RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE

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BRAZILIAN ELECTRICITY REGULATORY AGENCY - ANEEL
Hydrological Studies and Information Department - SIH






RESERVOIR SEDIMENTATION
ASSESSMENT GUIDELINE














Newton de Oliveira Carvalho
Naziano Pantoja Filizola Júnior
Paulo Marcos Coutinho dos Santos
Jorge Enoch Furquim Werneck Lima






Brasilia, DF – 2000
Reservoir Sedimentation Assessment Guideline

ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department
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RESERVOIR SEDIMENTATION
ASSESSMENT GUIDELINE

SUMMARY


1. Introduction.................................................................................................. 5

2. Reservoirs with sedimentation problems in Brazil...................................... 7

3. Deposition of sediments in reservoirs….............……….……………........ 7

4. The relevance of the sedimentation assessment survey for hydropower
plants ……....................………………………………………………....... 9
4.1 Inventory stage ……….................................……………………........10
4.2 Feasibility and basic project stages …… ......................................……10
4.3 Operational stage ..............……..........................……………….......... 11

5. Factors affecting sediments yield ……............………............................... 14

6. Reservoir sedimentation assessment …………................................................... 12
6.1 Reservoir data ............................................................…….............. 13

7. Sediment production determination...........................................…………......... 13
7.1 Erosion assessment....................................................................…....... 15
7.2 Sedimentometric gaging stations networking……..................…......... 15
7.3 Gaging station installation and measurement frequency …….............. 16
7.4 Measurement methods......................................................................... 17
7.4.1 Sediment sampling………................................................... 23
7.4.2 Laboratory analysis...............................................…........... 25
7.4.3 Sediment discharge computation............................................ 27
7.5 Data Analysis …………..................................................................... 30
7.5.1 Continuous, hourly and daily measurements.............…......... 31
7.5.2 Eventual measurements.....................................………......... 32
7.5.3 Data regionalization.............................................……......... 36

8. Reservoirs Trapping Efficiency ………………………...................................... 38
8.1 Medium and large reservoirs cases ……............................................ 38
8.2 Small reservoirs case …...............................………........................... 39

9. Specific weight of deposits.............................................……………………... 42
9.1 Computed ............................................................................................ 42
9.2 Measured .............................................................................................. 44
9.3 Estimate .............................................................................................. 44
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10. Estimation of sediment deposit in reservoirs..................................................... 45
10.1 Sedimentation assessment methods ……............................................ 45
10.2 Assessment of storage loss ......….….....………………………........ 46
10.3 Assessment of reservoir useful life …............................................... 47
10.4 Sediments distribution in reservoirs.................................................. 48
10.5 Assessment of erosion rates….......................................................... 48

11. Measurement of reservoirs sedimentation...............................………….......... 51
11.1 Purpose of the survey.......................................................................... 51
11.2 Survey frequency…............................................................................. 52
11.3 Survey methods................................................................................... 53
11.3.1 Contour survey ……………….................................…........ 53
11.3.2 Topo-bathymetric survey ..................................................... 54
11.4 Survey specifications........................................................................... 59
11.5 Bed mapping ....................................................….............………...... 61
11.6 Computation of reservoir volumes..............................................................
62
11.7 Computation of settled sediments volume ..........................………… 69
11.8 Outline of new level x area x volume relations.................….............. 66
11.9 Pivot point ................................................…………………….......... 66
11.10 Bed scanning and geophysics……................................................... 67

12. Control of a reservoir sedimentation................................................................. 68
12.1 Preventive control............................................................................ 69
12.2 Corrective practices ............................................................…............. 70
12.2.1 Dredged sediments discharge …….......................………… 70

13 Secondary effects due to sediments..........................................................…... 71
13.1 Effects on the reservoir backwater .................................................... 72
13.2 Changes on water quality..................................................................... 73
13.3 Ecological effects ............................................................................... 73
13.4 Erosion on reservoirs banks...................…..........................…............ 74
13.5 Deposit erosion.....................................................................................74
13.6 Downstream effects...................................…………..............…......... 74
13.6.1 Channel degradation ....................................................…..... 75
13.6.2 Main discharge .........................................…….................... 77
13.6.3 Channel hydraulic features…......................................…….. 77
13.6.4 Method of degradation constrained by the shield ...………. 77
13.6.5 Method of degradation constrained by steady slope....……. 81
13.7 Reservoir surveys supported by satellite imagery..............….............. 84
13.8 Erosion control at the downstream channel...................………......... 85

Bibliography (consulted and complementary) ........................................................ 86

Glossary of terms, symbols and units ...........................................…….................. 93

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1. INTRODUCTION

The construction of a dam and the creation of an impounded river reach area usually
change the stream natural conditions. Concerning the sedimentological aspect, the dams
cause a reduction on the flow velocity, thus causing the gradual deposition of those
sediments carried by the stream resulting in the sedimentation, gradually diminishing
the reservoir storage capacity. Therefore, it may come to hinder the reservoir operation,
besides causing several kinds of environmental problems.

Environmental and economic damages arising out of the sediments deposition in
reservoirs may be hard to solve, especially in arid and semi-arid regions (ICOLD,
1989). Apart of the reservoir size, this Guide seeks to deal with the problem in a simple
and objective way, presenting the critical conditions that may happen.

Surely, the reservoir may undergo an undesired sedimentation, thus requiring studies
each case. Small lakes are more susceptible to quick sedimentation, what may happen
even in a single flood (Carvalho/Guilhon/Trindade, 2000). On the other hand, large
reservoirs require more time to become sedimented. In Brazil, one can mention the
reservoirs of Itaipu, Itá, Sobradinho and Tucuruí, where the total time of sedimentation
assessed for each reservoir may overpass 1000 years. However, in a shorter period of
time – 20 to 30 years – the deposits at the backwater region - delta area - may be already
jeopardizing activities such as navigation. Furthermore, thin deposits at the banks may
give rising to suitable conditions for the growing of macrophytes plants that will surely
be displaced for areas nearby the dam and enter into the ducts, thus prejudicing power
production.

A tributary to the reservoir that is flowing nearby the dam, or its facilities, may
affect electric power production or other activities in a time shorter than the foreseen.
Sedimentation cases are becoming intensified due to the increase of erosion at water
basins. Therefore, it would be prudent to carry out sedimentological surveys for all
projects that require reservoir. In any case, the assessment carried out during the
planning stage shall be reviewed by a sedimentometric survey, including the operation
of gaging station and topo-bathymetric survey. Those studies shall be simultaneously
with environmental surveys.

Sedimentation processes may be complex. The sediments carried through the fluvial
system are primarily settled due to the lowering in the reservoir water speed. As
sediments are accumulated in the lake, its water storage capacity is reduced. While a
continuous deposition takes place, there is a distribution of sediments at the reservoirs.
The kind of distribution is influenced by both operation and occurrence of floods, which
are responsible for the transportation of great amount of sediments. When deposits
affect the reservoir useful life, it is necessary to change the reservoir operation or adopt
any other corrective measure (ICOLD, 1989). Other effects may happen such as, for
example, the delta area becomes more susceptible to problems with floods; downstream,
the river flume suffers erosion due to the absence of sediments at the runoff, and due to
floods attenuation and stream regularization as well.
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This Guide aims at defining and studying those features directly related to
planning and project of new dams, as well as to the operation of the existing ones, by
surveying the production of sediments, the reservoir sedimentation, the sediment control
and its secondary effects. Issues of that nature have not, up to this moment, been duly
managed in the country due to the lack of tradition for those studies. It is expected that
the experience acquired along time may bring stimulatio, information and additional
contributions for the development of the sediment survey area.

2. Reservoirs with sedimentation problems in Brazil

Sedimentological study is particularly important for Brazil since most electric power
plants in the country are hydraulic ones. Currently, over 90% of electric power
consumed comes from hydraulic sources, and it is foreseen to remain like that for the
next three or four decades. Despite that, it is observed that sedimentological studies are
not deep enough or are incomplete. Hydrologic studies concerning rivers’ regimen,
determination of discharges series and similar ones, are usually performed in a suitable
way, while most sedimentological studies are carried out in an incomplete way. It is
thought that this happens like that because most of the energy production in the country
is provided by large reservoirs, where the sedimentation issues are not regarded as very
important for production at short- and medium-term (Almeida and Carvalho, 1993).

