SUSTAINABLE COUNTERMEASURES USING FREQUENCYANALYSIS AND DESILTATION STRATEGY IN A RESERVOIR
Fong-Zuo Lee1*, Tetsuya Sumi2, Jihn-Sung Lai3, Yih-Chi Tan4 and Cheng-Chia Huang5

1

Ph. D. Student, Department of Bioenvironmental Systems Engineering, National Taiwan University,
Taipei City, 106, Taiwan.
2

Member of JSCE, Dr. of Eng., Professor, Water Resources Research Center, Disaster Prevention
Research Institute, Kyoto University (Goka-sho, Uji-shi, 611-0011, Japan)

3

Research Fellow, Hydrotech Research Institute, Adjunct Professor, Department of Bioenvironmental
Systems Engineering, National Taiwan University, Taipei City, 106, Taiwan.

4

Professor, Department of Bioenvironmental Systems Engineering, CEO, Center for Weather Climate
and Disaster Research, National Taiwan University, Taipei City, 106, Taiwan.
5

Research Assistant, Hydrotech Research Institute, National Taiwan University, Taipei City, 106, Taiwan.

Abstract: The annual mean loss of reservoir storage due to sedimentation is already higher than theincrease of capacity by construction of new reservoirs for water supply, irrigation and hydropower. Thus,
the sustainable use of the reservoir is more important and the storage is not guaranteed in the long term.
Therefore, for prolonging the reservoir life, the desiltation strategy of the reservoir is always animportant issue for reservoir long term operation. By dredging or excavation, deposited sediment may
be mechanically removed from reservoirs because of valuable material for construction and connivance
implementing. But, based on the characteristics of reservoir to classify suitable desiltation strategies ismore efficient to deal with sedimentation problem. This study depended on frequency analysis method
and venting efficiency of excited facilities for total inflow sediment volume and bypass with sediment
replenishment strategies to planned valuable desiltation methods in wushe reservoir in taiwan. The
characteristic of adopted reservoir in taiwan which have reliable and completed data in its historical
records has been identified by the parameters of the turnover rate of water (cap/mar=total capacity/meanannual runoff) and sediment (cap/mas=total capacity/mean annual inflow sediment). Based on thefrequency analysis, estimation of sediment transport capacity equation, experimental results with 3d
numerical model and historical data, the inflow and outflow boundary condition of sediment were
presented. In addition, using 1d numerical model, the flushing efficiency of bypass of objective case
had been simulated to suggestion, using and planning to implement in wushe reservoir. In addition, thevaluable historical excavation volume was also considered to implement sediment replenishment.
Therefore, the results of desiltation strategies in this study are reasonable to maintain reservoir capacity
as possible as we can and make sure the reservoir functions can prolong working.
Keywords: Sedimentation, desiltation strategy, frequency analysis, flushing efficiency

