DESIGNING AND OPERATING OF FLOOD RETENTION‘DRY’ DAMS IN JAPAN AND USA
1

Tetsuya Sumi 1

Associated Professor, Department of Civil and Earth Resources Engineering, Kyoto University
Kyoto University Katsura 4, Nishikyo-ku, Kyoto, 615-8540, Japan, e-mail: sumi@mbox.kudpc.kyoto-u.ac.jp

ABSTRACT
Flood retention dams designed only for the purpose of flood control whose bottom
outlets are specially designed at the original river bed level and can pass through inflow
projects. Masudagawa dam completed in 2007 is one of good examples in Japan and there
are also good examples in USA from 1920’s such as 5 dams at the Miami Conservancy
District in Ohio which are usually called as ‘Dry’ dams. This paper reviews concept as well as
designing and operating points including reservoir sediment management of these flood
retention dams and future challenges.
Keywords: flood retention dam, dry dam, Masudagawa dam, Miami Conservancy District,reservoir sediment management
1.

INTRODUCTION

Recently, flood retention dams designed only for the purpose of flood control whose
bottom outlets are specially designed at the original river bed level and can pass throughinflow sediments almost naturally during flood periods are often planned for the new flood
control projects.
Lempérière, F.(2006) has pointed out that ‘Future dams may generally be
multipurpose, but dams devoted only to flood mitigation which are completely dry except fora few weeks per century may be very acceptable environmentally; their design may be quite
different from multipurpose dams and their cost much lower for the same storage’.
These dams may have advantage not only on reservoir sedimentation but also on water
quality problems such as turbid water prolongment and eutrofication problems and on
maintaining longitudinal river basin ecosystem such as fish migration and various materialtransports throughout river basin.
Masudagawa dam completed in 2007 is one of good examples in Japan and other new
projects such as Tateno, Asuwagawa and Tatsumi dams are under planning. There are also
good examples in USA from 1920’s such as 5 dams at the Miami Conservancy District in
Ohio, Mount Morris dam at the USACE Buffalo District and Prado dam at the USACE Los
Angeles District, and they are usually called as ‘Dry dams’.
Generally, issues on these dry dams can be summarized as dam structural andhydraulic design, reservoir sediment management, quality of discharge water, maintaining of
ecosystem and land management in reservoir area, clogging problems of bottom outlets by big
stones or floating debris.
This paper discusses on these issues of dry dams from the point view of typeclassification, structural and hydraulic design, sediment management, quality of discharge
water and so on in Japan and USA, and future challenges.

2.

MASUDAGAWA DAM

Masudagawa dam located in Shimane prefecture is a flood control dam designed withprobable flow rate of 1/100 at the Masuda River as shown in Figure 1. Detail design of
Masudagawa dam is shown in Figure 2. Two gateless bottom outlets and overflow spillways
are installed. For energy dissipater, conventional hydraulic jump type stilling basin with anend sill is designed where two slits are installed for self sediment flushing from the stillingbasin. Inflow sediment can be naturally flushed through bottom outlets but dead capacity of
250,000m3 is planned. Bottom outlet and some part of stilling basin are protected withstainless steel plates against for abrasion damage (Kashiwai, 2000).
Catchment Area: 87.6km2
Dam Type: Gravity
Structural Height: 48.0m
Total Storage Capacity: 6,750,000m3
Gross Storage Capacity: 6,500,000m3
Dead Capacity: 250,000m3

Figure 1 Flood control plan at Masuda River

Slits for sediment flushingfrom stilling basin

Figure 2 Detail design of Masudagawa dam

3.

CLASSIFICATION AND HISTORY OF DRY DAMS

Figure 3 shows gross storage capacity and dam height of dry dams in Japan and USA
including Masudagawa dam. Because of geographical conditions, there is large difference
between dams in Japan and USA. Reservoir capacities to dam height of dams in USA are verylarge since they are constructed at the point in mild river slope and wide valley. Seven Oaks
dam is only constructed in a mountain river. Orden dam in Switzerland is also similar to
Japanese dams. River slope and reservoir capacity may affect way of sediment management.

