Geology Reference
In-Depth Information
2
Principles of Sediment Transport
Applicable in Tidal Environments
Ping Wang
Abstract
Physical processes of sediment transport in tidal environments are extremely
complicated and are influenced by numerous hydrodynamic and sedimentological
factors over a wide range of temporal and spatial scales. Both tide and wave forcing
play significant roles in the entrainment and transport of both cohesive and
non-cohesive particles. Present understanding of sediment transport is largely
empirical and based heavily on field and laboratory measurements. Sediment
transport is composed of three phases: (1) initiation of motion (erosion), (2) trans-
port, and (3) deposition. In tidal environments, the coarser non-cohesive sediments
are typically transported as bedload, forming various types of bedforms. The finer
cohesive sediments tend to be transported as suspended load, with their deposition
occurring mostly during slack tides under calm conditions. Rate of sediment
transport is generally proportional to flow velocity to the 3rd to 5th power. This
non-linear relationship leads to a net transport in the direction of the faster velocity
in tidal environments with a time-velocity asymmetry. Due to the slow settling
velocity of fine cohesive sediment and a difference between the critical shear stress
for erosion and deposition, scour and settling lags exist in many tidal environ-
ments resulting in a landward-fining trend of sediment grain size. The periodic
reversing of tidal flow directions results in characteristic bi-directional sedimen-
tary structures. The relatively tranquil slack tides allow the deposition of muddy
layers in between the sandy layers deposited during flood and ebb tides, forming
the commonly observed lenticular, wavy, and flaser bedding.
Notations and Conventional Units
c
:
suspended sediment concentration (dimension-
less for volume concentration, kg/m
3
for mass
concentration)
a
:
a reference level (typically defined at the top level
of the bedload layer) for suspended sediment con-
centration. (m)
c
a
:
reference concentration (dimensionless for vol-
ume concentration, kg/m
3
for mass concentration)
c
(
z
):
suspended sediment concentration profile
(dimensionless for volume concentration, kg/
m
3
for mass concentration)
