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Reservoir Sedimentation
Jolanda Jenzer Althaus and Giovanni De Cesare




ALPRESERV


Sustainable Sediment Management in Alpine Reservoirs
considering ecological and economical aspects



Volume 3 / 2006


Reservoir Sedimentation
Jolanda Jenzer Althaus and Giovanni De Cesare




www.alpreserv.eu




Neubiberg 2006
Die Deutsche Bibliothek – CIP-Einheitsaufnahme

ALPRESERV
Sustainable Sediment Management in Alpine Reservoirs considering ecological and
economical aspects
Neubiberg, 2006

Publisher:
Institut für Wasserwesen
Universität der Bundeswehr München
85577 Neubiberg
Germany
Tel.: +49-(0)89-6004-3493, - 3484
Fax: +49-(0)89-6004-3858
http://www.unibw.de/ifw/Institut/

Editor:
Dr.-Ing. Sven Hartmann
Dr.-Ing. Helmut Knoblauch
Dr.-Ing. Giovanni De Cesare
Dipl.-Bauing. Jolanda Jenzer Althaus
Dipl.-Math. Christiane Steinich


ISSN 1862-9636
Alpreserv (Print)
ISSN 1862-9644
Alpreserv (Internet)


© 2006 All rights reserved

Print: Universität der Bundeswehr München (Germany)

Contents
I

Contents

1 The Problem of Reservoir Sedimentation 1
2 Run-of-River Installations 5
2.1 Reservoir and Sedimentation 5
2.2 The Morphology of Reservoir Sedimentation - the Phenomenon 6
2.2.1 Influencing factors 6
2.2.2 The Mechanics of Sedimentation 6
2.2.3 Dead-Water Zones 8
2.3 Case study 9
2.3.1 Experience gathered in Reservoir Flushing on the River Drau in Austria 9
2.3.1.1 Plant Description 9
2.3.1.2 Sediment Management 9
2.3.1.3 Reservoir Flushing 9
2.3.1.3.1 Experiences 9
2.3.1.3.2 Discussion of Flushing Results as obtained at the Rosegg Dam 10
2.3.1.4 Summary 14
3 Deep, seasonal storage reservoir 15
3.1 Turbidity current - main cause of sediment transport in deep reservoirs 15
3.1.1 Propagation of sediments to dam 16
3.2 Theoretical Background 18
3.2.1 Flow over an obstacle 23
3.2.2 Flow through a screen 26
3.3 Experimental Studies 28
3.3.1 Experimental set-up at EPFL-LCH (De Cesare, 1998) 28
3.3.2 Experimental results 31
3.3.3 Experimental set-up at EPFL-LCH (Oehy, 2003) 32
3.3.3.1 Flume description 32
3.3.3.2 Properties of the sediment materials 33
3.3.3.3 Measuring Instrumentation 34
3.3.3.3.1 Flow velocity measurements 34
3.3.3.3.2 Front Velocity and Time Measurements 34
3.3.3.3.3 Density and Temperature Measurements 34
3.3.3.3.4 Auxiliary Measurements 34
3.3.3.3.5 Deposition Measurements 34
3.3.3.4 Experimental Procedure 35
3.3.3.4.1 Mixture and Flume Preparation 35
3.3.3.4.2 Preparation of Measuring Instruments 36
3.3.3.4.3 Experimental Run 36
3.4 Numerical Modelling 36
3.4.1 Two buoyancy-extended k-ε models or the standard k-ε formulation 38
3.4.1.1 Results 40
3.4.1.2 Conclusions 43
II
Reservoir Sedimentation
3.5 Case Studies 44
3.5.1 Submerged Dams in Lake Grimsel 44
3.5.1.1 Generalities 44
3.5.1.2 Turbidity current simulation of Flood Event in October 2000 45
3.5.1.3 Turbidity current passing over submerged dams 45
3.5.1.4 Conclusions 46
3.5.2 Luzzone 47
3.5.2.1 Numerical simulation 52
3.5.2.2 Selected numerical results of the physical model 52
3.5.2.3 Selected numerical results from Lake Luzzone 53
3.5.2.3.1 Grid Generation 53
3.5.2.3.2 Boundary Conditions 54
3.5.2.3.3 Simulation of a Flood Event 55
3.5.2.3.4 Results 55
3.5.2.3.5 Links to Observations 59
3.5.2.3.6 Conclusions 60
3.5.3 Livigno 60
3.5.3.1 Scope of the study 60
3.5.3.2 Lake topography 60
3.5.3.3 Hydrologic and operation data 63
3.5.3.4 Sediment characteristics 65
3.5.3.5 Numerical Simulations using CFX-4 66
3.5.3.6 Current situation 68
3.5.3.7 Conclusions and recommendations 75
4 Outline of the Historical Development of Reservoir Sedimentation 77
4.1 Scope of investigations 77
4.2 Evolution of knowledge regarding reservoir sedimentation 78
4.3 Evolution of management competence to master reservoir sedimentation 79
4.4 Statistical analysis of publications on reservoir sedimentation 79
4.5 Conclusions 82
5 Bibliography 83
5.1 References for chapter 4 86
5.1.1 Papers published before 1950 86
5.1.2 Papers published between 1950 and 1959 86
5.1.3 Papers published between 1960 and 1969 88
5.1.4 Papers published between 1970 and 1979 89
5.1.5 Papers published between 1980 and 1989 90
5.1.6 Papers published after 1990 95
6 Contact 107
Figures III
Figures

Figure 1.1-1:

Classification of impounding facilities 1

Figure 1.1-2:

Inventory of known measures against reservoir sedimentation (Schleiss and Oehy, 2002) 3

Figure 2.1-1:

Flow and sedimentation patterns (Westrich, 1988) 5

Figure 2.2-1:

Flow conditions and sedimentation process in a river sand trap (Mertens, 1987) 7

Figure 2.3-1:

Location map of the Rosegg power plant 10

Figure 2.3-2:

Duration and erosion volume for different reservoir flushings 11

Figure 2.3-3:

Effects of reservoir flushing in autumn 1993, differences in area [m
2
] before/ after flushing 11

Figure 2.3-4:

Effects of reservoir flushing in autumn 1998, [m
2
] before/ after flushing 12

Figure 2.3-5:

Cross section changes in the Rosegg reservoir 12

Figure 2.3-6:

Effects of reservoir flushing in Nov. 2004, differences in area [m
2
] before/ after flushing 13

Figure 2.3-7:

Cross section changes 13

Figure 2.3-8:

Effects of reservoir flushing in Oct. 2005, differences in area [m
2
] before/ after flushing 14

Figure 2.3-9:

Cross section changes, Profiles 2, 7 and 19/1 14

Figure 3.1-1:

Sediment-laden river entering a reservoir - plunging flow phenomenon and turbidity current
formation. 15

Figure 3.1-2:

Areas affected by sedimentation in the surroundings of a reservoir 17

Figure 3.1-3:

Maximal transportable grain sizes dependent on the flow velocity of the turbidity current
according to Fan (1986) 17

Figure 3.2-1:

Types of turbidity currents 18

Figure 3.2-2:

Turbidity current flowing at the bottom 19

Figure 3.2-3:

Schematic dimensionless velocity profile for turbidity currents (Graf and Altinakar, 1996) 19

Figure 3.2-4:

Flow over an obstacle (Oehy, 2003) 23

Figure 3.2-5:

Flow regimes of shallow-layer flow over an obstacle (Oehy, 2003) 25

Figure 3.2-6:

Flow over an obstacle in a laboratory flume (Oehy, 2003) 25

Figure 3.2-7:

Flow through a screen (Oehy, 2003) 26

Figure 3.2-8:

Ratio of heights down- and upstream of the screen, H
p
=h
3
/h
2
, as function of the effective
porosity, f (Oehy, 2003) 26

Figure 3.2-9:

H
j
=h
2
/h
1
as function of the porosity f and the upstream Fr
d1
(Oehy, 2003) 27

Figure 3.2-10:

Proportion of the incoming flow that is predicted to continue through the screen as a function
of the effective porosity f and the upstream Fr
d1
(Oehy, 2003) 27

Figure 3.2-11:

Flow through a screen in a laboratory flume (Oehy, 2003) 28

Figure 3.3-1:

Schematic drawing of the experimental installation : 1) mixing tank, 2) upstream tank,
3) recirculation pump, 4) free surface weir, 5) inflow gate, 6) turbidity current, 7) experimental
flume, 8) ultrasonic probes, 9) sharp crested weir, 10) flexible duct, 11) UVP instrument,
12) control computer 29

Figure 3.3-2:

Arrangement with 8 transducers looking with an angle of 60° against the main flow 30

Figure 3.3-3:

Axial disposition with 8 transducers looking straight against the main flow 30

Figure 3.3-4:

Square grouping with 4 transducers on each side looking straight at and perpendicular to the
main flow in the spreading part just after the inflow gate 31

Figure 3.3-5:

Expanding turbidity current in the experimental flume 25 s after opening of the gate; the
current spreads out almost radially, large eddies developing at the current front; these eddies
give the characteristic surface appearance of turbidity currents like clouds, 125 mm x 125 mm
grid on PVC bottom. 31

Figure 3.3-6:

Measured velocity values compared with theoretical vertical velocity distribution, u(z). Values
from run n° 2, 80 ms between two succeeding measurements. 32

Figure 3.4-1:

Scheme of numerical model with inflow conditions, discretized reservoir topography, bottom
user interfaces and erosion-deposition map. 38

