RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE · RESERVOIR SEDIMENTATION . ASSESSMENT GUIDELINE . Newton de Oliveira Carvalho . Naziano Pantoja Filizola Júnior . Paulo Marcos Coutinho

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 106m3 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.


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


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 xExQxEDS

γγ365

== (6.1)

S

VT res= (6.2)

where: S = volume of sediment trapped in the reservoir (m3/year); Dst = annual average for total bed load inflowing the reservoir (t/year); Er = trap efficiency for the sediment inflowing the reservoir (decimal); γap = deposits specific weight (t/m3); Qst = total average sediment discharge inflowing the reservoir (t/day); T = sedimentation time for a given volume (years); Vres = 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 m3; • Volume of minimum normal water level (dead storage), in m3; • Volume of intake sill, in m3; • Natural discharges series;


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• Long-term average discharge, in m2/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/(km2.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/(mi2.year) (t/(km2.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/(km2.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)



18

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 entrainment 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



19

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


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 90o 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


20


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


21


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


22


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


23


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 Cmv = 1,2 Csup

A point at the vertical at 0,5 or 0,6 in depth

Average concentration on the vertical section Cmv = C0,5 or = C0,6

Punctual Punctual

Two points at the vertical at 0,2 and 0,8 in depth

Average concentration on the vertical section

2,08,0 85

83 CCCmv +=


24


Three points at the vertical at 0,2, 0,5 and 0,8 in depth


38,05,02,0 CCC

Cmv

++=

or,

4.2 8,05,02,0 CCC

Cmv

++=

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)


nC

C imv

∑=

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 vt which must be computed due to the constant of the bill used and the average velocity at the vertical (equations 7.1


25


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 .4,0max, = (7.2)

Minimum sampling time: máxtvpt

,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).


26



27

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/m3, 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



28

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


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 Qss value is presented in t/day, and it requires a unity transformation factor. For the average concentration obtained through ILL and IID sampling methods: Qss = 0,0864.Q.cs (7.4) where, Qss = suspended sediment discharge, in t/day Q = net discharge, in m3/s cs = concentration, in mg/l If cs is a high value, presented in kg/m3, the equation is: Qss = 86,4.Q.cs (7.5) If the samplings are for several verticals separately analyzed, the following equation is used, with the due constant on unity transformation: Qss = Σ qss = Σq.Δl.csv (7.6) where qss = suspended discharge for width unity corresponding to the segment being considered q = partial net discharge for width unity corresponding to the segment being considered Δl = distance referenced to qss and q csv = sediment concentration at vertical. The average concentration on the vertical is equivalent to:

Q

Qq

qc 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) ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department

29



30

where Qb = sediment discharge, in t/day qb = sediment discharge at a given point, in kg/(s.m) l = distance between the measured points, in m Er = 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



31

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



32

table with the percentage of some diameters and characteristic values for usual bed material (D10, D35, D50, D65 and D90). 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 Ds), annual average transport (average annual sediment discharge Qs), sediments contribution (production of sediments Ps) 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.


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


33


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).


34


Table 7.11 –Manso River in Porto de Cima

Série de vazões anuais = annual runoff series Ano = Year


35


Table 7.12 – Manso River in Porto de Cima


36


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.


37


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 Ps value corresponding to the gaging station at that stream (point 6).


38


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 ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department

39


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 (ft3) divided by the daily average discharge

during the survey period (ft3/s); • Average velocity in the reservoir = average daily discharge (ft3/s) divided by the

average cross-section area (ft2). The average cross-section area may be determined by dividing the reservoir volume by its length (ft).


40


1

10

100

1.0E+04 1.0E+05 1.0E +06 1.0E +07 1.0E +08 1 .0E +09

S e d im en ta tion Index

S ed im ento loca l

S ed im ento fino desca rregado dereserva tó rio a m ontan te

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; Vres = Reservoir volume at the average operation level (ft3); Q = Daily tributary discharge average during the survey period (ft3/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

VIS.g 2

2res= (8.2)


41


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+11Sedimentation Index - IS

Sedimento local

Sedimento fino descarregado de umreservatório 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 ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department

42


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 m3/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 to the kind of operation for the specific reservoir, the level of sediment compaction and granulometry, which are the most influent factors for deposits consolidation. Less influent facts may be mentioned, such as the density of the reservoir’s stream sediment, the slope for the tributary stream thalweg and the vegetation effect on the reservoir headwaters area. 9.1 Computed

The computation for initial specific weight, and after compaction, for a given

time, is performed by using the following equations; the parcel factors for such equations are obtained based on the kind of operation for the reservoir (Table 9.1).

ssmmcci PWPWPW ... ++=γ

TKiT log.+= γγ for specific layer

or

( ) ⎥⎦⎤

⎢⎣⎡ −

−+= 1

1.4343,0 LnT

TTKiT γγ for total deposit

ssmmcc PKPKPKK ... ++= where: γi = initial specific weight (t/m3); Wc , Wm , Ws = coefficient of compaction for argyle, silt and sand, respectively, obtained according to the kind of reservoir operation (Tables 9.1 and 9.2); Pc, Pm, Ps = fractions of quantities of argyle, silt and sand contained in the tributary sediment; γT = average specific weight in T years (t/m3); T = settled sediment compaction time (years); K = constant depended on sediment granulometry and based on the kind of reservoir operation (Table 9.2); Ln = Neperian logarithm.

The values for coefficients γi, γT and K, as presented by Strand, were adjusted to be used in the metric system (Carvalho, 1994).


43



44

Table 9.1 – Kind of reservoir operation (adapted from Strand, 1974)

Kind Reservoir Operation 1 Sediment always, or almost always, submerged 2 Little to medium reservoir depletion 3 Reservoir reporting significant level variations 4 Reservoir usually empty

Table 9.2 - W and K constants for calculating the specific weight in relation to the kind of reservoir operation, to be used in the metric system (adapted from Strand, 1974)

Kind Argyle Silt Sand Wc Kc Wm Km Ws

1 0,416 0,2563 1,121 0,0913 1,554 2 0,561 0,1346 1,137 0,0288 1,554 3 0,641 0,0000 1,153 0,0000 1,554 4 0,961 0,0000 1,169 0,0000 1,554

Note: K constants for sand are null for any kind of operation.

To use the equations and their respective tables, it is necessary to obtain the average percentages of argyle, silt and sand contained in both suspended and bed sediments, as well as percentages for average suspended sediment discharge and the average bed sediment discharge. Following, the required composition should be performed in order to know the percentages of argyle, silt and sand (coarse) referring to total sediment discharge. If, for example, the average sediment discharge computation indicated 85% for suspended discharge and 15% for bed discharge, the average granulometry among the several analysis of the suspended sediment sampling for the observation period resulted in 45% of argyle, 50% of silt and 5% of sand, and the analysis for the bed resulted in 3% of argyle, 8% of silt and 89% of sand, therefore the computations for obtaining Pc, Pm and Ps may be performed as presented in Table 9.3.

Table 9.3 – Examples of computations for average percentage of argyle, silt and sand for use in Lara and Pemberton formulas, to obtain the specific weight in reservoirs

Argyle

% Silt %

Sand %

Qss %

Qsa %

Pc %

Pm %

Ps %

Suspended sediment 45 50 5 85 - 0,45x85= 38,25

0,50x85= 42,50

0,05x85= 4,25

Bed sediment 3 8 89 - 15 0,03x15= 0,45

0,08x15= 1,20

0,89x15= 13,35

Total 38,7 43,7 17,6 Once computed the total percentages for Pc, Pm and Ps, the trap efficiency for the reservoir must be verified. The percentage of fine sediments flowing through the ducts must be subtracted from the fine sediment, in order to calculate the specific weight.



45

9.2 Measured There are two processes for measuring the specific weight, namely direct and indirect. For the indirect process, or in situ, it is used the nuclear measurer, density radioactive-type. For the indirect process, it is used to collect a non-deformed sampling, by using equipment like gravity or piston-core; to measure the sampling volume, take it to the stove and determine the dry weight. Such measurements shall be performed along different positions in the reservoir, in order to verify the variation of specific weight and obtain the average value. 9.3 Estimate According to the equations, one may assess the variation of initial specific weight as follows: - If sediment is exclusively argyle, then γi shall range from 0,42 to 0,96; - If sediment is exclusively silt, then γi shall range from 1,12 to 1,17; - If sediment is exclusively sand, then γi shall be equivalent to 1,55; - If there is a composition reporting similar portions of argyle, silt and sand, there is a

variation from 1,02 to 1,22.