Dam Height（m）

200
Seven Oaks

USA
Japan
Switzerland

150

Asuwagawa100

Mount Morris

TatenoTatsumi

50

Orden

0
0

1

Masudagawa

Englewood
Germantown Prado
Lockington
Taylorville
Sepulveda
Huffman10
Gross Storage Capacity（10 6m3）

100

1,000

Figure 3 Gross storage capacity and dam height of flood retention ‘Dry dams’
These flood retention dams can be classified as Table 1. Viewpoints of classification are1) installing regulating gates in bottom outlets or not and 2) securing continuity of the riverthrough these outlets or not.
The simplest form is the one that outlet are installed in the riverbed level. Masudagawa
dam and five dams of MCD (Miami Conservancy District) correspond to this category. In
these dams, fish migration and sediment continuity can be secured. Moreover, similar type isadopted in small-scale farmland disaster prevention dams in Japan. Olden dam in Switzerland
is also classified into here.
The one that a regulating gate is installed to the bottom outlet according to the flood
control operation is similar to this. If the regulating gate is completely opened usually,
environmental impacts such as securing continuity of the ecosystem and the sedimenttransport are thought to be almost equal to the above-mentioned dam. Aswagawa dam inJapan and Mount Morris and Prado dams in USA are categorized here.
On the other hand, there is a dam where both capacities for the flood control and thesedimentation (100 years) are secured, and the outlet is installed in the middle level of dam
body. Seven Oaks dam in USA is categorized here. In such a dam, environmental impacts
should be very different, and it distinguishes from the above-mentioned dams.
Table 1 Classification of flood retention dams
Gateless bottom outlet

Gated bottom outlet

Dry dam
Masudagawa, Tateno,
Tatsumi, MCD, Orden,
Small-scale farmland disasterprevention dams in Japan
Asuwagawa,Mount Morris, Prado

Non dry dam
Natural flood control dams
for small basin in Japan
Seven Oaks

In USA, These dams are almost constructed as an embankment dam which may be
standard style except Mount Morris dam as a concrete gravity and Seven Oaks dam as a rockfill dam. The MCD dams and the Prado dam may be constructed most reasonably forembankment dams since dam height are about 30m or less and crest length are about as manyas 1,000m.
Bottom outlet is regularly installed in the embankment in USA, and there are two types
for spillway. Taylorsville dam in MCD is the example of the installation in the upper part ofthe bottom outlet in a concrete dam section and Englewood dam in MCD and Prado dam are
the ones that spillway is installed separately. Figure 4 shows aerial view of these dams.
Taylorsville Dam

Englewood Dam
Reservoir
Reservoir

Embankment dam

Embankment dam
Spillway
Bottom outlet

Bottom outlet

Spillway

Figure 4 Taylorsville and Englewood dams in MCD
The oldest dry dam in USA is five dams that MCD (Miami Conservancy District)constructed. MCD is an organization established in 1914 to execute flood control project of
the Great Miami River flowing through Dayton city in the state of Ohio. Arthur E. Morgan(1878-1975), a chief engineer of MCD, conducted the project and proposed dry dams forflood control.
He selected dry dams for the Great Miami River from the following point of views.
• There was little necessary for multipurpose such as domestic water supply or hydropower.
• Since the project was also supported by citizens’ fund-raisings, it was necessary to reduce
costs by site purchasing a minimum.
• The continuous use for the farmland was possible. (the farmer opposed the project)
He also referred to the precedent of dry dams, Pinay and La Roche, which wereconstructed in Loire River in France about 200 years ago. Pinay dam completion in 1711 wasa big slit dam set up in the river channel such as Figure 5. Local farmers accepted the dam
because of reducing downstream flood risks and bringing a fertile soil in the submerging areaat the winter and spring floods. The Pinay dam has been decommissioned because of theconstruction of Villerest dam completed in 1984.

Figure 5
Pinay dam in Loire
River in Francecompleted in 1711

4.

HYDRAULIC DESIGN OF DRY DAMS

It is necessary to examine the following items by a hydraulic design of dry dams in
consideration of the past experiences.
4.1 Bottom outlet
Appropriate abrasion proof design is needed such as with metal linings in case relatively
coarse sediment will be discharged. Especially, if the regulating gate is set up at the bottom
outlet for flood control, it may be necessary to install the sediment scouring sluice separatelyfrom the bottom outlet to secure the certainty of the gate operation.
It is necessary to set up trashracks at appropriate openings to prevent clogging and debris
damage to regulating gates. Engineering manual of USACE (EM 1110-2-2400) is
recommending to set up simple trash structure, usually of reinforced concrete, with clear
horizontal and vertical openings not more than two-thirds the gate width at the upstream endof the bottom outlets. Figure 6 shows trash structures of Prado dam. Trash boom constructedof logs or floating pontoons is also recommended to capture floating trash and debris.
According to the experience of MCD, it is necessary to install at least two conduits so as
to allow inspection during low flow periods in which all the flow is diverted to the otherconduit(s) since in case that the conduits have been completely blocked, intentional inspectionof the conditions of the conduits is needed.

