Fluvial systems: catchments and rivers - Fundamentals of the Physical Environment - page 335

Plate 14.12 The river Severn floods in Shrewsbury, December 2000, picking out the incised meander around the medieval core
of the county town of Shropshire, generated by high antecedent and intense rainfall in its Welsh catchment. The upstream Welsh
(centre left) and downstream English (centre right) bridges can be seen; the railway bridge crosses the river at the meander neck.
Photo: by courtesy of the Shropshire Star

They present smaller, 'streamlined' surfaces to the flow
and may also develop weak cohesive strength from sur-
rounding 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 14.23 ).
More mobile particles are lifted into the flow as pressure
falls in the wake of overlying accelerating and, espe-
cially, turbulent flow. Particles are drawn into the partial
vacuum and remain in suspension by incorporation
into even faster flow paths, or until their weight over-
comes 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

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.