Study of flood erosion efficiency and

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Study of flood erosion efficiency and its
adjustment in the Lower Yellow River during the
storage periods
LI Wenxue LI Xiaoping
2007 ? 6 ? 2 8 ?
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Content
1 Introduction 2 Data 3 Analysis of erosion
efficiency 4 Analysis of factors influencing
erosion efficiency 5 Remark and suggestion
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1 Introduction
In Oct 1999, the Xiaolangdi Reservoir began to
store water. During the preliminary storage
period, regimes of flow and sediment load
entering the Lower Yellow River changed
significantly. In the past, numerous scholars had
conducted remarkable studies on flow carrying
capacity and bed material variation for low
sediment concentration flow. However, studies
about erosion efficiency and the factors
influencing it for low sediment concentration
flow created by regulation of large reservoirs in
the master stem of the Yellow River are rarely
seen. This study focuses on erosion efficiency
and its variation characteristics, investigates
factors that affect it, and finally proposes flow
discharges for high erosion efficiency.
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2 Data
Data were selected from floods occurred during
the storage periods of the Sanmenxia Reservoir
and the Xiaolangdi Reservoir. Only floods of
clear water or low sediment concentration flow (
) formed by reservoir density flow
were considered. During storage period,
sediments discharged from reservoir by density
flow were very fine, and their influence on
erosion efficiency could be neglected.
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In this study, 41 flood events were selected.
Erosion had occurred in the Lower Yellow River
for all the selected floods. Among of them, 27
flood events happened during the storage period
of the Sanmenxia Reservoir, 14 happened during
the storage period of the Xiaolangdi Reservoir,
and sediment concentration was less than 15 kg/m3
in 38 flood events. The threshold values of flow
and sediment for these floods are listed in Table
1.
Table 1 The threshold values of flow and
sediment of floods
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3 Analysis of erosion efficiency
3.1 Erosion efficiency of total load
Fig.1 shows that erosion efficiency increases
with the increase of average flood discharge and
reaches its maximum (about 20 kg/m3) at a
discharge about 4000 m3/s. For discharge greater
than 4000 m3/s, erosion efficiency maintains at a
constant value basically.
Considering the facts that sediment concentration
for flow released from reservoir was low and
particles were fine, thus erosion efficiency
depended mainly on flood discharge and the supply
of bed material.
Figure 1 Relationship between erosion efficiency
and average flood discharge for total load in the
Lower Yellow River during storage period
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3.2 Erosion efficiency for grouped particles
In the Lower Yellow River, sediments are usually
classified as follows Fine particle if
median
particle if
coarse particle if
in which d is sediment
diameter.
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Figure 2 Relationships between erosion
efficiencies and average flood discharges for
total grouped particles
Fig. 2 illustrates the relationships of erosion
efficiency with grouped particles.
So we can see erosion efficiency for total load
does not increase when flood discharge is greater
than 4000 m3/s is mainly due to the decrease of
erosion efficiency of fine particles.
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4 Analysis of factors influencing erosion
efficiency
4.1 Adjustment mechanism of erosion efficiency
In the equation, J can be considered as constant
during a flood event, the viscosity coefficient v
can also be treated as constant during flood
season. Thus, sediment carrying capacity (S) is
mainly affected by flow rate (Q), roughness (n),
diameter of suspended particle (d), and channel
width (B).
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4.2 Analysis on factors affecting erosion
efficiency
1) Variation of hydraulic parameter (
)
To illustrate the influence of flow rate on
sediment carrying capacity, plots of hydraulic
parameter against flow rate are given in Fig. 3
and Fig.4 .
Figure 3 Relationship between hydraulic
parameter ( ) and flood discharge at
the Huayuankou station during storage period
It can be seen that hydraulic parameters
increased with the increases of flow discharge at
both stations for floods observed in the storage
periods of the Sanmenxia Reservoir and the
Xiaolangdi Reservoir. Consequently, sediment
carrying capacity increased with the increase of
flow rate.
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Figure 4 Relationship between hydraulic
parameter ( ) and flood discharge at
the Gaocun station during storage period
Fig. 3 and Fig. 4 also show that at the later
phase of storage period, value of hydraulic
parameter was smaller than it was in the earlier
phase (especially for Gaocun station). Due to
continuously erosion, channel became wide at the
later phase of storage period and channel
roughness became larger owing to armoring
process, which resulted in the decrease of
hydraulic parameter. Therefore, erosion
efficiency at the later phase of storage period
was smaller than it was at the earlier phase.
