hurricane protection system. Simultaneous tidal data are available for the 

 SWG South Jetty and Marsh Point gages. 



Between November 1971 and February 19/2 Prather and Sorensen (1972) oper- 

 ated tide gages in Rollover Bay, and at selected times measured current speeds 

 in the pass. Tide data are plotted in the report, but no current data were 

 published. 



(2) Data Analysis and Results . The present-day hydraulics of Roll- 

 over Pass were defined, and preconstruction and postconstruction data were 

 compared to determine any effect of inlet construction on the bay's hydrau- 

 lics. Tables 7 and 8 summarize the available information on tidal ranges and 

 lags in the vicinity of Rollover Pass. Before the pass was constructed, 

 Rollover Bay was filled entirely by water entering the Galveston entrance. 

 Tidal lags were about 1.3 hours longer than postconstruction conditions, and 

 bay tidal ranges were about 23 percent less than in 1965. In comparing two 

 postconstruction conditions (1965 and 1974) the bay tidal ranges were greater 

 for the earlier years, and the tidal lags were shorter, indicating that the 

 efficiency of the pass to fill the bay decreased. Another indication was the 

 lag between the Gilchrist and Marsh Point gages. In 1965, this time differ- 

 ence was 0.35 hour, while in 1971-72 Prather and Sorensen (1972) computed the 

 difference to be about 0.8 hour. Their study also discussed the propagation 

 of the tide into East Bay. They found that high and low waters occurred first 

 in Rollover Bay, then at Marsh Point, and finally at Hanna Reef, indicating 

 that East Bay tides were influenced primarily from flow through Rollover Pass, 

 an unusual condition in view of the pass's small effect on tidal range. To 

 evaluate this conclusion, tidal lags between Marsh Point and Hanna Reef were 

 calculated for the period 16 July to 9 September 1974. On the average, tide 

 extremes at Hanna Reef lagged those at Marsh Point by about 0.3 hour. How- 

 ever, many times the reverse was true, and it appeared that the direction of 

 tide propagation was influenced by at least two factors — bay water level and 

 wind effects. Figure 61 is a plot of the tidal lag between Marsh Point and 

 Hanna Reef versus the average daily water level at Marsh Point, computed as 

 the mean of the diurnal high and low. Note that for levels greater than +1 

 foot MSL, tide extremes usually occur at Marsh Point first, while for lower 

 levels they are evenly distributed. 



Seasonal variability in tide level and tidal range is plotted in Figures 

 62 and 63, respectively, for Marsh Point and Galveston South Jetty gages in 

 1974. Note the close correspondence in level variation. Range variability is 

 also similar except during June and July, when tidal ranges were below normal 

 at Marsh Point. 



d. Tidal Hydraulics . Tide and current data collected in 1965 by the 

 Galveston District were compared with 1971 data by Prather and Sorensen 

 (1972). The 1965 velocity measurements were made from the bridge channel 

 centerline at three depths, between 0530 hours, 4 May and 1300 hours, 5 May 

 1965. A plot of the average velocity variation is shown in Figure 64. Also 

 plotted are the tidal curves for the Galveston South Jetty and Marsh Point 

 locations. Note the strong predominance of flood currents (flood prism = 

 3.8 X 10^ cubic feet, ebb prism = 6.7 x 10^ cubic feet), due to 15- to 20- 

 mile-per-hour winds from the south during the entire measurement period. 

 Table 9 summarizes the 1965 measurements. Using the velocity and tidal 



69 



