First International Conference on“Coastal Zone Management of River Deltas and Low Land Coastlines “

CZMRDLLC – 2010

IMPACTS OF SEDIMENT REPLENISHMENT BELOW DAMS ON FLOWAND BED MORPHOLOGY OF RIVER
Sameh A. Kantoush32,33, Tetsuya Sumi1, Akira Kubota34, Takamasa Suzuki1

Abstract
Dams greatly influence flow and sediment discharge regimes and can have significant impact on
downstream reaches below dams. Bed load sediment management in reservoirs is required topreserve the capabilities of water resources facilities and to conserve the environment in rivers
and coastal areas downstream of the reservoir. The sediment deficit produces a large variety ofphysical (bed armouring, bank erosion, lowering water table, river incision), ecological(impoverishment of aquatic and riparian habitats) and environmental (oxygen stratification)effects. An effective means of such comprehensive reservoir sediment management is to
excavate and transport part of the coarse sediment, which accumulated in reservoirs to the river
channels downstream, and then the sediment is transported by flood water in the channels to thesea. Actual prototype sediment replenishment is carried out, to excavate some of the sedimentdeposited in reservoir of Murou dam, transported to Uda River bank, and discharged downstream
river. In the present paper, parameters and process of sediment replenishment characteristics are
discussed. The paper identifies the characteristics of sediment replenishment in the downstreamof Murou Dam and presents results of sediment replenishment on river bed changes. Appropriate
sediment excavation and treatment techniques from the upper part of the reservoir andreplenishing them to downstream rivers, contributes to maintain river bed levels, and mitigate thearmouring of river bed materials. The amount of sediment needed and the periodicity of thereplenishment should be adapted to the deposition volume in reservoir and seasonal behaviour of
every river. The field tests will provide data to validate and enhance modelling and restoration
efforts.
Keywords: Management of bed load, Sediment replenishment, River restoration, Monitoring,Reservoir sedimentation, Erosion mitigation.
Introduction
Modern human activities (e.g. dam construction, channelization, gravel mining, bankreinforcement), have transformed many rivers state from dynamically active and spatiallycomplex system to more static and homogenous ones, leading to detrimental effects on thephysical habitat and ecosystem (Pedroli et al., 2002). Dams and their reservoirs are constructed
and operated for multipurpose including hydroelectric power generation, flood protection,
32

Disaster Prevention Research Institute, Kyoto University, Goka-sho, Uji-shi, 611-0011, Japan,
kantoush@yahoo.com, sumi.tetsuya.2s@kyoto-u.ac.jp, suzuki@basewall.kuciv.kyoto-u.ac.jp
33
Swiss National Fund for Scientific Research (SNSF), Bern, Switzerland, sameh.kantoush@a3.epfl.ch
34
Kizugawa Dams Integrated Operation and Management Office, Japan Water Agency;Shimohinachi, Nabari-shi,
518-0413, Japan, Akira_Kubota@water.go.jp

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drinking water and agricultural water supply. Regardless of their purpose, all multipurpose dams
trap sediment and inevitably lead to physical and ecological changes downstream of the reservoir
site, as well as in the reservoir itself, and in some cases also upstream. Kondolf (1997) has
described that, by changing flow regime and sediment load, dams can produce adjustments inalluvial channels, the nature of which depends upon the characteristics of the original and alteredflow regimes and sediment loads. Downstream impacts develop through discontinuity indownstream gradients, e.g., sediment supply, water quality, temperature, flow and sediment
regimes.
Figure 1 classifies the impacts of sediment deficit downstream of dam to three groups;
morphological, hydrological, and ecological effects. Sediment deficit is not only anenvironmental issue but also a socio-economic problem, for instance due to loss of reservoir
capacity (e.g., Fan and Springer, 1993). In addition, dams alter the downstream flow regime ofrivers (Williams and Wolman, 1984), which controls many physical and ecological aspects ofriver form and processes, including sediment transport and nutrient exchange (Poff et al., 1997).
Morphological effects on the river channel (e.g., Kondolf and Matthews, 1993; Shields et al.,2000) that includes riverbed incision, riverbank instability, upstream erosion in tributaries,
groundwater over drafting, damage to bridges, embankments and levees (e.g., Kondolf, 1997;
Rinaldi and Simon, 1998; Batalla, 2003), and changes in channel width (e.g., Williams and
Wolman, 1984; Wilcock et al., 1996). Hydrological effects caused by dams include changes inflood frequency and magnitude, reduction in overall flows, changes in seasonal flows, andaltered timing of releases (Petts, 1984; Ligon et al., 1995). If flows released from dams havesufficient capacity to move coarse sediment, water becomes ‘hungry water’ (Kondolf, 1997),
which may transport sediment downstream without replacement from upstream and theecological effects on the fluvial and deltaic systems (e.g., Kondolf and Wolman, 1993), andpropagation of riparian vegetation into previously unvegetated or lightly vegetated areas (e.g.,
Church, 1995; Vericat and Batalla, 2005).
Effects of Depletion of Sediment Transport
Behind the Dam on:
MorphologicalHydrological