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2) Variation of particle size
At the end of storage period, bed materials in
the channel of the Lower Yellow River became
coarser (Fig. 5 and Fig.6). During the storage
period of Sanmenxia Reservoir, the medium
diameter of bed material at Huayuankou station
increased from 0.08 mm in 1961 to 0.17 mm in
1964. And at Sunkou station the medium diameter
of bed material increased from 0.064 mm in 1961
to 0.09 mm in 1964.
Figure 5 Size distributions of bed materials for
typical channel sections in the Lower Yellow
River during storage period of the Sanmenxia
Reservoir
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Samples taken in the storage of Xiaolangdi
Reservoir. Fig. 6 indicates the armoring effects
of channel bed after the Xiaolangdi Reservoir
impounding water. For instance, the medium
diameter of bed material at Gaocun station
increased from 0.05 mm in 1999 to 0.125 mm in
2004. The diameter of bed material was basically
greater than 0.05 mm.
Figure 6 Size distributions of bed materials for
typical channel sections in the Lower Yellow
River during storage period of the Xiaolangdi
Reservoir
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During the storage period of the Sanmenxia
Reservoir, river training works were imperfect,
channel widening and deepening occurred together
(Fig. 7a). From 1986, river training works has
been perfected gradually, thus bank collapse was
not significant and erosion in the Lower Yellow
River happened mainly in downward direction
during the storage period of the Xiaolangdi
Reservoir (Fig. 7b).
(b)
(a)
Figure 7 Channel configurations at the
Huayuankou section before and after the storage
period
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During storage period, suspended loads were
mainly supplied from riverbed. Mean diameter of
suspended particles varied simultaneously with
the mean diameter of bed material. For flood of
different magnitude, erosion capability increased
with the increase of flood discharge, so did the
sediment carrying capacity. Bed materials of
fine and medium size would be suspended and
carried away first. Bed materials of coarse
size, which was the major contents of bed
material, could hardly be carried away due to
inefficient sediment carrying capacity.
Therefore, the armoring process of riverbed was
quick for large flood (Fig. 8).
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Figure 8 Size variation of suspended material
with flood discharge at the Lijin Station during
storage period
With the development of armoring process, less
and less bed materials could be suspended and
transported and thus erosion efficiency of total
load could not be increased further. Therefore,
erosion efficiency of total load depended mainly
on the supplies of fine and medium size particles
from bed materials.
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3) Variation of roughness
Roughness represents the resistance of channel
boundary to flow. It is usually expressed by
Mannings coefficient . In the Lower Yellow
River, roughness is very large for small flow
discharge. With the increase of flow rate,
roughness decreases gradually and reaches its
minimum value at certain flow rate. Afterward,
roughness may increase slowly or maintain a
constant value. Some studies show that the
minimum value of roughness at the Lower Yellow
River happens for flow rate between 1000 m3/s to
1500 m3/s.
During storage period, the larger the flood was,
the more intensify the erosion and the quicker
the armoring process (represents by equivalent
roughness ) was. However, water depth (
) also increased with riverbed erosion, relative
roughness ( ) did not change much.
Thus, roughness did not vary significantly with
the increase of discharge during flood.
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5 Remark and suggestion
Analyses of erosion efficiencies during the
storage periods of the Sanmenxia Reservoir and
the Xiaolangdi Reservoir indicate that erosion
efficiency of total load increases with the
increase of flood discharge and reaches its
maximum value (20 kg/m3) at the discharge about
4000 m3/s, and afterward, erosion efficiency
maintains at a constant value (20 kg/m3)
basically. By analyzing erosion efficiencies of
grouped particles, we find that the decreasing
extension of the erosion efficiency of fine
particle is greater than the increase extension
of the erosion efficiency of coarse particles.
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Main factors affecting erosion efficiency include
average flood discharge, size of suspended
particle, configuration of channel cross-section,
roughness, and etc. Among them, size of
suspended particle is the dominant factor.
During storage period, size of suspended particle
is closely related to the size of bed material.
With the increase of flow rate, fine and medium
size particles are less available in bed
material, thus erosion efficiency does not
increase further with the increase of discharge.
Analysis indicates that the contents of fine and
medium size particles in bed material are
essentials to erosion efficiency of total load.
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Suggestion
Based on analysis given in this paper, it is
suggested that the average flood discharge during
the storage period of the Xiaolangdi Reservoir
should be controlled at about 4000 m3/s to obtain
the highest erosion efficiency, thus to make good
use of the limited water resources and improve
channel drainage capacity of the Lower Yellow
River.
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Thank you very much!