Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 272

(a)

(b)

mmol m -3 PSS78

53°N

31

16

32

30

52°N

33

Rhine 8 Rhine

2

34 4

4°E 5°E 4°E 5°E

Figure 9.3 The surface distribution of (a) salinity and (b) nitrate off the coast of the
Netherlands, as a result of the Rhine inflow. The dotted line is the course followed by the
research vessel during sampling. Figure courtesy of D. J. Hydes, National Oceanography
Centre, UK.

seen in the nitrate distribution. Nitrate levels in the coastal current are more than an
order of magnitude greater than in the North Sea outside the ROFI and, as the plots
show, there is a close correlation of salinity and nitrate. This high nutrient loading is
an important feature of many ROFIs, and we shall return to consider its impact on
the ROFI environment in Section 9.8.3.

9.2.2 Residual flows in a ROFI

Let us now have a look at the solution to the estuarine circulation problem, but this
time including the effects of the Earth's rotation. As well as the influence of the
Coriolis force we might expect the flow in the ROFI to be affected by frictional forces
as in the estuarine case. The simplest model which allows for the combined influences
of rotation and friction is that of Heaps (Heaps, 1972 ) in which it is assumed that the
flow is forced by a depth-uniform horizontal gradient in the x direction which is
normal to the coast, as shown in Fig. 9.4 . Conditions do not vary in the along
coast direction so that derivatives in y are zero (i.e.

0) and frictional stresses
in x and y are again controlled by the eddy viscosity. With these assumptions the
equations of motion ( Equation 3.13 ) for a steady state with hydrostatic pressure
( Equation 9.2 ) become:

@ / @ y ¼

2 u

1
0

@

x þ @

p t x

@

g @
@

gz 1

0 @

N z @ fv ¼ fv

x þ x þ z 2 ¼

0

@ z @ @

ð 9 : 11 Þ

2 u

0 @

1 t y

@

N z @

fu

z ¼

fu þ

z 2 ¼

0 :

@