River Flow 2010 - Dittrich, Koll, Aberle & Geisenhainer (eds) - © 2010 Bundesanstalt für Wasserbau ISBN 978-3-939230-00-7

Geomorphic response of rivers below dams by sediment
replenishment technique
S. A. Kantoush & T. Sumi

Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University, Japan

A. Kubota

Kizugawa Dams Integrated Operation & Management Office, Japan Water Agency, Japan

ABSTRACT: Sedimentation problems on regulated rivers are considerably different from natural rivers.
An effective means of reservoir sediment management is to excavate deposited sediments within reservoir and transport to the river downstream of a dam. The flood is designed to erode the placed sediment at
the banks of Nunome River, in an effort to restore the lower reach of the river to a more natural ecosystem. Since 2004, six attempts of sediment replenishment to rebuilding vital sediment sand bars below
Nunome dam are in progressing. Monitoring measurements pre- and post- replenishment show the effectof improvements in riverbed formation, grain size distribution, and erosion processes. Actual field experiments are presented and compared with scaled laboratory experiments, with the objective of identifying
parameters and process of sediment replenishment characteristics. The paper investigates the influence of
sediment replenishment below Nunome Dam on changes of river bed. Appropriate sediment materials,
which are excavated and treated from the upper part of the Nunome reservoir, and supplied to downstream rivers contributes to rebuilt sand bars, mitigate the armouring of river bed, and management of reservoir sedimentation. With the field experiments, the processes are directly visible and a wealth of valuable data is obtained, and will be used for numerical modeling in future application.
Keywords: Dam impacts, Reservoir sediment replenishment, Sediment management, River restoration,
Revitalization of regulated rivers, Sediment feeding, Geomorphic response

1 INTRODUCTION

Dam Impacts on:Morphological

1.1 Dam Impacts
Dams greatly influence flow and sediment discharge regimes and can have significant impacts
on downstream reaches below dam. Bed load sediment management is required to preserve thecapabilities of water resources facilities and toconserve the environment in rivers downstream of
the reservoir. The sediment deficit produces alarge variety of physical, ecological and environmental effects. To mitigate that coarse sediments,
by grain diameter, with specific grain size distributions artificially replenished to satisfy the river
bed load capacity (sediment replenishment). Figure 1 classifies the impacts of sediment deficit below dams to three groups; morphological, hydrological, and ecological effects. Sediment deficit is
not only an environmental issue but also a socioeconomic problem, for instance due to loss of reservoir capacity (Fan and Springer, 1993).

Hydrological

EcologicalRiver bankinstability

Water table lowering

Loss of aquatic andriparian habitats

Bed armouring

Reduction in overall flows

oxygen stratification

Changes in flood
frequency and magnitude

Nutrient exchange

Changes in seasonal flow

Gas-bubble disease

Changes hydrodynamic
within estuaries

Adverse effect onwater quality

River incision andbed degradation
Impacts on
infrastructures
Sediment deficit tocoastal zones

Figure 1. Possible impacts due to lack of sediment transport
in the downstream reach below dams

In addition, dams alter the downstream flow
regime of rivers (Williams and Wolman, 1984),
which controls many physical and ecological aspects of river form and processes, including sediment transport and nutrient exchange (Poff et al.,1997). Morphological effects on the river channel1155

(e.g., Kondolf and Matthews, 1993; Shields et al.,2000) that includes riverbed incision, riverbank
instability, upstream erosion in tributaries, damage to bridges, embankments and levees (e.g.,
Kondolf, 1997), and changes in channel width(e.g., Wilcock et al., 1996). Hydrological effects
caused by dams include changes in flood frequency and magnitude, reduction in overall flows,changes in seasonal flows, and altered timing of
releases (Ligon et al., 1995).

geometry are influenced by variability in waterand sediment release. Sediment replenishment
scenarios may, induce undesirable morphologicaland ecological consequences as well as significant
channel adjustments that can result in failure of
the restoration project itself. That is, it is necessary to better understand reversibility, direction
and time scale of changes, and the sustainabilityof a replenishment intervention before it is implemented.

