Advances in River Sediment Research – Fukuoka et al. (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00062-9

Sediment relocation trial by Ejector Pump Dredger System (EPDS)
in a dam reservoir
T. Temmyo & Y. Nakamura
Hazama Corporation, JapanY. Yamagami & M. Kaku
Kyushu Electric Power Co., Inc., Japan

T. Sumi & M.E. Meshkati Shahmirzadi
Kyoto University Disaster Prevention Research Institute, Japan

ABSTRACT: The Ejector Pump Dredger System (EPDS) is able to pick the sediment up from the bottom of reservoir by means of the pressure gradient generated by jet water. In this system, the maximum
expected size of gravel which can be transported is approximately 150 mm. The field tests on this newly
developed system was very successful as approximately 3,500 m3 sediment was dredged by ejector pumpfrom different depths in the reservoir, 3 to 15 m deep, and then the sediment relocated to the disposal site,
400 m upstream from the suction point, by floating transportation pipeline. The sediment transportation
rate was equal to approximately 70 m3/h.

1INTRODUCTION

Sedimentation in reservoirs causes severe negative impacts on environment such as: reduction ofreservoir storage capacity, damages for infrastructures, reduction of biodiversity for aquatic species
and so on. Up to now, a number of methods havebeen developing for dealing with the sedimentation
issues in a dam reservoirs such as free-flow flushing, pressure flushing, bypassing, dredging, anddensity current venting (Brandt, 2000).
The free-flow flushing (draw down flushing) is
one of the suggested methods, which is commonly
executed world wide to de-silt the deposited sedimentfrom reservoirs. It has been proven that free-flowflushing is much more effective than other methods
due to its large-scale effect (Morris and Fan, 1998).
In free-flow flushing, in fact, the water level in thereservoir is drawn down to a relatively low level, sothat the flow velocities over large areas of deposited
sediment are sufficient to effectively mobilize thesediment (White, 1984). Thus, it could effectively
retrieve the storage capacity of dam reservoir.
However, de-silting a huge amount of sediment from dam reservoir during free-flow flushingexecution is severely threatened downstream riverenvironment. In particular, the first free-flushingexecution may be the most threatening one for thelife of aquatic species in downstream river. In fact,
the deposited sediment in the vicinity of bottom
outlet, which is rather fine, contains the major ele-ments such as silica and iron as well as particulate
nutrients and contaminants (Teoduru and Wernli,2005). The de-siltation of this fine sediment fromreservoir, results the oxidation of particulate,organic matter and iron, which ultimately leads
to deoxygenated and anoxic conditions at downstream river. Thus, in turn, has resulted in the needto relocate this low quality sediment to an area far
enough from the bottom outlet, where it cannot bede-silted (Yamagami, et al. 2012).
To relocate the sediment in a reservoir, dredging of deposited sediment in the vicinity of bottom
outlet may require to be conducted. However, theconventional dredgers may not be suitable to use
in dam reservoirs; due to their impeller type pump.
This kind of pumps doesn’t let the gravel, debris
and rubbish pass through the system.
In present study, a newly developed dredger system has been used to relocate the sediment instead,
named Ejector Pump Dredger System (EPDS).
EPDS is distinguished by its ejector pump which isworking without any impellers. The detail information about this ejector pump can be found in previous publications of authors (Temmyo, et al. 2012).
This paper attempts to introduce the EPDS and
its components and accessories equipment whichused in the sediment relocation project. Moreover, the results of trial sediment relocation project
conducted at Mimikawa River reservoir in Kyushu,
Japan has been presented. Lastly, the capabilities
and constraints of EPDS have been discussed.

