5 Tides and Water Levels 



38 



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 2) by 

 using a Leupold and Stevens, Inc. (Beaverton, OR), digital tide gauge. 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 2.5-cm orifice and 

 a 21.6-cm-diam float. 



Operation and tending of the tide gauge conformed to NOS standards. The 

 gauge was checked daily for proper operation of the punch mechanism and for 

 accuracy of the time and water level information. The accuracy was 

 determined by comparing the gauge level reading with a level read from a 

 reference electric tape gauge. 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 orifice opening as required. 



The tide station was inspected quarterly by an NOAA/NOS tide field 

 group. Tide gauge 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 gauge and 

 handling the data. Any specific comments on the previous months of data 

 were discussed to ensure data accuracy. 



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 punch 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 



Chapter 5 Tides and Water Levels 



