tests. Unfortunately, neither the largest nor smallest scales were 

 tested simultaneously for perhaps more than two equivalent test condi- 

 tions. Most of the comparisons must he made separately between the 

 small and medium scales, and then between the medium and large scales. 

 The comparisons for the quadripods suggest that there is less scale 

 effect than for quarrystone. Results are given in Table 23, combining 

 values for both rough and smooth quadripods. 



Table 23. 



Scale effects for quadripod rubble mound (h c /d g 

 cot 8 = 1.5) (after Dai and Kamel, 1969). 



1.1; 



<V H i 



R {large scale) * 

 R (medium scale) 



^(medium scale) 2 

 ^■(small scale) 3 



4.0, 5.0, 8.0 



«1.025 



«1.09 



1 Large scale: 



R g = 2.72 



2 Medium scale: 



R g = 1.33 



3 Small scale: 



R ff = 4.69 



x 10' 



x 10 6 . 



x 10 5 . 



A greater increase is apparent between the small and medium scales 

 than between medium and large. The tests of Jackson (1968a) were con- 

 ducted at the same scale as the "medium" scale of Dai and Kamel (a few 

 of Dai and Kamel' s test conditions and results are the same as given by 

 Jackson). Thus, minimal scale correction (k as 1.03) appears necessary 

 for the steep structure slopes tested by Jackson. 



Dai and Jackson (1966) conducted tests on a rubble-mound breakwater 

 with 1 on 2 structure slope, fronted by a gently sloping beach repre- 

 sentative of the Dana Point, California, project. This structure was 

 tested at model-to-prototype scales of 1:5, 1:50, and 1:100; toe depths 

 were basically 2.16, 0.18, and 0.09 meters (7.1, 0.6, and 0.3 feet) 

 respectively, although depths were varied somewhat at each scale. How- 

 ever, evaluation of scale-effect differences is not possible for two 

 reasons: (a) the large-scale runup tests were very limited; only about 

 three runup values are available for comparison; and (b) the runup is 

 highly variable as measured in the two smaller scale tests; in many 

 cases the medium scale had lower runup than the small scale, and vice 

 versa. Trends in values of R/H^ for constant dg/gT 2 but varying 

 H o/gT 2 are so inconsistent that further 1 analysis is not possible. 



Few studies are available for evaluation of scale effects on riprap 

 slopes. Large-scale tests have been conducted, but the test conditions 

 are only comparable to those of small-scale tests for restricted condi- 

 tions. Hudson and Jackson's (1962) small-scale tests of riprap used two 

 different water depths (scales): d s = 0.30 meter (1 foot) and dg = 0.51 

 meter (1.67 feet). The test results for these two depths are roughly 

 equivalent. Ahrens (1975a) conducted large-scale testing of riprap on 

 slopes of cot 6 = 2.5, 3.5, and 5. His H^/k^ ratio at dg/H^ = 5.0 

 was somewhat larger than that tested by Hudson and Jackson; however, the 



116 



