************** EXAMPLE PROBLEM 



*************** 



GIVEN: A wave in depth d = 200 feet, with period T = 9 seconds, and a 

 maximum bottom velocity umaxr-d) 2: 1.0 foot per second. 



FIND : The minimxim wave height that will create the given bottom velocity. 



SOLUTION : Calculate 



Lo = 



and 



lit 



= 5.12(9)2 

 = 415 feet . 



200 



Lo 415 



= 0.482 



Entering Figure 4-20 with d/Lo = 0.482, determine 



u T 

 H 



H 



= 0.30 , 



"ma.C(_jjT 



0.30 



H =(l)(2) = 3ofeee. 

 0.30 



Thus a 30-foot minimum wave height with a 9-second wave period is needed 

 to create a bottom velocity equal to or greater than 1 foot per second 

 in 200 feet of water. Alternatively, a curve for a 9-second period can 

 be interpolated in Figure 4-21 and uma«(-tf)T/H can be read from the 

 curve's intersection with the 200-foot depth. 



************************************* 



As a wave moves into shallower water, both bottom velocity and size 

 of water-particle orbit increase. For some low velocities where the hori- 

 zontal orbit is small, sand moves as individual grains rolling across the 

 surface, but for most conditions, once quartz sand begins to move, ripples 

 form (Kennedy and Falcon, 1965; Carstens, et al., 1969; Cook, 1970). Thus, 

 the initiation of sediment motion is usually indicated by the formation 

 of sediment ripples. 



4-64 



