Sediment Replenishment on Floodplain Downstream of a Reservoir

Sumi, Tetsuya
University of Kyoto, Kyoto, Japansumi.tetsuya.2s@kyoto-u.ac.jp

Lee, Fong-Zuo, Lai, Jihn-Sung, Tan, Yih-Chi & Huang, Chen-Chia
National Taiwan University, Taipei City, Taiwan

ABSTRACT:The annual mean loss of reservoir storage due to sedimentation has been higher than the increase of capacity by construction of newreservoirs in many countries around the world. The sustainable usage of the reservoir is always an important issue for reservoir
long-term operation. By dredging or excavation, sediment deposits may be mechanically removed from reservoirs due to lack of
low-level outlet or flushing facilities. The characteristics of deposits, available disposal site, reuse options and environmental criteria
will affect the feasibility of sediment deposit removal. Recently, the replenishment technique has been implemented to prevent
downstream river from bed degradation and increase suitable living habitat for aquatic animals downstream of the dam. The
replenishment should be set before flood discharge released from reservoir. The released discharge and location of replenishment
should be investigated before the operation. A physical model was constructed to investigate the flow and replenished cohesive
sediment transport downstream of the Afterbay weir. In this research, the experimental results show that the flushing discharge does
not dominate the flushing ratio of sediment replenishment volume. More important parameter of flushing ratio is the flushingduration. Besides, the affection duration of water quality is about 1 hour and the investigated suitable replenishment volume is alsopresented in this study.
Keywords: Flushing, reservoir sediment, replenishment

1. INTRODUCTIONSWhen typhoon and heavy rainfall coming, the watershed
possesses steep slope would lead to soil washout andland collapse. Artificial development would make soil be
disturbed and increases frequencies of huge sediment be
flushed. Taiwan is situated at a such geographicallocation with special climatic condition that brings to theisland 3.6 typhoons per year on the annual average.
These typhoons often result in flood disasters that cancause serious damage to properties and sometimes withsevere casualties, especially after 1999 year Chi-Chi
earthquake. Large sediment would be flush out due tocollapses. However, Typhoons also bring heavy rainfall
as a valuable water resource in Taiwan. The Shihmen
reservoir acted as the primary and most important
infrastructure of water resource development over the
past forty years in northern Taiwan. As livelihood andindustrial water needs are increasing, the water supply
demand of the Shihmen reservoir is rising. The Shihmen
reservoir has a natural drainage area of 762.4 km2. It is
formed by the Shihmen dam located at the upstream
reach of the Dahan River and flows westward to the
Taiwan Strait. A map of the watershed area of the

Shihmen reservoir is presented in Fig. 1 (Lee et al., 2007).
The Shihmen dam constructed in 1963 is a 133.1m highand 360m wide embankment dam with spillways,
permanent river outlet, power plant intakes and flood
diversion tunnels controlled by tailrace gates. The
elevations of the spillway crest, permanent river outlet,
power plant intakes and flood diversion tunnels are
EL.235 m, EL.169.5m, EL.173m and EL.220m,
respectively. The design capacity of the three spillways is
11,400m3/s, the permanent river outlet is 34m3/s, thepower plant intake is 137.2m3/s, and flood diversion
tunnel is 2,400m3/s. With a maximum water level of
EL.245m, the reservoir pool is about 16.5km in lengthand forms a water surface area of 8.15km2. The initialstorage capacity was 30,912x 105 m3, and the active
storage was 25,188x 105 m3. In 2010, the capacity of
Shihmen reservoir remained 67% storage.
In 2004, Typhoon Aere attacked Taiwan and generated
more than 973 mm rainfall within 4 days in thewatershed of the Shihmen reservoir. Highly turbid
inflows affected the water supply system seriously andthe water supply suffered from a shortage for 14 days.
Sediment concentration of the inflow water duringDue to unique geological condition with extremehydrological climate and lack of venting facilities in
Shihmen reservoir, massive sediment would deposit inreservoir. Field survey showed the particle size inreservoir was small (Fig. 2) except backwater region.
The sediment classified sand at delta and during dam site
was silt and clay which diameter is less than 0.075 mm.
Therefore, the fine sediment deposition which near damsite would reduce reservoir storage and affect intake
operating period. In general, the excavation is adopted todeal with coarse material which located at upstream anddredging is adopted to deal with fine sediment which
located after delta. But, the characteristics of deposits,
available disposal site, reuse options and environmental
criteria will affect the feasibility of sediment deposit
removal. Based on transportation and treatment
efficiency, these two treatments spend much money tomove out sediment and drainage volume is limited.
Recently, the replenishment technique has been
implemented to prevent downstream river from bed
degradation and increase suitable living habitat foraquatic animals downstream of the dam. So, found out
sediment movement phenomenon in reservoir andimproved the occasion to desiltation were important.
This study based on the literature review in Japan andTaiwan to investigate replenished efficiency and theeffects of flushed sediment concentration.

