Sediment Management Strategies for Sustainable Reservoir
T. Sumi & S.A. Kantoush
Disaster Prevention Research Institute, Kyoto University, Goka-sho, Uji-shi, 611-0011, Japan

ABSTRACT: The worldwide sediment management techniques consist of three basic strategies:
sediment yield reduction, sediment routing, and sediment removal. The paper focuses on reservoir sustainability by considering the following points. One is how we should choose suitablesediment management options under the consideration of each sedimentation conditions. In thatcase, annual water and sediment inflow volumes comparing to reservoir storage volumes are thekey factors for the selections. The other is how we can design suitable volumes to be discharged
from dam reservoirs. Combination of excavation and sediment replenishment option is now becoming common in Japan. But it is still necessary to define target volumes to be excavated fromreservoirs and supplied to downstream river channels for river restoration. Currently these projects are still in trial stage with limited percentages of sediment replenishment ranging between
0.1 to 10 % of annual reservoir sedimentation volume and supplying grain sizes are mainly
sandy materials. Case study at Nunome dam is a good example of comprehensive sedimentmanagement in upper Kizu river basin, Japan, and field survey shows that it contributes positively to rebuilt sand bars and mitigate the armouring of river bed which create suitable habitats
for aquatic species.
1 INTRODUCTION
Sustainability of reservoirs and downstream reaches below dams are threatened by reservoir sedimentations. The objective of water and sediment incorporation is to manipulate the riverreservoir system to achieve sediment balance while maximizing the beneficial reservoir storageand minimizing downstream environmental impacts. Several methods for sediment management
are available and have been implemented in practice. At the same time, an attempt to return the
excavated and dredged sediment to the downstream river (hereafter we all ‘sediment replenishment’) has been undertaken aiming at balancing sediment inflow and outflow sediment budget.
An understanding of channel geomorphic responses to various sediment replenishment technique and natural disturbances is important for effective management, conservation, and rehabilitation of rivers and streams to accommodate multiple, often conflicting, needs. For example,
channel changes may have implications for various needs including water supply, infrastructure,navigation, and habitat. Possible geomorphic responses of a channel to disturbances include
channel-bed erosion or deposition, channel widening or narrowing, and channel straightening.
1.1 Sedimentation rates
Today’s worldwide annual mean loss of storage capacity due to sedimentation is already higher
than the increase of the capacity by construction of new reservoirs (Boillat et al., 2003). Thus,
sustainable use of the reservoirs is not guaranteed on the long term. The time evolution over thelast century of the water storage capacity and volume losses due to reservoir sedimentation inJapan, Switzerland and France are presented in Figure 1(a). These reservoirs are now facing a
critical question of sedimentation. To maintain the existing dams and their facilities over thelong term becomes an essential policy issue. Because of the following reasons: sedimentation isproceeding more than expected in many dams; the share of the dams having a design life of
more than 50 years, such as multi-purpose dams where maintaining storage capacity is absolutely necessary, will rapidly increase in the future; and due to social changes in environmentconscious trend and an era of low-growth economy. Figure 1(a) shows the relationship between
the changes in the reservoir storage capacity and the storage capacity loss due to sedimentationin Japan. While the storage capacity is being increased by the construction of new dams, it is being lost by an average rate of 0.24 %/year. The figure shows that the average in Switzerland
about 0.2% of the storage capacity and in France is 1.08%.
35000
Storage Capacity in Japan
Reservoir Sedimentation in Japan

30000

Storage Capacity in Switzerland

Storage Capacity（MCM)

Reservoir Sedimentation in Switzerland

25000

Japan: sedimentation rate 0.24%/yr

Storage Capacity in France
Reservoir Sedimentation in France

20000

15000
France: sedimentation rate 1.08%/yr

10000

(b)

(a)

5000
Switzerland: sedimentation rate 0.20%/yr

0
1900

1950

2000

2050

2100

Time (Years)

Figure 1. (a) Development and estimated evolution of installed water storage capacity and volumes lost
due to reservoir sedimentation; (b) Global reservoir sedimentation rates (Basson, G. 2009).

