156 



(i.e., the region where sea water mixes with fresh water). With sea level 

 rise and barrier migration, fine-grained sediment from the sea would become 

 available as a sediment source, stirred up by wave action and transported 

 by the residual current, depositing ultimately at the upland end of the 

 mixing zone. In some cases, e.g., in the long navigation channel through 

 the Maracaibo estuary in Venezuela (Partheniades , 1971) and in Zeebrugge 

 harbor, Belgium,^ the present source of fine sediment is marine rather than 

 alluvial . 



Fine sediment transport in estuaries is a complex process involving a 

 strong coupling between tides, freshwater flow and the coagulated sediment. 

 This process has been described extensively elsewhere (Postma, 1967; Krone, 

 1972). In Fig. 10.2, a schematic description is given. The case 

 considered is one in which the estuary is stratified, and a stationary 

 saline wedge is formed. Various phases of suspended fine sediment 

 transport are shown on a tide-averaged basis. In a partially mixed 

 estuary, the description must be modified, but since relatively steep 

 vertical density gradients are usually present even in this case, the 

 sediment transport processes are qualitatively similar. 



With reference to this figure, the vertical variation of the 

 horizontal residual flows (tide -averaged flows) can be conveniently 

 described by computing the ebb predominance factor, EPF, defined as 



J u(z,t)dt 



EPF = ;^^ 2 (10.3) 



E F 



Sq u(z,t)dt + J^ u(z,t)dt 

 e 



where u(z,t) = instantaneous longitudinal current velocity at an elevation 

 z above the bed, Tg = ebb period, Tp = flood period and T = Tg + Tp, where 

 T = tidal period (Simmons, 1965). If the strengths of flood and ebb were 

 the same throughout the water column, EPF would be equal to 0.5 over the 

 entire depth of flow. This is almost never the case; EPF is usually less 



^Robert Kirby, Ravensrodd Consultants Ltd. , Taunton, United Kingdom, 

 personal communication. 