Figure 1. Watershed area of Shihmen reservoir(Lee et al.,2007)

Percentage(%)

Typhoon Aere rose to 326,700 mg/L and this value was
far-exceeded water treatment capacity which can only
handle 6,000 ppm (Tan et al., 2009). Following,
typhoons such as Typhoon Haitang (2004) and Typhoon
Talim (2005) also brought high sediment concentration
inflow into reservoir to cause serious deposition problem
and water supply shortage. Therefore, the Water
Resources Agency of government had established atemporary pumping station with capacity 3x105 CMD(m3/day) on the top of the Shihmen dam to divert nearsurface lower-turbidity water that the water treatment
plant can accommodate people with clear water.
Recently, The temporary pumping station was installed
after Typhoon Aere(2004) and until now it also can beworked for emergency operating. In addition, due toserious sediment deposition problem from 2004 induced
by Typhoon AERE, the stratified withdraw facility wasbuilt at dam site to avoid the lack of public water and theone of tunnel of power plant was designed to ventturbidity current. The stratified withdraw facility withthree elevations (El. 220m, 228m, 236m) was finished at
Dec. 2009. In order to implements the sediment flushing
operation using existed and modified outlet structures,the feasibility study of Lake JhongJhuang engineering at
downstream reach of Yuanshanyan water intake forbackup water resources during Typhoon event was
executed in following years.

~0.005mm

Diameter(mm)Figure 2. Sediment grain size diameter

2. EXPERIENCE IN JAPAN AND TAIWANIn Japan, the replenishment technique had been
implemented to prevent downstream river from bed
degradation and increase suitable living habitat foraquatic organism at downstream of the dam. Okano et al.
(2004) also started to study reservoir sedimentation
management by coarse sediment replenishment below
dams and summarized sediment replenished project inJapan. Such as Tenryu, Otakine, Abukuma, Ara, Oi,
Naka, Kuzuryu, Yodo, Kanna, and Tone had been
constructed by Ministry of Land, Infrastructure andTransport (MLIT). Seto et al. (2009) investigatedreplenished sediment effects on the downstream river of
Yahagi dam. Sumi et al. (2009) produced appropriate
grain size with less turbidity using sediment treatment
system. Kantoush et al. (2010) analyzed the interactionof relative flow field and morphological evolution during
replenishment experiments in Kizu River. Fig. 3 shows
the field test of replenishment in Kizu River.

Figure 3.

Evolution of sediment replenishment experiment in
Kizu river[Kantoush et al. 2010]

Transportationroute

Water quality
measurement
point1

Shihkang
Dam
Replenishedarea
Water quality
measurement
point2

Figure 4. Replenished area and observation points
Typhoon Morakot
Replenished area
Norma l water level

Figure 5. Sketch map of replenished sediment situation
6000
5000

4500

Discharge
Water quality measurement 1
Water quality measurement 2

4000
3500

4000

3000
Supposed

2500

3000
2000
2000

1500
1000

Sediment concentration(ppm)