There are no accurate data on the rates of reservoir sedimentation worldwide, but it is commonly accepted that about 1–2% of the worldwide storage capacity is lost annually (Jacobsen,
1999). A detailed collection of sedimentation rates in regions all over the world can be found in
Figure 1(b). The volumes of water-storage capacity lost due to reservoir sedimentation and thevolumes of installed water-storage capacity in the world are presented in Figure 1(b). The graphshows the evolution over the last century, and the predicted future development.
1.2 Influences of sediment deficit on downstream reaches below dams
Reservoir construction and operation can have a substantial effect on the stability of the river
channel downstream from the dam (Sumi and Kantoush, 2010). Reservoirs can trap and permanently store virtually the entire sediment load delivered from the upstream basin (Petts, 1979;
Williams and Wolman, 1984). Thus, immediately downstream from a dam, a river’s sedimentload is greatly reduced. In addition, typical downstream changes in the flow regime include a
reduction in the magnitude of peak flows and a possible increase in the magnitude of low flows(Williams and Wolman, 1984). In response, the downstream river may adjust in an attempt tore-establish an approximate equilibrium between the channel and the discharge and sedimentload being transported. Possible adjustments include channel-bed erosion or deposition, channel
widening or narrowing, and changes in channel pattern or shape. 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 asocio-economic problem, for instance due to loss of reservoir capacity (e.g., Fan and Springer,1993). 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.

1.3 Objectives
The main purpose is to explain, how to select appropriate sediment management strategy in eachreservoir according to sedimentation conditions within reservoir and in downstream reaches.
The other is how we can design suitable volumes to be discharged from dam reservoirs. In thispaper, the results of study in Japan that used sediment replenishment strategy to investigate thegeomorphic responses of channels to disturbances are presented. The examples provided demonstrate the use of sediment excavation and replenishing to downstream reaches to reduce sedimentation in reservoir and improve downstream reaches. Given that currently about 20 reservoirs are excavated and supply the sediment to downstream rivers in Japan, the techniques
described here may have utility for sediment replenishment and geomorphic response investigations nationally.
2 SUSTAINABILITY OF RESERVOIRS
2.1 Concept and economical feasibility
The concept of sustainability applied to agriculture developed, ground water development,
and road engineering. The essential concept of sustainable development is that the welfare of future generation should logically figure into the project decision-making. Reservoirs arguably
represent today’s class of non-sustainable infrastructure. The objective of reservoir sedimentation management is to minimize the adverse effect that sediment deposition in a reservoir has on
its usability. The sustainability of reservoir should seek to balance sediment inflow and outflow
across the reservoir while maximize the long-term benefits, the concept of sustainability isshown in Figure 2. The optimization of sediment removal methods as a dam group is inexpensive if a dam group was linked with each other for sediment management. This may involve
strategies to minimize sediment inflow, enhance sediment release, or combination of several
countermeasures for coarse and fine sediments. Examples of each facilities and proper maintenance sustainable reservoir management under the limited budget are presented. Technically, efficient economically and environmentally countermeasures, the coordinating sediment management of multiple reservoirs in a river basin, are discussed. The main development patterns forreservoirs and sustainable development are summarized in Figure 2.
The latter essentially implies that the current generation uses resources in a consumptive
manner, leaving the problems of dealing with its remains to future generations without providing resources to do so. Integrated models of sediment flow and morphological dynamics in both
regulated and free flowing rivers are necessary. Integration of different skills and approaches
through provision of effective reservoir sediment management system to prolong the reservoir
lifetime as illustrated in Figure 3, should allow the research to obtain some significant advancesin the understanding of dam impacts.
Sediment
impacts
assessment

Namingof priority
dam

Actions

Sedimentation
process
assessment

Economicassessment

Screening of
Sediment
management
Implementation
countermeasures
Monitoringof control
measures

Long‐term
Sustainabilityof reservoir

Criteria

Maximizing
sediment
releaseCost andbenefits
Feasibility of
sediment
strategy

SafetyEnvironmental
and social
Biodiversity
Water
quality

Food
supplyEnvironmental
compatibility
Habitatdiversity

Figure 2. Concept of a sustainable reservoir.
time.