In small reservoirs, sand is the main material settled; therefore, the initial specific weight is established as ranging from 1,4 to 1,5 t/m3; medium-size reservoirs may report a composition with specific weight ranging from 1,2 to 1,4 t/m3, while for large reservoirs, where just a few quantity of fine sediments passes through the ducts and spillways, that amount may range from 1,1 to 1,3 t/m3. Obviously, knowing the basin and quality of existing sediments, as well, may allow for the technician to perform better assessments. For a more accurate assessment on apparent weight, one may use those values presented by Zhide (1998), Tables 9.4 and 9.5.

Table 9.4 – Average initial specific weight of deposits in reservoirs, in t/m3 (Zhide, 1998)

Kind of reservoir operation Argyle

( < 0,004mm ) Silt

(0,004-0,062mm) Sand

(0,062-2, 0mm) Sediment always, or almost always submerged 0,416 1,120 1,550 Little to medium depletion of the reservoir 0,561 1,140 1,550 Reservoir reporting significant level variations 0,641 1,150 1,550 Reservoir usually empty 0,961 1,170 1,550



46

Table 9.5 – Long-term average specific weight of deposits in reservoirs, in t/m3 (Zhide, 1998)

Sediment Granulometry

(mm) Specific weight

(t/m3) Argyle < 0,005 0,8 a 1,2 Silt 0,005 a 0,05 1,0 a 1,3 Medium and thin sand 0,01 a 0,5 1,3 a 1,5 Thick sand and thin gravel 0,5 a 1,0 1,4 a 1,8 Medium gravel > 1,0 1,7 a 2,1



47

10. FORECAST OF A RESERVOIR SEDIMENTATION 10.1 Sedimentation assessment methods The forecast methods for a reservoir sedimentation evaluation are tasks related to the intended objective. During the inventory stage, the main objective is to estimate the total sedimentation period, as well as the reservoir useful life. If there is any indication of serious problems during the useful life, surveys may be deepened in order to improve the economic estimates for the arrangements. During feasibility and basic project stages, the studies are a little harder; they seek to review the sedimentation effects and general solutions for controlling sediments (preventive control) as well. During the operation stage, the goal is to check on sedimentation through systematic surveys, sedimentometric monitoring, surveillance on basin changes and other studies, always aiming at the possibility of preventive control and, whenever that control is not possible, the most suitable corrective practices.

An evaluation comprising only volumes and sedimentation time may be performed by using equations 6.1 and 6.2. However, it is not enough for characterizing the sedimentation; therefore, it is necessary to perform more appropriate studies taking into consideration forecast, as indicated in item 4, and the stage of study. The tributary sediment inflowing the reservoir may become settled in or leave the dam. The deposits formed may be permanent or, in some cases, may move along the reservoir. During flood occasions some sediments may be displaced and cross the dam.

Usually, the fine sediment, reporting granulometry lower than 0,062mm, may move in suspension along the reservoir, thus forming density streams. For large reservoirs, part of that fine sediment may become settled closer to the dam, while part of it may run downstream. The coarse, with granulometry higher than 0,062mm, usually becomes settled in the reservoir and builds up the delta. As deposits are formed, coarses enter the reservoir, and become settled upstream, thus increasing backwater area. The process is a complex one, and its studies are properly performed by sediments hydraulics formulas. The study may be carried out by using Saint Venant equations for net runoff, or by using some modified sediments transportation formulas (Bruk, 1985). Currently, there are several methods for forecasting sedimentation and deposits distribution. The method most frequently used is the HEC-6, which allows for different types of studies, and is available in free-use software.

Simpler methods, semi-empirical, based on reservoirs systematic surveys are, for

example, the Borland & Miller’s empirical method for reducing the area, and the incremental area method. Both of them have been disseminated in several books (Strand, 1974; Vanoni, 1977; Annandale, 1987 and Morris/Fan, 1997). 10.2 Assessment of total sedimentation, dead storage and useful life


This evaluation may be performed through equations 6.1 and 6.2. Using as example Itaipu (Paraná River) and Itiquira (Itiquira River) reservoir, Table 10.1 displays computation data and results.

ap

rst

ap

rst xExQxEDS

γγ365

== and S

VT res=

where: S = volume of sediment trapped in the reservoir (m3/year); Dst = total annual average sediment tributary to the reservoir (t/year); Er = tributary trap efficiency in the reservoir (% and fraction); γap = average specific weight for deposits (t/m3); Qst = average total tributary sediment discharge to the reservoir (t/day); T = sedimentation time for a given volume (years); Vres = reservoir volume, total or dead storage (m³).

Table 10.1 – Assessment of reservoirs sedimentation of UHE's of Itaipu and Itiquira

(see Carvalho, 1994 and Carvalho et al, 2000)


48

Data Itaipu Reservoir (ITAIPU BINATIONAL)

Itiquira Reservoir (ITICON S.A.)

Normal maximum water level 220,00 m 412,00 m Usual minimum water level 197,00 m 411,50 m Water level at the intake sill 176,00 m Max. Normal Water Level Volume 29 x 109 m3 4,8 x 106 m3 Min. Normal Water Level Volume 10 x 109 m3 4,2 x 106 m3 Dead storage (at sill intake) 4,7 x 109 m3 3,9 x 106 m3 Long-term average discharge Qmlt 9.729 m3/s 72,9 m3/s Reservoir length 170 km 5.600 m Equations for sediments transportation

9034,8.3110704,1 Qxst

Q −=

for Q < 10000 m3/s

5146,2.610121,6 Qxst

Q −=

fpr Q > 10000 m3/s

(period 1988/1989)

Qst = 46,888 x Q0,9472

(period 1979/1982)

Annual total sediment discharge average Qst (obtained through the equation and discharges series)

71.063 t/day (period 1931/1992)

2.715 t/day (period 1931/1997)

Annual total solid runoff average Dst ( = 365 x Qst )

30.788.845 t/year 990.775 t/year

Obtaining trap efficiency Er

Brune Curve:

Affluence capacity = 0,098

Er = 86%

According to Roberts (Annandale, 1987), Churchill

curve

106,756002)9,72(

2)6108,4(8,9. x

x

xxgIS ==


Er = 45% (adopted 50%) Specific weight γap According to Lara and

Pemberton 1,13 t/m3

According to Lara and Pemberton

1,5 t m3 Annual average sediment volume (computed based on the equation for sediment transportation and discharges series)

23,37 x 106 m3/year

330.325 m3/year

Sedimentation time for total volume, in max. normal water level

1240 years 14 years

Sedimentation time for total volume, in min. normal water level

430 years 12,7 years

Sedimentation time for a volume equivalent to the volume at the intake sill (reservoir useful life)

200 years 12 years

Sedimentation time for total volume, considering the increase on sediment transportation since when sediment discharge measurements took place (1982)

------

15 months

10.3 Assessment of a reservoir useful life Under a sedimentological perspective, a reservoir useful life is considered as when sediments reach the intake sill and starts disturbing or hindering the operation.

For a computation more accurate than the one presented in Table 10.1, the sediment distribution in the reservoir and the increase on either erosion index or sediment transportation must be taken into consideration. One may calculate the height of the sediment deposit at the dam’s base, or at the intake position for different times, and depict an assessment graph in order to get the forecast on how long will it take for the deposits to reach the sill. The methods for performing such computation were presented in item 10.1. 10.4 Sediments distribution in reservoirs As presented in Chapter 3 (see Figure 3.1), the sediments deposits in a reservoir are irregularly formed, occurring the formation of a delta in the backwater area that expands towards the lake along time and in face of greater bed sediment. Fine sediments become settled inner and closer to the dam. The assessment of such distribution may be performed through several methods, as mentioned in item 10.1. 10.5 Increase on basin erosion

The increase on sediment transportation along a stream is a consequence of an increase on the basin’s erosion. Having data on annual average sediment discharge for ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department

49


several years, and their respective average discharges, one can calculate the rate of increase for sediment transportation, by using the mass curve. An illustrative example is presented, taken from the work of Carvalho/Guilhon/Trindade (2000) on reservoir sedimentation assessment for Itiquira, in Itiquira River, State of Mato Grosso.

The years near to 1980 presented major transformations for the region, due to the

expansion of agricultural area, which caused the intensification of erosion. By that time, huge formations of erosion gullies took place at São Gabriel do Oeste, located in the neighborhood of Taquari River basin. It led International Organizations to cooperate in the reconstitution of terrains and guidance on the proper soil management.

In order to review erosion development through the analysis of stream’s

sediments transportation, the data on net discharge and total sediment discharge, as provided by the gaging station in Itiquira River upstream the BR-163 Highway were used. Two sediments rating curves were prepared, being the first one for the years 1979/1980 (Figure 10.1) and the second one for the years 1981/1982 (Figure 10.2). It is recommended to have data enough in order to provide rating curves as accurate as desired, preferably for each year.

