10th EAST ASIA DAM CONFERENCE

HYDRAULIC DESIGN IN UPGRADING DAMS UNDER OPERATION
Toshiyuki Sakurai1, Takayuki Ishigami2
1

2

Japan Dam Engineering Center, Tokyo, 110-0008 Japan
Hydraulic Engineering Research Group, Public Works Research Institute, Tsukuba, 305-8516 Japan
Email: sakurai@jdec.or.jp

ABSTRACT: In Japan, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) presented a"MLIT's Vision to Upgrade Dams under Operation" in June 2017. The vision introduces countermeasures
to advance the upgrading of existing dams by applying both structural and non-structural measures toensure that they are used more effectively than in the past.
There are cases raising the dam body and increasing the discharge capacity of the discharge facilities etc.
in the past. At some existing dams, improvement projects are currently under construction.
There are challenges in hydraulic design of improvement and additional construction of discharge
facilities in dam upgrading different from the construction of a new dam. Methods to cope with thesechallenges have been examined by hydraulic model experiments and so on. In this report, the outline ofthree major technologies to increase the discharge capacity in dam upgrading, cutting dam body, drillingdam body and tunnel spillway, are introduced. And taking Nagayasuguchi dam remodeling project as arepresentative example, the contents of the hydraulic design of the existing dam redevelopment project
are explained.
Keywords: Dam upgrading, Increase of discharge facility, Cutting dam body, Drilling dam body,
Tunnel spillway, Hydraulic design

1. INTRODUCTION
In Japan, dams completed so far played a major role in both flood control and water use. On theother hand, it is becoming more important to effectively utilize existing infrastructure stocks
while reducing the total cost, under the constraints of a falling productive-age population and asevere financial situation. Based on this recognition, MLIT presented a "MLIT's Vision toUpgrade Dams under Operation" in June 2017 [1]. The vision introduces countermeasures toadvance the upgrading of existing dams by applying both structural and non-structural measures
to ensure that they are used more effectively than in the past (Table 1).
Regarding "(4) Improving Facilities for a more Advanced Functionality" listed as one of themeasures, there are cases raising the dam body and increasing the discharge capacity of thedischarge facilities etc. in the past. At some existing dams, improvement works are currently
under construction.
There are challenges in hydraulic design of improvement and additional construction ofdischarge facilities in dam upgrading different from the construction of a new dam. Methods tocope with these challenges have been examined by hydraulic model experiments. In this report,
the outline of major technologies to increase the discharge capacity in dam upgrading is

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introduced and the contents of the hydraulic design of the existing dam redevelopment project
are explained, taking Nagayasuguchi dam remodeling project as a representative example.
Table 1: Measures to develop and accelerate dam upgrading(1) Life Extension of Dam(2) Promoting Efficiency and Advancement of Maintenance(3) Flexible and Reliable Operation to Optimally Utilize the Capacity of the Facility(4) Improving Facilities for a more Advanced Functionality(5) Responding to Climate Change(6) Addition of Hydropower(7) Conservation and Revitalization of River Environment(8) Regional Promotion utilizing the Dams
(9) Promoting Dam Upgrading Technologies Overseas(10) Development and Implementation of Technologies to Promote Dam Upgrading

2. TECHNOLOGIES TO INCREASE THE DISCHARGE CAPACITY
In the facility improvement of the existing dams, efforts to increase discharge facilities and toraise dams are being carried out for various reasons, such as changing the flood control plan,changing the water usage plan, and improving functions and so on. The outlines of threetechnologies, which are typical when increasing comparatively large capacity of discharge
capacity, improvement of the crest discharge facilities, improvement of the discharge facilities ina dam body and constructing new tunnel spillways are described as bellow.
2.1 Improvement and expansion of the crest discharge facilities (cutting dam body)This technology is to improve or expand the overflow type discharge facilities installed at the
crest of the dam and the orifice type discharge facilities with a design water head of 25 m or less.
The technique of cutting from the top of the dam body is often used. Since the remodeled part isnear the top of the dam, there are few problems concerning dealing with large water pressure. In
addition, if it is not necessary to install a temporary coffering facility by lowering the reservoir
level, construction work becomes relatively easy.
In the dams constructed early period, both the flood control spillway and the emergency spillway
may be adopting a discharge facility with a gate. In such dams, when the catchment area is small
and the flow rate increases rapidly during flood, the discharge facility is remodeled gateless typefacility in order to improve the certainty of the flood control function.
Remodeling to gateless will also reduce the burden of operation and maintenance. In order toremodel the emergency spillway to gateless type, the method to expand the overflow width isusually adopted. The challenges of this method are whether it can be placed at the top of the dam
body, or whether it can safely flow down to the downstream river channel by dissipating thedischarged water flow energy. In order to solve these challenges, a new emergency spillway is
installed on the lake bank in the reservoir, or a training wall is installed on the downstream of thedam along with the expansion of the overflow width at the dam crest.
As a case with a purpose other than increase the discharge capacity, at the Yamasubaru dam andSaigo dam in Miyazaki Prefecture (managed by Kyushu Electric Power Co.), to improve the報－133

