158 



than 0.5 near the bottom, particularly in the saline wedge, and greater 

 than 0.5 in the upper layers. The residual upstream bottom current is due 

 to the characteristic nature of flow circulation induced by the presence of 

 the wedge, which means that the strength of this current will decrease as 

 the limit of seawater intrusion is approached, and is theoretically zero at 

 the limit (node) itself. Distributions of EPF at three locations - at the 

 mouth, in the wedge and at the node, would qualitatively appear as shown in 

 Fig. 10.2. When interpreted in terms of tidal flows, these distibutions 

 reflect the general observation that in the mixing zone of the estuary 

 flood flows landward at the bottom and ebb flows seaward at the surface . 



The trends indicated by the EPF distibutions suggest the dominating 

 influence of hydrodynamics on sediment movement. As noted in Fig. 10.2, 

 riverborne (alluvial) sediments from upstream fresh water sources arrive in 

 suspension in the mixing zone. The comparatively high degree of 

 turbulence, associated shearing rates and the increasingly saline waters 

 will cause sediment aggregates to grow in size as a result of frequent 

 inter-particle collisions and cohesion, and large aggregates will settle. 

 Aggregate settling velocities can be up to four orders of magnitude larger 

 than the settling velocities of the elementary particles. Some of the 

 sediment will deposit onto the bed, and some will be carried upstream near 

 the bottom until times close to slack water when the bed shear stresses 

 decrease sufficiently to permit deposition. The deposited sediment will 

 start to consolidate due to overburden. 



The depth to which the new deposit scours when the currents increase 

 after slack will depend on the bed shear stresses imposed by the flow and 

 the shear strength of the deposit. If the currents during both flood and 

 ebb are sufficient to scour all of the new deposit, the net movement will 

 be determined approximately by current predominance. However, if the bed 

 shear stress during ebb is less than sufficient to suspend all of the newly 

 deposited material, a portion of the material will remain on the bed during 

 ebb, and will be resuspended and transported during the predominant flood 

 flows, resulting in a net upstream transport. Net deposition, i.e., 

 shoaling, will occur when the bed shear during flood, as well as during 

 ebb, is insufficient to resuspend all of the material deposited during 

 preceding slack periods. Some of the fine material that is resuspended 