A World Bank study (Mahmood, 1987) illustrated that the average useful life of
existing reservoirs in all countries of the world decreased from 100 to 22 years. The
annual cost for promoting the removal of the volumes being sedimented is estimated in
US$ 6 billion. It has also shown that annual average of reservoirs volume loss due to
sediments deposition was of 1% varying from one country to another, as well as from
one region to another. Based on a survey carried out by Eletrobrás/IPH (1994) one can
conclude that, in Brazil, the reservoir’s annual storage capacity loss is of about 0,5%
(Carvalho, 1994). That index may correspond to storage capacity losses of 2.000 x
10
6
m
3
per year, corresponding to a volume greater than several existing medium-size
reservoirs (Estreito, Jaguari, Moxotó, Salto Osório, Porto Colombia etc.). On the other
hand, it is observed that erosion is increasing in the country in face of population growth
and soil management.

Brazil has already several reservoirs totally or partially sedimented. Usually, the
visible sedimentation is the smallest part of deposit. Due to the lack of systematic
surveys – and dissemination of their outcomes – the condition of Brazilian reservoirs is
not known as would be desirable. Table 2.1 presents a list of reservoirs partially or
totally sedimented, based on information collected by Carvalho (1994 and 1998).






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Table 2.1 – Some reservoirs in Brazil partially or totally sedimented (Carvalho,
1994 and 1998)

Reservoir Stream Owner Kind

Tocantins Basin
Itapecuruzinho Itapecuruzinho CEMAR UHE, 1,0 MW

North Atlantic Basins
Limoeiro Capibaribe DNOS Flood control

São Francisco Basin
Rio de Pedras Velhas CEMIG UHE, 10 MW
Paraúna Paraúna CEMIG UHE, 30 MW
Pandeiros Pandeiros CEMIG UHE, 4,2 MW
Acabamundo Acabamundo DNOS Control of floods
Arrudas Arrudas DNOS Control of floods
Pampulha Pampulha SUDECAP Control of floods

Atlantic/East Basins
Funil Contas CHESF UHE, 30 MW
Pedras Contas CHESF UHE, 23 MW
Candengo Una, BA CVI UHE, -
Peti Santa Bárbara CEMIG UHE, 9,4 MW
Brecha Piranga ASCAN UHE, 25 MW
Piracicaba Piracicaba B.-MINEIRA UHE, -
Sá Carvalho Piracicaba ACESITA UHE, 50 MW
Dona Rita Tanque - UHE, 2,41 MW
Madeira Lavrada Santo Antônio CEMIG Storage
Guanhães Guanhães CEMIG Storage
Tronqueiras Tronqueiras - UHE, 7,87 MW
Bretas Suaçuí Pequeno - -
Sinceridade Manhuaçu CFLCL UHE,1,416 MW
Mascarenhas Doce ESCELSA UHE, 120 MW
Areal Areal CERJ UHE, -
Paraitinga Paraitinga CESP UHE, 85 MW
Ituerê
Funil
Pombas
Paraíba do Sul
CFLCL
FURNAS
UHE, 4,0 MW
UHE, 216 MW
Jaguari Jaguari CESP UHE, 27,6 MW
Una Una, SP PM Taubaté Water supply

Paraná Basin
Pirapora Tietê - -
Caconde Pardo CESP UHE, 80,4 MW
Euclides da Cunha Pardo CESP UHE, 108,8 MW
Americana Atibaia CPFL UHE, 34 MW
Jurumirim Paranapanema CESP UHE, 22 MW
Piraju Paranapanema CPFL UHE, 120 MW
Pres. Vargas Tibaji Klabin UHE, 22,5 MW
Poxoréu Poxoréu CEMAT UHE, -
São Gabriel Coxim ENERSUL UHE, 7,5 MW
Rib. Das Pedras Descoberto CAESB Water supply
São João São João ENERSUL UHE. 3,2 MW

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Uruguay Basin
Caveiras Caveiras CELESC UHE, 4,3 MW
Silveira Santa Cruz CELESC UHE, -
Celso Ramos Chapecozinho CELESC UHE, 5,76 MW
Furnas Segredo Jaguari CEEE UHE, -

Atlantic/Southeast Basins
Santa Cruz Tacanica CCPRB UHE, 1,4 MW
Piraí Piraí CELESC UHE, 1,37 MW
Ernestina Jacuí CEEE UHE, 1,0 MW
Passo Real Jacuí CEEE UHE, 125 MW


3. DEPOSITION OF SEDIMENTS IN RESERVOIRS

The stream, when entering the reservoir, has its cross-section areas enlarged,
while the speed of the current decreases, thus creating conditions for sediment
deposition. The heaviest particles, such as gravel and thick sand, are the first ones to be
settled, while finest sediments enter into the reservoir. The dam hinders the passage of
most particles for downstream; therefore, the passage may come to occur upon the
runoff through the spillway and the ducts.

As the sedimentation increases, the reservoir storage capacity decreases, the
influence of backwater increases for the upstream, the velocities in the lake increase and
more sediment come to flow towards downstream, thus diminishing the particles trap
efficiency.

The sediments settled due to the influence of the reservoir, expand to upstream
and downstream, and are not equally distributed even within the lake. The upstream
deposition is called backwater deposit, named after the hydraulic phenomenon, being
also ascending since the deposits in that area increase. The depositions within the
reservoir are called delta, overbank and bottom-set deposit. Coarses make up the
delta, while the inland deposits are made up by finer sediments (Mahmood, 1987).
Floods produce another kind of deposition, occurring along both stream and reservoir,
being made up by thin and coarses, named flood plain deposit.

Such deposits cause different impacts or consequences. The backwater deposits
cause flood problems at upstream. The deposits in the lake cause reduction of the
storage capacity, and the variation of the water level shall determine the delta formation.
While most delta deposits gradually reduce the useful capacity of the reservoir, the
overbanks reduce the dead storage. Part of the delta is also contained in the dead
storage. Those sediments reaching the dam and passing through spillway and ducts,
cause abrasions on the structures, gates, piping, turbines and other pieces.

At downstream, the clean water – i.e., with no sediments - as well as the change
on discharges regimen, shall cause erosion on both bed and banks of the channel, or
even huge excavations that may develop towards upstream, jeopardizing the dam
structure.
Reservoir Sedimentation Assessment Guideline

Figure 3.1 illustrates, schematically, the sediment distribution due to the
existence of the reservoir, and indicates the main resulting problems as well.




Figure 3.1 Schedule on sediment deposits formation in reservoirs, indicating the main
issues resulting from it (Carvalho, 1994).
Legend:
Depósitos de remanso = backwater deposits
Declividade superior = higher slope
Delta = delta
N.A. max = maximum water level
N.A. min = minimum water level
Ponto de escorregamento = sliding point
Declividade frontal = front slope
Leito original (talvegue) = original bed (thalweg)
Declividade de fundo = bottom slope
Depósito do leito = bed deposit
Erosões, escavações no leito = bed erosion, excavation
Problemas de enchentes e ambientais = flood and environmental problems
Redução da capacidade do reservatório e problemas ambientais = reservoir capacity reduction and environmental problems
Redução de capacidade útil = useful capacity reductions
Redução no volume morto = dead storage reduction
Problemas de abrasão nas estruturas, comportas, tubulações, turbinas e peças = abrasion problems in structures, gates, tubes,
turbines and parts
Problemas ambientais e modificações na calha fluvial = environmental problems and changes on fluvial flume
Retirada de nutrientes e modificação da qualidade d’água = withdraw of nutrients and change on water quality



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Other problems deriving from sediments deposition may be noticed, and all of
them require study and present distinct environmental impacts (Carvalho, 1994).

Marginal deposits of fine sediments along stream and in the reservoir may
facilitate the growth of aquatic plants, which are removed by the raise in water level.
That fluctuating vegetation will cause several problems, such as its decomposition,
deposition at the lake bottom and transformation into minerals, in addition to the
sedimentation. Part of the vegetation will reach intakes, thus jeopardizing the operation,
if they are not removed.