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function of passing through sediment, cutting down the existing dam body and remodeling thedischarge facilities are in progress.
Regarding the method of cutting the dam body, Nagayasuguchi dam in Tokushima Prefecture(managed by MLIT), which is undergoing remodeling work, is the largest case in terms of
cutting depth in history in Japan. It is planned to cut the dam body at the maximum depth ofabout 37 m and to install two additional discharge facilities. As for Nagayasuguchi dam, it isdescribed the content of hydraulic design in chapter 3.
2.2 Improvement and expansion of discharge facilities in a dam body (drilling dam body)This technique is to drill a hole in an existing dam body and to install an additional discharge
facility. Usually it is applied to a gravity type concrete dam. In Japan, it is sometimes difficult tolower the reservoir level due to restrictions on the reservoir operation for the purpose of wateruse, and construction under large water depth conditions is required, so the technology of atemporary coffering upstream side of the dam is important.
A discharge facility with an acting water head exceeding 25 m is defined as a high pressure
discharge facility and the flow velocity becomes high speed during discharge, so it is necessary
to prevent cavitation damage due to a local negative pressure generation. For that purpose, it isrequired to reduce the flow velocity inside the pipe, but since the size and numbers of holes thatcan be drilled in the dam body are restricted by the dam structure, from the economical point ofview, increasing the flow velocity to increase the discharge capacity is advantageous.
Furthermore, it is difficult for the additional discharge facility to arrange the inlet and outlet ofthe discharge pipe straight on the relationship with the arrangement of the existing discharge
facilities. In this case, a complicated curved shape is adopted, but a local pressure drop tends tooccur in the curved part of the large diameter discharge pipe. In order to cope with these
problems, the examination of the water pressure acting at the inside wall of the bending part by ahydraulic model experiment was carried out [2]. Currently, it is possible to do rational design.
As an example of drilling of the dam body, the remodeling of Tsuruda Dam in Kagoshima
prefecture (managed by MLIT) is the largest scale previously recorded in Japan. Three additional
discharge conduit pipes (diameter: 4.8 m, drilled cross section: height 6.0 m and width 6.0 m)
and two replacement penstock pipes for power generation (diameter: 5.2 m, drilled cross section:height 6.4 m and width 6.4 m) will be installed (Figure 1). In the temporal coffering work at theconstruction of the remodeling of Tsuruda Dam, in addition to the conventional construction
method of assembling the steel coffering gates after installing the foundation pedestal concrete in
the water upstream of the dam, a new technology of a floating type coffering facility requiring nopedestal was developed. As a result, large depth diving work was reduced, cost reduction andsafety assurance were achieved.
If the discharge rate of discharge facility in a dam body is small, it can be discharged to theexisting energy dissipator, but in the case of large scale facilities such as Tsuruda Dam, aspecialized energy dissipator is newly constructed.
As for Tsuruta Dam, as well as Nagayasuguchi dam, the hydraulic design was examined while
confirming the flow condition by the hydraulic model experiments (Figure 1).

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Additional discharge conduits

Replacement penstock pipes

Additional
discharge
conduits

Existing
Spillway

Specialized energy dissipator
Energydissipator

Figure 1: Construction situation of Tsuruda dam (June 2017) (left) and hydraulic model (right)

