33. Bruun (1962) has developed a mathematical relationship between rel- 

 ative sea level rise and shoreline recession: 



Shoreline recession = - -■-- -'■' 



(h + d) 



where 



a = relative change in sea level 



b = distance from shore to 18-m depth contour which represents the 

 limit between nearshore and deep sea littoral drift phenomena 



h = dune height 



d = distance beyond which little sediment turbulence takes place 

 (Meyer-Arendt and Wicker 1982) 



Using a relative change in sea level of 1.2 m/100 years, the amount of shore- 

 line recession resulting from relative sea level rise for Isles Dernieres can 

 be calculated as follows: 



^ . 1.2 m/century (2.400 m) CoC . . 



Shoreline recession = t-t—t J -,\' n ~ = 626 m/century 



(0.9 m + j.f mj 



The value of 18 m (which represents the limit between nearshore and deep 

 sea littoral drift phenomena) was determined by Bruun (1962) by averaging 

 data obtained along the Atlantic, Pacific, and Gulf Coasts. From this 

 calculation, we find that subsidence accounts for 626 m/100 years (or approx- 

 imately 6.26 m/year) of shoreline recession on Isles Dernieres. The value 

 of 6.26 m/year accounted for 24 percent of the total recession for the cen- 

 tral Isles Dernieres during the 1887 to 1906 time interval. Since that time, 

 island migration due to overwash and longshore drift has decreased making the 

 impact of subsidence much more significant. For the 1953 to 1980 time inter- 

 val, the value of 6.26 m/year accounted for 36 percent of the total landward 

 retreat for the central Isles Dernieres. 



Longshore Drift 



34. The growth of spits was used as a measure to document the effec- 

 tiveness of longshore drift in controlling migration. On Isles Dernieres, 

 spits are found on both ends of the island group as a result of longshore 

 transport and tidal exchange forces. Because of its orientation perpendic- 

 ular to the dominant south and southeast winds, the Isles Dernieres group has 



22 