Qst = 2029,4Ln(Q) - 6286,3R2 = 0,3203

100

1,000

10,000

10 100 1000Net Discharge (m³/s)

Figure 10.1 – Total sediments rating curve in Itiquira, period 1979/1980 (Carvalho/Guilhon/Trindade, 2000)

Q s t = 4 1 3 0 ,6 L n (Q ) - 1 3 8 4 2R 2 = 0 ,6 6 0 1

1 0 0

1 ,0 0 0

1 0 ,0 0 0

1 0 1 0 0 1 0 0 0N e t D is c h a rg e (m ³/s )

Figure 10.2 - Total sediments rating curve in Itiquira, period 1981/1982 (Carvalho/Guilhon/Trindade, 2000)


50


Using the corresponding equations and data from the monthly discharges series, the annual average values and annual sediment discharge were obtained for the respective years, accumulated as showed in Table 10.2.

Table 10.2 – Values for accumulated discharges and sediment - Itiquira, from 1979 to 1982

Discharges Accumulate

d Discharges Sediment discharge

Accumulated Sediment discharge

Years

(m3/s) (m3/s) (t/day) (t/day) 1979 1980 1981 1982

112,1 109,1 88,3 88,3

112,1 221,2 309,5 397,8

3.036 3.040 4.473 4.374

3.036 6.075

10.548 14.923

Then, data on accumulated discharges and sediment discharges were used for the mass curve (Figure 10.3). By observing that curve, one may come to the conclusion that the sediment transportation through the stream increased in the period from 1979 to 1982, thus evidencing the basin’s erosion increase, due to anthropic actions.

0

4,000

8,000

12,000

16,000

0 100 200 300 400Accrued net discharge (m³/s)

Figure 10.3 – Sediments mass curve for Itiquira - period 1979/1982 (Carvalho/Guilhon/Trindade, 2000)

The variation rate for sediment transportation may be computed from the ratio between sediment discharges and corresponding discharges as (see Table 10.2):

5,271,1091,112

040.3036.31 =

++

=r and 3,883,88

374.4473.42 +

+=r


51


The rate of sediment transportation increase for the period is computed as:

82,01

12 =−

=r

rrEc

It means that there was an increase of 82%on the sediment transportation between 1979 and 1982 - a very high amount for the short period being surveyed, what may come to jeopardize the reservoir due to very quick sedimentation. The annual rate computation, considering the small sampling for 4 years, is performed by using the following equation: 82,1)1( 4 =+ iR thus resulting in the value of 16.15% for the annual increase on sediment transportation. For the percentage sedimented along 10 years or in a given period, t, the computation is as follows: % and 34747,31)1615,01( 10 ==−+ PR t

i =−+ 1)1( 11. MEASUREMENT OF A RESERVOIR SEDIMENTATION All reservoirs will become sedimented sooner or later. The main issue is to ensure that there will be no problem that comes to hinder the reservoir operation within its economical useful life. On the other hand, one should try to minimize the secondary effects resulting from sediment. Therefore, the sedimentation forecast is performed during the planning stage, and deposit formations, as well as sedimentation effects, are monitored during the operation stage, no matter the reservoir size. Surely, this kind of study always brings about experience and new knowledge in the field of Sedimentology. Therefore, the sedimentometric monitoring of gaging stations along the stream, the verification of erosion problems at the lake bed and downstream channel, as well as the topo-bathymetric survey for reservoirs bring subsidies for both undertakers and Science. 11.1 Purpose of the survey The survey includes both terrestrial and underwater parts, as may be relevant to the studies. The comparison between two surveys performed at different times is made, even if using aerophotogrametric interpretation map for the planning stage. Topographic


52



53

references must be the same. The surveys must report the same accuracy level in order to grant comparable results.

The determination of the new capacity and sedimentation level is the main purposes of the topo-bathymetric survey. Summarily, the following survey outputs may be mentioned: • Determination of either the reservoir volume or capacity under current conditions

(by the time of the survey), as the remaining capacity; • Determination of the new water body area; • Drawing new level x area and level x volume curves; • Drawing of the new bed geometry for the reservoir; • Drawing the new pivot point curve; • Verification of physical characteristics of accumulated sediments; • Quantification of sedimentation volume for the period, by comparing with previous

surveys or with the map at the reservoir formation time; • Determination of the reservoir trap capacity; • Determination of average tributary sediment discharge; • Verification of percentage of sediment settled in reservoirs, dead storage and the

volume lost in useful volume area. 11.2 Frequency of surveys The frequency of surveys in reservoirs depends on several factors; the main ones are: the reservoir’s total capacity and the likely amount of sediment deposit due to the river bed sediment. The small reservoirs, as well as those whose tributary bed sediment is high, shall be more frequently surveyed. On the other hand, the reservoirs with reduced tributary bed sediment shall have their survey frequency reduced. That is the case, for example, when the drainage area is reduced due to the construction of a dam at upstream (Vanoni, 1977), or even when the tributary basin had its sediment discharge value reduced due to protective measures. The financial cost is a factor that greatly influences the reservoirs survey frequency. Usually, resources for such works are limited, mainly because the sediment is submerged, where managers cannot reach.. Considering that the survey cost is justified by an update on the reservoir capacity verification and its sedimentation volume, the criterion indicated in Table 11.1 can be considered. It is evident that the surveys shall be performed more frequently in reservoirs where high indexes of sediments deposit are reported.



54

Table 11.1 – Desired frequency for topo-bathymetric surveys in reservoirs

Reservoir size Classification in volume

(m3) Survey frequency

Small Medium Large

< 10 x 106 from 10 to 100

> 100

Every 2 years Every 5 years Every 10 years

Note: The classification presented hereby is not strict, and may have different concepts in other countries

Some of the following reasons or steps may help in reducing the frequency, or assist the decision-making concerning the need of a survey (Vanoni, 1977): • Data on sedimentometric measurements at the tributary area report high runoff load; • Observations of areas that are usually submerged during reservoir depletion

occasions; • Review of the accuracy of the reservoir capacity curve, by occasion of tributary and

effluent volume computations during the operation surveys; • Measurements for identifying some reservoir bathymetric sections; • When special problems related to sediment deposition in a reservoir appear (for

example, a huge flood may cause the sedimentation of a small reservoir and must be verified; erosion and fall of big talus greatly contributing to deposits).

The studies on reservoir sedimentation monitoring are commonly performed

through a periodic survey in some sections. This practice does not offer accuracy enough for the monitoring of sedimentation and useful life. However, if the survey of such sections is carried out among comprehensive surveys, the result may be used in the decision-making process concerning extension for the next survey.

The stage of a comprehensive topo-bathymetric survey provides more accurately

level x area and level x volume curves when the reservoir is full, if compared to those surveyed through aerophotogrametric interpretation, which normally do not take into consideration the riverbed.

Other works such as, for example, suspended sediments and bed sediments

sampling for characterizing the material follow the studies. The bed sediment sampling shall include the determination of specific weight based on non-deformed samplings or direct measurements. This measure is necessary because of deposits compaction due to water weight or geological activities (ICOLD, 1989). 11.3 Survey methods The general procedures for reservoirs surveys have undergone changes due to scientific development and the emergence of new technologies and equipment. Basically, the general procedure consists of building up the bathymetric map for the



55

lake bottom, which may be compared to a previously prepared map (Bruk, 1985). The most frequent methods for reservoir survey are:

1) Survey method of reservoir contour; 2) Survey method of topo-bathymetric lines.

The method selection depends on the availability and conditions of previous

mapping, on the study objectives, on the reservoir size and on the accuracy level intended. 11.3.1 Survey on the reservoir contour This sort of survey is restricted to small reservoirs, or to reservoirs that may have its level lowered. Usually, the cost for this kind of survey is very high, but it is very accurate. The contour survey method basically uses the procedures of topographic mapping by aerophotogrametry, obtaining photos of the reservoir at several different levels. The method is especially suitable for aerial surveys, when one may program flights for different levels of reservoir depletion, in a relatively short time span. 11.3.2 Topo-bathymetric survey The reservoir topo-bathymetric survey using the cross-sections survey method is much more used for medium and large reservoirs (Bruk, 1985). Basic procedures are as follows: • Obtaining reservoir maps on suitable scale; • Preliminary exploration; • Search for altimetric survey marks and coordinates; • Planning of sections to be surveyed; • Selection of working methods and equipment (including proper boats,

communication means during works, well trained team, etc.); • Determination of the survey reduction level, usually the maximum normal level; • Installation of limnimetric rules along the reservoir for monitoring levels; • Installation of new reference marks; • Depth measurement and simultaneous location of such points (height or levels); • Interpretations, computations, mapping, cross-sections draws and others; • Preparation of the report containing maps, draws and conclusions.

