185 



outlined in paragraphs 304 and 305. The tidal heights on these 

 curves should, however, be taken off at lunar hourly intervals, begin- 

 ning generally either at a lunar transit or at the time of high water at 

 the initial entrance to the canal. 



355. If the tides at an entrance have a marked variation between 

 springs and neaps, it may be advisable to determine the tides and 

 currents in the canal at mean spring tides, or at ordinary spring tides 

 (par. 181); and perhaps at the corresponding neap tides as well. If 

 the daily tide curves show no marked distortions, the representative 

 spring and neap tides may be taken, without introducing errors which 

 will affect the results materially, as simple harmonic fluctuations with 

 the speed of the M2 component, and amplitudes of one-half of the 

 spring and neap ranges respectively; otherwise average or composite 

 curves of spring and neap tides may be prepared. 



356. Probably the most satisfactory method for dealing with an 

 entrance tide of the mixed type is the preparation of a composite 

 curve whose timing and elevations conform to the times and elevations 

 of mean lower low, higher low, lower high and higher high waters, as 

 outhned in paragraph 306; or a representative tropic tide could be 

 selected from the records. 



357. If the entrance tides are of the semidiurnal type, the currents 

 produced by the repetition of a single properly selected average, or 

 spring, or neap semidiurnal fluctuation will afford an adequate indica- 

 tion of the average, spring or neap currents and tides to be expected 

 in the canal. To facilitate the computations, the representative 

 entrance tides of this type should be adjusted as necessarj^ to afford 

 a smooth curve when identically repeated in successive periods of 12 

 lunar hours. When either or both of the entrance tides are of the 

 mixed or diurnal types, the representative tides similarly should follow 

 curves which are identically repeated every 24 mean lunar hours. 



358. The representative tides at the two entrances to a canal should 

 be referred to the same horizontal datum. If either end of the canal 

 takes off from the upper part of a generally shallow bay or river 

 estuary, the mean level of the tides at the two entrances may not be 

 at the same elevation. The same situation may arise if the canal 

 connects two oceans or seas in which the water density and mean 

 meteorological conditions are not the same. Any uncertainty may 

 be removed by connecting the tide stations at the proposed entrances 

 by a line of precise levels. Daily variations due to winds, freshets, 

 and other meteorological causes may produce a constant component 

 of the head between the entrances, of sufficient magnitude to have a 

 marked effect upon the currents. It may, therefore, be advisable to 

 select one or more typical concurrent storm tides at the two entrances, 

 which would produce large differences in the head through the canal, 



