Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 275

0

0.6

Sediment
concentration

Mean streamwise
velocity

Flood tide Ebb tide

Turbulent
shear flow

0.3

Deposition Settling

Resuspension
Deposition

Advection

Mobile turbulent
suspension

Resuspension

0.0

-0.8 0.0 0.8

Mean flow velocity, m s -1

Lutocline

Fig. 6.51 Variation of estuarine suspended sediment concentration
over several tidal cycles. Velocities are negative for the flood (incom-
ing) tide and positive for the ebb.

Mobile fluid mud

Bingham Plastic Flow

Stationary fluid mud

Cohesive sedimented bed

0

Sediment concentration or flow velocity

transport by the net upstream tidal flow. Settling of bound
aggregates of silt- and sand-sized particles creates large
areas of stationary and moving mud suspensions
(Figs 6.52 and 6.53), loosely termed fluid mud , that char-
acterize the outer estuarine reaches of tide-dominant estu-
aries. This may be mobile or fixed, the latter grading into
areas of more-or-less settled mud. Stationary suspensions
up to 3 m thick can show sharp upper surfaces on sonar
records and may deposit very quickly. Such suspensions
form during slackwater periods, progressively thickening
during the spring to neap transition. They are easily
eroded, to be taken up in suspension once more by the
accelerating phases of spring tidal cycles.
Type D estuaries are theoretical end-members of the
estuarine continuum in that they show both lateral and
vertical homogeneity of salinity. Such conditions apply
only in the outer parts of many type B and C estuaries;
they are clearly transitional to open shelf conditions.
Under equilibrium conditions, saline water is diffused
upstream to replace that lost by advective mixing.
Sediment movement is dominated entirely by tidal
motions, again with no internal sediment trap.

Fig. 6.52 To illustrate the process of fluid mud formation.

Concentration
profiles
after 1 hour

2

Initial conc.
1 g l -1

Initial conc.
5.5 g l -1

1.5

1

Concentrated
suspension

Dilute
suspension

0.5

Lutocline

0

0.1 1 10

Concentration, g l -1

Fig. 6.53 Experimental data to contrast the behavior of dilute and
concentrated settling sediment suspensions. Note the stepped profile
that forms in the latter case with the formation of a lutocline as hin-
dered settling and flocculation delay fall.

between closely located clay particles is made positive by
absorption of abundant cations from salt water. Also, the
higher the amount of suspended clay, the more likely
particle collisions will occur, leading to flocculation of
aggregates whose settling velocity is enhanced. At the
same time, the higher the particle concentration, the
lower will be the rate of settling as a result of the effects of
particle hindrance (Section 4.7). These two effects,
agglomeration and hindrance, lead to the formation of
distinct layers of suspended material during the period of

6.6.4 Estuarine sedimentation

The mixing of fresh and salt water causes estuarine circu-
lation in response to density gradients. Sedimentary parti-
cles may be of both marine and river origin, with
flocculation and floc destruction by turbulent shear and
resuspension of bed material as important controls upon
particle size. Flocculation is a process whereby the usually
repulsive van der Waals electrostatic forces present