2.3 New construction of tunnel spillway
When it is difficult to cut or drill the existing dam body due to restrictions on dam type and dam
structure in order to add large capacity discharge facilities, it is selected to construct a new tunnel
spillway in the mountains around the dam. As a representative example, there are two dams of
Kanogawa dam in Ehime prefecture and Amagase dam in Kyoto prefecture. Both dams aremanaged by MLIT and are currently under remodeling construction.
The tunnel spillway of Kanogawa dam is 11.5 m in tunnel diameter, 457 m in length, about
1,000 m3/s in discharge capacity. The tunnel spillway of Amagase dam is 10.3 m in tunnel
diameter, 617 m in length, about 600 m3/s in discharge capacity.
Because they are long pipe channels acting relatively large hydraulic pressure, they were
designed as pipe flow facilities where the cross section of the tunnel channels were large and theflow velocities were suppressed to around 10 m/s. Due to the pipe flow, air suction at the inflow
portion may cause vibrations of the gate at the outlet and disturbance of the flow condition. In
order to reduce air suction, it is necessary to deepen the position of the inflow part, but since they
are large-scale facilities and such arrangements are difficult, inlet part shapes were examined by
the hydraulic model experiment to suppress the generation of the suction vortex.
In construction of the inlet part in the reservoir of Amagase dam, a shaft-type underwater
working machine which can perform excavation work and so on underwater by remote control
from the land developed by a private company was adopted.
Both dams’ facilities have large discharge capacities and need energy dissipators, but due toconstraints on the arrangement within the downstream river channel, Kanogawa dam has acompact designed stilling basin with steps and baffle piers (Figure 2), the energy dissipator of
Amagase dam will be installed as underground type in the mountains.

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Tunnelspillway

Existing dam

Energy dissipator fortunnel spillway

Figure 2: Hydraulic model experimet situation of Kanogawa dam tunnel spillway

3. HYDRAULIC DESIGN IN NAGAYASUGUCHI DAM REMODELING PROJECT
3.1 Outline of Nagayasuguchi dam remodeling project
Nagayasuguchi dam is a multipurpose dam built in 1956 in Tokushima prefecture for flood
control, power generation and irrigation water supply. The dam type is a gravity type concretedam, with the dam height 85.5 m, total water storage capacity 54,278,000 m3, effective water
storage capacity 43,497,000 m3, catchment area 538.9 km2. Nagayasuguchi dam remodelingproject is to modify the dam for the purpose of increasing the flood control capacity, maintainingof normal function of the river water flow and improving the water quality of the discharged
water [3]. Regarding the increase of the flood control capacity, based on the flood control plan of
the Nakagawa River System, in order to secure the flood control capacity required, a part of thedam body on the right bank will be cut out and the two additional new spillways will be installed(Figure 3).
Newly installed spillways

Selective withdrawal facility

Remodeled energy dissipator

Figure 3: Outline of Nagayasuguchi dam remodeling project

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3.2 Hydraulic design by hydraulic model experiments
The hydraulic design by hydraulic model experiments of the newly installed spillways ofNagayasuguchi dam remodeling project was carried out at the Public Works Research Institutefrom 2008 to 2012. It was confirmed that required functions and safety as described below weresecured under conditions such as predetermined water level and discharge rate; whether therequired discharge capacity is secured, whether the water pressure acting on facilities such as thetraining channel part and the energy dissipator is within an allowable range, and whether a stableflow situation is formed. In the case that they were not secured, the design of the facilities wasrevised.
Specifically, the examinations on the overflow crest part, the training channel part, the energy
dissipator (the existing facilities and newly installed facilities released flood discharges merge
with each other) and the downstream river channel were implemented (Figure 4).
Overflow crest part

Existing spillways

Existing spillways
Energy dissipator

Energy dissipator
Training channel part
Overflow
crest partTraining channel partDownstream river channel

Figure 4: Plan view (left) and downstream view (right) of the original design shapeFirst, a hydraulic model of the original design shape by desktop design work was created, water
was flowed under conditions of predetermined water level, discharge rate, etc., and flow
conditions were checked. The overflow crest part was important to determine the flood control
function of the dam, it was necessary to confirm whether a predetermined discharge rate wasable to discharge at a predetermined water level and derive a relational equation between thewater level and discharge rate. Usually, it is studied with a large scale partial model (Figure 5),
which extracts only the overflow crest part, apart from the whole model (Figure 5) including theother parts of discharge facilities.
Figure 6 shows the situation of the hydraulic model experiment with the original design shape of
Nagayasuguchi dam.
The following problems were recognized from the experimental results of the original design. In
the overflow crest part, the discharge rate at the predetermined water level was insufficient. At
the training channel part, the water flow from the overflow crest hit the training wall with
momentum and dropped to the energy dissipator (stilling basin) without energy dissipation. In
the original design shape of the energy dissipator installing only sub-dam, the water stream on
the left bank largely creeped up in the area where the discharged water from the existing spillway
merged into the discharged water from the newly installed spillways.

