Worldwide Flood Mitigation Dams: Operating andDesigning Issues
T. Sumi1, S.A. Kantoush1 and A. Shirai2
1

Disaster Prevention Research Institute, Kyoto University, Goka-sho, Uji-shi, 611-0011,
Japan
2
Water Resources Environment Technology Center, 2-14-2, Kojimachi, Chiyoda, Tokyo,
102−0083, Japan
E-mail: sumi.tetsuya.2s@kyoto-u.ac.jp, kantoush@yahoo.com, shirai@wec.or.jp

Abstract
Worldwide dams designed only for the purpose of flood mitigation have different definitions
and classifications. In USA they are called “Dry dam”, in Austria “flood retention basins”,(“Hochwasserrückhaltebecken” in German), in Japan “in stream flood control dam”
(Ryusuigata dam, Japanese), others “Flood Mitigation Dam” (FMD). It is one of goodsolutions in dam engineering for sustainable management of reservoirs, downstream riverenvironment, and sediment transport. The paper summarizes field investigations for several
flood mitigation structures in Styria river basins (Austria) and comparison between Austrianand Japanese experiences, from several points of view as dam structural and hydraulic design,
reservoir sediment management, quality of discharge water, maintaining of ecosystem andland management in reservoir area, clogging problems of bottom outlets by big stones or
floating debris. There is direct relation between the flood magnitude and the necessary levelof dissipation by the stilling basin. During high magnitude of flood the stilling basin should be
more vulnerable to the impacts of flooding. An optimal stilling basin geometry withacceptable flood risk therefore requires a holistic approach, addressing the flow parameters,
design flood, upstream water level in the reservoir, dissipation energy, rivers, ecology andflood inundation as well as the human and socio-economic issues of planning, development
and design.

Introduction
Flood mitigation dam (FMD) is a gateless outlet dam designed only for the purpose of flood
control which provides long-term and efficient protection against floods. FMD is one of goodsolutions in dam engineering for sustainable management of reservoirs, downstream riverenvironment, and sediment transport. FMD is expected as environmentally friendly, sincealmost all incoming sediment during flood periods can pass through dam bottom outlets thatdesigned at the original river bed level and there will be fewer impacts to downstream riverenvironment.
Lempérière, (2006) has pointed out that „Future dams may generally be multipurpose, butdams devoted only to flood mitigation which are completely dry except for a few weeks per
century may be very acceptable environmentally; their design may be quite different frommultipurpose dams and their cost much lower for the same storage‟. There are still several
unknown factors such as sediment trap rates, patterns and flow regimes in the upstream of thedam, number of bottom outlets, and stilling basin dimensions (height, length, width),depending on flood hydrograph and water level.
Currently FMDs have different definitions and classifications. Several definitions for thesame hydraulic structure in different countries and languages are given. There is a need for
detailed design and operation guidelines coupled with research on biodiversity enhancement,reliability, economics, and social acceptance. Figure 1 shows the relationship between grossstorage capacity and dam height of FMD in Japan, Switzerland (Orden dam) and USA.
Because of geographical conditions, there is large difference between dams. Reservoir
capacities to dam height of dams in USA are very large since they constructed in mild riverslope and wide valley.
In the Japanese rivers, the flood wave propagates rapidly increases and decreases in a shortperiod, the peak discharge can be significantly reduced by using small storage capacity.
Therefore, an effective way to mitigate floods in Japan is to combine reservoirs and riverrestorations (Sakurai et al. 2009). In Japan, small FMDs less than 1 million m3 as shown inFigure 1 have been constructed from 1950s to protect mainly farmland against floods.
Recently, relatively large dams for flood mitigation for urban areas have been planned andconstructed. Masudagawa dam is one of them. In case of large reservoirs, careful attentions
should be paid for hydraulic design, reservoir sediment management, ecosystem and landmanagement, and water quality and clogging of bottom outlets. They are summarized inFigure 2.

