an alinement perpendicular to the predominant direction of wave attack 

 as established by the direction of the orthogonals in a wave refraction 

 diagram. The spacing between groins of a system should be correlated 

 with the length of the groins so that the final stabilized alinement of 

 the fillet between groins will provide the minimum width of beach 

 desired at the updrift groin with a sufficient margin of safety to pre- 

 vent flanking of the updrift groin„ The distance between adjacent 

 groins is in the range from one to three times the total groin length. 



6. Advantages of Treated Timber . 



Marine- treated timber is a recognized construction material for 

 building groins. It has the requisite strength to resist the applied 

 forces at locations with mild to moderate wave action. There is the 

 distinct advantage in withstanding abrasion from sand driven by the 

 wind without ill effects. The marine treatment provides an effective 

 protection for the timber against marine borers. 



7. Acting Forces . 



The two significant forces to be considered in design of groins are 

 earth pressure and wave action. The ordinary methods of computing earth 

 pressure from granular materials are applicable. The magnitude of 

 forces from earth pressure depend on the differential in level of the 

 material on the two sides of the sheet piling. For an impermeable 

 barrier wall, the maximum differential would occur at a time when the 

 accretion fillet has about reached its stabilized depth updrift of the 

 groin and the deposit of material on the downdrift side has only begun. 

 The height differential in sand level on the two sides of a permeable 

 groin would be considerably less than for an impermeable groin. 



Three types of wave action are: 



(a) Reflected oscillatory motion (nonbreaking waves); 



(b) breaking waves; 



(c) broken waves (onrush) « 



For a discussion of methods for computing wave forces for the three 

 types of wave action and illustrative examples, see U.S. Army, Corps of 

 Engineers, Coastal Engineering Research Center (1973). 



8. Sizes of Structural Members . 



Tlie conventional method of arriving at the sizes of members is to 

 follow successful examples from past engineering experience for similar 

 environmental conditions. Whereas this method does not lend itself to 

 close design tolerances, approximate coii;;)utations can be made to investi- 

 gate the strengths of the component parts shown for an existing design 

 in relation to the environmental conditions expected at a particular 

 location. 



