Sedimentation studies in China
-Present and Future
Zhao-Yin Wang1
and Bingnan Lin2
 1 Dept. of
Hydraulic Engineering, Tsinghua University and International
Research and Training Center on Erosion and Sedimentation, Beijing
100084, China.
2
International Research and Training Center on Erosion and
Sedimentation, P.O.Box 366, Beijing 100044, China
abstract
Sediment
management is always most challenging in the hydraulic engineering
on sediment-laden rivers. Chinese people have accumulated abundant
experiences in solving the problems like watershed erosion, heavily
sediment-laden rivers, reservoir sedimentation, estuarine and
coastal sedimentation, and debris flows and control strategies. This
paper summarizes the sediment issues, research approaches and
management strategies. The paper outlines a vision for the future
sediment research in the new century, especially reporting on an
ambitious plan of validation of sediment studies performed for the
Three Gorges Project. The need for multidisciplinary approaches to
conducting research is also discussed.
1
INTRODUTION
For milleniums, China has
been confronted with serious sedimentation problems. An outstanding
example is the strong aggradation that takes place as the Yellow
River carrying annual sediment load of about 1.6 billion tons enters
into its lower course on the flat Huan-Huai-Hai Plain. In time, a
perched river was formed that frequently breached its levees. In the
2000 years preceding the 1950’s, there were on the average two
breaches every three years, causing at each time very heavy losses
in lives and properties. Another major sediment-laden river is
the Yangtze River in Central China. The average annual sediment load
is 462 million ton at Cuntan near the inlet of the Three Gorges
Project Reservoir. Historically, the Yangtze River floods also
subject the riparian people to heavy losses in lives and properties.
In recent decades, in an
attempt to harness the major rivers, China has been carrying out a
large program of hydraulic constructions on such sediment-laden
rivers as the Yangtze, the Yellow, and the Pearl. As these rivers
either have alluvial beds in their upper reaches or are fully
alluvial in their lower reaches, many sedimentation problems are
encountered in the various stages of planning, design and operation
of the projects. Research leading to the solutions of these problems
constitutes the bulk of the present-day sedimentation studies in
China. Outstanding examples of the said hydraulic constructions
include Gezhouba Project and Three Gorges Project (TGP) on the
Yangtze River and Xiaolangdi Project on the Yellow River. In
addition to the applied research, some fundamental studies have also
been carried out.
More often than not, natural
rivers are irregular in cross section, profile and alignment.
Furthermore, the sediment they carry may vary in physical properties
from place to place and its quantity of occurrence may also vary
both spatially and temporally. These along with highly variable
hydrological conditions make a large alluvial river a very complex
system affected by many factors. Solutions of sedimentation problems
arising from planning and design of large projects to be built on
these rivers are bound to depend on a multitude of variables.
Simulation or modeling methods are thus often employed to provide
solutions to planning and design engineers, because more variables
can thus be taken into account with less assumptions made. The
sate-of –the-art of the science of sedimentation consists mostly of
the results of laboratory and theoretical studies carried out under
more or less simplified conditions. Verification with prototype data
is limited. A pressing question to be answered is then “how
realistic is our present-day knowledge in sedimentation?” Plans
have been drawn up in china to collect field data for the study of
impacts of the Three Gorges Projects on the Yangtze River with
respect to sedimentation after closure of the dam in 2003.
Besides the traditional
research interests of sediment transportation in rivers, the science
of sedimentation is growing up into a more interdisciplinary science
related to environment, economy and ecology. This paper reports the
main achievement of the sedimentation management in the past and
indicates the development of the science in the future.
2
Soil Erosion and Watershed management
Environmental health
encompasses the maintenance and quality of the natural sources:
soil, water, air and biota. According to the preliminary statistics
conducted for the world, the annual erosion of surface soil from
river basins amounts to 60 billion tons, of which 17 billion tons
are discharged into the oceans. In the process, as much as 5 to 7
million ha of farms are annually ruined. Moreover, aeolian erosion
of bare lands has intensified desertification after depriving the
ground of good top soils. Eroded soil contains nitrogen, phosphorous
and other nutrients and deposit in lakes and reservoirs, that
contaminate the waters through eutrophication and other biological
as well as chemical processes.
China is a country suffering
severe erosion, with 1.82 million km2 suject to water
erosion, 1.88 million km2 to aeolian erosion and 1.25
million km2 to glacial erosion. In other words, more than
46% of the territory of the country are being eroded. The soil
eroded from the Yangtze River basin is about 2.2 billion tons per
year and that from the Yellow River basin 2.3 billion tons per
year. The loess plateau in the central China is known for high
rates of erosion and sediment yield, which result in numerous
heavily sediment-laden rivers, such as the Yellow, Weihe and
Huangfuchuan Rivers. From 1950 to 1996, Chinese people have improved
0.7 million km2 of the water-erosion land, but in the
mean time, the total water-erosion land has increased from 1.16
million km2 to 1.82 million km2. As a result
the untreated erosion land remains at 1.13 million km2
(Wang, 2000). The main causes for the increase of erosion land are
deforestation, overgrazing, harvest of medical herbs, slope tillage,
development of mining and urbanization. For instances, people in the
Xiaojiang Watershed on the Yun-Gui Plateau of China cut trees and
burn the wood for iron and copper production in 1958. The forest
cover was reduced from 23% to 18% due to logging and the erosion
rate was almost doubled. In the Yangtze River basin alone, the area
of slope land people are still plowing is more than 11 million ha.
The Shenfu-Dongsheng coal mining removed 162 million tons of soil in
the period 1998-2000 and thus intensified the erosion rate greatly.
Erosion causes many
problems. More than 5 billion tons of fertile soil are lost every
year due to erosion, in which the amount of nitrogen, phosphorus and
potassium is about 50 million tons, much higher than the annual
national production of fertilizer (30 million tons). The farmland
deteriorates and grain production is reduced by erosion, too.
Moreover, the nutrients are transported with the sediment into the
environmental waters and causes eutrophication and red tide. More
than 240 harmful algal blooms occurred in the 1990s, causing huge
economic losses. Erosion exerts the highest ecological stress on the
vegetation cover. In the northern part of the loess plateau, the
vegetation can hardly develop because the extremely intensive
erosion carries off the top soil, on which the vegetation relies. In
the areas with vegetation, such as the upper reaches of the Yangtze
River, erosion damages and destroys the vegetation and scars the
land surface.
