onshore- offshore direction as well as in an alongshore direction. Figures 

 47 to 54 are selected bathymetric plots for the nearshore profiles (modi- 

 fied from Richter, 1974) . 



For all surveys the station displaying the greatest nearshore sand 

 level fluctuation was at pipe 7 on profile line III, located immediately 

 seaward of the beach-rock ledge. The sand elevation varied from a high 

 of -5.2 feet (-1.6 meters) MSL in April 1970, to a low of -11.6 feet 

 (-3.5 meters) MSL in May 1972. Minimum sand level fluctuation occurred 

 on the seaward end of profile line 1, where elevation varied between 

 -15.0 and -15.8 feet (-4.6 and -4.8 meters) MSL. Plots of the cumulative 

 nearshore volume change between surveys are presented in Appendix H. 



The average volumetric gain between nearshore surveys when accretion 

 occurred was 5 cubic yards per foot (12.6 cubic meters per meter); the 

 average loss was 4.9 cubic yards per foot (12.3 cubic meters per meter). 

 Since the nearshore section of the profiles is about five times longer 

 than the beach section, these volumetric changes indicate that the magni- 

 tude of change (per foot of profile length) is of the same order onshore 

 as it is in the nearshore zone. However, this conclusion is complicated 

 by the unknown changes during times of buried or missing pipes. Inter- 

 vals during which there were no measurable net volume changes between 

 nearshore surveys occurred about 11 percent of the time. 



The greatest nearshore loss (-24.3 cubic yards per foot, -61.0 ciibic 

 meters per meter) between surveys occurred on profile line I between 8 

 and 10 March 1971. During the same interval the beach lost 0.9 cubic 

 yard per foot (2.3 cubic meters per meter) and the shoreline eroded 3.3 

 feet. The greatest nearshore gain (+29.9 cubic yards per foot, +75.0 

 cubic meters per meter) occurred on profile line III between 30 March 

 and 1 April 1970. The beach showed no measurable change in volume or 

 shoreline position during the same interval. Although larger nearshore 

 volume changes are suggested by the data, they could not be quantified 

 due to the problem of lost or buried pipes. 



d. Onshore-Offshore Changes . An objective of this study was to 

 quantify the volume of material transported between the beach and near- 

 shore region. This onshore-offshore transport has been recognized by 

 many investigators (e.g., Johnson, 1919; King, 1959; and Shepard, 1963), 

 and is especially evident in some localities as a seasonal fluctuation 

 between a "summer" beach and "winter" beach. During storm conditions 

 the higher, steeper waves remove sand from the beach, depositing it as 

 a bar in the surf zone. As the storm conditions subside the bars begin 

 to migrate onshore, welding to the foreshore (Davis and Fox, 1972). 

 Other investigators have reported a more complex onshore-offshore bar 

 migration (e.g.. Goldsmith, 1969; Richter, 1974). 



Figures 55 to 58 are plots of the beach volume changes versus the 

 nearshore volume changes between surveys on each of the four profile 

 lines at Boca Raton. The expected trend of onshore-offshore sand trans- 

 port is plotted as a diagonal. However, this trend is not observed since 



74 