Figure 3. Illustration of prolongation of reservoir life-

Requiring a reservoir life measured in terms of thousands years instead of decades will demand new methods of analyzing costs and benefits. For all these reasons, developing new techniques to evacuate the fine and coarse sediment to maintain the functionality, and at the sametime ecologically rehabilitating the involved landscape would be economically and environmentally beneficial for all types of reservoirs.
3 SEDIMENT CONTROL STRATEGIES
3.1 Classifications of sediment management techniques
Controlling reservoir sedimentations means in fact the control of sediment deposition in reservoir.
It consists of three basic strategies:[1] Sediment yield reduction: to reduce sediment inflow to reservoirs. Apply erosion control techniques to reduce sediment yield from tributary catchments.
[2] Sediment routing: to pass sediment inflow around or through the reservoir so as not to accumulate in reservoirs, by techniques such as partial drawdown sluicing and turbidity release.
[3] Sediment removal: to remove accumulated sediment by drawdown flushing, dredging and excavation by mechanically or hydraulically techniques.
Figure 4 shows how sediment management is undertaken and classified. And some representativedams and examples from Europe and Japan, which exercise sediment management, are listed up.
Classification

Timing

Place

Methods and details of
sediment control measures

Examples ofdams in Japan

Examples ofdams in Europe

Afforestation and Vegetation practices

Sediment
yield
Reduction

Nonseasonal

Catchment,
River US ofreservoir

Settling and off-stream storage basins
Slope and bank protection, River regulation

Sediment Management Strategies

End ofreservoir

Sediment check dams

End ofreservoir

Sediment bypass
Sediment
sluicing

Seasonal
Sediment
Routing
Inside ofreservoir

Turbidity
current
venting

Sabo Area, Changing from sediment checkdams to sediment control dams
Miwa, Koshibu, Nagas
hima

BeninarAsahi, Miwa, Koshibu,
Yokoyama,

Egschi, Rempen, Pala
gnedraGated outlets

Sabaishigawa, Dashidaira, Unazuki

Luzzone, Livigno

Gatless outlets

Masudagawa

Orden, Ligistbach

Bottom outlet

Koshibu, Futase, Kiga
wa

LuzzoneNone-gate & curtain wall

Katagiri

Glockner-Kaprun

Selective withdraw inlets

YahagiTourtemagne

flushing outlet

Dashidaira, Unazuki

Bodendorf, Gebidem,
VerboisSediment scouring gate

Senzu, Yasuoka

Feistritz, Edling

Sediment scouring pipe

Ikawa

Annabrucke

Sediment
Replenishment

Nunome, Akiba, Futase, Murou

Margaritze, Emosson

Recycling for
concreteaggregate

Miwa, Yasuoka, Hiraoka

Bodendorf, Genissiat

Miwa, Yanase

Margaritze, Sylvenstein, Forni

GrimselSubmerged dam, groynes

Seasonal

Insidereservoir

Drawdown
flushing
Partialflushing

Sediment
RemovalDry
excavation

Nonseasonal

End ofreservoir

MechanicallyDredging

Insidereservoir

Hydraulically

Sediment siphoning

SakumaLuzzoneSediment redistribution

SakumaPieve di Cadore

Figure 4. Classification of strategies of sediment control in Japanese and European reservoirs.

4 SEDIMENT REPLENISHMENT TECHNIQUE
There are two techniques to reduce the amount of transported sediment: 1) countermeasure tocontrol sediment discharge which covers entire basin including the construction of erosion control dams; and 2) countermeasure to forcibly trap sediment by constructing check dams at theend of reservoirs. In the technique with sediment trap, a low dam is constructed at the end of reservoir as to deposit transported sediment, mainly bed load of relatively coarse grain size. The
accumulated sediment can be excavated on land except for flood time, and the removed sediments have been utilized effectively as concrete aggregate. As of 2000, the check dams havebeen constructed at 57 out of the dams under jurisdiction of Ministry of Land, Infrastructure,
Transport and Tourism.