To study the laws of soil
erosion Chinese scientists have conducted laboratory and field
experiments with artificial rainfall systems. They revealed the
relationships of the erosion rate with the splash power of
rain-drops, soil structure and composition, surface sealing and
crusting, vegetation, slope and topography, as well as human
activities (Cai et al., 1998). Based on the widely-applied numerical
models like USLE, RUSLE, CREAMS, ANSWERS etc, Chinese scientists
developed many soil erosion models for the typical erosion land,
such as the loess plateau and the upper Yangtze River basin, and
combined the models with GIS (Jin, 1995, Hu et al., 2000).
The main erosion control
strategies applied in China are building sediment barriers in
gullies and terracing the slope farmland in the arid and semi-arid
areas, building sediment-check dams and reforesting the hills in the
wet areas, and comprehensive reclamation of small river basins in
both arid and wet areas. Comprehensive reclamation of small basins
is the main strategy employed in the loess plateau. The area is arid
and semi-arid and, therefore, reforestation alone is difficult to
achieve the objective of erosion control. People built many sediment
barriers and created productive warped farmland. Terraced fields
enclosed with borders 20-cm high may trap almost all rainfall water
and greatly reduce erosion. Impounding water with dam on rivers
provides water for drinking, irrigation and reforestation. People
plant grass on the slope and trees around the fields and roads. As a
result, the sediment fed into the rivers by erosion from the loess
plateau has been greatly reduced since 1984 (Gu,1994). Fig.1 shows
the annual water and load of the Yellow River in the period from the
1950s to 1990s measured at the Lijin hydrological station (Wang and
Liang, 2000). The reduction in sediment load in the 1980s and 1990s
is due mainly to sediment trapping by reservoirs and sediment
barriers, terraced fields and reforestation.
Reforestation is effective
in wet areas (South China, for example) if mass movement of soil is
controlled by multiple check dams. The success of reforestation
relies more on the agricultural policy than technology. The change
of ownership from community to private households has incited
incentives in farmers on reforestation since the 1980s. After the
1998-flood event of the Yangtze River the state-funded reforestation
projects of the upper Yangtze River watershed was sped up. In many
mountainous areas people are still burning wood for cooking and
heating. Planting shrubs and fast-growing trees in selected zones
providing the local people fuel wood is an effective measure to
protect the forest.
 Fig.1
Variation of annual water and sediment load of the Yellow River
(Sediment load is almost halved since 1985)
3
Training of heavily sediment-laden rivers
A river is considered as
heavily sediment-laden if the ratio of annual sediment load to water
is over 1 kg/m3. The Yellow, Weihe, North Luohe, Yongding,
Liaohe and Daling Rivers in China, the Colorado River in USA, the
Ganges and Burahmaputra Rivers in India and Bangladesh, and the Nile
River in Sudan and Egypt are several examples of heavily
sediment-laden rivers. These rivers are dynamic and difficult to
harness. With the load/water ratio about 37 kg/m3 the
Yellow River is one of the most heavily sediment-laden rivers. The
history of river training of China is essentially a history of the
people's struggle against the Yellow River floods because this river
is the most disastrous and challenging. The experience of the Yellow
River training is perhaps one of the most painstaking and brilliant
chapter of Chinese history.
3.1
The Yellow River and Flooding Disasters
The Yellow River is the
second longest river in China (5,464-km) with total drainage area of
752,000-km2. The river watershed is mostly arid and
semi-arid with long-term average annual runoff depth only 77 mm and
total annual runoff 58 billion m3. The average annual
sediment load before the 1980s was 1.6 billion tons, ranking first
in the world with the recorded highest of 3.9 billion tons. The
sediment in the lower Yellow River is mainly composed of silt and is
liable to be suspended. The rainfall occurs mainly in July, August
and September with high rainfall intensity.
The river carries a huge
amount of sediment produced by soil erosion from the loess plateau.
The sediment deposits on the channel bed and the estuary. In time, a
perched river was formed that frequently breached its levees. From
BC 602 to 1949 the river witnessed 1,593 dyke bursts, flooding vast
areas in 543 years and claiming millions of human lives. The river
shifted its major course (600-700 km long) 26 times with the apex
around Zhengzhou with devastating calamities, left numerous old
channels, among them 8 major shifts (5 natural and 3 human-caused)
with river mouths alternating between the Bohai Sea and the Yellow
Sea. For its wild behavior, the Lower Yellow River was dubbed “the
sorrow of China”. Fig.2 shows the migration of the river in the
period from BC602 to 1855 and the abandoned channels (Li 1992).
The Yellow River floods are
very disastrous. For instances, a rainstorm in the middle reaches
during 6-8 Aug. 1843 generated the biggest flood in the history. The
crest discharge at Sanmenxia was recorded at 36,000 m3/s.
Twenty-seven counties and 15,000 villages were flooded and thousands
people were killed. From 1796 to 1855 the grand levees were breached
22 times and the major task for the river training was to close the
breaches in this period. A flood from the upstream magnified by a
heavy rainfall in the lower reaches breached the grand levee at
Tongwaxiang on 18 June 1855. The river poured out and inundated 8
counties, and finally pirated the Daqing River Channel.
Consequently, the Yellow River shifted its major course from south
to north and flowed into the Bohai Sea. Thousands people were killed
by the flood and several million people lost their shelters and
farmland. In 1933 the river swelled again by heavy rainfall during
5-10 Aug. with a crest discharge of 22,000 m3/s. The
flood caused 54 levee breaches, inundating 67 counties of flooded
area of about 8,600 km2 and killing 18,293 people.
In 1958, 198 mm rainfall within 5 days enhanced the crest discharge
at Huayuankou to 22,300 m3/s. The grand levee was not
broken but 1,700 villages on the floodplain were flooded.
Fig.2: Migration of
the Yellow River and the abandoned channels
3.2
Training and Regulation Strategies
The Yellow River training
has a history of more than 3,000 years. Levee construction was the
major strategy of flood control. The Qin Emperor united the country
and linked the flood defense dykes into an entire levee system about
2,200 years ago. The lower Yellow River was confined within the
levees but sediment deposition raised the riverbed and made the
river frequently shifting its courses. People developed many
strategies to harness the river, among them the wide channel and
narrow channel theories are the most influencing. The wide channel
theory is to confine the river within a wide river valley with
levees and divert the flood with diversion channels. Wang Jing- a
minister of the Han Dynasty- was the major practitioner of the
strategy. From BC168 to AD69, the river was active; it flooded and
changed its courses several times. Wang Jing implemented a
large-scale training project from AD69. He completed and enhanced
the dykes, built many diversion channels and weirs. The river was
confined by the enhanced levees of tens of kilometers apart. The
riverbed silted up at a low speed of about 1-cm per year. In the
following 800 years the river was calmed and no big flood disasters
occurred (Li 1992).