In Taiwan, replenishment was implemented in Shihkang
reservoir in 2009 (Central water resources bureau, 2009).
It is the first time to test replenishment strategy in field of
Taiwan. Shihkang reservoir was choose to test due toefficient water discharge and replenished environment.
Fig. 4 shows the sketch of replenished arrangement
around Shihkang reservoir. The excavation area locatedat upstream which area is provided with coarse material
and replenished area located at 200m downstream reachof Shihkang dam. Total replenished volume is about
50*103m3 and replenished duration is about 14 days.
Before typhoon was coming, the replenishment wasstarted to set up at flood plain as show in Fig. 5. Fig. 5
shows that the replenished sediment would not affect thewater quality at normal duration and during flood period,
the replenished sediment would be flushed. According to
the field observation, 1 month later, a huge rainfall was
occurred from typhoon Morakot and generated 5,400
m3/s peak flow discharge around this area. Figure 6
shows the hydrograph of inflow discharge and thesediment concentration. It is about two weeks later,
based on the field survey results, there is about 3%
sediment remained and the diameter of most remainedmaterial is more than 30cm (Fig. 7). Based on themonitoring data of sediment concentration, before
typhoon was coming, the turbidity in main channel was
not changing at measurement point 1 and 2 (Fig. 6). But,
during typhoon duration, the turbidity was reach to 3500
ppm and 4000 ppm at water quality measurement point 1
and 2, respectively. However, 1 week later, the turbidity
at point 2 of water quality measurement was decreasing
to 10% of peak turbidity during typhoon Morakot event.
The field experiment shows that replenished sedimentcan be flushed to the downstream reach by natural flood
which peak discharge is higher than 5,400m3/s andreplenished sediment which constructed at flood plane
would not cause water quality changing during normal

time. Refer to the experience of replenished volume inJapan is about 10% of annual deposition volume inreservoir, the similar replenished percentage of coarse
material in Shihkang reservoir is adopted to replenish
which value is about 8%. Until now, up to 10% of
replenished sediment is been proof that it can be
implement and the action does not affect water quality
before flood comes due to discharge control by reservoir
during normal condition.

Q(m3/s)

Before flood was coming, the sediment had been
constructed at the downstream of Nunome reservoir andafter flood was passing, the replenished sediment was
flushed to the sea. Such field test in Japan focused oncoarse material that deposition in the reservoir andreplenished at downstream for preventing bed
degradation and creating suitable environment foraquatic livings. Furthermore, the replenishment strategy
needed huge discharge to flush replenished sediment,
therefore, replenished sediment always constructed
before flood coming and implemented during wet season.
In Japan, it is common to practice and remove
accumulated coarse sediment by excavation and dredging
in reservoir. Making effective use of the removed
sediment possesses economic value and sediment
replenishment method is one of new measures ofsediment management. In this method, deposited
sediment in reservoir can be periodically excavated andthen transported to be placed temporarily downstream ofthe dam at floodplain. Replenished volume decided
according to the sediment transport capacity of thechannel and the environmental conditions.

1000
500
0
0
2009/7/11 2009/7/18 2009/7/25 2009/8/1 2009/8/8 2009/8/15 2009/8/22
Time

