5 Tides and Water Levels 



Measurement Instrument 



Water level data were obtained from an NOAA/NOS control tide station (sta 865-1370) located at 

 the seaward end of the research pier (Figure Figure 2) by using a Leupold and Stevens, Inc. 

 (Beaverton, OR), digital tide gage. This analog-to-digital recorder is a float-activated, negator-spring, 

 counterpoised instrument that mechanically converts the vertical motion of a float into a coded, 

 punched paper tape record. The below-deck installation at pier sta 19+60 consisted of a 30.5-cm- 

 diam stilling well with a 2.S-cm oriflce and a 21.6-cm-diam float. 



Operation and tending of the tide gage conformed to NOS standards. The gage was checked daily 

 for proper operation of the punch mechanism and for accuracy of the time and water level informa- 

 tion. The accuracy was determined by comparing the gage level reading with a level read from a 

 reference electric tape gage. Once a week, a heavy metal rod was lowered down the stilling well and 

 through the orifice to ensure free flow of water into the well. During the summer months, when 

 biological growth was most severe, divers inspected and cleaned the oriflce opening as required. 



The tide station was inspected quarterly by an NOAA/NOS tide field group. Tide gage elevation 

 was checked using existing NOS control positions, and the equipment was checked and adjusted as 

 needed. Both NOS and FRF personnel also reviewed procedures for tending the gage and handling 

 the data. Any specific comments on the previous months of data were discussed to ensure data accu- 

 racy. 



Digital paper tape records of tide heights taken every 6 min were analyzed by the Tides Analysis 

 Branch of NOS. An interpreter created a digital magnetic computer tape from the pimch paper tape, 

 which was then processed on a large computer. First, a listing of the instantaneous tidal height values 

 was created for visual inspection. If errors were encountered, a computer program was used to fill in 

 or recreate bad or missing data using correct values from the nearest NOS tide station and accounting 

 for known time lags and elevation anomalies. The data were plotted, and a new listing was generated 

 and rechecked. When the validity of the data had been confirmed, monthly tabulations of daily highs 

 and lows, hourly heights (instantaneous height selected on the hour), and various extreme and/or 

 mean water level statistics were computed. 



Results 



Tides at the FRF are semidiurnal with both daily high and low tides approximately equal. Tide 

 height statistics are presented in Table 7. Figure 17 plots the monthly tide statistics for all available 



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