Those sediments covering the bottom of the lake shall cause changes on both
fauna and flora of the bed. The clean water that flows towards the dam downstream,
already without the nutrients carried sediments, shall cause changes on fauna and flora,
with environmental impacts along the whole stream, specifically at the outfall. The
formation of estuary and delta at the sea may undergo severe environmental changes
(Carvalho, 1994).

4. THE RELEVANCE OF THE SEDIMENTATION ASSESSMENT SURVEY

Sedimentological studies must be carried out along all project stages, since
planning (inventory, feasibility and basic project) until the operation stage. During the
inventory, if there are no gaging stations for measuring the sediment load, one or several
gaging stations are installed and operated, thus building up a sedimentometric network,
which will be as large as the drainage area, and follow the importance of this study.

The studies show that there are several kinds of approaches for the distinct
stages of a reservoir project. As more serious the problems concerning erosion,
sediments transportation and sedimentation presented are, either at stream or regionally,
more detailed those approaches will be presented. Studies are carried out for
establishing the best sediment control measures that should be adopted.

In any stage o the studies, the first steps are (Carvalho, 1994):

Survey on basin erosion conditions (soil management, deforestation, etc.);

Survey on existing or deactivated sedimentometric gaging stations;

Existing studies on the theme for the basin;

Collection of the required hydrologic and sedimentological data (series of
discharges, sediment discharge, granulometry for suspended sediment and bed load and
others).

In face of the lack of sedimentometric and hydrologic data, there is the need of
installing and running, in short time, a hydrological-sedimentometric gaging station or
network.

The surveys to be performed concerning sedimentation forecast are as follows:

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Data processing (collection of parameters, average values, specific weight, sediment
trap efficiency in the reservoir, increase on erosion index or sediment transportation
and others);

Total sedimentation time for the reservoir;

Sedimentation time up to the intake level (useful life);

Height of deposits at the dam base for 50 and 100 years or other periods;

Distribution of sediments in the reservoir for 50 and 100 years, or other periods;

Tracing out of level x area x volume curves, both originals and for the sedimented
reservoir;

Percentage of the reservoir sedimentation for specific periods of time;

Amount of sediments settled in the volume set apart for controlling floods;

Top layer slope;

Front layer slope;

Effects of severe floods and sediments transportation (for small reservoirs);

If the sedimentation is a problem in a period twice the period of the reservoir useful
life (2x50 years), inclusively considering the sediment transportation rate along
time, so determine preventive measures for controlling the sediment;

Studies on the forecast of erosion effects on the channel of the dam downstream;

Prevention control of sediments during the planning stages;

Preventive and corrective practices during the operation stage;

Other studies may be contemplated, such the one on secondary effects due to
deposits and backwater monitoring, considering the reservoir sedimentation.

4.1 Inventory stage

Usually, during the inventory stage, one seeks for data from gaging stations from
the Country’s main network. That network is under ANEEL responsibility, and the
earlier gaging stations were installed in 1971 by the former DNAEE. The network was
expanded and some gaging stations were replaced. Therefore, it is always necessary to
review such discontinuity through information contained in DNAEE Inventory of
Fluviometric Stations. Old sedimentometric data, despite not reflecting current situation,
may indicate the increase or decrease of the erosion rate in the basin, by comparing
those data with current ones.

If there are not enough gaging stations, or if there is definitely no gaging station,
then it is necessary to install one or more sedimentometric gaging stations and take the
necessary steps for their proper operation. If there is no gaging station along the stream
course, primary studies may be performed by using sedimentometric data of neighbor
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basins reporting similar features. However, it is necessary to install gaging stations at
the focus area, in order to grant studies for the following stages.

Sedimentological studies for assessing sedimentation based on those data shall
indicate the need of short- or medium term preventive sediment control.


4.2 Feasibility and basic project stages

The sedimentological studies at the inventory stage should point out the
requirements for further stages. If there are no of such studies, the need of surveying the
existence of gaging stations nearby the project area will arise. The installation and
operation of a gaging station at the site of or nearby the forthcoming dam is the most
suitable solution.

The studies shall be more refined and expanded, for verifying the basin features
jointly with regional aspects concerning erosion occurrence.

The sedimentation assessment during those stages shall include computation of
reservoir life; the sediment deposit height at the dam base or at the water intake
position; the reservoir useful life and the sediment deposition after 100 years. The rate
of sediment transportation along the stream or the basin erosion rate shall be obtained
and considered while assessing the sedimentation and, mainly, when estimating the
reservoir useful life.


4.3 Operational stage

Sedimentological studies shall not cease upon the conclusion of the dam
building works. On contrary, at that stage, the monitoring of sediment effects in face of
the reservoir development should be even highlighted. Works like that necessarily bring
regional development and, therefore a territorial occupation that includes improved soil
management for agriculture – due to the increase on water availability -, the building of
roads and a set of changes whose consequences may have not been adequately assessed
during planning studies.

The steps for performing sedimentological studies at the operation level include
monitoring of secondary fluvial-sedimentometric network – installed during previous
stages -, and topo-bathymetric surveys for the reservoir, surveys and follow-up studies
on erosion effects at downstream, and sediment-related environmental impacts.

The secondary sedimentometric network shall monitor at least 80% of the dam
drainage area; the local gaging station shall be replaced by one station downstream and
another one upstream the backwater area.

The reservoir systematic topo-hydrograph survey is a requirement for
determining water availability through new level x area x volume curves, assessing the
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new reservoir contour, the pivot point, as well as several additional pieces of
information (please refer to the item on measurement of reservoir sedimentation). It
would be advisable to have small reservoirs surveyed at every two years; the medium-
size ones at every five years, and the large ones at every 10 years. It the new survey
presents small variation concerning sedimentation, so the survey interval may be longer,
and the changes taking place in the basin due to land occupation and consequent
increase of erosion should be monitored.

Comparative satellite-based studies for different periods of time allow for
obtaining several pieces of information on changes occurring in the concerned reservoir
area.

Data obtained from both operation of sedimentometric network and survey data
may allow for the assessment of the reservoir remaining useful life. For those
assessments, the surveys used for forecast shall be repeated.


5. FACTORS AFFECTING SEDIMENTS YIELD

Sediments reaching the reservoir come from the inflow drainage area and are
taken mainly through the major fluvial channels network.

The production of sediment deriving from drainage area – or corresponding to a
whole hydrograph basin – depends on erosion, rainwater runoff with the transportation
of sediments, and characteristics of sediment transportation along streams as well.

The main factors affecting the sediments yield at the drainage area are (ICOLD,
1989):


Precipitation – quantity, intensity and frequency;

Kind of soil and geological formation;

Soil coverage (vegetation, apparent rocks and others);

Soil management (cultivation practices, grazing grass, forest exploitation,
building activities and conservation measures);

Topography (geomorphology);

Nature of the drainage network– density, slope, shape, size and channels
configuration;

Surface runoff;

Sediments features (granulometric, mineralogical etc.);

Channels hydraulics.

Additional factors may be added, as well as likely combinations among the nine
above-mentioned factors. For the assessment of sediments yield in a drainage area
inflowing the dam position, it is necessary to have an expert assessment on the most
influencing factors. It shall, necessarily, lead to the measurement conclusions required
Reservoir Sedimentation Assessment Guideline
to accurately define the sediments amount, available techniques for foreseen such
sediment production or even to assess the quantity of sediments at basins where due
measurements have not yet taken place.


6. RESERVOIR SEDIMENTATION ASSESSMENT

The assessment on the sedimentation of the reservoir total volume and useful life
is essential for surveys about the lake formation, as well the evaluation of the reservoir
operation. The end of its useful life - in sedimentological terms - is considered as when
deposits come to interfere on the regular operation of either the plant or of the reservoir
purpose. Additional evaluations shall be performed, according to the time taken by the
sediment to reach the intake sill (useful life), sediments distribution along the reservoir -
corresponding to a given period -, the pivot point development and delta building (up
and frontal slope).

For the preliminary sedimentation computation, the following mathematical
expressions are used:


ap
rst
ap
rst
xExQxED
S
γγ
365
== (6.1)

S
V
T
res
=
(6.2)
where:
S = volume of sediment trapped in the reservoir (m
3
/year);
D
st
= annual average for total bed load inflowing the reservoir (t/year);
E
r
= trap efficiency for the sediment inflowing the reservoir (decimal);
γ
ap
= deposits specific weight (t/m
3
);
Q
st
= total average sediment discharge inflowing the reservoir (t/day);
T = sedimentation time for a given volume (years);
V
res
= reservoir volume, total or dead storage (m³).

