Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 16

T able 1.2 Main features of bed material load and wash load
Features Bed material load Wash load

Origin Soil erosion in watershed Soil erosion in watershed

Direct origin River bed upstream Sediment yield in watershed

Composition of bed
material

Main portion of the bed material usually does
not change except for heavily sediment-laden
rivers

On the bed surface, changing with
incoming amount and flow intensity

Composition of moving
sediment

A small portion of the moving sediment Main portion of the moving sediment

Patterns of movement Bed load and suspended load Suspended load

Transport rate Determined from the flow intensity and less

correlated with incoming sediment, but for
heavily sediment laden rivers depending also
on the incoming sediment

Determined mainly from the incoming
sediment

Relation between flow
and sediment transport

Relation established on basis of mechanics and
may be estimated by sediment transport
capacity formulas

Relations based on watershed
characteristics, determined by
measured or empirical data

Significance Transport rate determines bed stability Transport rate determines rate of

reservoir deposition

Criterion Coarser than D 5 of the bed material;

Rouse number Z larger than 0.06

Finer than D 5 of the bed material;
Rouse number Z smaller than 0.06

sediment and they interact with the flow because they change the roughness of the stream bed. An analog
to stream bed forms is desert sand dunes. In alluvial rivers with the bed consisting of sand and silt, various
bed forms may develop. At low flow velocity over an initially flat stationary bed, no sediment moves; but
once the flow velocity reaches a certain value, some particles are set in motion. Finally, ripples with a
regular shape form. The longitudinal cross sections of ripples usually are not symmetrical. The upstream
face is long and has a gentle slope, and the downstream face is short and steep. Ripples are the smallest
of the bed configurations.
With increasing flow velocity, the dunes develop with triangular profile that advance downstream due
to net deposition of particles on the steep downstream slope. Dunes move downstream at velocities that
are small relative to the stream flow velocity. The size of a dune is closely related to the water depth.
Dunes in large rivers can be kilometers long and several meters high. They may be less than half a meter
in other rivers. If the velocity continues to increase, the bed form develop into antidunes . Antidune is a
type of bed configuration that is in phase with the wave on the water surface, and these two waves
interact strongly. Antidunes move upstreamward is symmetrical in shape, like a surface wave.
In mountainous area the stream bed consists of boulders and cobbles. There is no ripples and dunes but
a step-pool system may developed, which occurs in high-gradient (>3% -5%) mountain streams with
alternating steps and pools having a stair-like appearance (Chin, 1999). The step-pool system occurs
usually on a stream with bed materials consisting of particles with diameters differing by several orders of
magnitude with the largest diameter on the same order as the water depth. Cobbles and boulders generally
compose the steps, which alternate with finer sediments in pools to produce a repetitive, staircase like
longitudinal profile in the stream channel.

1.1.5 River Patterns
Meandering rivers —Natural river is rarely straight. Sinuosity is defined as the ratio of the channel
centerline length to the length of the valley centerline. If the sinuosity is over 1.3, the stream can be
considered meandering. Figure 1.13 shows a typical meandering river, which is a tributary of the Yellow