Advances in River Sediment Research – Fukuoka et al. (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00062-9

Sediment management on the Arase Dam Removal Project
S. Fukuoka

Faculty of Science and Engineering, Chuo University, Tokyo, Japan

T. SumiWater Resources Research Center, Disaster Prevention Research Institute, Kyoto University, Kyoto, JapanS. Horiuchi

Arase Dam Removal Project Office, Enterprise Bureau, Kumamoto Prefecture, Japan

ABSTRACT: Sediment management during and after dam removal involves extremely important flood
control and environmental challenges, including the outflow of reservoir sediment and its effects downstream. The Arase Dam is the first dam to be completely removed in Japan. To assist in sediment disposal
planning and to predict both upstream and downstream changes after dam removal, riverbed change
simulations and environmental monitoring have been employed. Herein items to consider in sedimentcontrol for dam removal are discussed.
1

Introduction

Dam removal is one of important options for
future reservoir management. In such case, suitable
management of accumulated sediment in the reservoir is a crucial topic for effective and environmental friendly operation from the viewpoints of river
bed geomorphology, water quality and ecosystem
in and downstream of the reservoir. Basic understandings are explained by Morris and Fan (1997),
and Erosion and Sedimentation manual by USBR
(2006). Geomorphologic aspects focusing onchannel adjustment in the former reservoir area is
explained by Doyle, Stanley and Harbor (2003).
Complex ecological aspects are discussed by

Stanley and Doyle (2003). Cannatelli and ­Curran(2012) is reporting importance of hydrology onchannel evolution following dam removal.
The Arase Dam is the first dam to be completely
removed in Japan which is located approximately
20 km upstream from the mouth of the Kumagawa
River (Fig. 1). The dam is dedicated to hydropower
generation, and has been contributing to the economic development in the Kumamoto Prefecture
for over 50 years. However, plans are underway
to remove the Arase Dam over the next six years.
Because this is the first complete dam removal
project in Japan, the removal policies of the Arase
Dam must not only consider flood management
and the environmental impact, but also an economic removal method and the natural recoveryof the river environment in medium and longterms. Based on this background, a committee andworking group were created to study the removal
method, evaluate the environmental impact, andprovide a detailed review.
2

Figure 1. Dam facilities location.

Arase Dam Removal Committees
and Working Groups

The Arase Dam Countermeasures Review­Committee and Dam Removal Methods Working
Group were established in June 2003 to remove
the Arase Dam with considerations to flood control and the river environment. In January 2006, a
dam removal policy was developed, and a detailed
review of the removal method and other aspects

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were completed by March 2008. However, the dam
removal initiative was put on hold.
Since then the dam removal initiative has been
revived. The Arase Dam Removal TechnicalResearch Committee was established in April 2010
to reconfirm and verify the impact of dam removal
as well as to provide guidance and advice to theprefectural government. This committee’s work led
to the Arase Dam Removal Plan (Provisional) for
the actual removal work, which was to commence
in FY 2012, and included the latest findings andchanges in the environment since the first removal
decision related to the dam. Below a summary ofthe discussions within the Committee and Working Group.
• Arase Dam Countermeasures Review Committee
Based on the reports from the Dam Removal
Methods Working Group, the Arase Dam
Countermeasures Review Committee provided
a direction to the prefectural government fromthe specialists’ viewpoints for dam management, environment, dam removal method, andother areas.
° The 29-member committee held 9 meetings.
° Items reviewed:– Dam removal methods– Sediment removal and disposal methodfrom reservoir area
– Environmental investigations for dam
removal– Dam management– Current environmental countermeasuresand verification of their effectiveness:– Sediment removal from the dam reservoir– Repair of the embankment along national
and prefectural roads– Countermeasures against red tide
– Sediment supply downstream– Surveys on water quality, etc.
• Dam Removal Methods Working Group
This working group was established as a working group for the Arase Dam Countermeasures
Review Committee.
° The 9-member group held 12 meetings.
° Items reviewed:– Dam removal methods– Sediment removal and management– Environmental investigations– Challenges related to dam removal• Arase Dam Removal Technical ResearchCommittee
This committee confirmed and verified thereview findings of the committees and otherbodies as well as provided guidance and advice
to the prefectural government towards establishing a removal plan (prefecture's plan).
° The 14 members and 3 advisors met three times.

