Environmental Engineering Reference
In-Depth Information
ENTRAINMENT, TRANSPORT AND DEPOSITION
Entrainment incorporates particles into the flow when river velocity exceeds the
entraining velocity
for a particular particle size. More accurately, it occurs when bed
shear stress exceeds particle-bed friction and effective stress. This is a natural extension
of erosion and is vital to the movement of stationary particles in changing flow
conditions. Conversely, deposition occurs when
stream competence
, or ability to
maintain movement as bed load, falls below a given velocity. This applies when stream
velocity falls below the
fall velocity
of a particle in transit. Large debris particles
delivered to the channel by bank caving or landsliding may simply fall out of the flow.
All conditions are summarized in the version of Hjulström's diagram in Chapter 12
(Figure 12.11). This shows further important distinctions between
erosion
and
transport
velocities for particles below medium sand size (0·2-0·6 mm). Smaller particles require
higher initial velocities. They present smaller, 'streamlined' surfaces to the flow and may
also develop weak cohesive strength from surrounding water films. Dissolved load is
deposited by precipitation when solutions exceed saturation level.
Particle movement between points of entrainment and deposition is determined by
particle size, flow conditions and mode of entrainment. Particles above medium to coarse
sand size (over 0·2 mm) tend to roll or slide along the channel bed as
bed
or
traction
load
(Figure 14.16). More mobile particles are lifted into the flow as pressure falls in the wake
of overlying accelerating and, especially, turbulent flow. Particles are drawn into the
partial vacuum and remain in suspension by incorporation into even faster flow paths, or
until their weight overcomes buoyancy. Sand particles fall out rapidly and move by
saltation
or repeated bouncing. Silt particles (less than 0·06 mm) move as
suspended
load and clay particles (under 0·002 mm), indefinitely, as
wash
load. These modes and
overall catchment sediment transfers are summarized in Figure 14.17. Since particles
move by different styles and in distinct parts of the channel and river flow, a considerable
amount of
particle sorting
occurs which persists into the depositional environment.
Channel and over-bank (flood) sediments form predominantly sand-gravel and clay-silt
facies
respectively. Particle size, grading laterally within the same bed and changing
abruptly in a vertical sequence, demonstrates rapid changes in flow conditions (Plate
14.10). The global extent of sediment removal to the sea is shown in Figure 14.18.
FLUVIAL LANDSYSTEMS
Reference to landforms so far has been incidental and related largely to small-scale
channel morphology and bed forms, responding to changes in the power threshold and
search for efficiency. Their geomorphic impact is reviewed now at the catchment scale,
where fluvial landform assemblages and channel networks form a recognizable
fluvial
landsystem
. The transition from erosion-dominated upland to deposition-dominated
lowland is seen by following the trunk river from source to sea. Its typically concave long