4.1 Sediment replenishment projects in Japan: Volume and grain size
In Japan, it is common practice to remove accumulated coarse sediment by excavation anddredging, and to make effective use of the removed sediment. Sediment replenishment method
is one of new measures of sediment management. In this method, trapped sediment is periodically excavated and then transported to be placed temporarily downstream of the dam. In amanner decided according to the sediment transport capacity of the channel 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 by truck to downstream
river; (3) placing the sediment with specific geometry, and (4) monitoring flow, sediment, andenvironmental parameters.
Recently, sediment replenishment tests have been carried out in 20 dams in Japan. Okano etal. (2004) summarized sediment replenishment projects in Japanese Rivers. Kantoush et al.
(2010) investigated the morphological evolution and corresponding flow field during replenishment experiments in Uda River, Japan. Sediment treatment system is applied by Sumi 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.
Sediment replenishment volume and grain size are recognized as key factors for a successful
management in the river basin to create and maintain physical habitats, aquatic and riparian ecosystems. Figure 5 shows the relationship between the annual excavated sediment volumes fromreservoirs and annual reservoir sedimentation volumes within reservoir (Sumi and Fujita, 2009).
Percentages of sediment replenishment are very limited ranging between 0.1 to 10 % of annual
reservoir sedimentation since these projects are still in trial stage. Figure 6 shows the grain size
distributions of replenished sediment which are very much different in each dam because of thelocation to take sediment out from reservoirs. In Miharu and Nunome dams, relatively finesediments are dredged in upstream secondary reservoirs for trapping nutrient rich sediments.
Others are mainly excavating coarse sediments from conventional check dams.
Annual Sediment Replenishment
Volume（m3）100,000

1:10
1:100
10,000

1:1000
1,000

100
10,000100,000

1,000,000

Nibutani
MiharuFutaseKawamata
AimataKawaji
HachisuYahagi
Nagashima
Akiba
Nagayasuguchi
Urayama
Hitokura
Shimokubo
Agigawa
NunomeSagami

10,000,000
3Annual Reservoir Sedimentation Volume（m ）Percentage Passing（％）

Figure 5. Relationship between annual reservoir sedimentation and replenishment volumes in Japan.
100

Nibutani

90

Miharu80

Futase70

Hachisu

60

Managawa

50

Nagashima

40
30

Urayama20

Shimokubo

10

Akiba

0
0.001

Agigawa
0.01

0.1

1

10

100

Grain Size（mm）

Figure 6. Grain size distributions of sediment replenishment

Nunome4.2 Case study of Nunome dam actual sediment replenishment tests
Since 2004, six experiments of sediment replenishment projects with different sediment volumes are undertaken below Nunome dam. Tracking the history and performance of these projects is helping to understand the evolution of sandbars. Table 1 summarizes the replenishment
history, flushing flow and sediments characteristics. The volume of placed sediment is limited
to several hundreds of cubic meters each time. To implement this method, consideration has tobe 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. The data in Table 1 indicates that
there is a need to increase the amount of the supplied sediments in Nunome River every year.
The replenished sediment is placed at such an elevation; 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 and all sediment is eventually transported downstream (Figure 7). 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.
Table 1. History of the sediment replenishment
tests downstream of Nunome dam
Year

2004
2005
2006
2007
2008

2009

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

Flood period

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

* The remained 65 m3 sediment is removed.

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

Sediment volume 500m3
Remained sediment from 2008
Before 2009 Flood

Discharge 81 m3/s

NA: No placed sediment or No flood occurredAll
sediment
erodedPlaced sediment
geometry

Flood stage
Normal stageDuring Flood （8-10-2009）

Figure 7. Concept of sediment replenishment

Normal stage

Figure 8. Evolution of replenishment experiments.

