Integrated Management of Reservoir Sediment Routing by Flushing,
Replenishing, and Bypassing Sediments in Japanese River Basins
Tetsuya Sumi
Disaster Prevention Research Institute, Kyoto University
Goka-sho, Uji-shi, 611-0011, Japan
Sameh Ahmed Kantoush
Disaster Prevention Research Institute, Kyoto University
Goka-sho, Uji-shi, 611-0011, Japan
Abstract: Dam construction dramatically influences the river basin balance with respect to water
sediment inflow and outflow, which upsets downstream reaches. The objective of water and sediment
incorporation is to manipulate the river-reservoir system to achieve sediment balance while maximizing
the beneficial storage, and minimizing environmental impacts and socioeconomic costs. Such costs could
be avoided if reservoir sedimentation was minimized and dams were allowed to live forever. Sedimentflushing is one of the most attractive methods from the costs and sediment supply point of view. Attempts
to replenish the excavated and dredged sediment to the downstream river and sediment bypassing systemhave been undertaken. Therefore, it is important to predict how anticipated phenomena will impact on theenvironment and to conduct studies to develop feasible measures that minimize them. The paper aims at
identifying some of water and sediment management options in Japan. An analysis of sediment flushing
operations to date, the effect of sediment flushing on the river’s sediment balance and physicalenvironment is presented. The results show that, the successful reservoir sediment management proceedsfrom a permanent change in river management. This may involve strategies to enhance water andsediment release by a combination of several countermeasures. There is a need to define the water
discharge with disturbance and managing the sediment quantity and quality. Careful planning should bestudied for the flow and sediment regimes which may be changed frequently before and after a flood as aresult of maintaining the water and sediment release from the dams by sediment flushing, replenishing, orbypassing.
Keywords: Reservoir sedimentation, water and sediment release, flushing, sediment replenishment,
sediment bypassing, sediment routing, environmental impacts of dams, river restoration.

Introduction
Japan has geologically young mountains, steep and short rivers with flashy flow regimes, and densely
populated floodplains. Japan suffers from frequent heavy storms. Generally, the Pacific side of Japan has
heavy rain in June and July (rainy season) and between August and October (Typhoons). On the side of
the Japan Sea flow peaks during the snowmelt period in spring. The combination of steep catchments andheavy storms results in widespread hill slope failures and landslides and extensive flood discharges.
Reservoirs and dams are developed by modern human activities to deal with the variability of watersupplies over time. Dams and their reservoirs are constructed and operated for multipurpose including
flood control, drinking water, agricultural water supply, power generation, recreation and others. Until
now the common engineering practice has been to design and operate reservoirs to fill with sedimentslowly. With such an approach, the consequences of sedimentation and project abandonment are left to betaken care of by future generations. For many dams this future has already arrived. Therefore we need todevelop ingenious solutions for integrated sediment and water management; unless we will lose thestruggle to enhance the available water resources and ecological environment.
Regardless of their purpose, all multipurpose dams trap sediment and inevitably lead to physical andecological 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 in alluvial channels, the nature of which depends upon the characteristics of theoriginal and altered flow regimes and sediment loads. Downstream impacts develop through discontinuity
in downstream gradients, e.g., sediment supply, water quality, temperature, flow and sediment regimes.
Sediment deficit is not only an environmental issue but also a socio-economic problem, for instance due

831

to loss of reservoir capacity (e.g., Fan and Springer, 1993). In addition, dams alter the downstream flow
regime of rivers (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; Kantoush et al., 2010) that
includes riverbed incision, riverbank instability, upstream erosion in tributaries, groundwater over drafting,damage to bridges, embankments and levees (e.g., Kondolf, 1997; Batalla, 2003), and changes in channel
width. 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. If flows released from dams
have sufficient capacity to move coarse sediment, water becomes ‘hungry water’ (Kondolf, 1997), which
may transport sediment downstream without replacement from upstream and the ecological effects on thefluvial and deltaic systems.
Inventory of sediment management measures
Several methods for sediment management are available and have been implemented in practice. Thesemethods can be categorized as shown in Figure 1.

