SEDIMENT TRANSPORT
Pedro J. M. Costa
Centro de Geologia, Faculdade de Ciencias, Universidade
de Lisboa, Lisbon, Portugal
Definition
The simplest definition of sediment transport is the transport of granular particles by fluids. The main agents by
which sedimentary materials are moved include gravity
(gravity transport), river and stream flow, ice, wind, and
estuarine and ocean currents. Running water and wind
are the most widespread transporting agents. In both cases,
three mechanisms operate, although the particle size of the
transported material is very different, owing to the differences in density and viscosity of air and water. The three
processes are rolling or traction, in which the particle
moves along a sedimentary bed but is too heavy to be
lifted from it; saltation; and suspension, in which particles
remain permanently above the bed, sustained there by the
turbulent flow of the air or water (U.S. Army Corps of
Engineers, 2003).
Importance of sediment transport
Sediment transport has been studied for centuries and
remains a challenging area of research for earth and marine
scientists. The general term of “sediment transport” includes
a number of environmental processes that take place at
a wide range of spatial and temporal scales. The full understanding of sediment transport is fundamental to assessing
a range of heterogeneous geological, engineering, and
environmental processes. Over the recent years, the development of sediment transport research was transformed
from descriptions of simple empirical\phenomena to more
complex numerical models in which the flow and the
resulting sediment transport are detailed.
The study of sediment transport processes includes the
movement of particles, rocks, and other earth materials by
various processes. Transport is driven by gravity effects
and by friction effects with the air or the fluid containing
the sediment. Sediment transport due to fluid motion
occurs in rivers, lakes, estuaries, seas, and other bodies
of water due to currents and tides, in glaciers as they flow,
and on terrestrial surfaces under the influence of wind.
Sediment transport due only to gravity can occur on sloping surfaces in general, including hill slopes, scarps, cliffs,
and the continental shelf – continental slope boundary.
Sediment transport is usually divided into three types:
bed load, saltation, and suspension. Bed-load transport is
defined as the type of transport where sediment grains roll
or slide along the bed. Saltation transport is defined as the
type of transport where single grains jump over the bed at
a length proportional to their diameter, periodically losing
contact with the bed. Sediment is suspended when the flux
is intense enough such that the sediment grains are
suspended over the bed.
The interaction between fluid and solid particles is
greatly influenced by the sediment characteristics. Sediments are commonly divided into cohesive and
non-cohesive components. In cohesive sediments, the
resistance to erosion depends on the strength of the cohesive bond between the particles. The problem of erosion
resistance of cohesive soils is a very complex one, and at
present our understanding of the physics of it is still very
incomplete. The non-cohesive soils generally consist of
larger discrete particles (e.g., sand, pebbles, cobbles, and
boulders); the movement of these particles essentially
depends on the physical properties of the individual particles, such as their size, shape, and density. The most
important physical property of a sediment particle is its
size which has a direct effect on its mobility.
There are dozens of sediment transport functions that
predict sediment transport based on sediment size, weight,
fall velocity, water velocity, channel depth, channel width,
slope, roughness, and water temperature. However, in
many of these empirical solutions, some assumptions are
made. Moreover, in sediment transport, two important
concepts are settling rate and boundary shear stress. Settling rate describes the tendency for sediment particles to
fall out of suspension. Boundary layer shear stress
describes the tendency for a moving fluid to bring sediment particles into suspension.
Sediment entrainment or the threshold
for sediment motion
Sediment entrainment is defined as the transition from
repose to displacement. The incipient motion of sediment
occurs when the stability of a particle is disturbed. Such
instability can be attributed to the imbalance of forces
(or force moments) caused by the forces exerted on the
particle in the flow. The threshold of particle motion is
attained for a given ratio between driving and stabilizing
forces. Simplifying, the driving forces on a sediment particle resting on other particles on an originally plane horizontal bed are the tractive stress T $ (horizontal) and the
lift force (caused by the Bernoulli effect). The horizontal
drag, created by the flow, consists of skin friction acting
on the surface of the grain and from drag due to
a pressure difference on the upstream and downstream
sides of the grain because of flow separation.
The motion of a fluid flowing across its bed tends to
move the bed material downstream. A submerged grain
on the surface is subjected to a weight force and the hydrodynamic forces. Below some critical hydraulic conditions,
the hydrodynamic forces will be so small that particles
submerged weight will move very rarely or not at all.
However, a slight increase in flow velocity above this
hydraulic critical condition will initiate appreciable
motion by some of the particles on the bed. This hydraulic
critical condition is termed the condition of initiation of
motion and is computed in terms of either mean flow
velocity in the vertical or the critical bed shear stress
(also known as the tractive force or the drag force).