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2SEDIMENT RELOCATION SYSTSEM

2.1

The EPDS concept and components

EPDS aims to hydraulically dredge the deposited
sediment from reservoir bed. For this purpose, it
uses water to transport the dredged sediment. The
sediment is picked up by the suction of a pump,which is named ejector pump, and then transported
through a pipeline. In contrary with the conventional dredger system, as abovementioned, EPDS
has no rotary part (impeller wheel) inside of theejector pump. The necessary energy to vacuum
the sediment from the reservoir bed is provided
by differential pressure between inside of ejector
pump and outside of the system (Nakamura,et al. 2012).
The EPDS consists of three main parts: adredging barge, a transportation pipelines andan outlet barge. These parts are floating on thesurface of water and connected to each other.
Each part of EPDS also consists of the few components. The lists of these components are listed
in Table 1.
Table 1.

The list of main components of EPDS.

Main parts

Components

N. of sets

Dredgingbarge

Unit float
High pressure pump1.95 MPa, 5 m3/min
Air compressor 18 m3/min
Screw crusher 90 kW
Telescopic type excavator

14 set2 set

Transportationpipeline

Steel pipe f400 mmFloat
Rubber sleeve

400 m

Outlet barge

Unit float

6 set

Figure 1.

1 set
1 set
1 set

Both of dredging barge and outlet barge areassembled by unit floats which are equipped withwire to provide the required thrusters for the system
to move. The conceptual schematic view of an EPDS
and the schematic sediment relocation strategy are
respectively shown in Figures 1 and 2. Photographyof sediment relocation trial project in Mimikawa
River in Kyushu, Japan is shown in Figure 3.
2.2 Dredging barge
The dredging barge (27 m length and 16 m width)
includes an ejector pump, pressure pumps, air compressors and a screw crusher with telescopic typeexcavator. The main objective of dredging barge isto pick up the sediment from the reservoir bed andpush the sediment into the transportation pipeline. The plane view of dredging barge is shown inFigure 4. The pressure pumps located at dredging
barge aims to provide high water jet velocity forthe system. These pressure pumps are able to discharge 5 m3/min (1.95 MPa) of flow rate into theejector pump through the nozzle.
To effectively pick up and then transport themixture of water and sediment using an EPDS, it
is recommended by Meshkati Shahmirzadi et al.
(2012) to introduce an intermediate concentrationof air into the system. Therefore the air compressors have been used to inject 18 m3/min of air into
the system, where at the beginning of transportation pipeline, just after ejector pump.
2.3

Ejector pump

The ejector pump plays the main role in the suctionprocess, known as the heart of the system. Ejector pump is located at the conjunction of pressure
pumps, suction pipeline and transportation pipeline. As explained earlier, pressure gradient between
inside of Ejector Pump and Entrance of Suction

A conceptual schematic view of sediment relocation system.

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Figure 2.

The schematic layout of EPDS and its components.

A lifting arm is set to hold the ejector pump under
the water surface of water. Thus, the depth distance between the ejector pump and entrance ofsuction pipeline can be adjusted easily to reduce
the risk of blockage. Figure 7 depicts an ejector
pump lifting arm.
2.4

Figure 3. A photography of sediment relocation trial
project using EPDS.

pipeline (outside of EPDS) causes the mixture of
sediment and water picked up into the system.
Then, the sediment would push forward into thetransportation pipeline towards the disposal site.
Ejector pump is equipped by two specificaccessories respectively air controller inlet and astraight inner pipeline. Both accessories are vitalto preserve the ejector pump against the possible
cavitations and abrasion. By varying the combination of diameter of nozzle and diameter of inner
pipeline, the suction rate of EPDS can be adjusted.
A schematic side view of an ejector pump withnozzle and inner pipeline and the photography ofejector pump used in this study respectively can beseen in Figures 5 and 6.
Based on the field observations, it is found thatwhen the length of suction pipeline is relatively long,blockage in transportation pipeline often happens.
Therefore, to prevent the blockage in EPDS during dredging operation in particularly from deeparea, the ejector pump is placed under the surfaceof water to shorten the length of suction pipeline.

