When building an elevated roadway over wetlands, construction should 

 take place from the roadway structure and away from adjacent wetlands to 

 the maximum extent possible. Heavy equipment needed to place the roadway 

 pilings (cranes, dredges) should be operated from the roadway. This 

 topside construction is recommended to avoid the need for barge 

 construction canals. 



The choice of an elevated structure instead of a filled causeway has 

 important economic implications. The greater costs of a structure are 

 justified when the cost of "mucking out" and replacing the soft sediments 

 with firm fill is greater than the cost of constructing an elevated 

 structure. The costs of elevated structures, which minimize the use of 

 fill, have been comparable with those of solid-fill causeways when there 

 is about 10 to 12 feet of "muck" and usually have been lower when deeper 

 trenches are required [81]. According to a recent Louisiana study, 

 elevated structures may become a cost-effective alternative to solid-fill 

 causeways in only 3 feet of muck [82]. Also, the time period to reach the 

 95 percent compaction point, required before road pavement can be laid, 

 increases exponentially with any depth increase past 10 feet [81]. 



The cost efficiency of causeways appears to be declining because 

 of the increased value of wetlands and increased costs of fill. 



Spoil Disposal : Solid-fill causeway construction and barge 

 canal excavation often create a spoil disposal problem. Vital 

 habitat areas are not suitable disposal sites, and acceptable sites that 

 are easily accessible are becoming scarce. The remaining alternatives are 

 to transport spoil either well inland or to the ocean. To minimize 

 disposal problems, roadway designers should anticipate and eliminate when- 

 ever possible any requirement for dredging. 



4.8.5 Bridges 



Structures over the water should be designed to maintain the natural 

 water flow and circulation regime [83]. Bridges should be designed so as 

 not to impair tidal flow in respect to volume, velocity, or direction. 

 Abutments should be built back from the water edge, and clear spans used 

 rather than piers. Most simply, the cross-sectional area of a water- 

 course should not be reduced by abutments, support piers, pilings, and 

 so forth. 



Poorly designed and constructed bridges act as partial barriers to 

 natural water flow. In many urbanized bays, impairment of water circula- 

 tion by bridges results in a buildup of pollutants to adverse levels, 

 as well as shoaling problems. For example, the piers supporting the 

 Great South Bay Bridge on Long Island, New York, have been shown to slow 

 water flow. A Federal study concluded that restricted circulation west of 

 the Bay Bridge was a contributing factor to the degradation of water 

 quality in the area [84]. 



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