Table 1. Summary of results for application of model to high tide data from 1985 and 1986 

 winters, and calendar year 1985. 



No. inundation Highest high Deposition 



events tide (m) (g) 



1986 Winter 11 



1985 Winter 36 



1985 Jan.-Dec. 149 



of the deposition for the whole year. The most important aspect according to the model appears 

 to be the high frequency of inundation during the 1985 calendar year, which the model uses to 

 accumulate sediments on the marsh surface. 



DISCUSSION 



The results shown above suggest that the model overestimates sediment deposition on the marsh 

 surface during the summer months. It is important to note that the model is based upon data 

 collected during winter months. Thus, the high-magnitude inundation events shown in the original 

 data are those associated with cold fronts, when sediment is both mobilized by strong southerly 

 winds and transported onto the marsh surface as the water is forced up into the marshes from the 

 bays (Reed 1987). Examination of continuous water-level records for Cocodrie, LA, during analysis 

 of these data indicated an increase in the mean height of tides during the summer and fall and a 

 corresponding increase in the frequency of low-magnitude marsh inundation. 



Only 24% of tides which flood the marsh during the complete calendar year occur between 

 January and April 1985. The model assumes that all inundation events are the same in terms of 

 their potential to deposit sediments on the marsh surface. This is an inappropriate assumption 

 when applied to all seasons of the year. Although the marsh may be frequently flooded during 

 the summer months even in years without significant hurricane activity, sedimentation will not 

 necessarily occur if floodwaters have low suspended-sediment concentrations. 



The effect of pre-frontal winds in producing onshore waves and water-level set-up has also been 

 identified by using remote-sensing data (Roberts et al. 1987). Effects will be similar during the 

 passage of hurricanes, and the precise effects, as with cold fronts, will depend upon the orientation 

 to the coastline. Thus, the model of sediment deposition, which assumes available suspended sedi- 

 ment for deposition during marsh flooding, may be applicable to the high- magnitude inundation 

 events associated with the hurricane passage in 1985. However, it is not applicable to low- 

 magnitude summer tidal inundations associated with prevailing onshore winds which produce insuffi- 

 cient wave activity to significantly suspend bay-bottom sediments and transport them into the marsh. 



The implications of these results for managing sedimentation in Louisiana's coastal marshes are 

 that sediment will not be deposited on the marsh surface unless it is available for deposition and 



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