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maintenance. In regions where relative sea level is rising rapidly due to 

 ground subsidence or tectonics, a projected estimate of suitable length (on 

 the order of 10-50 years) may be most appropriate. "Hard" structures such 

 as dikes built with masonry, concrete, and rubble are usually very 

 expensive to maintain or improve, and should be designed using the maximum 

 long-term sea level estimate projected during the design life of the 

 structure. "Soft" structures built with sand or earth and armored with 

 vegetation usually require frequent (but relatively inexpensive) 

 maintenance and are more easily altered and improved. This permits raising 

 of the crest elevation in response to actual sea level rise, rather than 

 designing for a perhaps uncertain projection of sea level. 



The best example of the use of dikes and levees to prevent coastal 

 flooding, and their interaction with long-term sea level rise, is the 

 Netherlands. Dikes have existed in the Netherlands since pre-Roman times 

 and over 1,000 km now exist (Lingsma, 1966). However, several catastrophic 

 failures during storms have served to periodically demand a review of their 

 use and design, the most recent being the flood of February 1, 1953, where 

 1,783 people were killed and total damage was estimated at 250 million 

 dollars (Wemelsf elder , 1953). This disaster prompted construction of the 

 massive Delta Project, whose large storm surge barriers were just recently 

 completed (see Kohl, 1986). As shown in Fig. 6.6, almost half of the 

 Netherlands is below mean sea level and protected by dikes. The situation 

 here is a clear microcosm of the future of many regions around the globe if 

 the greatest estimates of sea level rise prove accurate. 



Seawalls, bulkheads, and revetments - are structures of concrete, 

 masonry, steel sheet pile, or rubble used to armor the shoreline and 

 prevent retreat due to the combination of wave activity and sea level rise. 

 Although performing much like dikes, they usually are not free-standing and 

 are always "hard" features, with vertical or steeply sloping faces. They 

 generally are used on a local rather than regional basis and are built to 

 protect the upland long a limited section of beach. Besides their cost, 

 the major drawback to seawalls is that as sea level rises progressively 

 less sandy beach is available for recreation and additional storm 

 protection. Periodic beach nourishment is often required as mitigation. 