The second strategy is to
narrow the river and confine the flood within the stem channel in
order to raise the velocity and keep high carrying capacity of the
flow, preventing sediment from depositing and even scouring the bed.
Pan Jixun- a minister of the Ming Dynasty- was the most outstanding
advocator and performer of the strategy. From 850 to 1600 the river
behaved active again. Pan regulated the levee system, blocked many
branches of the river and made the river flowing in a single channel
in the lower reaches in the period 1565-1592. The river became
relatively stable in the following decades.
Since the 1950s the Yellow
River Water Conservancy Commission (YRCC) has been the leading
institute for the river training (Wang, 1989). The nation has spent
$1 billion for flood control and saved $500 billion in flood loss
(Chen 1999). The riverbed profile has been maintained stable for
half century, with only parallel rising following the extension of
the river mouth into the sea (Zhang and Xie, 1985). The main
strategies are to reduce flood discharge with reservoirs, enhance
the capacity of the river channel by enhancing and reinforcing the
levees, and retending floodwater with detention basins. They are
referred in short as: upper reaches storing, lower reaches
discharging and two sides retending.
The strategies employed
are listed as follows: (1) Trapping sediment and controling flood
with reservoirs- Dam construction and flood regulation with
reservoirs is the most effective strategy. Eleven reservoirs have
been constructed on the river with total capacity of about 57
billion m3. More than 10 billion tons of sediment were
trapped reducing the sedimentation rate in the lower reaches. The
newly constructed Xiaolangdi Reservoir with sediment-trapping
capacity of 7 billion m3 may control sedimentation of the
lower Yellow River for 20 years; (2) Wide river valley with
reinforced levee system-The river in the Henan reaches is 5-20
km wide to accommodate sediment and retend water if rare floods
occur; (3) Narrow channel to prevent sediment from deposition-
The river width in Shandong reaches (0-600 km from the river
mouth) is 0.4-5 km to maintain high sediment carrying capacity of
the flow; (4) Enhancing and reinforcing levees-A total length
of 1320-km of levees has been raised by about 9-m in the past 50
years, with a total amount of 400 million m3 of earth
work and 4 million m3 of rock masonry work; (5)
Diverting flood with flood-detention basins- Five flood
detention basins were constructed. The Dongpinghu basin detended
floods in 1954,1957,1958 and 1982 and effectively reduced the
discharges to the lower reaches.
3.3
New Strategies to Train the Yellow River
On the 5 August 1996, a
flood discharge of 7,860 m3/s washed through the lower
Yellow River, breaking dikes and destroying 2,898 villages and 212
towns on the floodplain. The recurrence period of the flood with
peak discharge of 8,000 m3/s is only 2 years according to
1950-1996 data. It created the highest stage in historical records
and caused the highest economic loss. The main reason for the
highest flood stage is the quick siltation of the main channel in
the recent decades. The water flow was much lower than before
because the peak discharge of floods was clipped by reservoir
regulation. The river channel was in much less chances scoured by
turbulent flood, thus the main channel is silted up at high rate,
namely 0.1-0.2 m/year. In many sections the main channel is filled
up. The flood did not flow down the river in a well-defined channel
but flowed randomly within a valley confined of up to 10 km in width
by the main levees. The flood took 17 days to travel the 800-km from
Huayuankou to Lijin although the average time for floods of the same
discharge to travel over the same distance in 1950-1990 was only 7-8
days. Moreover, the population on the floodplains within the levees
has quickly increased to 1.7 million with rapid economic growth on
the land, which explains the high economic loss.
The amount of water diverted
annually from the river has increased to more than 30 billion m3
(70% of the total runoff). Less and less water flows and carries
sediment down the river to the sea. Sedimentation control and
maintaining the water conveying capacity of the channel are the main
aims of the river training. Besides the traditional and
currently-employed strategies, scientists and engineers have
suggested and tested many new methods. Among them dredging and
scouring sediment with seawater are the most promising auxiliary
measures.
Lin et al (1999) proposed
to take advantage of the topography and divert seawater laterally
from the Bohai Bay to pumping plants that would lift the seawater
for injection into the Yellow River. In a scheme proposed, the
diverting canal is about 50 km long and the point of injection is
chosen at Lijin about 110 km upstream of the existing river mouth.
As the seawater diverted is practically free of sediment, it would
degradate the river channel downstream of the point of injection. A
local drop in water surface at the point of injection would be
produced and would give rise to retrogressive erosion extending far
upstream. This would stop further rising of the riverbed. In a
scheme in which a seawater discharge of 1,500 m /s is injected at Lijin, it is estimated that
retrogressive erosion would extend upstream by a distance of 330 km.
Further upstream, the riverbed would be under the control of the
newly completed Xiaolangdi project. Also noticeable is the fact that
discharge of turbid seawater from the river mouth would assume the
form of density current that could climb over the mouth bar and
travel to more distant points of the Bohai Sea before deposition
takes place. This would effectively stop or slow down the seaward
extension of the estuary (Lin et al., 2000). The foregoing idea is
presently under further investigation.
Dredging can be used to: (a)
remove the sand bar from the river mouth; (b) widen and deepen a
short length of shrinking channel at special locations; (c) raise
the elevation of surrounding ground and reinforce the dykes with the
dredged sediment. The Yellow River has shifted its delta channel
(70-100 km from the river mouth) 11 times due to sedimentation and
extension of the channel since 1855. The recent shift of the delta
channel from the Diaokouhe channel to the Qingshuigou channel
occurred in 1976. The ending reach of the mouth (16-18 km long)
shifted to the Chahe Channel in 1996 mainly for the purpose of
creating land to facilitate oil well drilling. The 100-km long
Qingshuigou Channel has been used for 24 years, much longer than the
average life of the previous delta channels, partly due to the
dredging of the mouth sand bar in the 1980s and 1990s.
