Environmental Engineering Reference
In-Depth Information
exponential decrease of mass concentration and inventory with depth down an undisturbed soil profile.
The soil erosion in the watershed is moderate or intensive in cultivated land with the annual erosion rate
in the range of 2000-6000 t/km
2
yr. In general ultra-intensive or intensive soil erosion occurs at the upper
section of slopes, intensive or moderate soil erosion occurs at the middle section of slopes, and moderate
or slight soil erosion occurs at the lower section of slopes. On the slopes with vegetation consisting of
herbaceous and wood species the erosion rate is much lower or zero. On the lower section of slopes with
well-developed vegetation, however, no erosion but deposition occurs with a deposition rate higher than
1000 t/km
2
yr. The slope gradient and vegetation cover affect soil erosion and deposition rates, and in
general, the rate of soil erosion is proportional to the slope gradient and inversely proportional to the
vegetation cover. On average the erosion rate from slope or the value of
E
3
is about 2000 t/km
2
yr.
The values of
was about 0.8. The specific weight of the deposits was about 1.4 t/m
3
, therefore, the total sediment
yield is:
F DD
(
)
and
F
(
DD
)
were found to be around 0.1, and the value of
F
(
DD
)
1
84
2
84
3
84
2
The measured sediment yield per area from hydrological stations is about 2500 t/km
2
.yr. On the other
hand the total amount of erosion is
S
0.1
T
0.1(
ED
)
0.8
E
2482 t / km yr
1
2
2
3
2
The sediment yield is only a very small portion of the eroded sediment. The major portion of the
eroded sediment is stored on the hill slopes and gully bed. Most of the total erosion is to the result of
gravitational erosion and gully erosion, which play the most important role in the morphological process
in upper river basins. The sediment load in rivers is only a small portion of the fine sediment. The annual
load per area is the sediment yield, of which two thirds are from the slope soil erosion. The sediment load
is the main driver of the fluvial process of the alluvial river section in the lower reaches.
The method of sediment budget matrix can be used for small watersheds and the results may be used
to estimate the total erosion in the whole river basin. It may be derived that the total erosion in the
Yangtze River basin is much more than 2.2 billion tons. The total erosion in the Liujia Gully is about 30
times higher than the sediment yield. If this conclusion is used for estimating erosion from the whole
Yangtze River basin the total erosion would be more than 15 billion tons. The present estimation of the
total soil erosion from the upper Yangtze River basin is only 2.2 billion tons because the erosion due to
landslides and avalanches (
E
1
) is not included and the soil erosion is estimated by analyzing satellite
images which is calibrated with direct measurement for small tributaries. For the calibration, measurements
are performed at the downstream end of small watersheds and the amount of soil erosion from all small
watersheds is summed to obtain the total amount of soil erosion from the tributary basins.
EE E E
91,765 t / km yr
1
2
3
11.3
Methods of Integrated River Restoration
Integrated river management strategies are needed to achieve sustainable ecosystems that integrate
hazard mitigation, power generation and social development. There are various river management issues:
riverbed incision, and induced landslides and avalanches, soil erosion and debris flows, water diversion
and pollution, dam construction and habitat loss, urbanization and flood hazard, sediment transportation
and land creation, reservoir operation and ecological stresses, and fluvial processes and morphological
development. On incised mountain rivers a well planned cascade of dams may effectively control
riverbed incision while achieving power generation and habitat stabilization. A good habitat for benthic
invertebrates should have stable streambed consisting of boulders, cobbles, or gravel. Mud and aquatic
plants are also good substrate for invertebrates. Typical habitats for benthic invertebrates include mountain
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