Sustainable use of reservoir
Sediment management approach
Upstreamof reservoir

Withinreservoir

At the dam

Downstream

Soilconservation

Dry
excavation

Density
current

Sediment
replenishment

Slope & bankprotection

Dredging/
HSRS

Dam raise

Dilution

Sediments
traps

Flushing

Bottomoutletmodification

Monitoring

Bypassing

Off‐channel
storage

SettlingbasinsRinse out

Figure 1: Inventory of measures for sediment management
Downstream, water released from the dam possesses the energy to erode the channel bed and banks, but
has little or no sediment load. The rapid reservoir sedimentation not only decreases the storage capacity,
but also increases the probability of flood inundation in the upstream reaches due to heightening of the
bed elevations at the upstream end of the reservoir and the confluences of the tributaries (Liu et al., 2004).
The necessity for the reservoir sediment management in Japan can be summarized as: (1) to prevent thesiltation of intake facilities and aggradations of upstream river bed, accompanied by the sedimentation
process in reservoirs; (2) to maintain storage function of reservoirs, and realize sustainable water
resources management for the next generation; (3) to release sediment from dams as a perspective on
comprehensive sediment management in a sediment routing system.
The objectives of this paper are: (a) to make a general review of impacts of dam on downstream reaches
and river improvement methods; (b) to analysis the environmental impacts of sediment flushing at the
Kurobe River; (c) to illustrate and analyze the sediment replenishment through a regulated fluvial system,
Nunome and Managawa Rivers; (d) to discuss the role of slit type check dams and flood mitigation dams;(e) to draw some general conclusions in terms of future strategies for sediment management.

Dam Impacts and New Options of River Instream Flow Improvement
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 from the banks to compensate
for the supply limitation. Channel erosion may lead to an interruption of lateral connectivity with theadjacent floodplain. At the same time, dams often reduce flood discharges significantly, that reduce thetransport capacity of the river downstream. The number of the worldwide large rivers that do not reach832

the ocean any more is one rise and their river deltas suffer dramatic changes. If a river runs into the sea,
regulating the river will probably have an effect in and around its mouth (Sumi, 2006).
Environmental changes in downstream rivers and proposed mitigation measures
Environmental issues associated with reservoir sedimentation include the consequences of alteredsediment and flow release, serious environmental changes occurred in downstream rivers below dams asshown in Figure 2. These changes can be categorized as follows: River channel changes: Fixed sand bars; degradation and tree growth in river channels.
 Riverbed material change: Armoring and decrease of small porosity.
(a)

(b)

Figure 2: Environmental changes in downstream rivers: (a) Kazaya Dam and (b) Kuzuryu River
Sustainable management of sediment in river basins must be done on a regional basis, restoring thecontinuity of sediment transport where possible and encouraging alternatives to river-derived aggregate
sources. Different management and ecological restoration measures to compensate the sediment 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 andharbors has been implemented along many rivers (Everts, 1985; Sumi et al. 2006; Sumi 2009; Kantoush
et al 2009). These measures range from optimizing the compensation discharges, to reconnecting the riverwater bodies, and artificial sediment feeding (sediment replenishment). Today, rehabilitation of damaged
and modified aquatic ecosystems has become an integral part of catchment and river management.
Sediment transport and associated channel bed mobility are recognized as key processes for creating andmaintaining 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, reconnecting the Danube side-arm system to the main channel(Tockner et al. 1998), removing bank protection structures, and removing dams in North America (Poff et
al. 1997) to re-establish the river continuum.
Ikebuchi, et al. (2007) discussed new options in order to improve the above situations, the following
measures are currently proposed and implemented in Japanese River basin system: Conservation of flow regime
River instream flow
Flexible dam operation
Flushing flow
 Conservation of sediment flow
Sediment replenishment (restoration)
Large scale sediment management measures Suspended and bedload sediment bypass
 Sediment scouring gate
Flexible Dam Operation method (FDO)
The Flexible Dam Operation (FDO) is a dam management method to enhance conservation andrestoration of the river environment located in the downstream of a dam via utilizing “usable capacity”
without interrupting prime flood control. The “usable capacity” is a vacant portion of dam’s capacity,which is reserved for actual flood event during the rainy/typhoon seasons. The water stored in thecapacity is called “usable water”. The FDO requires storing water temporarily up to its design level or833

