Engineer District, Galveston (1942) (Fig. 52). In this time period, 6.6 

 million cubic yards of material was deposited between the jetties, and 28.3 

 million cubic yards was eroded, for a net erosion of 21.7 million cubic 

 yards. One major zone of erosion just north of the base of the south jetty 

 was discussed previously in connection with the cross-sectional area profiles; 

 about 5 million cubic yards eroded from this area. The other zone was 

 adjacent to the middle of the north jetty, where more than 10 million cubic 

 yards eroded. Unfortunately, the effects of extensive channel dredging could 

 not be assessed because the precise dredging limits are unknown. However, 

 from the configuration of the changes, it appears that a natural shift in the 

 thalweg resulted from the jetty construction and perhaps construction of the 

 Texas City dike. This shift could have contributed to increased deposition, 

 inside the jetties adjacent to the southwest fillet, which totaled about 2.8 

 million cubic yards. 



b. Galveston Bay Tides and Currents . 



(1) Summary of Hydraulic Data Collection. Tide measurements have 

 been taken in Galveston Bay since 1852; a summary of the tide stations and 

 dates of operation is given in Figure 44 for 1852-1940 and in Figure 53 

 for 1940-78. Current measurements from 1867 to present have been taken 

 extensively. 



(2) Data Analysis and Results . This analysis summarizes the 

 hydraulic characteristics of Galveston entrance for selected time periods. As 

 will be seen, these characteristics varied little over the years. 



(a) Tidal Ranges and Levels. Figures 54 and 55 show the tidal 

 range ratio distribution in Galveston Bay for November 1936 and June 1937, 

 respectively. The values shown are the ratios between the monthly mean range 

 at each station and the monthly mean range at the Galveston Pleasure Pier. 

 Considering Galveston and East bays as a combined system, the average November 

 1936 range ratio was 0.51, and the average June range ratio was 0.49. Tidal 

 ranges and ratios for each month in 1974 are presented in the Appendix. A plot 

 of the annual mean ratios (Fig. 56) shows that the average bay range ratio in 

 1974 was 0.56, about 12 percent more than the 1936-37 range, and in qualita- 

 tive agreement with the 25-percent increase in cross-sectional area during 

 that time. 



Unfortunately, stations 3, 6, and 7 were missing several months of data. 

 Therefore, the range and level variability plots (Figs. 57 and 58) show con- 

 siderably more scatter than those for a complete year of data shown in Section 

 II. In general, however, the same trends prevail. 



(b) Currents and Discharges . Table 6 summarizes the current and 

 tidal prism data used in this study. Data for 1936 and earlier were obtained 

 from U.S. Army Engineer District, Galveston (1942); those for 1962 were 

 obtained from raw data supplied by the Galveston District. The average 

 diurnal tidal prism is about 11 x 10^ cubic feet; for a mean tidal range the 

 prism is about 6.5 x 10 cubic feet. 



c. Theoretical Stability Analysis . O'Brien and Dean's (1972) stability 

 method was used to predict the response of Galveston entrance to 

 sedimentation. However, since Manning's n value could not be determined due 

 to the lack of appropriate tidal data, curves of V^„„ versus A„ were 

 plotted in Figure 59 for various values of n'^L. Thus, using a representative 



59 



