serious erosion unless measures are taken to transfer or bypass the sand from 

 the updrift side to the downdrift beaches. 



Several techniques of mechanical sand bypassing have been used where 

 jetties and breakwaters form littoral barriers. The most suitable method is 

 usually determined by the type of littoral barrier and its corresponding 

 impoundment zone. The five types of littoral barriers for which sand transfer 

 systems have been used are illustrated in Figure 6-39. The basic methods of 

 sand bypassing are as follows: fixed bypassing plants, floating bypassing 

 plants, and land-based vehicles or draglines. Descriptions of selected 

 projects illustrating sand bypassing techniques for various combinations 

 of littoral barriers are presented in the following sections. 



1 . Fixed Bypassing Plants . 



Fixed bypassing plants have been used at South Lake Inlet, Florida, and 

 Lake Worth Inlet, Florida (both type I inlet improvements, see Fig. 6-39), and 

 at Rudee Inlet, Virginia Beach, Virginia (type V inlet improvement). 



In the past, in other countries, fixed bypassing plants were used at 

 Salina Cruz, Mexico (U.S. Army Beach Erosion Board, 1951), and Durban, Natal, 

 South Africa (U.S. Army Corps of Engineers, 1956). Both were located at 

 breakwaters on the updrift sides of harbor entrances. The Salina Cruz plant 

 rapidly became land-locked and was abandoned in favor of other methods of 

 channel maintenance (U.S. Army Corps of Engineers, 1952, 1955). The Durban 

 plant bypassed about 153,000 cubic meters (200,000 cubic yards) of sand per 

 year from 1950 to 1954; afterward the amount decreased. Because of insuffi- 

 cient littoral drift reaching the plant, it was removed in 1959. No apparent 

 reduction in maintenance dredging of the harbor entrance channel took place 

 during the 9 years of bypassing operations. Starting in 1960, the material 

 dredged from the channel was pumped to the beach to the north by a pump-out 

 arrangement from the dredge with booster pumps along the beach. 



a. Sou th Lake Worth Inlet, F lorida (Watts, 1953; Jones and Mehta, 1977). 

 South Lake Worth Inlet, about 16 kilometers south of Palm Beach, was opened 

 artificially in 1927 to provide increased flushing of Lake Worth. The dredged 

 channel was stabilized by entrance jetties. The jetties caused erosion of the 

 downdrift beach to the south, and construction of a seawall and groin field 

 failed to stabilize the shoreline. A fixed sand bypassing plant began opera- 

 tion in 1937. The plant consisted of a 20-centimeter (8-inch) suction line, a 

 15-centimeter (6-inch) centrifugal pump driven by a 48 .5-kilowatt (65 horse- 

 power) diesel engine, and about 365 meters of 15-centimeter discharge line 

 that crossed the inlet on a highway bridge and discharged on the beach south 

 of the inlet. 



The original plant, with a capacity of about 42 cubic meters (55 cubic 

 yards) of sand per hour, pumped an average of 37,000 cubic meters (48,000 

 cubic yards) of sand per year between 1937 and 1941. This partially restored 

 the beach for more than a kilometer downcoast. During the next 3 years (1942- 

 45) pumping was discontinued, and the beach south of the inlet severely 

 eroded. The plant resumed operation in 1945, stabilizing the beach. In 1948 

 the plant was enlarged by installation of a centrifugal pump, a 205-kilowatt 

 ( 275-horsepower) diesel engine, a 25-centimeter (10-inch) suction line, and 



6-54 