Once identified the reference marks for heights and coordinates, the next step is the implementation of new marks at cross-sections and their identification. The current method, that uses DGPS, waivers the installation of marks in all sections, being necessary the installation of just a few. The installation shall be planned according to



56

the reservoir size. The different technologies available will guide the remainder work in terms of terrestrial support service, management equipment, staffing, and survey time as well as in clerical services, such as mapping, required computation and conclusive result. Modern methodologies allow for a more accurate survey, which may be performed in shorter time. Traditional and modern methods – The method to be used depends on the width of the section being surveyed, its depth, the reservoir size, available resources and other factors. It ranges from simple methods, using tape measure and ruler, to sophisticated methods, using DGPS. Table 11.2 presents a summary on the methods.

Table 11.2 – Methods used for topo-bathymetric surveys in rivers and reservoirs

Method Distance measurement Depth measurement

Use Note

Tape measure Ruler, graduated scale Rivers or narrow and plain lake channels

Ford measurement or up to 2m

Wire rope Probe or ballast Rivers or narrow and plain or deep lake channels, width of up to 300m

Ford or boat measurement

Sextant Probe or direct reading echobathymeter

Rivers or plain or deep lake channels, width of up to 2km

To install baseline at the bank, in such a way as to read angles over 30o

Teodolites (2 or 3) Digital or analogical echobathymeter

Rivers or plain or deep lake channels, width of up to 2km

To install basic topographic line at the bank, in such a way as to read angles over 30o

Distancemeter or total station

Digital or analogical echobathymeter

Cross-sections of up to 10km-width

May be electronically recorded to be used in plotter

Electronic positioning system Trisponder or Motorola

Digital or analogical echobathymeter

Cross-sections of up to 50km


DGPS Digital Echobathymeter Cross-sections and distances of up to 50km


Towfish equipment and positioning

Geophysics (side scan sonar)

Vertical and lateral scanning

Allows vertical and lateral survey on the bed, and for settled layers as well.

For works where depth is measured with probe, positioning with sextant or wire rope, the boat must remain still. For works with digital or analogical echobathymeter, the boat moves slowly, from 2 to 5 knots. For positioning works with sextant or teodolite, it is necessary to implement a topographic line on the bank, with leveled and counter-leveled references. For surveys with distancemeter, the boats may be tied with a high-resolution GPS. The fastening for marks position in the electronic system is performed with the equipment itself; the


stations must be “visible” among them, with no obstacle that may hinder transmission and reception. See Figures 11.1, 11.2, 11.3, 11.4 and 11.5. In any kind of survey, if the reservoir level is under the reference level (named reduction level) it will be necessary to complement the survey for each section and bank by using terrestrial topography. Currently, DGPS is the most used method and provides the best accuracy for points fastening. Records are all electronically, for use in plotter. A fixed GPS is used at the bank and the DGPS on the boat; this last is connected with the ground one. The DGPS on ground is connected to three or more satellites. The maximum error for a 50km-positioning is 3m (Figure 11.6).

Figure 11.1 – Simplified echogram for cross-section survey

Figure 11.2 – Location in depth points measured along a cross-section with sextant

Posição = position Reservatório = reservoir Posição do barco no instante de observação com o sextante = position of boat during observation with sextant Pontos de visadas na margem com o sextante no barco = observation points at banks with sextant in the boat Marcos de levantamento nas extremidades da linha = survey marks on the line ends Bandeira = flags


57


Figure 11.3 – Location in depth points measured along a cross-section with teodolites

Posição = position Reservatório = reservoir Posição do barco no instante de observação com o teodolito = position of boat during observation with teodolite Marcos de levantamento nas extremidades da linha = survey marks on the line ends Margem = bank Posição do teodolito = tedolite position

Figure 11.4 – Schedule for survey operation through electronic system

(Bruk, 1985) Estação remota de resposta = response remote station Transmissor = transmitter Antena = antenna Equipamento de controle = control equipment Relógio = clock Ecobatímetro = echobathymeter Indicador de rota = router Gravador = recorder Registrador de rota = route register Sinal de sonda de profundidade = probe signal


58


Figure 11.5 – Positioning of fixed and mobile stations on the survey electronic system Estação remota = remote station Base = basis Área de melhor cobertura = best coverage area Selection of sections to be surveyed – It is intended to densify the sections in order to obtain the suitable accuracy for drawing isobaths on the map with the selected scale. For small reservoirs, it is usually drawn on a sheet containing the whole lake, which can report a size similar to those maps presented by IBGE or the maximum of 1,0x1,0m. For large reservoirs, maps will be presented in more than one sheet, displaying the articulation draw. The scale must be suitable for both quality and accuracy required; therefore, according to DHN’s (from the Brazilian Navy) orientation, the sections for the draws must be 1,0cm far one from another. Table 11.3 presents a guideline.

Table 11.3 – Distance of cross-sections

Map scale Distance between sections (m)

Kind of Reservoir Note

1 : 2.000 20 Small 1 : 5.000 50 Medium 1 : 10.000 100 Medium to large 1 : 20.000 200 Large 1: 25 000 250 Large

Allows for drawing sections at every 1,0cm in the map

If the bed does not report major variations, a greater distance may be adopted, such as 2,0 or 3,0cm between cross-sections in the draw (the map scale is divided by 100 to provide such distance). Figures 11.6 e 11.7 present schedules for survey in small and large reservoirs (Vanoni, 1977).


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Figure 11.6 – Schedule for cross-sections to be surveyed for small reservoir (Vanoni,

1977)

Transversais de montante = upstream transversals Poligonal básica = basic poligonal Contorno do nível do sangradouro de emergência = emergy runoff level contour Marco topográfico = topographic mark Contorno mais baixo selecionado = selected lower contour Trasnversais de jusante = downstream transversals

Figure 11.7 – Schedule of survey lines for large reservoirs (Vanoni, 1977)

Barragem = dam Antigo canal do rio = old river channel Máximo nível d’água para o qual o reservatório é levantado = maximum water level for which reservoir is surveyed Linhas de levantamento = survey lines Canal submerso = underwater channel Area de delta do reservatório = reservoir delta area Canal a montante do reservatório = reservoir upstream channel


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11.4 Survey specifications Specification is a requirement for services like this, in order to provide guidance for the works. Following, it is presented a suggestion on the specification reach section where sections are more distant one from another, raising a longitudinal line to assist for a more accurate draw of isobathic lines.

Topo-bathymetric cross-sections and one longitudinal section must be surveyed; such survey shall be referenced to the reservoir maximum normal water level. Cross-sections will be distributed from upstream the reservoir backwater area up to nearby the dam, as well as along the river downstream channel. The longitudinal section shall be made along the old bed up to nearby the dam.

Sketches for locating the above mentioned cross-sections and longitudinal section should be provided.

The following items shall be observed for performing these services:

• Sections will be selected in such a way as to display, in the selected map, a distance of 2,0cm. From the backwater area up to the position where the initial delta formation is considered, sections were selected in order to have a distance of 1.0 cm in the map.

• Services will comprise definition of cross-sections, implementation and fastening of

level references - materialized through geodesic marks required for the survey-, installation and operation of limnimetric rulers, establishment of permanent cross-sections for further monitoring, location of points and their bathymetry, collection and analysis of bed sediment.

• Downstream, the sections to be surveyed shall be defined after a review on the

banks erosion conditions. For all cross-sections, level reference shall be installed for use in further surveys.

• The sections indicated on direct tributaries to the reservoir shall be constructed to

the backwater boundary. • Level reference – The geodesic marks for performing the survey and ensuring good

quality for field works, referenced at the reservoir maximum normal level, shall, whenever required, be implemented with known plani-altimetric coordinates. Basic marks for fastening shall be located nearby the dam and the reservoir, on the alignment of cross-sections. The densification of supporting network shall be made through points, materialized in concrete-made marks, identified according to IBGE rules in force. The points fastening shall follow parameters compatible with a work regarded as of the first order. For geodesic calculus, the “Reference Geodesic System – SAD/69” shall be used.



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• For all installed level references (RRNN), localization sketches shall be presented

with all required data for its accurate characterization. Besides that, they must be dully referenced to in the survey maps.

• Installation of ruler – Taking into consideration the long distances to be surveyed,

and the need for the work to be referenced to the reservoir maximum normal level, the rulers to be installed must be positioned in a proper way, with level reference materialized nearby it and fastened one to another, mainly altimetrically. During the survey, rulers must be read at short intervals - it may be hourly. The number of limnimetric gaging stations will depend on the distance between the reference stations to be used for the survey.

• For all those gaging stations, description cards on installation shall be presented. • Positioning – The positioning for each depth measured shall be satellite -based; it is

recommended to use the DGPS (Differential Global Positioning System). That system continuously registers the position of the vessel used, through a mobile receiver and a reference station, located on a known point of the coordinate on ground. That set works through data link, thus allowing the station based on ground to send data on positioning correction for the mobile station; therefore, a better accuracy is achieved for those coordinates obtained on board. The system shall operate through the continuous positioning of measured depths, with accuracy of 2 to 5,0m and ranging from 50 to 80km.

