Reservoir Sedimentation and Sediment Management in Japan
Team Leader, Dam Hydraulic Engineering Research Team
Hydraulic Engineering Research Guroupe
Incorporated Administrative Agency, Public Works Research Institute
Minamihara1-6, Tsukuba-shi, Ibaraki-ken, Japan
Abstract: This paper introduces the sedimentation condition of reservoirs and general of the
activities for sediment management in Japan. Quantities of sediment yield and component ratio of
grain size of sedimentation are shown. From the reservoir and sedimentation data, it is shown that
dams suffering from sedimentation problem will gradually increase in no distant future,
nevertheless now is a small number. With regard to the sediment management, many kinds of
works executed in Japan including reservoir sedimentation control methods are introduced. Also
concept and activities of integrated sediment system management is introduced. Integrated
sediment system management was proposed in 1998 and is expected to settle the sediment
problems appropriately. The relationship between each works concerning sediment flow and
integrated sediment system management is briefly discussed in this paper.
Key words dam, sedimentation, sediment management, reservoir
Almost all dam’s reservoirs in Japan have sediment capacity, same as other countries. Lifetime of
the capacity was varied in former dam’s planning, but one hundred years is fixed recently through
the establishment and the prevalence of planning technical standards.
Although the sediment capacity is still one of the main and effective countermeasures for
sedimentation in Japan, appropriate sediment management including sediment removal from
reservoirs, sediment supply to downstream rivers etc. is also required.
The reasons for the requirement from the viewpoint of reservoir usage are rapid loss of sediment
capacity than the estimation, aging of reservoirs in planning sedimentation condition and specific
site condition where the sediment inflow volume is too large to plan the sediment capacity and so
on. These are straightforward problems to set up the target. What engineers need to do is examine
and develop the methods to keep effective capacity of a reservoir or effective water head for a
power plant. In this case, however, engineers should also consider the influences of the methods on
the flooding disaster, functions of water use facilities and environmental issues.
Apart from the sedimentation countermeasures in reservoirs, above-mentioned items such as
environmental issues offer other viewpoints of requirement of sediment management. Since a
reservoir stores the large part of sediment inflow, a dam sometimes has important role in the total
sediment transportation system of the watershed, and is expected to perform not only as a storing
structure but also as a controlling structure to sustain safe, beautiful and vital national land. To find
optimum solution for this, approaches from various fields are required.
This paper will introduce the sedimentation condition and general of the activities for sediment
management in Japan’s reservoirs, with referring to the idea and activities of integrated sediment
system management through mountainous area to coastal area.
2. Conditions of sedimentation
2.1 Sedimentation volume
Sedimentation surface profile of a reservoir is usually measured every non-rainy season in Japan, if
the total storage capacity is larger than 1million m
. We can obtain annual sedimentation volume of
Figure-1 shows the distribution of average annual sedimentation volume till 1996. Total number
of reported dam is 786 (30%of total number and more than 80% of total storage capacity of Japan’s
dams) and 709 dams are plotted by eliminating minus data. The sites of large sedimentation dams
are distributed along Japan’s representative Tectonic line of Itoigawa-Shizuoka and Chuou, and
that indicates surrounding areas of those tectonic lines are easily eroded.
Figure-1 Distribution of average annual sedimentation volume
Early Neogene volcanic terraces (Green Tuff Area)
Main Paleozoic and Mesozoic sedimenatary rterranes
Main metamorphic terrances
Itoigawa-Shizuoka tectonic line
Chuou twctonic line
0 100 200 300
Number of dams
Total volume of resent reported annual sedimentation is about 20 million m
. Other survey
shows annual removal volume from reservoirs is about 3.9million m
as shown in Figure-2. So
deposition volume of sediment is 20% more than reported sedimentation volume shown in
Figure-1 on an average. About 60% of removal sediment is effectively used for concrete aggregates,
banking materials and so on.
Figure-2 Changes in Sediment Volume obtained from Reservoirs
2.2 Sediment inflow volume
2.1 showed measured sedimentation volume. It is necessary for the estimation of sediment inflow
(or sediment yield) to consider the removal volume as discussed before, the trapping rate and the
effects of dam construction in areas upstream.
With regard to trapping rate, inflow and outflow of suspended solids were measured during
flood in several reservoirs. Figure-3 shows the relationship between trapping rate of suspended
solid and flood turnover frequency (=total volume of a flood inflow/storage capacity before a
flood). Trapping rate decreases as the flood turnover frequency increases, and rapidly decreases in
the condition of turnover frequency is larger than 1. Trapping rate of fine sediments cannot be
assumed as 1 in large turnover frequency.