1.2 Sediment Replenishment Projects in Japan

1.3 Objectives

Large Japanese rivers are often trained to a large
extent, to maintain services such as navigation,hydropower generation and flood defense. Okano
et al., (2004) summarized sediment replenishment
projects in Japanese Rivers, such as Tenryu, Otakine, Abukuma, Ara, Oi, Naka, Kuzuryu, Yodo,
Kanna, and Tone, have been conducted by Ministry of Land, Infrastructure and Transport (MLIT).
Kantoush et al. (2010) investigated the morphological evolution and corresponding flow field
during replenishment experiments in Uda River,Japan. Sediment treatment system is applied bySumi et al. (2009), to produce appropriate grain
sized material with less turbidity. Seto et al.
(2009), analyzed sediment replenishment effects
on the downstream river of Yahagi dam.

The study focuses on the design criteria for sediment replenishment project in Japan to improveriver bed conditions. The investigations are basedon field and laboratory experiments. The objectives of this paper are: (a) to make a general review of impacts of dam on downstream reach; (b)to discuss the role of different factors influencing
the sediment replenishment research; (c) to illustrate and analyze the sediment replenishment
through a regulated fluvial system during actual
field tests.
2 FIELD EXPERIMENTS FOR SEDIMENTREPLENISHMENT IN NUNOME RIVER
2.1 Test Procedure and Replenishing Concept

Factors of Sediment Replenishment
Sediment
PhysicalCharacteristics

Flow PhysicalCharacteristics

River PhysicalCharacteristics

Replenishment
Characteristics

Monitoring
Characteristics

Economics
Characteristics

Porosity anddensity

Flood flow
discharges (Floodhydrographs)

Initial bed
morphology andslopesPlaced sedimentvolume andgeometry shape

Water quality(turbidity, Ph,temperature,

Excavation andsediment
treatment costs

Sediment type andcohesion

Flow water depths

Bed material size,
gradation andpermeability

Submerged or
partial or unsubmerged state

Environmental (fishspecies, stream
invertebrates,
algae)

Transportation cost

Grain size
distribution, andpercentage of finesediment

Flow duration
curve andfrequency

Sediment rating
curve (suspended
and bed loads)

Locations andplacement
arrangement along
the river

Flow field andvertical velocity
profilesTreatment of
excavated anddredged sediment

Flow patterns andvelocity profiles

River bankvegetation

Frequency of
feeding per year
and season

Cross sectional
survey and precise
surveying of riffles
and pools

Erosion rate andtype (side bank orvortex types)

Volume of Trapped
sediment inreservoirs

Indirect (side bankfeeding) or direct(in river) feeding
methodVisual inspection
and grain size
analysis

Sediment source

Feeding cost

Monitoring cost

Project benefits

Figure 2. Main groups of sediment replenishment characteristics and managing factors

Sediment transport and associated channel bed
mobility are recognized as key processes forcreating and maintaining physical habitats, aquaticand riparian ecosystems. In Japan, sediment replenishment projects are undertaking with different
configurations and characteristics of sediment anddischarges. Figure 2 summarizes factors that influence the sediment replenishment research.
These factors are classified in six groups as shownin Figure 2. Several significant gaps in the scientific understanding of these processes remain, particularly concerning how riverbed deposition and

In Japan, it is common practice to remove accumulated coarse sediment by excavation and dredging, and to make effective use of the removed sediment. Sediment replenishment method is one of
new measures of sediment management. In thismethod, trapped sediment is periodically excavated and then transported to be placed temporarily downstream of the dam. In a manner decided
according to the sediment transport capacity of thechannel and the environmental conditions. Therefore, the sediment is returned to the channel
downstream in the natural flooding processes. The
procedure of the experiments consists of four
steps: (1) extracting mechanically the accumulated sediment at check dam; (2) transporting it bytruck to downstream river; (3) placing the sediment with specific geometry (Figure 3), and (4)monitoring flow, sediment, and environmental parameters.
Flood stage

Placed sediment
geometry

Normal stage

Figure 3. Concept of sediment replenishment and placed sediment geometry on the bank of Nunome River
1156

2.4 Case Study of Nunome Dam Actual Sediment
Replenishment Tests

The replenished sediment is placed at such anelevation; in order to reduce the turbidity during
normal flow period. The top of the sediment is adjusted so that the sediment is completely submerged during flood at several times a year andall sediment is eventually transported downstream(Figure 3). Concrete means are being explored,taking into consideration the particle sizes of sediment, such as the scale of flood suitable for thesafe implementation, and appropriate ways toplace sediment.