“usable water level” within the flood control storage capacity as illustrated in Figure 3. This utilization
storage capacity that has been newly created is used for utilization discharge as follows:
1) Discharge to increase the instream flow: Its purpose is to improve the scenery and the habitat for
fishes.
2) Flushing discharge: Its purpose is to stir up the riverbed and flush out silt and slack water on theriverbed.
Since 1997, a “Flexible Dam Operation” has been underway at dam under the jurisdiction of MLIT withthe purpose of improving the river environment downstream of dams. The positive effects that have beenobtained by the investigation are summarized in Table 1 according to each discharge type.
prior flow

flood control

surcharge water level Table 1 Results of the FDO trails for different flow types

stage

normal water level
usable water leveltimenormal top water level usable flow
for flood control
flexibility dam control,
nomal dam control

Instream flow

Flushing flow

Objectives
Results
Improving fish habitat and Enhancing continuity of stretch andmigration opportunities
increasing in fish population
Improving water quality Maintaining desirable water quality
90% of community at the trial
Improving Vicinity
location observed vicinity improved
Conservation of
Restoring wetlands on downstreamof damsdownstream environment
Enhancement ofRemoving part of old colonies
rejuvenation of algae
Solving odor and vicinity
Flushing stagnant water
problemRemoving problematic floating algae,
Improving vicinity
sediments and attached mud

Figure 3: Multipurpose dam operation system – the “normal-top-water-for-flood-season” system

Reservoir Flushing in the Kurobe River, Japan
Drawdown flushing is the lowering the water levels in a reservoir. Hydraulic flushing involves reservoir
drawdown by opening the bottom outlet to generate and accelerate unsteady flow towards the outlet. This
accelerated flow possesses an increased stream power and consequently eroding a channel through thedeposits and flushing the fine and coarse sediments through the outlet. During this process a progressive
and a retrogressive erosion patterns can occurs in the tail reach and delta reach of the reservoir,
respectively. The Dashidaira dam and the Unazuki dam in the Kurobe River system have begun efforts toempty the reservoirs in a coordinated manner and to flush sediment deposited in the reservoirs. In July
2006, the Dashidaira dam flushes out 910,000 m3 of sediment which includes 420,000 m3 of newly
inflowing sediment. The Unazuki dam has an inflow of 1,380,000 m3 of sediment. From this inflow, thedam is passing through 1,010,000 m3, or 73%, which mainly consists of fine sediments smaller than 2 mm.
The amount of coarse sediments larger than 2 mm passing through the Unazuki dam is only 20,000 m3, or10%, and 90% is currently trapped at the reservoir. This method has been used for small dams but not forlarge dams because it takes time to refill a large reservoir after flushing and because it makes difficult tomaintain the dam’s power-generating functions. However, it was easy to introduce this method at thesedams because the risk relating to refilling the reservoirs is low due to the large volume of water flow
compared to the capacities of the reservoirs and because the impact of sediment flushing on the riverbed
downstream is relatively small due to the steepness of the river downstream from the dams and the shortdistance to the Japan Sea (30 kilometers from the two dams). The flushed water quality deteriorates when
sediment is flushed after being deposited in a reservoir for a long period of time. Therefore, the feature of
the dam sediment flushing has been conducted as frequently as possible at times of flood, and turbidityand dissolved oxygen levels in the river, mouth of a river and the sea area have been monitored whilelooking at the impact of sediment flushing on the lower river basin.
Physical environment changes in the downstream river channel
In the Kurobe River downstream, 0-6 km from the river mouth is braided channel, and 6-13 km from theriver mouth is single bar. Figure 4(a) shows the average riverbed level for each section from year to year
based on the one in 1980 before the Dashidaira dam was completed. While the riverbed degradation had
been occurred over all sections of the downstream river channel, after sediment flushing was begun, theriverbed began to rise again, mainly in the sections closest to the river mouth. Slightly riverbed
degradation started again after the Unazuki dam was completed, but we can now notice the riverbed
aggradations again, reflecting the coordinated sediment flushing. Figure 4(b) shows these changes in834

terms of the riverbed materials. This figure indicates that for a representative grain size of D60 (mm), theriverbed materials had generally been getting larger after the completion of the Dashidaira dam by“armoring”; however, after the start of sediment flushing, they began to get smaller in all sections. This isconsidered to be due to the supply of the sand component.
Based on 1980