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Figure 5: The whole model (scale:1/62.5) (left) and the patial model of crest part (scale:1/40) (right)

Figure 6: Flow stiation of hydraulic model experiment (training channel part: original desgin)

In order to improve these problems in the original design shape, as for the overflow crest part, bylowering the overflow crest top height on the river side discharge facility and changing theshapes of the overflow crests from the trapezoidal overflow spillway to the standard shape of
overflow spillway, the required discharge rate was secured.
Regarding the training channel part, various shapes such as changing the position and angle ofthe training wall, installing the slope and so on had been examined trial and error. The final
shape that separates and diffuses the discharged water from both spillways of the mountain sideand the river side was developed. As a result, the water level of the training channel part could be
kept low and the height of the training wall could be reduced.
As for the energy dissipator, a safe design shape for the dam and the downstream river channel
was obtained by improving the planar shape and installation of the side channel walls. The final
design shape is shown in Figure 7, and the situation of the hydraulic model experiment with thefinal shape is shown in Figure 8.
In addition, as shown in Figure 9, examinations on the flow situation in the case where discharge
from existing spillways occurred during the construction of the new energy dissipator were also
conducted by hydraulic model experiments.

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Overflow crest part

Energy dissipator
Energy dissipator
Training channel partTraining channel part
Overflow crest part

Figure 7: Plan view (left) and downstream view (right) of the final design shape

Figure 8: Flow stiation of hydraulic model experiment (training channel part: final desgin)

Figure 9: Flow stiation of hydraulic model experiment of the construction stage

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4. CONCLUSION
In the hydraulic design for upgrading dams, it is important to secure the required discharge
function and to ensure the safety of the dam body and the downstream river channel with respect
to the flow condition downstream of the dam which changes greatly due to an increase ofdischarge capacity.
In addition, since it is necessary to carry out the remodeling construction work while operating
the existing dam, the examinations how to secure the function of the existing dam underconstruction and how to minimize the influence of discharge from the existing dam on
construction work are important.
Regarding the technologies to increase the discharge capacity in Japan, in the past few years, thelargest historical projects of three representative methods of cutting dam body, drilling dam bodyand tunnel spillway have been started and are currently under construction respectively. Manytechnical experiences and knowledge have been accumulated in various fields through theimplementation of these designs and constructions, and it is expected to be utilized them for
future upgrading dams under operation.

REFERENCES[1] Water and Disaster Management Bureau, MLIT. (2017). MLIT's Vision to Upgrade Dams under
Operation. (in Japanese)[2] Kashiwai, J. and Miyawaki, C. (1999). Pressure characteristics of curved portion of circular conduit
pipe. Dam Engineering, Vol. 9, No. 4, 245-252. (in Japanese)[3] Nakagawa River Office, MLIT. Website “Nagayasuguchi dam remodeling project”.
http://www.skr.mlit.go.jp/nakagawa/dam/outline/index.html (in Japanese)

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報－141人

ダム

堤休を開削する手法については 現在改造1:事を施工中の徳島県の長安ロダム（同土交通省）がその開削深さ

No. 247 (2019-4)
発され ， 大深度潜水作業を軽減し ， コスト縮減や安全性向上が実現された。

において既往辰大規模の事例である。 蛾大で深さ約37m

堤体放流設伽は流呈が小さければ既設の滅勢工に放の開削が行われており ， 2つの人規模ロ ー ラ ー ゲ ー トを流する形式とすることができるが ， 鶴mダムのように人有する放流設備が増設される予定である。 長安ロダム に規模な場合には専用の滅勢エが新設される。

ついては 3古にて水坪設計の内容を報告する。

鶴田ダムについても長安r Iダム同様に水理校型実験で流況を確誇しながら設計形状が検討された（図ー 1参照）。

2.2 堤体放流設備の改良・増設（堤体削孔）
堤イ本放流設備の改良・ 増設は ， 既設のダム堤体に穴を2.3

トンネル洪水吐きの新設

あけ ， 放流設備を増設する手法で ， 某本的には霊力式コ

大容星の放流設備を増設する際にダム邸式やダムの構

ンクリ ー トダムに適用される。 且木では， 利水11的の貯遮上の制約から既設ダム堤体を切削または削孔すること

水池運用上の制約から貯水位を大きく低下させることがが困難な場合には ， ダム堤体周囲の地山内にトンネル洪難しい場合があり ， 大水深条件での施工が求められるた水吐きが新設される。 市例としては， 現在施上中の愛緩め ， ダム堤体上流側の仮締切の技術が重要となる。