3 Overview of the Dam Removal
Plan
Two major dam removal methods were considered: slit removal and slice removal. Eventuallyslit removal was selected. The dam is situated in a
curve of the river. When the dam was constructed,
there was a developed sandbar on the left bank,and the center of the water flow was near the rightbank. To recover this river channel geomorphology as soon as possible, the plan was to remove thedam from the right bank side.
The dam removal work is scheduled to occur insix phases from 2012 through 2017 during the nonflood seasons (Fig. 2).
• 1st phase (2012)
Install a water level—lowering facility to lower
the reservoir water level and remove the gate on
the right bank.
• 2nd phase (2013)
Remove the gate, pillars, and utility bridge fromthe right bank.
• 3rd phase (2014)
Remove the remaining gates and the water channel of the main dam body on the right bank.
• 4th phase (2015)
Remove the remaining parts of the water channel on the right bank.

Figure 2.

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Removal plan.

• 5th phase (2016)
Remove all remaining pillars and utility bridges.
• 6th phase (2017)
Remove the main dam body.
4

Sediment Disposal Plan

The Arase Dam was constructed more than 50
years ago. Its reservoir contains a lot of sediment,
including silt, sand, and gravel. Consequently, damremoval may impact flood control and the environment if a large quantity of sediment is allowed toflow downstream during a heavy runoff. To prevent
this runoff, the Sediment Disposal Plan removes
sediment from the reservoir prior to dam removal.
The current sedimentation was surveyed todevise a plan that is appropriate for each type of
sediment. Downstream riverbed changes associated with dam removal were simulated to establish
the amount of sediment to be removed for minimizing the impact.

generated, the silt removal method was basicallydry excavation during the non-flood season.
4.2 Sand and gravel disposal
Besides silt, approximately 700,000 m3 of sedimentremains in the reservoir. The plan is to allow mostof the sand and gravel to flow naturally. However,to minimize the impact on downstream flood control due to sand and gravel settlement associatedwith dam removal, the plan partially removed sandand gravel before the dam removal. The amount tobe removed was decided using a riverbed change
simulation.
4.2.1 Riverbed change simulation
The riverbed change simulation for sand and gravel
disposal was performed upstream and downstreamof the Arase Dam. A one-dimensional model enabled the large temporal and geographical span
to be calculated. The simulation period was fifty
years after the start of dam removal with actual
flow data repeatedly supplied.

4.1 Silt disposal
If silt is allowed to flow naturally from the reservoir without disposal as part of the dam removal
project, various environmental impacts are likely tooccur downstream. Specifically, if a large quantity
and high concentration of sediment flows during aheavy runoff and settles downstream, the riverbed
would change, which may negatively impact the
spawning fields of the Ayu fish.
To avert these issues, the Silt Disposal Planremoved approximately 100,000 m3 of silt fromthe reservoir mainly from upstream of Arase dam
and Kudaragi river which is the first tributary in
the left bank prior the first phase of dam removal(Fig. 3). To minimize the amount of turbid water

Figure 3.

Silt distribution.

Figure 4.

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Flood water level change after dam removal.

surrounding areas of the Arase Dam assuming thatthe dam is removed over six years and 100,000 m3
of sand and gravel is removed. This simulation
confirmed the appropriateness of the dam removal
project from a flood control viewpoint in terms ofchanges in the state of the river.
5.1 Calculation conditions

Figure 5. Sand and gravel removal area.

To determine the impact downstream, thesimulation used different combinations of dam
removal processes and the amount of sand andgravel removed. The results indicated that a phased
removal of sand and gravel in the area immediately
upstream of the reservoir can reduce the rise in theflood water level in the medium and long terms(Fig. 4).
Examined cases of sediment removal• Case 0: All in time of 100,000 m• Case 1: Before dam removal 80,000 m3, Afterdam removal 20,000 m3• Case 2: Before dam removal 50,000 m3, Afterdam removal 50,000 m3• Case 3: No sediment removal
3

4.2.2 Sand and gravel disposal plan
Based on the riverbed change simulation results,
a Sand and Gravel Disposal Plan was created
to remove approximately 100,000 m3 (Case 2:
50,000 m3 before and another 50,000 m3 during
dam removal) of the approximately 700,000 m3 ofsand and gravel in the area immediately upstreamof the reservoir (Fig. 5). In addition to restoring
the Kumagawa River basin and the Yatsushiro Sea
Area, the removed sand and gravel will be used forother purposes such as public works.
5