EcologicalRiver bank instability

Water table lowering

Loss of aquatic andriparian habitats

Bed armouring

Reduction in overallflows

oxygen stratification

River incision and bed
degradation

Changes in flood
frequency and magnitude

Nutrient exchange

Impacts on infrastructures(bridges)

Changes in seasonal flow

Sediment deficit tocoastal zones

Changes hydrodynamic
within estuaries

Gas-bubble disease
Adverse effect on water
qualityFig. 1. Possible effects due to lack of sediment transport in the downstream reach below dams onchannel morphology and environment
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Dam effects can vary, depending on the distance to the dam and other boundary conditions.
Depending on the supply from the tributaries the main river will have to erode sediments fromthe banks to compensate for the supply limitation. Channel erosion may lead to an interruption oflateral connectivity with the adjacent floodplain. At the same time, dams often reduce flood
discharges significantly, that reduce the transport capacity of the river downstream. In general,the combination of reduced supply of coarse material and reduced transport capacity
significantly reduces the potential for erosion and aggradations in the channel and floodplain,and causes a major morphological readjustment (Petts, 1984). The number of the worldwide
large rivers that do not reach the ocean any more is on the rise and their river deltas suffer
dramatic changes. If a river runs into the sea, regulating the river will probably have an effect inand around its mouth (Baxter, 1977). Since the construction of the Aswan High Dam, the area of
the Nile Delta has been reduced because of the disturbance of the equilibrium between erosion
by the sea and deposition of sediment by the river (Aleem, 1972). The sardine catches in the
Mediterranean near the mouth of the Nile have decreased, owing either to the absence ofnutrients formerly provided by the river or to the dispersal of the fish over a wider area (George,1972).
Sustainable management of sediment in river basins must be done on a regional basis, restoring
the continuity of sediment transport where possible and encouraging alternatives to river-derived
aggregate sources. Different management and ecological restoration measures to compensate thesediment deficit downstream from dams are conducted in some rivers (Kondolf, 1997). For
instance, controlled flow releases (flushing flows) and beach nourishment with imported
sediment dredged from reservoirs and harbours has been implemented along many rivers (Inman
1976; Everts 1985; Sumi and Kanazawa 2006; Sumi 2005; Kantoush et al 2009). These measures
range from optimising the compensation discharges, to reconnecting the river water bodies, andartificial sediment feeding (sediment replenishment). Sediment replenishment measures follows
several protocols, according to the material sources (banks, alluvial terraces, excavations due toworks, gravel mining, etc.) and the way of injection (pumping from ships, introduction frombank, recreation of artificial bed forms, etc.). The efficiency and potential negative effects must
be investigated through studies and local tests before large-scale operations, and by detailed
monitoring after such operations. Even if some reports of past test are available, the state of theart is not consolidated enough to draw general guidelines. There are gaps to quantitatively assess
the sediment deficit caused by large dams on a large river system, based upon regular, systematicand extensive direct measurements of total load (both in suspension and as bed load) upstream
and downstream of the reservoirs.
In Japan, Europe, and USA, large rivers are often trained to a large extent, to maintain services
such as navigation, hydropower generation and flood defence. Okano et al., (2004) summarize
sediment replenishment projects in large 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). Sediment treatment system is applied by Sumi et al.
(2009), to produce appropriate grain sized material with less turbidity. Investigations of effects
of sediment replenishment downstream of Yahagi dam is analysed by Seto et al., (2009). Large