There must be a PC coupled to the system, presenting a pre-established program for area and lines to be surveyed, containing the space between lines, the direction of profiles and the interval between the points examined, according to the mesh. It shall also allow for the repositioning of the vessel at any profile or position as required, being dully displayed at the computer screen.

• Bathymetry – For the bathymetric survey, a good digital echobathymeter must be

used, with a 208 kHz-transductor, or similar, capable of providing permanent and detailed records on the bed high resolution topography for defining the water-sediment interface, in such a way as to operate in very deep waters. The echobathymeter must be daily calibrated, by the beginning and by the end of works, through suspended card process, for purposes of correcting sound velocity and precisely defining depths.

The echobathymeter shall be coupled to the mobile receiver and to the computer through a program that allows for the automated and simultaneous record of depths, as well as their positioning, in magnetic means (diskette) for further processing.

If the water level is lower than the reference level, banks must be leveled - through topographic equipment or with GPS - up to the desired level.



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• Collection and analysis of bed sediment – Aiming at providing subsidies for the adequate estimate on Manning coefficient for backwater studies, a reservoir bed sediment collection shall be performed at every 4 cross-sections, as well as nearby the dam. Such sampling shall be forwarded to the laboratory for the granulometric analysis.

11.5 Bed mapping The selected map shall have a scale suitable for the reservoir size; the marks shall be plotted, as well as heights, position of rulers and other data such as topographic leveling line and others.

Once having the field cards (work with traditional methods) or the electronic

records (work with modern techniques) and additional information, the next step is to plot the sections in the map, with the selected scale, and record depths (Figure 11.8). The measured depths must be adjusted based on the rulers reading.


Figure 11.8 – Stream survey using DGPS (Microars, 1996)

Having registered all depths for the respective plotted points, the curves on the reservoir bed level – or isobathic – may be traced out, by interpolating depths at every 1,0, 2,0 or 5,0m, as allowed by the selected scale. 11.6 Computation of reservoir volumes The survey allows for determining the reservoir capacity that, compared with previous survey, provides the volume of settled sediment. That capacity is computed through two methods, with partial volumes, using either drawn lines or cross-sections. Planimetric methods of bathymetric curves – for this method, it is used to planimeter the bathymetric curves traced out in the map and, following, make the required computations to know the reservoir volume between two isobaths. Four processes are used: relation level versus bathymetric areas, average areas, Simpson rule and modified prisms (Vanoni, 1977 and Semmelmann, 1981). Other formulas can be seen in Vanoni, 1977, e Morris/Fan, 1997. a) The process level versus area uses the following formula: ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department

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V = E x A (11.1) the values of which are shown in Figure 11.9 The full line represents the plotting for isobathymetric areas. The dashed area A, between two lines, multiplied by the distance between them is equivalent to the volume V between two bathymetric curves. The total volume of the reservoir corresponds to the graph area between the curve and the levels axis.

Figure 11.9 – Relation level x area in the method of bathymetric curves planimetry

Cota = level Área = area

b) The process of average areas calculates reservoir volume by using the average of two successive bathymetric curves multiplied by the distance between them:

xEBAV2+

= (11.2)

where A and B are the area of two successive bathymetric curves and E is the distance between them (Figure 11.10).

Figure 11.10 – Process of average areas of bathymetric curves


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c) the process using the Simpson rule has the following equation:

( ) ([ ]2421310 ...2...431

−− +++++++++= nnn AAAAAAAAEV ) (11.3)

The condition for applying the Simpson rule is to divide depth into an even number of bathymetric curves.

d) the process of modified prisms, as illustrated in Figure 11.11, uses the following equation:

( BBAAEV ++= .3

) (11.4)

Figure 11.11 – Process of modified prisms for calculating a reservoir capacity (Vanoni, 1977)


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Method of cross-section planimetry – Once the map is available, one shall trace out cross-sections, parallel one to another, for obtaining the respective areas. The computation of reservoir volumes is performed through several processes, three of which are presented below: plotting cross-sections areas versus distance from dam; average of equidistant cross-sections; and Simpson rule. a) the process of plotting cross-sections areas versus distances from dam uses the

following equation:

AxDV = (11.5) where A is the dashed area between two cross-sections, and D is the distance between them. The reservoir total volume corresponds to the graph areas comprised between the curve and the distance axis (Figure 11.12).

Figure 11.12 – Cross-sections areas x distance from dam

Área = area Distância da barragem = distance from dam

b) the process of average of equidistant cross-section areas relies directly on established data and calculates the reservoir volume through the following equation:

( )2

2...22 1321DxAxAxAxAAV nn +++++= − (11.6)

c) for applying the Simpson rule, it is necessary to divide the reservoir total length in

an even number of cross-sections, equidistant one to another and parallel to the dam. The following equation is used:

( ) ([ ]2421310 ...2...43 −− +++++++++= nnn AAAAAAAADV ) (11.7)


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where V is the reservoir volume, D is the distance between the sections and A is the cross-sections area.

11.7 Computation of settled sediment volume The methods presented in item 11.6 refer to the computation of the reservoir remaining volume. Similar computation, using the same equations, is performed by using the primary or previous survey for comparison purposes, as well as for calculating the sediment volume through the difference between two reservoir volumes for the reservoir. Having the dead storage, the settled volume shall also be computed. By calculating the difference to the total volume, one can obtain the volume of sediment settled in the useful volume. Those values may also be computed as percentages, in order to review the reduction in the reservoir total volume, the useful volume reduction and to know the trap efficiency as well. 11.8 Outline of new level x area x volume curves Among the several results deriving from the survey, the knowledge on the settled volume and the new reservoir capacity are outstanding.

Having the areas of bathymetric curves and corresponding volume summed up to each isobathic being considered, one can trace out the level x area and level x volume curves. For comparison purposes, the original curves are also traced out. 11.9 Pivot point It is also important, for the survey, to verify the new geometry of the lake. For that, comparative cross-sections are traced out (Figure 11.13), selected from sites along the reservoir that may reflect the changes on geometry, concerning its original and new condition. If there are several surveys available, one shall trace out several sections on the same position for comparison purposes. The formation of crests, changes on the delta area and height of sediment settled at the dam base are also surveyed. The longitudinal line for the current thalweg is also traced out for comparing it with the previous line, intending to obtain the pivot point and its development, which characterizes the delta (Figure 11.14).


For visualizing the new bed morphology, if the survey is electronically available, it is useful to have software that allows the display of a draw showing the conformation variation.

Figure 11.13 – Comparative cross-sections for reservoir surveys

(Carvalho, 1994)

N.A. de redução = reduction water level Situação atual = current situation Situação primitiva = primary situation

Figure 11.14 – Longitudinal profiles for reservoir survey, where the pivot point can be

observed (Carvalho, 1994)

Altitude = height Altura de sedimento no pé da barragem = sediment height at the dam basis Cone de dejeção = pivot point Distâncias do talvegue acima da barragem = thalwegue distances above the dam


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11.10 Bed scanning and geophysics This work is performed by using special equipment. An echobathymeter working with high-frequency ultrasound allows for emissions that cross the settled layers, returning to the equipment and recording the changes on thickness (Figure 11.15).

Figure 11.15 – Seismic recorder, displaying the reservoir bed bottom line and lower

layers. – UHE Funil/FURNAS, on 02.02.1993 (Conage)

Another very useful equipment is the lateral scanning using audiography, which

shows the bed conformation. The equipment is submerged and a sonorous signal is sent, at regular time intervals, through two transductors located on coated water vehicles, named towfish, which carries the side scan sonar. The emission beams are addressed to the bottom surface sides. Each transductor acts independently, being responsible also for receiving the reflected signal. The signals from the bottom surface are recorded as they are received, on electro-sensitive sheet, therefore making up an image of the bottom of the area being surveyed, named sonogram (Geomap, 1991). 12. CONTROL OF A RESERVOIR SEDIMENTATION Forecast studies and the entire process of sedimentometric measurements aim at verifying the likely reservoir sedimentation and the need for sediment controlling, in order to mitigate its effects. The sediment control presents several implications for several Engineering fields, as a way of protecting works and patrimony involved. Several measurements are complex, since sediment is derived from erosion along the whole drainage area at the dam site, being hardly accessed by the entity responsible for the reservoir. Most of the time, only a governmental planning may establish and execute a program on erosion control along the whole hydrograph basin. ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department

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Many programs on sediment control by the reservoir owners are restricted to its action area, where they try to protect river and reservoir banks, in order to diminish the entry of sediments into the system. Programs on reservoir sedimentation prevention are the most important, and corrective measures are adopted just when there is no other alternative. 12.1 Preventive Control According to CIGB (ICOLD, 1989) the most obvious preventive measure for controlling sediments is, most times, disregarded by designers. It regards to the regions at the rivers headwaters - the upper basin – that have great runoff contribution but small parcel of bed sediment. It is extremely important to have the forests preserved at those regions, so that they do not become responsible for great production of sediments. Summarily, preventive measures may be listed as shown in Table 12.1, following the proper selection of work and reservoir sites, basin erosion control, sediments trap before entering into the fluvial system, and the automatic removal of sediments. They are used for all stages – inventory, feasibility, project and operation.

