reach a stable critical area of about 35,000 square feet, which will result in 

 maximum average velocities of about 3 feet per second. 



'Op Long Term 



60|- Recent 



Downdrlft Erosion , 



10 20 mi 

 Figure 39. Shoreline change rates, upper Texas coast (after Morton, 1975). 



3. Galveston Bay Entrance. 



a. Historical Review . Galveston Bay entrance is the largest pass on the 

 Texas coast and the best documented of the five passes studied. SWG and NOS 

 boat sheets were analyzed to document long-term changes in cross-sectional 

 areas of the pass and more recent changes in the adjacent beach and offshore 

 zones resulting from jetty construction. 



(1) Cross-Sectional Changes. Figures 40 and 41 show cross-sectional 

 profiles at the minimum inlet width made from the earliest survey, 1851, 

 through the latest available, 1975. Historical variations in minimum inlet 

 area, width, and hydraulic radius are plotted in Figure 42. 



From 1851 to 1908 (Fig. 40), the point of maximum depth in the inlet 

 throat moved consistently westward at a rate of about 30 feet per year; the 

 east side of the channel (Bolivar Peninsula) migrated westward at a slower 

 rate. Construction of the south jetty by 1888 fixed the west end of the 

 profile, and deposition on Bolivar Peninsula resulted in a constantly decreas- 

 ing width to 1933 (Fig. 42). Concurrent with the width reduction was a 

 general deepening of the profile. Postconstruction hydraulic radii were about 

 5 feet greater than during pre jetty conditions. Note that in 1888, a deep 

 channel began developing west of the main thalweg, and the continued growth of 

 this channel apparently caused the increase in cross-sectional area between 

 1867 and 1908. This rather sudden change in profile shape could have resulted 

 from either natural or manmade scouring action. Between 1874 and 1880, the 

 United States constructed a submerged jetty extending northeastward from the 

 north end of Galveston Island to the edge of the entrance channel (Fig. 43), 

 which could have constricted the channel and increased scour. 



Between 1908 and 1933 (Fig. 41) the thalweg increased its westward move- 

 ment rate to about 60 feet per year, double its rate from 1867 to 1908. This 

 movement could have been accelerated by the 1919-22 dredging projects which 

 increased navigation channel depths to 35 feet. The major reason given for 

 this movement is construction of the Texas City dike (Fig. 44) in 1915, which 



49 



