Rule -of- Thumb Methods 



36. Based on limited field observations, the most commonly used rule of 

 thumb states that maximum scour depth below the natural bed is roughly less 

 than or equal to the height of the unbroken deepwater wave height, or 

 S max/ H o ^ !• Data from 2-D laboratory tests conducted at the US Army Engineer 

 Waterways Experiment Station Coastal Engineering Research Center (CERC) by 

 Fowler (1992) fit within the bounds of this rule of thumb. These tests were 

 conducted using a wave flume with no currents and irregular waves on a sand 

 bed. The Fowler data are shown combined with data from other researchers in 

 Figure 13. As can be seen from the figure, some of the data from others 

 exceeds this rule of thumb. In each case, the laboratory tests by others were 

 conducted using regular waves on a sand bed. To investigate this further, the 

 CERC tests were extended to include monochromatic waves having similar heights 

 and periods. In all cases, scour depths associated with the monochromatic 

 tests exceeded scour depths associated with the irregular wave results by an 

 average of 15 percent. 



37. Dean (1986) used the "principle of sediment conservation" to 

 develop an "approximate principle" to predict the volume of local scour that 

 would occur during a 2-D situation (e.g., storm-dominated onshore-offshore 

 sediment transport). Dean proposed that the total volume of sediment lost 

 from the front of a structure would be equal to or less than the volume which 

 would have been lost if the structure were not constructed. In other words, 

 the amount (volume) of scour immediately in front of the structure would be 

 less than or equal to the volume of sediment which would have been provided 

 from behind the wall, had it not been there. 



Semi-Empirical Methods 



38. Jones (1975) used a number of limiting assumptions (including an 

 infinitely long structure and perfect reflection from the wall) to derive an 

 equation for estimation of scour depth at the toe of vertical walls. Jones' 

 equation relates ultimate scour depth S d to breaking wave height H b and X s , 

 the dimensionless location of seawall relative to the intersection of mean sea 

 level and the beach profile. Jones defined X s as follows: 



X - X 



X s ~ -^ (50) 



where X is the distance of the seawall from the point of wave breaking and X b 

 is the distance of the point of wave breaking from the intersection of MSL 

 with the pre-seawall beach profile (see Figure 14). Both distances are 



36 