1. GENERAL INSTRUCTIONSIn recent years, there are many huge earthquakes happened in the world. Terrible tsunami andnuclear radiation impacts were usually following the earthquake and many lives was passed away due
to serious damage. For example, the 8.8 Richter magnitude scale earthquake was happened at Chile in2010 and a 9.0 Richter magnitude scale earthquake with 40.5m Tsunami height was happened at East
Japan in 2011. In 1999, Taiwan also suffered from an earthquake in Chi-Chi area at central Taiwan
which magnitude reaches 7.3 Richter magnitude scale. After the earthquake, not only buildings weredestroyed in the urban area but also geological condition was changed in mountain vicinity. The soil
and rock of the mountainside were collapsed and decrease glued capacity between sediment particles.
Therefore, when heavy rainfall occurred in watershed, land collapse would happen and large sediment
would be washout into river, especially in steep slope area of mountains. And, the land development bymankind in the watershed would also accelerate soil erosion in recent years due to limited living land
and leisure purpose. As such huge sediment moved into a reservoir, serious sedimentation problem
reduced storage of reservoir due to decrease of flow velocity and without bottom outlet. However, there
are many countermeasures to deal with deposition sediment in the reservoir, such as check dam,
excavation, dredging, flushing, sluicing, venting, bypassing and replenishment sediment at
downstream. But, not all of the countermeasures are suitable for specific reservoirs and thecost-benefit ratio of each strategy is different. The suitable methods were mainly depended onhydrological patterns and geological conditions. Therefore, classified characteristics of reservoirs andinvestigated suitable countermeasures should be studied first.
In Japan, desiltation strategies were classified into four kinds in all reservoirs. Based on thecharacteristics of hydrological and geological conditions in reservoir watershed, suitable desiltation
strategies were adopted to implement for individual reservoirs. The four kinds of desiltation strategies
usually be distinguished to deal with sedimentation problem in Japan and such kinds can be identified
by the parameters of the turnover rate of water (CAP/MAR=total capacity/Mean annual runoff) andsediment (CAP/MAS=total capacity/Mean annual inflow sediment) (Sumi, 2011). The first kind of all,the flushing strategy was suited to execute if the annual quantity of inflow discharge is large enough toflush sediment. The second kind of all, the bypass and venting strategy is an option to take care inflow
sediment if the refilled rate of capacity by clear water is about 10 times in one year. The third kind ofall, the sediment replenishment technique at downstream river upland is useful for preventing
downstream river from bed degradation and increase sustainable living habitat for aquatic livings at thedownstream reach of dam if the capacity of reservoir is large enough or inflow sediment volume is not
so huge. In addition, in order to realize inflow sediment and suitable countermeasures, frequency
analysis of deposited sediment of large reservoirs and the applicability of the asset management forreservoir sediment management are investigated to classify the characteristics of reservoirs (Sumi etal., 2009; Sumi and Kantoush, 2010).
Based on the classification of desiltation strategies, when CAP/MAR is around 10, the bypass idea
might be implemented to prevent sedimentation in such reservoir. In Japan, sediment bypass tunnel at
Miwa Dam for fine sediment had been constructed in 2004 and successful operated in recent years.
Besides, for both fine and coarse material, the Nunobiki Dam was completed in 1908 and Asahi Dam
was completed in 1995(Harada, 1997). These two dams had been successfully introduced to realize
sustainable reservoir management (Kataoka, 2003). In addition, sediment bypasses at Matsukawa Dam
and Koshibu Dam are under construction from 2009 to 2013 year. Therefore, the bypass strategy wasplanned to divert more inflow sediment volume in selected reservoir. Based on designed informationof bypass system which under operating and construction in Japan, the literature contents show thatthe tunnel radius is between 2.9m and 5.2m, the tunnel length is from 258m to 4,300m, the tunnel
slope is from 1% to 4%, the designed discharge is from 39m3/s to 370m3/s, the operation frequency isabout 1 to 13 times per year and tunnel shape is hood and horseshoe shape. But, there is no stander
procedure for designing bypass system until now. Actual executed design is by the way of discussion
for most acceptable conditions of budget and objective. Integrated reviewing cases in Japan and underdesigning, the tunnel flow velocity, the path line of bypass tunnel, the intake and outlet location andimpacts of environment should be considered at the same time. According to the operation results of
Asahi dam and Miwa dam, more than 80% bypassed efficiency can be expected in past 10 year’srecord. Especially in Asahi dam, the bypassed sediment includes all coarse material and some part offine material can be bypassed. The designed discharge of bypass tunnel is only 140m3/s that value islower than 1 year return period (Kataoka, 2003).

