stone, for exaraple^ because of shortages of steel. The price which must 

 be paid for the constituent materials is a major item in first con- 

 struction and maintenance costs. If these materials are not available 

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

 of seawall or bulkhead may become economically infeasible to construct. 

 In some instances a compromise may have to be made and a lesser degree 

 of protection provided, 



260. Costs . - The analysis of costs must include the first costs of 

 construction, the estimated maintenance charges, and amortization of the 

 investment, over the economic life of the strjcture. Other things being 

 equal, that structure would be built which would provide the desired 

 degree of protection at the lowest annual or total, cost. Because of wide 

 variations in first and maintenance costs, this comparison is usually 

 made by reducing all costs to an annual basis, 



TYPE PROBLEMS 



261, High Semi-Gravity Type Concrete VJall . - It is desired to build 

 a protective seawall in an sxea where there is a 6-foot tide and a 

 possible additional 2-foot wind set-up. An erosion trend analysis in- 

 dicates that the expected ultimate ground line at tine wall's location will 

 be 2 feet below mean low water, with a bottom slope before the wall of 

 approximately 1 on 20. 



262, Analysis of refraction diagrams indicates that waves from a 

 certain direction may approach the area without decrease in height due to 

 refraction while waves from other directions will be significantly de- 

 creased. Analysis of synoptic weather charts indicates that waves from 

 this direction, under storm conditions, may have deep water heights up to 

 8 feet and periods from 7 to 11 seconds, 



263. The backfill material has a unit x\reight of 120 pounds per 

 cubic foot, and an angle of internal friction of 2^°. The backfill will 

 be subject to a uniform surcharge load of 2^0 pounds per square foot. 

 The foundation material has a bearing capacity of 2,000 pounds per square 

 foot. 



26I|.. Design, - General . - To reduce water overtopping, the face 

 of the wall will have a re-entrant angle of l5 to the vertical. To 

 .reduce beach scour, the face of the wall will be stepped. The wall 

 base is to be at least 2 feet below the ultimate ground line. A vSheet 

 pile cut-off wall is to be placed at the toe as a safety factor to prevent 

 damage to the vjall by undercutting of the foundation should the beach 

 lower to a greater depth than estimated. 



265= Computation of ^'''ave Forces . - Deep water wave lengths may be 

 computed from the relationship Lq ^ 5.12 T'^e With these the following 

 table should be drawn up to determine whether waves may break on the 

 structure, 



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