Figure 6 Prado dam bottom outlet and trash structures(Openings of Trash structures: B1.83m×D1.95m, Gate width: B=3.02m)
4.2 Stilling basin
According to the experience of MCD, the stilling basins are recommended to be built “inground” to provide the necessary staggered drops needed to dissipate the outflow watervelocity. The openings of the bottom outlets are built at the elevation in which the original
low-level flow of the rivers would be able to pass through the outlets. “In-ground” stillingbasin is friendly for fish migration since there is no obstacle on the river surface. In suchdesign, some sedimentation
problems may occur in thestilling basin between flood
events. But once a highflood occurs, the deposited
sediment can be easily
flushed out. Figure 7 shows“In-ground” stilling basinof Taylorsville dam of
MCD.
Figure 7 Stilling basin of Taylorsville dam

5.

RESERVOIR SEDIMENT MANAGEMENT OF DRY DAMS
5.1 Reservoir sedimentation

Dry dams may have advantage on reservoir sedimentation since almost all inflow
sediments can pass through bottom outlets during each flood events. Difference of reservoir
sedimentation between conventional impounding dams and dry dams can be summarized asshown in Table 2.
Table 2 Difference of reservoir sedimentation between impounding dams and dry dams
Dam type
Plannedsedimentation
volumeImpounding dam
Prepare 100 years
sedimentation volume foralmost all inflow sediments

Effects on continuity Sediment trap rate is generallyof sediment transport 80-90% though it depends on
the turnover rate of water andsediment grain size.

Dry damSedimentation volume can be
reduced by periodical sedimentflushing during natural draw down
period after flood impounding
Sediment trap rate is generally 1020% though it depends on theturnover rate of water andsediment grain size.

In Japan, reservoir sedimentation of dry dams should be prepared as the following
procedure. Total sedimentation volume is a) + b).
a) 100 years’ total volume of deposited sediment that is calculated by the one dimensional
numerical model for deposition and flushing of reservoir sediments.
b) Temporal volume of deposited sediment by the design flood on top of the final river bedof 100 year’s calculation obtained by a).
In case of Masudagawa dam, a) and b) are estimated as 230,000m3 and 20,000m3
respectively, and total sedimentation volume of 250,000m3 is 11.8% of total inflow sedimentvolume of 1,950,000m3.
Sedimentation in dry dams may be different depending on reservoir configuration as wellas the scale of the flood, the amount and the grain size of the inflow sediment and so on.
Figure 8 shows actual sedimentation of Orden dam in Switzerland. Relatively coarse sand andgravel have been deposited in reservoir area and channel erosion can be observed. From thehydraulic model study, continuous channel excavation was recommended to prevent excess
sedimentation but it seemed to be no such management works.

Figure 8 Reservoir sedimentation of Orden dam in Switzerland(Left: From dam to upstream view, Middle: Aerial view, Right: From upstream to dam on the model study)

Figure 9 shows actual sedimentation of Mount Morris dam. Relatively fine silt and sandhave been deposited in reservoir area and channel erosion can be also observed.

Figure 9 Reservoir sedimentation of Mount Morris dam in USACE Buffalo District(Left: Channel erosion, Right: Sediment deposited just upstream of dam is periodically dredged)

5.2 Environmental impacts to downstream river
Generally, dry dams have also advantage on environmental impacts to downstream river
since there may be almost no change in water quality and sediment transport in river with andwithout dams, rather than conventional normally impounding dams. But if we consider thesepoints more detail, there still remain so much unknown factors. For examples, some amountof sediment will be deposited in reservoir area during severe flood times and be graduallyeroded afterwards which may cause turbidity problem to the downstream river.
Kashiwai (2006) has conducted a study on sediment transport through dry dams. Figure
10 shows one of these results. Fine sediments such as silt and clay can be discharged inalmost the same concentration as the inflow one according to the same timing. However,relatively coarse sediments such as sand and gravel may be discharged in a low concentration
a little delaying from the inflow one. This delay may cause sedimentation in reservoir andsediment deposition in the downstream river channel.
160

280
計算条件：洪水吐き標高0m
Elevationof Bottom Outlet = 0m

245
210

100

175

80

Water
level
貯水位
Inflow
流入量
放流量
Outflow

60
40
20

0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00

105
70
35
0

0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00

Inflow流入（礫）(Gravel)
放流（礫）Outflow(Gravel)
Inflow

0.40
0.35
0.30
0.25

Inflow 流入（砂）(Sand)
放流（砂）Outflow(Sand)