Figure-3 Relationship between Trapping Rate and Flood Turnover Frequency
Trapping ratio can be calculated by numerical simulation of flow in a reservoir including
convection, diffusion and deposition phenomena of fine sediments. Recently, vertical
two-dimensional models for realizing fine sediment performances are generally used in Japan. The
0.1 1 10 100
Flood turnover frequency
Trapping rate (%)
models have been developed for the purpose of estimation and countermeasure’s examination of
turbid water problems. Although the numerical simulation is useful for estimating trapping rate,
calculation is not employed so frequently for the estimation especially in the analysis of many
dams. The reasons of that are calculation costs, time and a lack of grain size information of the
Ashida and Okumura
analyzed specific volume of sedimentation discharge q
using the data of dams having water storage capacities over two million m
and sedimentation rates
of less than 25%, on the assumption that all sediment is fully trapped. They concluded that macro
classification could be made for groups of river systems by using the relationship of q
drainage area, a: coefficient). Figure 4 shows the results of sediment data sorting including
additional data obtained by the Public Works Research Institute (PWRI). River systems are roughly
classified along with Ashida and Okumura classification. Groups of river systems are as follows.
① Large sediment yield group: Kurobe, Tenryu, Ooi
②-③ Relatively large sediment yield group located along tectonic line: Tadami, Shou, Kiso etc.
④-⑤ Small sediment yield group: Riversystems in Chugoku district
Figure-4 Relationship between q
From a comparison made between the data before 1970 and the data between 1977 and 1990, we
can find similar tendency with that of Ashida and Okumura as a whole. The data scattered widely,
however, as the number of dams increased, and classification was not sufficiently applicable. The
quantity changes from less than 100 m
/year to 10,000 m
/year as maximum. Clear
①Groupe(Kurobe,Tenryu and Ooi)
10 100 1000 10000
Ashida et al.
10 100 1000 10000
10 100 1000 10000
②～③Groupe(along Chuo Tectonic line)
10 100 1000 10000
differences in age could not be identified.
Table-1 shows the range of coefficient “a” of major river systems from the data 1977 through
1990. Although the rough value of sediment inflow volume can be obtained, the scattering degree
of “a” is not little in the same river system. It can be presumed that classification using only
parameter of drainage area is not sufficient. For more accurate estimation, further elucidation for
influencing factors is necessary. We should know that the sediment yield condition is very different
by each small area in Japan, and the way of appropriate sediment management is supposed to be
changed by each area.
Table-1 “a” of major river system
2.3 Reservoir condition concerning sedimentation
Figure-5 shows average annual rate of sedimentation capacity loss (loss rate herein after) with
operation years. Rate values (%) shown in the figure are the rate of number of dams, which loss
rates are less than the plotting position (dam rate herein after). In the same conditions of dam rate,
the loss rate tends to increase with operation years.
In the condition of less than 15years operation dams (recent dams), the loss rates of 50% dam
rate are nearly 1% (1% of loss rate corresponds to 100 years sediment capacity). On the other hand,
the loss rates of former dam (more than 40 years operation dams for example) of 50% dam rate
reach to 2%. That means not a little part of more than 40 years operation dams already lost the
whole sedimentation capacity.
Figure-6 shows the frequency of the ratio of sedimentation capacity to total storage capacity.
No. of Dams
Hokkaido Tokachi 3 388～533 31,000～100,000
Ishikari 19 12～1,779 280～160,000
Tohoku Aka 2 148～162 12,000～58,000
Kanto Tone 11 20～494 3,300～50,000
Tama 1 263 50,000
Sagami 2 947～1,201 16,000～180,000
Hokuriku Kurobe 1 185 380,000
Agano 17 41～6,728 3,300～170,000
Shou 6 20～1,100 5,300～510,000
Miomote 2 234～306 23,000～39,000
Shinano 19 5～2,787 1,800～180,000
Jintsu 3 128～2,060 3,100～54,000
Jyoganji 3 7～50 3,400～180,000
Chubu Ooi 1 459 820,000
Tenryu 5 58～4,895 13,000～1,100,000
Kiso 12 6～4,632 36～110,000
Chugoku Asahi 2 255～1,140 5,800～6,400
Takahashi 6 22
Saba 2 32～88 2,400～4,000
Oze 2 135～168 280～8,800
Hii 2 19～70 1,600～5,100
Shikoku Yoshino 10 15
Kyusyu Mimi 4 41～737 17,000～100,000
Hitotsuse 3 71～486 3,100～78,000
District Water System
1977 to 1990
The ratio is not large in Japan and the average is 16%. This indicates that the large part of the
storage capacity still remains in the full sedimentation condition of planning. The severity of
sedimentation problem is relieved, and only a small number of dams need emergent examinations
of countermeasures now. It can be predicted, however, that the dams suffering from sedimentation
will gradually increase in no distant future.