4
RESERVOIR SEDIMENTATION MANAGEMENT
4.1
Sedimentation of Reservoirs and Major Strategies
The problem of reservoir
sedimentation peeps in Table 1, in which only some major reservoirs
of capacity larger than 100 million m3 are presented (Qian,
1991). For small reservoirs the percentage of capacity loss due to
sedimentation is even higher. The major strategies to control
sedimentation and restore the capacity of the reservoirs are:
storing the clear and discharging the turbid; flushing by draw-down
and flushing by emptying the reservoir; and making use of density
currents as well.
Table 1
Capacity loss of large reservoirs due to sedimentation in China
Reservoir/River Total capacity
Year surveyed Reservoir sedimentation
(mi.
m3) volume
(mi.m3) (%)
Sanmenxia/Yellow 9640
1960-1981 5518 57.20
Yanguoxia/Yellow 220
1961-1978 160 72.70
Qingtongxia/Yellow 620
1966-1977 485 78.20
Liujiaxia/Yellow 5720
1968-1986 1078 18.85
Gongzui/Daduhe 357
1967-1987 286 80.11
Fenhe/Fenhe 721
1959-1988 327 45.30
Hongshan/Laohahe 2560
1960-1977 475 18.60
Guanting/Yongding 2270
1953-1985 612 26.96
Danjiangkou/Hanjiang 16050
1968-1986 1129 7.06
Sediment transportation in
the Yangtze and Yellow Rivers occurs, of 60-90% of annual sediment
load with 40-60% of annual runoff water, in 2-4 months of the flood
season. The Three Gorges Project (TGP) on the Yangtze River is
planned for flood control, power generation and inland navigation.
For all these purposes, it is important to maintain an adequate
storage in the reservoir. The main strategy to control sedimentation
is to draw down the pool level from 175 m to 145 m in the flood
season from June to September when the sediment concentration is
high and allow the turbid water wash through the reservoir to the
downstream. The reservoir stores water from October when the income
water becomes clear. Fig.3 shows the typical variation process of
sediment concentration at Yichang -the dame site of TGP- and the
operation scheme of pool level for sedimentation control. By storing
the clear and releasing the turbid, less sediment deposits in the
reservoir while the reservoir is still able to store enough water
for power generation in the low flow seasons. Numerical models
proved that after 150 years operation sedimentation in the reservoir
will reach an equilibrium, 17 billion m3 of the capacity
will be lost due to sedimentation but 22 billion m3 can
be permanently reserved.
Fig.3 Typical variation process of sediment
concentration at the dame site of TGP
and the operation scheme of pool level for
sedimentation control
For reservoirs on rivers
with high sediment concentration and low water runoff draw-down
flushing and empty flushing are employed. Low level outlets are open
in the flood season, so to draw-down or empty the reservoirs and
create riverine flows along the impounded reaches, which scour and
release the sediment deposited in the reservoirs. Retrogressive
erosion is induced by draw-down and empty flushing, which may extend
the flushing far upstream of the dam. A successful example is the
Sanmenxia Reservoir on the Yellow River, in which draw-down flushing
caused retrogressive erosion to a 100-km long reach upper from the
dam and effectively preserved the storage capacity. The Hengshan
Reservoir on the Changyuan River in the Shanxi Province is a gorge
type small reservoir with a capacity of about 13 million m3.
The area is arid and there is almost no flow in the non-flood
seasons. The reservoir was used to store water during the flood
season and provide water for irrigation in the non-flood seasons. It
was silted up quickly in the first 8 years and 30% of the capacity
was lost due to sedimentation. Then, the reservoir was emptied for
flushing sediment in the flood season of 1974 and 1979.
Consequently, about 2 million m3 of its capacity was
regained.
The Bajiazui Reservoir is on
the Puhe River in Gansu Province. Density currents consisting of
fine sediment occur in the reservoir if the inflow concentration is
over 200 kg/m3. The high density mixture flows under the
overlying clear water to the dam through the narrow and deep
reservoir channel, then it is vented out of the reservoir through
the bottom outlets. The ratio of the outflow concentration to the
inflow concentration is about 100%. In other words, the high
concentration of sediment can be discharged out of the reservoir
without loss of the stored water. The Heisonglin Reservoir on the
Yeyu River is another example, which discharges density currents
with the ratio of the outflow concentration to the inflow
concentration up to 91%.
4.2
Physical Modeling
The strategies controlling
reservoir sedimentation are usually studied with physical and
numerical models before it is employed. The Three Gorges Project (TGP)
on the Yangtze River is the largest reservoir in China with the
highest power generation capacity in the world. The TGP project
benefits shipping allowing 10,000-t tows as well as passenger boats
of 3,000-t class to sail to the inland metropolis Chongqing 620 km
upstream of the dam. Thus twin flight of five locks each and a ship
lift are provided in the left abutment of the dam to serve as
permanent structures of navigation. Altogether 21 physical models
have been built for the study of sedimentation problems related to
the project, among them six are for the reaches downstream of the
dam; one is for the Hanjiang River for a preliminary verification of
the technique of model testing presently employed. The rests are for
the sedimentation control of the reservoir and navigation channels.
As the river is meander, the model would often take up the whole
width of a laboratory, so that a large laboratory is generally
needed to house a single model. Lighter materials, such as lucite,
bakelite, coal powder and nut shell (treated), are used as model
sediment in order to achieve similarity of sediment transport. In
the case of TGP, movable-bed models are required to predict
sedimentation in the lock approaches, training of difficult reaches
in the backwater region, deposition in the great river port of
Chongqing and others. Prediction of sedimentation in 80 years or
more is usually required. This means that the models would have to
be run continuously for several months in a row.
River models are as a rule
distorted, except for the case in which the presence of structures
has a strong influence on sedimentation. An example in point is the
sedimentation in the lock approaches of TGP. In this case,
undistorted or slightly distorted models are required. Physical
model for the study of sedimentation is designed to satisfy the
criteria of similarity with respect to gravitation, resistance,
turbulent diffusion, sediment entrainment and deposition, capacity
of sediment transport and bed deformation. The procedure is by and
large conventional, save for the way to approximate similarity in
the entrainment of sediment. Observed bed load in the sand range was
plotted against mean velocity in the cross section. Two such plots
were made, covering a range of transport rate of about 1.3 to 2,000
kg/s. The lower ends of these plots are extended downward somewhat
to yield a velocity of 0.6 m/s, corresponding to a sediment
transport of about 1 kg/s. This velocity is taken as the threshold
at which sediment begins to move. It is scaled down to the model
value for the model sediment to match. This is a rather common
practice adopted in China.
