choice between the two will usually be 

 based on economic, environmental, or 

 hydraulic factors. In general, bridges 

 will be used where there is deeper wa- 

 ter to cross, or where navigation or 

 water passage and circulation must be 

 maintained. Causeways will usually be 

 economically attractive across marshy 

 land or the shallow water portions of 

 estuaries. 



Causeways can be used in conjunc- 

 tion with bridges. For instance, a 

 causeway can be used in the shallow 

 portion of a waterway and a bridge in 

 the deeper portion where a causeway 

 would be uneconomical or cause unac- 

 ceptable side effects to navigation or 

 water circulation. 



Site Characteristics and Environmental 

 Conditions 



The environmental conditions in 

 which bridges and causeways are con- 

 structed are variable. The literature re- 

 ferred to structures constructed across 

 marshes, tideflats, estuaries, and chan- 

 nels; but construction is not limited to 

 these locations. Bridges and causeways 

 extend from one shoreline to another 

 over the terrestrial zone, through the 

 tidal or subtidal zone, and back to the 

 terrestrial zone. 



Placement Constraints 



Engineering . Bridges and cause- 

 ways should be designed to minimize 

 changes in water circulation and flow. 

 Piers or pile support structures are 

 recommended over solid fill. Clear spans 

 are recommended over piers, if possible 

 (Clark 1974). The inclined approaches 

 should also be supported by piles as 

 opposed to fill to allow for "high-stage 

 water passage" (Bauer 1973) or high 

 water caused by storms. Bridge piers 

 should be as streamlined as possible 

 and piles should be adequately spaced 

 to minimize the interference with water 

 flow. According to Clark (1974), it may 

 be necessary to enlarge the watercourse 

 area to maintain the original cross sec- 

 tional area. Bauer (1973) recommends 

 locating bridges across straight chan- 

 nels rather than across meandering or 

 shifting channel systems to avoid inter- 

 ference with the dynamics of such a 

 system. 



103 



Where fill is used for support, the 

 ditches constructed through the causeway 

 may be an effective means to facilitate 

 tidal inundation. However, the movement 

 of water into and out of the wetlands 

 behind the causeway may be altered as 

 compared to natural conditions due to a 

 loss of hydraulic drag. This condition 

 has probably occurred in the Florida 

 Everglades due to channelization of wet- 

 land areas (Davis 1977). Clewell et al. 

 (1976) suggest considering the use of 

 many small culverts as opposed to ditch- 

 ing to achieve natural flooding and 

 drainage of a marsh area. Ditching will 

 generally cause faster drainage of the 

 marsh than would occur under natural 

 circumstances. 



S ocioeconomic . According to Gosse- 

 link et al. (undated), bridges through 

 marsh areas are more expensive than 

 causeways. They stated that the cost of 

 constructing a bridge is about four 

 times that of constructing filled high- 

 ways. When the estimated value of marsh 

 destruction is added to the cost of a 

 causeway, they become one-half to three- 

 fourths as costly as bridges. However, 

 in view of hydrologic considerations, 

 more extensive use of bridges may be 

 justified (Gosselink et al. undated). 

 Both bridges and causeways may have a 

 significant aesthetic impact on the 

 coastal environment. Bridges and cause- 

 ways are the major access modes from 

 mainland areas to barrier islands and 

 beaches which are utilized heavily for 

 recreation. 



Biological . When designing a road- 

 way, v/etland areas should be avoided 

 whenever possible. Existing dikes and 

 levees should be used if feasible. If 

 wetlands cannot be avoided, than care 

 must be taken to minimize biological im- 

 pact. According to Gosselink et al. (un- 

 dated) , bridges cause less marsh destruc- 

 tion than causeways because bridges have 

 less effect on water circulation. Steep- 

 er causeway and bridge approach slopes 

 might also aid in reducing habitat de- 

 struction (Bailey 1977). 



Environmental disturbances should 

 be minimized during construction. Mat- 

 ting and/or vehicles designed to prevent 

 soil compaction are recommended for use 

 in wetlands. The turbidity control de- 

 vices should be used if construction is 



