5. Winter Sedimentation . 



Summer sedimentation rates will not prevail during ice-cover conditions. 

 The submerged ice volume reduces the tidal discharge volume of water and sus- 

 pended sediment to the basin and that reduction can be estimated. Similarly, 

 a reduction in the amount of water and suspended sediment in the channel will 

 reduce the sediment mass inflow from that source. 



Because sediment size, water viscosity, and flocculation affect the 

 settling velocity of suspended material in the basin, these factors must be 

 considered on at least a seasonal basis. At Dillingham, median particle size 

 varied from 0.014 millimeter in summer to 0.005 millimeter in winter. 



The effect of salt on the settling rate of clay minerals becomes distin- 

 guishable at seawater concentrations of about 1 to 2 parts per thousand. 

 Flocculation, and hence the settling rate of the floes, increase to seawater 

 concentrations of 10 to 15 parts per thousand. Suspended sediment in water 

 samples with a salt content of 0.5 part per thousand at Dillingham during the 

 summer did not appear to flocculate. However, when the winter salinities 

 reached 3 parts per thousand, flocculation might have occurred. 



Temperature affects water viscosity and hence the fall velocity of the 

 sediment particles. For example, at 0° Celsius winter water temperature, the 

 fall velocity of an 0.008-millimeter particle is about 70 percent the velocity 

 it would be at 10° Celsius. This must be considered when using Figure 3 (see 

 Fig. 7 of example). 



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