Each removal phase was assigned a floodhydrographs, which caused the largest floods in thepast. Removal phases with similar flow rates weregrouped together. Thus, the number of removal
phases was reduced from six to four (Fig. 6). The
one-dimensional simulation results were used as
the boundary conditions for the two-dimensionalmodel (incoming flow rate, inflowing sedimentmaterials, and water level downstream) to maintain continuity with the one-dimensional model.
5.2 Calculation results of the simulations
Figure 7 shows the simulated temporal changesof the riverbed height and its planar distribution.
As dam removal progresses, sediment accumulates
from the center of the water flow to the left bankin areas immediately downstream. Over time, asandbar is formed, and it eventually connects withthe upstream left bank. In contrast, the upstream
riverbed is removed by the water flow on the outerbank of the curved part of the right bank.
River flow patterns in the vicinity of the dam
at high levels of water flow during and after dam
removal indicate that the main part of the flow isnear the right bank (outer bank) (Fig. 8). After thedam removal project is complete, the river flows
smoothly and a sandbar forms from the center ofthe river to the downstream left bank, restoring
this portion of the river to its original path before
the dam was constructed.

Riverbed Changes Associatedwith Dam Removal

Based on the one-dimensional model, the Sediment
Disposal Plan specifies the amount of sediment tobe removed during each phase of dam removal.
Although the one-dimensional model provides a
macroscopic view of sediment movement in themedium to long term, it does not provide yieldchanges in the planar distribution of shallow parts,
deep parts, and sandbars during a high level ofwater flow.
To resolve this shortcoming, a two-dimensionalriverbed change simulation was performed in the

Figure 6. Calculation condition. (Step of removal andhydrograph of flood dischage).

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­ nvironmental Monitoring Survey Plan wereE
established in 2007.
Environmental protection countermeasuresand environmental monitoring surveys have been
conducted since FY 2008 in accordance with theseplans. The flow rate and water quality were studied
between FY 2008 and FY 2010. Comprehensive
environmental monitoring surveys, including surveys on flora and fauna, commenced in FY 2011.
In April 2011, the Arase Dam Removal­Follow-Up Working Group was established to
discuss the survey results and assess the environmental impact of dam removal. Environmental
monitoring will continue until two years after
the dam removal project is complete (FY 2019).
­Figure 9 shows the review work to date.
6.1 Environmental protection countermeasuresplan

Figure 7. Contour of bed level and bed deformation.
(Simulation).

As the environmental impact prediction is assessed,
the plan is to provide “environmental protection measures” for items that have been predicted
to impact the environment. The plan also mitigates
items forecasted to have a minor environmental
impact by providing “measures to reduce further
impact”. Table 1 shows action items and policies
for each countermeasure.
6.2 Environmental monitoring survey plan
In addition to the aforementioned environmental
protection countermeasures, the plan includes anenvironmental monitoring survey to observe itemswith uncertainties in the basic items and prediction methodology. The figure below shows the survey items, while the table lists the environmental
monitoring survey items. The survey items coverecological systems, physical environment, chemical
environment, and biological environment as well as
items to assess the environmental impact due to theconstruction machinery during the removal project(atmospheric pollution, noise, and vibrations).

Figure 8. Velocity distribution (Simulation).

6

Environmental Protection
Measures and Environmental
Monitoring

Basic policies for the environmental impact prediction associated with the removal of the Arase
Dam were established by the end of FY 2005.
These policies are based on a survey of the current status (April 2004 to March 2005) with prediction and evaluation of environmental changes
due to dam removal. Furthermore, a detailed study
was conducted on the changes in living organisms
and their environment due to dam removal. Fromthe basic policies and study results, the Environmental ­Protection Countermeasures Plan and• Physical environment• Water flow, Sediment property (Grain size),
Ecological system, Landscape• Chemical environment• Water quality (Turbidity, Suspended Solid
Concentration),• Ecological environment• Fauna (Bird, Fish, Benthos, Important
species)
The surveys and assessments are designed to provide a comprehensive understanding of the overallchanges in the ecological system with an emphasis on habitats and reproductive environments ofliving organisms. Moreover, the plan evaluates the
distribution of plants and animals.

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Figure 9. Flow chart of examination.

If the surveys reveal unforeseen issues, the Arase
Dam Removal Follow-up Working Group willaddress them. This working group has the flexibility to add items to the surveys to address specificissues. Below are examples of surveys under consideration or in progress.

1. Belt transect survey on fauna
Because opening the dam gates will lower waterlevels in flooded areas and dam removal should
revive vegetation in flooded areas, the vegetation in upstream areas should change. To quantitatively assess these changes, five survey lines1206

Table 1.

Implementation items of the environmental conservation measures and policy.

Implementation items
Animals
Important species of benthic fauna
Paludinella devilis Lymnaea
PlantsImportant plant species
Water speedwell
Salvia plebeia
Siberian motherwort
WastesConstruction by productFigure 10.