562
SEDIMENT TRANSPORT
Pedro J. M. Costa
Centro de Geologia, Faculdade de Ciencias, Universidade
de Lisboa, Lisbon, Portugal
Definition
The simplest definition of sediment transport is the transport of granular particles by fluids. The main agents by
which sedimentary materials are moved include gravity
(gravity transport), river and stream flow, ice, wind, and
estuarine and ocean currents. Running water and wind
are the most widespread transporting agents. In both cases,
three mechanisms operate, although the particle size of the
transported material is very different, owing to the differences in density and viscosity of air and water. The three
processes are rolling or traction, in which the particle
moves along a sedimentary bed but is too heavy to be
lifted from it; saltation; and suspension, in which particles
remain permanently above the bed, sustained there by the
turbulent flow of the air or water (U.S. Army Corps of
Engineers, 2003).
Importance of sediment transport
Sediment transport has been studied for centuries and
remains a challenging area of research for earth and marine
scientists. The general term of “sediment transport” includes
a number of environmental processes that take place at
a wide range of spatial and temporal scales. The full understanding of sediment transport is fundamental to assessing
a range of heterogeneous geological, engineering, and
environmental processes. Over the recent years, the development of sediment transport research was transformed
from descriptions of simple empirical\phenomena to more
complex numerical models in which the flow and the
resulting sediment transport are detailed.
The study of sediment transport processes includes the
movement of particles, rocks, and other earth materials by
various processes. Transport is driven by gravity effects
and by friction effects with the air or the fluid containing
the sediment. Sediment transport due to fluid motion
occurs in rivers, lakes, estuaries, seas, and other bodies
of water due to currents and tides, in glaciers as they flow,
and on terrestrial surfaces under the influence of wind.
Sediment transport due only to gravity can occur on sloping surfaces in general, including hill slopes, scarps, cliffs,
and the continental shelf – continental slope boundary.
Sediment transport is usually divided into three types:
bed load, saltation, and suspension. Bed-load transport is
defined as the type of transport where sediment grains roll
or slide along the bed. Saltation transport is defined as the
type of transport where single grains jump over the bed at
a length proportional to their diameter, periodically losing
contact with the bed. Sediment is suspended when the flux
is intense enough such that the sediment grains are
suspended over the bed.
The interaction between fluid and solid particles is
greatly influenced by the sediment characteristics. Sediments are commonly divided into cohesive and
non-cohesive components. In cohesive sediments, the
resistance to erosion depends on the strength of the cohesive bond between the particles. The problem of erosion
resistance of cohesive soils is a very complex one, and at
present our understanding of the physics of it is still very
incomplete. The non-cohesive soils generally consist of
larger discrete particles (e.g., sand, pebbles, cobbles, and
boulders); the movement of these particles essentially
depends on the physical properties of the individual particles, such as their size, shape, and density. The most
important physical property of a sediment particle is its
size which has a direct effect on its mobility.
There are dozens of sediment transport functions that
predict sediment transport based on sediment size, weight,
fall velocity, water velocity, channel depth, channel width,
slope, roughness, and water temperature. However, in
many of these empirical solutions, some assumptions are
made. Moreover, in sediment transport, two important
concepts are settling rate and boundary shear stress. Settling rate describes the tendency for sediment particles to
fall out of suspension. Boundary layer shear stress
describes the tendency for a moving fluid to bring sediment particles into suspension.
Sediment entrainment or the threshold
for sediment motion
Sediment entrainment is defined as the transition from
repose to displacement. The incipient motion of sediment
occurs when the stability of a particle is disturbed. Such
instability can be attributed to the imbalance of forces
(or force moments) caused by the forces exerted on the
particle in the flow. The threshold of particle motion is
attained for a given ratio between driving and stabilizing
forces. Simplifying, the driving forces on a sediment particle resting on other particles on an originally plane horizontal bed are the tractive stress T $ (horizontal) and the
lift force (caused by the Bernoulli effect). The horizontal
drag, created by the flow, consists of skin friction acting
on the surface of the grain and from drag due to
a pressure difference on the upstream and downstream
sides of the grain because of flow separation.
The motion of a fluid flowing across its bed tends to
move the bed material downstream. A submerged grain
on the surface is subjected to a weight force and the hydrodynamic forces. Below some critical hydraulic conditions,
the hydrodynamic forces will be so small that particles
submerged weight will move very rarely or not at all.
However, a slight increase in flow velocity above this
hydraulic critical condition will initiate appreciable
motion by some of the particles on the bed. This hydraulic
critical condition is termed the condition of initiation of
motion and is computed in terms of either mean flow
velocity in the vertical or the critical bed shear stress
(also known as the tractive force or the drag force).
562
SEDIMENT TRANSPORT