For items 7, 8 and 9, equations 6.1 and 6.2 indicate how to determine the
parameters required for evaluating the sedimentation.

6.1 Reservoir data

The main project data required for such forecasts are:
• Maximum normal water level, in m;
• Minimum normal water level, in m;
• Intake sill height, in m;
• Volume of maximum normal water level, in m
3
;
• Volume of minimum normal water level (dead storage), in m
3
;
• Volume of intake sill, in m
3
;
• Natural discharges series;

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• Long-term average discharge, in m
2
/s;
• Spillway sill level, in m;
• Intake sill level, in m;
• Reservoir length, in m or km.


7. SEDIMENT PRODUCTION DETERMINATION

The entity responsible for building the hydroelectric plant - or any other kind of
water resources project available – and that comes to create an impounded river reach
area, should seek for hydrologic and sedimentological data with other entities existing
along the stream course. If there is no data available, the entity must, therefore, install
and operate gaging stations for that purpose. Bathymetric survey data for reservoirs
could also be used, but they are scarce. Other studies that might be obtained are data on
the basin erosion rates assessment, has required for the accurate sedimentation forecast.

It is necessary to regularly get suspended and bed granulometric data for
computing the specific weight. It is also essential to measure the sediment discharge in
sedimentological surveys for small- and medium-size reservoirs, since coarse (sand) is
never discharged through ducts and spillway; therefore, it remains deposited in the
reservoir. Exception is made to the small quantity of sand being discharged during
severe floods.

Studies concerning sediment production are presented in more details in the
Sedimentometric Practices Guideline and are outlined herein.

Generally, for implementing a program on sedimentometric measures –
according to the International Hydrologic Program – UNESCO (1982) has established
the criteria presented in Table 7.1, according to Yukian (1989).

Table 7.1 – Program on acquisition of sedimentometric data according to UNESCO
(1982) and Yukian (1989)

Gaging item

Survey Purpose
Bathymetric Survey Sediment
transportation
Other relevant items
Annual runoff Sediments
concentration,
suspended discharge,
total discharge in
hydrometric gaging
stations
Water level, net
discharge and others
1) Erosion and
deposition in river
reaches;
2) Reservoir capacity
depletion
Periodic surveys via cross-
section and longitudinal
lines in the river or
reservoir reach; full survey
on the reservoir
sedimentation
Total inflow or outflow
sediment discharge in
hydrometric gaging
stations
Sediment granulometry
and specific weight of
deposits
Fluvial processes in Periodic surveys along the Bed and bed load Relevant hydraulic and
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river reach or reaches
susceptible to
reservoir backwater
river reach or in interesting
sites; aerial photography, if
possible
discharge in affluent
hydrometric gaging
stations
sedimentological
parameters such as water
line slope, bed load
composition, velocity,
depth and width, water
temperature,
granulometry of
sediment being carried,
specific weight, etc.

High values of sediment production, such as 200 t/(km
2
.year), are very
prejudicial and may come to affect the reservoir with undesired deposits. According to
international criteria, the values reported in Table 7.2 may be used as indicators for
surveys.

Table 7.2 – Acceptable values for sediment production

Sediments yield
Tolerance
(ton/(mi
2
.year) (t/(km
2
.year)
High > 500 175
Moderate 200 to 500 70 to 175
Low < 100 35


7.1 Erosion Assessment

Soil erosion is a complex process presented in different ways in nature, and
whose measurement is also complex. Seet erosion surveys are the commonest
phenomena and are not measured. Similar studies from the USLE, Universal soil loss
equation that may be expanded to any area by using the modified equation MUSLE exist
only for agriculture, in some Brazilian regions. Despite that resource, the values
obtained through such equations are high and may not be used for studying sediment
transportation. For comparison purposes, the average results obtained as acceptable in
agriculture for rates from 3 to 15t/(ha.year), equivalent to 300 to 1500t/(km
2
.year), are
much higher than the values presented in Table 7.2 for sediment transportation rates.
That is true, since not all sediments eroded in the basin reach the stream, and, therefore,
part of sediment remains in depressions and plain areas.


7.2 Sedimentometric gaging stations networking

The sedimentometric gaging stations network for a basin may be dimensioned
following WMO criteria (WMO, 1994). It is regarded as the most useful network for
basic studies. Currently, ANEEL is responsible for that network in Brazil, monitoring a
little more than 300 gaging stations – an amount lower than WMO criterion – due to
operational costs. Countries such as Canada and Russia, reporting the same continental
dimension, also have sedimentometric networks with few gaging stations, such as ours.
Therefore, a secondary network must be usually considered for meeting the needs of
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specific surveys, with more frequent operations, as is the case for implementing gaging
stations for reservoir sedimentation assessment. That network shall remain operational
during the operation stage.

For implementing surveys about river or reservoirs reaches it is useful to know –
or measure/monitor – the inflowing of sediments for at least 80% of the inflow basin,
being necessary to obtain both suspended and total sediment discharge. For studies on
existing reservoirs, considering an investigation monitoring, it is necessary to monitor at
least 60% of the tributary basin and install a gaging station downstream for identifying
the effluent sediment. The tributaries that directly discharge into the lake, and report a
contribution of sediment higher than 10% of the total tributary shall also be monitored
(Yuqian, 1989).


7.3 Gaging station installation and measurement frequency

***Level readings and net discharge gaging shall be performed when measuring
the sediment discharge, and the gaging station shall be regularly operated. Therefore,
the sedimentometric gaging station may be selected among the fluviometric network
gaging stations holding historical data. For installing a new gaging station, the site shall
be selected following the same criteria as for the fluviometric gaging station.

In sedimentometric gaging station where it is intended to measure the bed load,
it would be useful to use a complementary gaging station, with the same reference and
duly located, in order to determine the water line slope for each measurement.

The measurement frequency for either the sedimentometric gaging station or
network must be planned jointly with the fluviometric network operations; special
attention shall be addressed to the phenomenon of variation for bed sediments during
rainy times and occurrence of precipitations.

Usually, the suspended load is the prevailing piece for the total bed sediment;
that is why the frequency is establishing aiming at measuring the suspended discharge.
The measurement may occur hourly, daily, weekly, monthly or even periodically.
Recording devices may perform the continuous operation at a stream point.

Hourly measurements may be performed with automatic pumping equipment
with rotating trays. Daily measures or collection are generally performed by the gaging
station observer in two or three pre-established vertical sections; during drought times,
measures are to be performed at every 15 days. In large streams, the sediment collection
may be performed weekly; however, recent studies on rivers of that nature have
evidenced that such variations may occur even daily.

Hydrometry team shall assist monthly or periodic measurements. Such
measurements shall be performed following the full sampling criterion and not just for
one to three selected vertical sections. Punctual measurement, either using automatic
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equipment or register such as hourly, daily or weekly collection, shall be followed by
measurements performed by the hydrometry expert for calibration purposes.

The measurement performed by the hydrometry expert shall include both
suspended sediment load and bed load collection. The measurement of water
temperature measurement and slope measurement are also required.

Most of the stream’s bed sediment occurs during rainy period, corresponding to
about 70 to 90% of annual total load. Therefore, it is useful that measurement frequency
comprises such period, and that few measurements remain for drought period.

Sediment measurements are relatively more expensive than the remaining
measurements for water resources surveys, due to the complexity of the phenomenon
and to its difficult computation, as well. Currently, by using computers that facilitate
such computations, it is possible to upgrade any measurement software in order to reach
greater accuracy and better outputs.


7.4 Measurement Methods

The different measurement methods for suspended bed load or total load are
classified as direct (or in situ) and indirect. Table 7.3 shows, in a simple way, such
methods.

