decrease and more sediment will be lost to the tidal deltas associated with 

 these inlets. Another factor adding to the decrease in longshore transport is 

 straightening of the coastline. In 1887, the curvature of Isles Dernieres was 

 greater and erosion was concentrated at the central-most point. As the island 

 migrates landward the curvature decreases and the concentration of erosion at 

 the central area diminishes, resulting in a decrease of sediment available for 

 longshore transport. Straightening of the coast reduces the angle of wave 

 impact causing waves to strike the shoreline more directly, thus reducing the 

 strength of the current parallel to the shore and as a result decreasing long- 

 shore drift. 



Tidal Inlets 



36. Tidal inlets are channels between two adjacent barrier beaches con- 

 necting the open ocean to the lagoon or bay area located behind the barrier 

 (Price 1962). On the Gulf Coast the term "tide" is used to describe the 

 twice-daily astronomical tides as well as sea-level change associated with 

 storms. The average range of astronomical tides for the Isles Dernieres is 

 0.4 m. Storm tides up to 7 m have been documented on the Gulf Coast (Hayes 

 1978). Tidal inlets on Isles Dernieres are most likely a result of ebb storm 

 tides overtopping the beach ridge from the bayside because the barrier is nar- 

 row and no extensive tidal flats are present on the bayside of the island 

 (Pierce 1970). Once an inlet is formed by storm tides, the normal astronomi- 

 cal tides are able to maintain and enlarge the inlet channel. The theory of 

 inlet formation proposed by Pierce is supported by Coupe Carmen, a tidal inlet 

 on the eastern end of Isles Dernieres. In 1974, Hurricane Carmen struck the 

 Louisiana coastline and produced this inlet. Since that time the inlet has 

 deepened and migrated westward due to tidal exchange and longshore transport. 

 Once the tidal inlets on Isles Dernieres have been formed, they rarely fill 

 back in with time; instead, they become wider and deeper. The main reason for 

 their enlargement seems to be the drastic increase in the volume of water 

 stored behind the island. The bay behind Isles Dernieres was approximately 

 2.4 km wide in 1853 and is now over 8 km wide and much deeper. As the volume 

 of water increases, the tidal flux, as compared to wave energy, becomes much 

 more significant in controlling island migration. The increase in tidal 

 exchange has also caused the development of a channel which runs behind Isles 



26 



