Appendix B are the average of four-rod measurements at each station, each 

 measured to the nearest 0.01 foot (0.3 cm) giving a probable error of 

 about 0.01 foot (0.3 cm) for the station. These measurements are rela- 

 tive to the top of the rod and are more accurate than the assigned sta- 

 tion elevation. The 0.01 foot (0.5 cm) accuracy is used in computation 

 of sand level changes as it is always referenced to the "top of the rod" 

 and not a change in absolute elevation. 



Measurements and soundings taken at each reference rod station were 

 used as absolute reference points for the fathometer data obtained from 

 the offshore surveys. Figure 4 shows a comparison of the reference rod 

 measurements and acoustic soundings made at the 33-foot (10 meters) sta- 

 tion on South Range. As can be seen, the acoustic sounding from the off- 

 shore surveys indicate depth variations of ±1 foot (±30 cm) relative to 

 the lead line sounding depth of 33 feet (10 meters). These differences 

 are related to errors in the survey procedure since the reference rod 

 measurements indicate little or no change in actual sand level. Similar 

 differences were observed by Inman and Rusnak (1956, Figure 9). 



4 . Recording Depth Gage . 



The beach profile surveys and reference rod data were supplemented 

 with soundings made with the recording depth gage that was developed for 

 this study. The instrument consists of a pressure sensor, logic cir- 

 cuitry, and a three-digit panel meter. An absolute pressure transducer 

 with a sensing range of about 13-30 psia was used in the instrument. 

 This sensor is capable of sensing water level changes of ±0.08 feet 

 (24 cm) in water up to -58 feet (-17.7 meters) in depth. Output from the 

 sensor was interfaced through an amplifier-low pass filter and a variable 

 gain amplifier to a digital display (Figure 5). The intent of instrument 

 design is to use an accurate pressure sensor to measure the water level 

 and then filter out the high-frequency oscillation caused by waves and 

 only read the mean depth. The low-pass filter used in the instrument has 

 a time constant of about 1 minute, so that only low-frequency water level 

 changes affect the measurements. 



The physical assembly of the instrument is such that the pressure 

 sensor and logic circuitry are one package called the sensing package; 

 and the recording panel meter and its power supply are another package 

 called the recording package. The sensing package is mounted on a tri- 

 angular-shaped metal plate that assured proper orientation of the pres- 

 sure sensor to the bottom and prevented scouring into the sand bottom. 

 This package is lowered to the bottom at a station for the 5-minute 

 recording period with a surface float to mark its location for retrieval. 

 The measurement of water depth made by the pressure sensor is averaged 

 and retained by the logic circuitry for display. At the end of the 

 measurement period the sensing package is retrieved and once at the sur- 

 face the recording package is connected to its output for display of 

 the measured depth. Once the measurement is recorded on a data sheet, 

 the instrument is cleared with a reset switch to prepare it for the next 

 measurement . 



