244 



471. Primary currents before improvement. — The velocity stations 

 are midway between the established tide stations, and the storage 

 stations, generally, should be midway between the velocity stations. 

 The coordinate amplitudes, A sin a and A cos a, of the primary tide 

 at the storage stations are obtained by linear interpolation between 

 those at the established tide stations. Taking any representative cross- 

 section area of the estuary as a base, Mo, the values of I = all /Mo 

 between the velocity stations are computed from the surface areas U 

 at mean tide, a being 0.0001405 radians per second. The summation 

 from the head of tide of the values of I A sin a and I A cos a then gives 

 the values of (B/m) sin 13 and (B/m) cos /S at the velocity stations, 

 from which the values of jS and B/m may be determined. The average 

 or effective cross-section area, M, in each subsection may be deter- 

 mined from a consideration of a sufficient number of plotted actual 

 cross sections. The multiplication of B/m by 7n=Mo/M then gives 

 the amplitude B of the primary current in the subsection before 

 improvement. 



472. Subsection coefficients before improvement. — Since the values of 

 H'^ and /S in each subsection have been found, the angular lag, 4>, of 

 the current is determined from the relation expressed in equation 

 (290), paragraph 373: 



0=^°-/3-9O°. 



This value should also satisfy the relation, from equation (289): 



sin (f)=Bla/gH. 



"Wliile the values of computed from these two equations should not 

 be widely apart, a complete agreement cannot be expected. The 

 value of (f) should therefore be computed from equation (290), the 

 length, I, in equation (289) taken as the virtual length of the subsec- 

 tion, and the value of the coefficient la/g computed from the relation: 



la/g=(H/B) sin 0. 



The value of the coefficient p in each subsection is determined from 

 the relation, from equation (288): 



p—B tan cj). 



473 . Subsection coefficients after improvement. — If the area, U, of the 

 water surface between any of the velocity stations is changed by the 

 proposed improvement, the coefficient I=aU/Mo must be recomputed 

 for the value of Mq originally chosen. The effective cross section 

 M/ in each subsection after improvement, determined by a procedure 

 paralleling that used in the selection of the value, M, before improve- 