European rivers such as the Rhône, the Rhine, or the Danube, have been undergoing ecologicaland hydraulic restoration measures, especially in reaches bypassed by hydraulic structures.
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Today, rehabilitation of damaged and modified aquatic ecosystems has become an integral partof catchment and river management. Sediment transport and associated channel bed mobility arerecognized as key processes for creating and maintaining physical habitats, aquatic and riparian
ecosystems. In this context, various river restoration projects have been initiated or implemented
in many countries. Some examples include creation of secondary channels along the Rhine River(Simons et al. 2001), reconnecting the Danube side-arm system to the main channel (Tockner etal. 1998), removing bank protection structures to initiate sediment transport in the Mur River(Kloesch et al. 2008), artificial gravel supply in the Ain River (Rollet et al. 2008), and removing
dams in North America (Hart and Poff 2002) to re-establish the river continuum. Gravels werebeing artificially added to enhance available spawning gravel supply below dams on at least 13
rivers in California Sacramento, Merced, Tuolumne, Mokelumne, Stanislaus, Russian, Dry, San
Juan, San Luis Rey, Cache, Dry, Stony, and Tujunga Wash Rivers (Kondolf and Matthews
1993).
In Japan, sediment replenishment projects are undertaking with different configurations andcharacteristics of sediment and discharges. Some of these projects are successful when placed
sediments are washed during high flows. But, in the other part of these projects, sediment
replenishment is not mobilized or transported according to the post- project monitoring. The
central challenge in sediment replenishment research is, however, to determine sediment andflow characteristics, which operation to implement, and how to determine sediment
replenishment factors. Figure 2 summarizes factors that influence the sediment replenishment
research. These factors are classified in six groups as shown in Figure 2. Indeed, rivermorphodynamics is a complex process that involves a high degree of interactions between theflow and sediment transport. And several significant gaps in the scientific understanding of theseprocesses remain, particularly concerning how riverbed deposition, mobility and geometry are
influenced by changes in the discharge and sediment supplies. Replenishment scenarios may,therefore, induce undesirable morphological and ecological consequences as well as significant
channel adjustments that can result in failure of the restoration project itself. That is, it isnecessary to better understand reversibility, direction and time scale of changes, and thesustainability of a replenishment intervention before it is implemented.

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Sediment Replenishment Factors
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

Fig. 2. Main groups of sediment replenishment characteristics and managing factors
Aim Of The Study
The study focuses on the design criteria for sediment replenishment projects in Japan to improveriver bed conditions. The investigations are based on field measurements for field tests. The
study will answer on the following questions:












How much sediment volume has to be supplied in order to avoid bed degradation or
aggradations?
What are the suitable sediment characteristics, grain size distribution and percentage of finesediment?
What is the most appropriate sediment source to use in terms of quality and economics?
What is the best method of sediment replenishment in order to maximize effects for
improvement and minimize adverse environmental impacts and riparian ecology?
Which geometry shape of the placed sediment has to be chosen in order to ensure sedimenttransport and erosion process?
Where the sediment should be supplied in the sediment deficit reach (several positions or
only one place)? What is the best arrangement for the placed sediment positions andlocations?
Which season and how often the sediment has to be injected?
What is interaction between the sediment supply and sediment bed? How, when and where
the sediment will be transported and deposited?
How much is the sediment transport capacity?
What are the impacts of the supplied sediment on the physical parameters of the river?

