139 



2 feet, the tide at the other end would be distorted only to the extent 

 shown. 



278. Comparison with measured current curves. — The form of the 

 computed tidal current curves derived in the preceding paragraphs 

 may be compared with curves of measured velocities, and with those 

 derived by the method of cubature hereafter described. It should be 

 recollected that the computed curves show the mean velocity at a 

 cross section of the channel during a tidal cycle. A meter measure- 

 ment of the mean velocity in a tidal channel is quite a difficult under- 

 taking, as the velocity in each area of the cross section is changing 

 continuously, while its fluctuations are not identically repeated from 

 day to day. Available records often show the velocities only at a 

 single point in the cross section; but these indicate the characteristic 

 shape of the current curve. Current curves derived by the cubature 

 of an estuary show, on the other hand, the mean velocities at the 

 cross section. 



279. The currents in the Cape Cod Canal afford a typical example 

 of the form of the current curve when the flow is markedly frictional. 

 The average measured midstream current velocities at 0.3 depth, at 

 station 225, after the time of a lunar transit, compiled from a series 

 made by the United States Engineer Department, September 28- 

 October 6, 1938; and the corresponding mean tide curve in the section 

 from station 1804-30 to station 225, are shown in figure 45. The 

 velocity curve has, it will be seen, the characteristics of the computed 

 curve of mean velocities, shown in figure 43. As is to be expected, 

 the midstream velocities are about 25 percent in excess of the mean 

 velocities throughout the cross section. 



280. The current in the estuary of the Delaware at the head o 

 Delaware Bay, as determined by a mean cubature made by the 

 United States Engineer Office in Philadelphia, shown in figure 49, 

 page 154, affords an example of a typical velocity curve when the 

 flow is of a less frictional character, and is not greatly modified by 

 overtides. This curve may be compared with the curve of computed 

 velocities shown in figure 42. 



281. The marked effect of overtides on the currents is illustrated 

 by the velocity curve at Philadelphia 63 miles further up the Delaware 

 estuary, determined by the same cubature, and shown in figure 50, 

 page 155. 



282. The even greater distortions of the current m some tidal chan- 

 nels is illustrated by the curve of measured channel velocities in 

 Seekonk River, R. I., shown in figiu^e 46, page 141, taken from the 

 Manual of Current Observations, United States Coast and Geodetic 

 Survey. 



192750—40 10 