Table 7.3 – Methods for gaging bed sediment (Carvalho, 1994)

Sediment
discharge
Measure
ment
Description
Measurement equipment or
methodology
Uses equipment that measures the
concentration or any other value, such
as turbidity or ultrasound directly in the
stream
Nuclear measurer (portable or
fixed), Optical ultrasonic flow
meter, Doppler Ultrasonic Flow
meter, Turbidimeter (portable or
fixed)
ADCP (Doppler)



Direct
Through sediment accumulation in a
measurer (graduated test tube)
Delft Bottle (punctual
measurement and high
concentration)





Suspended
sediment
discharge




Indirect


Sediment collection by sampling of the
water-sediment mixture, concentration
and granulometry analysis and further
computation on sediment discharge
Several kinds of equipment: -
pumping, equipment using bottles
or bags, being punctual
instantaneous, punctual through
integration and vertical
integrators (in Brazil, the North-
American series– U-59, DH-48,
DH-59, D-49, P-61 and bag
sampler are the mainly ones that
are used)
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Use of satellite pictures and comparison
with simultaneous field measures for
calibration in large rivers.
Equations are established in order
to correlate the values of picture
observation and measured
concentrations

Samplers or portable measurers of three
main kinds (the sampling is collected in
several points of the cross-section,
determining its dry weight, the
granulometry and calculating the
entrainment discharge); the measurer is
fixed on the bed from 2 minutes to 2
hours, in such a way as to receive in its
receiver from 30 to 50% of its capacity
1) Crate or case - Muhlhofer,
Ehrenberger, Switzerland
Authority and other measurers
2) Tray or tank – measurers
Losiebsky, Polyakov, SRIH and
others
3) Pressure difference –Helley-
Smith, Arnhem, Sphinx, USCE,
Károlyi, PRI, Yangtze, Yangtze-
78 VUV measurers and others





Direct
Crevasse or water well structures – the
bed crevasses are opened for a few
minutes and the sediment is collected
Mulhofer measurer (USA)
Bed load collection, granulometric
analysis, slope gauge, temperature
hydraulic parameters and computation
on entrainment discharge and bed load
through formulas (Ackers and White,
Colby, Einstein, Engelund and Hansen,
Kalinske, Laursen, Meyer-Peter and
Muller, Rottner, Schoklitsch, Toffaleti,
Yang and others)
Kinds of equipment:
1) horizontal penetration, like
dredge and shell bucket
2) vertical penetration, like
vertical tube, scraper bucket,
excavation bucket and gravel
excavation
3) piston-core, which holds the
sampling though partial vacuum

Dunes displacement – by measuring the
volume of the displacing dune, using
high-resolution echobathymeter
1) successive bathymetric surveys
along the cross-section
2) successive bathymetric surveys
along longitudinal sections
1) Radioactive trackers
2) Dilution trackers, being both
methods by setting the tracker on the
sediment and monitoring it by using the
suitable equipment (the tracker shall be
chosen in such a way as to avoid
polluting environment)
Methods:
1) by settling the tracker directly
on the bed sediment
2) by collecting sediment, settling
the tracker on the sediment and
returning it to the bed.

Lithologic properties – use of
sediments’ mineralogical features
Collection of tributaries and main
bed sediment, determination of
sediments’ mineralogical features
and comparison by using suitable
equations based on the quantity
of components existing in the
sampling
Acoustic method – used for stones
striking against the measurement
(Unsatisfactory)










Bed load
entrainmen
t discharge






Indirect
Sampling photograph method – used
for stones. A scale is settled and also
photographed
1) Photos of underwater stones
2) Photos of dry beds stones





Direct
Use of block-type structures, on the
bed, to cause turbulence and all
sediments become suspended
Sediment sampling is performed
and computed as suspended
discharge
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Topo-bathymetric survey for the
reservoir, determination of deposits
volume and trap efficiency in the lake
1) For small reservoirs, it allows
for the computation of bed
sediment
2) For large reservoirs, it allows
for the computation of total
sediment

Sediment
discharge
total
Indirect Collection of suspended and bed
material, concentration analysis,
granulometric analysis, temperature
measurement, hydraulic parameters and
computation of total discharge –
Einstein’s method modified and
Colby’s method simplified
Several kinds of equipment –
pumping, equipment using bottles
and bags, being instantaneous
point, points by integration and
vertical integrators (in Brazil is
mainly used the North-American
series U-59, DH-48, DH-59, D-
49, P-61 and bag sampler)

Several measurement or suspended sampling equipment may be classified in
different kinds, such as:

• Instantaneous or integrators, where the instantaneous quickly gets the sampling or
read them, while integrators admit sampling in a few seconds through a beak or a
bill, storing it in a recipient;
• Portable or fixed, where portable ones are manually operated, by pole or shrill, or
even fixed to a boat, while the fixed ones are installed in a adequate structure, either
on a bridge or at the bed;
• Beak or with bill, where the beak are of pumping or other, and those using bills are
the portable ones furnished with bottles, plastic recipient or plastic bag;
• Punctual instantaneous, punctual by integration and by vertical integration, where
punctual instantaneous are cylinder-like with a device for capturing the sampling
sending a messenger/weight that closes the valves. The punctual by integration
collects sampling in a few seconds at a vertical point. The vertical integrators or in
deep waters collect sampling by moving the equipment along vertical in a steady
movement that may be in a single way or back and forward from surface to bottom.
• Horizontal tube sampler, of bottle, collapsible bag, pumping, integration,
photoelectrical, nuclear, optical ultrasonic flowmeter, dispersion ultrasonic,
Doppler Ultrasonic Flowmeter– the horizontal sampler is a punctual instantaneous
one. The bottle sampler is hydrodinamically built and has a cavity for inserting a
collection bottle; the sampling is performed through a bill that may report several
diameters (1/4”, 3/16” e 1/8”) while the air is expelled through a tube. The
collapsible bag sampler is also hydrodinamically built and has an aluminum-made
recipient for holding the plastic bag, which is compressed in order to expel the air;
its capacity is greater than the bottle’s capacity and it also uses exchangeable bills.
The pumping device may be settled on a boat or installed at the bed; normally, it is
used a hose furnished with a beak or a bill adjusted for allowing in the sampling;
the pumping is monitored according to the stream velocity, and there are several
kinds of such equipment. The equipment working through integration is bottle or
bag collapsible . The photoelectrical and the nuclear ones operate through light and
rays, respectively, through a constant intensity source. The optical and the
dispersion ultrasonic work with sources that produce ultrasonic rays that are
received by adequate equipment. The Doppler Ultrasonic Flowmeter uses Doppler
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effect to measurement the intensity of acoustic energy reflected by the particles
suspended in the water, thus providing a correlation between the amount of decibels
(dB) received by the equipment (for example, ADCP) and the distribution of
suspended sediments along the gauging section.
• The equipment may also be classified according to its bills or beaks orientation,
such as on the stream direction or at 90
o
with the stream.

Note – The North-American collection equipment for suspended material have
denominations indicating their origin: US, for United States; kind of usage: D, for
depth, for vertical integration or in deep waters; and, P, punctual, for punctual sampling;
light equipment, manual, are represented by H, of hand; the number corresponding to
the project, 48, for 1948.

The most used equipment in the country for sediment load sampling is from the
North-American series, bottle-type, of collapsible bag and punctual measurer with
recipient, for determining the bed sediment by indirect method (Figures 7.1, 7.2, 7.3,
7.4, 7.5, 7.6 and 7.7). Bed load collection equipment, for indirect measurement as well,
is that of horizontal or vertical penetration (Figures 7.8, 7.9, 7.10 and 7.11).


Figure 7.1 – Single stage sampler US-U-59, punctual by integration, for fixed
installation and surface collection when water level increases


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Figure 7.2 – Sampler US-DH-48, integrator-type, for wading measurement or for use
on boat up to 2,0m in deep waters, and currently has two versions: DH-59 and DH-76


Figure 7.3 – Sampler US-DH-59, integrator-type, for use through shrill in deep waters
up to 4,50m and moderate velocity



Figure 7.4 – Sampler US-D-49, integrator-type, for use through shrill in depths up to
4,50m and high velocities, and currently has two versions: D-74 and D-74AL


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Figure 7.5 – Sampler US-P-61, punctual integrator-type, may perform collection
through vertical integration, on parts, at any depth, and has the following versions: P-
50, P-61A1, P-63 e P-72



Figure 7.6 – Collapsible bag sampler, integrator-type, for use with shrill at any depth


Figure 7.7 –Delft Bottle, punctual integrator-type, for direct measurement of
concentration also using a graduated test tube


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Figure 7.8 – Sampler of the U.S. Waterways Experimental Station for bed material


Figure 7.9 – Petersen sampler for bed load


Figure 7.10 –US-BMH-60 sampler for bed material in moderate depths and velocities;
it has a lighter version for hand use, the RBMH-80


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Reservoir Sedimentation Assessment Guideline

Figure 7.11 –US-BM-54 sampler for bed material for deeper water and higher
velocities


Note – The North-American series equipment identified by US, for United States, for
direct bed load measurement are indicated as BL, for bed load, while the simple
collection for indirect measurement, are indicated by BM, for bed material, and may be
hand-operated whenever labeled as H, for hand; the number corresponds to the project
year.


