This is the maximum moment , 

 The section modulus is 



S 



M 



^^.^P^Q ^ ^^ = 16.7 inches3 (l53; 



m ^ f^ ~ 111, 000 



From a steel pile handbook., an MZ22 or MPllO will be found to suffice. 

 A somewhat lighter section maj'- be used when the customary practice is 

 followed of using wales at the top of the sheet pile with secondaity 

 support furnished by round timber pile driven outside ^the wales, Tjjnber 

 sections or concrete pile sections can similarly be selected from respective 

 handbooks. In all probability a timber section would not be designed for 

 a differential loading as high as 9 feet, but could be used for lighter 

 loadings o 



ii6l4.o Wave Pressures o - A groin must also resist wave action during 

 periods of storm waves. The maximum wave force would occur immediately 

 following constmaction at the point where the unsupported height of the 

 pile would be a maximums However, at this point there would be no earth 

 loading to resisto As the earth pressure increased, the length of pile 

 which could be acted upon by wave action would decrease. Further., during 

 brief periods when the wave action might strike from the downdrift side, 

 the pile would be supported by the fill on the updrift side of the groino 

 Accordingly, maximum wave action and maximum earth pressure would not 

 apply simultaneously, but the pile should be designed either for one or 

 for the other o 



I|.65<- A wave will generally break in water ranging from 1,1 to 1.^ 

 times the height of the wave. Assuming a 6-foot tide, the maximum wave 

 that could break against the structure would be at about station 1 + 60 

 where the groin would extend from 2 feet below to 9 feet above mean low 

 water„ Using a breaking depth of l,3h with a 6-foot tide, the water depth 

 of 8 feet would pennit a maximum wave of 6 feet to impinge on the groin » 



ij.66o The period of the design wave would be determined from the 

 wave study. Assuming this period has been found as 10 secondSj, the length 

 in deep water is 



L^ . 5.12 T^ = 512 feet (l56) 



and the wave length at the site may be found as a function of d/LQ, 

 From Table 1^ Appendix D, for d/L^ of 8.0/512 = 0.0156, d/L = 0,050. 

 Therefore, L » d/0.050 m 8,0/0<,050 - l60 feet, 



i|67. From the profile, the beach wslope seaward of the groin was 

 found to be 1 on 20, With D = to the depth of ijater one wave length 

 from the seaward end of the groin 



223 



