°{ 20 



lo gio|-^-| = -1-2935 + 0.1917 log 10 (152 , 000) 

 = -0.300 



so that 



S 2 = 20 (10) -°- 300 

 = 10.02 ft 



By a similar method, total scour depth is determined to be 



A). 



20j 



log 10 |^-| = -1.4071 + 0.2667 log 10 (152 , 000) 



= -0.025 



so that 



Sj = 20 (10) -°" 025 

 = 18.87 ft 



Thus, local scour depth will be approximately 10 ft and the total scour will 

 be approximately 19 ft. 



Scour Prediction at Vertical Seawalls 



35. Researchers have typically developed non-dimensional relationships 

 for predicting scour, typically expressing relative scour in terms of ratio of 

 scour depth to incident deepwater wave height, S d /H . The following section 

 briefly describes various prediction methods, laboratory studies, and field 

 studies concerning prediction of scour at vertical structures in a wave 

 environment. The following paragraphs present various methods previously 

 developed to predict scour depths in front of vertical seawalls. In general, 

 these methods can be classified as being either rule-of -thumb or semi- 

 empirical methods. Where appropriate, sample calculations are provided, with 

 similar design parameters used to allow comparison of results among the 

 various methods. For additional discussion on prediction of scour at vertical 

 seawalls, consult Herbich et al. (1984), the SPM (1984), Jones (1975), Walton 

 and Sensabaugh (1979), Barnett (1987), Powell (1987), Kraus (1988), and Fowler 

 (1992). 



35 



