Geology Reference
In-Depth Information
deposition requires small velocity, settling of cohesive
sediment occurs mostly during slack tides. The regu-
lated acceleration and deceleration of tidal current, in
addition to the different bed shear stress for erosion
and deposition, is also the cause of the scour and set-
tling lags. Under most circumstances in tidal environ-
ments, fine-grain sediments (except rip-up mud clasts)
are transported as suspended load and the vertical
component of the turbulent fluctuation is typically
greater than the settling velocity of the cohesive parti-
cles. The movement of cohesive particles can be
approximated with the movement of water particles.
When the flow intensity increases, more sediment is
suspended into the water column resulting in an
increasing sediment concentration. As the flow inten-
sity decreases toward slack tide, deposition occurs
resulting in a decreasing sediment concentration
throughout the water column. Therefore, the fluctuat-
ing intensity of the tidal flow is crucial to the erosion
and deposition of fine-grain particles. Due to the large
temporal and spatial variations of sediment concentra-
tion and the slow settling velocity of the fine-grain par-
ticle, sediment transport by dispersion can be significant
(Eq.
2.23
). Accurately quantifying the critical shear
stress for erosion and deposition is essential to the
quantification of cohesive sediment transport and
deposition. Unfortunately, these two parameters are
influenced by many site specific factors including
physical, chemical, as well as biological ones.
In situ
data are crucial to the quantification of cohesive sedi-
ment transport and deposition.
influence the fluid motion and sediment transport. The
finer cohesive sediments tend to be transported as sus-
pended load; their deposition occurs mostly during
slack tides. Rate of sediment transport is generally pro-
portional to flow velocity to the third to fifth power.
This non-linear relationship leads to a net transport in
the direction of the faster velocity in the tidal environ-
ments with a time-velocity asymmetry. Due to the slow
settling velocity of the fine cohesive sediment and a
difference between the critical shear stress for erosion
and deposition, a scour lag and a settling lag exists in
many tidal environments resulting in a fining deposi-
tional trend landward. The periodic reversing of tidal
flow directions results in the commonly observed bi-
directional sedimentary structures (e.g., the herring-
bone cross-stratification). The relatively tranquil slack
tides between the flood and ebb tides allow the deposi-
tion of muddy layers in between sandy layers depos-
ited during the flood and ebb tides, forming lenticular,
wavy, and flaser bedding.
References
Allen PA (1997) Earth surface processes. Blackwell Science,
London, 404 p
Allen JRL, Duffy MJ (1998) Temporal and spatial depositional
patterns in the Severn Estuary, southwestern Britain: inter-
tidal studies at spring-neap and seasonal scales, 1991-1993.
Mar Geol 146:147-171
Bagnold RA (1956) Flow of cohesionless grains in fluid. R Phil
Soc Lond Trans 249:235-297
Bagnold RA (1966) An approach to the sediment transport prob-
lem from general physics. U.S. geological survey profes-
sional paper 422-I, Washington, DC
Bartholdy J (2000) Process controlling import of fine-grained
sediment to tidal areas: a simulation model. In: Pye K, Allen
JRL (eds) Coastal and estuarine environments: sedimentol-
ogy, geomorphology, and geoarchaeology. Geological
Society Lond, Spec Publ 175:13-29
Christie MC, Dyer KR, Turner P (1999) Sediment flux and bed
level measurements from a macro tidal mudflat. Estuar Coast
Shelf Sci 49:667-688
Davis RA, Hayes MO (1984) What is a wave dominated coast?
Mar Geol 60:313-329
Dyer KR, Cornelisse M, Dearnaley MP, Fenness MJ, Jones SE,
Kappenberg J, McCave IN, Pejrup M, Puls W, Van Leussen
W, Wolfstein KA (1996) A comparison of
in situ
techniques
for estuarine floc settling velocity measurements. J Sea Res
36:15-29
Dyer KR (1986) Coastal and estuarine sediment dynamics.
Wiley, Chichester, New York, 342 p
Dyer KR (1994) Estuarine sediment transport and deposition.
In: Pye K (ed) Sediment transport and depositional pro-
cesses. Blackwell Science Publication, Oxford, pp 193-216
2.3
Summary
Physical processes of sediment transport in tidal envi-
ronments are extremely complicated and influenced by
numerous hydrodynamic and sedimentological factors
over a wide range of temporal and spatial scales. Both
tides and waves play significant roles in the entrain-
ment and transport of both cohesive and non-cohesive
particles. Sediment transport is composed of three
phases, initiation of motion (erosion), transport, and
deposition. Various commonly used empirical formu-
las are provided in this chapter for the quantification of
the three phases.
In tidal environments, the coarser, non-cohesive
sediments are typically transported as bedload, form-
ing various types of bedforms. The bedforms in turn