Figure 6. Impacts of replenished sediment and dischargehydrograph

Figure 7. Remained sediment after Typhoon Morakot

3. PHYSICAL MODEL TEST
Although the replenishment of coarse material is
successes to implement, but, in Shihmen reservoir, it isdifficult to execute due to transportation limit and coarse
material is more valuable to be construction material. As
a result, only dredging material could be employed andthe other reason is course from the deposition pools (Fig.
8) which below the reservoir is almost filled up. In orderto solve the disposal area problem of dredging material,
replenishment strategy was considered to create a new
method for dealing with fine sediment due to limited
disposal space. According to the literature review, finematerial which possesses colloidal property is difficult toflush and move. Fig. 9 (Fortier and Scobey, 1926) and
Fig. 10 (Vanoni, 1977) show the results of thresholdvelocity for various sediment. Based on the results ofliterature experiment in Fortier and Scobey (1926) and
Vanoni (1977), the fine sediment (Fig. 2) which from the
Shihmen reservoir belongs to silt and clay needs morethan 1.5 m/s velocity to flush. Besides, the dragging
material is in liquid state which includes water and finesediment. Therefore, the replenished material needs filter
structure to make sure that replenished material could notaffect the water quality of main channel during normal
duration. According to the field survey of upstream
material, the diameter is about 4.5mm that belongs toaccumulative 80% volume of coarse sediment at Lofu
excavation area. Refer to the threshold velocity of 4.5mm,
the flushed velocity is about 1m/s which value is small
than colloidal material. Therefore, the coarse material of
Lofu was adopted to construct filter structure aroundreplenished area. In addition, according to the numericalsimulation results, the flow velocity reaches 1 m/s during
1700m3/s discharge at planned replenishment area (Fig. 8)
which value belongs to 2 year return period.
Possibility
replenishment
area
Main
channelDisposal area
Experimental
replenishmentarea

Physicalmodel area
Reservoir
area

Figure 8. Experimental area and surrounding environment

1.52
m/s

Figure 9. Threshold velocity [Fortier and Scobey, 1926]~1.5m/s~1.0m/s

Figure 10. Incipient velocity [Vanoni,1977]

Therefore, 500 m3/s, 1700 m3/s, 3500 m3/s and 6100 m3/s
were selected to be inflow discharge cases and thosevalue were 2 year, 5year and 20year return period, except
500 m3/s. On the other hand, the water depth is from
2.1m to 2.36m in replenishment area by numericalsimulation result. The height of filter structure sets to
2.0m and replenished height sets to 1.6m. Fig. 8 shows
the arrangement of replenishment and sketch of aroundenvironment and physical scale ratio is 1/64. The
possibility replenishment area was choosing at flood
plain and experimental replenishment area was adopted
at suitable location. Protection bank and possesses
enough space to replenish are two reasons to selectphysical model area. The solid line shows thereplenishment area is within physical model area, theupstream boundary is from some part of afterbay togenerate uniform inflow discharge and downstream
boundary is at narrowest section which flow mechanism
belongs to super critical condition.
From the report of Northern water resources bureau(2010), the water content of dredging material is from
39.9% to 45.2% after disposal 1 year. Therefore, 40% ofwater content was adopted to test the replenishment.
Moreover, based on flushing similarity, the flushingmechanism of physical model can be calculated byfollowing equation (Lai, 2001):

uc =

a d

b

Table1 Flushing efficiency and experimental items(1)

500 m 3 /s
Flushing
duration (hr)

500 m 3 /s
Flushing
ratio(%)

1700 m 3 /s
Flushing
duration (hr)1700 m 3 /s
Flushing
ratio(%)
67

Where, uc =critical flow velocity and  d = dry density.

Items

Lai (1998, 2001) investigated parameter a and b using
fine sediment of Akungtien reservoir, and result in 1.65
and 1.96. This result is used for this study due to similar
diameter of fine material between Shihmen and
Akungtien reservoir. Besides, according to thehydrograph of historical flood, the duration of discharge
500 m3/s, 1700 m3/s, 3500 m3/s and 6100 m3/s wereconsidered in this research. Fig. 11 shows thearrangement of replenished area. The sediment filter
which constructed by coarse material from Lofu
excavation was set around replenished area to keep thewater quality of main channel.

Experimental
values8428

Estimated values

38

96

14

81

Items

3500 m 3 /s
Flushing
duration (hr)3500 m 3 /s
Flushing
ratio(%)

6100 m 3 /s
Flushing
duration (hr)6100 m 3 /s
Flushing
ratio(%)

Experimental
values

8

84

1

87

Estimated values

5

79

1

87

Fig. 12 presented the variation of flushed sedimentvolume and it shows that after 3 hours, the flushed
sediment ratio possesses liner tendency.
120000110000

Sediment volume(m3)100000

Sediment
filter

90000
80000
70000
60000
50000
40000
30000

Replenished sediment volume
Flushed sediment volume_Q=500
Flushed sediment volume_Q=1700
Flushed sediment volume_Q=3500
Flushed sediment volume_Q=6100

