Geoscience Reference
In-Depth Information
The wash load is comprised of moving sediment particles that are derived from
upstream sources other than the channel bed. It is not found in appreciable quantities
in the bed. It is finer than the bed-material load and rarely exchanges with the bed
material. Einstein (1950) defined wash load as the grain size of which 10% of the
bed material mixture is finer.
It should be noted that the definition of wash load and bed-material load depends
on flow and sediment conditions. Some wash load in upstream channels may become
bed-material load in downstream channels due to the weakening of flow strength.
Some sediment particles are wash load in the main channel but may be bed-material
load in flood plains.
By definition, the bed-material load is the sum of bed load and suspended load. So
is the wash load. However, the wash load consists of fine particles that move mainly
in suspension, and thus dividing it into bed load and suspended load does not make
much sense in practice.
2.1.3 Properties of the water and sediment mixture
Fig. 2.4 shows a sketch of a mixture consisting of a volume of water,
V
w
, and a
volume of sediment,
V
s
. It is termed as “mixture” for short. Sediment concentration is
defined as
V
s
γ
s
V
s
c
=
or
c
=
(2.22)
V
w
+
V
s
γ
f
V
w
+
γ
s
V
s
where
c
is the concentration by volume, and
c
is the concentration by weight (mass).
They are related by
. Both them are unitless. In addition,
sediment concentration is sometimes given by weight or mass per unit volume of the
mixture (N
c
=
Gc
/
[
1
+
(
G
−
1
)
c
]
m
−
3
or kg
m
−
3
), which is obtained by
·
·
γ
s
c
or
ρ
s
c
. It is also given in parts
per million by weight (ppm), which is equivalent to 10
6
c
.
Note that the volumetric sediment concentration
c
is used in this topic, except where
stated otherwise.
Figure 2.4
Sketch of the water and sediment mixture.
