Continental High occurs fairly regularly. Its charac- 

 teristics during tlie summer are not as noticeable 

 as in tlie winter; however, it generally brings cooler, 

 somewhat drier air with fair skies. 



The Gulf Tropical Disturbance, which includes 

 hurricanes, occurs infrequently from late spring 

 through early fall. Winds can be extremely strong and 

 can approach from any direction except northwest 

 through northeast. This weather type is associated 

 with the most dramatic environmental responses. 



2.4 NEARSHORE HYDROLOGIC 

 PROCESSES 



The coastal waters of the Chenier Plain area are 

 kept in constant motion by the driving forces of 

 wind, waves, tide, atmospheric pressure gradients, 

 and semipermanent currents. Wave -driven currents 

 control the circulation patterns in the immediate 

 nearshore zone. Rainfall and freshwater inflows from 

 rivers, such as the Atchafalaya, mix with Gulf waters 

 to bring about density gradients and buoyancy effects 

 that are important in the circulation at tidal passes 

 and estuary moudis. Nearshore waters are very turbid 

 during high discharge periods of the Atchafalaya 

 River and when waves are breaking along the coast. 



Wind direction and intensity are the primary fac- 

 tors controlling orientation and size of wave trains 

 approaching the coasthne and consequently the over- 

 all circulation pattern. Winds along the Louisiana 

 and eastern Texas coast generally come out of the 

 east and southeast, at velocities of 4 to 10 kn (8 to 

 19 km/hr) (5 to 12 mi/hr) in summer, and at slightly 

 higher velocities in winter (fig. 2-6, Murray 1976). 

 These winds drive longshore currents toward the 

 west. 



Prevailing southeasterly winds often develop 

 swells that contact the bottom of the smooth, gently 

 sloping sliallow shelf and shoreface, causing wave 

 trains and currents that control deposition and ero- 

 sion along the coast. Investigations along muddy 

 coasts indicate that highly turbid waters have a 

 dampening effect on waves (Wells 1977). 



Approximately 92% of the waves along coastal 

 Louisiana are 1 to 2 m (3.3 to 6.6 ft) in height 

 and have a period of 4.5 to 6 sec when wind speeds 

 are greater than 10 km/hr (6.2 mi/hr) (Louisiana 

 Superport Studies 1972). Waves greater than 2.5 m 

 (8.2 ft) in height occur approximately 30% of the time 

 during winter but only 2% of the time in mid summer. 

 Thus, the Chenier Plain coast is a relatively low-to- 

 moderate energy coastline in temis of offshore waves. 

 The shallow slope of the Continental Shelf apparently 

 attenuates the offshore wave power sufficiently to 

 yield the low energy environment of the coast. 



Winds associated with winter frontal passages or 

 hurricanes produce large and sustained waves off- 

 shore. Hurricanes usually have a net drift toward the 

 northwest. They can cause considerable modification 



to the shelf waters and generally drive oceanic waters 

 onto the shore and into estuaries. The intense wave 

 action associated with hurricanes reworks the slielf 

 sediments and can transport large quantities of sedi- 

 ments shoreward. 



The significant inflow of fresli turbid water from 

 the Atchafalaya River reduces nearshore salinities. 

 During the flood season, the saUnity levels along the 

 entire open coast of the Chenier Plain are similar 

 to salinity levels in the estuaries, i.e., 10°/oo to 20°/oo 

 (part 3.3). 



Tides along the western section of the Chenier 

 Plain, especially in the vicinity of Sabine and Cal- 

 casieu lakes, are as high as 0.7 m (2.3 ft) and are 

 capable of producing significant tidal currents. 

 Currents of 3.3 kn (6.1 km/hr, 3.8 mi/hr) flood and 

 4.3 kn (8.0 km/hr, 4.9 mi/hr) ebb develop in re- 

 stricted passes in the Galveston Bay area, particu- 

 larly between Galveston and West Bay and between 

 Christmas Bay, Bastrop Bay, and West Bay (Murray 

 1976). 



Mudflats result from the net effect of sedimen- 

 tary input from local rivers, the Atchafalaya River 

 and its general westward drift, and the erosional 

 forces of the coastal waves and longshore currents. 

 When sedimentation exceeds erosion, mudflats may 

 develop offshore of the beach. During severe storms 

 the mud, along with whatever beach material is 

 present, may be driven landward over the adjacent 

 marshes. 



2.5 GROUNDWATER 



Rain surplus coupled with favorable geologic 

 conditions have enabled extensive ground-water 

 aquifers to develop in the Chenier Plain. These 

 aquifers are part of a regional ground-water area 

 that extends throughout most of the northern coast 

 of the Gulf of Mexico. 



Sands and gravels with over- and under-lying clays 

 have been deposited through geologic time along 

 the northern coast of the Gulf of Mexico. The tre- 

 mendous weight of these sediments has caused the 

 downwarping known as the Gulf Coast Geosyncline. 

 Two favorable conditions for ground-water develop- 

 ment are associated with this downwarping. First, 

 the resultant slope aOows for a gravity flow of water 

 from the outcropping areas in the north (the princi- 

 pal recharge areas) to the Chenier Plain. Second, 

 faults associated with the downwarping generally 

 parallel the coast and therefore transect all major 

 surface flows. This allows for additional ground- 

 water recharge from surface streams during periods 

 of higli surface flows and a discharge of ground water 

 to surface streams during periods of low flow. This 

 discharge maintains a minimum baseflow in surface 

 streams and acts as a buffer against drought con- 

 ditions for riparian vegetation. 



A third major source of ground-water recharge 

 is by downward seepage through the large surface 



21 



