75 



Typical cross -sections for a seawall, bulkhead and revetments are shown in 

 Fig. 6.7. 



Sea level rise affects the design, construction, and maintenance of 

 these structures in the same general manner as with dikes. As sea level 

 rises, higher crest elevations are required, but because the structures are 

 not free-standing the required cross-sectional area increases more linearly 

 than quadratically with sea level. As with dikes, a seawall, bulkhead or 

 revetment can either be designed with enough crest elevation to account for 

 projected sea level rise, or else the crest elevation can be periodically 

 raised in response to sea level. Because these are hard structures, it is 

 usually difficult and expensive to exercise the second option. 

 Galveston, Texas is fronted by a seawall constucted after the city was 

 demolished during a major hurricane in 1900, in which more than 6,000 

 people were killed. The wall, whose cross-section was shown in Fig. 6.7a 

 and planform is displayed in Fig. 6.8, is 4.9 m high and over 16 km long. 

 Nine million cubic meters of fill were placed behind the wall and much of 

 the city was raised in elevation. The seawall has been subjected to seven 

 major storms since 1915, during which overtopping and toe scour have 

 required additional fill and rubble toe protection. Subsidence of the wall 

 has also been a problem, especially in places where it is located over a 

 soft clay stratum. Relative sea level at Galveston has risen approximately 

 24 cm since 1904 (Leatherman, 1984) , and during that time most of the 

 original beach fronting the wall (up to 90 m wide) has been lost. 

 Leatherman also indicates that diking will be necessary in the future to 

 preserve the city. 



Breakwaters - are free-standing structures, usually of rubble mound 

 construction, attached to the shoreline or seaward (detached) of the 

 shoreline. Breakwaters cannot prevent inundation by sea level rise, but 

 can modify shoreline response by blocking some of the incident wave energy. 

 The resulting shoreline (for detached breakwaters) has a bulge associated 

 with each structure, and holds the mean shoreline at a more seaward 

 position. Effective in preventing beach erosion due to both longshore and 

 on/offshore transport, offshore breakwaters have been used for shore 

 protection in the U.S., Canada, Europe, and quite extensively (over 2,500) 



