Amodem mlw) " A 1933 mlw) = U. 1 / 1 HI 



AlGVD " Amodem mlw) = 0.34 m ( 1 ) 



AfGVD " Al933 mlw) = 0.5 1 1 HI 



&. Morphologic changes. By contouring the volume changes from 1933 to 1996, we can 

 clearly see how the semicircular ebb tide shoal has grown seaward of the mouth of Shinnecock 

 Inlet. In Figure 14, the green contours represent the sand that has accumulated on what was a 

 relatively smooth seafloor in 1933. The shoal projects seaward at least 1,400 m from the shore. 

 The contours at the seaward edge of the data coverage show a 1.8- to 2.4-m thickness, 

 suggesting that the ebb shoal must extend seaward a considerable distance further, possibly 

 100 m or more. The thickest sand accumulations, about 8 m, are southwest of the jetties 1,000 m 

 offshore. The shoal welds to the downdrift shore about 1,200 m west of the west jetty. Figure 14 

 also reveals that considerable scour has occurred between and immediately seaward of the jetties. 

 Sand loss exceeds 8 m in the deepest part of the channel. 



c. Volumes. The total accumulation of sand over 58 years (1938 to 1996) within the area 

 bounded by the box in Figure 14 was 6,410,000 m 3 (8,381,000 yd ). This represents average ebb 

 shoal accretion of 1 10,000 mVyear (145,000 yd 3 /year). This value understates the total sediment 

 transport in the area because not all longshore transport is trapped on the ebb shoal; some 

 proportion is certain to be bypassing the shoal and continuing on down the coast. In addition, in 

 1993 the NAN removed 363,000 m 3 (475,000 yd 3 ) from the deposition basin, a significant man- 

 made loss from the local shoal and inlet system. The computed annual accretion of 1 10,000 m 3 

 (145,000 yd 3 ) is similar to recent transport estimates from NAN and MME, which are discussed 

 later. 



Summary 



The ebb shoal growth rates for the two time periods (1933 to 1949 and 1933 to 1996) indicate 

 a similar and possibly relatively constant annual accumulation (1 17,000 m 3 /year versus 

 1 10,000 m 3 /year, respectively). This is supported by the linear nature of the plot shown in 

 Figure 15, which has a correlation coefficient of 0.99. Even though the ebb shoal may be 

 approaching equilibrium (see later section), no change in the rate of growth is apparent (Table 6). 



Chapter 2 Geologic Setting and Morphologic Development 1 7 



