The information available about 

 causeways in Coastal Region 3 is very 

 limited. Clewell et al. (1976) conducted 

 a study of seven fill-road sites on the 

 northern Gulf coast of Florida. Sites 

 were located in five tidal salt marshes 

 in Wakulla, Taylor, and Dixie counties. 

 This study will be used as a case his- 

 tory of causeways in Coastal Region 3. 



According to Clewell et al. (1976), 

 tidal marshes in the area studied exist 

 "where waves penetrate only during se- 

 vere storms and hurricanes." Marshes 

 are periodically flooded as a result of 

 tidal sheet flow. The height of high 

 tide is dependent on lunar positions and 

 is, therefore, variable. A marsh located 

 at a higher elevation may not be inun- 

 dated as often as one at a lower eleva- 

 tion. The sites that are inundated daily 

 usually have a uniform salinity similar 

 to that found in tidal creeks or rivers. 

 Sites not flooded daily have a higher 

 salinity due to evaporation. Sites high 

 enough in elevation to receive more 

 fresh water from runoff and rain than 

 the salt water inundation have low salin- 

 ities. The vegetation is dependent upon 

 the salinity levels of the site. 



The distribution of three mollusc 

 species sensitive to particular regimes 

 of salinity and inundation were studied. 

 These species reacted to disturbances 

 by alterations in density. Plant zonation 

 was also determined along with salinities 

 and elevations. 



The Porter Island site involves a 

 paved fill-road built 22 yr prior to the 

 study that traverses a 1.5-mi (2.4-km) 

 long marsh protruding into Apalachee 

 Bay. The fill is not culverted. The 

 only opening consists of a 25-ft (7.6m) 

 long bridge span. Fill canals run along 

 the entire length of a roadway on both 

 sides. The study revealed that other 

 than the presence of the roadway and 

 canals, the marsh environment was not 

 adversely affected because tidal inunda- 

 tion occurs independently on both sides 

 of the unculverted marsh since it is 

 bounded by Apalachee Bay on both 

 sides. 



The Levy Pond site consists of a 

 marsh separated from a creek by an 



unculverted fill-road built 38 yr prior 

 to the study. The roadway has blocked 

 the sheet flow so that the marsh (Levy 

 Pond) contains fresher water than the 

 tidal creek on the other side of the fill- 

 road. Photographs taken from years 

 after construction showed that various 

 salt-intolerant plant species have grown 

 in Levy Pond. No vegetation can be 

 seen in similar ponds on the seaward 

 side of the road. At the time of the 

 Clewell et al. (1976) study, Levy Pond 

 was "completely choked with cattails, 

 sawgrass, and other emergent marsh 

 species, all characteristic of fresh water 

 or very slightly brackish habitats." 



The Evans Creek site contains a 

 fill- road, built 38 yr prior to the study, 

 that traverses a tidal creek (Evans 

 Creek) approximately 0.5 mi (1.3 km) 

 from its mouth. The salt marsh on the 

 landward side is isolated from the creek 

 except for a box culvert (5 x 5 ft or 

 1.5 x 1.5 m). The creek was ditched 

 to facilitate tidal inundation of the land- 

 ward side of the road. Only slight dif- 

 ferences in salinity, animal density, and 

 vegetational zonation were discovered 

 between the landward and seaward side 

 of the fillroad. Clewell et al. (1976) 

 state that it is uncertain if these dif- 

 ferences are due to the roadway or if 

 they always existed between the two 

 areas. It is suggested that the ditching 

 "increased the frequency of tidal flood- 

 ing but decreased the length of time 

 that the marsh was inundated in each 

 tidal cycle." They suggest that culverts 

 might be substituted for ditching to 

 maintain more natural inundation and 

 drainage in such marshes. 



The Cedar Island study involves a 

 north and a south marsh. The two sites 

 are landward of a fill-road, built 8 yr 

 prior to the study, that runs parallel to 

 the coast 0.3 mi (0.5 km) inland. The 

 north site can only be inundated by the 

 sheet flow. Only one culvert opens up 

 to the seaward side of the road. A 

 ditch and tidal creek flowing into a cul- 

 vert allow inundation at the south site. 

 The results of the study indicate that 

 sheet flow was blocked at the north 

 marsh except when severe storms occur- 

 red. This allowed for the invasion of 

 salt-intolerant species. The effects of 



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