Recently, the replenishment technique also has been implemented to prevent downstream river frombed degradation and increase suitable living habitat for aquatic animals downstream of the dam [Sumi etal, 2012]. Such restoration methods could be implemented in bad hydrological and geographic area
which drawdown flushing could not be practiced, for example Taiwan and Japan. Therefore, thedownstream replenishment technique had been implemented to prevent downstream river from bed
degradation and increase suitable living habitat for aquatic organism at downstream of the dam in past 15
years in Japan. Okano et al. (2004) started to study reservoir sedimentation management by coarse
sediment replenishment below dams and summarized sediment replenished project in Japan from 1999
year to 2004year. Such as Tenryu, Otakine, Abukuma, Ara, Oi, Naka, Kuzuryu, Yodo, Kanna, and Tone
had been constructed by Ministry of Land, Infrastructure and Transport (MLIT). The replenishedsediment effects on the downstream river of Yahagi dam and appropriate grain size with less turbidity
using sediment treatment system in Kizu River were following to investigate (Seto et al.,2009 ; Sumi etal.,2009). The interaction of relative flow field and morphological evolution during replenishment
experiments were also analyzed (Kantoush et al., 2010). The conceptual idea was before flood coming,
the sediment had been constructed at the downstream flood plain and after flood was passing, thereplenished sediment was flushed to the sea. Such field test in Japan focused on coarse material that
deposition in the reservoir and replenished at downstream upland for creating suitable environment foraquatic livings. However, the replenishment strategy needed huge discharge to flush replenishedsediment, therefore, replenished sediment always constructed before flood coming and implemented
during wet season. Using this method, deposited sediment in reservoir can be periodically excavated andthen transported to be placed temporarily downstream of the dam at floodplain. But, replenished volume
was based on the sediment transport capacity of the channel and the environmental conditions. Refer tothe experience of replenished volume in Japan; it is up to 10% of annual deposition volume in reservoir
could be successful executed.
Therefore, this study followed the research process of Sumi and Kantoush (2010) to classify themainly reservoirs and weirs in Taiwan and adopted frequency analysis method to estimate inflow
sediment volume to recognized suitable desiltaion strategies for objective reservoir. Numerical modeland physical model was executed to realize releasing efficiency of existed structures. Then, alternative
strategies were investigated to discuss sustainable countermeasures.

2. DESCRIPTION OF FILED RESERVOIR
2.1 Selection of objective reservoir
Based on the classification of desiltation strategies using CAP/MAR and CAP/MAS parameters,
the main important reservoir in Taiwan were figured out to show suitable desiltation strategies. As
mentioned before, the serious sedimentation reservoirs, Shihmen reservoir, Wushe reservoir andTsengwen reservoir were also classified to show suitable countermeasures in Figure 1. The Figure 1
shows that most of reservoirs in Taiwan were suited to execute bypass, venting strategy and sediment
replenishment combining with check dam. According to the analysis, the reservoir life is obviously
decreased due to Chi-Chi earthquake and the earthquake almost affects all kind of reservoirs. The
Figure 1 also shows the priority countermeasures of the classified kind, the Shihmen and Wushe
reservoir were suited for using bypass, venting strategy combining with sediment replenishment andTsengwen reservoir was agreed with replenishment and check dam strategy. Due to mostsedimentation problem and similarity magnitude scale of reservoir, the Wushe reservoir was choose toinvestigate sustainable countermeasure using frequency analysis and desiltation strategies. Besides, themore popular desiltation strategies of bypass and sediment replenishment technique were adopted toinvestigate.