Screw crusher and suction pipeline

The suction section includes a telescopic type excavator arm, a screw crusher and a suction pipeline.
The telescopic type excavator arm is used to bring
down the suction pipeline deep into the reservoir
and keep its entrance close to the sediment surface.
A screw crusher is attached to the excavator arm
to crush the deposited gravels and ease their suction by ejector pump. The screw crusher’s shaft isconical shape, which could crush the gravels placed
between conical shaft and lining plates in the shell.
The gravels with maximum size of 300 mm could
be crushed with screw crusher’s shaft. The usedscrew crusher is shown in Figure 8.
2.5

Transportation pipeline

Transportation pipeline made by steel is assembled
by attaching cylindrical pipeline with diameterof 400 mm. Each steel pipeline is connected withrubber sleeve. The transportation pipeline is also
floated on the water surface and it connects thedredging barge to the outlet barge. As it is apparent, the transportation pipeline aims to convey themixture of sediment and water to the outlet barge.
2.6

Outlet barge

The outlet barge (11 m length and 7 m width) is
the last part of the EPDS system, dedicated tokeep the end of transportation pipeline floated at
the surface of water, and also dissipates the energy

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Figure 4.

The layout of dredging barge.

Figure 5.

Schematic side view of ejector pump.

Figure 6.

The photography of ejector pump.

Figure 7.
arm.

The photography of ejector pump and lifting

Figure 8.

The photography of screw crusher.

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of outflow. Dissipating the energy of outflow is notonly for reducing water turbidity around the outlet
barge but also for the stability of the outlet barge.
To dissipate the energy of outflow from an EPDS,
a dissipation unit named cyclone is set at the outlet barge just at the end of transportation pipeline.
The layout of outlet barge is shown in Figure 9.
3
3.1

SEDIMENT RELOCATION TRIAL
PROJECT
Properties of sediment

Figure 10 demonstrates the sample of dredged
sediment by EPDS. The sediment excavation wasconducted at different points range from 3 m to15 m depth in the reservoir. The dredged sedimentfrom these depths, neglecting the stones and debris,comprises fine sediment (sand) with maximum sizeof 2 mm. Investigation on the dredged sedimentshows that 88% of dredged sediment range from
0.425 mm to 0.85 mm diameter, and mean grain
diameter ratio (D50) is 0.39 mm. However, in addition of fine sediment, there are lots of stone, gravel,
branches and debris in the dredged sediment. The
photography of dredged stones and debris areshown in Figure 11.

Figure 9.

The layout of outlet barge.

Figure 10.

A sample of dredged sediment by EPDS.

3.2

Effect of the location of ejector pumpon suction flow rate

As above mentioned, the vertical distance of ejector pump from the reservoir bed can be adjusted in
EPDS using a lifting arm. In this section, the variation of water depth of ejector with variation ofwater flow rate is investigated. Figure 12 illustrated
the relationship between the water depth of ejector and flow rate when the pressure pumps set
to 1.86 MPa and the length of suction pipeline
is 15 m. The square and triangular points in thisfigure respectively represent the situation in which
the water depths of suction points are 7 m and
11 m. As it can be seen, when placed ejector is thesame elevation, by increasing the depth of suction
point, the flow rate decrease. And then each casesof water depth of suction points are 7 m and 11 m,
by increasing the water depth of ejector, flow rate
linearly increase. It could be concluded that when

Figure 11.
sediment.

Stones and blanches in the dredged

Figure 12.

Water depth of ejector and flow rate.

