Geology Reference
In-Depth Information
Fig. 9.16
Variation in hydrodynamic and morphodynamic
processes within a tidal cycle on the Skeffling intertidal flat
(Humber Estuary, UK) during the calm (
a
and
c
) and stormy
(
b
and
d
) conditions. W
c
, the critical shear stress of sediment,
was set to 0.3 N m
-2
. Erosion occurs when W
b
> W
c
(After Christie
et al.
1999
)
9.4.1.1 Short-Term Cycles
(A Few Minutes to Days)
The interactions of tides and waves over a tidal cycle
are fundamental processes and mechanisms to induce
deposition and erosion on the open-coast tidal flats.
Bottom shear stress is the important parameter to
assess the deposition and erosion of sediments (e.g.,
Christie et al.
1999
; Le Hir et al.
2000
). Over the inter-
tidal flats, purely tide-induced stresses are generally
low to trigger significant erosion, while wave-induced
stresses are much higher due to their orbital movement
character. It has been widely acknowledged that even
presence of small waves potentially enhances erosion
of the surficial sediment significantly (Anderson et al.
1981
; Christie et al.
1999
; Le Hir et al.
2000
; Lee et al.
2004
). Enhanced erosion by large waves was nicely
described by a comparison study of Christie et al. (
1999
)
on a semi-closed and mega-tidal mudflat in the Humber
Estuary (UK) with the average tidal range of ~6 m. The
bed shear stresses (W
b
) of combined flows and waves
were highly elevated toward the large-wave condition,
producing several centimeters of erosion, which is
strongly contrastive with a few millimeters of accre-
tion during the small-wave condition (Fig.
9.16a, b
).
The difference is highly outstanding between wave
and tide processes over a tidal cycle. Tidal currents and
tide-induced bottom stresses decrease landward from
the lower intertidal flats, and drop to nearly zero at the
high, slack tide. However, waves and wave-induced
bottom stresses are significantly strengthened by the
increased water depth during the rising tide owing to
the relationship between wave height and water depth
(Green et al.
1997
; Le Hir et al.
2000
).
So peak waves and the related processes hypotheti-
cally attain at high tide for any given location at the
intertidal flats, and the slack (quiet) condition is conse-
quently detained at high tide by the presence of large
waves (Christie et al.
1999
). Th
e
phenomena were
clearly shown by the variations in
uc
value (mean sus-
pended sediment flux, the product of mean current
velocity and mean concentration of suspended sedi-
ment). The curve was flat with roughly zero value over
approximately 1.5 h at high tide during small-wave
condition (Fig.
9.16c
), but was drastically replaced by
a seesaw curve section during large-wave condition
(Fig.
9.16d
, Christie et al.
1999
).
The shallow wave-break/swash zone should shift
several 100-1,000 m landward and seaward over a
