event of vertical stratification, the denser water along the bottom has a net 

 flow landward (when averaged over a tidal cycle) providing a mechanism for 

 sediment to move into the inlet. The less dense surface waters have a net 

 flow seaward when averaged over the tidal cycle, thus satisfying continuity of 

 water mass in the system. 



4 . Inlet Migration and Stablilization Effects on Adjacent Shorelines . 



Shorelines in the vicinity of inlets are subject to considerable change, 

 much more so than typical shorelines remote from inlets. Many shorelines have 

 undergone little or not change prior to inlet creation. Following the opening 

 of an inlet, significant changes occur. Figure 4-77 shows a natural barrier 

 island on the gulf coast of Florida whose shoreline, although receding, 

 underwent little historic change prior to 1926 when hurricane currents broke 

 through the barrier island and created Redfish Pass. After Redfish Pass had 

 been created, the shoreline on each side of the inlet receded. A maximum 

 recession rate of 275 meters (900 feet) in 30 years (9 meters/year) occurred 

 on the downdrift side of the inlet (University of Florida, Coastal Engineering 

 Laboratory, 1974). 



When long-term historical records are examined, it is clear that tidal 

 inlets undergo spectacular changes over a period of a century. Examples of 

 long-term natural inlet migration is illustrated by the inlets shown in 

 Figures 4-78 to 4-80. 



Short-term changes in shorelines in the vicinity of inlets are no less 

 dramatic, as demonstrated in Figure 4-81 which shows changes of 150 meters 

 (500 feet) in the shoreline adjacent to an inlet at Brown Cedar Cut, Texas, 

 within a one year survey period. 



Often the inlet can migrate in a direction counter to that expected from 

 its dominant longshore sand transport direction. Brown (1928) has noted that 

 Aransas Pass, Texas, among others, has migrated in a direction opposite that 

 of the net longshore transport for many years; Walton and Dean (1976) have 

 noted a northward movement of Redfish Pass, Floida, in the Gulf of Mexico for 

 a period of 20 years, although the dominant sand transport direction in the 

 area is southward. 



The effects of inlet stabilization works (e.g., jetties, terminal groins, 

 offshore breakwaters) on the shorelines adjacent to inlets are often difficult 

 to assess in view of the dynamic character of natural inlets (i.e., inlets can 

 change significantly within a short time). Shoreline accretion in the wave- 

 sheltered areas of jetties and offshore breakwaters has been discussed in 

 Sections V,2, V,3, and VIII, 1. Also, changes are induced due to the con- 

 striction of the channel by entrance jetties. Typically, a confinement of the 

 inlet flow between jetties causes stronger velocities within the inlet channel 

 and a consequent displacement of sand from the area between the jetties to 

 seaward, thereby making the inlet a more effective littoral trap (i.e., 

 decreasing natural sand bypassing) and distorting the natural ebb-tidal 

 delta. Figures 4-82 and 4-83 provide a historical perspective of St. Mary's 

 River entrance, Florida, where long jetties of 5-6 kilometers were constructed 

 between 1880 and 1902. Figure 4-82 documents the forcing of the natural ocean 

 bar offshore during the 5-year period (1902-1907) after completion of the 

 jetties. Figure 4-83 documents the changes, natural and jetty-induced, in the 

 period 1870-1970, in which the area seaward of the jetties accreted 92 x 106 



4-167 