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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 analyse the sediment replenishment through a regulated fluvial system, Uda
River, during actual field tests on 2008 and 2009; (d) to draw some general conclusions in termsof future strategies for sediment management.
Replenishment Procedure
In Japan, it is common practice, as a means of reservoir maintenance, to remove accumulatedcoarse sediment by excavation and dredging, and to make effective use of the removed sediment.
Sediment replenishment method is consisting of trapping coarse sediment in the upstream area ofthe reservoir (Figure 3). In this method, trapped coarse sediment is periodically excavated (ordredged depending on the site conditions) and then transported and placed temporarily on thechannel downstream of the dam, in a manner decided according to the sediment transportcapacity of the channel and the environmental conditions, so that the sediment is returned to thechannel downstream in natural flooding processes.

①

Flood stage②

4

Beforeflood Normal stage③

sediment placed
for replenishment

Fig. 3. Concept of sediment replenishment: (1) Extracting mechanically the accumulated
sediment; (2) Transporting it by truck to the downstream river bank; (3) Putting the coarse
sediment and pours; (4) Monitoring of river flow, sediment, and environmental parameters
The procedure of the sediment replenishment test is shown in Figure 3. Four procedure steps toprepare the sediment replenishment test are shown in Figure 3. Sediment hauled to the river
channel is placed so that it does not get waterborne in times of normal flow and it is graduallytransported downstream after flooding river water begins to become turbid. Thus, replenishment
sediment is placed at such an elevation that it does not become a source of turbidity in times ofnormal flow and bank erosion occurs as stream flow increases. The top of the sediment placed
for replenishment is adjusted so that the sediment is completely submerged in flood water several
times a year and all sediment is eventually transported downstream (see Fig. 3).
The Study Area
Figure 4 shows the 75 km long Yodo River (Yodo-gawa) system, located in the central part ofJapan. It encompasses 6 prefectures, namely Mie, Shiga, Osaka, Hyogo and Nara, and is the6 – 10 March 2010290

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CZMRDLLC – 2010seventh largest river basin in Japan with a catchment area of 8,240 km 2. Flowing south out of
Lake Biwa, the largest lake in Japan, 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, Murou, Shourenji, Hinachi, Takayama, andNunome dams as shown in Figure 4. The Japan Water Agency (JWA) maintains water resource
development of the five dams. Murou dam is a gravity dam for flood control and water supply,
located in Uda River, was completed 32 years ago, in 1974. The dam height is 63.5 m and has acatchment area of 169 km2. The Murou reservoir has storage capacity of 16.9 Mm3.

Study Area

Uda River0

612

18 24kmFig. 4. Map of Yodo River system showing location of Murou dam in Kizu River system.
Cumulative sediment in Kizu River upstream reservoir
Dam reservoirs are normally designed to have a capacity to store 100 years’ worth of sediment in
the deepest parts close to the dams, but sediment flowing from the upper rivers is often depositedin areas of effective storage capacity near the entrances to the reservoirs. Sediment accumulationin the Kizu River upstream group dams 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 the originalplan. In Japan, the planned sedimentation capacity is set by the sedimentation of 100 years.
Sedimentation of Murou dam that were constructed more than 30 years ago have already reached
40 to 50% of planned sedimentation. Regarding annual fluctuation of sediment load of Murou
dam, maximum annual sedimentation in a single year is 314,000 m 3. The maximum annual
sedimentation is about 12% of planned sedimentation capacity.

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Table 1. Sedimentation rate of Kizu River upstream dam group (Sumi, 2009)
Name ofDam
Murou
ShorenjiNunomeHinachiTakayama