In 1979, the late Dou
proposed another procedure for physical modeling of sediment-laden
flow (Dou, 1979). This procedure features unification of time scales
for suspended and bed loads. Still many more schemes exist for
physical modeling, including the tidal model of sedimentation for
the Bay of Hangzhou and the large models for the study of the Yellow
River.
4.3
Numerical Modelling
The Three Gorges Project on
the Yangtze River is planned for flood control, power generation and
inland navigation. For all these purposes, it is important to
maintain an adequate storage in the reservoir. Therefore, estimating
reservoir deposition or loss of reservoir capacity is of great
importance. In 1972, Han presented a one-dimensional mathematical
model of sedimentation based on the equation of non-equilibrium
transport of sediment, which was first presented by Dou (1963) from
a heuristic approach without formal derivation. Later, Han (1979)
and Lin et al (1983) presented different derivations and arrived at
the same expression:
(1)
where h is the depth
of flow, S the mean concentration of sediment in a cross
section, q the water discharge per unit width,
 the fall velocity of sediment,
 the mean concentration of sediment corresponding
to the capacity of sediment transport and
a
variable coefficient. Also subscripts x and t denote
the independent variables in partial differentiation. On the basis
of this equation of non-equilibrium transport of sediment as well as
the equations of momentum and continuity of the mean flows of water
and mixture, Han developed a 1-D model for the computation of steady
flows over a movable bed. In this model, Han specifies the values of
 to be 0.25 for the case of deposition and 1.0
for the case of erosion on the basis of considerable amount of field
data. This model has been widely applied to the long-term forecast
of deposition in the reservoir of TGP and degradation of the
alluvial channel downstream of the dam. Depositions in the reservoir
for a period up to 120 years have been computed (Han et al, 1993).
The coefficient
 was analyzed for two-dimensional cases (Zhou,
1990). Curves were given for the evaluation of the coefficient. In
1998, Zhou and Lin (1998) presented an extended 1-D model that can
also yield approximately lateral changes of cross-sectional area.
This model has also been applied to the computation of deposition in
the lower part of the TGP reservoir. Many versions of 2-D models
with sediment have been developed and applied in China, using
Cartesian and boundary fitted coordinates. The former model has been
employed for the case of movable lateral boundary. A 3-D model has
also been developed. Its use is still limited.
The reservoir of TGP has
three characteristic levels, namely, the normal pool level (NPL),
the flood control level (CFL) and the dry season control level (DCL).
At the beginning of the flood season in June the reservoir is to be
drawn down to FCL for the releasing of floods. At the beginning of
the dry season in October, the reservoir is to be impounded to NPL
for full generation of power and for 10,000 ton tows to sail about
600 km to Chongqing. As power is generated, the reservoir will be
gradually drawn down to DCL. It may be further drawn down to FCL
when flood provides enough depth of flow for the safe passage of the
tows.
Lin (1992) suggested a new
scheme of reservoir management to lower the reservoir pool from 145
m to 135 m in the flood season when an incoming flow is between
45,000-m /s and 56,700-m /s. On the basis of the long-term average, there
would be 7 days a year during which the pool in front of the dam
would be lower than 145 m. During the short period the operation of
the locks would have to be suspended. This scheme, if adopted,
should start applying not later than the 11th year after
the commission of the project. In this way, deposition may be
shifted forward and more sediment would then be carried by the flow
and be discharged out of the reservoir. Thus there would be less
deposition in the reservoir. Mathematical modeling indicates that by
adopting the proposed scheme, an increase of storage in the amount
of 3.5 billion m between elevations of 145 and 175 m may be
obtained for flood control. This is 15.9 % of the initial storage of
22 billion m available between the same elevations. Except for
the short suspension of lock use, the scheme is particularly good
for navigation. It would increase the depth of flow in the reach of
fluctuating backwater, and even more important it would also greatly
reduce the deposition in the Chongqing harbor. As this scheme adopts
another FCL at 135 m, it is dubbed double FCL scheme. Additional
variants of the scheme is studied and proposed by Zhou et al.
(2000).
5
ESTUARY SEDIMENTATION
5.1
Mouth Bars and Sedimentation in Navigation Channels
The Yangtze River is the navigation artery of
China. Sedimentation at the river mouth generates the so-called
mouth bars, which jam the navigation channels. As shown in Fig.4 the
Chongming Island appeared in the river mouth 800 years ago and
bifurcates the river into the North Branch and South Branch. Until
the 18th century the North Branch was one of the main
discharge channels. Due to the Coriolis effect the mainstream
shifted to the South Branch and the North Branch shrank in the past
century. Furthermore, the South Branch is bifurcated into North
Channel and South Channel again by the Changxing Island, which was
born by sedimentation at the wide South Branch after a 100 years
flood in 1860 (Le et al, 1998). A sand bar, which is named
Jiuduansha Shoal, appeared after the 1954 flood and bifurcates the
South Channel into North Passage and South Passage. Nowadays, 50% of
the river water flows through the North Channel. The rest 50%
flowing into the South Channel is divided again by the Jiuduansha
Shoal half-to-half into the North Passage and South Passage.
Nowadays, the North Passage is the main navigation channel because
it is relatively stable.
Fig.4
The Yangtze River mouth and the dredging project
improving the navigation channel
At the river mouth the runoff velocity is low and
sediment deposits forming a huge sand bar under water. The mouth
sand bar makes the navigation channel shallow (-7 m only) and large
container ships can not navigate through it to the Shanghai Harbor
and Nanjing Harbor. In order to create a 12.5 m deep navigation
channel the government launched a project with a budget of $2
billion to dredge the north passage in 1998, as shown in Fig.4.
Parallel levees and numerous groins are to be built along the north
passage. Sediment is dredged from the channel and filled onto the
enclosed Jiuduansha Shoal and East Hengsha Shoal to create land for
harbor construction. The project will be constructed in three
phases: in the first phase the channel will be dredged to 8.5 m
deep, allowing the 3rd and 4th generations of
container ships to navigate into the river; in the second phase, the
channel will be dredged to 10.5 m deep and in the third phase to
12.5 m deep and a new harbor in the river will be created. The
project will be completed in 2008 and the shipping throughout of the
river mouth will then increase from 196 million tons to 350 million
tons, including 16 million TEU containers. Dredging will continue to
maintain the deep channel at an intensity of 10-15 million tons per
year after the completion of the project.
