A channel approximately 2.5 m deep has formed behind Isles 

 Dernieres as a result of increased tidal exchange. This chan- 

 nel is capable of removing sediment from the bayside of the is- 

 lands, hence decreasing the rate of landward migration. 



The tidal currents behind Isles Dernieres allow removal of 

 washover sediments from the bay to the tidal inlets. From 

 there, strong ebb currents, especially those produced during 

 storms, are capable of carrying sediments to depths too great 

 to be returned by normal wave activity (greater than 3.7 m) . 



Subsidence 



32. Much of the shoreline recession on Isles Dernieres can be attrib- 

 uted to subsidence. Some of the factors causing subsidence on Isles Dernieres 

 include sea level rise, compaction, geologic downwarping, and extraction of 

 hydrocarbons from subsurface strata (Meyer-Arendt and Wicker 1982). Although 

 no absolute value for subsidence on Isles Dernieres has been determined, an 

 approximate value of 1.2 m/100 years has been derived for this area of the 

 Gulf Coast using tide gage data (Baumann 1980). Of this 1.2 m/100 years, only 

 0.12 m/100 years is related to eustatic sea level rise (Nummedal 1983). Sub- 

 sidence exposes previously subaerial sediments to wave and current action 

 while lowering the height of the island above sea level, making the island 

 more susceptible to overwash. As relative sea level rises due to subsidence, 

 Isles Dernieres are forced to retreat landward (Figure 8). 



SHORELINE RETREAT 



NEW SEA LEVEL 



RISE IN 

 SEA LEVEL 



/ 



EROSION 



Figure 8. Shoreline response to relative sea level rise 

 (after Bruun 1962) 



21 