Figure 3.4-2:

Comparison of velocity and density profiles of buoyancy-extended (Model 1: Burchard and
Petersen, 1999; Model 2: Rodi, 1980) and the standard k-ε models against experimental data
(SALT08; Altinakar, 1988). 41

Figure 3.4-3:

Comparison of the propagation of the turbidity current head for the buoyancy-extended (Model
1: Burchard and Petersen (1999); Model 2: Rodi, 1980) and the standard k-ε models against
experimental data (SALT08; Altinakar, 1988). 42

Figure 3.4-4:

Comparison of velocity and density profiles of buoyancy-extended (Model 1: Burchard and
Petersen, 1999; Model 2: Rodi, 1980) and the standard k-ε models against experimental data
(TK1305; Altinakar, 1988). 42

Figure 3.4-5:

Comparison of velocity profiles of buoyancy-extended (Model 1: Burchard and Petersen
(1999); Model 2: Rodi, 1980) and the standard k-ε models against experimental data (NOVA7;
Garcia 1993). 43

IV
Reservoir Sedimentation
Figure 3.5-1:

Overview of the investigated obstacles in a 1:25'000 map 44

Figure 3.5-2:

Sedimentation after the flood in October 2000 (40 m
3
/s and 15 g/l) with a single obstacle at
1845 m a. s. l. 46

Figure 3.5-3:

Luzzone Reservoir during emptying in 1985, looking upstream. 47

Figure 3.5-4:

Picture of the 225 m high Luzzone arch dam and its reservoir. 48

Figure 3.5-5:

Measured precipitation, discharge, temperature, optical turbidity and suspended sediment
samples for a typical small flood 49

Figure 3.5-6:

Location and vertical disposition of current meters of the underwater measuring network
consisting of 3 stations A, B and C with 2 levels at each station. 50

Figure 3.5-7:

Observed directions at the bottom of the reservoir at station C, 4 meters above ground,
classified by current velocity. The flow is well oriented along the longitudinal axis of the lake51

Figure 3.5-8:

Computed and measured 2D flow field close to the bottom and limits of the spreading turbidity
current a) 5 and b) 10 seconds after opening of the gate. Numerical simulation : black velocity
vectors, turbidity current as a blue surface; Physical model : white velocity vectors, limits of
the turbidity current as a bold line 53

Figure 3.5-9:

Method of decomposition of the real reservoir geometry a) to a simplified physical space
without shore and islands b) and then to the rectangular computational space c) 53

Figure 3.5-10:

Location of stations s11 to s61, used for extraction of local values of the significant parameters
for data analysis. Also shown are the directions of the turbidity current and the generated single
grid mapped on the reservoir bottom 54

Figure 3.5-11:

Measured and adjusted non-dimensional time evolution of inflow (

,

) and sediment
concentration (+) 55

Figure 3.5-12:

Pictures a) to f) in plane view and along the axis of Luzzone Reservoir showing the advancing
turbidity current 10, 20, 30, 40, 50 and 210 minutes after the rise of the hydrograph 56

Figure 3.5-13:

Evolution in time and space of the horizontal turbidity current velocity along the axes of the
reservoir at station s11 to s61 in the bottom cell 57

Figure 3.5-14:

3D view of the bottom of the Luzzone Reservoir showing the numerically simulated turbidity
current flow, 30 minutes after the rise of the hydrograph 58

Figure 3.5-15:

Calculated sediment depth change due to a simulated 1000-year flood on the bottom of the
reservoir 58

Figure 3.5-16:

Qualitative representation showing the location and the measured sediment deposits magnitude
after 31 years in service 59

Figure 3.5-17:

Map showing the location of the Livigno Reservoir on the Swiss-Italian border. 61

Figure 3.5-18:

Overview of the EKW hydraulic scheme 62

Figure 3.5-19:

3D topography of the Livigno Reservoir 63

Figure 3.5-20:

October 2000 flood event 64

Figure 3.5-21:

Simulated flood events 65

Figure 3.5-22:

Log-normal fit for the peak flood discharge at the Livigno Reservoir 65

Figure 3.5-23:

Unit inflow hydrograph and unit concentration curve 66

Figure 3.5-24:

Scenarios of simulations 67

Figure 3.5-25:

Computational domain for the study 68

Figure 3.5-26:

Final condition for the annual flood with maximum inflow concentration 15 g/l 69

Figure 3.5-27:

Final condition for the October 2000 flood event with maximum inflow concentration 15 g/l
(left) and 30 g/l (right) 69

Figure 3.5-28:

Final condition for the 1960 flood event with maximum inflow concentration 15 g/l (left) and
30 g/l (right) 69

Figure 3.5-29:

Final condition for the 100 years of return period flood event with maximum inflow
concentration 15 g/l (left) and 30 g/l (right) 70

Figure 3.5-30:

Evolution of the current in the cross section of the foreseen obstacle. 71

Figure 3.5-31:

Longitudinal cross section for time step 9 h without the effects of obstacle and screen 71

Figure 3.5-32:

Evolution of the current in the cross section of the foreseen screen. 71

Figure 3.5-33:

Longitudinal cross section for time step 16 h without the effects of obstacle and screen 72

Figure 3.5-34:

Zoom of the body-fitted grid around the obstacles 72

Figure 3.5-35:

Final condition for the October 2000 flood event with a maximum inflow concentration of 15
g/l for the present situation (up left) and obstacles with 4 (up right), 8 (down left) and 12 m
height (down right) 73

Figure 3.5-36:

Longitudinal cross section at time step 9 h with the effects of obstacles of 4, 8 and 12 m height
(up to down) 74

Figure 3.5-37:

Deposition profile along the longitudinal cross section of the reservoir 75

Figure 4.4-1:

Evolution of the total number of publications related to reservoir sedimentation 79

Figure 4.4-2:

Evolution of the number of publications separated into five major categories 80

Figure 4.4-3:

Parts of number of publications separated into the five major categories 81

Tables V
Tables

Table 3.4-1:

Experimental conditions of data sets used for computations (D
sg
geometric mean diameter; ρ
a

density of ambient water). 41

Table 3.5-1:

Boundary conditions used in the numerical simulation 54

Table 3.5-2:

Average and peak discharge values 64

Table 3.5-3:

Summary of boundary conditions used in the numerical model 67

Table 3.5-4:

Characteristics of the discretisation of the grid for the current situation (SYM G.P = symmetric
geometric progression; G.P = geometric progression) 68

Table 3.5-5:

Ratio between the depositions upstream of the foreseen section for the alternatives and the total
deposition in the full west arm of the reservoir at the end of the simulations 70

Table 3.5-6:

Characteristics of the discretisation of the grid for simulations with obstacles 72

Table 3.5-7:

Ratio of the depositions upstream of the foreseen sections for the alternatives and the total
deposition in the full west arm of the reservoir at the end of the simulations for different
obstacle heights 73


VI
Reservoir Sedimentation

The Problem of Reservoir Sedimentation
1
1 The Problem of Reservoir Sedimentation
Reservoir sedimentation is a problem that will keep those responsible for water resources
management occupied more than usual during the decades to come. Although the aim behind
the efforts to create reservoirs is storing water, other substances are carried along by the water
and are usually deposited there. Lasting use of reservoirs in terms of water resources
management involves the need for de-sedimentation.
The alterations of the flow behaviour due to dam constructions lead to transformations in a
fluvial process, where deposition of solid particles transported by the flow can be cited
(Chella et al., 2003). Each reservoir, independent of its use (water supply, irrigation, energy or
flood control), can have its capacity decreased due to deposition over the years. In an extreme
case, this may result in the reservoir becoming filled up with sediments.
A reservoir, like a natural lake, silts up more or less rapidly. In actual fact, they may
completely fill with sediments even within just a few years, whereas natural lakes e.g. in our
Alpine foreland, may remain as stable features of our landscape for as much as 10,000 or
20,000 years after they were formed during the last Ice Age.
Impounding structures include large dams, in the form of fill or concrete dams, as well as
river barrages comprising weirs, power plants, locks, impounding dams and dykes (Figure
1.1-1). The artificial lakes formed by such closure structures may be called reservoir lakes and
backwater reservoirs, respectively. In addition, there are flood-retention basins, pumped-
storage basins and sedimentation basins.

