70 



Fig. 6.4. Quisitis Point and Wya Point are two natural headlands 

 responsible for the large embayment in between. Although historical 

 shoreline changes for the bay are not readily available, the general 

 behavior in response to future sea level rise is expected to be as 

 described. Part of the erosive potential of the rise will be spent on 

 lengthening the shoreline of the bay as it enlarges. So, the average 

 retreat of the shoreline will be less than on the open coast. 



6.3 CONSTRUCTED WORKS 



Dikes and levees - are free-standing, elongated mound- like structures 

 used to prevent coastal and riverine flooding and to create usable land 

 from low- lying, previously inundated wetlands. They are usually 

 constructed of earth or sand (armored by clay, asphalt, rubble or 

 vegetation), masonry, and concrete, and are often assisted by pumps to 

 remove seepage. A few typical design cross-sections are shown in Fig. 6.5. 

 Although costly to construct and maintain, dikes are the only means of 

 totally preventing shoreline retreat (both inundation and wave- induced) due 

 to a rise in relative sea level. They "modify" shoreline response 

 essentially by creating a new shoreline at the structure location, and have 

 been successful in many places throughout the world, more notably the 

 Netherlands. If long-term sea level rise is significant, dikes may be the 

 only workable means of protecting coastal cities. 



The effect of sea level rise on existing dikes and the design of new 

 ones is manifested predominantly in the required crest elevation of the 

 structure. This is the elevation that prevents significant overtopping 

 during the design storm. Crest elevation in turn determines the 

 cross -sectional area of the structure and the volume requirements for 

 material. A crude relationship is that the height increases directly with 

 the rise in sea level and area increases with the square of the increase in 

 sea level. More precise estimates depend on the actual design 

 cross -section. The major sea level related question confronted in the 

 design of new dikes and levees is whether to include projected long-term 

 estimates or not. Answers depend on the site specific estimate of the rate 

 of sea level rise, the type and method of construction to be used, the 

 expected life span of the structure, and the expected frequency of 



