and deposition occur due to diminishing flow intensity,
estuarine stratification, and flocculation (Wright, 1977;
Geyer et al., 2004). Hence, sediment resuspension
becomes the key process to further deliver sediments offshore before the journey of sediment source to sink can
be completed (Wright and Nittrouer, 1995). Although sediment resuspension is driven by the overlaying hydrodynamics, resuspension also leads to a variety of seabed
characteristics, such as bedform and fluid mud, which in
turn determines the bottom friction (dissipation) experienced by the overlaying hydrodynamics.
Mechanisms of sediment resuspension
Sediment resuspension is generally driven by bottom
boundary layer flow, which is a layer of sharp transition
of flow velocity from nearly zero at the bed to a large magnitude of overlaying hydrodynamics. Hence, the flow
shear in a bottom boundary layer, defined here as the vertical gradient of streamwise flow velocity, is significant,
and it is the main ingredient of turbulence production
and dispersion of solutes (Pope, 2000). Consequently, surficial bed sediments are mobilized and suspended in the
bottom boundary layer. Boundary layer flow and sediment
resuspension can be driven by tidal currents, river outflows, surface waves, and even internal waves. Moreover,
anthropogenic causes such as dredging and ship waves
can also cause sediment resuspension. Once sediments
are suspended in the bottom boundary layer, depending
on the bed slope and the characteristics of suspension
(see next section), downslope gravitational force may also
play a critical role to transport sediment in the downslope
direction.
Resuspension processes
Although turbulence is the key mechanism of sediment
suspension, when a significant amount of fine sediments
is suspended in the bottom boundary layer, flow turbulence can be attenuated through sediment-induced density
stratification. This is typically observed through the formation of the “lutocline,” a sharp negative gradient of
suspended sediment concentration at some elevation
above the bed. Such turbulence-sediment interaction leads
to several critical processes. In moderate sediment concentration (or relatively intense overlaying flow), the
lutocline separates the lower turbulent boundary layer
from the upper nonturbulent layer. Hence, sediments are
mostly accumulated below the lutocline and may establish
sufficiently large buoyancy anomaly to drive offshoredirected gravity flow (Traykovski et al., 2000; Wright
and Friedrichs, 2006). Because flow turbulence eventually
leads to energy dissipation through energy cascade (Pope,
2000), when turbulence is attenuated by the presence of
sediments, mean flow kinetic energy increases. This phenomenon is called drag reduction. During AMASSEDS
(A Multidisciplinary Amazon Shelf Sediment Study;
Nittrouer et al., 1991), a significant reduction of bottom
drag coefficient is observed as tidal currents propagate
over a muddy bed (Beardsley et al., 1995), where a thick
layer of mud up to several meters is present (Trowbridge
and Kineke, 1994). The existence of the lutocline also
implies significantly suppressed mixing of other solutes
and hence effects on benthic boundary layer. Since the
presence of sediment can attenuate turbulence, there exists
a carrying capacity for both current-dominant and wavedominant sediment resuspension (Winterwerp, 2001;
Ozdemir et al., 2011), and that for a given flow intensity
and sediment characteristics (e.g., settling velocity), the
flow can only sustain a maximum amount of sediment
load. When the carrying capacity is exceeded (e.g., flow
intensity decreases when approaching slack water),
turbulence in the boundary layer is significantly
suppressed and catastrophic sediment settling occurs.
Also facilitated by hindered settling effect, a thick layer
of concentrated sediments is accumulated near the bed,
called fluid mud, and experiences a slow consolidation
process (Mehta, 1991). The formation of fluid mud and
a laminarized boundary layer give rise to a greatly
enhanced bulk flow viscosity near the bed, which
appears to also cause a large surface wave dissipation rate
(Sahin et al., 2011).
In the intertidal environments, large sediment
resuspension at the intratidal time scale is often observed
at very shallow water depths near the land-water interface,
called turbid tidal edge (e.g., Christie and Dyer, 1998;
Nowacki and Ogston, 2013). On the daily and fortnightly
time scale, a significant amount of sediment is exchanged
between the tidal channel and adjacent flats. The net
exchange is highly dependent on seasonal variability due
to vegetation. For example, observations at the intertidal
mudflats of Willapa Bay (USA) show that during the winter period, flats are not vegetated and channels are filled
with thick layers of muddy deposits delivered from the
flats during ebb flow. On the contrary, with only limited
vegetation during the summer period, sediments appear
to be trapped on the flats, and hence the channels are
observed to be free of mud (Boldt et al., 2013).
Summary
New sensor technology has provided a wealth of insights
into the sediment-laden bottom boundary layer regarding
fluid-sediment interactions and sediment properties that
are essential to the understanding and modeling of sediment resuspension (Mikkelsen et al., 2004; Traykovski
et al., 2007). In recent years, numerical simulations based
on two-phase flow principles are capable of resolving
most of the three-dimensional turbulence-sediment interactions; they also reveal new insights on the mechanism
of sediment resuspension (Cantero et al., 2009; Ozdemir
et al., 2010). Several critical aspects of sediment
resuspension warrant future studies. A robust flocculation
module needs to be incorporated into models for sediment
resuspension. On the same note, appropriate parameterization of erosion flux for cohesive sediment bed capturing
consolidation and characteristic of aggregates is necessary
SEDIMENT RESUSPENSION
559
estuarine stratification, and flocculation (Wright, 1977;
Geyer et al., 2004). Hence, sediment resuspension
becomes the key process to further deliver sediments offshore before the journey of sediment source to sink can
be completed (Wright and Nittrouer, 1995). Although sediment resuspension is driven by the overlaying hydrodynamics, resuspension also leads to a variety of seabed
characteristics, such as bedform and fluid mud, which in
turn determines the bottom friction (dissipation) experienced by the overlaying hydrodynamics.
