Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 268

into the main flow region where the velocity profile was logarithmic. They observed two characteristic
features. ķ Before almost every uplift of a low-speed streak and the appearance of an oscillation at the
boundary, a disturbance originating in the main flow region occurred just upstream. Originating in the
logarithmic velocity distribution region, i.e., usually in the range of 20 < y+ < 200, the disturbance
possessed an eddy-like flow pattern with a mean motion toward the wall. The oscillation in the boundary
region was always located downstream of that disturbance. ĸ The low-speed streak grew and was
gradually lifted up. At the end of the growth stage of the oscillating low-speed streak, the mutual action
of the fluid in the streak and the fluid in the logarithmic region induced another large eddy-like structure.
This eddy system grew downward toward the boundary, and a disturbance in the main flow region formed
that moved toward the boundary. Thus. inducing another uplift and oscillation of a low-speed streak at a
location further downstream.
The momentum of the faster water is transmitted to the slower boundary water. In doing so, the faster
water tends to roll up the slower water in a spiral motion. It is this shearing motion, or shear stress, that
also moves bed particles in a rolling motion downstream. Particle movement on the channel bottom
begins as a sliding or rolling motion, which transports particles along the streambed in the direction of
flow.
5.1.4 BedForms
5.1.4.1 Development of Bed Forms
Stream channels and their floodplains are constantly adjusting to the stream power and sediment supplied
by the watershed. Channel response to changes in water and sediment load may occur at differing times
and locations, requiring various levels of energy expenditure. Daily changes in stream power and
sediment load result in frequent adjustment of bed forms and roughness in many streams with movable
beds. Streams also adjust periodically to extreme high and low-flow events. Similarly, long-term changes
in runoff or sediment yield from natural causes, such as climate change, wildfire, etc., or human causes,
such as cultivation, overgrazing, or rural-to-urban conversions, may lead to long-term adjustments in
channel cross section and planform that are frequently described as channel evolution. Stream channel
response to changes in stream power and sediment load has been described qualitatively in a number of
studies (e.g., Lane, 1955; Schumm, 1977).
In alluvial rivers with the bed consisting of sand and silt, various bed forms may develop. While
sediment is being transported, the bed load particles move collectively in all sorts of ways along the
riverbed. Their motion in turn can cause changes in the configuration of the riverbed in accordance with
the variation of the sediment transport rate. The collective movement of large quantities of sediment
particles on the bed is called bed form movement.
With low rates of flow over an initially flat stationary bed, as shown in Fig. 5.8 (a), no sediment moves;
but once the flow velocity reaches a certain value, some particles are set in motion. Soon after that, a few
particles may gather on the bed and form a small ridge; this ridge gradually moves downstream and tends
to increase in length. Finally, the ridges connect and ripples with a regular shape form, as shown in
Fig. 5.8(b) (Chien et al., 1998).
The longitudinal profile of ripples usually is not symmetrical. The upstream face is long and has a
gentle slope, and the downstream face is short and steep. The former is generally 2-4 times as long as the
latter. The height of ripples is usually between 0.5 and 2 cm; the highest ripple is not more than 5 cm. The
wave lengths normally do not exceed 30 cm, and they are usually within the range of 1 to 15 cm.
Ripples are the smallest of the bed configurations. They are related to the physical parameters near the
river bed and have little correlation with the water depth. Their occurrence is the result of the unstable
viscous layer near the boundary. They can form in both shallow and deep water. In plan, they either are