Environmental Engineering Reference
In-Depth Information
11.2.4 Sediment Budget Matrix
The difference between the amount of sediment erosion and sediment yield (sediment transported by
river flow per drainage area) is great. Most of the newly eroded material from overland ares is deposited
in the upstream gullies. A part of the deposited sediment may be eroded again. Therefore, erosion is a
multiple event process and the amount of sediment erosion can be counted multiple times. A sediment
budget matrix can be used to clearly show the amount of sediment in consecutive erosion processes:
EDT
ª
º
1
1
1
«
»
B
«
E
D
E
(11.26)
»
2
2
«
»
¬
¼
3
in which
E
1
is the annual amount of erosion due to landslide and avalanches per unit area,
D
1
is the
sediment deposited in the gullies after landslides and avalanches,
T
1
is the portion of sediment
transported by flowing water after landslides and avalanches,
E
2
is the eroded sediment amount due to
bank failure and gully erosion,
D
2
is the sediment deposited in the gully and on the riverbed after bank
failure and gully erosion,
E
3
is the soil erosion from slopes. The total amount of erosion is given by:
(11.27)
The sediment yield from a drainage area is defined as the annual sediment load over the drainage area.
The sediment yield can be measured at the hydrological stations. The amount of sediment yield is much
smaller than the amount of erosion because only the fine portion of the eroded sediment is able to be
transported for a long distance by flowing water. In general, the sediment yield may be given by
EE E E
1
2
3
(11.28)
where
f
1
, f
2
and
f
3
are sediment delivery ratios with values less than 1. The factors
f
1
, f
2
and
f
3
can be
determined by using the following formulas:
Sf Tf
(
ED f E
)
11
2
2
2
3 3
f
FD D
(
)
(11.29)
1
1
84
f
FD D
(
)
(11.30)
2
2
84
f
FD D
(
)
(11.31)
3
3
84
in which
1
is the sediment fraction in
E
1
with diameter smaller than
D
84
of the suspended
sediment sampled from the stem river at hydrological stations;
F DD
(
)
84
is the sediment fraction in
E
2
with diameter smaller than
D
84
of the suspended sediment sampled from the stem river at hydrological
stations;
F DD
(
)
2
84
is the sediment fraction in
E
3
with diameter smaller than
D
84
of the suspended
sediment sampled from the stem river at hydrological stations; and
D
84
is the diameter of the sediment
particles which is larger than or equal to 84% of the sediment. Moreover, if
f
1
and
f
2
are known,
f
3
may
also be determined with Eq. (11.28).
In general,
E
1
is much greater than
E
2
and
E
3
by an order of magnitude or more difference. Nevertheless,
E
1
is extremely high in some individual years, but is zero in most years, as shown in Fig. 11.50.
E
3
is
continuous and fairly stable.
E
2
fluctuates but is not intermittent, like
E
1
.
The values of the components in the matrix
B
may be determined by field investigation, measurement
and analysis of satellite images and digital topographical maps. Selection of a typical small watershed is
important, it should be one of the high soil erosion watersheds. The scale of the watershed is large
enough to have all types of erosion. Figure 11.51 shows a small watershed in the upper Jialing River
basin—the Liujia Gully watershed. The Liujia Gully is 6.82 km long with a drainage area of 11.76 km
2
.
The Liujia Gully is a second order stream and flows into the Yanzi River, which flows into the Xihanshui
River. The Xihanshui River is in fact the upper Jialing River (the second largest tributary of the Yangtze
River).
F DD
(
)
3
84
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