Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 287

the increasing stratification and reduced vertical mixing on the ebb tend to decouple
the surface layer, which then moves faster in the offshore direction. On the flood the
increased vertical mixing reduces the vertical shear in the flow, thus increasing the
shoreward movement of the bottom layer relative to the surface. Both ebb and flood
half cycles, therefore, act to increase residual circulation, in the same direction as the
density-driven circulation, relative to that which would occur in the absence of
straining. This enhancement of the circulation by tidal straining, identified by Jay
and Musiak (Jay and Musiak, 1996 ) , has been recognised in observations (Stacey
et al., 2008 ) and demonstrated in a numerical models (Prandle, 2004 ) .
Modulated residual flows in estuaries and ROFIs will have impacts on the flushing
or retention of material. As an example, consider the asymmetry in vertical turbulent
mixing as a result of tidal straining and its effect on the transport of suspended
sediments. We saw in Section 9.4.2 how the stratification which develops on the ebb
in a standing wave regime leads to a severe damping of turbulence. On the flood, the
straining acts to erode stratification and, therefore, to promote turbulence. Towards
the end of the flood, the water column can become vertically mixed and the straining
mechanism then tends to produce instability and convection which will further
enhance turbulence. The result is a pronounced variation in vertical mixing with
much higher diffusivity, K z , on the flood than on the ebb. This contrast in vertical
mixing means that flood flow is able to maintain much higher levels of sediment in
suspension than on the ebb, thus favouring the transport of sediments in the direction
of the flood. The operation of this asymmetry in the transport of suspended sedi-
ments has been observed in the York River estuary (Scully and Friedrichs, 2007 ).

9.6 The influence of wind stress

...................................................................................
So far in this chapter, we have concentrated on the interaction between freshwater
input and the action of the tides in stirring the water column and straining the density
field. In many ROFI systems, tidal influence predominates for much of the time, but
episodes of strong winds can modify the ROFI regime especially in cases where the
tides are weak. In extreme cases like the ROFI of the Rhone River debouching in
to theMediterranean Sea, where the tidal flow is almost negligible, wind stress becomes
the dominant mechanical forcing (Reffray et al., 2002 ) . In conditions of light winds, the
Rhone discharge forms a plume which flows to the west along the French coast under
the control of the buoyancy forcing and the Coriolis force. At times of strong winds,
however, the surface stress out-competes the buoyancy forcing and, depending on the
wind direction, the Rhone plume may be driven away from the coast or even reversed,
so that the Rhone ROFI regime is highly variable (Forget and Andre, 2007 ) .
This influence of both the magnitude and direction of the wind stress on the flow
and structure is apparent in other ROFIs even where the tidal influence is substantial.
In a narrow estuary where the flow is constrained by the boundaries, wind stress
operating in the seawards direction tends to strengthen the estuarine circulation and,
hence, to increase stratification. Conversely, a landward component of wind stress