232 



Wood 

 Beach 



re 



ary 



ne 



OOO lOOOOO 



I I I I I 



feet 



Figure 78. — Delaware River, Philadelphia to 'Woodland Beach. 



444. The actual mean tidal range in the Delaware increases from 

 5.63 feet at Woodland Beach to 5.85 feet at the contraction at Artificial- 

 Island, then decreases to 5.09 feet at Philadelphia. The rate of 

 advance of the tide up the ideal estuary would be ■yl21.5g=26.3 feet 

 per second. The actual rate of advance of the high water from Wood- 

 land Beach to Philadelphia averages 23 feet per second, and of low 

 water, 19.5 feet per second. 



The amplitude of the primary current, computed from equation 

 (319) with the given data, is 2.09 feet per second. The actual mean 

 current velocities at various cross sections of the Delaware, determined 

 by cubature, have strengths generally of about 2.00 feet per second, 

 increasing to 2.6 feet at contracted sections. 



445. The equation of the tide in an ideal estuary (equation 303) 

 shows that high water at a point distant x from the entrance occurs 

 when atQ--77x=0, or when to=nila. Similarly equation (304) shows 

 that the current turns from positive to negative, or from flood to ebb, 

 when ati—7ix—(l)=0, or when ti=(nx-{-cf))la. The interval between 



