An important question is whether the Long Island barriers are still retreating. Historical 

 evidence indicates they are. In a mapping project based on charts and aerial photographs, 

 Crowell and Leatherman (1985) measured annual net erosion of 0.3 to 1.2 m along most of the 

 south shore barriers between 1834 and 1979. Accretion occurred in the immediate vicinity of 

 Shinnecock Inlet due to the trapping of sand on the updrift fillet. Table 2 summarizes Crowell's 

 and Leatherman 's findings. Their evidence points to accelerated erosion after 1933; presumably, 

 much of this occurred after the inlet opened in 1938. Reports indicate that vast amounts of 

 overwash occurred during the September 1938 hurricane. Unfortunately, there is not enough 

 evidence to determine if this one event might have caused a majority of the post- 1933 erosion. 



Table 2 



Shoreline Changes Near Shinnecock Inlet 



Period 



Zone 3,000 m west of inlet 

 (average) 



Near inlet 



Zone 3,000 m east of inlet 

 (average) 



1834/1838-1873/1892 



Variable: 1 .2-m accretion west of 

 Ponquogue Pt; 0.6- to 0.9-m 

 erosion east of Ponquogue Pt. 



Variable 



0.6- to 1 .2-m retreat 



1873/1892- 1933 



0.6- to 0.9-m accretion 



0.6- to 0.9-m accretion 



0.3- to 1 .5-m accretion 



1933-1979 



1 .2- to 2.4-m retreat 



3.0-m accretion (updrift 

 fillet) 



0.3- to 0.9-m retreat 



Annual average change 

 1834/1838-1979 



0.3- to 0.9-m/year erosion 



0.3- to 0.9-m/year 

 accretion (mostly updrift 

 fillet) 



0.3- to 0.6-m/year erosion 



Source: Scaled from Figure 4-3 in Crowell and Leatherman (1985)~Note that the accuracy of maps made in the 1830s is 

 questionable due to the lack of cartographic standards (Shalowitz 1 964). Therefore, shoreline change statistics based on the 

 1834/38 charts must be used with caution. 



All tide gauges near Long Island have recorded a rise in rsl during this century. As examples, 

 tide level curves for New York City and Montauk are plotted in Figure 3, and Table 3 lists rsl 

 trends at four stations near Long Island. The New York station, located at the Battery at the 

 southern tip of Manhattan Island, is a remarkably long, 122-year record showing an average 

 2.72 mm/year (0.0089 ft/year) rise in rsl. This means that over the 63 years since the 1933 

 U.S. Coast and Geodetic Survey (USC&GS) hydrographic data were collected off Shinnecock 

 Inlet, a span less than the lifetime of some of Long Island's inhabitants, the sea has risen about 

 0.17 m. Even without including sediment losses due to overwash, assuming a beach slope of 

 1:20, a 0.17-m rise in water level translates to a 3.4-m horizontal movement. This is slightly 

 greater than the retreat rate calculated by Crowell and Leatherman (1985) for the east end of 

 Westhampton. At all four stations listed in Table 3, the 1950 to 1993 trend suggests that the rate 

 of sea level rise has decreased slightly compared to the longer-term average, but at this time we 

 cannot speculate whether such a decreasing trend will continue. 



In summary, geologic studies and historic evidence from maps and photographs verify that 

 the Long Island barriers have retreated during the Holocene era. It seems likely that the barrier 

 retreat has been largely a result of rising rsl. Sea level is still rising in this area, and all 



8 



Chapter 2 Geologic Setting and Morphologic Development 



