The following steps outline the method used to estimate the annual growth rate of the two 

 shoals. We assumed that most of the sand eroded from the barrier moved north onto the flood 

 shoal. 



Flood shoal - Barrier erosion = Sand supplied by littoral drift to flood shoal 

 2,220,000 - 960,000 = 1,260,000 m 3 

 (2,900,000 - 1,260,000 = 1,640,000 yd 3 ) 



Flood shoal + Ebb shoal = Combined shoal volume supplied by littoral drift 

 1,260,000 + 1,284,000 = 2,540,000 m 3 

 (1,640,000 + 1,680,000 = 3,320,000 yd 3 ) 



Combined shoal volume ■=- Years since inlet formed = Rate of flood & ebb shoal growth 

 2,540,000 H- 11 = 231,000 m 3 /year 

 (3,320,000 -=-11 = 302,000 ydVyear) 



Ebb shoal volume -=- Years since inlet formed = Rate of ebb shoal growth 

 1,284,000 + 11 = 1 17,000 mVyear 

 (1,680,000 -=-11 = 153,000 ydVyear) 



If we assumed that the inlet, at least during the first decade of its existence, formed an almost 

 complete barrier to littoral drift, the growth of the shoals indicate that the gross annual longshore 

 transport was in the range of 230,000 mVyear (301,000 ydVyear). The growth of the ebb shoal 

 alone understates the total amount of sediment in motion. 



Volumetric analysis, 1933 - 1996 



a. Data analysis. To evaluate morphologic changes in the region near Shinnecock Inlet, the 

 1933 USC&GS bathymetry was subtracted from the 1996 SHOALS bathymetry using the 

 VOLUME function in Terramodel© software. The 1933 data were referenced to mean low water 

 (mlw). However, in 63 years, rsl in this area has risen about 0.171 m, based on the annual trend 

 computed by NOAA for the Battery in New York Harbor (Table 3). In other words, the 1933 

 mlw datum is lower than the contemporary mlw datum, and therefore any individual 1933 depth 

 must be lowered to be directly comparable to contemporary data. We adjusted the 1933 

 soundings by 0.171 m, a value obtained by multiplying the trend, 2.72 mm/year x 63 years. Note 

 that the adjustment is based on the average trend, but in any one year actual rsl may deviate 

 greatly from the trend due to numerous oceanographic and climatologic factors. Finally, the 

 1933 depths were lowered another 0.34 m to adjust from mlw to National Geodetic Vertical 

 Datum (NGVD) (1929 adj.) to allow direct comparison with the 1996 SHOALS data. 

 Adjustments are summarized in Equation 1 : 



1 6 Chapter 2 Geologic Setting and Morphologic Development 