Figure 1. Reservoir characteristics in Taiwan

Figure 2. Sketch of Wushe reservoir and river
system

2.2 Description of Wushe reservoir
Wushe reservoir located at central Taiwan (Figure 2) and it belongs to the Taiwan Electric Power
Company. The Wushe reservoir has a natural drainage area of 219 km2. The Wushe dam was formed by
the concrete gravity and located at the upstream reach of the Jhuoshuei River and flows westward to the
Taiwan Strait. The Wushe dam constructed in 1951 is a 114 m height and 7 m width at the top of archedgravity dam with two spillways, one permanent river outlet, one tunnel spillway and one power plantintake controlled by gates. And, there is a new power plant intake with 22 m3/s intake discharge at
983m elevation is under construction for electric power at. The elevations of the spillway crest,
permanent river outlet, flood diversion tunnel and power plant intake are 998.9m, 927.65m, 989.8m and
938m, respectively. The designed capacity of the two spillways is 850m3/s, the permanent river outlet is
87m3/s, the tunnel spillway is 1,200m3/s and the power plant intake is 24m3/s, respectively. With amaximum water level of 1005m, the reservoir pool impounds about 7.5 km in length and the initialstorage capacity was 1.5x 108m3. Incoming sediment particles have settled down significant along thereservoir since the dam was completed. Based on the survey data, the longitudinal bed profile along thereservoir is plotted in Figure 3. As shown in Figure 3, the Wushe reservoir has accumulated a greatamount of sediment after Chi-Chi earthquake and the impact effects reached 3 times deposition volume
from 1.36*106m3 to 3.33*106m3 per year. Specifically in 2004 year and 2008 year caused by Typhoon
Aere and Typhoon Sinlaku, the deposition volume is about 6.00*10 6m3 and 6.76*106m3, respectively.
Therefore, the serious sedimentation problem was caused bed elevation arising 6 m in those two years(Figure 4). By recent survey data in 2010, the storage capacity was estimated to be 35.86% of its initialstorage capacity. From longitudinal variation of Wushe reservoir and relative elevation of facilities in
Figure 4, the delta position is starting to move forward downstream from 2007 year due to Typhoon
Sinlaku and the location of delta is about 2 km from the dam site. Moreover, the bed elevation ishigher than power plant intake and permanent river outlet (PRO). Due to frequently operation of
power plant intake, the facility is still been worked well. But, unfortunately for PRO can't be operated
after dam was constructed due to never open it in past years. And, the figure also shows that huge
sediment settled down after delta location and also the upstream area of delta. According to the fieldsurvey of sediment size in 2005 (Taiwan Electric Power Company, 2005 ; as show in Figure 5), themean size of coarse material at NO-6 is about 3mm and fine sediment at NO-1 and NO-2 is about
0.02mm. The measurement locations are showed at Figure 6. The Figure 5 also shows that, from NO-5
to the dam site, the deposition sediment belongs to silt and clay (size less than 0.075mm) and thedeposition volume is about 70 % of total deposition volume due to the location of NO-5 is about at
70% volume of reservoir impounding area. Moreover, the grain size also indicated that the finesediment would deposit after delta location and coarse material settled at the upstream of delta areawith 30% total deposition volume.

1959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010

0
1

100

2
After Chi-Chi
earthquake80

y = 4.2371x +b
1999 year
Chi-Chi earthquake

60

4
5
6

Before Chi-Chi
earthquake

40

3

Typhoon Aerey = 1.3467x +a

Typhoon Sinlaku

20

7
8
9

0

Average annual deposition volume(106m3)

Accumulative deposition volume(106m3)

120

101959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010

Year

Figure 3. Variation of sedimentation in Wushe reservoir
Figure 4. Longitudinal variation of Wushe reservoir and relative elevation of facilities

100NO-1-S1
NO-1-S2
NO-2-S1
NO-2-S2
NO-3-S1
NO-3-S2
NO-4-S1
NO-4-S2
NO-5-S1
NO-6-S1
NO-6-S2

Accumulative percentage (%)

90
80

70
60
50
40
30
20

10
0
100.0000

10.0000

1.00000.10000.01000.00100.0001

Grain size(mm)Figure 5. Grain size of deposition sediment

Figure 6. Sketch of investigation location of
sediment grain size(Taiwan ElectricPower Company, 2005)

3. METHODS
3.1 Frequency analysis of deposited sediment volume
In this study, frequency analysis methods were adopted to estimate the performance of deposition
sediment. The field data was also mainly used to select proper frequency analysis method. According
to the historical records of 19 years deposition volume in Wushe reservoir, the most popular usedstochastic methods of Gumbel distribution, Normal distribution and Lognormal distribution were
employed to estimate deposition volume for each return period. According to the biggest inflow
discharge of historical record is up to 50 year return period of Typhoon Sinlaku (as show in Figure 7),
the frequency analysis is estimated from 2 year to 200 year return period. Figure 8 shows the results offrequency analysis. Comparison of three methods and historical data of field, the relative good
approach was selected on Gumbel distribution as shown in Figure 8. Before 10 year return period, theNormal distribution and Gumbel distribution is closed to each other. However, entirety considerationof covering 200 year return period, the Gumbel estimation is the best fitting and finally been adopted
for deposition sediment volume of objective case,50 year return period.
30
Typhoons of 2007~2009 year
1400

