154 



station determined by multiplying the area U, between this station 

 and the head of tide, by the half -hourly increases in the tide at the 

 station, as shown in tabulated computations in figure 48. 



311. Graph oj discharges and velocities. — ^The fluctuations in the dis- 

 charge and in the velocity at the successive tidal stations during the 

 tidal cycle, and the relative importance of the tidal and fresh-water 

 discharges, are made apparent by plotting the tidal heights, discharges 



*\soo 



I 



§ +IOO0 



k 



\ +50O 

 



V -soo 



X -1000 



Q-I500 



rOE/»N Solar Hour 



600 



500 



Figure 49. — Discharges and velocities at Ship John Light, Delaware Bay, from mean cubature. 



and velocities through the tidal cycle. The discharges and velocities 

 at Ship John Light, near the head of Delaware Bay, and at Phila- 

 delphia, 64 miles up the estuary of Delaware Eiver, as derived from 

 the mean cubature of this channel, are shown in figures 49 and 50. 



312. The tidal discharges computed in the cubature of a channel 

 (column 15 of fig. 48) are the mean discharges during successive half- 

 hour intervals. These are shown by the stepped lines in figures 49 

 and 50. The curve of instantaneous tidal discharges is then so drawn 

 that the area under the curve during each time interval is the same as 

 the area of the rectangle of mean discharge for the interval. 



The fresh-water discharge is plotted as a horizontal line above the 

 zero line of tidal discharges. Since the fresh-water discharge is flowing 



