Q. 89 – R 5
COMMISSION INTERNATIONALE
DES GRANDS BARRAGES------VINGT TROISIÈME CONGRÈS
DES GRANDS BARRAGES
Brasilia, Mai 2009-------PRIMARY TREATMENT OF DREDGING SYSTEM FOR DAM SEDIMENTREPLENISHING TO THE RIVER *Tetsuya SUMI
Associate Professor, Dr. of Eng.
Department of Civil and Earth Resources Engineering, Kyoto Univercity
Hiromi KOTSUBO
Director of Dam Engineering Group, Water Resources Engineering Department
Kentaro KIDO
Dam Engineering Group, Dr. of Eng., Water Resources Engineering Department
Akira KUBOTA
Kizugawa Dams Integrated Operation & Management Office, Japan Water Agency
Ichiro YOSHIKOSHI
Chief Engineer of Mechanical Division,
Isamu SANDANBATA
Chief Research Engineer of Technical Research Institute,Toshiyuki TEMMYOManager of Civil Engineering Division, Hazama Corporation
Shiro KODAKA
Director, Development Division, Maezawa Industries Inc.
JAPAN

*

Traitement primaire du système de dragage pour les sédiments de barrage rejetés dansla rivière

Q. 89 – R 5
1.

INTRODUCTION

Sediment replenishment that is to supply dredged sediment artificially to thedownstream river is one of attractive solutions to solve reservoir sedimentation
problem and to maintain sediment continuity for downstream river from theenvironmental point of view. In case of the sediment replenishment, it is importantto estimate and mitigate water quality changes if the reservoir sediments containvery fine materials which may cause turbidity problems or nutrient discharge1)-3).
The purposes of our study are how to dredge sediment from reservoirs
safely, and how to produce the appropriate grain sized material from the sediment
which contains very fine sediment nutrients.
In this paper, we present the application of a system utilizing an ejector
pump to suck up sediment and two spiral classifiers to produce the appropriate
grain sized materials from the sediment. To confirm the effectiveness of thedeveloped treatment system, field tests were executed near a check dam of the
Nunome dam reservoir in Yodo river system4). The result shows that our proposed
method is effective to achieve the purpose of our study.

2. FEATURE OF MACHINE USEDEJECTOR PUMP

By spouting the high-pressurized jet water from the nozzle, the ejector pump(Figs. 1, 2) is able to produce the energy required to transport the dredged
sediment slurry4). The ejector pump is structurally simple and easy to maintainsince it has no rotary parts such as an impeller wheel. This pump has two special
characteristics: one is a controlled air inlet into the pump; the other is the innerstraight pipe which is not throttled. These are very effective for the cavitation
control and abrasion resistance of the pump. By changing the water pressure andthe nozzle diameter, it is easy to adjust the potential head and the suction rate.
Another advantage is that sediment can be washed out by water turbulence with
air while passing through the pump.

Q. 89 – R 5

Fig. 1
Ejector pump
Pompe à éjecteur

Pressurized

Fig. 2
Schematic chart of ejector pumpSchéma de pompe à éjecteur

SPIRAL CLASSIFIERS

The spiral classifiers named Escargot (Fig. 3) are compact devices that
separate sand from slurry5). These cylindrical containers compose spiralwatercourse shown in Fig. 4. Diagonal plates with slits (Fig. 5) are installed in thedevice to settle sand particles on. The settled sand falls through the slits to thebottom of the container. The lower part of the container is cone-shaped to collectsand effectively, and the sand is discharged by a screw conveyor attached to thecontainer. For the field test described later, the system consists of two spiralclassifiers connected, the first one (Small Escargot) collects relatively coarse sand,and the second one (Big Escargot) is lengthened watercourse to collect relatively

Q. 89 – R 5
fine sand. The grain size distribution of the classified sand will be able to becontrolled by designing the length of the watercourse properly.

Fig. 3
Spiral classifiers (Escargot)
Classificateur à spirale (escargot)

Fig. 4
Schematic plane of streamlines in spiral classifiers
Schéma de circulation de l'eau dans les classificateurs à spirale

Q. 89 – R 5

Unit: mmFig. 5
Cross section of spiral classifier (Escargot)
Vue en coupe d'un classificateur à spirale (Escargot)

3.