Deposition volume(106m3)

Discharge(m3/s)

1200

968 m3/s> 5 year return period

1000

708 m3/s< 5 year return period
523 m3/s~ 2 year return period

800

Historical data
Gumbel distribution
Normal distribution
Lognormal distribution
discharge

27

1493 m3/s~ 50 year return period

600

400

24
21
18

0

50

100

150

200
250
Time(hr)

300

350

400

450

Figure 7. Flow hydrograph from 2007 year to 2009 year

2500
2000

12

1500

9

1000

6

500

0

0

3000

15

3

200

3500

Discharge(m3/s)

1600

1

10

100
1000
Return period(year)

0
10000

Figure 8. Frequency analysis of deposition volume

3.2 3.2 Relationship between inflow discharge and sediment
The actual flood event is unsteady and inflow discharge with sediment is strongly depended on
upstream flow condition. As a result, the useful suspended sediment transport equation of reservoir
was employed to estimate fine sediment. The adopting equation (Chien, 1999) as following:*  K (Where * =

K=
V=
g=
R=

w=
m=

V3 m
)
gRw

(1)
3

Transport capacity (kg/ m )3

Coefficient (kg/ m )Average flow velocity(m/s)
Gravity(m/s2)
Hydraulic radius (m)Falling velocity(m/s)
Index

The coefficient K and index m can be decided from the value of V 3 gRw (Chien, 1999). Basedon the field survey of grain size in 2005 as show in Figure 5 and deposition volume in Figure 3, thefine sediment quantity from field measurement was compare to the simulation results using equation(1). Based on the mainly record inflow discharge of rainfall in 2005, the volume ratio of total
deposition volume between field measurement and simulation results is 77% and 76 %, respectively.
The calibrated values of coefficient K and index m are 1.0 and 0.6, respectively. The relative error isabout 1% and such accuracy possesses confidence to use equation (1) for generating inflowhydrograph of fine sediment. And, the quantity with hydrograph of fine and coarse material could be
distinguished and obtained. Figure 8 shows the relationship between inflow discharge and inflow
sediment volume using equation (1). The estimation case was depended on Typhoon Sinlaku (Figure9(a)) and also suitable for 50 year return period of inflow discharge. The figure indicated that thevolume of fine sediment is about 68% and coarse material is about 32% (Figure 9(b)).

1600

Water elevation(m)

1005
1000

45

1200

40

1000

995

800
600

990

50

1400

Seiment discharge(m3/s)

Water elevation
Inflow discharge

Discharge(m3/s)

1010

400
985
980
0

(a)

50

100
150
Time(hr)200

y = 0.0003x1.6281
R²= 0.9999
Suspended sediment 68%
Bed load sediment 32%

35
30
25

y = 0.0001x1.6281
R²= 0.9999

20

15
10

200

5

0

0
0

250

(b)

200

400

600

800

1000 1200 1400 1600 1800 2000
Q (m3/s)

Figure 9. (a)Flow hydrograph of Typhoon Sinlaku (b) Estimation of inflow sediment discharge

3.3 Numerical model and physical model
The 3D numerical model and physical model were adopted to investigate venting efficiency ofoutflow discharge of existed structures. The 1D model was employed to investigate flushing efficiency
of bypass tunnel and topography after bypass system was operated.

3.3.1 3D simulation model
Because of the total inflow sediment was decided not only the deposited sediment but also theoutflow sediment volume of existed facilities. Therefore, a commercial CFD solve model, CFX-11.0
from ANSYS, Inc. (2006), was adopted to simulate venting efficiency of suspended sediment inWushe reservoir. The simulation fluid was assumed to be isothermal and incompressible. The boundary
condition of bed was set to be no slip condition, the inflow flux was set as mentioned in paragraph 3.2,
the outlet flux was set by reservoir operation and free surface was rigid-lid approximation. Simulation
area was from section 48 to the dam site.

