time allowed, the water content decreases and the sediments become less 

 erodable. In the present study of Mississippi Sound sediments, however, we 

 have found that the water content of the sediments does not change appreciably 

 with the settling time allowed. Despite this fact, the erodability of the 

 sediments shows a decrease with the settling time. For the sediments from 

 site 1, the settling time is allowed to increase from 1 day (Figure 6.8(a)) to 

 3 days and 5 days (Figure 6.10). The equilibrium concentration has dropped 

 substantially with increasing settling time. The water content has only 

 changed from 89.2% to 88.16%, a much smaller variation when compared with 

 changes found in the Lake Erie study. 



A similar trend has also been found for sediments from other sites in 

 water of 30 ppt salinity. For example, the entrainment of Site 3 sediments 

 for a 3-day settling and a 10-day settling are shown in Figures 6.11(a) and 

 6.11(b), respectively. In this case, the water content has actually increased 

 slightly from 79% to 81%. 



This trend is best summarized in Figure 6.12. It is clear that for the 

 Mississippi Sound sediments, the erodability decreases with the settling time 

 while the water content remains largely unchanged over a 10-day period. Hence 

 the water content alone is not a good index of bed property. Some dynamic 

 process which is present in both fresh and salt waters, is apparently 

 responsible for this anomalous trend. 



We have examined several possibilities to try to explain this trend: 



(1) Compaction and increase of grain-grain contacts . This is not likely to 

 occur since the entrained sediments are the very uppermost surfacial 

 sediments and, as such, are not subject to any significant overburden 

 pressure. Moreover, water content, even in the top 1 cm of sediments, 

 does not change appreciably over the 10 day period. 



The effect of overburden pressure on the entrainment process cannot 

 be examined easily in a laboratory flume such as the one used for our 

 study. The water column is only a few centimeters high in the 

 experiment, while it is on the order of several meters or more in the 

 field. To reproduce the overburden pressure experienced in the field 



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