structures instead 

 of fill to allow nearly unhindered circu- 

 lation of water. 



There are several nonstructural 

 alternatives to bridges. One, of course, 

 is routing of the highway or railway 

 over existing bridges or by circuitous 

 routing not requiring a bridge. Another 

 nonstructural alternative is to use a 

 ferryboat instead of a bridge. 



Tunnels and rerouting are also al- 

 ternatives to the causeways, although a 

 tunnel is so much more costly than a 

 causeway that it is a theoretical rather 

 than a practical alternative. Since the 

 causeways normally cross marshes or 

 shallow water, it is unlikely that a 

 ferryboat would present a viable alter- 

 native in many instances. Structures 

 associated with ferryboats, such as 

 piers, also have environmental impacts. 

 In addition, the convenience of a bridge 

 relative to a ferryboat is obvious. 



Regional Considerations 



The only specific regional consid- 

 erations mentioned in the literature 

 were in reference to the Overseas High- 

 way through the Florida Keys in Coastal 

 Region 4. The case study should be 

 consulted for further information. 



Besides various methods of design- 

 ing and building a bridge to alter the 

 impact, there are some alternatives avail- 

 able. The most common structural alter- 

 native to a bridge is a tunnel. After 

 construction, a tunnel provides no inter- 

 ference to water flow or circulation and 

 no interference with the substratum or 

 intertidal zone. If the tunnel is placed 

 in a dredge trench, there might be sub- 

 stantial alteration of the substratum 

 during construction, as well as other 

 problems normally associated with the 

 dredging or underwater excavation. As 

 a general rule, tunnels are significantly 

 more expensive than bridges. 



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