NOS tide station and accounting for known time lags and elevation anomalies. 

 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. 



Following a year of comparison tests at the FRF the Leupold and Stevens, 

 Inc., digital tide gauge was replaced in June 1995 by an NOS acoustic tide gauge 

 (Next Generation Water Level Measurement System, NGWLMS) located at pier 

 sta 19+20. 



The following brief discription of the NGWLMS was condensed from a 

 paper found on the NOAA World Wide Web site (Gill 1 990). 



The NGWLMS system's primary sensor is a self-cahbrating, downward- 

 looking acoustic system that transmits a short acoustic pulse through a 1 .3 -cm- 

 diameter soxmd tube to the water surface and to a cahbration point referenced to 

 the station's datum. Because the major potential sovnce for errors is vertical 

 temperature changes between the sensor and the water surface, sound tube air 

 temperatures are monitored and accompany the transmitted data. The sensor takes 

 181 1-sec samples in 3-min periods centered every 6 min. A new mean value and 

 standard deviation are computed every 6-minutes. Each NGWLMS also includes 

 a less accurate, strain-gauge type sensor as a backup. The systems relay data 

 every 3-hrs to NOAA's Geostationary Operational Environmental Satellite 

 system. NOAA's NGWLMS Data Processing and Analysis System retrieves 

 data on an hourly basis, decodes and then performs automated quality control 

 checks. 



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 data, and Figure 18 

 compares the distribution of daily high and low water levels and homrly tide 

 heights. The monthly or annual mean sea level (MSL) reported is the average of 

 the hourly heights, whereas the mean tide level is midway between mean high 

 water (MHW) and mean low water (MLW), which are the averages of the daily 

 high- and low-water levels, respectively, relative to NGVD. Mean range (MR) is 

 the difference between MHW and MLW levels, and the lowest water level for the 

 month is the extreme low (EL) water, while the highest water level is the extreme 

 high (EH) water level. 



NOTE: Due to a mistake in converting feet to centimeters the tide height statistics 

 from 1987 through 1993 (as pubhshed in the 1987 through 1993 Annual Data 

 Summaries) found in Table 7 and Figure 17 were in error. These were corrected 

 begindng with the 1994 report. 



Chapter 5 Tides and Water Levels 



39 