Impounding facility
Large dam
Reservoir

Reservoir
basin

Run-of-River
dam
Backwater
area
Backwater
reservoir

Structure

Storage
basin

Lake

Figure 1.1-1: Classification of impounding facilities
Impounding facilities are always costly, but this is justified by their various potential uses.
Reservoir sedimentation, however, reduces the value of or even nullifies this investment. The
use for which a reservoir was built can be sustainable or represent a renewable source of
energy only where sedimentation is controlled by adequate management, for which suitable
measures should be devised.
All lakes and reservoirs created on natural rivers are subjected to reservoir sedimentation.
There are no accurate data on the rates of reservoir sedimentation worldwide, but it is
commonly accepted that about 1 – 2 % of the worldwide capacity is lost annually (Jacobsen
1999). Analysis of data of 14 reservoirs in Switzerland showed that this percentage is only
2
Reservoir Sedimentation
about 0.2 % in Alpine reservoirs. The lower filling rates are results of geologic characteristics
of these basins at high altitude.
Nevertheless, sedimentation is also a subject of major importance in Alpine reservoirs and is –
in big reservoirs - mainly related to the phenomenon of sediment transport by the means of
turbidity currents. The sediment discharge of the inflowing rivers is usually significant during
flood events. Turbidity currents are often the governing process in reservoir sedimentation by
transporting fine materials in high concentrations and following over long distances the
reservoir bottom along the thalweg through the impoundment down to the deepest point in the
lake normally near the dam. At the dam the eroded and transported sediments settle down and
can cover the bottom outlet adversely affecting the operation of the power intake.
The turbidity currents belong to the family of sediment gravity currents. These are flows of
water laden with sediment that move downslope in otherwise still waters like oceans, lakes
and reservoirs. Their driving force is gained from the suspended matter (fine solid material),
which renders the flowing turbid water heavier than the clear water above. If the difference in
density between the lake water and inflowing water is high enough, it may cause the flow to
plunge and turbidity current can be induced. Turbidity currents are encountered in fluvial
hydraulics, most prominently if a sediment-laden discharge enters a reservoir, where, during
the passage, it may unload or even resuspend granular material.
Thus sediment deposition in reservoirs not only reduces storage capacity, but also increases
the risks of blockage of intake structures and sediment entrainment in waterway systems and
hydropower schemes. Sediments removed by flushing have a detrimental impact on the
downstream river. The design of a sustainable reservoir requires the accurate prediction of
sediment transport, erosion and deposition. For existing reservoirs, deeper knowledge of the
phenomenon involved is needed to better understand and solve sedimentation problems for
improvement of reservoir operation.
Accepted practice has been to design and operate reservoirs to fill with sediment, generating
benefits from remaining storage over a finite period of time. The consequences of
sedimentation and project abandonment are “left” to the future. These consequences can be
summarized as: sediments reaching intakes and greatly accelerating abrasion of hydraulic
machinery, decreasing their efficiency and increasing maintenance costs; blockage of intake
and bottom outlet structures or damage to gates that are not designed for sediment passage
(Boillat and Delley, 1992), etc. Considering reaches downstream of the dams, one important
problem can be the augment of the erosions risk in the river since the sediment equilibrium
was affected. This ‘future’ has already arrived for many existing reservoirs and most others
will eventually experience a similar fate, thereby imposing substantial costs on society
(Palmieri et al., 2001).
Normally, the measures against reservoir sedimentation can be divided in three groups, after
Schleiss and Oehy, 2002: measures in the catchment area, in the reservoir and at the dam.
These measures are schematically presented in Figure 1.1-2.
The Problem of Reservoir Sedimentation
3

Measures against
reservoi
r
sedimentation
In the catchment area
In the reservoir
At the dam in the reservoir
• Soil conservation
• Settling basins
• Slope and bank
protection
• Bypassing structures
• Off-stream storage
reservoi
r


Dredging
• Dead storage
• Flushing
• Hydrosuction, air lift

Sluicing
• Turbidity current venting
• Turbining suspended
sediments
• Dam heightening
• Heightening of intake and
bottom outlet structures
Figure 1.1-2: Inventory of known measures against reservoir sedimentation (Schleiss and Oehy, 2002)
For Alpine reservoirs, sedimentation phenomenon due to turbidity currents represents a large
part of the problem and has been extensively studied in the recent years.
Sediment deposition in reservoirs causes mainly loss of water storage capacity (Graf, 1984,
Fan and Morris, 1992), the risk of blockage of intake structures as well as sediment
entrainment in hydropower schemes (Boillat et al., 1994, Schleiss et al., 1996, De Cesare,
1998). Finally the sediment ends to some extent in the downstream river during flushing
(Rambaud et al., 1988). The planning and design of a reservoir requires the accurate
prediction of sediment transport, erosion and deposition in the reservoir. For existing artificial
lakes, more and wider knowledge is still needed to better understand and solve the
sedimentation problem, and hence improve reservoir operation.
4
Reservoir Sedimentation

Run-of-River Installations
5
2 Run-of-River Installations
2.1 Reservoir and Sedimentation
Run-of-river installations can be classified into various basic types according to their flow-
through characteristics (
Figure 2.1-1
).

Figure 2.1-1: Flow and sedimentation patterns (Westrich, 1988)
6
Reservoir Sedimentation
Developed bodies of running water with relatively regular flow cross-sections generally show
a unidimensional sedimentation profile. By contrast, backwater reaches with complex cross-
sections (channels with groyne fields, river beds with flooded washlands) show a clear
differentiation with respect to the transport and sedimentation processes across the main flow
direction. The parameters determining reservoir sedimentation are sediment delivery and
discharge.
Starting from the end of the upstream reach, flow velocity and bottom shear stress decrease as
the cross section of discharge increases, and the solids begin to settle. Sedimentation within
the reservoir is structured according to grain size. The coarser bedload material (gravel) is
deposited at the end of the upstream reach, developing a sediment tongue over time, which
gradually moves towards the barrage. The finer, suspended particles, such as fine sand and
silt, are swept far into the reservoir and down to the dam structure, and diffuse transverse
transport carries them at low flow velocities to border zones and peripheral shallow water
zones, where they are allowed to settle. Only the very fine suspended particles are discharged
from the reservoir. Fine material can be deposited wherever the flow velocity falls below the
respective limit shear-stress velocity for the sedimentation range. Natural remobilisation of
fine sediment will take place as soon as the shear-stress building up during a flood exceeds
the appropriate velocity for the size of sediment.
A factor of particular importance is cohesive inorganic sediment. Deposition is followed by a
major consolidation phase, after which the erosion shear-stress may have increased by more
than a factor of 10. Substances of man-made origin may intensify this effect. Such
sedimentation zones will rarely be washed out even by major floods.

2.2 The Morphology of Reservoir Sedimentation - the
Phenomenon
2.2.1 Influencing factors
The sedimentation process in a reservoir is governed by a wide variety of highly complex
factors. Sediment influx, reservoir geometry and flow are considered as the determining
factors. These in turn depend on a number of parameters and conditions such as catchment,
reservoir management and climate. In addition, the upstream river morphology is important;
in particular, other reservoirs, dams, retention basins or lakes may substantially reduce
sediment delivery to the reservoir under study. Other man-made measures, such as sewers,
may also have a direct effect on the sedimentation process.

2.2.2 The Mechanics of Sedimentation
Flow conditions change substantially as the water travels from the end of the upstream reach
in the direction of the dam. As the cross-section increases, the bottom shear-stress as a factor
governing sediment transport decreases, and the solids start settling. Bedload is deposited near
the upstream end of the reservoir, while the suspended particles settle further downstream.
Another important factor influencing the sedimentation process, in particular its morphology,
is reservoir geometry. In a long, narrow backwater reach (above a river barrage) and under
idealised constant conditions (water level, inflow, sediment influx), the bedload deposit
progresses relatively evenly from the upstream end of the reservoir in the direction of the
dam. By contrast, the sedimentation process is irregular in reservoirs of major width. Even
small bedload bars, sudden widenings of the channel (such as lakes), etc. may generate
unexpected sedimentation conditions. Other factors of some importance are the changes over
time in inflow, water level and sediment supply, resulting in a constant alternation between
7
Run-of-River Installations
sedimentation, a state of equilibrium and erosion. Stream bends intensify the difference in
transport capacity of the current between the inner and the outer bends, the sedimentation
tendency being higher on the inner side.
An experimental study on a sand trap some 50,000 m³ in capacity (Mertens, 1987) showed a
narrow main current to flow first along the left bank of the basin, while by far the largest
portion of the sand trap volume was taken up by an extensive recirculation zone (
Figure 2.2-1

a). A small bedload bar in the intake area, however, altered the flow pattern substantially
(
Figure 2.2-1
b). Flow over the bar was uniform and fanned out over the whole basin. Later, a
wide main current developed (
Figure 2.2-1
c and
Figure 2.2-1
d), which slowly swung
between the two banks, alternating between deposition and erosion of sediment.
The hydraulic and sedimentological processes are even more complex and irregular in wide
reservoirs (lakes) with rapidly widening inflow cross-sections. The bedload is deposited in
delta-shaped formations (Mangeldorf, Scheuermann, Weiss, 1990) which, at a later stage,
become traversed by several distributary streams transporting the settling particles to the
channel edges. These distributaries keep migrating, causing the delta to spread approximately
radially from the inlet (Mertens, 1987).
In rivers developed by a series of dams, the location of the reservoir under study is another
influencing factor, and so is the commissioning year, which is important for the sedimentation
condition (initial sedimentation followed by erosion and then sedimentation in the
downstream reservoirs, state of equilibrium).
In practice, sedimentation processes are also influenced by changes in time of inflow, water
level (in the basin) and sediment supply, which results in an alternation of sedimentation, state
of equilibrium and erosion of already deposited solids. While the transport behaviour of non-
cohesive sediment obeys the limit curves of the diagram by Shields (1936) or others,
considerable changes in the critical erosion and sedimentation shear stresses may result for
inorganic fine material with aggregation and flocculation properties. Thus, the sedimentation
shear stress for fine sediment with a high mineral percentage in the clay fraction may be 100
times higher than suggested by the Hjulström criterion (1935) or even above the erosion limit.
After settling, such fine sediment undergoes a major consolidation phase during which the
erosion shear stress may increase by more than a factor of 10. In consequence, in reservoirs
with a high inflow of inorganic and organic fine sediment, once this is consolidated, the
sediment may fail to be carried along even by floods.


