range-area method average prism is 1.63 x 10^ cubic feet, which is in remark- 

 ably close agreement considering the assumptions made in delimiting the bay 

 areas and other possible sources of error. For the 1974 annual mean West Bay 

 range of 1.02 feet, a mean annual prism of 1.61 x 10^ cubic feet is obtained. 

 On the average, the July 1976 prisms showed no flood or ebb predominance, but 

 wind effects may have been important in this regard. In addition. Figure 5 

 shows that some seasonal variation in the monthly mean tidal range occurs, so 

 seasonal variations in tidal prisms can be expected, with slightly greater 

 than average prisms in January, May, June, November, and December, and lower 

 than average prisms in March, August, and October. 



e. Stability of San Luis Pass . The stability of San Luis Pass was 

 analyzed using O'Brien and Dean's (1972) method. This method combines a 

 stability relatioiiship between maximum average velocity, V x> ^nd inlet 

 cross-sectional area, A^, similar to that of Escoffier (1940), with the 

 simplified hydraulic analysis of Keulegan (1967) discussed previously. At San 

 Luis Pass, the following constants were used: T = 89,000 seconds; A- = 2.42 x 

 10^ square feet and la^ = 2.17 feet. The most difficult variable to define 

 was the inlet length, L. This was obtained by using the recent values of A 

 = 25,550 square feet, R = 6.2 feet, f = 0.042, and the observed long-term 

 value of K in equation (1). The bay to gulf tidal range ratio of 0.44 was 

 determined from the tidal data; a corresponding K value of 0.40 was 

 determined from Figure 37(c). Solving for L yields an effective length, L 

 = 4,325 feet. 



O'Brien and Dean's (1972) stability method is based on the fact that at 



the two extremes of a cross-sectional area, A = and A = «>, the velocity 



through the inlet will be 0. For intermediate values, the relationship 

 between V and A is given by 



V'iT2a A^ 



V = ^, °^ (7) 



max TA . 



c 



where V is a dimensionless velocity coefficient related to K by Figure 

 37(b). The stability curves shown in Figure 38 were developed as follows: for 

 each value of A , a corresponding value of K was calculated from equation 

 (1) using one of four lengths: L = L^ , L = 432 feet (0.1 L^), L = 1,000 feet 

 (0.23 Lg)j and L = 2,000 feet (0.46 Lg)- Various lengths are necessary 

 because the length over which the cross-sectional area may change will infl- 

 uence the hydraulic response of the inlet. O'Brien and Dean (1972) refer to 

 "deposition lengths" (i.e., channel segments in which sand is deposited to 

 change the cross-sectional area), and they suggest a standard length of 1,000 

 feet. It was found that the hydraulic radius varies with A as follows: 

 R = 0.5 + 2.3 x lO"'* A^. 



K values were calculated for individual values of A , R, and L; the 

 corresponding values of V were obtained from Figure 37(b); values of V 

 were calculated using equation (7); and V x versus A was plotted in 

 Figure 38. The peaks of each curve represent the critical cross-sectional 

 area for that particular length. For areas less than the critical value, the 

 inlet is unstable; i.e., an increase (decrease) in area causes an increase 

 (decrease) in maximum velocity. For areas greater than the critical value, 

 the inlet is stable, and an increase (decrease) in area produces a decrease 



46 



