209 



402. Primary discharges and storage. — Multiplying the area of the 

 cross section at the entrances by the amplitude of the primary cur- 

 rents, it is found that, with the given tidal fluctuations, the primary 

 discharge at Reedy Point reaches a maximum of 4,096 c. f. s.; and at 

 the Chesapeake City entrance, 6,960 c. f. s. Evidently, the larger 

 part of the filling and emptying of the canal is through the latter 

 entrance. 



403. Effect oj the flow through the canal upon the primary entrance 

 tides. — At Reedy Point the canal opens into the wide estuary of the 

 Delaware, and the flow in and out of the canal obviously is insuffi- 

 cient to produce a measurable effect upon entrance tides. At the 

 other entrance, at Chesapeake City, the discharge is into the com- 

 paratively restricted channel in Back Creek, whose area of cross 

 section, in its upper part, is given as but 3,700 square feet. The 

 discharge through the canal therefore, is sufficient to effect the cur- 

 rents and tides in this approach to the canal. The computation of 

 the tides and currents in the canal has been made from the actual 

 recorded elevations at Chesapeake City, after the canal was opened. 

 If equally good records were available at the mouth of Back Creek, 

 and physical data were at hand to determine the constants for the 

 successive reaches in that approach, the computations profitably 

 could have been extended to include this approach as a part of the 

 canal prism. 



DISTORTIONS OF THE PRIMARY CURRENTS 



404. The distortions of the primary current in a short section of a 

 tidal channel have been developed in paragraphs 260 to 276 of chapter 

 V. In a long tidal canal, further distortions are introduced by the 

 variation, with the rise and fall of the tide, in the area of the water 

 surface between successive velocity stations, and in the area of the 

 cross section of the water prism at these stations. The corrected 

 velocities at any stations at which a determination is desired may be 

 computed by a procedure which now will be described. 



405. Intervals. — The corrected velocities are computed at selected 

 intervals of time which, like those chosen for deriving the corrections 

 in a short section of a tidal channel, should be parts of the component 

 hour of the primary tides and currents. This component hour usually 

 is the lunar hour of 1.035 mean solar hours. Intervals of one-half a 

 lunar hour, or 1,863 mean solar seconds, ordinarily are sufficiently 

 small to aft'ord reliable results. 



406. Procedure. — A fu'st adjustment of the currents, which usually 

 is sufficient for all practical purposes, may be accomplished through 

 the following procedure: 



(a) The primary tides at the ends of the subsections of the canal 

 are adjusted to the selected representative tides at the entrances, if 



