126 



252. The relation between the head in a section 5,000 feet in length 

 and the velocity is shown in figure 39, the origin of time being taken 



Lunar Hour 



Figure 39. — Primary current and head in section of Delaware River near entrance. 



at a moment when the head turns from negative to positive. 



253. For a third example, a channel may be taken with a hydraulic 

 radius of 100 feet, and a maximum slope, S, of 0.000,01 during the 

 tidal cycle. An appropriate value of Cis 150. 



Then: 



P = 5. 138 F/^=513,800 



= 79° 5=5.135Vcos 79° = 2.24 feet per second. 



The relation between the head in a 5,000-foot section of such a 

 channel and the velocity is shown in figure 40, the origin of time being 

 as in the preceding example. 



ho.od- ^ 



■^ 



-O.05' 



Lunar Hour^. ~ 



FiGUKE 40. — Primary current and head in deep channel. 



254. Lag of primary tidal current. — It may be observed that positive 

 directions have been so assigned to heights and velocities that the 

 water is running down hill when the head is positive and the velocity 