7.4.1 Sediment sampling

There are several kinds of sediment load sampling, which may be punctual or by
vertical integration. Table 7.4 presents the usual sampling methods.

Table 7.4 – Methods for sediment sampling

Sampling Positions Average concentration
In pre-established position when using an
automatic equipment (pumping) or
measurer (turbidimeter, nuclear or other)
Average concentration in the section
determined through calibration and
based on the correlation with the
hydrometrist's measurements
A surface site with sampler or directly
with the semi-sunk bottle, in every
vertical section

Average concentration on the vertical
section
C
mv
= 1,2 C
sup


A point at the vertical at 0,5 or 0,6 in
depth
Average concentration on the vertical
section
C
mv
= C
0,5


or = C
0,6





Punctual








Punctual
Two points at the vertical at 0,2 and 0,8
in depth
Average concentration on the vertical
section
2,08,0
8
5
8
3
CCC
mv
+=



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Three points at the vertical at 0,2, 0,5 and
0,8 in depth
Average concentration on the vertical
section

3
8,05,02,0
CCC
C
mv
+
+
=

or,
4
.2
8,05,02,0
CCC
C
mv
+
+
=

Several points on the vertical section, at
0,1, 0,3, 0,5, 0,7 and 0,9 (if concentration
values vary too much, the average should
be computed by weighing it with depths
among the measured points)
Average concentration on the vertical
section

n
C
C
i
mv

=

Using different transit rates for the
sampler at each vertical section.

Concentration is the average at the
vertical section.
The suspended sediment discharge
should be determined by multiplying
segments for the partial discharge,
where the total suspended discharge is
equivalent to the sum of partial values
and the average concentration for the
section is equivalent to the total
suspended discharge, divided by the
total net discharge.
Method of Equal Increment of Width,
(IIL), using the same transit rate for all
verticals and the same bill along the
entire cross-section
All vertical sub-samplings are gathered
(from 10 to 20) and a single analysis is
performed, thus providing the average
concentration and, if required, a single
average granulometric curve for the
section




Vertical
integration
Method of Equal Increment of Discharge
(IID), performing the sampling at the
middle point of equivalent discharge
increments along the whole cross-
section, where the bill may be changed
and one may use different transit rates for
each vertical, however sampling equal
volumes of the mixture water-sediment
All vertical sub-sampling are gathered
(from 5 to 15) and a single analysis is
performed, thus providing the average
concentration and, if required, a single
average granulometric curve for the
section

For those sampling methods, the bottle should never be totally full; it is
recommended to collect no more than 400ml for bottles with total capacity of 500ml.
The samplers using that kind of bottle cannot collect samplings in very deep waters,
being the DH-48 for depths up to 2,0m, and the DH-59 and D-49 for depths up to
4,50m.

For the vertical integration process, the sampler is submerged and moved in a
steady velocity, from surface to the bottom, then returning to surface. Each up or down
movement happens in a constant velocity, but not necessarily in equal velocities. The
sampler transit rate shall not be higher than a given value v
t
which must be computed
due to the constant of the bill used and the average velocity at the vertical (equations 7.1

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and 7.2). The minimum sampling time is computed by using a course equivalent to
twice the depth (equation 7.3).


1/8” Bill:

mmáxt
vv.2,0
,
=

(7.1)

3/16” and 1/4" Bills:
mt
vv.4,0
max,
=
(7.2)

Minimum sampling time:
máxt
v
p
t
,
min
.2
=
(7.3)

The IIL and IID methods are regarded as the best ones, since they allow for
determining the average concentration and average granulometry upon one single
analysis (Table 7.4), besides facilitating sediment discharge computations. The total
volume of the sub-sampling to be collected should allow the analysis following the
restriction criterion for each process available at laboratory.

It is usual to collect enough suspended material - from 10 to 15% of
measurements performed - with mixture of water-sediment, in order to allow the
granulometric analysis of that material (ICOLD, 1989).

Bed material sampling is performed at some intermediary positions among the
same verticals, as for the IIL and IID methods, using from 5 to 10 sub-samplings. The
total weight for sub-samplings should be equivalent to 2kg, or a little higher, in order to
allow the successful analysis by the laboratorist.


7.4.2 Laboratory Analysis

The sediment analysis for suspended material is performed in laboratories like
the Chemistry ones, while the bed material analysis is performed in laboratories such as
the Soil Mechanics ones. Therefore, the laboratorist must combine the procedures by
using the equipment suitable for each method.

The sediment load analysis, despite being performed with the equipment used
for Chemistry – such as analytical balance, becher, pipette, capsules, test tubes and so
on – is not a chemical analysis; rather it is a sedimentometric analysis. It means that all
samplings, when delivered at the laboratory, shall be analyzed and must not be divided
or reduced for a sub-sampling for a supposed homogenization. Particles contained in a
mixture water-sediment report several distinct densities and sizes, like colloids, argyle,
silts and even sand, as well as different mineralogy (quartz, iron, calcium, etc.);
therefore it is impossible to have them homogenized. All sediments received by the
laboratory must be analyzed.

The different usual analysis and methods, or equipment, may be viewed in Table
7.5. For a better understanding on the methods, it is useful to see Guy (1969).

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Table 7.5 – Methods and equipment for sedimentometric analysis

Filtration method
Evaporation method
Settling tube method

Total concentration analysis

Settling tube method
Pipetting
Densimeter



Suspended sediment
sampling

Granulometric analysis


Siftering
Densimeter
Pippeting
Visual accumulation tube


Bed load sampling


Granulometric analysis
Settling tube method


Each method has its own restriction, thus demanding suitable quantities of
sediment contained in the sampling. The filtration method is used for low-concentration
samplings – lower than 200mg/l – and small volume, in order to avoid obstructing the
filter. The evaporation method is used for higher-concentration and higher-volume
samplings. Both methods require the reduction of the sampling volume, by decantation
or water-bath, in such a way as to hold all particles along the process. According to
WMO (WMO, 1981), the required volumes for an accurate analysis are those presented
in Table 7.6.

Usually, concentration is determined as the ratio between the dry sediment
weight and the volume of the water-sediment mixture, in mg/l, or the ratio between the
dry sediment weight and the water-sediment mixture weight, in ppm (= mg/kg =
mg/1.000.000mg). The ppm values may be used as mg/l up to 16.000ppm with no
density adjustment. Data may be presented with three significant algorisms up to 999
(0,32ppm, 3,21ppm, 32,1ppm, 321ppm).

Table 7.6 – Volumes of sampling required for suspended sediments concentration
analysis (WMO, 1981)

Expected concentration of sediment
load
Sampling volume
(g/m
3
, mg/l, ppm) (liters)
> 100 1
50 to 100 2
20 to 30 5
< 20 10

Granulometric analyses for suspended material are performed with small
quantity of sediments, using the principle of the particles water dropping velocity. Each
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method, thought based on Stokes law, has its restriction for reaching the desired
accuracy when surveying the percentage of sediments for a given granulometry in liquid
means. Table 7.7 presents the major restrictions to be obeyed.


Table 7.7 – Amplitude of several granulometric analysis methods for
fine material using water-dropping velocity
(SUBCOMMITTEE ON SEDIMENTATION, 1943)

Method Approximate limit of the
particle diameter
Approximate limit in
concentration
(mm) (ppm)
Settling tube 0,001 to 1,0 300 to 10.000
Decantation 0,001 to 0,0625 1.250 to 19.000
Pippeting 0,001 to 0,0625 3.000 to 10.000
Hydrometer (densimeter) 0,001 to 0,0625 60.000 to 116.000
Siltmeter (TAV, visual accumulation tube) 0,0625 to 2,0 125 to 25.000


Bed load analysis is performed mainly through siftering, using the Tyler series
of sifters. For small quantities of sandy material, the TAV method can be used. If the
remainder for the last sifter – the finer material – is equivalent to 5% of the material, or
higher, it is necessary to complement the analysis by defining the curve lower segment.
One of the methods presented in Table 7.7 should be used for that. The analysis
procedures may be consulted in Normas e Recomendações Hidrológicas – Anexo III,
Sedimentometria (DNAEE, 1970).