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the ejector pump system dredge sediment fromthe bottom of reservoir, locating the ejector pumpdeep under water, EPDS is more advantageous andeffective to transport the sediment.
3.3

Sediment transportation rate

Measuring the sediment transportation rate in EPDS
is difficult in a sediment relocation project, simply
due to the re-deposition of dredged sediment into
reservoir, which is not detectable directly. Herein,
three different indirect methods have been proposed
to calculate the sediment transportation rate:
1. Setting a valve on the transportation pipeline
and take samples through this valve over time.
Then, sediment transportation rate is estimatedby calculating Equation 1:
Qs1 QrQVsV60(1)where, Qs1 is the sediment transportation rate
obtained by samples of the sediment (m3/h), Qr
is the flow rate in the transportation pipeline
measuring by magnetic flow meter (m3/min), Vs:
is the volume of sampled sediment (m3) and V isthe volume of sampled water and sediment (m3)
2. Setting a G-ray density meter along the transportation pipeline. Sediment transportation
rate then can be expressed by calculating
Equation 2:
Qs 2 QrQ¤] rlw ³60¦ l t lw µFigure 13. The result of estimated sediment transportation rate between Qs1 and Qs2.

The figure demonstrates a plausible correlation
between method 1 and 2. Moreover, these results
are not far from the result by bathymetric survey.
Therefore, the sediment relocation rate is obtainable through either sampling the sediment or measuring the density meter.
In conclusion, setting density meter and magnetic flow meter enable the EPDS operators tomonitor sediment transportation rate in real time.
This monitoring system is very effective to manage
the sediment transportation rate.

(2)
4

where Qs2 is the sediment transportation rate
obtained by G-ray density meter (m3/h), Qr isthe flow rate measured by magnetic flow meteron the transportation pipeline (m3/min), A is thedensity correction factor (in this study equal to1.02), R is the density of mixed flow in the transportation pipeline measured by magnetic flow
meter (m3), Rw ( 1 t/m3) and Rt ( 1.833 t/m3) arerespectively the density of water and wet densityof sediment in the transportation pipeline.
3. Lastly, the sediment relocation rate can be calculated by considering the total operation time
and the result of bathymetric survey before andafter dredging operation. The calculated sediment relocation rate using bathymetric results
and total operation time is equal to 73.8 m3/h
as the bathymetric result was equal to 3,572 m3
and the total operation time was 48.8 h.
In Figure 13, the sediment relocation rate estimated using Equation 1 is plotted versus thesediment relocation rate obtained by Equation 2.

CONCLUSION

The special ejector pump system is able to suck thesediment up from the bottom of reservoir by meansof the pressure gradient generated by jet water. The
field tests on this newly developed system was very
successful as approximately 3,500 m3 sediment wasdredged by special ejector pump from different
depths in the reservoir, 3 to 15 m deep, and thenthe sediment relocated to the disposal site, 400 m
upstream from the suction point, by floating transportation pipeline. The sediment transportation
rate was equal to approximately 70 m3/h. However,the possible maximum capacity of the system isdepends on the condition such as particle size ofthe sediment, water depth, transportation lengthand so on.
Field research of sediment relocation system also
shows some of other useful results. When the ejector pump system dredge sediment from the bottomof reservoir, locating the ejector deep under water
is more advantageous and able to increase the sediment transportation rate.

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The sediment transportation rate is able to be
calculated by sampling sediment or by bathymetry
survey. However, setting density meter and magnetic flow meter enable to monitor transportation
rate in real time. This monitoring system is effective in case of trouble happen during dredging aswell.
Reservoir sedimentation poses the greatest challenge toward the sustainable management in reservoir. In this regard, one of the major problems
is deposition of coarse sediment at the upstream
end of reservoir, in areas far away from the bottom
outlet. Thus, it is difficult to transport coarse sediment by free flow even in flood events. This problem, transport of deposited coarse sediment, canbe solved using the ejector pump dredger system
discussed in this paper.
Using the ejector pump dredger system to pickup the deposited coarse sediment, transport themby horizontal pipeline, and finally release them in
the areas in the vicinity of bottom outlet. Relocation of coarse sediment from upstream end of reservoir to the areas near the bottom outlet, would
increase the chance of sediment removal by flood
events.
Above mentioned sediment relocation system
can be effectively used at dam sites without any
interruption in their daily operations. The authors
hope that this system contributes to the sedimentmanagement for reservoirs, and is considered as apositive jump for environmental remediation ofrivers.
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