Figure 2.2-1: Flow conditions and sedimentation process in a river sand trap (Mertens, 1987)

8
Reservoir Sedimentation
2.2.3 Dead-Water Zones
In running waters, the presence of water-retaining structures, groynes and training structures
usually involves the formation of zones where flow discharge is very low. Such dead-water
zones include groyne fields, dead river branches and harbours. They often develop into
biotopes of ecological importance, which in the long run ought to be protected from
detrimental sedimentation. Depending on size and configuration, groyne fields and harbours
act as sediment traps where major amounts of sediment (usually suspended solids)
accumulate.
Where backwater reaches above barrages have flooded washlands, the coarse bedload
material is deposited within the river channel, while the finer suspended solids settle on the
washlands. Lateral transport of suspended sediment is greatly influenced by water exchange
between main channel and washland. A cross-current from the main channel to the washland
(e.g. washland widening) favours sedimentation on the washland, while a cross-current in the
opposite direction (e.g. narrowing washland) reduces sediment deposition. Washlands with
shallow water will tend to fill up with sediment over the years. In conjunction with
vegetation, such areas may even end up again as dry land.
Fine sediment may be resuspended during major flood flows. Convective and diffuse water
exchange between the dead-water zone and the river is responsible for suspended sediment
delivery, which is mainly a function of the size of the exchange area, flow velocity and
suspended load concentration in the river as well as inner circulation determined by the shape
and size of the swirl area. Of the suspended material supplied by the main stream, the coarser
fraction is deposited, while the very fine particles are kept in suspension by turbulent
movement, their residence in the dead-water zones thus being temporary.
The relationship between the shape and size of dead-water zones of simple configuration,
such as groyne fields, on the one hand, and water exchange on the other hand is as follows:
the smaller the exchange coefficient, ε, (dispersion coefficient), the larger the half-life time,
t
1/2
, within which water exchange takes place. Unsteady water-level fluctuations (ship traffic,
flood waves, tide), density flows (mainly in tidal waters) and turbulence-generating factors
(ship propulsion) may substantially increase the delivery of suspended sediment to such dead-
water zones.
Ship traffic not only whirls up sediment and keeps it longer in suspension, but also produces
substantial wave-induced cross-currents, which increase the lateral transport of suspended
sediment and bedload towards the dead-water zones. Studies conducted on the River Main
have demonstrated the influence of navigation on the transverse transport of suspended
material towards near-bank water zones. Water exchange and, hence, suspended load delivery
increases considerably under the influence of water-level fluctuations (Westrich, 1988).

Run-of-River Installations
9
2.3 Case study
2.3.1 Experience gathered in Reservoir Flushing on the River Drau
in Austria
2.3.1.1 Plant Description
The River Drau between the Austrian border with Slovenia and almost up to the town of
Spittal forms a practically continuous series of 10 power plants. Its total length is about
134 km with a total head of approximately 176 m. The individual heads all exceed 20 m at the
larger dams. The damming stages have not been constructed by proceeding in one direction,
but as dictated by economy.
The damming stages with the high heads have usually wide reservoirs and are equipped with
bay power stations, while those with the lower heads tend to have canal-shaped backwater
reaches and pier power stations.
The series of power stations is operated by Austrian Hydro Power (formerly Österreichische
Draukraftwerke AG).

2.3.1.2 Sediment Management
It was decided at the outset that no coarse material – that is, bedload – should enter the
reservoirs. This is mainly achieved by dredging in bedload traps and sedimentation basins
where and when necessary.
The sediment management strategies selected depend on the size of the respective reservoir.
No de-sedimentation measures are considered necessary in minor reservoirs. Sedimentation is
allowed to progress until the basin fills up. Floods will wash out the accumulated material
without the need for previous water-level drawdown, so that even large floods may be routed
through the reservoir without risking dam overtopping.
By contrast, silting in the larger reservoirs is not allowed to exceed a certain level. When this
is reached, the reservoirs must be flushed with the help of partial drawdown during discharges
corresponding to not less than about 0.7 x HQ1. In addition, weir operation rules dictate what
water level must be maintained, depending on the inflow. Water-level drawdown starts at
approximately HQ1 and varies according to the degree of sedimentation and the respective
reservoir.
These strategies were already included in the application for the Water Right Permit and
received approval by the Austrian Supreme Water Right Authority.

2.3.1.3 Reservoir Flushing
2.3.1.3.1 Experiences
Meanwhile the reservoirs have advanced in years, and this strategy which, as mentioned
above, was made part of the design, has proved efficient. The uppermost of the wide
reservoirs, Rosegg, has been flushed consistently for 20 years. In contrast to the 5.50 m
drawdown provided in the Water Right Permit, the water level has been lowered by not more
than 2.50 m from 1984 on. Hydraulic model analyses and extensive monitoring after flushing
events have proved so far the smaller drawdown to be sufficient.
In connection to reservoir Rosegg, partial water-level drawdown has also been practised at the
downstream Feistritz and Edling developments over the past few years, but so far by no more
than 1.50 m (Edling) and 2.50 m (Feistritz).
10
Reservoir Sedimentation
The drawdown process is not started until the installed hydrological forecast model predicts
an inflow larger than 700 m³/s. Water-level drawdown at the downstream reservoirs is
controlled by their respective hydraulically determined drawdown plans and weir operation
rules.

2.3.1.3.2 Discussion of Flushing Results as obtained at the Rosegg Dam
Rosegg is characterised by a number of detail problems resulting from its specific features.
Rosegg is a derivation-type power station, (located at the end of a river derivation where it
discharges into the main river). That means that any surplus water runs through the original
riverbed. Vegetation that has developed there "combs out" part of the passing suspended
solids, especially during reservoir flushing. More than 1/2 Mio m
3
of material has thus been
dredged and hauled from this river section since the plant was commissioned in 1973.
Furthermore, the design provided for a hydraulic constriction in reservoir width by means of
dyke structures.
The meander cutoff provided to isolate a river bend upstream of the plant had to be improved
in terms of sediment hydraulics after about 15 years of service.
A large tributary, the River Gail, joins the Drau at the upstream end of the backwater reach.
The Gail occasionally transports large amounts of sediment. Normally, this material is
dredged out in a sediment trap directly above the junction of the two rivers. During major
floods, however, some of the sediment finds its way to the Drau, and this must then be
removed by means of a pontoon dredger.
Meander cutoff
Weir
Derivation
Headrace channel
Rosegg Power Station
Villach Power Station
Tailwater
Erosion
Meander cutoff
Weir
Derivation
Headrace channel
Rosegg Power Station
Villach Power Station
Tailwater
Erosion

Figure 2.3-1: Location map of the Rosegg power plant
The erosion problems in the tailwater of the Villach power station, within the town of Villach,
will not be discussed in greater detail here. Suffice it to mention that even minor floods have
caused severe riverbed degradation, which has called for extensive stabilisation measures.
By far the largest proportion of accumulated sediment is removed by reservoir flushing.
Since 1991, with the use of the above mentioned partial drawdown processes which transport
the sediments through the reservoir, an additional siltation of the Rosegg reservoir was
avoided. These partial drawdown events have taken between a few hours and a few days,
depending on the nature of the respective hydrological event. To assure the absolute safety
11
Run-of-River Installations
against floods according to the city of Villach, in 2006 and 2007 substantial dredging
activities (800,000 m
3
) have to be performed.
To perform the desilting process in the future by means of flushing, substantial approving
processes are necessesary in a medium term.

Figure 2.3-2: Duration and erosion volume for different reservoir flushings
The volume changes over the length of the reservoir for four events are shown below:

Figure 2.3-3: Effects of reservoir flushing in autumn 1993, differences in area [m
2
]
before/ after flushing
12
Reservoir Sedimentation
The accretions immediately upstream of the weir are certainly due to re-sedimentation after
erosion during the desiltation according to the drawdown process (flushing). The above graph
clearly suggests that this particular desiltation process was stopped relatively early. The
whirled-up sediment accumulated and settled near the weir during reservoir refilling.
In general, it may be assumed that the greater part of the fine sediment eroded during flushing
tends to be carried along near the bottom. Measurements of suspended load concentration
over the whole series of dams has shown that the amounts delivered to the chain are not much
different from those discharged from the flushed reservoir and further downstream.

Figure 2.3-4: Effects of reservoir flushing in autumn 1998, [m
2
] before/ after flushing
The desiltation processes represented above in autumn 1998 show degradation extending over
the whole reservoir length up to the junction with the River Gail. As demonstrated by the
graph, this is a result of the long flushing period and the relatively high peak discharge.
By way of example, the following graphs demonstrate the changes in cross section profile
resulting from reservoir flushing.

Figure 2.3-5: Cross section changes in the Rosegg reservoir
The change in cross section shown in Profile 2 is mainly a result of landfill measures having
moved the channel line to the left.
13
Run-of-River Installations

Figure 2.3-6: Effects of reservoir flushing in Nov. 2004, differences in area [m
2
] before/ after flushing
By way of example, the following graphs demonstrate the changes in cross section profile
resulting from reservoir flushing.

Figure 2.3-7: Cross section changes
14
Reservoir Sedimentation

Figure 2.3-8: Effects of reservoir flushing in Oct. 2005, differences in area [m
2
] before/ after flushing
By way of example, the following graphs demonstrate the changes in cross section profile
resulting from reservoir flushing.

Figure 2.3-9: Cross section changes, Profiles 2, 7 and 19/1
On the whole, the consistent desiltation strategy has ensured de-sedimentation. Failure to
draw down the water surface to a lower level even during a short flood event has proved to
result in substantial sedimentation. Consequently, the intention is to provide for adequate
drawdown even during short events, so as to ensure the passage of incoming fine sediment.