OUTLINE OF THE FIELD TEST AT THE NUNOME DAM

LACE OF THE FIELD TEST

Field test was executed to confirm the effectiveness of the treatment system
for reservoir sediment near the check dam in the Nunome dam reservoir6). Outlineof the Nunome dam is shown in Table1.
The check dam located upstream of the Nunome dam as shown in Fig. 6
was constructed both for reservoir sedimentation and water quality management.
In the shallow sub-reservoir upstream of the check dam, inflow sedimentcontaining fine materials and nutrients can be settled and deposited. If these
deposited sediments will be removed timely, the check dam will highly contributeto reduce sediment and nutrient loads to the main-reservoir.

Q. 89 – R 5
Table 1
Outline of the Nunome dam
Périmètre du barrage de NunomeRiver

Dam height / length

Effective storage
capacity

Nunome river(in Yodo river
system)

72 m / 322 m(Concrete gravity
dam)

15,400,000 m

3

Dam manager
Japan Water
AgencyStorage yard for reservoir sediment

Check dam

Nunome dam

Field test place

Address: Kitanoyama-cho Nara-city, Nara, Japan

Fig. 6
Outline of the Nunome dam
Périmètre du barrage de Nunome

3CHARACTERISTICS OF RESERVOIR SEDIMENTATION

Dredging of deposited sediment in front of the check dam is regularly
executed. When the water level of the check dam is drawn down, the dredging bya backhoe is executed at shallow areas. A cutter suction dredger is used in every
three years for relatively deep areas with keeping the water level. Dredged
sediments are dried in a large excavated pit made at the park near the reservoir,
and trucked to a temporary storage yard (Fig. 6). The locations of dredging in2000 and 2004 are shown in Fig. 7. Grain size distributions of those sediments areshown in Figs. 8, 9. In case of dredging by backhoe, the fine fraction ratio of
0.074mm or less in grain size (Fc) is in the range of 10-40%. In case of that
dredged by cutter suction dredger in 2004, Fc is up to the range of 90%.
The purpose of the field test is to develop the compact system for classifying
the reservoir sediments and replenishing them to the downstream river. The

Q. 89 – R 5
reservoir sediments dredged by backhoe in 2000 were used in the test. The Fc ofthese sediments is in the range of 5-30% as shown in Fig. 8 in which the sedimentwith Fc being under 10% (Represented by curves 2000-2, 3) is called “coarse
dredged sediment”, and the sediment with Fc being nearly 30% (Represented bycurves 2000-1) is called “fine dredged sediment”.

2004

2000

2004-3， Cutter suction dredger
Flow

Flow

Check
dam

Park
(Test site)

Flow

Park
(Test site)

Flow

Check
dam

2004-1,2

10

100

2000-1,2,3
BackhoeFig. 7
Dredging locations in the check dam
Zones de dragage du barrage de correction

100

Percent Passing (%)

90
80

2000-1
2000-2
2000-3

70
60
50
40
30
20
10
0
0.001

0.01

0.1

1

Grain size (mm)Fig. 8Grain size distribution of dredged sediment in 2000
Granulométrie des sédiments dragués en 2000

Q. 89 – R 5100

2004-1
2004-2
2004-3

Percent Passing (%)

90
80
70
60
50
40
30
20
10
0
0.001

0.01

0.1

1

10

100

Grain size (mm)Fig. 9Grain size distribution of dredged sediment in 2004
Granulométrie des sédiments dragués en 2004

TEST PROCEDURE

The flow chart of the field test is shown in Fig.10. Photo of the field test is
shown in Fig. 11. In Fig. 10, (a) to (d) are the soil sampling points, and (e) to (j) arewater sampling points for testing, respectively.
The sequence of the field test is described as follows.
1) Dredged sediments from the check dam are trucked and temporally
stored next to the treatment plant. Then these sediments are supplied properly
into the grand hopper by a pay loader.
2) Dredged sediments pumped up from pump station are sent to the firstspiral classifier called “Small Escargot”. Clear water is pumping up from the lakeof check dam to the pump station.
3) Dredged sediments and water sending by ejector pump are treated by“Small Escargot” and the recycled coarse sands (called Recycle (1)) are taken out
by the screw conveyer.
4) Dredged sediments and water after taken out the coarse sands are sent
to “Big Escargot”, and the relatively fine sands are taken out. The classified sands
are sent to the Mesh Separator (vibration sieve) for dewatering. The productionsof “Big Escargot” are called Recycle (2).
5) Drained water from “Big Escargot” is sent to the lake of the check dam
after getting rid of very fine materials in the sedimentation ponds (1) and (2).