Since 2004, six experiments of sediment replenishment projects with different sediment volumes
are undertaken below Nunome dam. Tracking thehistory and performance of these projects is helping to understand the evolution of sandbars. Table
2 summarizes the replenishment history, flushingflow and sediments characteristics. The volume ofplaced sediment is limited to several hundreds of
cubic meters each time. To implement this method, consideration has to be given to environmental problems in the lower river basins, to theoccurrence of turbid water, and to safety risks due
to sediment deposition in the channel. The data in
Table 2 indicates that there is a need to increase
the amount of the supplied sediments in Nunome
River every year.

2.2 The Study Area
Yodo River (Yodo-gawa) system is 75 km in
length, located in the central part of Japan, is theseventh largest river basin in Japan with a catchment area of 8,240 km2. Flowing south out of
Lake Biwa first as the Seta River, and then the Uji
River, it merges with the Kizu and Katsura Rivers
near the border between Kyoto and Osaka Prefectures. There are five completed dams in the Kizu
River System, Nunome, Shourenji, Hinachi, Takayama, and Murou dams. Nunome dam is a gravity dam for flood control and water supply, completed on 1994, 16 years ago.

Table 2. History of the sediment replenishment tests downstream of Nunome dam
Year

2004
2005
2006
2007
2008

2.3 Cumulative Sediment in Kizu River Upstream
Reservoirs

2009

ActualPlannedsedimentation sedimentationin 2006
capacity in 100(MCM)*
years (MCM)*
Nunome16
1.9
0.243
Shorenji
36
3.4
1.484
Murou
32
2.6
1.12
Hinachi9
2.4
0.41
Takayama
37
7.6
3.648
* MCM is Million Cubic Meter

29-9-2004
4, 5-10-2005
19, 21-7-2006
23, 29-8-2007
8-7-2008
5, 19 -9-2008
NA
2-8-2009
7,8-10-2009

Volume ofremainedsediment (m3 )
0
0
460
90
0
0*
0
500
0

Volume ofVolume ofplacederoded3
sediment (m ) sediment (m3 )
190
190
540
80
0
370
720
810
100
35
100
100
500
0
0
500
500
500

NA: No placed sediment or No flood occurred

Supplied sediment geometry of 500 m3
Supplied sediment geometry of 100 m3
L: Length; W: Width; t: thickness

500 m3
L =40m

100 m3
W=10m

L = 40 m
t = 0.5 m
W=5m

Figure 4. Dimensions and shapes of two different sediment
geometries placed on the left bank of Nunome River

Table 1. Sedimentation rate of Kizu River upstream dams
Elapsedtime(Years)Flood period* The remained 65 m3 sediment is removed.

Dam reservoirs are normally designed to have acapacity to store 100 years, worth of sediment in
the deepest parts close to the dams. Sediment accumulation in the Kizu River upstream groupdams up to the end of fiscal 2006 are shown in
Table 1. The table shows that sedimentation in all
five dams has progressed faster than that of theoriginal plan. In Japan, the planned sedimentation
capacity is set by the sedimentation of 100 years.
Sedimentation of Nunome dam has alreadyreached 12 to 18% of planned sedimentation. Regarding annual fluctuation of sediment load of
Nunome dam, maximum annual sedimentation ina single year is 230,000 m3.
Name of
Dam

Settingsediment
period28-9-2004
9-8-2005
NA
9-8-200727-6-2008
7-8-2008
12-11-2008
NA
2-10-2009

The increasing amount of the placed sedimentshould be proportional to the bed load transportcapacity of the river. Furthermore, the period andthe discharge of artificial flow flushing should besuitable for the sediment transport capacity. The
natural flood duration for Nunome dam in 2008 is
approximately 10 hours during one day only. Neither the flushing discharge nor the flushing period
is able to transport all the placed sediment volume. Figure 4 shows two geometries with thesame length of 40 m and two reduced widths of 10

Sedimentation
rate
(%)
12.8
43.6
43.1
17.1
48.1

1157

and 5 m, and geometry thicknesses of 1.2 and 0.6m, which have rectangular shape in the plan view.

