concrete caisson breakwater described in the previous example (see para- 

 graph 392) on a rubble-mound substructure at elevation -10 feet, values 

 for H and H' should be found for this depth by Iribarren's method. H' is 

 used in computing the weight of the individual stone at elevation -10 feet, 

 and H is the shallow water wave height at the superstructure. The wave 

 height H should be used in the stability analysis of the concrete super- 

 structure. Assuming the same conditions as in the previous examples, the 

 wave characteristics are computed to bes 



H' - 8.1 H - IU.3 d - 10 d/Lo -O.05U3 

 d/L =0.09859 L = 101. U 



Using these wave data, the concrete caisson would be designed according 

 to the procedure described in paragraphs 392 to 398. 



SAND BY-PASSING 



14.06. It is often desired to nourish beaches downdrift from a 

 natural or artificial barrier within the littoral zone, the trapped 

 material acting as a source of supply. Such a littoral barrier may be 

 a jettied entrance to a harbor, a natural inlet, an offshore breakwater, 

 or a shore connected breakwater, A sand by-passing plant is a plant 

 designed to mechanically transport littoral drift past a barrier to a 

 point on the downdrift shore from which it will again be moved by natural 

 forces. Several methods for by-passing sand have been considered with a 

 view to reducing the cost of the operation. Methods considered at 

 various times include the following: 



a. Fixed land-based dredging plants; 



b. Portable land-based dredging plants j 



c. Eductor method with a fixed plant j 



d. Floating plant. 



Ii07. In this section those methods, already in operation, will be 

 described. 



U08. Fixed Plant , - A small fixed plant has been used for some 

 time at South Lake Worth Inlet, Florida, (see Figure 132), After con- 

 struction of jetties at South Lake Worth Inlet, erosion occurred on the 

 downcoast beach, A seawall and groin field failed to protect the 

 shore line and a small by-passing plant was placed in operation in 1937. 

 The basis of design of the pump and pumping plant as initially installed 

 was not related to the rate of littoral drift along the shore. It was 

 designed to transport the quantity of sand over a period of 2 years es- 

 timated as required to fill the groins and give adequate protection to 

 the seawall. Pumping was continued until 19i;2. 



197 ,. 