Table 12.1 – Preventive measures for sediments control and reservoir sedimentation

Preventive measures

Selection of reservoir site If there is more than one site available for the dam and reservoir formation, select the one presenting the lowest allocation of sediments

The site selection depends on financial costs including protection for the most deprived area

Suitable dead storage forecast Increase the dam height Forecast on volume set apart for sediment

Increase the dam height For the reservoir project

Forecast of sediment discharger with gates (for density streams and bed sediment)

Plants far from the dam needs a de-sander after the intake

Vegetative practices: - Forestation and reforestation - Grazing grass - Coverage plants - Strip crops - Vegetation belts

Control of erosion in the basin (brings several benefits, where the most efficient is hard to be applied by the dam worker; it is necessary to ask for support from other entities to manage

Soil conservation and management in agriculture (Bertoni and Lombardi Neto, 1990)

Pedology practices: - Fire control - Green fertilization - Chemical fertilization - Organic fertilization



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Mechanical practices: - Rational distribution of ways - Counter crops - Leveling - Groove and raised bed - Drainage channels

the basin)

Sediment control on roads, cities, several civil works, control of both urban and rural erosion;

- Refrain or protection of taluds - Drainage works - Control of erosion in ravines and gullies

Control of erosion at water courses and reservoir banks

Erosion on the channel Filling in gullies

- Protection with infusorians vegetation - Structural protection (break-water construction, etc.)

Dams at upstream (may be submerged, if desired)

Periodically remove the sediment trapped

Vegetation By-pass derivation channels - Channel

- Duct

Control of sediment affluence in the flume

Deviation of floods for inundation areas

Decantation basins

Discharger with gate (planned eration) op

- Density streams - Bottom sediment

Reservoir depletion dead storage

Control of sediments deposition

Reservoir planned operation There are computer software aimed at sediment accommodation

12.2 Corrective practices Sedimentation corrective practices are performed during the operation stage. Usually, deposits surprise the operator, since they are submerged and increase slowly. If there is no monitoring the surprise happens. It is tried to recover the lost volume with mitigating measures, which are expensive and repetitive. Table 12.2 presents a summary on corrective practices measures.

Table 12.2 – Corrective measures for controlling sediment and reservoir sedimentation

Corrective measures

Eventual Dredging (the deposition site is important) Almost permanent

Channel By-pass derivation works Duct

Siphon Filtering At the dam or sometimes removing farer sediments through the bottom discharger

Removal of sediment from the reservoir

Bottom discharger Sometimes is must be built after the dam is ready

Dam raising Perform an adequate dimensioning

Whenever possible, because it will increase the level and water body



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12.2.1 Discharge of dredged sediments The removal of a reservoir’s sediments by dredging it is expensive. Sometimes, it is cheaper to raise the dam or another solution. Therefore, costs must be reviewed in relation to the convenience of dredging. Usually, this solution is applied to small reservoirs to relieve problems caused by deposits in given sites, as for example, at the intake basin. One of the major problems involving dredging is the material settlement. Usually, the dredged material is not economically used because of several costs and other factors, such as sediment pollution or issues concerning the material transportation for reservoir sources. One could suppose that the coarse settled in the delta area could be used for construction, and the fine material closer to the dam, containing nutrients could be put into agricultural areas. However, in small reservoirs, that natural selection is not so good and the deposits may have too many impurities, such as waste and others. There is experience enough and suitable solutions for each kind of problem referring to dredging – width, depth, consolidated sediment, presence of materials such as stones, gravel, tree trunks and rigorous limitation of disposal – for all cases, (ICOLD, 1989). There are several kinds of equipment for removing deposits, which are basically the airlift system, the mechanical system (drag-line or clam-shell) and suction and deposition dredge, using centrifuges pumps to perform the material hydraulics transportation (Engevix, 1980). Therefore, one must look for the suitable equipment for each case, allowing greater savings. The disposal of dredged material is a topic involving economic and environmental issues. The simple disposal at the reservoir bank, at the area closer to the dredging site, or is thrown at the dam downstream channel, may be an inadequate solution. For the first case, most of the sediments may come back in short time, during the first rains, for the reservoir. For the second case, there will be several problems at downstream, including channel sedimentation.

Many countries have laws regulating water quality, prohibiting the disposal of dredged material in the stream. Countries as China and Formosa, where sites for dam building are scarce, have improved agricultural fields by putting selected material deriving from dredging, simultaneously recovering the reservoirs’ water storage capacity. The material may also be used for building marginal dikes to the rivers where there is the need for protection against floods (ICOLD, 1989) 13. SECONDARY EFFECTS DUE TO SEDIMENT Besides the physical effects deriving from reservoir sedimentation over its objective functions, there are several additional secondary impacts that must be taken


into consideration, and that may expand beyond the reservoir limits and the actuation of the responsible company. Such secondary impacts shall be previewed, assessed and conciliated, both during planning, project and construction stage and during the reservoir operation stage (ICOLD,1989). 13.1 Effects on the reservoir backwater The bed sedimentation in the reservoir entry with delta formation causes deformations on the river channel which, along time, becomes strangled. The deposits advance downstream and a little upstream, the channel gradient becomes reduced, while the underground water table remains in high level, thus hindering the dredging. Upon the channel narrowing, as the delta increases, the effects on the reservoir backwater also increase, thus increasing the frequency of floods upstream (ICOLD, 1989). The effects may be analyzed by surveying the formation and expansion of the delta; however, the study is complex due to reservoir operation, quantity of sediment affluence and other factors. The use of HEC-6 model for computation backwater, taking into consideration the sediment affluence, may display water line profiles for floods reporting different recurrence times. Delta formation is represented in 13.1, where the top layer, the sliding point, the front layer and the overbanks are displayed.

Figure 13.1 – Typical delta formation: (1) top layer slope,

(2) coarse, (3) thalweg original slope, (4) sliding/pivot point, (5) front layer slope,

(6) overbanks slope, (7) normal/average operation water level, (8) maximum water level, (9) intake,

(10) fine sediments (ICOLD, 1989)


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For preliminary evaluations, the delta formation starting point is considered as being on the intersection of bed line with the maximum level of the reservoir; and the sliding point (pivot) is on the intersection with average operation level. In this case, the value 1.5 of that bed is used for the top layer slope, and for the front layer slope, a value equivalent to 6,5 times that top layer slope is used. Once having that volume, one may calculate the formation time for that condition. When the sliding point reaches the dam, the top slope disappears (ICOLD, 1989, and Strand/Pemberton, 1982). 13.2 Changes on water quality The impacts of sediments on the reservoir and on the quality of downstream water have not yet been fully explained or surveyed. Eutrofization is the term applied to describe the effects and changes on waters confined by the increase on nutrient level, reduction of dissolved oxygen and increase on biologic productivity.

The torrents deriving from precipitation carry many types of sediment for streams and, together with such sediments, nutrients, agro-toxic and whatever those waters may carry. Once in the reservoir, those substances undergo several changes and may, inclusively, affect the downstream water quality. Proliferation of algae and other effects are consequence of such transformations. 13.3 Ecological effects Both fauna and flora suffer ecological effects. The deposits in reservoirs modify the bed quality, thus affecting the life of fishes by changing their natural habitat. Species disappears and only the strong ones survive.

The sediment load in water also reduces the penetration of sunlight, thus hindering the required transformations for life existing there. On the other hand, the full removal of sediments with nutrient, through disposal on the bed, also causes changes. In anyway, Nature suffers, loosing some species that cannot overcome the changes.

Concerning flora, the formation of macrophytes at the reservoir banks, due to the

disposal of fine sediments with nutrients, may be highlighted. Vegetation quickly spreads and is wrenched by the water level increase and, following, is carried towards the dam and intakes.

Some vegetal species, upon the fluviometric level raising, may be quickly

displaced to the lake bottom, thus raising the quantity of flooded terrestrial biomass. Later on, that biomass decomposes through aerobic and anaerobic processes, starting the process of gas emission to atmosphere, mainly the CH4 (Methane) that may contribute to worsen the thermal heating of low terrestrial atmosphere – Greenhouse Effect (UNEP, 1997).



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The natural formation of river beaches provides leisure for riparian population.

