296
8 Transport in the Oceans and Coastal Zone
and
(8.118)
Following the same method as before we find that the salinity deviation, corresponding to the velocity distribution, takes the form:
h2uo as
S(z) = --12(z) + so,
Kz ax
in which:
7
1 (Z)2 1 (Z)4
12(z) = -120 + 4" h - 8" h .
(8.119)
(8.120)
In Fig. 8.17, the vertical distribution of the salinity deviations, S(z) - So,
represented by the functions h (z) and 12 (z) is shown. All equations indicate
that the vertical salinity deviation is proportional to the salinity gradient and
inversely proportional to the coefficient of vertical diffusion. Therefore, in an
estuary with strong diffusion, the vertical salinity deviation is very small.
We will revisit the dynamics of a partly mixed estuary in Chap. 13, where we
discuss the ecological implications of salinity intrusion and pollution dispersion
in estuaries.
8.5.5 Dynamics of a Well-Mixed Estuary
When tidal energy dominates over river input, the total water column is completely mixed by the turbulent motion of eddies. The salinity field is mostly
governed by tidal motion, river flow, channel geometry and bed roughness. The
shear effects tend to increase vertically and transverse salinity gradients result
in an increase of gravity induced currents. On the other hand, turbulent diffusion works towards reducing density gradients. Observed density gradients
strongly depend on the relative magnitudes of both mechanisms.
Field observations are the best method to investigate the structure of turbulence in well-mixed estuaries. Mean velocity and salinity measurements in
well-mixed flows by Anwar (1983) shows that the velocity profile is logarithmic
(see Eq. 2.54), i.e.:
U
1 (Z)
u* = ~ In Zo .
(8.121)
Field measurements were undertaken in the mouth of the River Carron, Scotland. The freshwater discharge of the river was small (about 30 m 3 Is in winter
and less than 1 m 3 /s in summer) while the tidal discharge at peak spring ebb
8 Transport in the Oceans and Coastal Zone
and
(8.118)
Following the same method as before we find that the salinity deviation, corresponding to the velocity distribution, takes the form:
h2uo as
S(z) = --12(z) + so,
Kz ax
in which:
7
1 (Z)2 1 (Z)4
12(z) = -120 + 4" h - 8" h .
(8.119)
(8.120)
In Fig. 8.17, the vertical distribution of the salinity deviations, S(z) - So,
represented by the functions h (z) and 12 (z) is shown. All equations indicate
that the vertical salinity deviation is proportional to the salinity gradient and
inversely proportional to the coefficient of vertical diffusion. Therefore, in an
estuary with strong diffusion, the vertical salinity deviation is very small.
We will revisit the dynamics of a partly mixed estuary in Chap. 13, where we
discuss the ecological implications of salinity intrusion and pollution dispersion
in estuaries.
8.5.5 Dynamics of a Well-Mixed Estuary
When tidal energy dominates over river input, the total water column is completely mixed by the turbulent motion of eddies. The salinity field is mostly
governed by tidal motion, river flow, channel geometry and bed roughness. The
shear effects tend to increase vertically and transverse salinity gradients result
in an increase of gravity induced currents. On the other hand, turbulent diffusion works towards reducing density gradients. Observed density gradients
strongly depend on the relative magnitudes of both mechanisms.
Field observations are the best method to investigate the structure of turbulence in well-mixed estuaries. Mean velocity and salinity measurements in
well-mixed flows by Anwar (1983) shows that the velocity profile is logarithmic
(see Eq. 2.54), i.e.:
U
1 (Z)
u* = ~ In Zo .
(8.121)
Field measurements were undertaken in the mouth of the River Carron, Scotland. The freshwater discharge of the river was small (about 30 m 3 Is in winter
and less than 1 m 3 /s in summer) while the tidal discharge at peak spring ebb