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 of81 m3/s. When the discharge exceeds 8 m3/s, the erosion starts and about 40 m3 of sediments aretransported. Figure 8 shows the photos of field tests phases. All of the remained sediments are
removed, and then new dredge sediments are placed.
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 9. Aerial photographs of Nunome dam with the downstream reach of Nunome River. Morphological changes and the self forming sand bar due to sediment replenishment below the dam (14-10-2009)By using satellite images and aerial photos, a map of Nunome River in 2009 is constructed
for 1 km below the dam. The river channel, island, point bars, and vegetation area are identified
and distinguished by color as shown in Figure 9. The downstream reach of the dam, first 150 m
from dam, experienced the greatest change in channel structure and loss of bars and islands. The
replenishment processes are efficient to restore the bed load transport and the associated habitedby coupling reintroduction with floodplain habitat restoration. In Figure 9 along the Nunome
River, several cross sections are identified to survey after replenishment. Newly depositions
over sand bars and in the river channel are shown in Figure 9. Moreover, a completely new sandbar is formed after 600 m from dam. With the field experiments the processes are directly visible, and will be used for validation of numerical models.
5 SUITABLE SELECTION OF SEDIMENT MANAGEMENT TECHNIQUE
Figure 10 shows flow chart for setting up sediment management strategy. Firstly, we have topick up dams for sediment management (1) and select high priority dams (2). We should consider design life cycle for redevelopment project for these dams to calculate investment cost andbenefit (3). Based past record of reservoir sedimentation, we can estimate future expected sediment inflow (4). Here, stochastic approach considering extreme flood events is recommended toassess long term sediment yield and transport process (Sumi and Kantoush, 2010).
After that, we should consider parallel studies; one is to design necessary reservoir capacity
volume for maintaining original functions of dams (5) and design necessary sediment supply
volume to realize it (6). The other is to design sound river environment (7) and design necessary
sediment supply volume and grain size (8).
①Pick up Dams for Sediment Management②Select Dams for Priority
Low
Maintenance
Current Priority
Works
Feed back if that projectsare not economically feasible High
③Design Life Cycle④Assess Inflow Sediment Volume
Dam View Point
Downstream View point⑤Design Necessary Volume for⑦Design Sound
Maintaining Original Functions
River Environment⑧Design Necessary Sediment⑥Design Necessary
Supply Volume and grain size
Sediment Supply Volume
Feed back if projects scenarios can not be proposed⑨Propose Sediment Discharge Scenarios
Feed back in case that ⑩Select Optimal Sediment Management Measuressignificant problems arise⑪Project Evaluation(Benefit/Cost Analysis)⑫Implement Sediment Management Measures⑬Implementation and Monitoring

Figure 10. Flowchart for setting up sediment management strategy

Finally, we should combine these two needs and decide appropriate scenario to discharge sediment from dams (9). If there do not exist suitable scenarios, we should go back to (3). Basedon the proposed scenarios, we can select optimal sediment management measures from several
possible options such as reservoir flushing, sediment replenishment, bypass tunnel etc (10). If
technical significant problems may arise, we should modify the scenario.
Selected sediment management measure should be evaluated by benefit/cost analysis (11)and, if the project is not economically feasible, we should revise the project from the masterplanning stage. Sediment management of reservoirs is very much complicated project which
will affect reservoir sustainability and also maintaining downstream river health. So we should
start to implement step by step basis (12) by conducting filed monitoring under the adaptive
management concept (13).

6 CONCLUSIONS AND DISCUSSIONSIn case of sediment management, it is necessary to find out appropriate combination of flow
and sediment release which can meet demands of various functions based on data of hydrology,
water quality, river morphology and ecosystem, etc. Furthermore, the integrated sediment management approach should be considered in sediment routing system which covers not only river
basin but also coastal area. Since suitable sediment replenishment volumes and grain sizes are
key parameters to design effective management strategy, these values should be decided fromboth view points of reservoir sustainability and downstream environmental improvements.
Sediment replenishing case study at Nunome dam is a part of comprehensive sediment management in upper Kizu river basin containing fiver multi-purpose dams. We are now starting toclarify how to sustaining reservoir functions and how to improve river environment at the sametime by coordinating operation. The replenishment directs future supplements for a more widespread dispersal of suitable sand for fish habitat including spawning beds. We shall intensively
conduct these research and development to pursue the goal of harmony between human beingsand other creatures. Regarding the continuation of this research project, the major goal is to findout which sediment geometry, volume, and feeding frequency leads to environmental andphysical improvements below dams. This requires long duration tests combined with numericalmodeling techniques.
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