100
90

Percent finer (％)

80

2.5 Experimental conditions
The test site is placed by the Japan Water Agency(JWA) on the left side bank of Nunome River(Figure 5). The sediment is located in the downstream reach at 300 m from the Nunome dam.
Planning, design, implementation and long-termmonitoring of tests are guided by JWA. Figure 5
shows the location map of the placed sedimentand reservoir shape. Check dam is a small dam
constructed at the upstream end of the reservoir. It
has low trap efficiency for suspended sediments
and traps primarily sand and gravel sediment. In
general, reservoir deposits are attractive sourcesof aggregates to the extent that they are sorted bysize. Designers of replenishments projects are
faced with the challenges of estimating the sediment deficit volume and determine sediment grain
size distribution. Sand can be obtained from thedeposition upstream of check dam.

70
60
50
40
30
20
10
0
0.001

0.01

0.1

1
10
Grain size (mm)100

Check dam

Nunome Dam

Nunomereservoir

Check dam2009 Natural flood

Inflow
Outflow
Storage capacity

Maximum inflow = 188.82m3/s
Maximum outflow = 81.01 m3 /s

Monitoring after flood
Preparation and placing
sediment before flushing

May 2009

June

July

AugustSeptember

Monitoring after
natural flood

October

Storage capacity (x10 3 m3)

Location of
500 m 3 placed
sediment

Figure 7 shows the monthly inflow and outflow
discharges of Nunome reservoir in 2009, with thereservoir storage capacity. The reservoir has thehighest flow discharges during July and October.
There are two monitoring phases; (1) before flood,(2) after flood. The sediments are placed during
the normal flow stage where the water level islow. Then waiting the coming flood to erode andtransport the sediment. During one day in 2008
flood the dam gate is opened from 8:00 with discharge of 1.34 m3/s and gradually increases tillreach to the peak discharge around 12:00 with
15.36 m3/s. The peak discharge remains two hours
then a gradually decrease till recover the normal
flow stage by the end of the day at 18:00.
Inflow and outflow discharge ( m3/s)

Figure 6. Grain size distribution of placed sedimentNovember December 2009

Figure 7: Flow duration curves before and after flood at the
Nunome dam gauging station for the study year 2009.

The duration of the peak flow during flood
should be longer to effectively erode and transport
sediment. During 2008, field test of 100 and 500
m3 are supplied in a successive periods. The sediments volume of 100 m3 is completely eroded tofurther downstream. However, the volume of 500
m3 is not eroded because no flood occurred. The
remained volume of 500 m3 is transported after
the natural flood of 2009 (see Figure 8).

Figure 5. Location map of Nunome reservoir, dam, andplaced sediment. Photos of geometry and check dam

The deposited sediments usually require more
processing and often require longer transportation.
The supplied sediment is composed of sand andgravel with grain size of 19 mm or less. There area lot of medium gravel with size of 10 mm andmedium sand size of about 0.25 mm. The placed
sediment median grain size d50 is 0.38 mm asshown in Figure 6.

2.6 Results of Replenishment DS of Nunome Dam
The evolving of bed topography and grain size
distribution is monitored, along with water surface, velocities and rate of sediment transport at
the downstream end of the Nunome River. Figure
8 shows the remained sediment of 2008 and during the heavy rain with peak discharge of 81 m3/s.
When the discharge exceeds 8m3/s, the erosion
1158

starts and about 40 m3 of sediments are transported. Figure 8 shows the photos of field tests
phases. All of the remained sediments are removed, and then new dredge sediments areplaced.

getation. The reach after cross section 7, has asmall sand bars, islands, and riparian vegetations.
The replenishment processes are efficient to restore the bed load transport and the associated habited by coupling reintroduction with floodplain
habitat restoration.
(a)

9:30
Q =9.46m 3/s

(c)

13：
00
Q =5.71m 3/s(b)

Sediment volume 500m3
Remained sediment from 2008

Ｂ=30ｍ

Before 2009 Flood
Ｈ=0.7ｍ

Ｈ=5ｍ

Discharge 81 m3/s
Allsediment
erodedDuring Flood （8-10-2009）11：50
Q =15.36m 3/s(d)100m 3 のうち約 40m

