impounding capacity is less than it would be if the angle were obtuse. 

 Structures perpendicular to the shore have greater impounding capacity 

 for a given length, and thus are usually more economical than those at 

 an angle, because perpendicular jetties can be shorter and still reach 

 the same depth. If the angle is acute, channel maintenance will be re- 

 quired sooner due to littoral drift passing arotmd the end of the struc- 

 ture. Planning for jetties at an entrance should include some method of 

 bypassing the littoral drift to eliminate or reduce channel shoaling and 

 erosion of the downdrift shore. (See Section 5.5 - SAND BYPASSING.) 



tSiaVfePi 



Ballona CreeK, California -Jan. 1946 

 Figure 5-15. Effects of Entrance Jetties on Shoreline. 



5.8 



BREAKWATERS - SHORE-CONNECTED 



5.81 



DEFINITION 



A breakwater is a structure protecting a shore area, harbor, anchor- 

 age, or basin from waves. Breakwaters for navigation purposes are con- 

 structed to create calm water in a harbor area, and provide protection for 

 safe mooring, operating and handling of ships, and protection for harbor 

 facilities. 



5.82 TYPES 



Breakwaters may be rubble mound, composite, concrete-caisson, sheet- 

 piling cell, crib, or mobile. In the United States, breakwaters built on 

 the open coast are generally of rubble-mound construction. Occasionally, 



5-49 



