Most of the effects are indirect, e.g., the hydrody- 

 namics of the system modify other abiotic factors 

 which in turn affect the biota. The major hydrody- 

 namic features of the Chenier Plain are water circula- 

 tion pattern, current velocity, water replacement rate, 

 and turbidity. 



Circulation patterns are affected by the topogra- 

 phy of the area, flow volume, wind, and tides. These 

 patterns determine the direction and movement of 

 salts, dissolved organic compounds, nutrients, sedi- 

 ments, plankton, and contaminants. Current velocity 

 affects size of suspended sediinents, sediment load 

 distribution, and strongly influences depositional and 

 erosional patterns in inland open waters and the near- 

 shore Gulf. The rate of water displacement is deter- 

 mined by flow volume and discharge rate which af- 

 fect rates of nutrient replenishment and eutrophica- 

 tion. The volume and velocity of rivers, currents, and 

 tides determine, in part, the degree of turbidity, the 

 distribution of nutrients and contaminants, and indi- 

 rectly affects the distribution of aquatic plants and 

 benthic filter feeding organisms. 



Circulation patterns and current velocities affect 

 the distribution of living organisms. Larvae of oysters 

 and other sliellfishes are distributed almost entirely 

 by prevailing currents. Many estuarine-dependent 

 species, such as shrimp, spawn offshore, but their larvae 

 are carried by currents into bays and estuaries which 

 are their principal nursery grounds. 



Currents are essential for carrying nutrients to 

 clam beds and oyster reefs; consequently , the location 

 or abundance of these forms is determined largely 

 by the circulation pattern. Oyster reefs always build 

 across prevailing currents (Hedgepeth 1953). Currents 

 across fringing wetlands help transport nutrients and 

 organic detritus throughout interconnecting aquatic 

 habitats. 



Water volume renewal relates the total volume of 

 a body of water to the volumetric flow through it. 

 The renewal time is a function of the inflow and out- 

 flow rate and the volume of the body of water. In 

 general, the shorter the renewal time the more nu- 

 trients a body of water can receive without accumu- 

 lating excessive nutrients. 



The coastal waters of the Chenier Plain are kept 

 in constant motion by the driving forces of wind, 

 waves, tides, and atmospheric pressure gradients. 

 Wave-driven currents control the circulation patterns 

 in the immediate nearshore zone. Large volumes of 

 freshwater from the typically abundant rainfall, as 

 weU as watershed runoff, mix with coastal salt waters 

 to bring about density gradients and buoyancy effects 

 that are important in the circulation of waters through 

 tidal passes and estuaries. 



A primary factor controlling the orientation and 

 size of wave trains approaching the coastline, and 

 consequently the overaU circulation pattern, is the 

 direction and intensity of the consistent winds along 

 the Louisiana and eastern Texas coasts. Prevailing 

 southeasterly winds with average velocities of 4 to 

 lOkm/hr (2 to6mi/hr)in summer, and slightly higher 

 in winter (Murray 1976), develop swells that contact 

 the bottom of the smooth, gently sloping inner shelf 

 and shoreface (Fisher et al. 1972). The resulting wave 

 trains and currents control deposition and erosion 

 along the coast (table 4.16). 



There is an obvious lack of westerly winds 

 throughout the year. As a result, local wind-driven 

 currents are predominantly toward the west. Although 

 winds other than the predominant southeasterly winds 

 do occur, they are significantly less effective in 

 generating waves, currents, and tidal effects. 



Approximately 92% of the waves along coastal 

 Louisiana are 0.9 to 1 .5 m (3 to 5 ft) in height and 

 have a period of 4.5 to 6.0 sec when wind speeds are 

 greater than lOkm/hr (6.2 mi/hr) (Louisiana Super- 

 port Studies 1972). Seasonal variability of waves also 

 is demonstrated. Waves greater than 2.4 m (8 ft) in 

 height occur approximately 30% of the time during 

 winter, as opposed to 2% of the time in midsummer. 



The Chenier Plain coast is a low to moderate 

 energy coastline in terms of offshore waves. During 

 spring and summer the intensity of offshore waves is 

 relatively low, but during fall and winter intensity 

 increases two- to three-fold. The shallow slope of the 

 Continental Shelf apparently attenuates offshore 

 wave power sufficiently to yield the low energy 

 environment of the coast. 



Table 4.16. Annual wave climate summary for coastal Louisiana (Becker 1972). 



The percentages cited are relative to portion of time during the year when wind velocities exceed 10 km/hr (6.2 mi/hr). Winds 

 greater than 10 km/hr prevail during 43.3% of the year on the average. 



196 



