Table 9. Summary of Rollover Pass hydraulic data. 



Tidal data by date 



Flow 

 area 



(ft2) 



Average head 

 differential 



(ft) 



Average 

 velocity 



(ft/s) 



Maximum 



average 



velocity 



(£t/s) 



Tidal prism 



(ft3) 



4 and 5 May 1965 

 Flood 

 Ebb 



1971 



One tidal cycle 



(Prather and Sorensen, 1972) 



Nov. 1971-Feb. 1972 



1,900 



1,130 

 1,290 



0.67 

 0.80 



1.5 



0.67 

 0.67 



^Unknovm. 



^Predicted from 1965 V versus AH. 



^Predicted by Prather and Sorensen ( 197 



3.26 

 1.22 



2.71 



3.1 ^ 

 0.663 



6.02 

 1.95 



4.23 



3.8 

 6.7 



X 108 

 X 10^ 



8 X 



2). 



differential data, the total friction loss through the inlet during the 1965 

 measurements was determined. Manning's friction coefficient is given by 



1.49R^/^ AH^/^ 



(9) 



where 

 R 

 AH 

 V 



L 



hydraulic radius 



head differential between bay and gulf 

 average velocity through the pass 

 inlet length 



Throughout the tidal cycle, L, n, and R are assumed to remain constant 

 which means V varies as a function of AH^'^. To find the function, all 

 instantaneous values of V and AH^'^ were plotted (Fig. 65). A line was 

 best fit to the points using a linear regression analysis, and the slope of 

 the line was substituted into equation (9), using values of R = 6.5 feet and 

 L = 1,200 feet. The resulting value of n was 0.032, which includes the 

 resistance due to the weir. 



Velocity measurements by Prather and Sorensen (1972) were made 500 feet 

 north of the bridge at two depths at eight locations across the channel. 

 Although instantaneous current data were not published, the data were calcu- 

 lated from plotted discharge values and the assumed average cross-sectional 

 area of 1,130 square feet. In plotting the data in Figure 65, note that 

 although the best-fit line does not pass close to the origin, the slope of the 

 line is almost exactly the same as that for 1965, indicating the same value of 

 Manning's n of 0.032. Table 9 summarizes the 1971 hydraulic parameters by 

 Prather and Sorensen (1972) for both their tidal cycle measurements and their 

 predicted long-term averages. Note also that because of their average veloc- 

 ity versus head difference relationship, their long-term average tidal prism 

 was only one-tenth of the 1965 prism, and only about one-fifth of the prism 

 calculated using their average differential and the 1965 relationship. In 

 view of commonly reported high-velocity conditions in the pass, it is 



73 



