PART IV: SCOUR PREDICTION METHODS 



General 



26. For many scour problems, the primary concern is the amount of 

 scour that will occur, both in terms of area and depth. Depth of scour S d has 

 been studied by numerous investigators and a functional relationship is: 



S d = F x (p, p,, d g , g, a, h, U, v, H, T. SE ,L) (36) 



where 



d = median grain diameter, ft 



u = near bottom maximum horizontal orbital velocity, ft/sec 



/ = characteristic size of structure, ft 



H = wave height, ft 

 T = wave period, sec 



Now, through dimensional analysis techniques, Equation 33 may be reduced to 

 the following dimensionless parameters: 



m 



9 s d g U \Ps-P\9d\ ur Ud uT 



p h o> \ p A if- I 2 v H 



(37) 



A more common expression for dimensionless scour depth in cases where waves 

 are important is given by the ratio of scour depth to wave height, S d /H. 

 In the above equation, the effect of p s /p is accounted for in the fourth 

 parameter in Equation 34 so that the first parameter may be dropped. 

 Additionally, when sediment particle size is small compared to water depth, 

 the second term can be neglected as well. Finally, studies by Carpenter and 

 Keulegan (1958) showed that for oscillatory flows, scour at the bed was not 

 strongly related to the Reynold's number, so that the next- to -last term may 

 also be omitted. As before, dimensionless quantities may be formed as shown 

 below: 



£5? 



9 d \ irr 



if ) S£ 



(38) 



From the above relationship, near-bottom fluid velocity, sediment fall 

 velocity, relative sediment density ((p e -p)/p) , sediment particle mean 

 diameter, wave height, wave period, and characteristic structure length are 

 generally the most important parameters for description of local scour. Under 



27 



