Using this value of E, in equation (6) gives the relative rundown, i.e., 



Rd 98 "2-46/? 

 -^- = -2.32e = -0.99 

 H s 



which can be confirmed in Figure 8. Then 



Rd 98 = (7.0) (-0.99) = -6.9 feet (-2.10 meters) 



and using Appendix A to correct this rundown for scale effects gives 



Rd 98 (corrected) = -6.9(1.128) = -7.8 feet (-2.38 meters) 



The same scale correction factor used for runup is used for rundown. 



*************** EXAMPLE PROBLEM 3*************** 



This example illustrates how the results of tests with irregular waves on 

 smooth slopes can be applied to situations where the structure is rough and 

 porous. 



GIVEN: A rubble-mound breakwater is to be built with a slope on the seaward 

 face of 1 on 2 which will be overtopped by wave action only occasionally 

 under the design conditions. The design conditions include a significant 

 wave height, significant wave period, and water depth at the toe of the 

 structure of 15.0 feet (4.57 meters), 12.0 seconds, and 45.0 feet (13.72 

 meters), respectively. The core of the breakwater will be slightly above 

 the design water level, i.e., a high core breakwater. 



FIND : The height at which the breakwater will only occasionally be overtopped 

 during the design conditions. 



SOLUTION : The period of peak energy density is 



T p = 1.05(T S ) = 1.05 (12.0) = 12.6 seconds 

 and the steepness parameter is 



H s 15.0 



gT p 2 32.2(12.6) 2 



= 0.00293 



Using equation (2) with the coefficients in Table 1 for a plane, smooth slope 

 of 1 on 2 and R2H/ S gives 



R 2 



— = 3.2083 + 71.879 (0.00293) = 3.42 



H s 



(this value can be checked in Fig. 3) and 



R 2 = 3.42(15.0) = 51.3 feet (15.64 meters) 



17 



