variety of environments more characteristic of coastal processes described 

 elsewhere, including beaches, inlets, beach ridge plains or dune fields. 



Once an active delta is abandoned, marine, estuarine, and paludal processes 

 take over the landscape. Such abandoned deltas may prograde or remain 

 stable if marine processes provide sufficient sediment. More likely, they may 

 deteriorate because of marine reworking, increased subsidence and compaction 

 inland, and decreased fluvial sedimentation rates. The Mississippi Delta Plain 

 has undergone such transformations, where the sands of abandoned deltas 

 have been reworked into barrier headlands and flanking barrier islands by 

 marine processes. As the marsh behind these barriers subsides, barrier island 

 arcs are left standing in open water. Through continued marine reworking 

 and subsidence, the barrier islands progressively disintegrate to form inner 

 shelf shoals. Transformation from one form to another may take several 

 decades or hundreds of years, while delta abandonment to shoal formation 

 may take a few thousand years to complete. 



A delta plain, such as that of the Mississippi River, has unique environ- 

 ments that are highly vulnerable to both natural events and human activities. 

 Because of storms such as tropical and extratropical cyclones, transgression of 

 the low-profile barriers is rapid, with erosion rates over 5 m/year in places. 

 In wetlands, many factors are involved in relative sea level rise and 

 subsidence, including eustatic factors and consolidation of Quaternary and 

 older sediments. In Louisiana, land loss has averaged over 100 sq km per 

 year. Decreased sediment supply to wetlands because of levee building along 

 the rivers and in the marshes, as well as decreased sediment loads due to 

 trapping of sediments by reservoirs upstream, has prevented accretion of the 

 marsh surface as sea level rises. Brine discharges, and dredging of canals and 

 channels for navigation and oil-drilling activities are some other human 

 activities that influence land loss. 



Strand Plains: Ridges and Cheniers 



On coasts where there is an abundant supply of unconsolidated sediments, 

 and in wave-dominated settings, including in the vicinity of river mouths, it is 

 common to find beach ridge plain coasts and chenier plain coasts. Both are 

 called strand plains and display shore-parallel ridges, which represent succes- 

 sive seaward accretion and reworking (Figure 14). Ridges are shore-parallel 

 bodies of coarse materials, with ridge crest elevations well above mean high 

 tide and troughs near the mean low tide level. Ridges originate primarily due 

 to marine processes, caused by several possible mechanisms. They generally 

 develop immediately behind the active beach as a flood-level ridge, as an 

 eolian accumulation, or by water deposition below and eolian deposition 

 above. 



Cheniers are shore-parallel bodies of sand and shell enclosed by prograding 

 marsh and mudflat deposits. They develop in response to fluctuating supplies 



Chapter 3 Variable Coastal Features 



63 