PolicySurvey where they may be transplanted and consider measures since drawdown
expected during the construction period may affect the river, including temporary loss of their habitat.
Consider measures thinking of the condition of growth in the areas along theKuma River since the part of the land area including sandbar are expected
to be submerged due to the water level rising in the recession area during
construction and after removal.
Consider how to control and reuse soil and concrete expected to be generated
during construction in order to minimize their disposal.

Shino.

have been selected, including group scale, coverscale, and tree height.
2. Ayu fish apawning field survey at Gedainose
Ayu fish symbolize the clarity of Kumagawa
River water, and many people enjoy Ayu fishing from early summer to fall. For these reasons, many local residents are concerned about
the Ayu fish and their habitat. Thus, the survey aims to assess the impact of sediment due
to dam removal on the downstream spawning
fields of the Ayu fish as well as obtain comparison data for the Ayu fish spawning fields
that are expected to appear upstream of thedam. In particular, there is an artificial spawning field for the Ayu fish at Gedainose where asurvey is to be conducted. Ayu fish spawn theireggs on riverbeds where suitable size of gravels
are softly placed and flows underneath them.
Instead of counting the number of spawning
fish, the survey uses a simpler method; a survey
worker places a steel construction tool called a“Shino” ­(Fig. 10) into the riverbed with his/herown body weight, and its measured penetration
is an indicator of how the riverbed is packed.
3. Physical environment survey
To directly assess the impact of sediment outflow
due to the removal of the Arase Dam (causedby the methodology and equipment such as
employing a water level–lowering facility) on the

Figure 11.

Monitoring point.

downstream physical environment during normal and heavy runoff, the cross-sectional profile,water level, flow rate, and the size of sedimentson the riverbed will be measured on two surveylines immediately downstream the dam using thelinear lattice and planar methods.
6.3 Interim report on the environmental
monitoring survey
Major changes have not occurred downstream ofthe dam. However, opening of the gates in April
2010 lowered the water level of the reservoir, andwater has returned to a flowing state in some
upstream sections (Fig. 11).
Water flow is particularly noticeable in the
Kudaragigawa River, which is a tributary on theleft bank upstream of the dam, and in Nishikamase, which is located near the upstream end of
the dam reservoir. In addition to plants returning to its banks, the Kudaragigawa River now has
streams with deep paths, multiple riffles as wellas pools (Fig. 12). Nishikamase now has large
sandbars on the bank or in the middle of the river(Fig. 13).

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Figure 12. Kudaragi creek.

Figure 13. Nishikamase.

Figure 14. Annual cycles of stream benthos.

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Additionally, riffles and pools, which used tohave their own names when the river was flowing
prior to construction of dam, have reappeared.
For example, riffles such as Miyase and Fujinose
and pools such as Fuchinomaki exist once again.
­Furthermore, the numbers of benthos living inrunning water, including ephemeroptera, plecoptera, and tricoptera, have noticeably increased since2010 after the Kudaragigawa River and Nishikamase returned to flowing environments (Fig. 14).
7

Conclusions

The Arase Dam is the first dam in Japan to becompletely removed. To assist in sediment management planning and to predict the upstream anddownstream changes in the riverbed, simulations
and environment monitoring have been employed.
Based on the reports from the Asase Dam RemovalTechnical Research Committee and the Arase Dam
Removal Follow-Up Working Group, this paper
discusses items to consider in sediment control fordam removal.
To provide a comprehensive assessment of thechanges, the physical environment, including theriver profile, riverbed composition, the ­chemical

environment in terms of water quality such as
turbidity, the biological environment of flora andfauna, and the physical environment of the riverwill continue to be monitored.
The authors would like to thank committee
members and others involved who provided withguidance based on their area of expertise.
References
Cannatelli, K. and Curran, J. 2012. Importance ofHydrology on Channel Evolution Following Dam
Removal: Case Study and Conceptual Model. J.
Hydraul. Eng., 138(5), 377–390.
Doyle, M., Stanley, E.H., Harbor, J.M. 2003. ­Channel
Adjustments Following Two Dam Removals in­Wisconsin, Water Resources Research, 39(1), 15–22.
Morris, G.L. and Fan, J. 1997. Reservoir Sedimentation
Handbook, McGraw-Hill, New York.
Randle T.J. and Greimann, B. 2006. Erosion and Sedimentation manual, Chapter 8, “Dam Decommissioning and Sediment Management”, Erosion and
Sedimentation manual, USBR.
Stanley, E.M. and Doyle, M.W. 2003. Trading off: theecological effects of dam removal, Front Ecological
Environment, 1(1), 15–22.

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