Q. 89 – R 5(g)

Small Escargot(b)
Big Escargot
Recycle (1)

(c)
(i)

(f)
Recycle (2)Mesh separator(a)(h)
Ejector Pump

Screw conveyor

Pay loader

Dredged soil
Turbid
water

(e)
Fresh
water

Sedimentation pond (2)(j)(Channel)(d)

Check dam lake
Sedimentation pond (1)(Excavated)

Fig. 10
Flow chart of the field test
Schéma du procédé de l'essai in situ

Fig. 11
Photo of the field test
Photo de l'installation utilisée lors de l'essai in situ

Q. 89 – R 5
4.

TEST RESULTPERFORMANCE OF THE SYSTEM

The field test was carried out in September 2007. The amount of therecycled sand produced by the system was 108.5 m3 from 155.5 m3 of thedredged sediment treated in 12 days. Three cases in the field test were executed
as shown in Table 2. Fine dredged sediment was used in Case (1) and (2). Coarse
dredged sediment was used in Case (3). In Case (1) and (3), the treatment
system was operated normally (as smoothly as possible). In Case (2), thedredged sediment was supplied into the hopper in a massive amount (plentifully;
as much as possible).
Performances of the system during the tests are shown in Table 2. The
speeds of treatment in Case (1) is lower than Case (3). The reason is that the finedredged sediment is more cohesive than coarse dredged sediment. Therefore,
the screw conveyor under the hopper does not work effectively. The speed oftreatment in Case (2) is slightly higher than in Case (3), but the total recycle rate islower down to 49% in Case(2) compared to 72% in Case(1) and 85% in Case(3).
It was found that to operate this system properly, it is important to adjust
supplying speed according to the cohesion of the sediment, which mainly
depends on the grain size distribution and the organic matter content.
Table 2
Performance of the reservoir sediment treatment system
Performance du système de traitement des sédiments du réservoir

Case
(1)
Case
(2)
Case
(3)

Type ofdredgedsediment(volume)Fine(118.5m3)
Fine(23.5 m3)
Coarse(13.5 m3)

Total

(155.5 m3)

Case

Average
Operation
treatment
days
speed
10 days

2.8 m3/h

1 days

4.5 m3/h

1 days

4.2 m3/h

12 days-

Recycle volume / ratio
Recycle(1)

Recycle(2)

Total

61.5m3 /52%
8.5m3 /
36%
10.0m3 /74%
80.0 m3 /51.4%

24.0m3 /20%
3.0m3 /
13%
1.5m3 /
11%
28.5 m3 /
18.3%

85.5m3 /72%
11.5m3 /49%
11.5m3 /85%
108.5 m3
69.7/ %

Q. 89 – R 5
ROPERTIES OF DREDGED SEDIMENT AND RECYCLED SAND

The grain size distributions (GSDs) are almost the same in Case (1) and (2)as shown in Fig. 12. After the treatment, the Fc (fraction ratio of fine materials
under 0.074mm) is reduced from around 30% to less than 10%.
In Case (2), despite the recycle rate was down to 49% as described before,
the GSDs of Recycle (1) and (2) are almost the same as in Case (1).
In Case (3) as shown in Fig. 13, the GSDs of Recycle (1), (2) are different
from that of Case (1).
100

Recycle(2)90Percent Passing (%)

80

Recycle(1)Case(1), (2) : Fine
dredged sediment70
60

Dredged 9/6
Dredged 9/7
Dredged 9/10
Dredged 9/11
Dredged 9/12
Dredged 9/13
Dredged 9/14
Dredged 9/19
Dredged 9/20
Dredged 9/21
Dredged 9/25

50
40
30
20
10

Recycle(1)9/6
Recycle(1)9/7
Recycle(1)9/10
Recycle(1)9/11
Recycle(1)9/12
Recycle(1)9/13
Recycle(1)9/14
Recycle(1)9/19
Recycle(1)9/20
Recycle(1)9/21
Recycle(1)9/25

Recycle(2)9/6
Recycle(2)9/7
Recycle(2)9/10
Recycle(2)9/11
Recycle(2)9/12
Recycle(2)9/13
Recycle(2)9/14
Recycle(2)9/19
Recycle(2)9/20
Recycle(2)9/21
Recycle(2)9/25