Dam Height （m）

200

USA
Switzerland

150

Japan
Austria

Seven Oaks

Medium dams

Small dams

10050

Large dams
Asuwagawa

TatenoMount Morris

TatsumiOrdenPradoEnglewood
Gabriachbach1
Ligistbach
Masudagawa
Germantown
Felberbach
Lafnizt-Reinbergwiesen
HuffmanSepulveda Lockington TaylorvilleLafnizt-Waldbach

00.01

0.10

1.00
10.00
100.0063
Reservoir Capacity（10 m ）Figure 1: Classification of worldwide FMD based on reservoir capacity and dam height

Features of Flood Mitigation Dams
From the point of view of:

HydraulicdesignBottomoutletNumber ofgates: min 2

Stilling basin

Geometry

Reservoir
sediment
management

Maintaining ofQuality ofEcosystem and land
watermanagement
discharged

Reservoir configurations

In reservoir
regions

Turbid water

Flood scale

In the river
regions

Eutrofication

Clogging
problems
Floatingdebris
damageBig stones

Trashracks
Protection
againsterosionSluicing

Energydissipaters

Amount and grain size ofsediment inflow

In-groundEnvironmental impacts
on downstream river

Figure 2: Features of designing and operating of flood mitigation dams.

Field Investigation in Styria, Austria
Key parameters
In the republic of Austria, Styrian government has actively constructed FMD from 1960s.
More than 100 dams are located at small tributaries nearby city of Graz and mountain regions.
In 1992, an interesting guideline for planning, designing and operation of FMD which
explains engineering, economical and ecological aspects is published. In order to determine
and characterize all relevant and particularly the key classification variables for FMDs inJapan and Austria, Table 1 summarizes these key points: rainfall; dam height, dam length;
dam arrangement- concentrated or distributed; construction material, fish passage; screensystem; basin and channel connectivity; design flood frequency; outlet arrangement; gate
operation; catchment size; stilling basin design; and landscape planning and aesthetic.
Table 1: Classification and comparison of Flood Mitigation Dams in Austria and Japan
Item
Names of field investigation
flood mitigation basin anddams
Dam Height (min-max)
Dam Length (min-max)
Gross Capacity (min-max)
Catchment area
Dam arrangement in riverbasin
Mean Annual Rainfall
Utilization of reservoir area
Fish passages
Screen system design
Design Flood frequency

Austria
12 dams (Bärndorfbach,Dobelbach, Felberbach,Gabriachbach1 & 2, Labuchbach,Lafniz-Reinbergwiesen, LafnizWaldbach, Ligistbach, Sauhaltbach, Stullneggbach, Gamlizbach)
5.8-23.2 m
84-241 m
14,000-1,100,000 m3
0.8-162 km2

8 dams(Sotomasuzwe,Rentaki, Kawachi,Matsuo, Sagatani,Ootouge, Sasakura,Takaono)
17-37.7 m
63.6-169 m
186,000-6,500,000 m3
5.5- 16.8 km2

Distributed set of dams

Concentrated dam

865 mm/yr
Playground, habitat
Well design (Stepped ladder withnatural sun light)
Bar pitches are designed byguideline

1700 mm/yr
Playground

Return period is 30-50 years

Only one with bypass outlet foremergency
With gate (Automatic and FixedGate Operation
opening)
In-ground stilling basin andStilling basin design
hydraulic jump
Earth fill with concrete outlet
Construction materialsections
Landscape planning
Well match with natureRiver and basin bed gradient Mild slope
Sediment load
Medium sediment yield
Reservoir sedimentation
Less deposition
Outlet Arrangement

Japan

Under development
Under development
Return period is 80100 years
Usually two bottom
outletUsually gatelessHydraulic jump withend sill
Mainly concrete forgravity dams
Under development
Steep slope
High sediment yieldLess deposition

The examined Styrian basins capacities range from 14,000 m3 to 1,100,000 m3, the damheights are between 5.8 m and 23.2 m, and they were all earth fill dam combined withconcrete outlets. While the Japanese Flood mitigation dam heights are between 17m to 37.7 mfrom concrete, the reservoir volume is ranging between 186,000-6,500,000 m3. During ourvisit, we have discussed several unique points in Styrian case studies which will be very much
valuable to improve performances of FMDs.
Unique points of Styrian flood mitigation dams
1- Bottom outlet design
Based on the guideline, bottom outlets are all gated. They are classified into (a) fixed small
gate opening, (b) closed gate with small gate opening section, and (c) circular small diameterwith automatic gate as shown in Figure 3. All gates are designed large enough formaintenance.

