Atlantic coast (4-61a) 



Atlantic coast (4-61b) 



gulf coast (4-62a) 



gulf coast (4-62b) 



Pacific coast (4-63a) 



A = 1.19 X 10~^ pO-^1 Pacific coast (4-63b) 



where A is the minimum cross-sectional area in square meters (square feet) 

 and P is the tidal prism in cubic meters (cubic feet). A plot of inlet 

 tidal prisms versus minimum cross-sectional aras for all of Jarrett's data is 

 given in Figure 4-70. 



Jarrett's (1976) work pertains to equilibrium minimum cross-sectional 

 areas at tidal channels as ascertained from one survey at a given date. 

 Byrne, De Alteris, and Bullock (1974) have shown that inlet cross section can 

 change on the order of ±10 percent in very short time periods (see Figure 

 4-71). In one case Byrne, De Alteris, and Bullock (1974) noted a 7 percent 

 reduction of cross-sectional area in 3 days followed by a 10 percent cross- 

 sectional area increase I week later for an inlet with an equilibrium cross- 

 sectional area of approximately 4,500 square meters. 



Ebb-tidal deltas of inlets can also change significantly in short periods 

 of time. Brown (1928) notes that for Absecon Inlet, New Jersey, "a single 

 northeaster has been observed to push as much as 100,000 cubic yards of sand 

 in a single day into the channel on the outer bar, by the elongation of the 

 northeast shoal, resulting in a decrease in depth on the centerline of the 

 channel by 6 to 7 feet." 



Such changes also effect changes in the hydraulics of the inlet system, 

 which in turn remodify the shoaling patterns. Shoal changes at inlets may 

 simply be perturbations around an equilibrium geometry, dynamic changes in a 

 cyclic pattern of inlet geometry change, or a permanent inlet geometry change. 



2. Circulation Patterns at Tidal Inlets. 



Typical flood and ebb current patterns on the ocean side of a tidal inlet 

 are shown in Figure 4-72. The important aspect of this general circulation 

 pattern is that the currents always flow toward the inlet near the shoreline 

 (in the flood channels), even on ebbtide. The reason for this seeming paradox 

 is the effect of the wave-driven currents caused by wave refraction around the 

 outer bar. On the downdrift side of the inlet the waves are turned toward the 

 inlet due to refraction over the outer bar and, hence, cause currents toward 

 the inlet; although further down the downdrift coast, currents are directed 

 away from the inlet. An example of this effect is given in Figure 4-73. 



4-157 