Case(1)

Case(2)

0
0.01

0.10

1.00

10.00

100.00Grain size (mm)Fig. 12Grain size distributions(all data for fine dredged sediment and recycled sand)
Granulométrie (toutes les données concernent des sédiments dragués fins et du sable recyclé)

Q. 89 – R 5

100
90

Percent Passing (%)

80

Case(3) : Coarse
dredged sediment70

Case(1) : Fine
dredged sediment

60
50
40
30
20
10

Dredged 9/14

Recycle(1)9/14

Recycle(2)9/14

Case(1)

Dredged 9/18

Recycle(1)9/18

Recycle(2)9/18

Case(3)

0
0.01

0.10

1.00

10.00

100.00Grain size (mm)Fig. 13
Grain size distributions(comparison between fine and coarse dredged sediment)
Granulométrie (comparaison entre sédiments dragués à grain fin et à gros grain)

Figs. 14 and 15 show the grain size frequency and the recycle ratio of eachcase. Each grain size frequency is sorted out Recycle (1), (2), and discharged
with water.
As shown in Fig. 14, the grain size frequency of Case (3) is considerably
different from Case (1), but the ratios of Recycle (1) and (2) are almost the samein every range of grain size. This result suggests that the grain size distribution of
the recycle sand produced by the treatment system is able to be estimatedpreviously from that of dredged sediment. However, as shown in Figs. 14 and 15,
the ratio of recycle may be varied according to the sediment supplying speed.

80%
60%
Discharged

40%

Recycle(2)20%Recycle(1)

0%

80%
60%
40%

Discharged

20%

Recycle(2)9/18
Recycle(1)9/18

0%

Fig. 146
5

Discharged

4

Recycle(2)9/25

3

Recycle(1)9/25

2
19.500～19.00

4.750～9.500

2.000～4.750

0.850～2.000

0.425～0.850

0.250～0.425

0.106～0.250

0.075～0.106

0.075＞

0100%
Ratio of recycled sandCase(2) : Fine dredged sand plentifully （23.5m3)

7

(mm)

Grain size frequency in volume (m3)

Grain size frequencies and recycle ratio(comparison between fine and coarse dredged sediment)
Fréquences granulométriques et taux de recyclage(comparaison entre sédiments dragués à grain fin et à gros grain)

8

80%
60%
40%

Discharged
Recycle(2)9/25
Recycle(1)9/25

20%
0%

Fig. 15
Grain size frequencies and recycle ratio(fine dredged sediment, provided plentifully)
Fréquences granulométriques et taux de recyclage(sédiments dragués de grain fin, fournis en abondance)

9.500～19.00

4.750～9.500

100%

2.000～4.750

0Ratio of recycled sand

9.500～19.00

4.750～9.500

2.000～4.750

0.850～2.000

0.425～0.850

0.250～0.425

0.106～0.250

0.075～0.106

010.850～2.000

50.425～0.850

Recycle(1)10Discharged
Recycle(2)9/18
Recycle(1)9/18

20.250～0.425

153

0.106～0.250

Recycle(2)（mm）

Discharged

20

Case(3) : Coarse dredged sand （13.5m3)

40.075～0.106

25

50.075＞

30

100%
Ratio of recycled sandCase(1) : Fine dredged sand （Total 118.5m3)

35Grain size frequency in volume (m3)40

0.075＞（mm）

Grain size frequency in volume (m3)

Q. 89 – R 5

Q. 89 – R 5
Ignition losses of dredged, Recycle (1) and (2) sediment and also the
portions of ignition loss after treatment are shown in Fig. 16,
Approximately 6% of organic material in the fine dredged sediment isreduced to 2% in Recycle (1), and 4% in Recycle (2) respectively. In Case (3),initial organic content of coarse dredged sediment is only 2.5%, and this valueslightly reduced in Recycle (1), but doesn’t reduced in Recycle (2).
The portions of ignition loss after treatment are calculated by multiplying
ignition loss and recycled ratio. As shown in the right graph of Fig. 16, it isassumed that approximately 4% of organic matter in fine dredged sediment isdischarged with the water.
DredgedRecycle(1)100Recycle(2)Recycle(1)Ratio of ignition loss
after treatment (%)