Figure-5 Average annual rate of sedimentation capacity loss
Figure-6 Frequency of the ratio of a sedimentation capacity to a total storage capacity
Accuracy of planed sedimentation volume is also indicated in Figure-5. Because of the complex
geological, climate, vegetation condition etc., there is no general formula for estimating
sedimentation volume in Japan. Simple formula is always limited its application. In resent planning
of sedimentation capacity, analysis of neighbor and similar dams’ sedimentation data are usually
executed. This can be done because the sedimentation data have been stocked. Ratio of
non-vegetation area appears to be one of the useful parameter in the analysis.
.4 Grain size of sediment
Grain size information is necessary for the sedimentation management. Figure-7 shows the Grain
size component ratio of sedimentation volume. Ministry of Land, Infrastructure and Transport
(MLIT) manages all of dams in the figure. Dams are located whole area of Japan.
0～5 5～10 10～15 15～20 20～25 25～30 30～35 35～40 40～
Storage capacity for sedimentation / Total storage capacity (%)
Average : 16.4%
Number of data : 695
5 10 15 20 25 30 35 40 45
Time length of operation till 1996 (year)
Ratio of annual sedimentation volume to
storage capacity for sedimentation (%)
Rate of number of dams
Figure-7 Grain size component ratio of sedimentation volume
The definition of grain size classification is as follows:
Clay : diameter is 0.005mm or less.
Silt : diameter is 0.005mm ～ 0.075mm.
Sand : diameter is 0.075mm ～ 2mm.
Gravel : diameter is larger than 2mm.
Fine component : clay + silt.
This figure shows that the fine component occupies about 55% of total sedimentation volume
(liquid part included) on the average. This makes it clear that the influence of fine component is
considerably large for the sedimentation problems in Japan’s dam reservoirs.
Figure-8 shows a relationship between porosity and 60% diameter for each sample. The porosity
increases as the 60% diameter becomes smaller. In case the content ratio of fine component is near
100%, the porosity is 60～80%. In case 0%, 10～40%. Using this relation, we can obtain the
average ratio of fine component in the solid part is about 44%.
Figure-8 Relationship between D
The ratio of fine component, however, considerably varies by dams. This may be a reflection of
the complex condition of dam’s drainage area. Resent analysis of those dams indicates that the
large part of the drainage area of high fine component ratio dam is composed by sedimentary rock;
on the other hand, of low fine component ratio dam is composed by igneous rock.
Name of dam
0.001 0.01 0.1 1 10 100
3. Sediment management
3.1 Activities for sediment control
Many kinds of works concerning sediment control have been executed in Japan. Those works have
contributed to the land development and conservation in each place. Activities of the works, along
with sediment flow, are as follows. These activities are usually pursued as the countermeasures of
the problems at the area.
1) Mountain and foot of a mountain area, alluvial fan –steep and rapid flow
hillside works :reducing sediment yield from hillside slope
check dam: conserving forest area, preventing excess sediment flow to areas downstream
preventing excess sediment flow to areas downstream
countermeasures for reservoir sedimentation: reducing reservoir sedimentation
2) Areas downstream
foot protection works: stabilizing embankment
groundsill: preventing scoring, stabilizing riverbed
prohibition of sand and gravel removal: preventing riverbed degradation
riverbed excavation: preventing riverbed aggradation, conserving water quality
spur dike: restoration of pools
3) Coastal area
Most of the activities have executed for the problems of coastal erosion from 60’s. Several
reasons are considered for the erosion such as littoral transport direction change by coastal
structures, sediment supply reduction by sand and gravel removal in rivers and dam construction
etc. Activities are as follows.
wave absorbing works, jetty, offshore breakwater, artificial reef, head land (Figure 9), sand
bypass, artificial nourishment
Figure-9 Example of head land
3.2 Idea of integrated sediment system management
Problems concerning sediment flow are not satisfactorily settled in spite of the activities
introduced above. The General Sediment Control Subcommittee
the General Policy Committee
of Rivers Council, Ministry of Construction (former MLIT), Govt. of Japan proposed the outline of
sediment management policy -integrated sediment system management- in the future in July 1998.
This report introduced the idea of a new concept, called the “sediment transport system” that
considers all aspects, from mountains include forests to shorelines, and proposed the promotion of
total sediment management along with the new concept.