Normal stage

Figure 8. Evolution of sediment replenishment experiment

To understand the process of erosion and deposition of the placed sediment during flood, seriesof pictures are taken at different time and discharges are shown in Figure 9. The erosion at thebase of the placed sediment causes the sand massof placed sediment bank to slide downward anddeposit. Deposited sand mass is then eroded due
to water flow. Deposition occurs when the bottom
shear stress is less than the critical shear stress.
Only sediment with sufficient shear strengths towithstand the highly disruptive shear stresses in
the near bed region is deposited and adheres to thebed. The erosion region developed from a straightbank line into the alcove shaped as seen in Figure
9 (b). At the peak discharge the water level increased at 11h50 produces a greater erosion area
and the water submerges the placed sediment asshown in Figure 9(c).
The peak discharge is lasted for two hours andpermits a deeper cut in the inner side bank. With
peak flow, the eroded volumes increased in therange of 45 percent. Reduction of flood discharge
reduces the erosion rate, therefore 50 m3 of theplaced sediment remains as shown in Figure 9(d).
By using satellite imagery and aerial photos amap of Nunome River in 2009 is constructed for 1
km below the dam. The river channel, island,
point bars, and vegetation area are identified anddistinguished by color as shown in Figure 10. The
downstream reach of the dam, first 150 m fromdam, experienced the greatest change in channel
structure and loss of bars and islands.
The surface flow area in the first reach is greater than the bars and vegetation areas. Between
fourth and seventh cross sections, 300 m fromdam, reach exhibits variable patterns of channelchange and intermediate in loss of island and ve-

Figure 9. Evolution of sediment replenishment experiment

In Figure 10 along the Nunome River, several
cross sections are identified to survey after replenishment. Newly depositions over sand bars andin the river channel are shown in Figure 10.
Moreover, a completely new sand bar is formed
after 600 m from dam. With the field experiments
the processes are directly visible, and will be usedfor validation of numerical models.
The effects of sediment replenishment are investigated for cross section bed deposition, flow
velocity, grain size distribution, water quality andorganisms. Eleven monitoring points are shown in
Figure 11. The distribution of river bed materials
are analyzed by visually determining the sizes ofriver bed material in quadrate of 1 to 2 m in dimensions and preparing a two dimensional map,
which enabled changes in distribution before andafter sediment replenishment to be compared.
Figure 11 shows bed material size in three different monitoring times and 10 observation points
along the river. By comparing Figure 11(a) and11(b), a significant riverbed changes can be identified from cross sectional surveys, and visual inspections such as at section No. 1 and No. 11. After 600 m from the dam the survey at point No. 1
shows that the rate of gravel of less than 50 cm
increased after natural flood, grace to the replenishment. Similar sediment grain sizes are foundat point No 11 after 8200 m from dam point. Before sediment replenishment in point No 1 and No
11, the material is coarser gravel than in the middle points No 4 and No 19 (Figure 11(a)). Afterflood, the fine material content of the channel de1159

posits strongly increased. The sediment depositionin point No 1 and No 11 after replenishment consist of a nearly continuous layer of fine sand, but
no change occurs from No 2 to No 9 (Figure11(b)). But after second flood where anotherplaced sediment conducted, the bed material istransported and much of the coarsest sediment is
supplied to point No 1, and No 11 (Figure 11 (c)).

Deposition and vegetation after
replenishment over point sand bar

Stilling basin

Location of placed sediment (yellow area)

Point bar and vegetation
Stream flow

Under water
deposition
Newly formed sand bar after
sediment replenishment

Figure 10. Aerial photographs of Nunome dam with the downstream reach of Nunome River. Morphological changes and theself forming sand bar due to sediment replenishment below the dam (14-10-2009)＞20cm
Nunome Dam

5～20cm

＜5cm

No 4
No 19

Percentage of bed grain size(a)Placed sedimentat 0.3 km
No 13
No 1 at 0.6 km
No 16
No 2
No 3

Monitoring section on 18-12-2008 before flood

(b)
No 9

No 8
8.2 kmNo 11

Monitoring section on 5-8-2009 after flooda series of laboratory tests are conducted with six
sediment grain sizes and heights. The primary aimof laboratory tests is investigating the nature of
the relationship between sediment grain size anderosion processes. The placed sediment erosion isin general a very complex problem because it involves multi-processes such as bank surface erosion, bank toe erosion and bank material mechanic
failure, etc. Each of these processes is related toseveral parameters: sediment size distribution, sediment cohesion, slope, homogeneity, consolidation, soil moisture and ground water level, as well
as sediment height.
3.1 Experimental Facility and Conditions(c)

An extensive series of laboratory tests are carriedout in large flume facility at the Ujigawa Open
Laboratory of the Kyoto University. A schematic
view of the experimental setup is shown in Figure
12. The facility measures a length of over 20 m, awidth of 1 m and a depth of 0.3 m (Figure 12(c)).
The channel slope is adjusted to 1/250 (Figure12(a)). Rectangular sediment geometry has a fixed
length and width of 1.0 and 0.3 m, respectively(Figure 12(b)). Several sediment heights t of 5,
7.5, 10, 12.5, and 15 cm are investigated (Figure12(c)).