Ignition loss (%)

6

4

2

Recycle(2)Discharged

80

60

40

20

0

0
9/6

9/7

9/10

9/11 9/12 9/13 9/14

Test date (month/day)

9/18 9/19
CoarseCase(3)

9/20 9/21

9/25
PlentifullyCase(2)

9/6

9/7 9/10 9/11 9/12 9/13 9/14 9/18 9/19 9/20 9/21 9/25
CoarsePlentifullyCase(3)
Case(2)

Test date (month/day)Fig. 16
Ignition loss analyses
Essai de perte au feu (calcination)

PROPERTY OF THE DREDGED SEDIMENT BY ELUTION TESTThe elution test was carried out by stirring 200cc of dredged sediment or
recycled sands with 1,000 cc of the clear water in the plastic container in 5
minutes. Discharged water was also tested. Fig. 17 show the test results.
The water turbidity and suspended sediment concentration (SS) of the Recycle(1) and Recycle (2) are reduced to about 10-20% of that of dredged sedimentrespectively. Similarly, such as chemical oxygen demand (COD) value, the index
of organic matter content , the content rate of nitrogen (T-N) , the content rate ofphosphorus (T-P), in Recycle (1) and Recycle (2) are reduced to approximately
10-20%.

Q. 89 – R 5

20000

DredgedRecycle(1)Recycle(2)30000

Discharged

DredgedRecycle(1)Recycle(2)Discharged

9/20

9/25
plentifully

25000

SS (mg/L)

Terbidity

15000

10000

20000
15000
10000

5000
5000
0

0
9/10

9/18
Coarse

9/20

9/10

9/25
plentifully

9/18
CoarseTest date (month/day)

Test date (month/day)

COD (mg/L)

2000

DredgedRecycle(1)Recycle(2)Discharged

9/20

9/25
plentifully

1500

1000

500

0
9/10

9/18
CoarseTest date (month/day)
150

DredgedRecycle(1)Recycle(2)50Discharged

DredgedRecycle(1)Recycle(2)Discharged

9/18
Coarse

9/20

9/25
plentifully40

T-P (mg/L)

T-N (mg/L)

125
100
75
50

30
20
10

25

0

0
9/10

9/18
Coarse

9/20

9/25
plentifully

9/10

Test date (month/day)

Test date (month/day)Fig. 17
Elution test resultsRésultats du test d'élution

CONCLUSION

In the field test for treatment of reservoir sediment in Nunome dam, several
results are found as follows.
1) Performance of the system
The reservoir sediment treatment system successfully worked. It treateddredged sediments at the rate of 3 to 4 m3 per hour. We found that the production
rate is dependent on the cohesion of the sediment mainly depending on the grain

Q. 89 – R 5
size distribution and the organic matter content. The system is able to producecoarse and fine sands approximately 70% to 80% from the dredged sediment.
2) Classification impact
Despite of the type of dredged sediment, the system is able to reduce thecontent of fine materials under 0.074mm usually causing turbid water problem. It
is possible to get high quality sand materials for replenishing to the river fromreservoir sediment in consideration of the water turbidity and the nutrientsdischarge.
3) Washing impact for organic matter and nutrients
The content of COD, nitrogen, and phosphorus can be reduced to between
20% and 10% by using the first spiral classifier “Small Escargot”. It is proved thatthe washing impact by the ejector worked effectively.
4) Comprehensive evaluation
Generally, in reservoir sediments, there are a lot of very fine materials andnutrients that may cause eutrophication and turbidity problems in reservoirs. It isconsidered that the ejector pump and the proposed spiral classifiers are verymuch effective to wash and classify reservoir sediments not only for the
replenishment to the downstream river but also for the control of eutrophication or
turbidity problems in reservoirs. This study confirmed the ability and feasibility ofthe proposed new sediment treatment system for reservoir sediment.

