irregular vave field, the significant wave description may be appropriate in 

 this application (see Ch. 3, Sec II). 



**************** EXAMPLE PROBLEM 2************** 



GIVEN : Quartz sediment in seawater, with a median sediment diameter 

 dcQ = 0.15 millimeter . 



FIND: 



(a) With wave period T = 10 seconds , the minimum wave height for sand 

 motion in water depth d = 10 meters . 



(b) With wave period T = 8 seconds , the maximum water depth for sand 

 motion with wave height H = 2 meters . 



(c) With wave height H = 1 meter and water depth d = 20 meters , the 

 minimum wave period for sand motion. 



SOLUTION ; From Table 4-2, y = 2.65 , and y = 1.026 , so that the threshold 

 condition for sand motion from equation (4-23) is 



(-d) 



2.65 , ^ ,„ „,, ,^ ^^o,rM0.5 



= [8 (jT^f^-O (9.81) (0.00015)] 



= 0.1365 meter/second 

 (a) For d = 10 meters (32.8 feet) and T = 10 seconds , Figure 4-29 gives 



(-^I-= 4.4 



H 



so that 



H = u T 



max , jx 

 (-g) 



4.4 



^ (0.1365) (10) 



4.4 



= 0.310 meter (1.02 feet) 



This is the required minimum wave height, since higher waves will induce 

 near-bottom velocities larger than the threshold, according to equation (4- 

 24). 



(b) With T = 8 seconds and H = 2 meters , calculate 



4-69 



