Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 274

a more gradual salinity gradient from bed to surface water
by intense turbulent mixing. The tidal : river discharge
ratio is between about 20 and 200. Down-estuary changes
in the salinity gradient at the mixing zone occur so that the
zone moves upward toward higher salinities. Earth rota-
tional effects cause the mixing surface to be slightly tilted
so that in the northern hemisphere the tidal flow up the
estuary is nearer the surface and strongest to the right.
Sediment dynamics is strongly influenced by the upstream
and downstream movement of salt water over the various
phases of the tidal cycle. The resulting turbidity maximum
is particularly prominent in the upper estuary (around
1-5 ppt salinity) on spring and large neap ebb and flood
tidal phases, and less prominent at slackwater periods due
to settling and deposition. Turbidity maxima are affected
by the magnitude of freshwater runoff. A seasonal cycle of
dry-season upstream migration of the turbidity maximum
and locus of maximum deposition is followed by wet-season
downstream migration and resuspension by erosion. The

turbidity maximum is also acted on by gravity-induced
circulations arising from excess density.
Type C well-mixed estuaries are those in which strong
tidal currents completely destroy the salt-wedge/fresh-
water interface over the entire estuarine cross-section. The
ratio of tide : river discharge is greater than 200.
Longitudinal and lateral advection processes dominate.
Vertical salinity gradients no longer exist but there is a
steady downstream increase in overall salinity. In addition,
the rotational effect of the Earth may still cause a pro-
nounced lateral salinity gradient, as in Type B estuaries.
Transport dynamics are dominated by strong tidal flow,
with estuarine circulation gyres produced by the lateral
salinity gradient. Extremely high suspended sediment con-
centrations may occur close to the bed in the inner reaches
of some tidally dominated estuaries. Sediment particles of
river origin, some flocculated, will undergo various trans-
port paths, usually of a “closed loop” kind (Fig. 6.51), in
response to settling into the salt layer and subsequent

Freshwater buoyant
plume

River flow 3D Salinity gradients River flow

Mixing around
internal waves

Inte nse t urbul ent m ixing

salt wedge

Type A: well-stratified estuary

Type B: partially stratified estuary

2D Salinity gradients in horizontal

River flow

Negligible river flow

Near-homogenous salinity

Inten se tur bulen t mix ing

Type C: well-mixed estuary

Type D: completely mixed estuary

Fig. 6.49 A useful classification of estuaries according to the dynamic processes of mixing and salinity gradients.

sed. conc. (mg l -1 )

Flow velocity (m s -1 )

10 5 Salinity (‰)

0

0

5
10

20 30

2

15
20
25

River
Water

4

40

6

8

Salt
Wedge

10

10

50

12

High-sediment
concentration
gradients

Estuary bed

Estuary
mouth

5 km

Fig. 6.50 Salinity, velocity, and suspended sediment profiles taken during high tide along transect of the well-stratified (salt wedge) Fraser River
estuary.