Examples of sediment problems picked up by the report are,
① Disasters caused by landslide and debris flow in mountainous area
② Disasters caused by aggradation of riverbed in alluvial fan etc.
③ Excess accumulation of sediment in dam reservoirs; worse scenery of reservoirs by
④ Longtime turbid water release from dam reservoirs (influence on ecosystem and scenery)
⑤ Riverbed armoring caused by sediment trapping at reservoirs (influence on ecosystem)
⑥ Influence on river management facilities by large river bed change
⑦ Progress of coastal erosion caused by reduction of sediment supply from rivers, changes
of littoral transport direction etc.
And goals of sediment management are as bellows.
① To prevent disaster
② To conserve river and coastal environment like ecosystem and sightseeing, etc.
③ To apply river and coast in appropriate manners
To reach the goals, following viewpoints of sediment management are required.
① To consider the continuity of place and time
② To consider quantity and quality (river morphology and grain size etc.)
③ To consider the relationship with water flow
Proposed activities are as follows.
① To promote monitoring and identify problems concerning sediment transportation
② To establish restoration or conservation system of integrated sediment system
a. To promote sediment supply type Sabo works
b. To establish sediment management system including sediment supply measures for
③ To control extraction of sand and gravel in an appropriate manner
④ To maintain river structures in an appropriate manner
Some activities, especially for monitoring the sediment transportation system in each grain size,
are being pursued. This means activities are on the first stage of the integrated management. It is
expected that the activities, which are executed now in each areas as introduced 3.1, will
organically and totally combined by this integrated approach. We may obtain appropriate managing
plans by this approach.
New concept of Sabo works was worked out parallel to the integrated sediment management.
The concept is as follows.
Short period: stopping and storing debris flow to prevent disaster
Medium period: controlling sediment flow in a certain level
Log period: discharging storing sediment
Figure-10 shows the example of slit type check dam, which aimed to cope with the concept.
Riverbed change simulation, that can realizes wide range grain size performance, should be
applied to ascertain the effects of these structures. Other activities also require the simulation
model, and 1D or 2D models are developing and used.
Figure-10 Slit type check dam
3.3 Sediment control methods around dam reservoirs
From the viewpoint of the integrated sediment management, dam reservoirs are expected to have
functions of sediment flow monitoring facilities and of sediment control facilities as large check
dams. Annual measurement of sedimentation volume should be continued for a monitoring facility.
Also more data of grain size distribution of sedimentation should be stocked.
Since dam’s reservoirs only store inflow sediments if they left as they are, some activities should
be done for the function of controlling sediment. Operated or test operated methods of sediment
control around dam reservoirs in Japan are as follows.
1) Sediment flushing
Draw down operation is executed for flushing large amount of sediments. Partially draw down
operation is also executed to control released sediment volume or recover store water.
2) Sediment bypassing
There are both cases. Bypassing wide range of grain size and fine sediment only.
3) Excavating and dredging
60% of removed sediment is effectively used. Some dams have tried to resettle in river area of
dam downstream for flushing during flood.
4) Discharging turbid water
Outlet conduits, selective withdrawal facilities or special structures to release turbid bottom
water are used.
5) Empty dam
Gateless bottom outlets are placed near riverbed elevation if a dam is planed only flood control.
Main purpose of the operation or test operation is different by each example. Results of the
operation or test operation, however, have various phases such as countermeasures of
sedimentation, sediment supply method to the areas downstream, influential activity on river
eco-system conditions and so on. We have to find the position of the activity in the sediment
transport system. That may be obtained by the concept of integrated sediment system management.
Situations of sedimentation in reservoirs and sedimentation management in Japan are introduced.
Situations of sedimentation are as follows.
1) Quantity of sediment yield in Japan changes from less than 100 m
/year to 10,000
/year. Sediment yield varies by river system and widely scattered by dam site in the same
2) The range of sediment grain size is wide in Japan. This condition should be considered in
3) Only a small number of dams need emergent examinations of countermeasures now in Japan. It
can be predicted, however, that the dams suffering from sedimentation will gradually increase in
no distant future.
Situations of sediment management are,
1) Although many kinds of works concerning sediment flow have executed in Japan, problems are
not satisfactorily settled
2) Activities of integrated sediment system management are expected to settle the sediment
3) Dam reservoirs are expected to have functions of sediment flow monitoring facilities and of
sediment control facilities from the viewpoint of integrated sediment system management.
Operations of sediment control methods around reservoirs have various phases, and we have to
find the position of the operations in the sediment transport system.
K. Ashida and T. Okumura: Study on sediment deposits of dam, Annual Report of Disaster
Prevention Research Institute, Kyoto University, 1974