iillo A by-passing plant on a large scale has been constructed at 

 Salina Cruz, Mexico. Salina Cruz is an artificial harbor in the Gulf 

 of Tehuantepec on the Pacific coast of Mexico. (See Figure 133) This 

 plant essentially consists of six l8-inch suction pipes operating through 

 two dredge p\imps. Each dredge pump is motivated by a k^O h.p. Diesel 

 engine. An l8-inch discharge line crosses the entrance to the inner 

 harbor on a drawspan and discharges on the beach downcoast from the eastern 

 breakwater, 



I|.12. The Salina Cpuz installation immediately encountered operational 

 difficulties. The suction pipes of the plant were not long enough to 

 open a channel in the beach that would permit .the free entrance of the 

 sand to the dredge. An attempt has been made to ppen this channel by 

 means of a dragline operating across the beach by a cable supported from 

 two posts. As yet it is not known whether operational difficulties have 

 been overcome. 



I4I3. In theory, the plant is designed with a capacity somewhat 

 greater than the average rate of littoral drift. Its purpose is to pump 

 initially a sufficient quantity to pull the shore line back to the future 

 alinement shown on Figure 133. Thereafter the plant would pump all 

 material coming to it along the coast. This is expected to prevent 

 shoaling of the harbor as well as to prevent damage of downdrift areas. 

 The plant has cost approximately $2 million to date. It remains to be 

 seen if it can be made to function as designed, 



hlh» Portable Land-Based Pla nt. - Because of the wide range in 

 the daily rate of littoral drift, any by-passing plant designed to pump 

 substantially all of the material moving along the coastj must necessarily 

 have a capacity of several times the average daily rate computed from an 

 annual basis, ThuSj after the operating sequence has been established 

 the plant would be required to operate only a fraction of the time - pro- 

 bably not over U to 5 hours a day. In an effort to reduce plant costs, 

 consideration has been given to a portable land-based dredging plant. 

 This plant would probably be operated from a fixed pier or other founda- 

 tion. It would have to be highly portable so that it could be rapidly 

 and easily assembled^, disassembled, and transported between sites. By 

 working full time, such a plant xjould be able to by-pass sand from several 

 sites 5 provided adequate sand traps were constructed to impound littoral 

 material between dredging. However, no known working plants of this 

 nature are presently in operation, 



iil5» Eductors . The successful use of eductors in moving li; million 

 cubic yards of sand from dunes to the beach during the preparation of 

 the Hyperion sewage treatment plant site on Santa Monica Bay, California, 

 led to a consideration of the possibilities of its use for by-passing 

 sand. In principal the eductor is merely a venturi activated by a high 

 pressure jet. It can be effectively used to move sand for distances up 

 to 2jOOO feet^, depending on the size of the eductor- and high pressure jet. 



199 