Inflow

0.20
0.15
0.10
0.05
0.00
1,330

1,360

1,390

1,420
1,450
時間(hour)

Hour

1,480

1,510

1,540

transport rate m3/s
Sediment流砂量(m3/s)transport rate m3/s Sediment流砂量(m3/s)

Sediment transport
rate m3/s
流砂量(m3/s)

Sediment流砂量(m3/s)transport rate m3/s

0140

流量(m3/s)

120

Discharge rate m3/s

Waterlevel m
貯水位(EL.m)

140

2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1,330

Inflow 流入（シルト）(Silt)
放流（シルト）
Outflow(Silt)

Inflow流入（粘土）(Clay)
放流（粘土）
Outflow(Clay)

1,360

1,390

1,420
1,450
時間(hour)

Hour

1,480

1,510

1,540

1

Figure 10 Changes on sediment fluxes depend on sediment grain sizes (Kashiwai, 2006)

The turbid water discharge may also become a problem if the deposited sediments in thereservoir include large amount of fine materials. Difference of these environmental impacts todownstream river between impounding dam and dry dam relating reservoir sedimentation issummarized in Table 3.
Table 3 Difference of environmental impacts to downstream river between
impounding dam and dry dam relating reservoir sedimentation
Dam type
WaterturbidityRivermorphology

Impounding dam
Usually clear, but long-term
turbidity may occur after large
floods if inflow sedimentcontains very fine particles hard
to settle.

Dry dam
Usually clear, but short-term turbidity may
occur during flood latter term because once
deposited sediment is re-suspended alongwith the reservoir drawdown. Turbidity
may also occur because of sedimenterosion during small and middle floods or
gully erosion on the sedimentation createdby the heavy rainfall.
River morphology may largely Deposition of fine materials may occur onchange by river bed degradation river bed surface after floods.
and armouring. Sand bar andriver channel may be fixed andsevere vegetation growth may
occur in the river channel.

5.3 Future challenges on reservoir sedimentation management of dry dams
Dry dams are much more advantageous than usual impounding dams in the point that
sediment is not trapped as much as possible. However, as shown in the case of Orden andMount Morris dams, some sedimentation will occur depend on the reservoir condition.
To predict the ultimate sedimentation volume in dry dams, one dimensional sedimentdeposition and erosion model should be studied. RESCON model based on the following
Atkinson’s formula to assess reservoir sustainability by sediment flushing may also be useful.
Figure 11 shows schematic diagram and Eqs.1, 2 and 3 show the formula where SBR(Sediment Balance Ratio)>1.0 and LTCR (Long Term Capacity Ratio)>0.5 are requested forreservoir sustainability.

⎛ QsT f ⋅ 86400 ⎞
⎜⎜
⎟⎟
N
⎝
⎠
SBR =

LTCR =

M in ⋅ TE

B
A+ B

(1)
(2)

1.6

Q f S 1.2
0.6

(3)

W f = αQ 0f .5(4)

Qs = Ψ

Wf

where，Qs=Flushing sediment volume(t/s)，Tf =Flushing duration (day) ，N=Flushinginterval (year)，Min=Annual sediment inflow volume (t/year)，TE=Sediment trap ratio by
the Brune curve, Qf=Flushing discharge (m3/s)，S=River bed slope，Wf =Erosion channel
width (m)， Ψ andα = constants.

水みち幅
Erosionchannel width
Wf
Normal water level
常時満水位

Modeling
モデル化

1
SSres

1
SSs

B
A
Wbot

排砂時
Water level duringの水位

flushing
元河床高
Original
river bed

Original貯水池底面の幅
river channel width

モデル化
Modeling

Representative cross section
代表断面

Figure 11 Schematic Diagram of the Atkinson’s Formula for Sediment Flushing
In RESCON model based on the Atkinson’s formula, Ψ mainly depends on dataobtained in China as Ψ = constant such as Loess sediment; 1500，D50＜0.1mm; 650，D50≥
0.1mm; 300，small flushing discharge; 180. Regarding Wf, α=12.8 is also proposed in
Chinese reservoirs. As Atkinson has already pointed out, Qs is calculated much larger thanactually expected in other reservoirs outside of China and he recommended one third of thesevalues. From Japanese experiences, Ψ andαshould be modified in case for relatively steep
rivers with coarse river bed materials.
Sumi et al.(2005) examined Ψ by the following equation with existing sedimenttransport equations and field data.
cΨ (d , S , Q f ) = K ⋅ d a ⋅ S b ⋅ Q f(5)
where d=sediment grain size, K , a , b and c = constants. By comparing Eq.5 with bed loadtransport by Ashida-Michiue formula, and suspended load transport by Lane-Kalinske
formula for concentration profile with the equilibrium concentration derived from AshidaMichiue formula, Eqs. 6 and 7 were obtained for bed load ΨB and suspended load ΨS whered=0.1～2.0mm, S=1/50～1/500, Qf =30～400m3/s, Wf is obtained by Eq.4 asα=6 which is
generally used in Japan, Manning’s roughness n=0.030, density of sediment σ=2.65 (t/m3).
(6)