20000

Replenishedsediment

100000(a)Upstream of replenishment

0

1

2

3

4

5

6

7

8

Time(hour)MainchannelFigure 12. Variation of flushed sediment volume

Replenishedsediment(b)Middle of replenishment

The first 3 hours dominates the main flushing process
and possesses higher erosion efficiency. Therefore, theestimated flushing ratio was estimated from 4 hours to 8
hours. Fig. 13 shows the equations of regression results
between discharge 500 m3/s and 1700 m3/s.
120000110000

Replenishedsediment(c)Downstream of replenishment

Figure 11. Sketch of experimental set up and arrangement

Sediment volume(m3)10000090000
80000
y = 2186.2x + 44991
R² = 0.9745

70000
60000

y = 1672x + 25944
R² = 1

50000
40000
30000

Replenished sediment volume

20000

Flushed sediment volume_Q=500

10000

Flushed sediment volume_Q=1700

03

4. RESULTS AND DISCUSSION
Table 1 lists the information of experience items.
Flushing durations of experimental values in table 1 werebased on the analysis of historical record. But, it is better
flushing efficiency cases, the same test duration was
choose to analysis from discharge 500 m3/s, 1700 m3/s
and 3500 m3/s due to the limited duration of 3500 m3/s is
8 hours. In addition, according to the recent 10 years,
more than 6100 m3/s is only 1 hour, therefore, such caseonly implemented for 1 hour in this experiment.
However, the estimated value was also presented inTable 1 from 8 hours results.

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

Time(hour)

Figure 13. Tendency of flushing rate

The table 1 lists the estimated values of discharge 500
m3/s and 1700 m3/s. The data of table 1 presents thehigher flushing ratio due to higher flushing discharge inexperimental values. But, according to the happened
duration of each discharge, the flushing ratio possesses
different sequence. The best flushing efficiency isdischarge 500m3/s, the second is discharge 6100 m3/s, thethird is discharge 1700 m3/s and the worst is discharge
3500 m3/s. This means that the first suitable discharge is1700 m3/s due to more confidence and easier to reach theobjective. Fig. 14 shows the variation of outflow
sediment concentration from 500 m3/s to 6100 m3/s. Dueto the sediment filter which as an armour effect, theearliest peak sediment concentration is discharge 6100
m3/s and the latest is discharge 500 m3/s due to water
discharge energy. However, due to water quantity, thehighest sediment concentration is discharge 500 m3/s andthe lowest is discharge 6100 m3/s. Besides, thehydrograph shows that the main variation was obviously
happened in first hour. After 1 hour, the sedimentconcentration is lower than 6000ppm which value can betreated by water treatment facilities. Combining theresults of Fig. 12 and Fig. 14, the flushing impacts
presented obviously effects at first two hours. The Fig.
12 also shows that during the test period, the replenishedsediment cannot be flushed 100%. Therefore, the suitable
replenished volume should be reduced to 60000 m3 toreach optimal flushed ratio.
30000
Q=500

27000

Q=1700

Q=3500

Q=6100

Sediment Concentration(ppm)

Treatment limitted~23000ppm

24000
21000
18000
15000
12000

~1hour

9000
6000
3000
0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Time(hour)

Figure 14. Tendency of flushing sediment concentration

5. CONCLUSIONS
According to the experience of replenishment strategy inJapan and Taiwan, coarse material was usually adopted
to implement for preventing downstream river from bed
degradation and increase suitable living habitat foraquatic organism at downstream of the dam. In recent
years, up to 10% of replenished sediment was been proof
that it can be executed and the performance did not affectwater quality before flood coming due to replenishedarea was at flood plain. However, in this study, the finesediment was first time to try to be replenished inphysical model of Shihmen reservoir. The flushingsimilarity, flushing discharge, replenished area,
replenished volume and operation duration were bothconsidered in this study. Based on the experimental
results, the peak sediment concentration timing is result
from water discharge. On the other hand, due to water
quantity, the highest sediment concentration is discharge
500 m3/s and the lowest is discharge 6100 m3/s. Besides,
combining the results of variation of flushed sedimentvolume and tendency of flushing sediment concentration,
the main effect of water quality was obviously happened
in first 2 hours. After 2 hours, the sediment concentration
was reduced to 6000ppm and such water turbidity can betreated by water treatment facilities. From flushingefficiency and the variation of flushed sediment volume,the suitable flushed discharge was suggested to 1700m3/s
and replenished sediment volume was presented to60000m3 due to more confidence and more easy to reachthe objective.

