Scour Prediction at Piles or Other Vertical Supports 



34. For a more complete description of scour prediction methods for 

 scour at vertical piles, consult Herbich et al. (1984) and Einstein and Weigel 

 (1970) . Based on test results from a laboratory study in which 39 flume tests 

 were run to examine effects of waves, currents, and the combination of the 

 two, Herbich et al. concluded that scour at the base of vertical piles caused 

 by wave action alone is insignificant and proposed the following: 



For local scour, S 1 , in ft, which occurs in the immediate vicinity of the 

 obstruction causing the scour: 



logJ-^M = -1.2935 + 0.1917 log 10 (i (*5) 



where 



3 _ & L uj D p \u b + (1/r - u b /L ) H L /2hf 

 [(p. - p)/p] v g 2 h* d g 



In the above , 



D = pile diameter, ft 



u b = near -bottom current velocity, ft/s 



For total scour depth, S t , in ft, which occurs over a much larger area and 

 includes local and general scour: 



logj-^j = -1.4071 + 0.2667 log 10 P ( 47 ) 



For the 39 tests conducted, the correlation coefficient r for Equation 45 was 

 reported to be 0.970 and 0.905 for Equation 47. In addition to the above 

 equations, an additional parameter a, which can be used to determine whether 

 general scour will occur, was developed and is given as: 



a = "° L ° U » \ Ub+ {1/T ~ Ub/L ° ] H ° L ° /2h ? (48) 



[(p s - p)/p] g 2 h* d g 



According to Herbich et al., general scour will not occur when a < 0.02 and 

 total scour will be limited to that associated with local scour. The above 

 relations were not verified using prototype data. A useful relationship 

 between a and /3 is 



33 



