78 



in Japan. Although initial construction costs can be high, proper design 

 usually ensures low maintenance. The shoreline response and functional 

 design of offshore breakwaters is extensively discussed in Dally and Pope 

 (1986) . 



As sea level rises, an existing breakwater project will lose sediment 

 from its salient(s) as its relative position moves offshore and overtopping 

 becomes more frequent. In order to maintain shoreline position and a 

 prescribed level of protection, the structure will need to be lengthened 

 and its crest elevated. Otherwise, projected sea level must be used in 

 both structural and functional design, with the margin of safety 

 diminishing as sea level rises during the life of the project. 



An example of a segmented breakwater project installed to provide 

 shoreline protection and a recreational beach is found at Presque Isle, 

 Pennsylvania. The project, shown in planform in Fig. 6.9, consists of 

 three segments, each 38 m long and placed 46 m offshore of a beach fill. 

 There is a substantial longshore drift (from left to right) from which the 

 structures have entrapped additional sand to form a series of salients 

 which progressively diminish in size in the drift direction. These 

 salients erode during storms and accrete in calm weather, but the placed 

 fill has remained relatively unscathed. 



Groins - are shore -perpendicular structures made of timber, steel or 

 concrete sheet pile, or rubble, whose purpose is to entrap sediment moving 

 alongshore. The shoreline accretes on the updrift side and erodes on the 

 downdrift until sand is able to pass around the end of the structure and 

 restore the longshore drift. If the fillet is placed artifically during 

 groin construction, much of the downdrift erosion can be prevented. For 

 long stretches of beach, a groin "field" of many structures is used, 

 examples of which are found at Rehoboth Beach, Delaware; West Hampton 

 Beach, Long Island, New York; and Madeira Beach, Florida. It is stressed 

 that groins are only useful if local erosion is due to spatial variation in 

 the longshore drift, and have little positive impact if erosion is due to 

 on/offshore sediment transport. Consequently, the use of groins will do 

 little to modify shoreline response to sea level rise. 