The effects of the reservoir are felt on those sand banks, both when the lake floods such areas, making them disappear, and through effects at downstream. Once the reservoir is formed and the sediment is being settled therein, there is no sand feeding at downstream, thus causing the disappearance of sand banks on that reach. The beaches will appear just very downstream when the erosion on the downstream incremental basin – and the associate transportation of sediment along the stream – allows for the occurrence of new sand banks, known as sand bars.

The effects of the lack of sand provision at downstream are felt up to the outfall

of those rivers, and the changes may appear in long-term. This phenomenon may be the reason for the changes occurring at the outfall of Paraíba do Sul and São Francisco Rivers. 13.4 Erosion on reservoirs banks The reservoir banks must be always protected with infusorians vegetation, or by using conservative practices. Nevertheless, erosions may occur on its banks, be it due to waves impact or due to the high level of drenching during rainy periods, causing the fall of taluds. When that happens nearby the dam, it is necessary an immediate protection. Those sediments will become incorporated to sedimentation, while erosion development may bring several consequences. 13.5 Deposit erosion The settled sediment may undergo accommodation, slipping into the reservoir bed. When it is settled in the dead storage, it is regarded as benefic. There are computational models for reservoir operation using quicker depletion, thus facilitating the accommodation process, and enlarging part of the volume occupied by the sediment. However, when sedimentation is closer to the dam, the sliding of deposit may pose risk for its structure or suddenly reach the intake. 13.6 Downstream effects

The sediment trapping in the reservoir causes a runoff of clean water downstream.

On the other hand, the regulation of downstream discharges causes major actions over beds and banks of that channel.

Jointly, those two effects – besides others – may cause the deepening of dam

downstream channel’s bed and erosion of its banks. For small reservoirs, these are minor effects and may occur at the closest channel, while for large reservoirs those effects are greater and may be felt even hundred kilometers downstream the barrier.


Degradation at the downstream channel may have several undesired consequences to environment. Structures at the channel, such as bridges or piping crossing the river by the bed, are subject to lowering that could damage its structural integrity. If the channel banks are on the stream attack point, valuable agricultural, industrial or residential properties may be damaged, unless protective measures are adopted. The biological community at the downstream channel may be seriously affected by the increase of the channel bed’s thicker material and by a change on the vegetation growth along the banks (ICOLD, 1989).

There are several methodologies for forecasting the effects occurring downstream

the reservoirs (Bruk, 1985). One of them is the HEC-6 model, and there are others that also perform computations by applying sediment hydraulics formulas. Simpler methods were suggested in Strand (1974) and ICOLD (1989) trying to approach the survey through the formation of the bed shield, through the transportation of finer material, or through the steady slope computation. The items below correspond to a bibliographical survey using bed transformation and mainly based on the two previously mentioned papers.

13.6.1 Channel degradation Usually, the river natural runoff carrying sediment is in a quasi-equilibrium

regime, with no long-term tendency for sedimentation or degradation. That equilibrium regime may be expressed by equation 13.1 (see Figure 13.2)

Q D kQSs = (13.1)

where: Qs = sediment discharge D = Particle diameter Q = Net discharge S = Channel slope k = Proportionality constant


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Figure 13.2 – Relation among factors contributing to the establishment of a steady

equilibrium in a river channel, according to Lane (WMO, 1981).

Granulometria = granulometry Plano íngrime = sloped plan Alta degradação = high degradation Alta agradação = high aggradation Carga sólida = sediment Vazão = runoff Diámetro da partícula = particle diameter Declividade do leito x vazão = bed x runoff slope

If one of the four variables is altered, one or more of the remaining shall undergo

changes in order to return the channel back to the equilibrium status. Therefore, the reduction of the dam slope at downstream may be foreseen if changes occur. If there is enough coarse, then fine particles may be carried and the thick material is trapped. Those processes resulting into the removal of bed and bank sediment particles are known as degradation (Strand, 1974).

The degradation process gradually moves towards downstream, until it reaches a

site where the sediment being carried results in a stable channel, or in equilibrium. Any amount of coarse passing through the dam shall have a compensation effect over the channel degradation.

There are two different ways for estimating the height or quantity of degradation

that may occur downstream, or in a similar structure, each of them depending on the kind of material making up the river channel bed.

When there is enough thicker material or material reporting greater granulometry -

that cannot be carried by the river normal discharges –on the bed, a protective layer will be developed as finer material is displaced and carried downstream. A vertical degradation shall occur in a value progressively slower, until a shield stands in such a height as to inhibit greater degradation. However, if the bed is made up by material that


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may be transported, and the material moves towards waters deeper than those where the channel may become degraded, so the channel will change its slope until it reaches a steady slope, which will be computed together with the expected degradation volume. (Strand, 1974).

The determination of the channel main discharge and features are a requisite for

those estimates.

13.6.2 Main discharge The main discharge is defined as the discharge that, if a constant runoff occurred,

it should have the same effect all over the channel shape, such as would be the unsteady natural discharge. The main discharge used for surveys on channel stabilization is usually considered as the overflowing discharge, or peak discharge, reporting an occurrence interval of about two years for a non-monitored river (Strand, 1974).

By regulating the discharge through an upstream dam, the problem becomes more

complex at the downstream channel, since data required for further discharge by the dam would be no longer available. If the reservoir runoff is almost uniform and flood discharges are relatively rare, the daily average discharge may be used as the main discharge. However, if the runoff is subject to a considerable variation due to floods, the peak discharge that is equivalent to or exceed the average once at every two years would be considered as the main discharge (Strand, 1974).

13.6.3 Channel hydraulic features The next step for calculating the degradation at a dam’s downstream channel is

the determination of hydraulic characteristics for the main discharge nearby the channel. Usually, those data may be obtained from the survey on the discharge tributary to the reservoir backwater. The features of all backwater cross-sections, when draining the main discharge, are proportionally divided in order to reach a cross-section that may represent the channel degradation. The water surface slope may be considered as equivalent to the hydraulic gradient (Strand, 1974).

13.6.4 Method of degradation constrained by the shield The first procedure to be tested for calculating the degradation downstream is the

method of shield formation review. This method is specially applicable if there are enough big stones or thick material – that cannot be carried by the normal river discharge – on the bed, in such a quantity as to build up a shield layer (ICOLD, 1989).

During shield building process, the finer material transportable is removed, and

degradation takes place in a progressively lower index, until a shield high enough to hold back a greater degradation is built. Usually, a shield layer may be foreseen if there


is about 10% or more of bed material reporting the same diameter of the shield, or greater. The shield computations suppose that a coarse layer will be built, as shown in Figure 13.3.

y = height of original bed at the bottom of the shield layer ya = degradation height or thickness of shield layer Dc = diameter of the material building up the shield yd = height of original bed at the top of the shield layer or degradation height Figure 13.3 – Schedule for defining the shield (ICOLD, 1989)

NA = Water level Escoamento = runoff Leito original = original bed Material original do leito = original bed material Leito degradado = sedimented bed

From the figure, one may deduce that:

y y ya = − d

y

(13.2)

By default:

( )y pa = Δ (13.3)

where, Δp = percentage of material with diameter greater than the shield thickness

diameter. Matching both equations, the degradation height is equivalent to:

y ypd a= −

⎛⎝⎜

⎞⎠⎟

1 1Δ

(13.4)

The thickness of shield ya shall vary according to the particle diameter; however it

is usually adopted as equivalent to 3 times the diameter Dc of the shield particle, or 15cm (0,5ft), or a little smaller.


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The average diameter for the sediment particles required for building the shield

might be computed through several methods, being one considered as the verification of the other. Each method shall report a different shield diameter, thus requiring experience when evaluating the most suitable option. Basic data for computations require:

• sampling of bed material along the reach being surveyed, as well as in different

heights along the entire zone where degradation may occur; • selection of the main discharge, usually adopted as the discharge peak with two

years of recurrence; • channel’s average hydraulics features corresponding to the selected main discharge,

obtained from the computation of uniform backwater runoff throughout the selected river reach.

Following, are presented four methods for computing the diameter Dc.

Use of the bed capable velocity – Several laboratory surveys have evidenced that the diameter of a particle taken from bed is proportional to the velocity of the stream nearby the bed. The velocity in which the particle starts its movement is considered as the bed capable velocity, Vb, which was observed to be approximately equivalent to 0,7 times the average velocity for the channel Vm:

Vb = 0,7 Vm (13.5)

Figure 13.4 represents the bed capable velocity Vb in relation to the diameter of a mobile sediment particle, and has been used to determine the shield thickness.

