a. Foundation Conditions . Foundation conditions may have a significant 

 influence on the selection of the type of structure and can be considered from 

 two general aspects. First, foundation material must be compatible with the 

 type of structure. A structure that depends on penetration for stability is 

 not suitable for a rock, bottom. Random stone or some type of flexible 

 structure using a stone mat or geotextile filter could be used on a soft 

 bottom, although a cellular-steel sheet-pile structure might be used under 

 these conditions. Second, the presence of a seawall, bulkhead, or revetment 

 may induce bottom scour and cause failure. Thus, a masonry or mass concrete 

 wall must be protected from the effects of settlement due to bottom scour 

 induced by the wall itself. 



b. Exposure to Wave Action . Wave exposure may control the selection of 

 both the structural type and the details of design geometry. In areas of 

 severe wave action, light structures such as timber crib or light riprap 

 revetment should not be used. Where waves are high, a curved, reentrant face 

 wall or possibly a combination of a stepped-face wall with a recurved upper 

 face may be considered over a stepped-face wall. 



c. Availability of Materials . This factor is related to construction 

 and maintenance costs as well as to structural type. If materials are not 

 available near the construction site, or are in short supply, a particular 

 type of seawall or bulkhead may not be economically feasible. A cost com- 

 promise may have to be made or a lesser degree of protection provided. Cost 

 analysis includes the initial costs of design and construction and the annual 

 costs over the economic life of the structure. Annual costs include interest 

 and amortization on the investment, plus average maintenance costs. The best 

 structure is one that provides the desired protection at the lowest annual or 

 total cost. Because of wide variations in the initial cost and maintenance 

 costs, comparison is usually made by reducing all costs to an annual basis for 

 the estimated economic life of the structure. 



III. PROTECTIVE BEACHES 



1. General, 



Planning analysis for a protective beach is described in Chapter 5, 

 Section III. The two primary methods of placing sand on a protective beach 

 are by land-hauling from a nearby borrow area or by the direct pumping of sand 

 through a pipeline from subaqueous borrow areas onto the beach using a 

 floating dredge. Two basic types of floating dredges exist that can remove 

 material from the bottom and pump it onto the beach. These are the hopper 

 dredge (with pump-out capability) and the hydraulic pipeline dredges. A 

 discussion of the above dredges and their application to beach nourishment is 

 presented by Richardson (1976) and the U.S. Army Corps of Engineers (1983a). 

 Hydraulic pipeline dredges are better suited to sheltered waters where the 

 wave action is limited to less than 1 meter (3 feet) , but many of the recent 

 nourishment projects have used an offshore borrow source. This has resulted 

 in specially equipped dredges and new dredging techniques. 



One of the earliest uses of a hydraulic pipeline dredge in an exposed 

 high-wave energy offshore location was at Redondo Beach, Malaga Cove, 

 California in 1968 (see Ch. 6, Sec. III,2,b). This dredge was held in 

 position by cables and anchors rather than spuds and used a flexible suction 



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