REFERENCES

[1]

[2]

[3]

[4]

OYA, M., SUMI, T. and KAMON, M., “Study on environmental approaches
for recycling reservoir sediment”, Proceedings of the 20th Congress on
Large Dams, 117-197, 2006.
OKANO, M., KIKUI, M., ISHIDA, Y. and SUMI, T., “Reservoir
Sedimentation Management by Coarse Sediment Replenishment below
Dams”, 9th International Symposium on River Sedimentation, Ⅱ ,pp.1070-1078, 2004.
SUMI, T., HAYASE, H. and OYA, M., “Study on environmental impact ofrecycling reservoir sediment with lots of fine particles”, Advances in River
Engineering, vol.11, 297-302, 2005(Japanese).
SUMI, T., KUBOTA, A., FUCHIGAMI, G., and SANDANBATA, I., and
YOSHIKOSHI, I. and KODAKA, S., “Study on compact treatment systemof dredging dam sediment for replenishing to the river”, Advances inRiver Engineering, vol.14, 2008(Japanese)Q. 89 – R 5
SUMMARY

Sediment replenishment that is to supply dredged sediment artificially to thedownstream river is one of attractive solutions to solve reservoir sedimentation
problem and to maintain sediment continuity for downstream river from theenvironmental point of view. In case of the sediment replenishment, it is importantto estimate and mitigate water quality changes if the reservoir sediments containvery fine materials which may cause turbidity problems or nutrient discharge.
In this study, we applied a system utilized an ejector pump to collectsediment and two spiral classifiers to produce the appropriate grain sizedmaterials from the sediment. To confirm the effectiveness of the developed
treatment system, field tests were executed near a check dam of the Nunome
dam reservoir in Yodo river system.
During the field test of 12 days in September 2007, 108.5 m3 recycled sandwas produced from 155.5 m3 dredged sediment by the system. The sediment
which contains about 30% of fine materials under 0.074mm was normally treatedat speed of 2.8m3/hr and the recycle rate is 72%.
After treatment, the very fine materials under 0.074mm were reduced to lessthan 10% from 30%, and the water turbidity and SS (suspended sedimentconcentration) obtained by the elution tests were reduced to 10-20% incomparison with the original dredged sediments. The COD (chemical oxygen
demand), nitrogen, and phosphorus of the recycle sand were also reduced to
10-20%.
It is considered that the ejector pump and the proposed spiral classifiers arevery much effective to wash and classify reservoir sediments not only for the
replenishment to the downstream river but also for the control of eutrophication or
turbidity problems in reservoirs.

RÉSUMÉ

Le rejet artificiel en aval dans la rivière des sédiments dragués est une
solution attrayante pour résoudre le problème du dépôt de sédiments dans leréservoir et pour maintenir la continuité de la sédimentation en aval de la rivière,d'un point de vue environnemental. Dans le cas du rejet des sédiments, il est
important d'évaluer et d'atténuer les changements dans la qualité de l'eau lorsqueles sédiments du réservoir contiennent des matériaux très fins qui peuvent
entraîner des problèmes de turbidité ou de déversement d'éléments nutritifs.

Q. 89 – R 5
Dans cet essai, nous avons utilisé une pompe à éjecteur pour recueillir lessédiments et deux classificateurs à spirale pour obtenir des matériaux avec une
taille de grain appropriée à partir des sédiments. Pour réaliser la phase dedéveloppement et mettre au point un système de traitement efficace, des essais
pratiques ont été effectués à proximité d'un barrage de correction du réservoir dubarrage de Nunome, appartenant au réseau fluvial de la rivière Yodo.
Au cours des essais pratiques qui se sont déroulés sur 12 jours en
septembre 2007, 108,5 m3 de sable recyclé ont été produits à partir de 155,5 m3
de sédiments dragués par le système. Les sédiments, qui contenaient environ
30 % de matériaux très fins de taille inférieure à 0,074mm, ont été traités à undébit de 2,8 m3/h et le taux de recyclage obtenu a été de 72 %.
Après traitement, la proportion de matériaux très fins de taille inférieure à
0,074 mm est passée de 30 % à moins de 10 %, et la turbidité de l'eau ainsi quela concentration de sédiments en suspension (SS) obtenue par les tests d'élutiona été réduite de 10 à 20 % en comparaison des sédiments dragués avant
traitement. La demande chimique en oxygène (COD), la teneur en azote et lateneur en phosphore du sable recyclé ont aussi été réduites de 10 à 20 %.
Nous estimons que la pompe à éjecteur et les classificateurs à spirale
proposés sont une solution très efficace pour laver et trier les sédiments desréservoirs, non seulement pour ce qui concerne le rejet des sédiments en avaldans le cours d'eau, mais aussi pour le contrôle de l'eutrophisation et desproblèmes de turbidité dans les réservoirs.