県肱illの鹿野JI Iダム（国土交通省）と京都府淀川の天ヶ作用水頭が25mを超える放流設備は閥圧放流設備と定装され ， 放流時に流れが高速となるため ， 局所的な負！土ー

瀬ダム（同土交通省）の2つがある。
鹿野ILLダ ムのトンネル洪水吐き は ，トンネル直径ション損偽を防止する必要が11.5m, 延長457m, 放流呈約l,OOOrrl Is, 天ヶ瀬ダムは．

ある。そのためには管内流速を小さくする必要があるが ，トンネル直舒10.3m, 延長617m. 放流盈約600rrl Isである。

堤体にあけることができる穴の大きさや数はダムの構造

比教的大きな水斤の作用する延長の長い設備となるたの発牛等によるキャビテ

により制約があるため ， 経済性からは流速を大きくしてめ ， トンネル水路の断面が大きく ， 流速をlOm/s程度に放流能力を増加させることが有利である。 さらに 増設抑えた管路流の施設として設計されている。 筈路流のた放流設備では ， 既設の放流設備の配料との関係から放流

め ， 流入部で空気を吸い込むと出11でのゲ ー トの振動や管の呑口と出口を直線．卜.に配惜することが困難なことが流況の乱れを招く懸念がある。 空気混入を減らすには流あらその場合複雑に湾曲する形状が採） 11されるが，入部の位置を水面から深くする必要があるが大規模な

大口径放流岱の湾曲音訳では局所的な圧力低下が生じやす施設であり ， そのような配罹が難しいため， 水理模型実

こうした課題に対して ， 水理校型実験による湾験によって吸い込み渦の発生を抑える流入部形状が検討

くなる

3

曲部の壁而作Jtj圧力に関する検討,2) によって現在ではされた c
天ヶ瀬ダムの貯水池内流入部施設の施工においては，合理的な設計が可能となっている。
堤体削孔の事例としては ， 鹿児島県） II内川の鶴圧ダム（国七交通省）にて ， 増設放流貸3条（直径4.8m, 削孔

民間企業が開発した陸上から遠隔操作で水中の掘削等の作業が行えるシャフト式水巾作業機が採用された。

断而：閾さ6.0m X幅6.0m), 付替発電着2条（直径5.2m,

両ダムとも放流呈が大きく ， 滅勢エが必要になるが，削孔断面：高さ6.4m X幅6.4m)を増設したものが既往

河道内の配置の制約があり ， 鹿野川ダムでは段差やバッ（ ー 1参照）。 鶴固

フルピアを配惜することでコンパクトな減勢池形状とし巌大規模であるrn ダム増設放流設備の施工における仮締切では ， ダム上流の水中に台座コン（ ー2参照） ， 天ヶ瀬ダムでは地山内に減勢工を設匿す
図クリ ー トを設箇後， 鋼製の仮締切扉体を組み上げる従来る工夫がされている。

の工法の他に ， 台座が不要な浮体式仮締切の新技術が開

、..

図 ー1

鶴田ダムの改造状況(2017年6月）（左）と水理模型（右）66- 報－142

ダム再生における水坪設叶

図 ー2

鹿野川ダムトンネル洪水吐きの水理模型実験状況

3.

長安ロダム改造事業における水理設計

3.1

長安ロダム改造事業の概要

部はダムの洪水調節機能を決定する重要なもので ， 所定の水位で所定の流呈が流れるかを確認し ， 水位と放流呈の関係式を導く必要があり ， 多くの場合は ， 他の減勢工

長安ロダムは ， �JIS賀川水系那賀川に ， 洪水調節 ， 発宙 ，等の部分と合わせた全休模堕（脳 ー 5参照）とは別に ．

かんがい用水の補給を目的として ， 1956年（昭和31年）

越流部のみを抽出した大きな縮尺の抽出模型（図 ー 5参に建設された多且的ダムである。 ダム形式は ， 直力式コ照）で検討される。

ンクリ ー トダ ム で あ り
54,278,000叫

堤忘85.5m,

総貯水容笹

有効貯水容益43,497,000叫

流域面積

長安ロダムで実施した）京案形状での水理模型夷険の状況を図 ー 6に示す。

538.9kniである。 長安LIダム改造事業は ， 長安l lダム の原案形状の越流部では所定の水位での放流呈が不足し洪水調節能力の増強 ， 那賀川における流水の正常な機能ていた。 導流部については越流部からの水流が勢いをの維持及び放流水の水質改善を目的としてダム改造を行持ったまま導流墜に当たり さらに減勢することなく減うものである凡洪水調節能力の増強については ， JJIS賀勢エ（減勢池）に落下していた。 副ダムのみを設匿した川水系の治水計画に基づき ， 長安ロダムに要求される洪減勢Lの原案形状では ， 既設洪水吐き及び新設洪水吐き

