designed for Biscayne Bay, Florida, their model has 

 been adapted for this study of the Chenier Plain. 



Despite local geomorphic variation, all Chenier 

 Plain basins can be characterized by the following 

 general subunits (fig. 3-15): 



A) Tidal region - responds to direct exchanges 

 of water with the Gulf via tidal action. 



B) Riverine region - primarily controlled by 

 freshwater inflows. 



C) Wind-driven region - responds to tides and 

 river discharge but is dominated by wind- 

 driven circulation. 



D) Wetland region - responds to tides, winds, 

 rain, and evaporation. 



Many graphs and tables presented in this section 

 represent a synthesis of long-term tidal records main- 

 tained by the U.S. Army Corps of Engineers (USAGE). 

 Salinity data are also primarily from USAGE records 

 (app.6.1). 



3.3.3 SUBUNITS HAVING DIRECT EXCHANGE 

 WITH THE GULF 



Tidal fluctuations can often be detected through- 

 out a drainage basin. Due to the fiat topography of 

 the Chenier Plain much of the land is inundated, dis- 

 sipating tidal energy. The part of an estuary within 

 which water is directly exchanged with the Gulf, how- 

 ever, is usually restricted to the vicinity of tidal inlets. 

 The direction of water flow depends upon the surface 

 water slope set up by the astronomical tide. The 

 strength of the current in the pass depends upon the 

 magnitude of the surface water slope; that is, the dif- 

 ference in water height across the pass. The volune of 

 water exchanged througli the opening depends on the 

 width, length, depth, and straightness of the tidal pass. 

 Marine waters scour tidal passes and directly exchange 

 a semicircular estuarine area whose radius extends 

 from the pass to a distance of about 500 times the 

 mean depth of the tidal pass (Lee and Rooth 1972). 



Characteristics of Gulf waters. The influence of 

 the Gulf waters on estuaries and wetlands depends on 

 the physical and chemical character of the nearshore 

 Gulf habitat. For purposes of this discussion, pertinent 

 parameters are saUnity, tides, and water levels. 



Murray (1976) has demonstrated that offshore 

 salinity patterns are similar along the coast east of the 

 Sabine Basin; no data are published for the section 

 west of the Sabine Basin for the same period. He found 

 the salinities were close to deep ocean sahnity during 

 months of low river discharge. Salinity values were 

 low during months of high river discharge (fig. 3-16). 

 During the flood month (December) of 1963 (a dry 

 year in Louisiana), estuarine levels of salinity existed 

 along most of the open Louisiana cost. The dilution 

 of Gulf waters occurred because of the large amount 

 of river discharge. 



Dissolved salts are usually diluted by mixing with 

 fresher waters as tidal currents carry them inland. Ver- 

 tically stratified salt wedges can form in deep channels 



and allow saline water to move into a basin along the 

 channel bottom. Tidal currents also mix this saline 

 water with overlying fresher waters. Chenier Plain 

 estuaries are shallow (1 to 2 m or 3.3 to 6.6 ft), and 

 in the past the Gulf passes had shallow sills that pre- 

 vented formation of a salt wedge. However, deep 

 dredged channels (15 m or 49 ft) now exist in the 

 Calcasieu and Sabine basins and these have allowed 

 significant salt water intrusion. (Details about specific 

 basins are addressed in parts 3.6.2 through 3.6.7). 



Coastal ecosystems have adapted to tides for mil- 

 lenia. As a result, virtually every biotic response of 

 these systems is keyed to some component of the tide. 

 Important aspects of tide are period, range, and eleva- 

 tion or level. 



There are semidiurnal, diurnal, and mixed tides 

 in the Gulf of Mexico. A semidiurnal tide has two 

 liigh waters and two low waters in a tidal day with 

 comparatively little diurnal inequaUty (Coastal En- 

 gineering Research Center 1973). A diurnal tide has 

 one high water and one low water in a tidal day. A 

 mbced tide is one in which there is a large inequality 

 in either the high or low water heights, with two high 

 waters and two low waters usually occurring each tidal 

 day. In the Gulf of Mexico, in contrast to most of the 

 world oceans, a diurnal tide pattern predominates. 



Along the Chenier Plain, the tidal phases shift 

 slightly (fig. 3-17). Tides reach the center of the region 

 first, lagging slightly both east and west (Byrne et al. 

 1976). The tide shows a nearly pure diurnal curve at 

 Bayou Riguad, east of the Chenier Plain and in East 

 Bay, Texas, on the west end of the Chenier Plain but 

 has a distinct semidiurnal character in between. (For 

 comparison. United States east and west coasts are 

 botli characterized by semidiurnal tides, but the east 

 coast tides are generally equal whereas the west coast 

 tides have large inequahties.) 



In the Gulf of Mexico there is an orderly progres- 

 sion in time between the two typesof tides, with the 

 semidiurnal tides never being fully developed. The di- 

 urnal tide fades into a semidiurnal tide over a two- 

 week period. The diurnal tides that occur when the 

 moon is over the Tropics of Capricorn and Cancer at 

 maximum angle relative to the equator have the largest 

 range and are called "tropic tides." The tides exhibit- 

 ing the most semidiurnal character are called "equa- 

 torial tides" and occur when the moon is over the 

 equator. 



Tidal ranges along the Chenier Plain are low in 

 comparison to tides along otlier coasts. They fall in 

 the micro-tidal range, i.e., tidal ranges less than 2 m 

 (6.6 ft). The mean tidal range at the coast is about 60 

 cm (24 in), varying from about 30 cm ( 1 2 in) at East 

 Bay to about 75 cm (30 in) at Calcasieu Pass (table 

 3.46). This range attenuates upstream depending on 

 the depth, bottom, and shape of the channel. 



The diumal tidal cycle is superimposed on a sea- 

 sonal water level cycle and on a long-term water level 

 trend on the northern Gulf coast. Both the seasonal 

 and the long-term trend are of major significance in 

 the way the Chenier Plain ecosystem functions. The 



62 



