92
4 TRANSPORTATION AND SEDIMENTATION
J
Fig. 4.3. The mechanics of particle movement. (A) Suspension. (B) Bouncing (saltation). (C) Rolling.
For flow in open channels the Froude number is expressed thus:
F
U
where D is the depth of the channel and U is the average current velocity.
A Froude number of 1 separates two distinct types of fluid flow in open channels. Each
flow regime generates specific bed forms and sediment structures. These are described
in detail in the section on aqueous traction sedimentation.
Consider now the mechanics of particle movement. Essentially a grain can move
through a fluid (liquid or gaseous) in three different ways: by rolling, by bouncing, or in
suspension (Fig. 4.3). In a given situation, the heaviest particles are never lifted from
the ground. They remain in contact with their colleagues, but are rolled along by the current. At the same velocity, lighter particles move downcurrent with steep upward trajectories and gentler downward glide paths. This process is known as saltation. At the
same velocity the lightest particles are borne along by the current in suspension. They
are carried within the fluid in erratic but essentially downflow paths never touching the
bottom or ground.
In a situation such as a river channel, therefore, gravel will be rolling along the bottom,
sand will sedately saltate, and silt and clay will be carried in suspension. Sand and gravel
are generally referred to as the traction carpet or the channel bed load. The silt and
clay, loosely termed "fines," are referred to as the suspended load. Considerable importance is attached to the critical flow velocity needed to start a particle into motion.
The critical flow velocity for a particle is a function of the variables contained in the
Froude and Reynolds equations. A number of empirical, experimental, and theoretical studies have been made to determine the critical flow velocity for varying sediment
grades, notably by Shields (1936), Vanoni (1964), and Hjulstrom (in Sundborg, 1956).
Of these the latter is the better known (Fig. 4.4). As one might expect, the critical fluid
flow increases with grain size. An exception to this rule is noted for cohesive clay bottoms. Because of their resistance to friction, they need rather higher velocities to erode
4 TRANSPORTATION AND SEDIMENTATION
J
Fig. 4.3. The mechanics of particle movement. (A) Suspension. (B) Bouncing (saltation). (C) Rolling.
For flow in open channels the Froude number is expressed thus:
F
U
where D is the depth of the channel and U is the average current velocity.
A Froude number of 1 separates two distinct types of fluid flow in open channels. Each
flow regime generates specific bed forms and sediment structures. These are described
in detail in the section on aqueous traction sedimentation.
Consider now the mechanics of particle movement. Essentially a grain can move
through a fluid (liquid or gaseous) in three different ways: by rolling, by bouncing, or in
suspension (Fig. 4.3). In a given situation, the heaviest particles are never lifted from
the ground. They remain in contact with their colleagues, but are rolled along by the current. At the same velocity, lighter particles move downcurrent with steep upward trajectories and gentler downward glide paths. This process is known as saltation. At the
same velocity the lightest particles are borne along by the current in suspension. They
are carried within the fluid in erratic but essentially downflow paths never touching the
bottom or ground.
In a situation such as a river channel, therefore, gravel will be rolling along the bottom,
sand will sedately saltate, and silt and clay will be carried in suspension. Sand and gravel
are generally referred to as the traction carpet or the channel bed load. The silt and
clay, loosely termed "fines," are referred to as the suspended load. Considerable importance is attached to the critical flow velocity needed to start a particle into motion.
The critical flow velocity for a particle is a function of the variables contained in the
Froude and Reynolds equations. A number of empirical, experimental, and theoretical studies have been made to determine the critical flow velocity for varying sediment
grades, notably by Shields (1936), Vanoni (1964), and Hjulstrom (in Sundborg, 1956).
Of these the latter is the better known (Fig. 4.4). As one might expect, the critical fluid
flow increases with grain size. An exception to this rule is noted for cohesive clay bottoms. Because of their resistance to friction, they need rather higher velocities to erode