RIPRAP STABILITY SCALE EFFECTS 



bij 

 Laurie L. Broderick and John P. Ahrens 



I. INTRODUCTION 



Small-scale wave tank tests of riprap stability were conducted at the U.S. 

 Army Coastal Engineering Research Center (CERC) , and the results were compared 

 with previously conducted large-scale tests of riprap stability (Ahrens, 1975) 

 to determine the nature and magnitude of scale effects. The large-scale tests, 

 conducted in CERC's large wave tank (LWT) , used wave heights which exceeded 



1.5 meters in some instances and can be regarded as prototype scale. The small- 

 scale tests replicated the LVJT tests at a 1:10 (model: prototype) Froude scale. 

 Both small-scale and LWI tests were conducted using monochromatic waves (waves 

 of constant height and period) . The results of the experiments were used to 

 evaluate scale effects correction factors. These results will also be used to 

 evaluate future small-scale tests of riprap stability using irregular waves, 

 the next phase in this study. 



It has only been in the last few years that irregular wave conditions could 

 be satisfactorily generated in the laboratory but only at small scales, because 

 of the unavailability of a prototype-scale irregular wave research facility. 

 This investigation of scale effects will allow, with some confidence, the 

 extrapolation of the small-scale test results to prototype scale when the 

 model/prototype-scale ratio is 1:10. It will also give general insight into 

 the nature of scale effects to be expected when conducting model experiments at 

 other scale ratios. 



II. TEST SETUP AND PROCEDURE 



1. Large Wave Tank (LWT) Tests . 



Ahrens' (1975) large-scale tests were conducted in the LWT which is 193,6 

 meters long, 4.6 meters wide, and 6,10 meters deep, A Stillwater depth of 



4.6 meters was used for all tests. The distance between the toe of the embank- 

 ment and the mean position of the wave generator blade varied from about 119 to 

 137 meters, depending on the slope of the embankment being tested. Details on 

 the wave tank and generator are given in Coastal Engineering Research Center 

 (1980). 



The embankment was made up of core material, a filter layer, and an armor 

 layer. The core material was compacted bank-run gravel, graded to the desired 

 slope, and was essentially impermeable to wave penetration. The filter layer, 

 15 to 21 centimeters thick, was placed between the core material and the armor 

 layer. The filter stone was sized such that the ratio of the 15 percent finer 

 diameter of the riprap stone, d2 5, to the 85 percent finer diameter of the 

 filter stone, dgs, was usually less than 4 and always less than 5, The armor 

 stone, which was a diorite with a specific gravity of 2,71, was divided into 

 three stockpiles according to the median weight, W50. One stockpile ranged 

 from 12,2 to 16,3 kilograms, another from 33,1 to 35,4 kilograms, and the third 

 was constant at 54,4 kilograms. The relative size gradations of the three 

 stockpiles were the same; the specified maximum stone weight was four times the 

 median weight and the minimum weight was one-eighth of the median weight. 



