change relative to the initial survey taken before any waves were riin. 

 The volumetric, changes were a measure of the damage to the embankment, 

 and were plotted versus wave height to estimate the zero-damage wave 

 height. As each test progressed there was usually some wave height at 

 which an abrupt increase in the damage occurred. The wave height that 

 preceded this abrupt change in the rate of damage is considered the zero- 

 damage wave height, ^d=0- '^^ damage that occurred to the embankment be- 

 fore reaching the zero-damage wave height was usually insignificant, and 

 resulted from the slight movement of a few stones which were in unstable 

 positions at the completion of construction of the embankment. As an 

 additional aid to establishing the zero-damage wave height, extensive 

 written records of visual observations during the testing were made. 

 Normally, the visual method of estimating the zero-damage wave height was 

 used to confirm the survey data method, and there was always close agree- 

 ment between the two. 



Stability of riprap is often measured as the stability number, Ng . 

 The stability number, which is a dimensionless zero-damage wave height, 

 can be used to compute a stable armor unit weight (Hudson, 1958), and is 

 defined by: 



where W^q is the median weight of the stone (pounds), Wj, is the unit 

 weight of the stone (pounds per cubic foot) , and Sp = ^r/^W' ^w i^ the 

 unit weight of the local water. Since the weight distributions for the 

 overlay stone used in the small tank tests were not obtained, it was con- 

 venient to define a stability number based on average weight. The average 

 weight stability number is given by: 



^D=0 



J^V/^ (S, - 1) 



(3) 



where W is the average weight of the stone (pounds) . N* was useful 

 for making comparisons between various small-scale tests and between small- 

 scale and prototype tests since the average weight was known for all tests. 

 However, it should be noted that N* is not the stability number normally 

 used. 



An embankment with a stone overlay was considered a failure when enough 

 of the stone overlay and riprap underlayer had been removed by wave forces, 

 leaving the filter layer clearly exposed. An embankment without a stone 

 overlay was considered a failure when enough riprap had been removed to 

 clearly reveal the filter. 



Another useful definition is the tolerable-damage wave height, Hp_^, 

 which is the largest wave height that does not remove either filter or rip- 

 rap material through voids of the stone overlay. A wave height of ^jj^-f; 

 will move some overlay stones around but the damage is considered tolerable 



16 



