II. TIDE CHARACTERISTICS 



1. Definitions of Tide Variations . 



Tides in the Gulf of Mexico control the daily exchange of water through 

 the many Texas passes, enhancing the quality of the water in the bays and 

 estuaries. The exchange also provides a means of transport for larval and 

 adult animal populations, and transports sediment into and out of the passes, 

 significantly affecting their stability. Knowledge of the variability in gulf 

 tides is important to understand the variability in bay tides and the hydrau- 

 lic characteristics of the Texas passes. This section presents the results of 

 an analysis defining the short- and long-term variations in gulf tides, using 

 data collected from the U.S. Army Engineer District, Galveston (SWG) and NOS 

 (formerly U.S. Coast and Geodetic Survey) tide gages. The following defini- 

 tions are used: (a) Diurnal tidal range, daily higher high water minus lower 

 low water; (b) diurnal tide level, average of higher high water and lower low 

 water; and (c) mean water level, average of hourly water levels over some time 

 period. 



In analyzing tide data collected simultaneously from the NOS and SWG gages 

 at about the same location, it was found that mean water levels often differed 

 significantly with respect to stated mean low water (MLW) datums. Whether 

 this was caused by a difference in the MLW datums or by errors in the gage 

 elevations is unknown. Regardless of the cause, quantitative level compar- 

 isons between the gages of the two agencies could not be made. Therefore, 

 much of the level data are presented as the difference between selected values 

 and a long-term mean computed from those values. 



2. Seasonal Variability in Tidal Range and Level . 



Behrens, Watson, and Mason (1977) found that seasonal variability in 

 monthly mean diurnal tidal ranges and levels affects the stability of tidal 

 inlets on the Texas coast. Therefore, data from the NOS gages at Freeport 

 Harbor, Galveston Pleasure Pier, and Sabine Pass were used to examine seasonal 

 fluctuations between 1955 and 1975. Figures 2 and 3 show the mean deviation 



_ 5 

 S. 0.4 



<z 0.3 



z 



E 0.2 



' 0.1 



1 1 r 



1 1 r-— T r — r 



Long-Term Meon Diurnol Tidol 



r 



Range 



1 



(111 





Golveslon Pleosure Pier 



>.I7 





_ 



Freeport Horbor 



.85 







Sobine Poss 



.85 







^. 





/ 





j^ \. 





// 



^^5l^ 



/"^''^^^ 





A " 





V\ 



J 



/ 



" 



■i;j\ 



/ 



- 



_ 



^^-r=. 







Golwesl 



on Pleosure Pier ( 1958-1974 1 















Sobine 



Poss 11960-1975) 







1 1 1 



_l 1 1.. ,1 1 1 



L 



1 



Jon. Feb Mor. Apr. Moy June July Aug. Sept. Ocl. Nov. Dec. 



Figure 2. Long-term monthly mean 



tidal range variability. 



13 