(a)

(b)

(c)

2- Safeguard for clogging of bottom outlet
Preventing from clogging by floating woods
or big stones, all bottom outlets are coveredby screens. These bar pitches are ranging
from 15 to 50 cm based on design dischargeof bottom outlets as Figure 4. These screens
are installed at not only inlet level but alsoon top of the bottom outlets to maintainenough discharge for safeguard. Periodical
cleaning or screen design modification isrequested because unexpected water storageis occurred by sediment and tree leaves
trapping (Figure 5).

Screen bar pitches (m)Figure 3: Bottom outlet design: (a) Lafiniz-Waldbach, (b) Ligistbach and (c) Labuchbach.

Design discharge (m3/s)

Figure 4: Screen bar pitches based on design
discharge of bottom outlets

Figure 5: Screen bar design, and sediment and tree leaves trapping in front of a bottom outlet

3- Fish passage
Bottom outlets are also designed for fish passage. Big stones or stepped pools are used tocreate natural stream in the channel by reducing velocity (Figure 6). Natural sunlight is also
introducing to the channel by mesh opening at both upstream and downstream sides.

Figure 6: Bottom outlet design for fish passage by using natural stones and stepped pools
4- Reservoir area design
Total landscape design in reservoir area is well discussed with local communities and experts.
Biotopes are designed in reservoir area for river restoration (Figure 7). In a reservoir,swimming pool is created for recreational use.

Figure 7: Reservoir area and biotopes on the right side

Conclusion and Future Challenges
Recent flood events in Austria and Japan have shown the need for improved flood mitigation(retention) dams along the rivers. Therefore, several further research works is needed to
update planning, designing and operating of flood retention dams. Flood mitigation dams
(FMD) are considered eco-friendly because of their peak reduction reputation without
rupturing the normal flow regime of the river. The FMD individualized and characterized forthree future challenges parts have to be studied, reservoir area and the inlet, outlets and gate
operation, and stilling basin with downstream reach of the dam.
1- In the upstream reservoir and the dam inlet: During the flood discharge retardation, thecharacteristics of sediment (sand and gravel) outflow rate are changeable and unknown
compared with the normal stage. The degree of change varies according to flood control
plans, inflow sediment properties, and scale of the flood, so dams must be studied
individually. Moreover, the development of a prediction and an optimum management
measure of sedimentation in flood mitigation dams should be investigated.
2- Outlets and gate operation: As measures to mitigate changes of sediment transport
properties, the geometry of the outlet works and stilling basins should be further studied inorder to smooth the fish and sediment passages at the end of the flood period. The mosteffective approach is to accept variability of the reservoir water level less frequently within arange that satisfies flood control plans. By expanding the cross-section of outlet works
installed on the elevation of riverbeds, it is possible to raise the reservoir level less frequently.
But, in Japan, the peak cut rate of flood at dam site is generally large, so in order to achieve aflood control plan, it is necessary to make the outlet works section small when the reservoir
water level is raised. The measure that is considered at this time is to install large outlet works
for sediment discharge and separate small outlet works for flood control, and switch overfrom the former to the latter during flood control. To rationalize equipment and simplify its
operation during a flood, at normal times, a large cross-section ensures the movement ofsediment, stream, and aquatic life. But for flood periods, discharge equipment that permits theoperation of gates to reduce the flow section, thereby controlling the flood discharge, should
be developed. In that sense, automatic gate in Styrian examples are one of possible solutions
for small discharge.
3- Stilling basin and downstream reaches: To improve current design method of outlets andstilling basins effective dissipation of energy, optimal design of stilling basin leading tooptimal geometry is required. The main question is what are the optimal SB configuration andupstream conditions to maximize the energy dissipation (∆E), fish passage and minimize thecost? An optimal stilling basin geometry with acceptable flood risk therefore requires aholistic approach, addressing the flow parameters, design flood, upstream water level in thereservoir, dissipation energy, rivers, ecology and flood inundation as well as the human andsocio-economic issues of planning, development and design.

Acknowledgements
Authors express their gratitude to Mr. Rudolf Hornich for his kind guidance in our field trip
and participation to the symposium on Flood Mitigation Dams in Tokyo in 2010.

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