3.3.2 1D simulation model
The NETSTARS model (Lee and Hsieh, 2003) is employed to simulate the impacts of upstream
river during bypass operation of the Wushe Reservoir. The NETSTARS model is an uncoupled
sediment routing model. It consists of hydraulic routing and sediment routing. Suspended load and bed
load are treated separately in sediment routing.
The NETSTARS model solves the nodal point problem as same as CHARIMA model. The node isassumed to be a virtual section that could not accumulate water and sediment. The allocation of linkdischarges at a node is considered in proportion to the discharges resulted from previous time step, andthe sum of link discharges is assumed to be zero at a nodal point. The allocation of the suspended load at
the node is assumed to be proportional to the allocations of the flow discharge, and the net flux of thesuspended sediment due to longitudinal dispersion is assumed to be zero at a nodal point.
The NETSTARS model adopts some good ideas of CHARIMA and GSTARS models to develop a
powerful tool for resolving the problems of unsteady sediment process in a channel network. Since the
NETSTARS model is a 1-D model, secondary current and local scour can’t be simulated. But, thetransverse bed evolution can be computed due to using the stream tube concept. Considering the tube
boundary for satisfying equal conveyance requirement across the channel, the transverse bed evolution
can be estimated. Besides, adding transverse transport term into the convection-dispersion equation, thesuspended load would be simulated.

3.3.3 Physical model
In addition, a physical model was also constructed to verify the venting efficiency as shown in
Figure 10. In this study, the Froude similarity was adopted to scale the dimensions of flow patterns dueto the free surface condition. According to the specific ratio (X-Y-Z scale =  =50) of physical model,
the theoretical scale of flow patterns were derived from similarity process and the results were listed inTable 1. The used grain size of physic sediment is the same as field because of the relaxation time of
0.02mm which grain size was deposited in physical model area is smaller than 10-4 and this value ismuch smaller than the flow time scale (Manninen et al., 1996). That means during experimental
duration, the settling performance of fine sediment can be ignored. Besides, according to the similarity
using eq. (1), the sediment concentration ratio would become to 1. Therefore, the mainly ratio andscale were listed in Table 1.
Table 1 Theoretical scale of flow patterns and sediment properties
Items

X ,Y scale[m]

Z scale
[m]

Ratio

X

Z

Scale

50

50

Flow patterns
Water
Velocity
level
[m/s]
[m]

Sediment properties
Discharge[m3/s]

Time[s]sediment
size[m]Sediment
concentration[]

ZX1/ 2

X 5 / 2

X1/ 2

1

1

50

50

505 / 2

50

1

1

30

Accumulative venting efficiency(%)

Venting efficiency_S.

25%

Venting efficiency_P.

25

Spillway_S.
Spillway_P.
Tunnel spillway_S.

20

Tunnel Spillway_P.
Power plant intake_S.

15Power plant intake_P.

10
5
0
0

10

20

30
Time (hr)

40

50

60

Figure 10. Experimental area with outflow structures of physical model Figure 11. Venting efficiency of existed facilities

4. RESULTS AND DISCUSSION
4.1 Venting efficiency and numerical model verification
According to the numerical simulation and physical experiment, the figure 11 shows the total
venting efficiency of existed structures of objective case, Typhoon Sinlaku, and the value is about
25% which value is from the experiment results of Typhoon Sinlaku and compared to the numericalmodel. In figure 11, the venting efficiency belongs to spillway, tunnel spillway and power plant intake
are about 15%, 8% and 2%, respectively. This means that the deposition volume is about 75%.
Therefore, according to the frequency analysis and assumed outflow venting efficiency of other return
period have the same value of Typhoon Sinlaku, the total inflow sediment volumes were obtained astable 2 listed. According to the historical measurement of deposition volume (Figure 3), the annual
average deposition volume after Chi-Chi earthquake is about 3.33*106m3 and this value is 3.58 year
return period of sediment. This means the incoming sediment volume do not accompany withhydrological condition anymore due to earthquake impact. Therefore, owning to designed purpose was
Typhoon Sinlaku condition, the 3.58 year return period of sediment is chosen and 50 year return
period of clear water is adopted due to objective inflow discharge is about 1493 m3/s that value wasrecorded from Typhoon Sinlaku and deposited sediment volume was closed to annual average.
Besides, this study employed sediment transport equations and historical hydrological record with2005 and 2008 river bed investigation to verify 1D numerical model at upstream area. Based on thesimulation results in figure 12, the sediment transport equation of Meyer-peter & Muller formula was
the best fitting to the field measurement and the manning N was ranged from 0.08 to 0.1. Therefore,
the coefficients that had been decided can be used to simulate the bed evolution of upstream river andbypass efficiency for selected bypass case.
Table 2. Performance of frequency analysis value
Deposited sediment volume(106m3)
Returnperiod(year)
2
3
3.58
4
5
10
20
30
50
100
200

