erosion zone to R-48 (45,000 ft from jetty) advanced an average of 32 ft. For the period 

 September 1972 to February 1998, the shoreline advanced an average of 42 ft for most of the 

 coast south of the south jetty except for a 5,000-ft-long segment located between R-4 and R-9 

 that experienced an average 9 ft of recession (Figure 3-5). These trends indicate that, since 1972, 

 nearly all coastal impacts (beach erosion and shoreline recession) caused by the Harbor have 

 been mitigated by placement of sand just south of the entrance channel. 



200 



-700 



April 1948 - February 1970 (NOS - NOS) 

 April 1948 - September 1972 (NOS - FDEP) 



I ' I ' 1 



10,000 20,000 30,000 



Distance from South Jetty, ft 



40,000 



Figure 3-4. Shoreline-position change prior to the 1974/75 beach fill comparing beach response derived 

 from NOS data to that derived from the 1948 NOS shoreline and the 1972 FDEP beach-profile data. 



Shoreline-position change plotted in Figure 3-5 shows that the maximum distance of 

 downdrift impact of Harbor construction after the 1974/75 beach fill is about 17,000 ft. 

 Historical shoreline-position change prior to this beach fill (Figure 3-3) exhibited an impact zone 

 located about 7,000 ft south of the Harbor. The difference in impact distances is interpreted to be 



19 20 



associated with beach adjustments (equilibration and spreading losses) after fill placement, 

 unrelated to response of the natural or native beach. After placement of sand on the beach in 

 1974/75, the beach south of the area of erosion located adjacent to the south jetty benefited 

 substantially from southward sand transport. 



Initially, sand placed on the beach will not be in equilibrium and will have a different slope and shoreline orientation as 

 compared with the natural (pre-fill beach). Over several months to years, the placed material will be transported at rates 

 greater than the naturally occurring rates. These processes are referred to as equilibration (across shore) of the profile and 

 spreading (alongshore). Both equilibration and spreading appear as volume losses at the original site of the placement. 



20 



Placement of sand on the beach in 1974/75 was the least-cost disposal alternative of sand dredged from the Harbor 

 channel. As such, the operation was not a beach-fill project, which would have its own Federal authorization and shore 

 protection as a main objective. However, for simplicity of language, the 1974/75 sand placement will be called a beach fill. 



Chapter 3 Assessment of Coastal Change 



3-9 