Elapsedtime(Years)
32
36
16
9
37

Planned sedimentation
capacity in 100 years
(MCM)
2.6
3.4
1.9
2.4
7.6

Actual sedimentationin 2006
(MCM)
1.12
1.484
0.243
0.41
3.648

Sedimentation
rate
(%)
43.1
43.6
12.8
17.1
48.1

Sand replenishment projects below Kizu River upstream dam
In Japanese reservoirs the check dams are effectively used to trap sediment due to the higher
percentage of sediment bed load to the total sediment load. In general, reservoir deposits are
attractive sources of aggregates to the extent that they are sorted by size. Figure 5(a) shows thelocation map of check dam and reservoir shape. Check dam is 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. These coarse sediment deposits can be removed periodicallyas shown in Figure 5(b). Designers of replenishments projects are faced with the challenges of
estimating the sediment deficit volume and determine sediment grain size distribution. Sand for
replenishment can be obtained from the deposition upstream of check dam. The deposited
sediment usually require more processing and often require longer transportation. Although theirproduction costs are commonly higher, the sediment excavation provide other benefits, such as
partially restoring reservoir capacity lost to sedimentation and providing opportunities for
ecological restoration in downstream rivers.

(a)

(b)
Placed sediment
Uda
River
Murou Dam

Check Dam

Fig. 5. (a) Location map of the check dam at the end of Murou reservoir in the Uda River; (b)Mechanical removing of deposited sediment in front of check dam
Recently, several sediment replenishment field tests are conducted. Some cases in below Kizu
River upstream dam are summarized in Table 2. The volume of sediment replenishment is small
compared to the deposited volume in reservoirs. Below Murou dam the sediment replenishment
has been started in 2006 with volume of 225 m3, which is 0.6% of the annual deposited sediment
in reservoir (see Table 2). In fact the effective volume ratio of sediment replenishment should be
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CZMRDLLC – 2010at least 30 to 40% of the annual deposited volume in reservoir. Furthermore, that sedimentshould be from coarse sand. The present paper deals with the results and analysis of Murou dam
sediment replenishment field test.
Table 2. Sediment replenishment projects downstream of dam group of Kizu River
Name of
Dam

Replenishment
project starting
year

Murou
NunomeHinachi
Shorenji

2006
2004
2008
2009

Annual mean
volume of sediment
replenishment
below dam (m3)
225
430
65
70

Annual mean
volume of actual
deposition inreservoir (m3)35,60018,60051,80041,600

Sediment volume
ratio of supplied
over deposited
(%)
0.6
2.3
0.1
0.2

Case Study Of Murou Dam Actual Sediment Replenishment Tests
Since 2006, several replenishment projects with different sediment volumes have been
undertaken below Murou dam. Tracking the history and performance of these projects is helping
to answer some of questions in objectives section. Table 3 summarizes the replenishment history,
flushing flow and sediments characteristics. The volume of placed sediment is limited to several
hundred cubic meters each time. Two types of flow are used to transport the placed sediment;
natural flood and artificial flushing flows (Table 3). To implement this method, consideration has
to be given to environmental problems in the lower river basins, to the occurrence of turbid
water, and to safety risks due to sediment deposition in the channel. Concrete means are being
explored, taking into consideration the particle sizes of sediment, such as the scale of flood
suitable for the safe implementation, and appropriate ways to place sediment in riverbeds.
Fishery workers association and local residences are often against the occurrence of turbid waterand excess sediment deposition in river channel.
Table 3. History of the sediment replenishment tests downstream of Murou dam
Year
2006
2007
2008
2009

Setting andplacing sedimentperiodFrom 12-5-2006
To 16-5-2006
From 8-5-2006
To 10-5-2006
From 12-5-2006
To 15-5-2006
From 7-5-2006
To 12-5-2006

Time ofartificial or
natural flow
13,14-5-2006
17,18-5-2006
18-5-200724-5-2007
16-5-2008
25-5-2008
14-5-200931-5-2009

Type of flushingflow
Natural flood
Natural flood
Artificial flushing
Natural flood
Artificial flushing
Natural flood
Artificial flushing
Natural flood

Volume ofplacedsediment (m3)
90
50
250
230
280

Volume ofsediment
transport
90
50
150
100
170
60
230
50

Monitoring efforts of pre- and post- replenishment methods show the effect of improvements inriverbed formation, riverbed materials, benthic organisms, and algae. The data in Table 3
indicates that there is a need to increase the amount of the supplied sediment downstream of
Murou dam every year. The increasing amount of the placed sediment should be proportional to6 – 10 March 2010