Mechanisms of sediment resuspension
Sediment resuspension is generally driven by bottom
boundary layer flow, which is a layer of sharp transition
of flow velocity from nearly zero at the bed to a large magnitude of overlaying hydrodynamics. Hence, the flow
shear in a bottom boundary layer, defined here as the vertical gradient of streamwise flow velocity, is significant,
and it is the main ingredient of turbulence production
and dispersion of solutes (Pope, 2000). Consequently, surficial bed sediments are mobilized and suspended in the
bottom boundary layer. Boundary layer flow and sediment
resuspension can be driven by tidal currents, river outflows, surface waves, and even internal waves. Moreover,
anthropogenic causes such as dredging and ship waves
can also cause sediment resuspension. Once sediments
are suspended in the bottom boundary layer, depending
on the bed slope and the characteristics of suspension
(see next section), downslope gravitational force may also
play a critical role to transport sediment in the downslope
direction.
Resuspension processes
Although turbulence is the key mechanism of sediment
suspension, when a significant amount of fine sediments
is suspended in the bottom boundary layer, flow turbulence can be attenuated through sediment-induced density
stratification. This is typically observed through the formation of the “lutocline,” a sharp negative gradient of
suspended sediment concentration at some elevation
above the bed. Such turbulence-sediment interaction leads
to several critical processes. In moderate sediment concentration (or relatively intense overlaying flow), the
lutocline separates the lower turbulent boundary layer
from the upper nonturbulent layer. Hence, sediments are
mostly accumulated below the lutocline and may establish
sufficiently large buoyancy anomaly to drive offshoredirected gravity flow (Traykovski et al., 2000; Wright
and Friedrichs, 2006). Because flow turbulence eventually
leads to energy dissipation through energy cascade (Pope,
2000), when turbulence is attenuated by the presence of
sediments, mean flow kinetic energy increases. This phenomenon is called drag reduction. During AMASSEDS
(A Multidisciplinary Amazon Shelf Sediment Study;
Nittrouer et al., 1991), a significant reduction of bottom
drag coefficient is observed as tidal currents propagate
over a muddy bed (Beardsley et al., 1995), where a thick
layer of mud up to several meters is present (Trowbridge
and Kineke, 1994). The existence of the lutocline also
implies significantly suppressed mixing of other solutes
and hence effects on benthic boundary layer. Since the
presence of sediment can attenuate turbulence, there exists
a carrying capacity for both current-dominant and wavedominant sediment resuspension (Winterwerp, 2001;
Ozdemir et al., 2011), and that for a given flow intensity
and sediment characteristics (e.g., settling velocity), the
flow can only sustain a maximum amount of sediment
load. When the carrying capacity is exceeded (e.g., flow
intensity decreases when approaching slack water),
turbulence in the boundary layer is significantly
suppressed and catastrophic sediment settling occurs.
Also facilitated by hindered settling effect, a thick layer
of concentrated sediments is accumulated near the bed,
called fluid mud, and experiences a slow consolidation
process (Mehta, 1991). The formation of fluid mud and
a laminarized boundary layer give rise to a greatly
enhanced bulk flow viscosity near the bed, which
appears to also cause a large surface wave dissipation rate
(Sahin et al., 2011).
In the intertidal environments, large sediment
resuspension at the intratidal time scale is often observed
at very shallow water depths near the land-water interface,
called turbid tidal edge (e.g., Christie and Dyer, 1998;
Nowacki and Ogston, 2013). On the daily and fortnightly
time scale, a significant amount of sediment is exchanged
between the tidal channel and adjacent flats. The net
exchange is highly dependent on seasonal variability due
to vegetation. For example, observations at the intertidal
mudflats of Willapa Bay (USA) show that during the winter period, flats are not vegetated and channels are filled
with thick layers of muddy deposits delivered from the
flats during ebb flow. On the contrary, with only limited
vegetation during the summer period, sediments appear
to be trapped on the flats, and hence the channels are
observed to be free of mud (Boldt et al., 2013).
Summary
New sensor technology has provided a wealth of insights
into the sediment-laden bottom boundary layer regarding
fluid-sediment interactions and sediment properties that
are essential to the understanding and modeling of sediment resuspension (Mikkelsen et al., 2004; Traykovski
et al., 2007). In recent years, numerical simulations based
on two-phase flow principles are capable of resolving
most of the three-dimensional turbulence-sediment interactions; they also reveal new insights on the mechanism
of sediment resuspension (Cantero et al., 2009; Ozdemir
et al., 2010). Several critical aspects of sediment
resuspension warrant future studies. A robust flocculation
module needs to be incorporated into models for sediment
resuspension. On the same note, appropriate parameterization of erosion flux for cohesive sediment bed capturing
consolidation and characteristic of aggregates is necessary
SEDIMENT RESUSPENSION
559