Geology Reference
In-Depth Information
Fig. 2.7
Schematics of scour lag (
a
) and settling lag (
b
) for
fine-grain sediments. (
a
) Scour lag: a particle on the bed is sus-
pended into the water column when the threshold velocity is
exceeded at point 1. It does not, however, achieve the depth-
averaged velocity till point 2, a relatively seaward position. It
then travels with the water trajectory to point 3, where we
assume it is instantaneously deposited. On the following ebb
tide, the particle is suspended, but again lags the flow till point 4
is reached. It is eventually re-deposited at point 5. Considerable
landward movement has occurred during the tidal cycle because
of the scour lag. (
b
) Settling lag: at position 1, the particle is
entrained from the bed and travels with water till point 2, where
it starts to settle. Because of the settling lag, it reaches the bed at
point 3. On the following ebb tide, it is not entrained till later in
the tide cycle when the threshold velocity (greater than the
velocity for settling) is reached. The deposition at low water is at
position 6. Consequently, the particle has moved shoreward due
the settling lag (Modified from Dyer
1994
)
their deposition discussed above. Erosion of cohesive
sediment from the bed occurs when the bed shear stress
from the fluid (current, wave, or combined) exceeds a
critical value for erosion. The critical value for erosion
is greater than the critical value for full deposition
(Mehta
1986a
). In other words, more fluid power is
needed to erode the cohesive sediments than that to
deposit them. This difference in the shear stresses for
erosion and deposition, in addition to the time needed
for the fine grains to settle, is responsible for the
so-called settling lag and scouring lag, which is an impor-
tant sedimentary process on many mud flats (Postma
1961
; Dyer
1998
; Bartholdy
2000
). The schematics of
scour lag and settling lag are well illustrated (Fig.
2.7
)
and explained by Dyer (
1994
). Theoretically, the scour
and settling lag should result in a net deposition of fine
grain sediment landward in the area where maximum
velocity during the tidal cycle equals the grain's threshold
velocity. At many tidal flats, the net sedimentation on
the upper intertidal zone due to settling lag during calm
season is often eroded by storm waves during storm
season, result in a seasonal cycle (Allen and Duffy
1998
; Dyer et al.
2000
; Talke and Stacey
2008
).
Erosion of cohesive sediment is strongly influenced
by the degree of consolidation, as well as the strength
of the newly deposited flocs. Therefore, time history of
the sediment, which is highly variable temporally and
spatially, plays a significant part in cohesive sediment
erosion. Consolidation history, as examined in labora-
tory studies, is usually evaluated using the bulk density
of the bed sediment at different depths. The bulk den-
sity typically increases with depth below the sediment
surface (Partheniades
1986
). Under many cases, the
erosional process is stopped when it reaches a well-
consolidated layer, although the fluid power may
remain the same. Bioturbation can play a significant,
