The third method based on ETL 1110-2-221 uses equation (8) with the 

 rounded-off coefficients, i.e., 



1.0 



H s 0.4 + (2.56/76.46) 1 /2 (2.5) 



= 1.17 



and. R s = 2.56 (1.17) = 3.00 meters (9.84 feet). Increasing the significant 

 runup by 50 percent gives 



Kmax = 3.00(1.5) = 4.50 meters (14.76 feet) 

 Table 5 provides a summary of methods used in this problem. 

 Table 5. Example problem 4 summary. 



Method 



^nax 



(m) 



(ft) 



Stoa (1979) 

 This report 

 ETL 1110-2-221 



5.04 

 3.67 

 4.50 



16.54 

 12.04 

 14.76 



*************************************** 



The rather wide range of estimates for Rmax shown in the example 4 sum- 

 mary (Table 5) is partly due to the inherent difficulty in estimating extreme 

 values and the specific difficulty of adapting the results of monochromatic 

 wave tests to irregular wave conditions in relatively shallow water. To 

 evaluate which of the three methods would produce the best estimates of Rmax* 

 a comparison was made with observed values from the laboratory tests of Ahrens 

 and Seelig (1980) . These tests measured the maximum wave runup on a riprap- 

 protected dike using various irregular wave conditions. The dike had a slope 

 of 1 on 2 and a submerged fronting slope of 1 on 15; some of the water levels 

 tested had wave conditions similar to those in example 4. All three methods 

 overpredicted the observed maximum runup on an average, and overpredicted for 

 most of the individual conditions compared. Stoa's method overpredicted Rmax 

 by an average of 38 percent, the method of this report by 29 percent, and the 

 method of ETL 1110-2-221 by 38 percent. Since data were available only for 

 one slope with which to compare predicted and observed values, it is not clear 

 how general the tendency to overpredict is. Based on the comparison, the 

 method of this study is regarded as the best estimate of maximum runup; however, 

 the value from another method might be selected in order to be conservative. 

 Laboratory tests to improve the existing guidelines for estimating the charac- 

 teristics of irregular wave runup are now underway at CERC. 



9. Overlays . 



Overlays are single layers of larger stone placed on top of existing rip- 

 rap which is too small to provide adequate protection to the embankment . The 

 concept of an overlay as a simple and logical method to upgrade existing re- 

 vetment was developed by the U.S. Army Engineer Division, Missouri River (see 

 McCartney and Ahrens, 1976). Overlays using 100-percent coverage are recom- 

 mended to upgrade existing riprap; this means that all stones touch adjacent 

 stones. Photos in McCartney and Ahrens show 100-percent coverage. 



23 



