but mean monthly water levels can vary by as much as 0.9 foot during 

 the year as a result of astronomical and meteorological influences. 

 The variability of the predicted tidal range is less than that actu- 

 ally measured. 



(b) Mean monthly range variability in 1974 was greater than nor- 

 mal (about 25 percent), and mean monthly levels varied as much as 1.4 

 feet. Thus, considerable variation in tidal prisms can be expected, 

 which impacts not only on the natural scouring capacity of the inlets, 

 but also on the exchange rate of gulf and bay waters. 



(c) Hicks (1972) showed that the trend in sea level rise between 

 1940 and 1970 at Galveston was 0.014 foot per year. This rate is less 

 than that shown in Figure 8 for 1900 to 1970 (about 0.018 foot per 

 year), and an order of magnitude less than the post-1970 rates of as 

 much as 0.15 foot per year at Freeport and Galveston. The annual mean 

 water levels at Freeport and Galveston should be monitored to deter- 

 mine whether these present trends continue. Continuation of the SWG 

 tide gage network is also recommended to document localized trends in 

 interior waters. 



(d) In the mid-1970' s mean annual tidal ranges have been at a 

 minimum in the 19-year cycle (Fig. 9). However, ranges within the 

 following decade will increase about 20 percent, reaching a maximum 

 between about 1986 and 1988. If the mean annual water level remains 

 at the 1975 value, by 1986 bay tides will penetrate to areas which are 

 as much as 1 foot above high water values in 1970. More importantly, 

 if the present trend in sea level rise continues, this penetration 

 between 1986 and 1988 could reach areas that are more than 2.5 feet 

 above 1970 high waters. 



2. Freeport Entrance. 



a. Bathymetric Changes . Since 1946, the areas offshore of Freeport 

 (Figs. 12 and 13) have generally been eroding (Seelig and Sorensen, 1973; 

 Morton, 1977). Most of the downdrift offshore change to the west is attrib- 

 uted to erosion of the former Brazos River delta. Between 1960 and 1975, 

 shoreline recession on the east side of the entrance occurred at about twice 

 the rate as on the west side. This difference may have resulted from the 

 protection afforded the west side by the relic delta and by the wave shadow 

 zone created by the jetties. However, the actual causes of this erosion are 

 difficult to determine due to the lack of definitive process data. 



b. Tides and Hydraulics. Lack of prototype data also hindered analysis 

 of the Freeport entrance hydraulics. In the absence of current measurements, 

 the long-wave equation is assumed to be the most applicable model for predic- 

 tion of entrance channel currents. However, a harbor resonance model shows 

 that amplification of long wave amplitudes in the harbor and currents in the 

 entrance channel will be reduced if the harbor is expanded as planned. 



c. Stability. The offshore part of the Freeport navigation channel is 

 plagued with rapid sedimentation, particularly between 3,000 and 4,000 feet 

 gulfward of the jetty ends. Between the jetties, currents are apparently too 

 weak to scour the bottom, and deposition also occurs. In view of the lack of 



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