Inflow sedimentvolume(106m3)

Inflow water discharge(m3/s)

Gumbeldistribution

Normaldistribution

Lognormal
distribution

Gumbeldistribution

Taiwan Electric Power
Company (2009)

2.17
3.00
3.33
3.53
3.92
5.08
6.19
6.83
7.63
8.71
9.78

2.50
3.35
3.65
3.83
4.16
5.03
5.75
6.13
6.56
7.10
7.59

1.64
2.47
2.86
3.12
3.66
5.55
7.85
9.39
11.57
15.00
19.01

2.90
4.00
4.44
4.71
5.23
6.77
8.26
9.11
10.17
11.61
13.04

531

851
1063
1266
1529
1726
1923

4.2 Bypass efficiency
According to the designed bypass discharge of Asahi was 140 m3/s which value was small than 1
year return period and had high performance of sediment bypass effect, a similar conceptual idea wasadopted to implement in Wushe reservoir and bypass discharge 450 m3/s that value was closed to 2
year return period (531 m3/s) and 10.5 m height of check dam was designed at section 54. Based on
the 50 year return period of discharge and 3.58 year return period of sediment, the performance of
bypass using 1D numerical model showed that 71% inflow sediment would be bypassed to thedownstream and 21% sediment would over flow into the reservoir. This means that about 8% sediment
would deposit at the upstream of check dam.

4.3 Desiltation performance
Based on the results of sustainable research, the reservoir life can be prolonged due to desiltation
strategies (as shown in figure 13). The effect of Chi-Chi earthquake in Wushe reservoir led to the life
reduced from 110 year to 36 year. But, according to the desiltation strategies, the reservoir life would
cover to 214 year using bypass system. It shows obviously improvement of reservoir life by using
desiltation strategies. But, there is 21% deposition volume need to deal with by valuable desiltation

strategy. The Figure 2 indicated that the Wushe reservoir also suitable for replenishment strategy todeal with sediment. As a result, the remained sediment volume can be take care by sediment
replenishment. Besides, according to the experience in Japan and Taiwan (Central water resources
bureau, 2009), up to 10% replenished sediment of annual deposition volume in reservoir had beensuccessful implement. Therefore, a reliable experiment of more than 21% replenished sediment shouldbe planned to test. However, according to the historical excavation volume of Wushe reservoir isabout 1x105m3, the same execution volume is adopted to discuss reservoir life in this study.
The figure 13 shows the effects of desiltation strategies. When sediment replenishment strategy is
executed, the reservoir life would prolong from 36 year to 46 year after Chi-Chi earthquake. If bypass
system and sediment replenishment are both implemented to reduce sedimentation, the reservoir life
would prolong from 36 year to 250 year. This effect shows that 2 times original designed reservoir life
would present. However, there is still 16% sediment remained in the reservoir. The detail of desiltation
strategies were listed in Table 3. Based on the construction schedule, the designed bypass tunnel wasplanned to execute from 2023 year. Therefore, when bypass tunnel was constructed, the remained
reservoir capacity would become to 10.50x106m3. Moreover, if replenishment was implemented from2011 year, the remained storage in 2023 year is about 11.80 106m3. Besides, according to the sedimentrouting after bypass system is executed at upstream river of 50 year return period(figure 14), the local
scour was happened at downstream of check dam and delta deposition at the upstream of back water area.
It is about 2 m degradation and aggradation of local scour and delta deposition location, respectively. In
addition, the effect area of downstream local scour is about 600m. Therefore, for the stability of river bed,
the degradation control should be covered during this distance to protect check dam.