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the bed load transport capacity of the river. Furthermore, the period and the discharge of artificial
flow flushing should be suitable for the sediment transport capacity. The artificial flushingduration for Murou dam is approximately 10 hours during one day only (see Table 3). Neither
the flushing discharge nor the flushing period is able to transport all the placed sediment volume.
The remained sediment is transported during the natural flood periods that have large discharge
and period. The descriptions and the results hereafter related to the last year sediment
replenishment below Murou dam in right side bank of Uda River.
Field Methods
The test site for this study is placed by the Japan Water Agency (JWA) on the right side bank of
Uda River downstream of Murou dam in Nara prefecture (Figure 6). Planning, design,
implementation and long-term monitoring of Murou dam replenishment tests have been guided
by JWA. Photographs in Figure 6 show the 2009 field test of Murou dam stilling basin and theplaced sand in downstream reach at 150 m from the dam site.

Fig. 6. Sediment replenishment site in Uda River downstream reach of Murou dam.
Placed sediment geometry and grain size distribution
Below the Murou dam 280 m3 of sand are artificially placed to the Uda River in 2009. Figure 7
shows plan view and cross section of the placed sediment geometry. To construct the placed
sediment geometry 80 sand package of 1.1 m x 1.1 m were used. The placed sediment has a
trapezoidal shape with a groove channel near the river right bank. The top width of the geometryis 12 m.

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A

Rock
Placedsediment

Rock

Placed sediment
Placedsediment

Cross section A of placed sediment and river bed

A

Fig. 7. Plan view and cross section geometries of the placed sediment.
Grain size is the most fundamental physical property of sediment. The supplied sediment iscomposed of sand and gravel with grain size of 19 mm or less. There are a lot of medium gravelwith size of 10 mm and medium sand size of about 0.25 mm. The placed sediment median grain
size d50 is 1.25 mm as shown in Figure 8. The grain distribution is well sorted.
Clay

Silt
Fine

100

Sand
GravelMedium Coarse Fine Medium Coarse

90

Percent finer (％)

80
70
60
50
40
30
20
10
0
0.001

0.01

0.1

1

10

100

Grain size (mm)Fig. 8. Grain size distribution of placed sediment used for replenishment in Uda River
Flow regimes and conditions
Figure 9 shows the monthly inflow and outflow discharges of Murou reservoir in 2009, with thereservoir storage capacity. The reservoir has the highest flow discharges during July andOctober. The post-dam hydrograph shows a significant reduction in flood peaks and the
maximum use of the outflow discharge is about 14.0 m3/s. The timelines of sediment setting,placement, starting of artificial flushing, and monitoring measurements are shown in Figure 9.

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CZMRDLLC – 20102009 Natural flood

Inflow
Outflow
Storage capacity

Maximum inflow = 557.4 m 3/s
Maximum outflow = 249.6 m3/s

Preparation andplacing sediment
before flushingartificial flushing
Monitoring after
natural flood

Monitoring after
flushing
May 2009

June

July

AugustSeptember

October

Storage capacity (x10 3 m3)

Inflow and outflow discharge ( m3/s)

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November December 2009

Fig. 9. Flow duration curves for before and after artificial flushing and natural flood discharges
at the Murou dam gauging station for the study year 2009.
There are three monitoring phases; (1) before artificial flushing, (2) after artificial flushing, and(3) after natural flood. In fact, the sediments are placed during the normal flow stage. Then, toerode and transport the supplied sediment artificial flood by opening the Murou dam lateral gate
is discharged as shown in Figure 10. During one day the dam gate is opened from 8:00 with
discharge of 1.34 m3/s and gradually increases till reach to the peak discharge around 12:00 with
14 m3/s (Figure 10(a)). The peak discharge remains two hours then a gradually decrease tillrecover the normal flow stage by the end of the day at 18:00.
16

Artificial flushing discharge during replenishment

14

Uda River

12

Discharge （m3/s）

1086Artificial
flood from
Murou damlateral gate

4
2
0
8:00

(a)

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
Clock time(b)

Fig. 10. (a) Artificial flushing discharge curve on 7-5-2009; (b) Jet of Murou dam gate
The duration of the peak flow during artificial flood should be longer to erode and transport all
the placed sediment. Figure 10(b) shows the jet plunging in the stilling basin of Murou dam
before flowing over the weir to the downstream reach of Uda River. During 2009, field test of
280 m3 supplied sediment, only 230 m3 are transported further downstream. The rest of 50 m3 aretransported after the natural flood. To investigate the influence of the dam and flow duration
curves another scenarios will be simulated in future study.

