Note that acoustic and laser profiling instruments were available as 

 alternatives to the mechanical system used, but the signal returns off the irregular stone 

 surfaces were poor resulting in unacceptably high data analysis requirements and low 

 accuracy. Also, these backscatter-type sensors were generally an order of magnitude 

 more costly. The spheroid diameter was chosen to coincide with that of Davies et al. 

 (1994) so that results could be compared and was nearly twice the diameter of the 

 sounding disc in the WES eroded volume method. In the present case, the sphere 

 provides varying resolution of voids depending on the depth of penetration between 

 stones and so corresponds to a range of sounding disc sizes. No attempt is made herein 

 to relate the sphere size to the WES sounding disc size since there exist no damage 

 progression data from the historical records that are directly comparable with data 

 obtained for the present study. The profiler arm pivot point was at a fixed elevation just 

 above the crest of the structure with the profiler arms extending back toward the 

 structure. The angle of an arm at any point during the profiling process was determined 

 through recorded voltages from rotational potentiometers attached to the pivot on the 

 carriage. The location of the carriage was determined using a linear potentiometer fixed 

 to the flume wall. 



It was required that the profiler arms be just heavy enough to continuously 

 follow the structure without plowing, or moving stones. Another requirement was that 

 the structure be profiled without lowering the water level between runs. Therefore, the 

 hinge point, rod length, rod and ball weight, and counter weight were chosen to provide 



77 



