In the Shinnecock General Design Memorandum (USAE District, New York 1988), two reasons 

 are cited for the continuing erosion: First, the growth of the sandbar across the inlet absorbed 

 about 76,500 mVyear (100,000 yd /year) of the littoral material. Second, the growth of the bar 

 caused the ebb flow to be diverted more to the southwest and closer to the beach. 1 The loss of 

 sand was estimated to be 46,600 mVyear (61,000 yd /year) between 1955 and 1984 for the 

 1,800-m stretch of shore west of the inlet. 



Volumetric analysis, 1933-1949 



The morphology of the 1949 shoal is revealed in a plot based on a volumetric comparison 

 with the 1933 USC&GS (pre-inlet) data. Unfortunately, the 1933 survey only covered the ocean 

 and bay sides of the barrier but not the topography of the barrier where the inlet formed in 1938. 

 The 1926 USGS topographic map displayed dune and marsh features but no elevation data. 

 Therefore, the pre-inlet barrier was simulated by using contemporary topography and assuming 

 that the pre- 1938 barrier was similar. Adjustments to convert the 1933 data to a modern datum 

 are described in Appendix C, and the procedure for generating the topography is outlined in 

 Appendix D. 



In Figure 13, accretion is shown by green contours and erosion by red. For the ebb and flood 

 shoals, volumes were only computed for areas where the 1933 and 1949 data sets overlapped. 

 Volume changes, computed using Terramodel© software, are summarized in Table 5. 



Table 5 



Volumetric Comparison, 1938 - 1949 1 



Region 



Accumulation, m 3 (yd 3 ) 



Loss, m 3 (yd 3 ) 



Ebb shoal 



1,284,000 (1,680,000) 



145,000(190,000) 



Barrier and inlet 



23,000 (30,000) 



960,000(1,260,000) 



Flood shoal 



2,220,000 (2,900,000) 



495,000 (647,000) 



'Note: Preinlet data collected in 1933, but inlet opened in 1938. 



The total accumulation of sand on the Atlantic side of the barrier was 1,300,000 m 3 

 (1,700,000 yd 3 ), while the flood shoal grew 2,200,000 m 3 (2,880,000 yd 3 ). Since there are no 

 important inland sand sources, the growth of the flood shoal must have been fed by a 

 combination of sand funneled through the inlet from the interrupted littoral drift and from erosion 

 of the barrier after the 1938 breach. 



The General Design Memorandum (USAE District, New York 1988) states that these explanations are based on bathymetric 

 surveys. The value of 76,500 m 3 /year (100,000 yd 3 /year) was computed from 1955, 1984, and 1985 surveys. 



Chapter 2 Geologic Setting and Morphologic Development 1 5 



