Plans to divert sediment from the Mississippi River into marsh areas to alleviate the problem of 

 sedimentation deficit must aim to manipulate these natural processes of sediment transport to 

 enhance surface deposition. This study attempted to elucidate some of the natural processes which 

 transport sediment onto salt marshes in east Terrebonne Parish and to develop a simple model of 

 the delivery of sediments to the marsh surface. This model can be used to identify the potential 

 role of events of differing magnitudes in contributing to marsh accretion, and how those events can 

 be manipulated and managed to increase marsh surface sedimentation. 



METHODS 



Short-term variations in sediment deposition on the marsh surface were identified by using a 

 filter paper sediment trap technique outlined by Reed (1987) in studies conducted near the 

 Louisiana Universities Marine Center at Cocodrie, LA The results of those studies for December 

 1986 to April 1987 show great variation in the sediment deposition between different periods 

 (Figure 1). The results are expressed as mean sediment deposition per day for periods varying from 

 1 to 4 weeks, the dates shown being the limits of the sampling periods. Reed (1987) attributed 

 these variations to sediment availability for transport onto the marsh surface, and opportunity for 

 sediment transport onto the marsh. The passage of winter cold fronts was seen to be especially 

 important in both mobilizing sediment and transporting it onto the marsh surface. 



These data were used to develop a regression model relating the amount of sediment deposited 

 to the magnitude of the inundation event producing the deposition. To do this, the data were 

 reduced to the total amount of sediment deposited during each sampling period and the total "area" 

 of the inundation peaks during each period. Water-level records were taken from the National 

 Ocean Service (NOS) tide gauge at Cocodrie, LA, within 200 m of the sampling site. The 

 inundation-peak area was calculated by using graphs of water-level variation and summing the areas 

 above the line representing the general level of the marsh surface at the sampling site (Figure 2). 

 The area of an individual tidal peak over the marsh surface reflects both the depth and duration 

 of the inundation event. The sediment deposited during each period was regressed against the total 

 area of the inundation peaks by using a simple regression model (Starview, Brainpower, Inc.). 



RESULTS 



The results, which include 44 individual inundation events, show a clear relationship between 

 the amount of sediment deposited and the depth/duration of marsh flooding (Figure 3). Variation 

 in "inundation area" explains 83% of the variation in sediment deposited. 



The results of the regression analysis confirm that regular tidal inundation during the winter 

 months contributes very little sediment to the marsh surface, while the passage of cold fronts 

 increases the inundation of the marsh and the amount of sediment deposited. This experiment 

 was limited to the winter months of 1 year, but examination of water-level variations over other 

 periods allowed the importance of these deposition events to be assessed as part of longer-term 

 marsh-surface accretion. A similar model was applied to December-April water-level records in 

 1984-85 and 1985-86, and to records from January-December 1985, a year which included the 

 passage of hurricanes Danny, Elena, and Juan. These data were obtained from the U.S. Army 

 Corps of Engineers tide gauge at Cocodrie, LA, and the records converted to the same datum as 

 that of the NOS gauge. The regression model was similar to that described above but used high- 

 tide elevations rather than inundation-peak area for ease of data acquisition from the charts supplied 



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