2.3.1.4 Summary
Consistent de-sedimentation management has been practised according to the de-
sedimentation plan on the River Drau in Austria since 1991. This includes water-level
drawdown of the Rosegg reservoir by 2.50 m, resulting in adequate suspended loads while
achieving a sufficiently large washing-out effect in downstream reservoirs. The riverbed is
thus maintained at a state of equilibrium within the permitted range of water-level variation,
so as to comply with requirements of regulatory authorities.
The water surface of the next downstream reservoirs Feistritz and Edling are also partial
drawn down. Likewise, the progress of sedimentation will require these partial drawdowns to
be extended to the next two reservoirs downstream, Ferlach-Maria Rain and Annabrücke.
Deep, seasonal storage reservoir
15
3 Deep, seasonal storage reservoir
3.1 Turbidity current - main cause of sediment transport in
deep reservoirs
If water of higher density (ρ
w
+Δρ) flows over a bottom slope into stagnant water, such as
oceans, lakes and reservoirs with a smaller density ρ
w
, the inflow pushes the ambient water
until it reaches a balance of forces, at which point the denser water sinks beneath the ambient
water. This point is referred to as “plunge point” (see
Figure 3.1-1
). The parameters
influencing the plunging behavior are slope, geometry, inflow and density distribution. The
plunging phenomenon has been observed in the field and in the laboratory. The plunge
phenomenon can be defined as the transitional flow from a homogeneous open channel flow
to stratified, entraining underflow, which is commonly referred to as “density current”, or
more specifically “turbidity current”.
Clear stagnant water
Plunge point
Sediment-laden inflow
Turbidity
current
Head
Erodibl
e
deposits
sedim
ent
Clear stagnant water
Plunge point
Sediment-laden inflow
Turbidity
current
Head
Erodibl
e
deposits
sedim
ent

Figure 3.1-1: Sediment-laden river entering a reservoir - plunging flow phenomenon and turbidity
current formation.
Turbidity currents are driven by the density difference in the turbidity current and the ambient
water. The difference in density is caused by suspended fine solid material, which renders the
flowing turbid water heavier than the ambient water above. To travel long distances, the
velocity of a turbidity current must be sufficient to generate the turbulence required to
maintain its sediment load in suspension, thereby maintaining the density difference between
the gravity-induced current and the ambient water. During passage the turbidity current may
unload or resuspend fine granular material.
The sediment exchange at the bottom of the reservoir can be described by a flux between the
bed and the current, separated into a sediment entrainment and a sediment deposition term
evaluated at a reference height slightly above the real bed level (Parker et al., 1986, De
Cesare, 1998). Suspended sediment is constantly falling out of the current at a rate given by
the sediment fall velocity and the mean volumetric concentration of suspended particles near
the bed.
The motion of the turbidity current exerts a stress on the bed and is capable of entraining
sediments from the bed into suspension. If the entrainment rate is less than the depositional
rate, then the turbidity current experiences a net loss of granular material, and the sediment
concentration in the current decreases. Consequently, the driving force acting on the current
decreases, causing the current to decelerate and eventually to vanish.
On the other hand, a higher flow velocity of the turbidity current can produce a rate of
sediment entrainment from the bed that is greater than the depositional rate. The density of the
current increases, and the turbidity current accelerates. As bed stress increases further and
more sediment is entrained, a self-reinforcing cycle is created which allows the development
16
Reservoir Sedimentation
of a self-sustaining turbidity current that can gradually reach high speeds. Only the
availability of bed sediment for entrainment, the reservoir geometry or eventually the
damping of turbulence at high concentrations will limit the growth of such a gravity-driven
flow.
When a turbidity current reaches a barrier such as a dam, the forward velocity is converted
back into potential energy as the current rises up against the face of the dam, and subsequently
falls back down, initiating the formation of a muddy layer. The top surface of this highly
sediment charged layer will extend along a nearly horizontal profile upstream from the dam.
The volume of the muddy layer will increase and the interface will rise, as long as turbid
inflow exceeds losses by the upward seepage of clear water from within the muddy layer due
to sedimentation and compaction of solids.
Sedimentation within the highly sediment charged layer causes the fluid density to increase at
the bottom of the layer. Subsequent turbid inflows will spread across the top of the higher
density fluid in the sediment charged layer rather than mix. The inflowing density current is
thus converted from underflow to interflow in areas affected by muddy lake accumulation.
With time the muddy lake creates a horizontal deposit of fine-grained material extending
upstream from the dam, which typically consists of material significantly finer than the delta
deposits.

3.1.1 Propagation of sediments to dam
Depending upon the sediment supply from the watershed and flow intensity in terms of
velocity and turbulence, river flows usually carry sediment particles within a wide range of
sizes. Bed load consists on the coarser components which are transported near the river bed.
The suspended sediments are generated by superficial erosion as well as by smashing and
abrasion of coarser components. In the events of floods the fraction of sediments smaller than
sand reaches 80 to 90 % of the total sediment carried by the river. When a river flows into a
reservoir, the coarser particles deposit gradually and form a delta in the headwater area of the
reservoir that extends further into the reservoir as deposition continues. Finer particles, being
suspended, flow through the delta stream and pass the lip point of the delta, first entering a
quasi-homogeneous (non-stratified) flow region and subsequently being deposited along the
path due to a decrease in flow velocity caused by the increased cross-sectional area (
Figure
3.1-3
). The quasi-homogeneous flow is shorter in cases of smaller discharge and/or higher
sediment concentration. On the contrary, the region of quasi-homogeneous flow is longer in
cases of larger discharge and/or lower sediment concentration. As a consequence close to the
dam the deposition of the finest particles takes place.
Deep, seasonal storage reservoir
17

Figure 3.1-2: Areas affected by sedimentation in the surroundings of a reservoir
Due to bigger erosive forces and transport capacities with increasing discharge, the sediment
content in a river gets especially high during floods. The flow with high concentration on
sediments is carried into the artificial lake by means of the turbidity currents. This
phenomenon takes place several times a year. The sediment-laden river, driven by a density
difference, plunges into the reservoir, where it follows the thalweg of the lake to the deepest
area, which is normally close to the dam. It can move for several kilometres (
Figure 3.1-2
).

Geschwindigkeit (m/s)
Mittlere Geschwindigkeit
Max. Geschwindigkeit
Geschwindigkeit (m/s)
Mittlere Geschwindigkeit
Max. Geschwindigkeit
Average velocity
Max. velocity
Flow Velocity

Geschwindigkeit (m/s)
Mittlere Geschwindigkeit
Max. Geschwindigkeit
Geschwindigkeit (m/s)
Mittlere Geschwindigkeit
Max. Geschwindigkeit
Average velocity
Max. velocity
Flow Velocity

Figure 3.1-3: Maximal transportable grain sizes dependent on the flow velocity of the turbidity current
according to Fan (1986)

18
Reservoir Sedimentation
3.2 Theoretical Background
Turbidity currents are flows driven by density differences caused by suspended fine solid
material in an ambient fluid. They can appear as different forms, depending on the density of
the mixed (water/sediments) fluid. If the density of the mixed fluid (ρ
m
) is superior to the
density of the ambient fluid (ρ
w
), the turbidity current will be formed on the bottom. If the
density of the mixed fluid is less than the ambient fluid, the current will be propagated at the
surface. Sometimes, the reservoir presents a stratification of density, and the mixed fluid can
flow like as an intrusion. Generally, it is the first type of current which can transport the
largest quantities of sediments downstream.
ρ
w
ρ
w
ρ
m
ρ
w1
ρ
m
ρ
m
a) ρ
m
> ρ
w
b) ρ
m
< ρ
w
c) ρ
w1

m

w2
ρ
w2
ρ
w
ρ
w
ρ
m
ρ
w1
ρ
m
ρ
m
a) ρ
m
> ρ
w
b) ρ
m
< ρ
w
c) ρ
w1

m

w2
ρ
w2

Figure 3.2-1: Types of turbidity currents
Turbidity currents consist of two regions of a steady flow, one region dominated by the
momentum and another dominated by the buoyancy. For the currents at the bottom, at a
position determined by the balance between the momentum of the inflow and the baroclinic
pressure resulting from the density difference between the inflow and the receiving water, the
inflow plunges below the surface.
The physical parameters that control the plunging depend on inflow conditions as well as the
geometrical characteristics of the reservoir. For a channel with a constant width and lateral
slopes of the trapezoidal section (m) between 0.2 and 0.8, Savage and Brimberg (1975)
propose a depth of plunging (h
p
) given by

3
1
0
2
0
3
1
2
p
p
g
q
F
1
h
















=
ε
Eq. 3.2-1
where q
0
is the initial inflow per unit width, ε
0
is a relative density difference between inflow
(mixed fluid) and ambient fluid, g is the gravitational acceleration and F
p

is the densimetric
Froude number at the plunge point.
The density of the mix fluid can be expressed by

C)(
wswm
ρ
ρ
ρ
ρ
−=
Eq. 3.2-2
where C is the volumetric concentration.
The term F
p
, after Savage and Brimberg (1975) is empirically defined by

478.0
d
p
C
S
m1
05.2
F








+
=
Eq. 3.2-3
In this formula S is the bottom slope and C
d
a total friction coefficient of the underflow
(0.01 < C
d
< 0.09).
Deep, seasonal storage reservoir
19
After plunging, turbidity currents normally can be separated in 2 parts. The front or head,
which has as driving force essentially the pressure gradient due to the density difference
between the front and the ambient fluid ahead of it and the body, with the driving force the
gravitational force of the heavier fluid (mixed water/sediment). The flow in the front is
unsteady while for the body, the flow can be considered as being steady.
In turbidity currents, the quantity of the suspended sediment is not conserved and it is free to
exchange with the bed sediment by means of bed erosion and deposition. It can cause self-
acceleration of the turbidity by entrainment of bed sediment.
Figure 3.2-2
illustrates the
typical turbidity current.

