only the peak measured vel(xity values were used herein, drop-outs did not pose a 

 particular problem. 



For this experiment, the LDV probe was pointed through the glass flume 

 wall into the voids within the armor layer and measurements were made of the internal 

 flow within the porous media. Many of the voids between armor units were more than a 

 nominal armor diameter deep, so measurements could be made outside the flume wall 

 boundary layer. One drawback to the LDV was the requirement of a full time operator 

 with continuous attention to detail. Also, because of the small measurement volume, 

 small changes in measurement location in the sheltered region behind a stone or within 

 the armor layer often yielded large variations in measured average peak velocities. 

 Therefore, the instrument required many measurements to map the flow field. So data 

 analysis requirements were substantial for this experiment. 



The wave heights were determined using free surface measurements from a 

 vertical capacitance-type gage positioned at the location of the structure toe with no 

 structure in place. Synthetic rubberized horse-hair mats were placed landward of the 

 structure location to absorb the waves. The sampling rate for free surface measurements 

 was 20 hz. The zero-downcrossing wave height was computed as the average height 

 from a burst of approximately ten regular waves. 



16 



