PART V: PROCESSES CONTROLLING MIGRATION 

 Overwash 



28. A washover occurs when sand, eroded from the beach and coastal 

 dunes by overwash, is deposited on the backside of a barrier beach. The most 

 important washover deposits are generated by hurricanes that strike the 

 Louisiana Gulf Coast an average of once every 2.9 years (Neuman 1978). Hurri- 

 canes can elevate sea level up to 4 m for a period of 2 to 5 hr as a result of 

 wind and low barometric pressure. A core taken in a known washover deposit on 

 Isles Dernieres shows that these deposits are thin (less than 1 m) and are 

 underlain by bay deposits. The main result of storm-generated overwash is the 

 migration of Isles Dernieres landward. From 1887 to 1980, Isles Dernieres 

 have migrated approximately 1.9 km landward due, primarily, to overwash 

 processes. Figure 7 illustrates the landward migration of Isles Dernieres from 

 1887 to 1980. Note the location of Ship Shoal offshore of Isles Dernieres. 

 Wind action plays a small role in the movement of sediment landward, as demon- 

 strated by minor dune deposits. Compared to the process of overwash, however, 

 it is insignificant and was not considered in this investigation. 



29. Another important result of overwash is the formation of tidal in- 

 lets. Tidal inlets on Isles Dernieres most likely form as a result of ebb 

 storm tides overtopping the beach ridge from the bayside (Pierce 1970). The 

 absence of tidal flats behind Isles Dernieres aids inlet formation, because 

 tidal flats limit the ability of storm tides to retreat through areas of low 

 elevation on the barrier, thus creating new inlets. 



30. To better understand the role overwash plays in the migration of 

 Isles Dernieres, the following procedure was observed. Using overlays of to- 

 pographic maps from the years 1887, 1906, 1935, 1953, and 1980, the total 

 amount of frontal beach erosion and the total amount of backshore deposition 

 was estimated for each interval using the planimeter (as described pre- 

 viously). By comparing the amount of sediment eroded versus the amount re- 

 deposited, a trend for washover deposition was determined. Table 1A lists the 

 values for total erosion and total washover deposition for each time period. 



31. From these data it is clear that the percentage of eroded sediment 

 that was deposited by overwash has decreased with time. To further evaluate 

 the role of overwash deposition in the migration of Isles Dernieres, the data 