7.4.3 Sediment discharge computation

Once all field and laboratory data are available, sediment discharge
computations may be performed. The required data are obtained from net discharge
measurement and sediments sampling, sediments concentration, granulometric
distribution and others. For calculating the bed discharge by using formulas, one must
obtain some additional values, such as water temperature, energy line slope, shearing
tension, kinematic viscosity, particle-dropping velocity; usually, those last ones are
included in the computation programs available.

The maximum error expected for sediment discharge determinations is 10%,
even including the bed discharge collection, which is very inaccurate. Suspended
discharge is usually the prevailing part of the total discharge, representing more than
90% for most measurements. However, the bed discharge may report values from 10 to
150% in relation to the suspended discharge, according to ICOLD (1989). On the other
hand, the sedimentometric data consistency analysis is very hard, due to the several
processes required for determining it, mainly, the phenomenon complexity. Therefore, it
is essential to try to eliminate errors during the measurement and for the laboratory
work. Consequently, the sediment discharge measurement shall be performed as
accurately as possible in the field, by a successful hydrometologist, using the suitable
Reservoir Sedimentation Assessment Guideline
equipment, and the analysis should be performed by an experienced chemistry
expert/technician. That shall allow repeating the computations, if required. If field and
laboratory services report errors, the adjustment of the sediment discharge value
becomes impossible.

Suspended sediment discharge computation
– In both direct and indirect
measurement for suspended discharge, the concentration value is obtained. The
computation is performed by multiplying the net discharge by the concentration.
Usually, the Q
ss
value is presented in t/day, and it requires a unity transformation factor.
For the average concentration obtained through ILL and IID sampling methods:

Q
ss
= 0,0864.Q.c
s
(7.4)

where,
Q
ss
= suspended sediment discharge, in t/day
Q = net discharge, in m
3
/s
c
s
= concentration, in mg/l

If c
s
is a high value, presented in kg/m
3
, the equation is:

Q
ss
= 86,4.Q.c
s
(7.5)

If the samplings are for several verticals separately analyzed, the following
equation is used, with the due constant on unity transformation:

Q
ss
=

Σ
q
ss
=
Σ
q.
Δ
l.c
sv
(7.6)

where
q
ss
= suspended discharge for width unity corresponding to the segment being
considered
q = partial net discharge for width unity corresponding to the segment being
considered

Δ
l = distance referenced to q
ss
and q
c
sv
= sediment concentration at vertical.

The average concentration on the vertical is equivalent to:

Q
Q
q
q
c
ssss
s
=


=
(7.7)


Computation of sediment discharge and bed load –
The direct measurement
determines the dry sediment and the bed discharge is calculated as:


(7.8)


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where

Q
b

= sediment discharge, in
t/day
q
b

= sediment discharge at a given point, in
kg/(s.m)
l
= distance between the measured points, in
m

E
r
=
equipment sampling efficiency.
For that kind of measurement, the formula must take into consideration the
equipment trap efficiency, the value of which is determined in laboratory.

For indirect measurement, the sediment discharge computation is through
formulas
. Stevens & Yang (1989) have studied the several formulas available and
selected 13 as the most recommendable (Table 7.8). Besides that, they have prepared
computer programs that are available in the above-mentioned publication.

Table 7.8 – Summary of the main formulas for calculating sediment discharge and bed
material, as presented by Stevens & Yang (1989)


Formula author

Year
Entrainment
discharge (B) or
bed material
discharge (BM)
Kind of
formula
(1)
Kind of
sediment
(2)
Granulometry
Ackers & White (*) 1973 BM D S S, G
Colby 1964 BM D S S
Einstein (bed load) 1950 B P M S, G
Einstein (bed material) 1950 BM P M S
Engelund & Hansen (*) 1967 BM D S S
Kalinske 1947 B D M S
Laursen 1958 BM D M S
Meyer-Peter & Muller (*) 1948 B D S S, G
Rottner 1959 B D S S
Schoklitsch (*) 1934 B D M S, G
Toffaleti 1968 BM D M S
Yang (sand) (*) 1973 BM D O S
Yang (gravel) (*) 1984 BM D O G
(1) Deterministic (D) or Probabilistic (P)
(2) Granulometric sand fraction (S), composition or mixture (M) or optional (O)
(3) Sand (S) or gravel (G)
(*) Regarded as the most reliable by Stevens & Yang




Total sediment discharge computation –
The approximate total sediment discharge
may be obtained by summing up the suspended discharge and the bed material
discharge. Nevertheless, that procedure is questionable due to the inaccuracy reported
by non-sampled zones.

The total sediment discharge may be obtained through computation processes of
Einstein’s modified method
and by
Colby’s simplified method.
Otto Pfafstetter
converted the first method into the metric system, where the abacus depends on units,
adjusted by Carvalho (1994). Stevens (1979) prepared a computer program for using
Reservoir Sedimentation Assessment Guideline

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that method. Carvalho (1981) has also converted the second method into the metric
system.

If there are some measurements using Einstein’s modified method – which is
very hard to do – such values may be used for correcting Colby’s simplified method or
for obtaining correlations for correcting the total discharge (Yuqian, 1989).

Establishing the sediment discharge value
– Considering that the only data available
are for suspended sediments only, the calculator tries to establish the value of the non-
measured discharge, in order to have the total discharge required by sedimentation
assessment. In Brazil, it is usual to determine such value as 10%, while there are
countries where utilities establish up to 30% of the suspended discharge. ICOLD (1989)
presents a suggestion for selecting the method for obtaining sediment discharge, in
relation to bed material and sand percentages existing in the suspended sampling (Table
7.9). The table shows the complexity of establishing just the %.

Table 7.9 – Guide for correcting the sediment discharge and for orienting the method
for obtaining such discharge (ICOLD, 1989)


Condition
Concentration of
sediment load
(mg/l)

Bed material

Granulometry of
bed material
% of bed load in
relation to the
suspended load
1 (1) < 1000 Sand 20 to 50% of sand 25 to 150
2 (1) 1000 to 7500 Sand 20 to 50% of sand 10 to 35
3 > 7500 Sand 20 to 50% of sand 5
4 (2) Any concentration Compacted argyle,
gravel, rolled
stones or stones
Any amount up to
25% of sand

5 to 15
5 Any concentration Argyle and silt No sand < 2
(1) Special sampling for computations through Einstein’s modified method are required for this
condition
(2) A program for direct measurement using a Helley-Smith sampler, or other measurer, or even using the
formulas for thick material


7.5 Data processing

Data processing is intended to obtain average discharge and average runoff load
either annual or for a period, as well as to obtain representative parameters for the
phenomenon.

The first step is an adequate revision of both field and laboratory documentation
and, following, the tabulation of measurements performed. The table shall have the
following items: number of measurement, date, values for average level, section width,
area, average depth, average speed, net discharge, concentration of dissolved solid
material, sediments concentration, suspended sediment discharge, entrainment bed load
or bed material discharge, total sediment discharge and method for obtaining such data.
Granulometric curves shall be always available for further use. One can also make a
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table with the percentage of some diameters and characteristic values for usual bed
material (D
10
, D
35
, D
50
, D
65
and D
90
).

7.5.1 Continuous, hourly and daily measurements

Continuous, hourly and daily measurements shall also be tabulated and the
sediment discharge must be computed. The preliminary work consists of calibrating
concentration values, as based on the correlation with the hydrometrist’s data. If a value
is not available because it was not measured, than a graph with a discharge hydrogram
is prepared, as well as the respective plotting on concentration or suspended discharge,
for obtaining the lacking values. Those values may also be obtained from the rating
curve sediments equation prepared with the values measured.

