expected to result in the significant in- 

 creases in turbidity (Figure 51). Con- 

 struction roads should be designed to 

 cause the minimal adverse effects and 

 should be removed when construction is 

 finished. The bottom grade should be 

 restored to what it was before alteration 

 (Florida Game and Fresh Water Fish 

 Commission 1975). Dredging and filling 

 should be kept minimal. Clark (1974) 

 advises segmental construction to avoid 

 dredging for access. Gosselink et al. 

 (undated) also recommend against ac- 

 cess canals. When solid fill causeways 

 are constructed, Gosselink et al. (un- 

 dated) recommended side casting to re- 

 duce water quality degradation, long- 

 term environmental damage, adverse 

 aesthetic impacts, and the time required 

 for revegetation. If hydraulic dredges 

 are used they recommend disposal in 

 diked waste areas to facilitate settling 

 of suspended materials. 



Construction Materials 



Construction materials for bridges 

 include steel, concrete, or wood. A. 

 causeway embankment may be construct- 

 ed from soil, sand, or rock. 



Expected Life Span 



Information was not found in the 

 literature about the expected life span 

 of bridges and causeways. Both of 

 these structures should be considered 

 as extremely long lived and essentially 

 a permanent change to existing condi- 

 tions. 



Summary of Physical and Biological 

 Impacts 



Construction effects. Construction 



activities are likely to cause increased 

 turbidity and sedimentation, particularly 

 when excavation and spoil disposal are 

 involved. Spoil disposal may cause hab- 

 itat loss, change in species composition, 

 and water quality deterioration (Gosse- 

 link et al. undated). Revegetation is 

 almost impossible where sandy spoil is 

 deposited and is slow and variable when 

 spoil is taken from brackish or saline 

 marshes (Gosselink et al. undated). 



Many aquatic and terrestrial sedi- 

 ments are spongy and are subject to 



shifting due to stress. The potential 

 exists for shifting of sediments due to 

 the weight of materials deposited during 

 causeway construction. In some cases, a 

 mud wave had been created which advanced 

 ahead of the causeway construction. 



Chronic effects. The most prominent 

 chronic effects of bridges and causeways 

 mentioned in the literature are an al- 

 teration in current, velocity, and water 

 circulation patterns resulting from de- 

 creased cross sectional area. Salinity 

 may be affected in estuarine environ- 

 ments and other areas subject to tidal 

 flow. Marsh circulation may also be af- 

 fected. Concomitant alterations in the 

 flora and fauna will be dependent on the 

 degree of salinity change. Scour pits 

 and deposition behind abutments may re- 

 sult where current velocity is increased 

 by bridge piers and approaches (McAllis- 

 ter 1977). Blocking of longshore cur- 

 rents and sedimentation may result from 

 causeways. This is shown dramatically 

 in Figure 52 where the silt laden water 

 of the Fraser River is directed offshore 

 by the deadend Roberts Bank Causeway in 

 British Columbia. An atypical unturbid 

 environment results between the Roberts 

 Bank Causeway and the more southerly 

 Tsawwassen Ferry Terminal. This can be 

 highly detrimental to filter feeding 

 benthos (Rounsefell 1972). Impoundment 

 of water upstream from a causeway can 

 adversely affect marsh vegetation, re- 

 ducing the amount of plant biomass for 

 the food webs and decreasing the value 

 of the marsh as wildlife habitat (Sipple 

 1974a). The impoundment of water above a 

 causeway can lead to secondary environ- 

 mental alterations, such as stream chan- 

 nelization to prevent flooding. A study 

 of the causeway in the Strait of Canso, 

 Nova Scotia, revealed that the once dom- 

 inating tidal currents were superseded 

 by wind driven currents as a result of 

 the causeway. The currents were not only 

 slower, but also more variable. Salinity 

 and temperature stratification were also 

 altered (Vilks et al. 1975). 



The weight of material used for 

 causeway fill can cause changes in the 

 elevation in adjoining areas. Marshlands 

 are especially vulnerable to these types 

 of changes due to their relatively 

 spongy composition. Most wetland plants 

 are very sensitive to changes in their 



104 