5.2
Shrinking River Mouths
The Bohai Bay is an
inland-sea water of China with 12 major rivers, including the
Yellow, the Haihe, the Yongding and the Ziya, pour into it. Since
the 1970s the water runoff to the river mouths have been greatly
reduced because the quickly increasing water demand causes
over-diversion of the river water. Moreover, about 600 reservoirs
and thousands of wells, numerous dams and tide locks constructed in
the past decades have changed the river flow, sediment load and
fluvial processes. Consequently, many of the estuarine channels have
been shrinking quickly. The runoff water of the Haihe River flowing
into the Bohai Bay reduced from 7.3 billion m3 in 1950s
to 0.17 billion m3 in the 1980s. The Yongding River began
to be dry in 1960s and has become an emergency floodway.
Although the sediment load
from the rivers to the estuaries is limited (2 million tons per
year), a tremendous volume of sediment is carried into the river
mouths by sea currents, which is initiated by waves from the
surrounding silty coast. Because of the reduction of runoff
discharge, the sediment deposited in the river mouths is hardly
scoured away and the river outlets are clogging up. Studies find
that there is a high turbidity belt along the coast, which acts like
a sediment transportation belt and brings continuously sediment into
the river mouths. For instances, a volume of 18-million m3
sediment deposited in the 11-km long channel below the Haihe Tide
Lock from 1958 to 1989 (Fang, 1996). The channel was narrowed from
250 m in 1958 to 100m in 1990. And the channel bed at the tide lock
was silted up by 6 m. The bed elevation of the mouth bar raised from
-3.2m to +0.5m. The outlet of the Yongding River is filled with
sediment at a rate of 3.64 million m3 per year. The river
mouth sedimentation poses a great flooding risk to the highly
industrialized zone.
Laboratory experiment and
numerical modeling have been conducted to study the laws of sediment
transportation and deposition at the river mouths. The present
strategies are mainly dredging and scouring sediment by using tidal
water. Dredging is undertaking at many river mouths but the dredged
mouths are quickly silted up again in the next year. In the period
1981-1994, people dredged 7.36 million m3 of silt from
the Haihe River mouth before the flood seasons but in the meantime
more than 9 million m3 of sediment from the sea deposited
in the river mouth. People store sea-water by using the tide lock
during flood tide and released the sea-water to scour sediment
during ebb tide. The strategy does not work well because the stored
sea-water always carries sediment thus causes sedimentation in the
upstream channel of the lock. Double guiding dike is a new strategy
to cutoff the high turbidity belt, which is about 5 km wide along
the coast depending on the wind and waves. If the strategy is
adopted, high turbidity water can not enter into the river mouths
and the river mouths will be prevent from siltation. The strategy is
still under investigation.
6
Debris Flow and Debris Flow Control
6.1
Debris Flow Disasters
Debris flow is a
wide-distributed and frequently occurred sedimentation disasters in
China. Chinese people call the phenomenon as "dragon" which stand
for power and irresistible. More than 800 counties (about 40% of the
total ) have witnessed debris flows and more than 100 cities and
towns have been hit by debris flows. According to an investigation,
there are more than 10,000 debris flow gullies throughout the
country. Rainfall debris flow frequently occurs in Yunnan, Sichuan,
and Gansu provinces. The Xiaojiang watershed in Yunnan province has
a drainage area of 3,220 km2. There are 107 debris flow
gullies in the area. Annually 100-2,000 debris flows take place and
10-40 million tones of solid material are carried into the Xiaojiang
River from the gullies. Glacial debris flow takes place mainly on
the Qinghai-Tibet Plateau. The Guxiang Gully in the plateau watches
more than 10 glacial debris flows every-year. A glacial debris flow
of huge scale occurred in the gully in 1953, with depth 40-95m and
discharge about 28,000 m3/s (Du and Zhang, 1985).
Debris flows played many
tragedies. A debris flow occurred in the suburbs of the Xichang City
of Sichuan Province in 1981, which damaged 5 streets and caused more
than 1,000 casualties. On July 8, 1984, a debris flow from the
Guanmiao Ravine carried 60 huge stones of diameter 5-10 m and 430
stones of diameter 2-5 m and rushed down to the Nanping County town
at a velocity of 9.2 m/s. It cut half of a three story building and
destroyed a 1 m thick concrete wall of a prison. There are 1,368
debris flow gullies along the railways in China. About 300 debris
flow disasters happened in the past 5 decades, which buried 41
railway stations. The railway transportation was cut off for 7,500
hours (Shen et al., 1991). On July 9, 1981, a debris flow with a
“dragon head” 8-m high flowed from a gully to the Chengdu-Kunming
railway at a velocity of 13.2 m/s. It carried rocks of several
meters in diameter and the density of the mixture was estimated at
2.32 t/m3. The debris flow damaged the 110 m long
Liziyida Bridge on the Dadu River, destroyed a pier and an abutment,
overturned the No.422 passenger train and killed 300 passengers.
More than 5 million dollars were lost and the railway was blocked
for 384 hours. Debris flows also destroy forests and results in
temperature difference enlarged. The debris flow areas become drier
in dry season and suffer from more intensified rainstorms in wet
season.
6.2
Studies on Mechanism of Debris Flow
Quite a few scientists adopt
one or several constitutive equations to study the mechanism by
assuming the debris flow a homogeneous non-Newtonian continuum. It
is successful in some cases. For instances, the visco-plastic
conceptualization of debris flow with fine materials explains the
high gravel-carrying capacity, laminar flow, velocity profile with a
plug, and the roll waves and intermittence of debris flow (Yano and
Dido, 1965, O’Brien and Julien, 1988, Wang et al., 1990). The
dilatant fluid model interprets the mechanism of supporting force
for the moving gravel and stones and the particles velocity profile
(Bagnold, 1954, Takahashi, 1978, Savage and McKeown , 1983).
To study the mechanism and
control strategies of debris flow, Chinese Academy of Sciences
established the Dongchuan Debris Flow Observation and Research
Station on the Jiangjia Ravin of the Xiaojiang Watershed in 1988.
Kang (1985) reported that debris
flows in the Jiangjia Ravine occur during or after rainstorm in
summer. A typical debris flow begins with torrential flood.
Following erosion of the gully bed the flow develops from
low-viscous turbulent flow into high-viscous laminar flow as the
specific weight of the flowing mixture increases from 1.1 g/cm3
to 1.9 g/cm3. Highlight of the process is intermittent
flow with a series of waves rolling downstream when the density
reaches 1.9-2.3 g/cm3.