Average river bed level

1.00

Dashidaira dam
completion 1985

Unazuki dam
completion 2000

Start of Dashidaira
dam flushing1991
Unazuki dam
completion 2000

0.00
-0.50

D60 (mm)

-1.00

100

-1.50
-2.00

Ave. 0-3.0k
Ave. 6.4-11.2k
Ave. 13.4-15.6k

0-3k
7-11k
14-15k
Ave. of all sections

Ave. 3.2-6.2k
Ave. 11.4-13.2k
Ave. 15.8-20.4k

-2.50

10

1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007

(a)

4-6k
12-13k
16-20k

(b)

1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009

Variation from 1980 (m)

0.50

1000
Dashidaira dam Start of Dashidairacompletion 1985 dam flushing 1991

Figure 4: Relationship between (a) river bed level; (b) representative grain size (D60), and annual
sediment flushing at different cross sections

Sediment Replenishment
In Japan, it is common practice to remove accumulated coarse sediment by excavation and dredging, andto make effective use of the removed sediment. Sediment replenishment method is one of new measuresof sediment management. In this method, trapped coarse sediment is periodically excavated (or dredged
depending on the site conditions) and then transported and placed temporarily on the channel downstreamof the dam, in a manner decided according to the sediment transport capacity of the channel and theenvironmental conditions. The replenishment processes are efficient to restore the bed load transport andthe associated habited by coupling reintroduction with floodplain habitat restoration. In Figure 5 along the
Nunome River, several cross sections are identified to survey after replenishment.
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 5: Aerial photos of Nunome dam and the downstream reaches with morphological changes
due to sediment replenishment below the dam and the self forming sand bar (14-10-2009)835

Grain size
distribution (%)
通過質量百分率（％）

100
90
80
70
60
50
40
30
20
10
0
0.01

試 料 A:採 取 地

Replenished
試 料 B:採 取 地
sediment
試 料 C:設 置 土 砂

0.1

1
粒径（ｍｍ）

10

100

Diameter (mm)Figure 6: Placed sediment before and after flushing flow and its grain size distribution respectively
Newly depositions over sand bars and in the river channel are shown in photos Figure 5. Moreover, acompletely new sand bar is formed after 600 m from dam. Large Japanese rivers are often trained to alarge extent, to maintain services such as navigation, hydropower generation and flood defense. Okano etal., (2004) summarize sediment replenishment projects in large Japanese Rivers such as Tenryu, Abukuma,
Ara, Oi, Naka, Kuzuryu, Yodo, and Tone, have been conducted. Kantoush et al., (2010), investigate themorphological evolution and corresponding flow field during replenishment experiments in Uda River. In
case of Managawa dam, above mentioned sediment restoration trial is also planned to enhance river
refreshment. Figure 6 shows placed sediment before and after flushing flow below Managawa dam and its
grain size distribution respectively. The main objective of the flushing flow is to move bed gravels andinitiate detaching attached algae on them which are very important to provide fresh food sources to ‘Ayu’
fish species. Peak discharges of 30m3/s and 50m3/s, and with and without sediment restoration werecompared from the view points of water level, flow velocity, temperature, turbidity, suspended sedimentconcentration, movement of river bed materials and so on. Rate of detached algae on river bed gravels
were also measured both upstream and downstream of sediment placed point.