ΨB = 12.7d −0.043S −0.11Q −f 0.33
ΨS = 0.042 d −2.6 S −0.35Q f

0.073

(7)

Finally, Ψ is described as follows.
Loess sediment;d < 0.1mm;
0.1mm ≤ d ;Ψ = 1500

Ψ = 650 / 3

Ψ = ΨB + ΨS = 12.7d −0.043S −0.11Q f−0.33+ 0.042d −2.6 S −0.35Q f

0.073

(8)

In dry dams, it is very much important to predict Qs by assuming Tf, N, Qf and Wf since
every flood events can be counted for flushing events but flushing discharge is always smaller
than inflow discharge. How to set up TE is also a question. It is, therefore, important to pickup significant flood events and assume these values based on each hydrograph for the
calculation. In dry dams, SBR>1.0 and LTCR>0.5 will be easily secured. However, if
reservoir area is very wide (that is Wbot is larger than Wf ), LTCR may be dropped.
In order also to predict turbid water discharge, deposition and re-suspension model offine materials is needed. Through these studies, optimum design and operation of bottom
outlets to minimize sediment deposition, especially fine materials, should be examined.

6.

Conclusions

This paper discuss on type classification, structural and hydraulic design, sedimentmanagement, quality of discharge water and so on of flood retention dry dams in Japan andUSA. From the sustainable management of reservoirs and river environment, thesesedimentation free reservoirs may be one of good solutions in dam engineering.
Major conclusions are as follows.
1) The flood retention dams can be classified from the viewpoints of 1) installing regulating
gates in bottom outlets or not and 2) securing continuity of the river through these outlets
or not. Masudagawa dam in Japan and five dams of MCD (Miami Conservancy District)in USA are categorized into the simplest ‘Dry dam’ that gateless outlet is installed in theriverbed level where fish migration and sediment continuity can be secured.
2) In hydraulic design of dry dams, trashracks at appropriate openings to prevent clogging
and debris damage to regulating gates of bottom outlet and “In-ground” stilling basin thatis friendly for fish migration might be considered.
3) Dry dams may have advantage on reservoir sedimentation and environmental impacts todownstream river since almost all inflow sediments can pass through bottom outlets
during each flood events. However, studies on reservoir sedimentation and fine sediment
discharge are needed to secure appropriate sedimentation capacity and to obtain optimum
design and operation of bottom outlets.
4) To predict reservoir sedimentation of dry dams with the Atkinson’s formula, it isnecessary to consider periodical sediment flushing from bottom outlets and to modify for
relatively steep rivers with coarse river bed materials.
ACKNOWLEDGMENTS

This paper is mainly based on the technical committee for designing and operating of drydams in Japan which was organized by the Ministry of Land, Infrastructure, Transport andTourism, and Dam Engineering Center. Through the committee, technical visit to dry dams in
the U.S.A has been organized to research Miami Conservancy District, USACE Buffalo
District, USACE Los Angeles District and Orange County Water District under the kindarrangement of Dr. George Annandale. Author would like to express sincere thanks for theirkind cooperation.
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Atkinson, E.(1996), The feasibility of flushing sediment from reservoir，HR Wallingford
Report OD137.
Kashiwai, J.(2000), Sediment flushing operation and facilities at flood control dams,International workshop on reservoir sedimentation management, Toyama, pp.69-77.
Kashiwai, J.(2006), Sediment Management Techniques in Reservoirs, Symposium on
Reservoir Sediment Management, Kyoto University.
Lempérière, F.(2006), The role of dams in the XXI century, Achieving a sustainable
development target, Hydropower and Dams, Issue Three, pp.99-108.
Morgan, A.E.(1951), The Miami Conservancy District, McGRAW-HILL BOOK COMPANY.
Sumi, T., Takata, Y. and Iguchi, M. (2005), Feasibility study of sediment flushing operationin reservoirs using RESCON Model, Advances in River Engineering, JSCE, pp.285-290.
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