Field studies find
that there two types of debris flow with distinguished dynamic
characteristics and mechanisms: 1) Viscous debris flow, which is
composed of clay, sand and gravel and exhibits non-Newtonian
features. Such type is characterized by the striking phenomena of
intermittent flow, “paving way process”, low resistance and drag
reduction, extremely high superelevation at bends and well-mixed
deposit materials. 2) Two-phase debris flow, which is composed of
stones and gravel as the solid phase and the fluid mixture of water
and low concentration of clay and sand as the liquid phase. Typical
two-phase debris flow exhibits high, steep head consisting of
rolling, colliding and noising large gravel.
Recent studies
indicate that the resistance of debris
flow can not be approached by using the constitutive equations
because the viscosity and other rheologic parameters represent much
greater resistance than the real debris flows. Fig.5 shows the
Manning’s roughness n of viscous debris flows (o) and water flows (△)
in the Jiangjia Ravine as a function of the depth. Drag reduction
must occur in debris flow because the debris flow velocity is higher
than the clear water flow at the same flow depth. The rate of drag
reduction of viscous debris flows RD is shown as a
function of the gas concentration in the debris mixture. The rate of
drag reduction RD is defined as
(2)
in which nw
and nd are the Manning’s roughness n of
water flow and debris flow. For the viscous debris flow in the
Jiangjia Ravine, the rate of drag reduction is as high as 60%. In
other words, debris flows are 2 times faster than water in the same
gully although debris mixture has much higher viscosity than water.
The drag reduction is perhaps due to the paving way process (30%)
and air cushion (30%) (Wang et al., 2001).
Fig.5 (a) Bed roughness of
viscous debris flows (o) and water flow (△)
in the Jiangjia Ravine as a function of the depth of the flows; (b)
The rate of drag reduction of viscous debris flows as a function of
the gas concentration in the debris mixture
6.3
Prediction and Warning of Debris Flows and Control Strategies
Prediction of rainfall
debris flow can be made by using the 10 minutes rainfall intensity I10
and a parameter of accumulated precipitation Pa, which is
defined by (Chen, 1985):
(3)
in which P0 is
the precipitation just before the most intensive 10 min. rainfall, Pi
is the precipitation on the day i-days before the debris flow.
Debris flow occurs if:
(4)
The critical value Pc
is different for different areas For the debris flow gullies in the
Xiaojiang Watershed on the Yunnan Plateau, for instance, Pc
equals 60 mm. By combining the results and rainstorm forecasting,
debris flow can be roughly predicted.
Many detecting and warning
systems have been developed for preventing railways, highways,
bridges, factories and mines from debris flow disasters. For
instance, vibration detector receives vibration induced by debris
flow and transmits warning signal to the protected objects. Debris
flow level detector can send warning signal to the protected objects
when a debris flow is over a given stage. UJ-2 type ground sound
wave probe and warning system were developed by the Chengdu
Institute of Mountain Hazards Studies in 1984. The system can
automatically work for 3 months with a group of batteries. The
system had successfully sent warning signals of 12 debris flows to
the protected area 2.8 km downstream.
The most important strategy
to control debris flow is building dams. The Daqiao Creek lying on
the right side of the Xiaojiang River was an active debris flow
gully. Five detention dams reduce solid material transported into
the Xiaojiang River and basically control debris flow. Nowadays
lattice dam, window dam, slit dam and comb-shape dam are widely
employed because, on the one hand, these dams can trap large
boulders and mitigate debris flow disasters, and on the other hand,
they have much longer life span because the siltation rate is much
lower than normal dams (Kang, 1996). Debris flow flume is an
important strategy to protect railways and highways from debris
flows and is widely employed in China. In Gansu Province alone there
are 25 flumes which guided debris flows across over the highways and
railways and consequently protect the transportation arteries from
damages. Another measure is to establish debris silt basins. By
guiding debris flow into a given area, the farmland, hydraulic works
and dwelling area are protected from disasters of debris flow. A 2.5
km-long diversion channel has guided debris flows from the Jiangjia
Ravine to a deposition area and prevented the Xiaojiang River from
blocking for twenty-two years. In some debris flow gullies willow
piles are planted in the gully bed, with 1 m underground and 0.5 m
emerged. Sediment carried by debris flows is trapped by the willow
brush. While more and more sediment deposits in front of the willow
piles the willows grow up and become strong. Thus debris flow is
controlled. Comprehensive debris flow control projects are conducted
in many debris flow watersheds. The projects are composed of
reforestation, building dams, constructing debris diversion channels
and basins on the debris cone and constructing reservoirs on the
main stem and tributaries. The projects are very successful (Wang,
1999).
7
SEDIMENT STUDIES IN THE NEW CENTURY
7.1
Validation of Sediment Studies
In addition to the
conventional hydrological gauging, collection of field data for
future validation of sedimentation studies carried out for the Three
Gorges Project (TGP) was started in 1993. The timing is a year ahead
of starting the construction of TGP. It is to collect the data of
the Yangtze River before the river is disturbed by the construction
of the project. Reaches both up- and down- stream of the dam were
observed. For the period from 1993 to 2009, a fund of over 30
million USD has been appropriated. Additional funds would be
appropriated after 2009 for field studies to monitor the scheme of
reservoir filling. The observation will cover, among other things,
deposition in the reservoir, depth of water in the fluctuating
backwater reach for the passage of large tows, deposition in the
lock approaches, degradation of river channel downstream, especially
the part of the river down as far as to Hankou. All these field
studies will help validate or modify the results of the present
studies and place people in a better position to answer how
realistic is the present knowledge in sedimentation. It would take
at least 10 years, i.e., up to 2019, to complete the initial phase
of verification. Our knowledge on sedimentation should then be
advanced by a large margin.
Closure of the TGP dam will
be effected in 2003, when the pool will reach the elevation of 135
m. The normal pool will be raised further to 156 m in 2007. Although
structurally the reservoir will then be ready for impoundment to 175
m in the year 2009, considerations based on sedimentation studies do
not recommend hasty impoundment. It is suggested in the feasibility
study, a period of several years was allowed for sedimentation
observation before full impoundment to pool 175 m. Sedimentation
engineers have recommended schemes for impoundment in steps.