Figure 3.2-2: Turbidity current flowing at the bottom
The current moves in the longitudinal direction x, over a bottom slope S, and under a deep
layer (H>>h) of an ambient stagnant fluid (U
a

o) with a density ρ
w
. This density is less than
the density of the turbidity current, ρ
w

m
.
Altinakar et al. (1996) compared the body of the current to a wall jet with 2 regions (
Figure
3.2-3
), the wall and the jet region. The height h
max
attended where the velocity is maximum,
u=U
max
, is the separation of these regions.
Water entrainment
Jet region
Maximum velocity
Wall region
Erosion / Deposition
z = h
max
z/h
max

Figure 3.2-3: Schematic dimensionless velocity profile for turbidity currents
(Graf and Altinakar, 1996)
20
Reservoir Sedimentation
• Wall region : z<h
max
, turbulence is created at the wall and entrainment of sediments can
take place;
• In the free region, z>h
max
, turbulence is created by friction and by entrainment of the
ambient fluid.
Experimentally they found that

3.0
h
h
max
=

3.1
U
U
axm
=

3.1
h
h
t
=
Eq. 3.2-4
In these expressions, h is the average height of the current and h
t
is the height of the current at
which u ≡ 0.
A simple hydraulic approach using the Chezy-type relationship between the front velocity
(U
f
) and height of the front (H
f
) can be described after Turner (1973) for a large range of
slopes and roughness as

f1f
H'glU =

Eq. 3.2-5
where l
1
is a constant that can vary between 0.63 (Altinakar et al.,1990) and 0.75
(Middleton,1966 and Turner,1979) , being 0.63 more appropriate for small slopes and g’ is a
reduced gravitational acceleration expressed as








⎛ −
=
w
wm
g'g
ρ
ρρ

Eq. 3.2-6
After Britter and Linden (1980), the front velocity can also be expressed by

3
1
02f
BlU =

Eq. 3.2-7
where l
2
is a constant depending on the bottom slope and the Reynolds number (Re). For
slopes less than 5%, Altinakar et al. (1990) found values varying linearly between 0.7 and 1.0.
Choi and Garcia (1995) proposed l
2
=1, based on numerical experiments and laboratory data of
Altinakar et al. (1990).
B
B
0
is the initial buoyancy flux per unit width and can be defined as
Eq. 3.2-8
000000
''qgUhgB ==
with g
0
’ as initial reduced gravitational acceleration, h
0
is the initial depth and U
0
is the initial
velocity.
For the description in a simple model, the body of a turbidity current can be considered two-
dimensional and plane, and the flow turbulent and incompressible. The velocity of the body is
normally greater than the velocity of the head. This difference grows as the angle of the
bottom slope increases and in order to maintain the flow continuity, the height of front will
always be larger than the height of the body. The height of the front increases with
entrainment of ambient fluid depending on the distance covered.
After Parker et al. (1986), the description of the flow in the turbidity currents with
entrainment can be based on a conventional three-equation model, conservation of mean
Deep, seasonal storage reservoir
21
momentum, fluid mass and sediment. This model is an extension of the unidirectional steady
gravidity flows developed by Ellison and Turner (1959).
• conservation of the mean momentum

2
b*s
2
s
2
uhSgRC)hC(
dx
d
Rg
2
1
)hU(
dx
d
)Uh(
dt
d
−+−=+

Eq. 3.2-9
where t is time, R is the submerged specific density, (ρ
s
−ρ
w
)-1, and u
*b
is the shear
velocity, that can be defined as ,

ku
ksb
α
=
Eq. 3.2-10
with α
k
as a dimensionless constant and k the mean kinetic energy of turbulence.
• The conservation of fluid mass (mixed water/sediment)

h
W)Uh(
dx
d
dt
dh
=+
Eq. 3.2-11
where W
h
is the entrainment velocity of the ambient fluid into the current often assumed
to be proportional to the velocity of the turbidity current, U.
Eq. 3.2-12
UEW
Wh
=
with the constant of proportionality, E
w
, being the entrainment coefficient of ambient
fluid and depending on the Richardson number, Ri, that represents the ratio of inertia to
reduced gravity forces.

2
d
2
Fr
1
U
cos'g
Ri ==
α

Eq. 3.2-13
with Fr
d
as a densimetric Froude number. The parameter E
w
is given after empirical
relation as
Eq. 3.2-14
(
5.0
4.2
7181075.0

+= RiE
W
)


Conservation of mass of sediment

DEss
SS)UhC(
dx
d
)hC(
dt
d
−=+

Eq. 3.2-15
where S
E
is the sediment erosion and S
D
is the sediment deposition.

The equation of continuity for the solid phase in a steady two-dimensional case approximates
the equation of diffusion of granular material.

2
s
2
s
s
ss
ss
z
c
z
c
z
)wc(
x
)uc(


+


=


+


ευ

Eq. 3.2-16
22
Reservoir Sedimentation
In this formula, the horizontal and vertical velocities are designed by u and w . The local
volumetric sediment concentration is designed by c

while
υ
ss
represents the settling velocity
and it can be calculated with different methods. For very fine particles the Stokes equation
(Graf, 1971) can be used

2
w
ws
ss
d
18
1
g
νρ
ρρ
υ

=

Eq. 3.2-17
with d as the sediment particle diameter and
ν
the kinematic viscosity of water.
The
ε
s
is the diffusion term and can be expressed by,

)'w'c(
z
z
c
ss
2
s
2
s


−≅


ε

Eq. 3.2-18
The terms w’ and c’ represents the fluctuations of the vertical velocity and volumetric
sediment concentration. The term
w'c'
s
expresses the vertical Reynolds flux of sediments.
The sediment’s mass exchanges at the bottom can be found integrating vertically the
expression
)'w'c(
ss
over the entire height (h
t
) and analyzed close to the bottom at
z = b = 0.05 h
t
. Thus,

sssbzssE
E|)'w'c(S υ==
=

Eq. 3.2-19
This represents the erosion of sediments per unit area. E
s
is the entrainment coefficient of
sediments from the bed and can be found after an empirical relation,

5
u
7
5
u
7
s
Z10*33.41
Z10*3.1
E


+
=

Eq. 3.2-20
Where Z
u
is a dimensionless parameter defined by

( ) ( )





<

==
5.3ReRe586.0
5.3ReRe
Re,Re
23.1
6.0
*
pp
pp
pp
ss
b
u
fwithf
u
Z
υ
Eq. 3.2-21
The term Re
p
represents the particle Reynolds number and is based on the characteristics of
the characteristic sediment size, D
s
, and the kinematic viscosity of water.

υ
ss
p
DRgD
Re =

Eq. 3.2-22
For the sediment deposition term, expressed by deposition of sediments per unit area on the
bed,

b
sssbzsssD
c|cS
υ
υ
−=−=
=

Eq. 3.2-23
where c
sb
is the local volumetric sediment concentration at z=b.
The numerical solution for this three-equation model under certain initial conditions leads to
high acceleration of the current and high consumation of the turbulent energy, more than
Deep, seasonal storage reservoir
23
available. This led to introducing a fourth equation in which the turbulence production rate is
specifically dealt with through the conservation of the mean value of kinetic energy of
turbulence.
)cEs(Rgh
2
1
RgChE
2
1
hRgChEU
2
1
u)KUh(
dx
d
)Kh(
dt
d
sbsswsss0w
3
2
b*
−−−−∈−+=+
υυ

Eq. 3.2-24
E
S
where K is the mean kinetic energy of turbulence and ∈
0
represents the mean rate of turbulent
kinetic energy dissipation due to viscosity. Parker et al. (1986) proposed

h
K
2
3
0
β=∈
with
2
3
d
d
*d
w
a
C
C)
a
C
2Ri1(E
2
1






+−−

Eq. 3.2-25
where,
β
is a volume porosity and a is a reference level for equilibrium concentration. The
reference concentration c
sb
is calculated close to the bed at b = 0.05h
t
.

3.2.1 Flow over an obstacle
When a turbidity current meets an obstacle some of the denser fluid may flow over the
obstacle while a hydraulic jump or bore travels upstream as shown in
Figure 3.2-4
.
Sometimes, it can be partially or totally blocked by the obstacle. In this figure, there is a fluid
traveling with constant depth h
1
and velocity U
1
and a moving hydraulic jump propagating
upstream with the velocity U
j
as well as the conjugated depth h
2
with a smaller velocity U
2
.
The obstacle has a height h
m
.