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Results Of Replenishment Downstream Of Murou Dam
The evolving bed topography and grain size distribution are monitored, along with water surface,velocities and rate of sediment transport at the downstream end of the river. Measurements are
performed at various cross sections along the river. With the field experiments the processes are
directly visible and a wealth of valuable data are obtained with relative ease, and thereby will be
used for further calculation or validation of numerical models.
Morphological evolution and corresponding flow field
To understand the process of scouring and deposition during the artificial flushing of the placed
sediment series of pictures are taken at different time and discharge as shown in Figures 11 and12. The figures clearly depict the process of erosion and flow behaviours. The erosion at the baseof the placed sediment causes the sand mass of placed sediment bank to slide downward anddeposit. Deposited sand mass is then flushed due to water flow. The width of erosion during thewhole flushing periods is one to three times the height of the placed sediment. The erosion
region developed from a straight bank line into the crenulated shaped (half moon shape) as seenin Figure 11 (b). At the peak discharge the water level increased at t3 produce a greater erosion
area and the water enters the grove channel in the placed sediment as shown in Figure 11(c).
Artificial flushing increases the erosion rate and transports the sediment out of the placed
sediment region.
The peak discharge lasts for two hours and permits a deeper cut in the placed sediment bank.
With peak flow, the eroded volumes increase in the range of 45 percent. Reduction of discharge
at t5 reduces the erosion rate, therefore 50 m3 of the placed sediment remains as shown in Figure11(f). By looking downstream in Figure 11, the erosion start at the inner bank of the placed
sediment till the half moon shape is formed due to gradually increase of the discharge at t1 and t2,
see Figure 11(a, b). Looking upstream to the dam side, during the peak discharge one part of theplaced sediment is isolated and collapsed as a block (Figure 12 (c,d,e)). Apparently, 20 percentof the placed volume that were placed in higher elevation on right bank is not transported
because of low water level during peak flow.

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First International Conference on“Coastal Zone Management of River Deltas and Low Land Coastlines “(a) t 1 = 10:19, Q1 = 3.48 m 3/s(b) t 2 = 11:47, Q2 = 12.0 m 3/s(d) t4 = 12:34, Q4 = 13.0 m3/s (peak) (e) t 5 = 14:26, Q5 = 5.0 m3/s

CZMRDLLC – 2010(c) t3 = 12:04, Q3 = 13.0 m3/s (peak)(f) t6 = 15:05, Q6 = 2.5 m 3/s

Fig. 11. Looking downstream photographs of morphological evolution during artificial flushingof placed sediment at Uda River bank(a) t 1 = 10:19, Q1 = 3.48 m3/s(b) t 2 = 11:47, Q 2 = 12.0 m3/s(d) t 4 = 12:34, Q4 = 13.0 m3/s (peak) (e) t 5 = 14:26, Q 5 = 5.0 m3/s(c) t 3 = 12:04, Q 3 = 13.0 m3/s (peak)(f) t 6 = 15:05, Q 6 = 2.5 m3/s

Fig. 12. Looking upstream photographs of morphological evolution during artificial flushing ofplaced sediment at Uda River bank
Changes in river bed materials
The effects of sediment replenishment are investigated on cross section bed deposition, flow
velocity, grain size distribution, water quality and organisms. Thirteen monitoring points below
Murou dam are investigated and their locations are shown in Figure 14. The distribution of river

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bed materials are analysed by visually determining the sizes of river bed material in quadrates of
1 to 2 m in dimensions and preparing a two dimensional map, which enabled changes in
distribution before and after sediment replenishment to be compared. Figure 13 present thechanges in the river bed material before and after sediment replenishment and after natural flood.
By comparing Figure 13(a) and 13(b), a significant riverbed changes can be identified from crosssectional surveys, and visual inspections. But after the natural flood the newly deposited
sediment grace to the replenishment is eroded again and replaced by a larger boulders and rocks.