Bypassing Tunnels
Another possible approach to sediment management, next to the reduction of sediment inflow itself, is toroute sediment inflow so as not to allow it to accumulate in reservoirs. In Japan, the following techniques
are adopted: 1) sediment bypass by directly diverting total load of sediment transport flow, and 2) density
current venting by using a nature of high-concentration sediment transport flow. Although this techniqueinvolves high cost caused by tunnel construction, there are several advantages such that it is also
applicable to existing dams; it does not involve drawdown of reservoir level and therefore no storage
capacity loss; and it has relatively small impact on environment because sediment is discharged not so
rapidly as sediment flushing, which is described later. Recently, the bypassing effect is clarified after
completion of the Asahi dam (Figure 7), and the planning and construction of the bypass have been
advanced also in the Miwa, Matsukawa and Koshibu dams.
Low head dam to divert coarse and fine sediment

To Asahi bypass tunnel

TunnelintakeFigure 7: Coarse sediment load bypass around an Asahi instream reservoir using a bypass tunnel

836

Open Check Dams and Flood Mitigation Dams
Check dams have been built to stop sediment and prevent from sediment disasters downstream. Opencheck dams can be divided into two categories, beam dams and slit dams characterized by a different
management of sediment transport: beam dams are characterized by wide openings. To be more precise,the erosion control dams in the upstream region are planned to be converted to slit dams with notches,
which are so designed as to pass, not to trap, as much fine sediment carrying less risk of sediment disasteras possible. In recent years, check dams assembled from steel pipes or concrete dams having slit(s) havebeen built to stop sediment only at times when sediment outflow is great and to let sediment rundown into
lower river basins at times when water and sediment outflow is small (Figure 8 (right)).

Figure 8: (left) Masudagawa flood mitigation dam; (right) Slit check dam from steel pipes
Flood mitigation dam (FMD) is a gateless outlet dam designed only for the purpose of flood control
which provides long-term and efficient protection against floods, e.g. Masudagawa dam in Figure 8. Its
bottom outlets are installed at the original river bed to facilitate the sediment transport during flood andflush out the deposited sediment at the end of flood. FMD is one of good solutions in dam engineering forsustainable management of reservoirs, downstream river environment, and sediment transport. FMD is
expected as environmentally friendly, since almost all incoming sediment during flood periods can passthrough dam bottom outlets and there will be fewer impacts to downstream river environment.

Design of water and sediment release
Water and sediment management in river basins has tended to deal with local issues associated with water
discharge and water disturbance, managing the quantity and quality of sediment, and timing. Water
discharge and associated turbulence are important and required by environmental aspect independently
from the sediment issues, to enhance the biodiversity. Both of low-flow conditions and high-flow
conditions can be characterized by the magnitude, duration, frequency, timing and rate of change of flow,
and by their temporal and spatial variability. Furthermore, the release of water has influence to scour andtransport the sediment according its capacity. Not only the water has importance for the river basin but
also the sediment should be supplied with specific quantity and quality. To select a reasonable sedimentmanagement measure for a specific reservoir-river system, the above characteristics for water sediment
release should be clarified. There are many different sediment management options available and thatmanagement options may impact differently on different river basin. For example, the necessary water isdischarged artificially or naturally according to the river basin conditions for sediment replenishment
option. Furthermore, during drawdown flushing there is a time delay between water and sediment release.
This introduces the need to involve in recognition of the detrimental effects with the sediment quality(grain size distribution); we should define both of quantity and quality in the design criteria.

Conclusion
Various integrated management strategies of water and sediment incorporation to preserve riverenvironments downstream of dams in Japan are presented. Water management measures are presented as“Flexible Dam Operation” and instream flow which discharge rate is defined to maintain habitats of riverbiota, water quality in the each studied section of the rivers depending on each season. For sedimentmanagement measures are reservoir flushing, sediment replenishment, bypass tunnel, and flood
mitigation dams are discussed. The present problem for maintaining river health is to determine the837adaptive range of the river system to the flow and sediment regimes, which is the basic of all the measure.
In detail, it is necessary to find out flow and sediment release which can meet demands of various
functions based on data of hydrology, water quality, ecosystem, etc. Furthermore, the integrated
management measure for flow and sediment should be researched further, which means how to realize thereasonable and order exploitation and how to regulate and control reasonably by large hydropower project.
The sediment flushing of the Kurobe River is extremely important from both sides of the sustainable
management of dam reservoir and the securing of the sediment mobility in the sediment routing system.
By replenishing sand at different locations of the Nunome and Managwa Rivers within the downstream
reaches, the replenishment may direct future supplements for a more widespread dispersal of suitable
sand for fish spawning.

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