Monitoring section on 28-10-2009 after flood
Figure 11. Variations of river bed material size at 10 crosssections along the Nunome River

3 PHYSICAL MODEL TESTS
It is difficult to compare and examine theprocesses of different sediment types in the fieldtests, due to the differences in discharge condition
and measuring limitation during flood. Therefore,1160

Turbidity meter
COMPACT-HTW

CameraL =1.0 m

00

08

16

24

28

32

W =0.3m

t = 5, 7.5, 10, 12.5, 15 cm
7m
Pump Flow meter Valve(c)

Rectifierbasin

B = 1.0m

Settling
basin

F0 group. Figures depict the erosion process ofdifferent materials. The erosion process is a combination of lateral erosion of the placed sedimenttoe, and mass failure of the sediment. Erosion of
the placed sediment toe by hydraulic forces is acommon mechanism of bank over steepening,leading to collapse of the upper part of the bank.

(b)

(a)

6m

20.0m

Figure 12. Laboratory setup and sediment geometry, (a)general view of flume, (b) placed sediment geometry, (c)plane view of the experimental installation circuit.

The sediment geometry is placed over a fixed
bed which located at 6.0 m from the inlet offlume. In each of the tests the flow rate of Q = 20
l/s remained constant. Near the downstream end, aturbidity meter is installed for suspended sedimentconcentration measurements (see Figure 12(a)).
The erosion mechanism is recorded at successive
instants by video camera which is fixed perpendicular on the flume covering the plane sedimentarea (Figure 12(c)). The sediments grain characteristics used in the tests are based on those of the
Nunome River (Figure 13).

Figure 14. Time series of erosion sequences of C0
00

08

12

20

24

100
90
80

Percentage finer (%)

70
Nunome Field experiments material

60

16

50
Laboratory experiments materials
F: Fine dredged sediments
C: Coarse dredged sediments

40
30
F0

20

F1

F2

C0

C1

C2

10
0
0.01

0.10

1.00

Grain size (mm)10.00

Figure 15. Time series of erosion sequences for sediment F0

Figure 13. Sediment size distribution of laboratory tests andactual material for Nunome field tests

This typically occurs in sediment having acomposite grain size, particularly if noncohesivematerials comprise the geometry toe. Larger shear
stresses exerted by the flow at the base of thegeometry and the greater erodibility of noncohesive sediment material leads to undercutting andthe formation of cantilevers. Once the stability criterion is exceeded, a sediment mass failure would
occur. The failed material deposits first on the bed
near the placed sediment toes and then is eroded
away by the flow. Cohesive material is generally
more resistant to surface erosion because its low
permeability. As water flows along the eroded sediment, secondary currents remove the noncohesive material at the toe creating a cantilever overhang of cohesive material. At the toe of the

Two groups of fine (F) and coarse (C) sediments by grain diameter are used, the size distributions of which are seen in Figure 13. Eachgroup contains different treatments to change thesediment cohesion.
3.2 Experimental Results
In order to explain the mechanism of the erosion
features; time sequence investigations for fine andcoarse materials with constant sediment height of
10 cm were conducted. The pictures of erosion
evolution are described in Figures 14 and 15,
every 8 minutes for coarse C0 and fine sediment1161