Res erv o i r Li fe=CA P/ MAS

1000

Effects by sediment management options(Bypass, Replenishment, Bypass+ Replenishment)100
W u S h e ( b e f o r e-CChhi ie a r t h q u a k e )W u S h e ( a f t e r-CChhi i e a r t h q u a k e )Impact ofC h -Ci hiearthquake

W u S h e ( a f t e-CC hh ii e a r t h q u a k e ) _ B y p a s s
W u S h e ( a f t e r-CChhi i e a r t h q u a k e ) _ R e p l e n i s h m e n t
W u s h e ( a f t e r-CChhi i e a r t h q u a k e ) _ B y p a s s + R e p l e n i s h m e n t

10
0.1

1

10

C a p a c i t-inf
y l ow r a t i o = C A P /M AR

Figure 12. Verification of 1D numerical model at upstream river

Figure 13. Evolution of Wushe reservoir
characteristics due to desiltation strategies

1035

1030

Section 54

Elevation(m)

1025
1020
1015
1010
Original bed elevation

1005

Water elevation of 50 year return period
Bed elevation of 50 year return period

1000
4700

4950

5200

5450

5700

5950

6200

6450

6700

6950

7200

Distance from dam site(m)Figure 14. Water elevation and bed elevation after bypass operation of designed flood

Table 3. Detail information of desiltation strategies in Wushe reservoir

Items

2010
Capacity(106m3)

Construction
finished(year)

No strategy
Bypass

53.79
53.79

—
2023

Replenishment

53.79

2011

Bypass + Replenishment
53.79
2023+2011
Remark：1.Annual incoming sediment volume 4.44*106 m3
Remark：2.Venting efficiency of existed facilities 25%
Remark：3. Flushing efficiency of Bypass 71%
Remark：4. Sediment replenishment volume 1x105m3

Capacity
whenConstruction
finished(106m3)
—
10.50
11.80
(2023year)
11.80

BeforeDesiltation strategy
AverageannualReservoi
depositionrvalue(106m3 life(year)
)
3.33
36
3.33
36

After
Desiltation strategy
AverageannualReservoir
deposition
life(year)
value(106m3
)
3.33
36
0.70
214

3.33

36

3.23

46

3.33

36

0.60

250

5. CONCLUSIONS
Due to the serious sedimentation problem in Wushe reservoir after Chi-Chi earthquake in 1999
year, desiltation strategies were investigated to obtained sustainable operation from 2009 year.
Therefore, in this study, the suitable desiltation strategies of bypass system with venting strategy had
been classified and planned to implement in Wushe reservoir. Based on the historical data ofdeposition volume and frequency analysis method, the Gumbel estimation was adopted to investigate
deposited sediment volume. The fine sediment transport equation is employed to generate the quantityof suspending load and bed load using inflow hydrograph and transport sediment is also classified byfield grain size survey. The valuable venting efficiency of existed facilities were obtained fromexperiment and calibrated from 3D numerical model. Therefore, the total inflow sediment volume wascombined with deposited sediment and outflow sediment. Besides, according to the field survey ofdesiltation strategies in Japan, up to 10% sediment replenishment can be executed, more than 80%
moved efficiency can be implemented by bypass strategies, and almost 100% transported efficiency
can be implemented using drawdown flushing strategies. It shows that the flushing strategy possesses
highest moved efficiency to move out deposited sediment in a reservoir. However, the drawdown
flushing strategies can not be executed in the objective reservoir. Therefore, based on the efficiency of
bypass and sediment replenishment, the divert sediment volume were calculated from 1D numericalmodel and historical valuable excavation volume, respectively. According to the study results, thebypass efficiency is about 71% of objective case and the reservoir life is prolonged from 36 year to
250 year after bypass and sediment replenishment countermeasures is executed.
The presented study was financially supported by the Taiwan Power company. Writers wish to
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