Table 3.49. Minimum fetch and duration required 

 for full development of set-up associated 

 with various wind speeds. 



''Wax 1977, modified from Bascom 1964. 



SYNOPTIC We*TMEB TYPES 



-e;. 



^/ /\ no wslvr aurplut 

 [1 1 wa1«r turplul 



Figure 3-21. Relationship between water levels and 

 different weather types for Calcasieu 

 Lake near Hackberry, Louisiana (Wax 

 1977); text describes synoptic weather 

 types. 



3.3.6 SUBUNITS OF EXCHANGE BETWEEN 

 WATER BODIES AND WETLANDS 



Wetlands are built by the deposition of sediments 

 carried by flooding waters. The growth of vegetation, 

 the use of wetlands by aquatic organisms, the export 

 and import of organic matter, and the cycling of nu- 

 trients are dependent on periodic flooding. It is dif- 

 ficult, therefore, to overemphasize the importance of 

 the exchange of water between wetlands and adjacent 

 water bodies. 



Wetland processes tend to stabilize marsh eleva- 

 tion somewhere around mean sea level (MSL) (Sasser 

 1977). If elevations become too low, the vegetation is 

 flooded for longer periods than it can survive and the 

 marsh erodes. At the other extreme, as wetlands ag- 

 grade they are flooded less and less often until they 

 no longer receive the sediment supply necessary for 



continued growth. Sasser (1977) found Uttle differ- 

 ence in elevation within different marsh vegetation 

 types in eastern Louisiana. There, marshes varied in 

 elevation between about -9 cm and +9 cm (± 3.5 in) 

 relative to local MSL. 



Saline marshes receive regular flooding from tidal 

 waters. As tides attenuate upstream meteorological 

 forces affecting water levels become more dominant. 

 If marsh elevation is assumed to be at local MHW, at 

 Calcasieu Pass water exceeds this elevation 243 times 

 per year and remains over the marsh for an average 

 of 6.9 h/inundation. Upstream at Hackberry where 

 freshwater runoff primarily controls water levels, the 

 same elevation (e.g., local MHW) was exceeded only 

 200 times but the mean inundation was of longer 

 duration. In Barataria Bay, Louisiana, where gage sta- 

 tions span the distance from the Gulf pass at Bayou 

 Rigaud to the intermediate marshes upstream at Bara- 

 taria, the same relationship holds but is even more 

 dramatic. The frequency of inundation decreases up- 

 stream but the duration, both in terms of total hours 

 per year and hours per event, increases (table 3.50). 



Table 3.50. Duration of water above local MWH, 

 frequency of inundation, and mean 

 inundation duration in 1971 in the 

 Calcasieu Basin and in Barataria 

 Bay (data from U.S. Army Corps 

 of Engineers). 



Hydrologic Time above Frequency Mean duration 

 unit MHW (inunda- of inundations 



(% of yr) tions/yr) (hrj^ 



The seasonal inundation regime of the marshes 

 near Hackberry in Calcasieu Basin, which is fairly typ- 

 ical for the entire Chenier Plain, shows that marshes 

 are inundated most often during the fall and winter 

 months (fig. 3-22). This does not correspond with the 

 months of highest mean water, May through Septem- 

 ber (fig. 3-18). However, it does correspond with the 

 months when tlie variation in water level is high, as 

 indicated by the standard deviation curve. During the 

 summer, water levels are high but are below marsh 

 elevation; they are less variable than water levels dur- 

 ing winter montlis so they usually do not exceed marsh 

 elevation. In the early winter and to a lesser degree in 

 the spring, althougli the mean water level is lower, the 

 range of fluctuation is much greater and marshes are 

 flooded more often. The increased variability in win- 

 ter is probably associated with storms and illustrates 

 the importance of rain and wind for marsh flooding. 



70 