After the daily tabulation, it is possible to obtain monthly and annual tabulation,
containing the summaries of average net and load discharges. Following, an annual
summary is prepared, presenting total annual transport (runoff load
D
s
), annual average
transport (average annual sediment discharge
Q
s
), sediments contribution (production of
sediments
P
s
) and other values. Table 7.10 presents an example of semi-annual bulletin
of computations performed by CEMIG. The average for average annual values will be
used for the sedimentation assessment computations.
Reservoir Sedimentation Assessment Guideline
Table 7.10 – Semi-annual bulleting on suspended discharge - São Francisco River in Porto das
Andorinhas


Transporte total anual = annual total transportation
Máximo transporte diário = maximum daily transportation
Mínimo transporte diário = minimum daily transportation
Máxima concentração anual = maximum annual concentration
Mínima concentração anual = minimum annual concentration
Sumário anual = annual summary
Deflúvio total anual = annual total runoff
Transporte médio anual = annual average transportation
Escoamento específico = specific runoff
Contribuição de sedimento = sediment contribution

7.5.2 Eventual measurements


Data processing for eventual measurements is performed by preparing the
sediment transportation rating curve using either concentration or sediment discharge in
connection with the net discharge. A common practice is to work with the bi-
logarithmic sheet, such as the example in Figure 7.12. The curves may be obtained
through visual process or through the minimum squares method, as used for Excel. One
should be very careful when using the computer, mainly when there is a data
concentration that may come to influence the curve direction. It is a common practice to

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assimilate one or more straight lines and get the respective exponential equations,
similar to the one presented below. For obtaining more than one line, the sediment
discharge or the net discharge shall be ascending ordered.

(7.9)
n
s
QaQ
.
=



Figure 7.12 – Sediments rating curve for Manso River in Porto de Cima –
measurements for the period 1977/1981 (Carvalho, 1994)

Descarga líquida = net discharge
Descarga sólida total = total sediment discharge

The respective loads, as well as the average values and required parameters may
be obtained through the rating curve equations for a given period. When a series of
discharges for several years is available, it is used for obtaining the sediment discharge
series, by accepting the equation as valid for the period (see examples in Tables 7.11
and 7.12).

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Table 7.11 –Manso River in Porto de Cima

Série de vazões anuais = annual runoff series
Ano = Year

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Table 7.12 – Manso River in Porto de Cima



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Descarga sólida total media mensal = Total monthly average sediment discharge

7.5.3 Data regionalization


If there are data for at least two gaging stations along the stream, average values
for each gaging station shall be computed, a line concerning the drainage area is drawn
and the runoff value is obtained by using the gaging station drainage area (see Figure
7.13 of the example for São Francisco River and Rio das Velhas, according to Carvalho,
1994).


Figure 7.13 –São Francisco Basin – Sediments yield lines (Carvalho, 1994)

Produção de sedimento = sediment yield
Área = area


The regionalization for data concerning the same basin may also be performed
through the analysis of local features in relation to the basin features (see Figure 7.14
where the sediment discharge value in UHE Mascarenhas, at the Rio Doce was
searched). The regionalization of sedimentometric data is tricky, and must be carefully
performed; therefore, it is not recommended.

Scientific works like global curves shall not be used for studies, rather, they are
used just for curiosity purposes.

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Figure 7.14 – Example of sedimentometric data regionalization – Relation between
discharges and load discharges in basins neighboring Rio Doce basin –
Measurements from 1960 to 1971 (Carvalho, 1994)

Rio = river
Posto = station
Período = period

For data regionalization concerning different basins, one should try to verify the
curves that may be obtained and use the one whose features are compatible with the
gaging station position. In the example for Figure 7.15, the higher curve was used for
obtaining the sediment production in dam construction sites in Doradas River. Note that
the curve reports a P
s
value corresponding to the gaging station at that stream (point 6).

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Figure 7.15 –Regionalization with data from several basins (Carvalho, 1994)

Produção de sedimento sólido total = total sediment yield
Rio = river
Local = site


8. TRAP EFFICIENCY IN RESERVOIRS

The sediments trap efficiency value in reservoirs may be obtained based on
systematic measurements of tributary load discharges and discharges downstream. For
surveys previously to damming, the curves obtained from existing reservoirs surveys are
used. For medium and large reservoirs, the Brune curve is used; for small reservoirs the
Churchill curve is used.

8.1 Medium and large reservoir cases

The Brune curve presents at the ordinate axis the value for
trap efficiency
in the
reservoir, either in percentage or in fraction and, at the abscissa axis, the
affluence
capacity
, corresponding to the reservoir volume divided by the annual average tributary

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runoff. For that, it is used the reservoir volume corresponding to the normal maximum
water level. The Brune curve may be obtained in Carvalho (1994), Morris/Fan (1997),
Strand (1974) or Vanoni (1977).


Figure 8.1 – Curves on reservoirs trap efficiency, according to Brune (Vanoni, 1977
and others)

Sedimentos retidos = trapped sediments
Relação capacidade/volume afluente anual = ratio capacity/annual tributary volume
Sedimento grosso = coarse
Sedimento fino = fine sediment
Curva média = average curve

8.2 Small reservoirs

The Churchill curve is presented in three versions, and requires attention when
being used. In any of them, the ordinate axis represents the percentage of tributary
sediment passing downstream. Therefore, the trap efficiency is obtained by difference
and shall be expressed in fraction for computation purposes.

The Churchill curve presented by Morris/Fan (1997), Strand (1974) or Vanoni
(1977) is illustrated in Figure 8.2. In it, the abscissa axis corresponds to the value of the
Reservoir Sedimentation Index IS
that is equivalent to the
Retention period
divided by
the
Reservoir average velocity.
Those parameters are computed as follows:



Retention period
= reservoir volume (ft
3
) divided by the daily average discharge
during the survey period (ft
3
/s);


Average velocity in the reservoir
= average daily discharge (ft
3
/s) divided by the
average cross-section area (ft
2
). The average cross-section area may be determined
by dividing the reservoir volume by its length (ft).

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1
10
100
1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09
Sedimentation Index
Sedimento local
Sedimento fino descarregado de
reservatório a montante

Figure 8.2 – Curve on the trap efficiency, according to Churchill, version
presented in Vanoni, 1977

Sedimento local = local sediment
Sedimento fino descarregado de reservatório a montante = fine sediment discharged by upstream reservoir

The reservoir volume corresponds to the capacity at the average operation level.
Usually, small reservoirs operate at run-of-river level, and the volume of that level is the
one to be used. Deriving from the above information, one can reach the following
expression for the
Sedimentation Level
used for the Churchill curve version presented in
Figure 8.2:
IS
= Retention Period
(8.1)
Average Speed

where:

IS
= Reservoir sedimentation index;
V
res
=
Reservoir volume at the average operation level (ft
3
);
Q
= Daily tributary discharge average during the survey period (ft
3
/s);
L
= Reservoir length (ft).

Another version of the Churchill curve, presented by ICOLD [1989], has on its
ordinate axis, at the upper corner of the illustration, the Churchill sedimentation index
multiplied by the gravity acceleration
g ,
where:

g
.
.LQ
V
IS.g
2
2
res
=
(8.2)

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Figure 8.3 – Curve on sediments trap efficiency, according to Churchill, version
presented in ICOLD (1989), where:

1: Annual Average Tributary Reservoir Capacity /
Flow; 2: Sediment Trapped, in %; 3: SIxg – Sedimentation Index x g (gravity
acceleration constant); 4: Average Brune Curve and; 5: Churchill Curve

A third view on Churchill curve, modified by Roberts, is presented by Annandale
(1987), to be used in metric system. In the graph (Figure 8.4), the ordinate axis is
expressed as in Figure 8.2; the difference is according to the curve presentation.

1
10
100
1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E+11
Sedimentation Index - IS
Sedimento local
Sedimento fino descarregado de um
reservatório a montante

Figure 8.4 –Sediment trap in the reservoir, according to Churchill (Annandale, 1987)

Sedimento local = local sediment
Sedimento fino descarregado de reservatório a montante = fine sediment discharged by upstream reservoir


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9. SPECIFIC WEIGHT OF DEPOSITS


The runoff load is usually computed in terms of weight by time, as t/year, and
shall be converted into equivalent volume, as m
3
/year, by knowing the specific weight.
Lara and Pemberton realized - by performing researches with samplings from existing
reservoirs – that the specific weight for sediment deposits may be computed according