(a) before flushing(b) after flushing and replenishment(c) after natural flood

Fig. 13. Photographs of Uda river bed depositions in different months in 2009
Figure 14 shows bed material size in three different monitoring times and 13 observation points
along the river. Before sediment replenishment in point No 2 and No 2-1, the material is coarser
gravel than in the farther downstream points No 4 and No 7 (Figure 14(a)). After flushing, thefine material content of the channel deposits strongly increased. The sediment deposition in point
No 2 and No 2-1 after replenishment consist of a nearly continuous layer of fine sand, but nochange occurs further downstream (Figure 14(b)). But after natural flood where a less placed
sediment volume, the bed material is transported and much of the coarsest sediment is supplied
to point No 2, and No 4 (Figure 14 (c)). The mount of supplied sediments is not effectively
influenced on the river bed after natural flood occurs.

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First International Conference on“Coastal Zone Management of River Deltas and Low Land Coastlines “Placedsediment

Murou Dam
No 1 (0.1 km)
No 2 (0.2 km)
No 2-1(0.2 km)

Percentage of bed grain size

＞20cm

No 5 (0.8 km)

No 9-1 (3.2 km)

No 10 (4.0 km)

Percentage of bed grain size

No 9 (2.6 km)

5～20cm

＜5cm

80%
60%
40%
20%
0%

(a)

1

2 2-1 34
5
6
7 7-1 8
9 9-1 10
Monitoring section on 16-8-2009 befor artificial flushing

＞20cm

100%

Percentage of bed grain size

No 6 (0.9 km)
No 7 (1.0 km)

100%

CZMRDLLC – 2010

5～20cm

＜5cm

80%
60%
40%
20%
0%

(b)

1

2 2-1 34
5
6
7 7-1 8
9 9-1 10
Monitoring section on 19-5-2009 after artificial flushing＞20cm
5～20cm＜5cm

1

2 2-1 34
5
6
7 7-1 8
9 9-1 10
Monitoring section on 9-11-2009 after natural flood

100%
80%
60%
40%
20%
0%

(c)

Fig. 14. Variations of river bed material size in 13 cross sections along Uda River
Conclusions
The first results of ongoing research on the artificial sediment replenishment downstream dams
are presented. This paper has pointed out the alteration of sediment transport and, subsequently,of the fluvial dynamics due to dam impacts in a large river system. The downstream sediment
replenishment method is currently being field tested at Murou dam, but so far not enoughknowledge has been accumulated about its influence on lives in rivers and coastal areas. The
authors think that the only way to gain necessary knowledge is to conduct a monitoring study onchanges in the physical environment of the river downstream and their influence on various
forms of lives.
By replenishing sand at different locations of the Uda River within the downstream reaches, thereplenishment may direct future supplements for a more widespread dispersal of suitable sandfor fish spawning. In order to put the downstream sediment replenishment method to practicaluse, it is also important to share information with the people concerned in the same river basin,such as fishery workers associations and environmental groups, and endeavor to make themethod socially acceptable. Sediment replenishment for the purpose of keeping a dam functional
needs to be performed semi-permanently. If the downstream replenishment method is to be used
as a means of reservoir sedimentation control, cost reduction is also an important consideration.

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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 tests of long duration combined with numerical modelling techniquesthat include the processes as observed in this study. Firstly, validation and verification of 2D
numerical model will be conducted by using actual data of sediment replenishment projects.
Secondly, investigation of selected scenarios to examine sediment replenishment geometry,volume, erosion rate and positions for different grain size distribution. Finally, examine theapplication of the numerical model by using the field data will be carried out.
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