7.319 Calculation of Forces and Moments on Cylindrical Piles Due to 

 Breaking Waves . Forces and moments on vertical cylindrical piles due to 

 breaking waves can, in principle, be calculated by a procedure similar 

 to that outlined in Section 7.312 by using the generalized graphs with 

 H = Hjy. This approach is recommended for waves breaking in deep water. 

 (See Section 2.6, BREAKING WAVES.) 



For waves in shallow water, the inertia force component is small 

 compared to the drag force component. The force on a pile is therefore 

 approximately 



Fm - ^Dm ^ ^D^P^^^'^Dm (7-55) 



Figure 7-44, for shallow-water waves with H = H^, gives Kj^ = 0.96 =» 1.0; 

 consequently the total force may be written 



Fm = C^ -^PgD Hg (7-56) 



From Figure 7-46, the corresponding lever arm is dj,S^^ "^ '^b (^-H) ^^"^ ^^^ 

 moment about the mudline becomes 



^m - Fm (1-11 dfo) (7-57) 



Small-scale experiments (Rg as 5 x 10^ by Hall, 1958) indicate that 



F^ « 1.5 pg D 4 (7-58) 



and 



M^ * F^ H, (7-59) 



Comparison of Equation 7-56 with Equation 7-58 shows that the two 

 equations are identical if Cd = 3.0. This value of Cd is 2.5 times 

 the value obtained from Figure 7-5 8. (C^;; = 1.2 for Rg »> 5 x lo'*.) From 

 Section 2„6, since H^, generally is smaller than (1.11) dj, , it is con- 

 servative to assume the breaker height approximately equal to the lever 

 arm, 1.11 dj,. Thus^ the prooedure outlined in Section 7.212 may also he 

 ■used for breaking waves in shallow water. However^ Cj) should be the 

 value obtained from Figure 7-58 multiplied by 2.5. 



Since the Reynolds number generally will be in the supercritical 

 region, where according to Figure 7-58, C^ = 0.7 it is recommended to 

 calculate breaking wave forces using 



N 



= 2.5(0.7) = 1.75 (7-60) 



breaking 



The above recommendation is based on limited information; however, 

 large-scale experiments by Ross (1959) partially support its validity. 



7-124 