geometry, where shear stress exceeds critical
shear stress, particles are detached from the bankby the flowing water.
This oversteepens the bank, causing noncohesive particles higher up on the bank to fall off in
thin, vertical slices. When the cohesive silt layer isundercut, the cantilever overhang collapses into
the eroded pocket. This loose, fallen material isthen transported downstream, resulting in a repositioned bank line or bank retreat.
According to real-time observation and film
analysis, the following were found; (1) The eroding speed of C2 > F2 > C1 > F1 > F0 > C0., (2) alump of sediment is eroded and the erosion rateincreased rapidly, (3) C1 and C2, rate of bed loadtransport is governing the total eroding speed, (4)C0 rate of mass failure is governing the total eroding speed (5) C1, C2, F1, and F2 have about 30°
steepens of the bank. Figure 16 shows the time
variation of erosion depth for different sedimentmaterials. The erosion of a material proceeds rapidly for the first 8 minutes, and after that the erosion rate decreases gradually according to the sediment type (see Figure 16). This may be due tosome extent to the decrease of shear stress as theerosion depth grows larger. For the sediment typeF0 that contain large amount of clay, there is asudden drop in the erosion rate as shown in Figure
16. The erodibility of sediment type F2 and C1 isalmost same behavior, with different transport
rate. The erosion rates comparison are C2 > C1 >F2 > F1 > F0 > C0.
35
30
C0 C1 C2 F0 F1 F2
25
20
15
10
5
0

replenishment may direct future supplements for amore widespread dispersal of suitable sand forfish spawning.
In order to prepare the downstream sediment
replenishment technique to the practical use, it isalso essential to share information with the people
concerned in the same river basin, suchminas fishery
workers associations and environmental groups,
and endeavor to make the method socially acceptable. Sediment replenishment for the purpose of
keeping a dam functional needs to be performed
semi-permanently. If the downstream replenishment technique is to be used as a means of reservoir sedimentation control, cost reduction is also
an important consideration.
Regarding the continuation of this research
project, the major goal is to find out which sediment geometry, volume, and feeding frequency
leads to environmental and physical improvements below dams. This requires long duration
tests combined with numerical modeling techniques.

Erosion depth (ｃｍ)

REFERENCES

0

10

20

Time (min)30

40

Figure 16. Time variation of erosion depth for six materials

4 CONCLUSIONS
The first results of ongoing research on the artificial sediment replenishment downstream dams arepresented. Furthermore, several series of systematic experiments are conducted to investigate theerosional characteristics of original and treatedmaterials. The effect of the sediment grain sizeand clay content on the erosion rate is evaluated.
By replenishing sand at different locations of the
Nunome River within the downstream reaches, the1162

Fan, S., Springer, F.E., 1993. Major sedimentation issues at
the Federal Energy Regulatory Commission. In: Fan, S.,
Morris, G. (Eds.), Notes on Sediment Management in
Reservoirs. Water Resources Publications, Colorado,
USA, pp. 1–8.
Kantoush, S. A., and Sumi, T., 2010. The impacts of sediment replenishment below dams on flow behavior andbed morphology of river channel and river mouth. FirstInternational Conference, Coastal Zone management,
River Deltas and Low Land Coastlines, Alexandria,
Egypt, 2010.
Kondolf, G.M., 1997. Hungry water: effects of dams andgravel mining on river channels. Environmental Management 21 (4), 533–551.
Kondolf, G.M., Matthews, W.V.G., 1993. Management ofCoarse Sediment in Regulated Rivers of California. University of California, Water Resources Center, Davis,
CA.
Ligon, F., Dietrich, W.E., Trush, W.J., 1995. Downstream
ecological effects of dams: a geomorphic perspective.
Bioscience 45 (3), 183–192.
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.
Poff, N.L., Allan, J.D., Bain, M.B., Karr, J.R., Prestegaard,
K.L., Richter, B.D., Sparks, R.E., Stromberg, J.C., 1997.
The natural flow regime: a paradigm for river conservation and restoration. Bioscience 47 (11), 769–784.
Seto, K, Sakamoto, T., Suetsugi, T., 2009. Sediment control
measures and improvement effects of physical conditionand environment by sediment flushing – a case study in
the Yahagi dam. International Congress on Large Dams,
Brasilia, Q.89-R.3.
Shields, F.D., Simon, A., and Steffen, L.J., 2000. Reservoir
effects on downstream river channel migration. Environmental Conservation 27 (1), 54–66.

Sumi, T., Kobayshi, K., Yamaguchi, K., and Takata, Y.,
2009. Study on the applicability of the asset management for reservoir sediment management. International
Congress on Large Dams, Brasilia, Q.89-R.4.
Wilcock, P.R., Kondolf, G.M., Matthews, W.V., Barta,
A.F., 1996. Specification of sediment maintenance flows
for a large gravel-bed river. Water Resources Research
32, 2911–2921.
Williams, G.P., Wolman, M.G., 1984. Downstream effects
of dams on Alluvial Rivers. US Geological Survey Professional Paper 1286.

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