Figure 3.2-4: Flow over an obstacle (Oehy, 2003)
The flow over an obstacle is characterized by two independent variables: the densimetric
Froude number of the approaching flow Fr
d1
and the ratio H
m
between the obstacle height h
m

and the height of the incoming flow h
1
.
If motion of the upper fluid can be ignored (U
a

0), the flow has many features in common
with free-surface flow and the shallow-water approximation is valid (Rottman et al., 1985).
Neglecting the water entrainment and the friction (E
w
= 0; u
*b
= 0), as well as the erosion and
deposition, the inflow can be described by
24
Reservoir Sedimentation

0)Uh(
dx
d
dt
dh
=+

Eq. 3.2-26
and

0)hh(
dx
d
'gU
dx
d
U
dt
dU
m
=+++
Eq. 3.2-27
Steady solutions to these equations are sought by setting the partial derivatives with respect to
time equal to zero. The equations then reduce to
Uh= U
1
h
1
Eq. 3.2-28
and

1
2
1
m
2
h
'g2
U
hh
'g2
U
+=++
Eq. 3.2-29
With U
1
and h
1
defined as the upstream values of U and h. It can be shown that for each value
of Fr
d1
, a limit H
m
=H
mc
exists at which the flow on the crest becomes critical. Applying the
Bernoulli’s equation between sections 1 and 3 (
Figure 3.2-4
), this limit can be found by

3
2
1d
2
1d
mc
Fr
2
3
2
Fr
1H −+=
Eq. 3.2-30
In order to define limits for the flows over an obstacle, equation Eq. 3.2-30 and the next two
can be combined, as shown is
Figure 3.2-5
.








⎛ +
−=
m
m
2
m
2
1d
H2
1H
)1H(Fr
Eq. 3.2-31
given by Baines (1995) and

3
2
1d
2
1d
2
3
2
1d
m
Fr
2
3
4
1
Fr16
1)1Fr8(
H −−
++
=
Eq. 3.2-32
if considered that U
j
=0 (Long, 1970).
Deep, seasonal storage reservoir
25

Figure 3.2-5: Flow regimes of shallow-layer flow over an obstacle (Oehy, 2003)
In
Figure 3.2-5
, steady flow is only possible in the left part of the curve from the equation Eq.
3.2-30. In the regions B and C, the flow is supercritical whereas in region A, subcritical. In
region D, the flow will be partially blocked when H
m
is increased above H
mc
.
In this case, an internal bore, i.e. a moving hydraulic jump is then formed and propagates
upstream. This internal bore dissipates energy in order to match this steady solution to the
upstream flow condition. For the calculation of the hydraulic jump, the Belanger’s equation
for single phase open-channel flow can be utilized, considering the densimetric Froude
number.







−+= 1Fr81
2
1
h
h
2
d1
1
2
Eq. 3.2-33
In the region E, the obstacle is high enough to block the approaching flow completely.

Figure 3.2-6: Flow over an obstacle in a laboratory flume (Oehy, 2003)

26
Reservoir Sedimentation
3.2.2 Flow through a screen
The interaction of a gravity current with a screen, after the transient effects have disappeared,
can be solved by a similar method to that used in the case for the obstacle, as shown in
Figure
3.2-7
and depends mainly on its porosity. Since the screen is permeable, the current does not
climb as high as in case of an obstacle (Rottman et al., 1985).

Figure 3.2-7: Flow through a screen (Oehy, 2003)
The theory of the flow through a screen presented in the following is presented in the thesis
document of Oehy (2003). In these analyses, the deposition was not considered.
In
Figure 3.2-7
, h
1
represents a constant upstream flow depth with a velocity U
1,
U
j
the
upstream propagation velocity of a hydraulic jump, h
2
and U
2
the depth and the velocity of the
current immediately upstream of the screen and h
3
and U
3
the conditions of the current, depth
and velocity downstream of the screen. Also in this figure, f represents the porosity of the
screen.
The ratio H
p
between down- and upstream depth of the screen can be determined as a function
of the porosity as shown in
Figure 3.2-8
.

Figure 3.2-8: Ratio of heights down- and upstream of the screen, H
p
=h
3
/h
2
, as function of the effective
porosity, f (Oehy, 2003)
Deep, seasonal storage reservoir
27
The relationship H
j
between h
2
and h
1
can be found as a function of the porosity of the screen
and Fr
d1
and is illustrated in
Figure 3.2-9
.

Figure 3.2-9: H
j
=h
2
/h
1
as function of the porosity f and the upstream Fr
d1
(Oehy, 2003)
The proportion
η
of the flow which continues over the screen is illustrated in
Figure 3.2-10

also as a function of the porosity of the screen and the upstream Fr
d1
.

Figure 3.2-10: Proportion of the incoming flow that is predicted to continue through the screen as a
function of the effective porosity f and the upstream Fr
d1
(Oehy, 2003)
This proportion can be calculated by

1hU
hU
1
22
=
η
Eq. 3.2-34
h
1
The force F exerted on the screen can easily be calculated by
28
Reservoir Sedimentation

2
2
2
h'g)f1)(
f1
f
2
1
(F
ρ

+
−=
Eq. 3.2-35

Figure 3.2-11: Flow through a screen in a laboratory flume (Oehy, 2003)

3.3 Experimental Studies
Turbidity currents are flows driven by density differences which are caused by fine solid
material suspended in the fluid. They belong to the family of sediment gravity currents. These
are flows of water laden with sediments that plunge in a mass of stagnant water such as an
ocean, a lake or a reservoir.
The turbidity currents have been observed by experimental studies.

3.3.1 Experimental set-up at EPFL-LCH (De Cesare, 1998)
Figure 3.3-1
shows the general schematic view of the flume, two adjacent mixing and storing
tanks and the measuring equipment. The flume used in this investigation is 8.4 m long, 1.5 m
wide and 65 cm deep. On the bottom a 6 m PVC plate is placed, which makes it possible to
vary the slope between 0 and 6%. Water and sediments are mixed in a separate tank (2 m
3
) by
a propeller-type mixer. This tank is connected to an upstream tank by a recirculation pump.
The turbid water returns to the mixing tank over a free surface weir that controls the water
level in the upstream tank. A gate with variable width and opening allows the controlled
release of the turbidity current into the flume.
Deep, seasonal storage reservoir
29
v = v ( t, y )
1
2
3
4
7
9
8
11
5
6
12
10

Figure 3.3-1: Schematic drawing of the experimental installation : 1) mixing tank, 2) upstream tank,
3) recirculation pump, 4) free surface weir, 5) inflow gate, 6) turbidity current,
7) experimental flume, 8) ultrasonic probes, 9) sharp crested weir, 10) flexible duct,
11) UVP instrument, 12) control computer
Before passing the open gate, water is constrained to flow through a set of 20 cm long
horizontal tubes, with increasing diameter from the base to the top of the opening. This set-up
is mainly used so that there is a one directional horizontal velocity field at the open gate.
Thus, when the turbidity current enters the experimental flume it is perfectly perpendicular.
The arrangement of more than 40 tubes gives an almost uniform velocity over the total height
of the gate. A reinforced PVC wall with a sharp crested weir that controls the water level in
the flume closes the downstream end of the flume.
All the experiments were conducted with fine homogenous clay as suspended matter. The
density of the sediments is ρ
s
= 2'740 kg/m
3
. The particle size distribution ranges from
d
10
= 0.002 mm to d
90
= 0.1 mm, with a mean particle diameter of d
50
= 0.02 mm. The
corresponding settling velocity calculated using Stokes law is v
ss
≈ 0.4 mm/s for the
representative particle size in calm water. In all experiments the clear water from the main
reservoir of the hydraulic laboratory was used as the ambient fluid. The water-sediment
mixtures were prepared in the mixing tank by adding the dry clay to the clear water. The
density of the water-sediment mixture, ρ
m
varied between 1'002 and 1'005 kg/m
3
, and the
mixture was considered to be a Newtonian Fluid.
The measuring section is located above the PVC bottom. This allows the monitoring of the
spreading turbidity current and its uniform flow over the total width of the flume. The
turbidity currents in the laboratory were monitored by means of ultrasound probes functioning
with the Doppler Method (UVP), thus giving a complete velocity profile along the ultrasound
beam in a very short time. The spatial resolution is smaller than a millimetre and it takes less
than one tenth of a second to measure and compute a whole profile (Takeda, 1995).
Measurements were made in the flume with three different configurations of the UVP
transducers. Vertical and frontal velocity profiles were taken, and 2D flow mapping close to
the bottom was performed. Because the UVP instrument allows only one transducer to be
connected at a time, the eight transducers used in the experimental set-up were connected to
the UVP via a multiplexing unit.
30
Reservoir Sedimentation
The beam directions and the penetration length were chosen in order to cover the interior of
the advancing turbidity current. As the model is symmetric, the profiles were taken on the axis
of symmetry and the flow-mapping region was situated on one side of the flume only. The
arrangement of the transducers are described as follows:
Vertical arrangement with 8 transducers looking with an angle of 60° against the main flow.
The measurements give the projected vertical velocity profiles over 2 m flow length from the
gate, the distance between the transducers was 25 cm.
α


Figure 3.3-2: Arrangement with 8 transducers looking with an angle of 60° against the main flow
Axial disposition with 8 transducers looking straight against the main flow. The
measurements give the horizontal velocity profiles over the whole 3 m flow length from the
gate; the distance between the transducers was 50 cm. The probes were installed 12 mm
above the bottom and were slightly set off laterally in order to reduce interference by
reflection of US signal from different transducers.


Figure 3.3-3: Axial disposition with 8 transducers looking straight against the main flow
Square grouping with 4 transducers on each side looking straight at and perpendicular to the
main flow in the spreading part just after the inflow gate. The side of the square plane where
the flow mapping took place was 62.5 cm long, the distance between transducers was 12.5 cm
close to the inflow gate, and 25 cm elsewhere. The transducers were installed with plastic
clamps 12 mm above the bottom on an aluminium frame.
Deep, seasonal storage reservoir
31


Figure 3.3-4: Square grouping with 4 transducers on each side looking straight at and perpendicular to
the main flow in the spreading part just after the inflow gate
The echo of the flowing turbidity current was strong enough to allow rapid measurements,
only 4 successive profiles were taken with each transducer, and the averaged profile is used as
velocity profile at one location. The temporal resolution was therefore less than ½ second per
profile. The duration to sweep all transducers was around 3 seconds and the cycle was
repeated every 5 seconds, thus giving a quasi-instantaneous velocity information every 5
seconds. The vertical profiles were obtained with 8 measurements per profile, thus doubling
cycle time; repetitions were made every 10 seconds.