水調節能力を確保するため ， 堤休の右岸の

一

部を切り欠

いて2門の新設洪水吐きの増設を行うものである（図一からの水流が合流する笥域で左洋側の水脈が大きく追い上がる状況となった。
これらの原案形状での課題に対して ， 越流部について

3参照）。

は ． 川側の越流頂標嵩を低くし ． 越流頂の形状を台形越
3.2

水理模型実験による水理設計

流頂から標準越流頂に変更することで ， 必要な放流旦が長安ロダム改造事業に関して， 土木研究所にて2008年確保された。

度から2012年度に， 水理模型実験により新設洪水/I七き等

導流部については ． 導流壁の位置・角度の変更 ， スロ ーの水理設計が実施された。 所定の水位· 流呈等の条件にプの設慨など様々な形状が試行錯誤的に検討され， 山側

おいて求められる放流能力が確保されているか ． 尊流とJII側の両者の洪水吐きからの水脈を分離・拡散する形

部や減勢工等の施設に作用する水圧が許容される範囲内

状が考案された。その結呆導流部の水位を低く抑えるこ

か ， 安定した流れが形成されているかなど， 必要とされとができ荘流壁の高さを低くすることが可能になった。

る機能や安全性が確保されていることを確認し ， 確保さ

減勢工については ， 乎面形状の修正や側壁天端の水路れていない場合は施設の形状等が修正された。
具体的には ． 図ー

壁の設梢により ， 一部減禁エ側碓を越水する場合がある

4に示す新設洪水吐きの越流部 ， 遵

流部， 既設及び新設洪水吐きからの放流水が合流する減が ， 側壁忘を低く抑えることができ ， 下流河道に対しては十分な安全性を確保することができた。
図ー 7に修正した兼終的な形状及び図ー 8にその形状

勢．［及び下流河道を対象に検討された。
最初に机J.:.設計による原案形状の縮小模型が製作さでの水理模型実験の状況を示す。

れ ， 所定の水位 流呈等の条件で通水され ． 流況等が確

また ． 図 ー 9に示すように ． 減勢工の施1こ途中に既設

卜油［から越流部 ， 導流部． 減

洪水吐きからの放流が生じた場合の流況についての水理・忽された。 千順としては，

酪工 ， 下流河道の順に機能が確認される。 ここで ． 越流

模型実験も実施されている。

報－143
- 67-

大

図ー3

ダ

ム

No. 247 (2019-1)

長安□ダム改造事業の概要

図一4 原案形状の平面図（左）と下流面図（右）図 ー5

全体模型（縮尺1/62.5) (左）と越流部抽出模型（縮尺1/40) (右）

図一6

水理模型実験での流況（導流部：原案形状）
- 68-

報－144

ダム再生における水罪設計

越流剖

図ー7 最終形状の平面図（左）と下流面図（右）

図一8 水理模型実験での流況（導流部：最終形状）

,,

図 一9

4.

＇

璽

J

身璽i

施工途中の水理模型実験での流況
3つの手法について それぞれ既往最大規模の工事が開おわりに始され ， 施工が進められている。 これらの設計や施工をダム再生における水理設計上の留意点は ， 必要な放流機能の確保と放流機能の増強によって人きく変化するダ―

実施する中で多くの技術的な知見が各分野で蓄積されてきており ， 今後のダム再生への活川が期待される。

ム下流の流況について ， ダム堤休と ド流河道の安全性を確保することである。

参考文献

また ， 既設のダムを運用しながら施工する必要があり ，施工中の既設ダムの機能をいかに確保するか ， また既設ダムからの放流が工事に及ぼす影碧をいかに小さくする

2017年
2)柏井条介宮脇千睛：円形放流符湾曲部の圧力特件 ， ダ
ム工学 ， Vol.9, No.4, pp.245-252, 1999年

かの検討が重要になる。
日本のダムの放流設価増強に関しては ， この数年の間に ， 堤体切削 ， 堤体削孔 ，

1)国十交通省水笞埋 • 国土保全局： ダム再生ビジョン ，トンネル洪水叶きの代表的な- 69 報－1453)国土父通省那賀川河川串務所ホームページ（長安ロダム
改造車棠）