33. Based on these percents of the storm wave hydrograph (Table 5, 

 column 6) , the appropriate regression curve was used to obtain the overtopping 

 rate for each hour of the storm. The last three columns in Table 5 are the 

 overtopping rates from the regression curve and the upper and lower prediction 

 intervals, respectively. The values are cubic feet per second per linear foot 

 of seawall and are plotted in Figure 9. Complete results of the study are 

 presented in Part VI. 



Relative Freeboard Method 



34. The RFM is an expression which was originally developed during the 

 Roughan's Point physical model study (Ahrens, Heimbaugh, and Davidson 1986). 

 Because seawall overtopping rates are dependent on local wave heights and 

 wavelengths, relative freeboard parameter was defined. It is best applied to 

 situations where the shallow-water wave climate is known or has been simulated 

 for the design storm conditions. Relative freeboard is defined as 



F' = 



\ mos p/ 





where 



F = average freeboard, defined as the distance between the 

 crest of the seawall and the local swl 



H = zero-moment wave height at the toe of the structure 

 mos 



L = significant wave length associated with the peak period 



at the toe of the structure 



35. The STHM was developed to supplement the RFM because the RFM merges 

 all test conditions (extratropical storm and hurricane sea conditions with 

 nondesign storm related wave heights) into a single data set to calculate 

 overtopping rates. The two storms include different wave spectra (Figure 4) 

 and related overtopping characteristics. Combining the physical model data 

 did not appear appropriate for computing the design peak overtopping rate. 



36. The design data of interest, which occurred at high wave energies, 

 did not fit the general RFM exponential curve based on the entire data base. 

 Figure 10 shows the exponential relation which does not account for the higher 

 data points located near the center of the graph. The deepwater wave height 



24 



