Little Chenier. There has been a progressive west- 

 ward shift of river mouths through time in response 

 to tlie dominant littoral drift to the west. The sedi- 

 ment buildup at Hackberry Beach was shifting 

 the Memientau River moutli westward until con- 

 struction of the navigation channel (through lower 

 Mud Lake) and the jetty system at the coastline 

 interrupted nonnal riverine and littoral processes. 



Recessional beach ridges, the most common sand 

 ridges, are the characteristic type in the Chenier Plain 

 (fig. 2-2a and b). These ridges were constructed by 

 erosional processes but may be laterally continuous 

 with progradational ridges at bay or river mouths. 

 The ridges were formed along sections of the coast 

 undergoing coastal retreat, and their development 

 coincides with Mississippi River shifts eastward and 

 the resulting lack of sediments to maintain coastal 

 buUdout. As a consequence, existing beach front and 

 nearshore deposits are eroded and are deposited 

 landward over marsh or bay deposits. Storm con- 

 ditions accelerate this process. Most of the present 

 shoreline in the Chenier Plain is experiencing re- 

 treat; the existing beaches are pushing back over 

 marshes. As evidence of this process, exliumed peat 

 and marsh plant remains are exposed along the strand- 

 Une. It is likely that sediments being transported 

 westward from the Atchafalaya delta will reverse 

 the erosional trend along the coast in the eastern 

 section of the Chenier Plain. The present coast- 

 line contains many examples of seaward buOdup of 

 progradational ridges at river mouths over near- 

 shore deposits that grade laterally into recessional 

 ridges overlying marsh deposits. 



2.2.5 NEARSHORE TOPOGRAPHY 



Turbidity is high along the nearshore zone when 

 waves are breaking, and each breaking wave injects 

 plumes of fine-grained sediment into the water 

 column. On long stretches of the coast, water energies 

 are essentially working against shoal mud bottoms. 

 Coarse-grained sediments are deficient except at 

 locations where the strandhne is either holding 

 its own or experiencing slight buildout. Coarse- 

 grained sediments are winnowed westward and 

 accumulate at inlets or river mouths. Thus, pro- 

 gradation is occurring on the shores fronting Chenier 

 auTigre, at river mouths, and along the Bolivar 

 Peninsula. The remainder of the coast is experiencing 

 retreat over marshlands and bay bottoms that provide 

 the source of fine-grained sediments and much of 

 the broken shell that makes up the beach. The 

 Atchafalaya Bay is becoming an increasingly im- 

 portant source of fine-grained material, which drifts 

 westward and enhances the sediment supply. Evi- 

 dence indicates that Atchafalaya suspended sedi- 

 ments extend westward to the Sabine River (Wells 

 1977). 



Deficiencies of course -grained sediments are also 

 reflected in the general absence of extensive dune 

 fields along the coast and of well-developed offshore 

 bars. Hackberry Beach and Bolivar Peninsula con- 

 stitute the only areas of important dune activity. 

 Offshore bars that constitute conspicuous features 



along most coasts are only subtly developed along 

 the Chenier Plain. Where sand is more plentiful, 

 such as along the Bolivar coast, offshore bars are well- 

 developed. Depending on offshore conditions, there 

 may be two or more sequences of bars seaward 

 paraUeUng the shore. 



2.3 HYDROCLIMATE 



CUmate combines with the biological and physical 

 components of the ecosystem to determine the 

 character of the physical environment. At the regional 

 level, emphasis is placed on the dynamic aspects of 

 climate that interact with water and water movement. 

 Climate is highly variable and exerts both short- and 

 long-term influences on the region. 



Time scale is important when considering vari- 

 ability and climatic trends such as the long-term 

 variability of global temperature (fig. 2-4). The 

 Chenier Plain's development has spanned approxi- 

 mately the last 3,000 to 4,000 years, and the cli- 

 matic variability associated with that time period 

 has influenced conditions in the study area. Sea level 

 rise to its approximate present position resulted from 

 long-term climatic influences. Note that the global 

 temperatures of the mid-1970's were warmer than 

 the mean when one views temperatures over both the 

 last 1,000 or 10,000 years; during the last 100 years 

 the temperatures have not shown as large a variance. 



The most important aspects of cUmate in the 

 study area are precipitation, temperature, and wind. 

 A generalized hydrologic cycle across the Chenier 

 Plain is depicted in figure 2-5. The parameters illus- 

 trated are in a constant state of flux and the move- 

 ment of water between the ground-water aquifers 

 and tlie overlying marshes is known to occur but 

 has not been quantified. 



Winds are of primary importance in water 

 movement. Several of the basins in the area align 

 in a north-to-south direction that gives maximum 

 exposure to southerly and northerly winds. Southerly 

 winds drive Gulf waters shoreward into the estuaries, 

 resulting in raised water levels. The magnitude of rise 

 in water levels depends on the strength and duration 

 of the winds, on tidal conditions, and on the amount 

 and duration of rainfall. Southeasterly winds are 

 dominant throughout the study area (Atturio et al. 

 1976, Murray 1976). The frequency of southeasterly 

 winds are higher in the spring, when they occur 

 approximately 30% of the time, and decrease in 

 winter to a low of 17% (fig. 2-6). These winds cause 

 the dominant westerly longshore drift. Coastal land- 

 fonns in this area indicate that winds from the 

 southerly and easterly quadrants were prevalent 

 during the past 3,000 to 4,000 years. 



North winds occur, on the average, 16% of the 

 time from October through March, and decrease 

 to less than 5% of the time during the summer 

 months. Winds are usually strongest during the 

 winter, coinciding with the high frequency of north 

 and northeasterly winds. These winds, which are 

 associated with the passage of cold fronts, lower 



15 



