u h D 

 — 2 — a (49) 



Example 1. Calculate local and total scour associated with a vertical pile 

 given the following information: 



D = pile diameter = 2.0 ft 



H Q = 6.0 ft 



T = 7.5 sec 



h = depth = 20.0 ft 



d g = 0.25 mm = 0.00082 ft 



p s = sediment density =2.65 lb/ft 3 



p = fluid density =1.00 lb/ft 3 



v = kinematic viscosity = 0.00001 ft 2 /s 



u b = near bottom current velocity = 1.50 ft/s 



Solution: The initial step is to determine whether scour will be confined to 

 the immediate vicinity of the pile or if general scour will also occur. This 

 is accomplished by calculating the parameter a: 



First, determine the wavelength from the widely known relation 



L = 5.12To = 184.0 ft 



Then from Equation 48 , 



(6.0) 2 184.0 (2.0) 2 [2.0 + (1/6.0 - 2.0/184.0) 6.0(184.0) /2 (20) I 2 

 [(2.65 - 1.0)/1.0] (32. 2) 2 (20.0)" (0.00082) 

 = 4.68 



Since a is greater than 0.02, general scour will also occur. Equation 45 

 will be used to calculate local scour but first Equation 49 is used to 

 calculate @: 



B = — £-a 

 v 



= (2-0) (2.0) ( 68) 

 0.000015 



= 152,000 



Now, from Equation 45 



34 



