Gulf tropical disturbances are important erosion 

 factors; approximately one-half of the shoreline 

 erosion on lakes in the Calcasieu area over the past 

 25 years, as deduced from maps and photos, was the 

 result of Hurricane Audrey (Adams et al. n.d.). 

 Tropica] disturbances cause both wind and water 

 erosion. Storm surges and heavy rains produce 

 an abnormally large volume of water that must 

 exit to the Gulf through restricted passes. 



Tropical disturbances are low frequency events; 

 however, Muller (1977) included them as one of 

 eight synoptic weather events that, when combined, 

 represent the climate of the northern Gulf coast. 

 The probability that a tropical disturbance will cross 

 the Chenier Plain in any given year is 0.5% based 

 on data reported by Cry (1965). However, tropical 

 disturbances centered outside the Chenier Plain may 

 also cause dramatic changes in the area. 



Annual precipitation decreases from east to 

 west in the study area from a mean of 144 cm 

 (57 in)/yr in Vermilion Basin to 113 cm (44 in)/yr 

 in the East Bay Basin. 



The temperature pattern is not as evident; how- 

 ever, it may be slightly warmer in the western portions 

 of the Chenier Plain. On a seasonal basis a strong 

 temperature gradient is found in a south-to-north 

 (coast-to-inland) direction. Sea and land breezes tend 

 to moderate the climate, cooling it in summer and 

 wanning it in winter. Table 2.1 shows the inter- 

 regional differences that can be expected in the 

 average number of freeze days between Rustin and 

 Hackberry, Louisiana. Rustin is about 261 km 

 (162 mi) north of Hackberry. Freezes in the coastal 

 environment are more moderate, occuring later in 

 the fall and earher in the spring. 



Table 2.1. Average number of freeze days annually 

 at various places in Louisiana (U.S. De- 

 partment of Commerce, Weather Bureau 

 1964, 1965). 



Station 



Area 



Freeze days 



Hackberry Chenier Plain 13 



Alexandria Central Louisiana 27 



Rustin Northeast Louisiana 46 



2.3.1 WATER BUDGET 



The climatic water budget originally developed 

 by Thomthwaite (1948) combines the effects of 

 precipitation and temperature into an accounting 

 system for water. Although it was devised for upland 

 agricultural areas, the model has been modified for 

 wetland situations (Wax et al. 1978). The water 

 budget of wetlands differs from that of the uplands 

 in the following ways: 



1. Since soils are always saturated, a soil 

 moisture storage term is not necessary in 

 the wetlands; more water is available to the 

 plants. Thus, the model predicts that plants 

 in the wetlands will have higher transpiration 

 rates. 



2. In the uplands, water surplus in the form of 

 runoff flows away from the area through 

 constricted streams, whereas the same 

 runoff in coastal areas flows into the wet- 

 lands and provides an additional source of 

 water. 



Rain surplus is the amount of water available for 

 surface runoff and ground-water recharge. Evapora- 

 tion and transpiration rates are low during the winter, 

 thus a high percentage of the precipitation is sur- 

 plus. Throughout the Chenier Plain, average con- 

 ditions show tliat the surplus period extends from 

 December through April. The effect of this surplus 

 on the streams and eventually on the marshes varies 

 with the size, slope, watershed area, and substrate 

 of the individual streams. Small streams will respond 

 almost immediately to rain surplus; however, in rivers 

 such as the Calcasieu, it may take several months 

 before all of the surplus generated from rainfall has 

 drained into tlie marshes. Rain deficits occur when 

 evaporation and transpiration exceed precipitation; 

 this is common during May, June and July (fig. 2-7). 



2.3.2 SYNOPTIC WEATHER TYPES 



Synoptic climatology classifies all observed 

 weather in a region into designated types. Muller 

 (1977) devised a synoptic climatology for the north- 

 ern Gulf of Mexico based on data from the New 

 Orleans weather station. Muller and Wax (1977) 

 extended this synoptic analysis to Lake Charles, 

 Louisiana. Table 2.2 lists the weather types, and 

 enumerates and contrasts the averages for several 

 parameters for four Januaries during 1971 through 

 1974. 



A strong seasonal pattern for many of the 

 weather types is apparent (table 2.3). Cold fronts 

 occur frequently on the Chenier Plain during winter. 

 The weather type sequences associated with these 

 fronts begin with the Frontal Gulf Return, when 

 the cold front is still several hundred miles to the 

 west or north but is affecting the weather by lifting 

 the warm Gulf air. Rain is common at this time. 

 Frontal Gulf Return accounts for 31% of the average 

 annual precipitation although it occurs only 1 1% 

 of the time. As a cold front passes through, it often 

 stalls out in the northern Gulf (Frontal Overrunning), 

 bringing on precipitation from the western Gulf. 

 This weather type accounts for 32% of the average 

 annual precipitation. After the front has passed, 



19 