Execution of these schemes is to be monitored by field studies as
well as real-time mathematical modeling. The main concern is the
deposition in the upstream metropolis Chongqing. Under natural
conditions, there is deposition in the port of Chongqing during the
flood season. During the low flow season from November to March in
the following year, the deposition is scoured away, thus maintaining
equilibrium between erosion and deposition. It is estimated that
change in this process will begin when the reservoir pool reaches
approximately the elevation 160 m. Should harmful deposition be
found thereafter, mitigation measures would be applied and
observation continued.
7.2
Theoretical Studies
Sediment theories have been
developing for about a century. Traditional sediment research
programs have fallen out of step with the needs of the profession.
Sediment engineering is moving toward becoming more
multidisciplinary. The sediment researchers have to think bigger and
broader. Unsteady sediment transport, environmental sedimentation
and eco-sedimentation, and economic sedimentation are the new
directions of theoretical studies in the new century.
The main theories and
formulas of sediment dynamics were established based on steady and
uniform flows. Nevertheless, the theories and formulas often fail to
apply in engineering projects because sediment in nature is
transported by unsteady and non-uniform flows. It is more often so
following development of the application scope and requirement of
high accuracy estimation of the rate of sediment transport. The
parameter
 in Eq.(1) represents the mean concentration of
sediment corresponding to the capacity of sediment transport. But it
is calculated with the formulas for steady and uniform flows. In the
numerical models, a coefficient
 is introduced to offset the error. However, new
formulas, which can be directly applied in unsteady sediment
transportation, are needed. Sediment-removing capacity is defined as
the capacity of the flow to remove sediment from per unit length of
a river section to other places per time. Differing from the
well-defined sediment-carrying capacity, which is the feature of the
mean flows and represents the amount of sediment load the flow can
transport through the channel, the sediment-removing capacity is the
feature of unsteady, non-equilibrium flows and represents the
capability of the flow to change the channel shape and location. The
sediment-removing capacity is found proportional to the fluctuation
intensity of discharge of the unsteady flow (Wang and Wu, 2001). The
movement of a river channel within the fluvial plain is defined as
the river motion. The speed of the river motion is a function of the
sediment-removing capacity.
Environmental sedimentation
is an important aspect of sediment studies. Sediment-oxygen demand,
contents of ammonia, nitrate, phosphorus, silica in sediment,
sulfide, methane, and heavy metals, and the chemical interaction
between water and sediment are the major indicators of sediment
quality. A primary interaction is the exchange of solutes between
the sediment and the overlying water. The flux-the transport of
mass-of dissolved and particulate chemical species to and from the
sediment are important components of the chemical and biological
cycle that take place. For example, the consumption of dissolved
oxygen by organic matter that settles to the sediment in the spring
is usually the primary cause of summertime oxygen depletion in the
bottom water of lakes and estuaries (DiToro, 2001). Eutrophication
and harmful algal bloom (red tide) are often related to the
nutrients from sediment. The study and modeling of the
bio-geo-chemical process is one of the new job of sediment
researchers.
Vegetation-erosion dynamics
is a new interdisciplinary science, studying the laws of evolution
of watershed vegetation under the action of various ecological
stresses, especially soil erosion. By introducing the mathematical
expressions of various ecological stresses, the vegetation
development and the erosion process is modeled. The vegetation of a
watershed or an area may exists in three states: vegetation
developing and erosion reducing, vegetation deteriorating and
erosion increasing, and the transitional state between the two.
Human may change a watershed from one state into the others, the
effort required depends on the distance of the present position to
the destination position on the vegetation-erosion chart. The theory
may predict the vegetation development of an area under the action
of various ecological stresses and answer the questions: whether it
is possible to permanently change the landscape by human activities
and how much effort is required to achieve it (Wang et al., 2001).
Sediment is also regarded as
a kind of precious resources (Qian, 2000). Sediment depositing in
reservoirs cost capacity, but sediment transported to the estuaries
can be used for land creation. The so called artificial peninsula
project at the Yangtze River mouth will create 300 km2
land by trapping the sediment from the river, which is of economic
value of $10 billion. The Yellow River mouth channel was
artificially shifted to the Qingshuigou-Chahe Channel in 1996 to
create a land oil field with the sediment. Sediment value-addition
is defined as the sum of all economic values (positive or negative)
due to sediment transportation and deposition. The sediment at the
abandoned Yellow River mouth is scoured by waves. A part of the
scoured sediment is transported into the mouths of the rivers
flowing into the Bohai Bay (see Chapter 5), causing shrinking of the
mouths and enhancing the dredging cost. Therefore, wave breakers for
protection of the Yellow River delta may not only stop the beach
erosion and the land loss, but also greatly enhance the sediment
value-addition. The study of sediment value-addition will help
people for better sediment resources management.
8
CONCLUSIONS
The main erosion control
strategies applied in China are building sediment barriers and
terracing the slopes in the arid and semi-arid areas, sediment-check
dams and reforestation in the wet areas, and comprehensive
reclamation of small river basins in both arid and wet areas. The
Yellow River was unstable and disastrous. It has been harnessed by
employing reservoirs, levees, wide river valley and narrow channels,
and flood diversion basins. The sedimentation problems in the TGP
project and operation schemes are studied with physical and
numerical models. With the strategy of storing the clear and
releasing the turbid a permanent capacity of 22 billion m3
can be preserved. Numerical calculations recommend a double FCL
scheme to flush more sediment and preserve 3.5 billion m3
more permanent capacity. Dredging of the Yangtze River mouth bar is
performing for improving the navigation and allowing the 4th
and 5th generations of container ships to navigate into
the river. The shrinking of the river mouths by the Bohai Bay is a
result of the movement of the turbidity belt along the bay, which is
generated by silty beach erosion and tidal currents. The mechanism
of debris flow is studied by field investigations and laboratory
experiments. Flume, dam train, debris diversion channel and basin,
and comprehensive control project composed of reforestation and
engineering measures are proved effective in reducing and
controlling debris flow disasters.
In the new century the
sediment approaches and theories will be validated through
continuous observation of sedimentation of the TGP reservoir with
appropriated fund of over 30 million USD. The studies will place
people in a better position to answer how realistic is the present
knowledge in sedimentation and promote the discipline of the science
by a large margin. The science of sediment transportation is moving
toward becoming more multidisciplinary. Unsteady sediment transport,
environmental and ecological sedimentation, and economic
sedimentation will perhaps become the new directions of research in
the new century.
Acknowledgement
The study is supported by
the National Natural Science Foundation (No. 59890200) and the
Ministry of Science and Technology of China (G1999043604).
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