condition, it would be necessary to generate a gravitational acceleration 

 more than an order of magnitude of g. This would require a very 

 different laboratory apparatus. 



(2) Slow variation in clay properties (other than water content) with time . 

 The influence of clay mineralogy properties on the entrainment of 

 cohesive soils has been studied by several workers. Anderson (1951) 

 claimed that Middleton's dispersion and erosion ratios are important 

 indices in characterizing the erodability of sediments from 14 mountain 

 watersheds in the Coastal Range of the United States. Smerdon and 

 Beaseley (1959) concluded that plasticity index and dispersion ratio have 

 pronounced effect on the critical shear stress of cohesive soils in open 

 channels. Wallis and Stevan (1961) defined an erosion index in terms of 

 cation exchange capacity. Krone (1963) related cation exchange capacity 

 with Bingham shear strength. However, contradictory conclusions have 

 been reported by the various workers (Kandiah, 1974). Moreover, none 

 have examined the transient variation of the various clay properties of a 

 given sediment type. It also seems highly unlikely that some clay 

 properties would show the same transient behavior in two waters of vastly 

 different salinity and ionic strength. 



(3) Increase of microbial (algal, bacterial) exudates with time . The 

 appearance and odor of the Mississippi Sound sediment samples, along with 

 the relatively high organic content of some of the sediments, support 

 this possibility. Actively growing bacteria and benthic algae are known 

 to secrete micropolysaccharides that can bind fine-grained sediments 

 (Rhoads, et al., 1978). This would decrease the erodability of the 

 cohesive sediments. Bacteria growth increases in high organic 

 environment and when the growth is not regulated by benthic organisms 

 (macro-fauna). If the growth continues, the effect of bacteria and 

 benthic algae may play a dominant role in determining the erodability of 

 cohesive sediments. During this process, it may also render the water 

 content of the sediments relatively unchanged with time. 



In a natural environment, the bacteria growth in sediments is regulated 

 by the presence of the benthic organisms and hence shows a seasonal 

 variation. Grant (1981) has found a twofold change in the critical shear 



156 