3.3.2 Experimental results
Figure 3.3-5
shows a photograph of the spreading turbidity current 25 seconds after its initial
release. After its spreading to the total flume width, the current adjusted itself rapidly to a
uniform flow advancing steadily within the tank. When the current reached the downstream
end of the flume, the turbid water was evacuated by opening the bottom gate. During the total
duration of the experiment a constant turbid water flux was maintained through the inflow
gate.

Figure 3.3-5: Expanding turbidity current in the experimental flume 25 s after opening of the gate; the
current spreads out almost radially, large eddies developing at the current front; these
eddies give the characteristic surface appearance of turbidity currents like clouds,
125 mm x 125 mm grid on PVC bottom.
Turbidity currents can be separated into two characteristic regions of flow behaviour:
32
Reservoir Sedimentation


A bottom surface layer region, where turbulence is created from the bottom surface
roughness and where sediment entrainment may occur. The velocity distribution is
assumed to be logarithmic.


A jet region above the nose, where main turbulence comes from mixing with
surrounding clear water. The presumed velocity distribution is half the normal
(Gaussian) distribution.


The separation of these two behaviours is located at the point of maximum velocity,
often referred to as the nose of the turbidity current head.
The measured velocity profiles were compared with the distributions described above; the
result fits well as shown in
Figure 3.3-6
.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.0 0.2 0.4 0.6 0.8 1.0
z / h
m
j
et region Gaussian
distribution
bottom layer region
logarithmic distribution
z = h
m
u / U
m

Figure 3.3-6: Measured velocity values compared with theoretical vertical velocity distribution, u(z).
Values from run n° 2, 80 ms between two succeeding measurements.
The measured velocity profiles were compared with a theoretical distribution; the result
agrees well as shown in
Figure 3.3-6
: A bottom surface layer region, where the velocity
distribution is logarithmic, and a jet region, where the velocity is the half Normal (Gaussian)
distribution.

3.3.3 Experimental set-up at EPFL-LCH (Oehy, 2003)
3.3.3.1 Flume description
The experiments were carried out in a modified multi-purpose tilting flume with a total length
of 8.55 m. A second flume was inserted into the multi-purpose flume, and the sides of the
inner flume consisted of 8 mm thick transparent PVC plate to allow the observation of the
underflow. The inner experimental flume had a section of 27.2 cm in width and 90.0 cm in
depth. The flume can be tilted in a slope range S between 0% and +5% (2.86°).
The entire flume was divided into two sections of unequal length by a sliding gate. Adjacent
to the experimental flume, a mixing tank with a maximum capacity of 1,500 liters was used to
prepare and store the dense fluid mixture. The mixing tank was equipped with a propeller-
type mixer to create a homogeneous sediment mixture. After filling the experimental flume
Deep, seasonal storage reservoir
33
with tap water, the dense fluid (water-sediment mixture) was pumped up into the stilling box
through a 60 mm diameter flexible plastic pipe, passing a calibrated electromagnetic
flowmeter. The water-sediment mixture entered the stilling box through the bottom turning
horizontally behind the entrance gate into a slotted pipe. During preparation the sluice gate
was closed and the flow returned through a 30 mm orifice and a PVP pipe back into the
mixture tank. This circulation ensured a uniform mixture in the stilling tank and an accurate
regulation of the pump.
The shorter upstream section served as a stilling box and head tank for the mixture to be
released by opening the sliding gate to create the turbidity current. The movement of the
sliding gate was controlled by a lever mounted on the flume wall. Downstream of this gate a
box containing rectangular tubes of 1.5 cm diameter reduced the scale of the initial
turbulence.
The long downstream section simulates a reservoir in which a turbidity current is flowing. A
compartment at the end of the flume bottom trapped the turbidity current for withdrawal. The
rate of the outflow is controlled by a drainage valve where a rotameter allowed the regulation
of the water level. To prevent a contamination of the hydraulic system of the laboratory with
fine sediment, a filter was installed to retain the sediment particles.

3.3.3.2 Properties of the sediment materials
Various materials can be used to create turbidity currents in the laboratory. They include
different types of clay, bentonite, quartz or marble powder obtained by grinding. For the
present study a cohesionless, light weight homogeneous material was chosen, which had a
well known and relatively narrow particle size distribution, as well as a small settling
velocity. Specifically, the material was ground polymer with a density of 1135 kg/m
3
. The
grain size distribution of the sediment material was determined with a Mavern Mastersizer
(Laser refraction method). The material had a fairly narrow grain size range, but the
frequency histogram was skewed towards large grain sizes, which is typical for ground
particles. The particles have a median diameter, d
50
, of 90 μm. With a standard deviation, σ
g
,
of 1.6 the grain size distribution cannot be considered uniform, therefore, some grain sorting
effects may occur.
The selection of a characteristic grain size of the sediment is important for the calculation of
the representative settling velocity of the sediment, which greatly affects the sediment
transport.
Altinakar (1988) proposed that the grain size which has a settling velocity equal to the
average settling velocity of the sediment material to be the representative particle size for the
sediment material.
The settling velocity, v
ss
, can be calculated by means of different methods described in the
literature. Here, Stokes' law has been used, considering the small, almost spherical particles
and the low Reynolds number, Re=dv
ss
/ν < 0.2. It is expressed as

2
18
1
dgv
w
ws
ss
υρ
ρ
ρ

=
Eq. 3.3-1
To prepare the water-sediment mixtures the particles first had to be wetted in a small pot with
a mixer before they were added to the mixing tank. Due to the low sediment concentrations of
up to 5% by volume, no corrections for the concentration are made on the settling fall velocity
and the viscosity of the water. The mixture is considered to be a Newtonian fluid.

34
Reservoir Sedimentation
3.3.3.3 Measuring Instrumentation
3.3.3.3.1 Flow velocity measurements
An ultrasonic velocity profiler (UVP) was used to measure flow velocities. With this device
an instantaneous velocity profile in a liquid flow can be measured along the ultrasonic beam
axis.

3.3.3.3.2 Front Velocity and Time Measurements
The front velocity of the head of the turbidity current is determined from video recordings by
reading the time at which the head passes predetermined positions. In turbidity current
experiments, the interface between the current front and the ambient fluid could be easily
observed. Since the camera was only 4 m away from the flume, a parallax correction was
made; the actual positions of the head were measured from scales drawn on both the front and
back walls of the flume. Unfortunately, no measurements could be taken on the first 2 meters
because of the opaque wall of the multi-purpose flume. A digital chronometer was used to
link the internal clock of the digital camera with the start of the experiment. The frame rate of
the camera is 25 frames per second and the screen resolution is 720 x 576.
The digital video records were analyzed to determine the position of the turbidity current head
as a function of time. Generally, 35 to 40 measuring points were taken. In all the experiments
the front velocity was nearly constant over the whole length, and was determined from a least-
square fit. The relative change of the front velocity measurements was less than 1%.
At the end of each experiment the watch was stopped with the closing of the gate and the
duration of the experiment was read from the chronometer.

3.3.3.3.3 Density and Temperature Measurements
In all experiments clear tap water was used as the ambient fluid. During the experimental
work it was observed that the temperature of the water shows diurnal variations. The density
of the sediment mixture and the clear water was measured before and after the experiment by
means of a hydrometer, which allowed determination of the density to a precision of ± 0.1 g/l.
Care was taken to obtain a turbidity current mixture of approximately the same temperature as
the clear water in the flume, such that the density difference between turbidity current mixture
and the ambient fluid was only due to the presence of sediment. Prior to the experiment the
temperatures of the mixture prepared in the mixing tank and the temperature of the clear water
in the flume were measured using a mercury thermometer with a resolution of ± 0.1 °C.

3.3.3.3.4 Auxiliary Measurements
The flowmeter of the inflowing pump is monitored during the experiments, which allowed
checking the stability of the inflow.
During each experiment the depths of the free surface at the upstream end of the flume, and in
the stilling tank were measured by means of ultrasonic level probes. This allowed a
verification that the boundary conditions were constant during the experiments.

3.3.3.3.5 Deposition Measurements
It is greatly desirable to measure the deposition thickness during physical experiments of
turbidity currents. Most of the experimental techniques described in the literature are designed
to measure the deposition after the experiment, but give no information on the variation in
Deep, seasonal storage reservoir
35
time (Bonnecaze, 1993, Garcia, 1994, Altinakar, 1988). They often used a suction method, by
which they siphoned up the particles deposited on a known area at several locations in the
tank, dried them and weighted the samples. The advantage of this method is that by means of
a sieve analysis the sample can be analyzed and the segregation of the particles can be
investigated, see Altinakar (1988) for example.
In the present work, a new device to measure the local sediment layer thickness was
developed based on work done by De Rooij et al. (1999). The technique is based on the fact
that the electrical resistance of a layer of particles depends on its thickness. In the present
experiments a polymer powder was used. The electrical resistance of this material is much
higher than that of clear water, which implies that the current passes through liquid phase,