ranges differed little between stations: 1.80 feet for the NOS gage and 1.78 

 feet for the SWG gage. However, the annual mean diurnal levels differed 

 significantly — 0.50 foot above the National Geodetic Vertical Datum (NGVD) of 

 1929 or 1.90 feet above MLW for the NOS gage, and 0.80 foot above SWG MSL 

 datum or 2.36 feet above SWG MLW. This exemplifies the datum problem dis- 

 cussed previously. Figures 4 and 5 show almost identical monthly variations 

 in level and range for the two gages. 



c. Tidal Hydraulics . Unlike the other passes analyzed, the Freeport 

 Harbor system has a very small tidal prism. No tidal current data were found; 

 therefore, several theoretical approaches were taken to determine the hydrau- 

 lic regime. Due to time and funding constraints, two-dimensional numerical 

 models were not applied. The simplest method to compute inlet hydraulic 

 characteristics is that of Keulegan (1967), but recent refinements by Seelig, 

 Harris, and Herchenroder (1977), based on less restrictive assumptions, have 

 been found to provide more accurate results in most cases. 



(1) Keulegan Method . Keulegan (1967) defined the ability of an inlet 

 to fill its bay in terms of a repletion coefficient, K, given by 



TA I 



where 



T = gulf diurnal tidal period (89,000 seconds) 



2a = mean diurnal gulf tidal range (2.17 feet) (20-year mean at 

 Galveston Pleasure Pier) 



A = inlet cross-sectional area (square feet) 



Ai = bay area (square feet) 



R = hydraulic radius of inlet cross section (feet) 



1/3 

 f = Darcy Weisbach friction coefficient = 116n /R ; 



where n is Manning's coefficient, and 0.025 is the 



average value from field data on many inlets 



L = channel length (feet) 



g = acceleration of gravity (32.2 feet per second squared) 



From charts and other sources, the Freeport entrance parameters of A = 15,000 

 square feet. A, = 1.9 x 19^ square feet, R = 30 feet, L = 4,660 feet, and f = 

 0.023: K = 22.4, indicating complete filling of the bay. However, if this 

 were the case the tidal range in the harbor would be close to that at the 

 Pleasure Pier. Instead, it is about 0.3 foot le^s, indicating either the 

 open-coast range is less at Freeport or there is some minor loss through the 

 entrance channel unaccounted for by the Keulegan method. Multiplying the 

 long-term average harbor tidal range of 1.84 feet by the harbor area alone 

 yields an average tidal prism of 3.5 x 10 cubic feet. However, this is a 

 minimum value, since the extent to which filling of the gulf Intracoastal 



25 