ACKNOWLEDGEMENT
The presented study was financially supported by the Northernwater resources bureau, Water Resources Agency Ministry of
Economic Affairs. Writers wish to thank the National Taiwan
University Hydrotech Research Institute and Disaster
Prevention Research Institute of Kyoto University in Japan for
manpower and technique supporting.
REFERENCES
Central water resources bureau, Water resources agency,
Ministry of economic affairs (2009) : Report of
implemention and efficiency of replenishment in Shihkang
dam downstream river (Chinese).
Fortier, S., and F. C. Scobey (1926) : Permissible Canalvelocities, Transactions of the ASCE, Vol. 89, No. 1, pp.
940-956.
Kantoush, S. A., Sumi, T. and Kubota, A. (2010) : Geomorphic
response of rivers below dams by sediment replenishment
technique, River Flow 2010 - Dittrich, Koll, Aberle &Geisenhainer (eds), pp. 1155-1163.
Lai J. S. and Chang F. J. (2001) : Physical modeling of
hydraulic desiltation in Tapu reservoir, Intl. Jour. of
Sediment Research, IRTCES (International Research andTraining Center on Erosion and Sedimentation), Vol. 16, No.
3, pp. 363-379.
Lai J. S., Chang F. J. and Yang, H. C. (2001) : Movable bed
physical modeling of flushing cohesive reservoir deposits,
Journal of the Chinese Institute of Civil & HydraulicEngineering, Vol. 13, No. 1, pp. 99-109.
Lai, J. S. (1998) :「Research of threshhold velocity on finesediment in Akungtien reservoir, Journal of Taiwan Water
Conservancy, Vol.46, No.3, pp.76-83.
Lee, F. Z., Lai, J. S., Hwang G. W. and Tan, Y. C. (2007) :Influence of High-turbid Inflow on Irrigation and Water
Supply in the Shihmen reservoir, 2007_58th ICID IEC
Meeting and Fourth International Conference on Irrigation
and Drainage.
Northern water resources bureau, Water resources agency,
Ministry of economic affairs (2010) : Report of arrangement
and planning on depsition silt in Shihmen Reservoir.
Okano, M., Kikui, M., Ishida, H. and Sumi T. (2004) :Reservoir sedimentation management by coarse sediment
replenishment below dams, Proceedings of the Ninth
International Symposium on River Sedimentation, Yichang,
China, Volume Ⅱ , 1070-1078.
Seto, K., Sakamoto, T. and Suetsugi, T. (2009) : Sedimentcontrol measures and improvement effects of physicalcondition and environment by sediment flushing – a casestudy in the Yahagi dam. International Congress on Large
Dams, Brasilia, Q.89-R.3.
Sumi, T., Kobayshi, K., Yamaguchi, K. and Takata, Y. (2009) :Study on the applicability of the asset management forreservoir sediment management, International Congress on
Large Dams, Brasilia, Q.89-R.4.
Tan, Y. C., Lai, J. S. and Lee, F. Z. (2009) : A study oftyphoons induced high serious sediment concentration
deposition into a reservoir and reduced the water supply in
Taiwan, 60th IEC Meeting and 5th Asian Regional
Conference, New Delhi, India.
Vanoni, V. A. (1975) : Sedimentation Engineering, ASCE Task
Committee for the Preparation of the Manual on
Sedimentation of the Sedimentation Committee of theHydraulics Division(Reprinted 1977).