parameter to determine for Rollover Pass was the effective bay area, since 

 East Bay fills both from the pass and from Galveston entrance. Before the 

 inlet was cut, the ratio of Rollover Bay tidal range to Galveston South Jetty 

 gage was 0.57. In 1971, this ratio was 0.63. Therefore, it is assumed that 

 the difference is due to flow through Rollover Pass, or that the effective 

 range due to this flow equals (0.63 minus 0.57) times the South Jetty 

 range. From continuity, the effective bay area is equal to the tidal prism 

 divided by the effective range. Using the 1971 average prism of 1.8 x 10^ 

 cubic feet and an effective range of 0.06 (2) = 0.12 foot, an effective area 

 of 1.5 X 10 square feet is found, which is exactly the same as the measured 

 area of East Bay. 



Letting the minimum inlet width remain constant at 200 feet (i.e. , letting 

 the hydraulic radius vary directly with A ), values of the maximum average 

 velocity and minimum cross-sectional area are plotted in Figure 68 for an 

 inlet length of 1,200 feet, bay area of 1.5 x 10 square feet, tidal period of 

 89,400 seconds, and Manning's n of 0.032. Also plotted are measured values 

 of V and A ^lin* Note that the theoretical curve does not agree with 

 actual values. Better agreement could be achieved if the curve was developed 

 using a much smaller bay area. For instance, if the bay range was assumed to 

 result only from flow through Rollover Pass, then the effective bay area 

 filled by this flow would be about 4 x 10 square feet. This would produce a 

 stability curve, shown as the dashline in Figure 68, which exactly fits the 

 field data. In either case, the pass is on the unstable side of the critical 

 cross-sectional area. 



r 6- 



- 5 



1 1 1 I I I I I 



<- critical 



)= Locoticn of A^ and y^ox ^O"" dotes shown 



J 1 I I I I I I I I I I I I I I 



200 



400 600 1,000 2,000 4,000 6,000 10,000 20,000 



Minimum Cross-Sectional Area, A^ift^) 



Figure 68. Average maximum velocity versus cross- 

 sectional area. Rollover Pass, Texas. 



76 