Figure 13.4 – Bed capable velocity in relation to the average diameter of the

transportable sediment (Strand, 1974)


81


Velocidade de fundo = bottom velocity Velocidade capaz do leito = bed capable velocity Velocidade média = average velocity Diâmetro de partículas = particles diameter Use of tractive power – Tractive power, or shearing strength, is the tension acting over the channel bed wet area, and may be expressed as:

τ γ= pS (13.6)

where, τ = tractive power ( kg/m2 or lb/ft2 ) γ = water specific weight ( kg/m3 or 62,4 lb/ft3) p = average depth (m or ft) S = hydraulics gradient When the tractive power is computed for main discharge, the curves for tractive

power presented in Figure 13.5 may be used in order to determine the average diameter of the the bed material shield thickness diameter.


82


Figure 13.5 – Tractive power in relation to the transportable sediments diameter (Strand, 1974)

Linha de força gradual = gradual tension line Força gradual = gradual tension Valor recomendado para canais com alta concentração de finos = value recommended for channels with high concentration of fine material Recomendado para canais com areia fina com água contendo colóides = recommended for channels with fine sand with water containing colloids Recomendado para canais com areia = recommended for channels with sand Valores recomendados para canais com baixa concentração de finos = values recommended for channels with low concentration of fine material Valores recomendados para canais com mat. não coesivos = values recommended for channels with non-cohesive material Valores recomendados para águas claras = values recommended for clear water Valores de Straub de força trativa crítica = Straub values for critical tractive power Recomendado para canais de areia fina e águas claras = recommed for channels with fine sand and clear waters Força trativa crítica = critical tractive power

Use of Meyer-Peter & Muller equation – The Meyer-Peter & Muller equation for null sediment discharge is expressed by:

S

QQ

nD

D

pB

s

=

⎛

⎝⎜

⎞

⎠⎟0 19

901 6

3/2

, /

(13.7)

where,

Q = total net discharge (ft3/s) QB = part of the net discharge influencing the bed (ft3/s) ns = Manning roughness coefficient for the total section D90 = diameter of the particle for which 90% of bed sediment is lower (mm) p = average channel depth (ft) D = minimum average diameter transportable present in the bed material (mm) Based on that equation, D may be computed and, afterwards, the shield thickness.

Therefore, from Meyer-Peter & Muller equation, considering Q/Qb = 1:

D D Sp

nD

c

s

= =⎛

⎝⎜

⎞

⎠⎟

5 26

901 6

3/2

,

/

(13.8)

Use of Schoklitsch equation - Schoklitsch equation for null sediment discharge has the following expression:

S DBQ

=⎛⎝⎜

⎞⎠⎟

0 000213/4

, (13.9)

where B is the channel width (ft). Evidencing the value of D for the equation, we have:


83


D D S QBc= =

4762 4 3/

(13.10)

13.6.5 Method of degradation constrained by steady slope

The method for calculating the steady slope - in order to define the downstream degradation - is used when there is not enough coarse for building a shield layer. The method is used when the main purpose is to calculate the height of downstream bed erosion, for the work project; it may result in the indication of protective measures at downstream, in order to avoid diggings on the bed. It is also used for previous planning on levels with small amount of field data, and when the costs for a more detailed survey are extremely high (ICOLD, 1989).

The steady channel method is illustrated in the Figure 13.6 schedule. The steady

slope is defined as the river slope where bed material can no longer be transported.

Sb = natural bed slope SL = threshold or stable slope

Figure 13.6 – Typical schedule of degraded channel using the triple slope method (ICOLD, 1989)

As shown in Figure 13.6, the process is also defined as the triple slope method

because this is the expected variation on total slope, concerning steady slope and the slope existing a little farther at downstream. The computations for steady channel may be performed by applying several methods, such as:


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85

• Meyer-Peter & Muller equation for bed load, expression 13.7, for the beginning of the transportation;

• Schoklitsch equation for bed load, expression 13.9, for null sediment transportation; • Shield graph for no movement or no displacement of particles; • Lane relations for critical tractive power, presuming a clean water runoff in

channels. The discharge to be used for any of the above-mentioned methods is the main

discharge, and it is also necessary to have hydraulics features determined. Besides the restraints or steady slope for the degraded channel, it is also necessary

to determine the volume of material that may be removed from the channel. If there is no control downstream in order to restrain degradation, sometimes, one may suppose that the river will report a coarse load (> 0,0625 mm) equivalent to that share of historical load concerning the same granulometric band. It necessarily assumes little depletion in discharge amount and less regulation at upstream. If the discharge is drained or regulated, the sediment load to occur will be lesser than historical load, and the due adjustment on that load shall be made (ICOLD, 1989).

Once established the steady slope and the volume of the material that may be

removed, the degradation height nearby the dam, and the degraded channel profile as well, may be estimated if the following assumptions are reasonably met:

• The degraded reach is uniform enough to allow for the use of average cross-section

and average slope along its full extension; • The bed and bank material, along the entire channel, are similar enough in order to

allow for the use of an average composition, and to ensure that there are no non-erodible obstacles, either at bed or banks, for avoiding the stream to reach the average steady section at the steady slope;

• Degradation is such that the vertical component will prevail and horizontal movement will be restricted to the small layer on the bank, resulting from vertical degradation.

Experiments have proved that a degraded stream course profile may be

represented by a typical schedule equivalent to three times the slope, as presented in Figure 13.6 (ICOLD, 1989).

There are several ways for determining the volume of the material to be removed

by using the steady slope method. The volume may be visualized from the figure, as follows:

V = AT × B (13.11)

where, V = volume of material to be degraded (m3) AT = longitudinal degradation area (m2) B = width of degraded channel (m)


If there is no control at downstream or no limiations for length L for degradation,

the two ways for calculating the volume are (ICOLD, 1989):

• To presume that the river will remove a coarse load (> 0,062 mm) equivalent to the quantity of historical sediment load > 0,062 mm;

• To calculate the influenge at the degraded reach by using sediment rating curve and the discharges permanence method, or any other method.

For the second case, the sediment rating curve may be defined by using one or

more of the bed sediment formulas, and the reservoir influenge permanence formula. Evidencing AT for the previous equation (13.11):

A VBT = (metrical system) (13.12)

Once computed the value for AT, the degradation height may be determined

through the following equation:

DA ST=

× ×⎛⎝⎜

⎞⎠⎟

6439

12Δ (13.14)

where: ΔS = the difference between the existing and the steady slope The degraded reach length may be computed through:

L DS

=××

138 Δ

(13.15)

If any lateral degradation foreseen for the river - caused by erosion on the bank –

is regarded as a significant factor, a complementary survey shall be required in order to determine the width of the degraded channel. The amplitude of vertical degradation is not necessarily so high, because a portion of the material will come from banks. Lateral movement shall always be assessed when banks are made up by the same material as the bed, and there is not enough vegetation to hold the material.

If there is a permanent control at any point of the degradation reach, equation

13.14 may be used to directly solve the degradation height issue (ICOLD, 1989).

13.7 Reservoir surveys supported by satellite imagery

Satellite imageries are used either isolated or compared to previous imageries. Landsat TM imageries are especially suitable for performing works aimed at analysis referring to reservoirs sedimentation; development of aquatic vegetation and in erosive


86



87

processes along the reservoir banks and downstream channel. Those products are useful for identifying the features through imageries interpretation. They may also be useful for guiding field works. Such imageries are periodically obtained by the satellite, thus allowing comparison and analysis aiming at the dynamics of geomorphological and fluvial processes and, therefore, their trends.


(A) (B)

Figure 13.7 – (A) Landsat Imagery covering part of Tucuruí reservoir – Water tones represented by bluer colors correspond to the areas reporting greater percentage of suspended material than the darker tones. (B) –Landsat TM Imagery illustrating the

process of aquatic vegetation development at a branch of Tucuruí reservoir with intensive agricultural and cattle raising use, nearby it (picture of the work performed by

Eletronorte).

The interpretation of Landsat TM imageries are digitally processed and analyzed jointly with data on level curves obtained from existing cartographic material and thematic maps issued by project Radam.

13.8 Erosion control at downstream channel

The downstream channel erosion may advance towards upstream and damage

the dam, even though dams are always planned taking into consideration such possibility. However, works at downstream, such as bridges, marginal dams and intakes may be affected by the erosion on the river channel. Table 13.1 shows the preventive and corrective measures to be adopted.

Table 13.1 – Erosion control at the downstream channel -

Preventive and corrective measures Armoring To review, through models, if

thick bed sediment is enough for protective purposes

Preventive measures (Studies during the project stage)

Change on slope To review, through models, if slope will not change beyond a given limit

Building a rocky sea-wall Corrective measures Structural works


88



89

BIBLIOGRAPHY (consulted and complementary) Note: Not all the bibliography below was referred to in the Reservoirs Sedimentation Evaluation Guide • ABRH, Associação Brasileira de Recursos Hídricos (Brazilian Water Resources

Association) (1991). Carta de Ouro Preto. I National Meeting on Sediments Engineering. Sediments Engineering Commission. Ouro Preto, MG

• ABRH, Associação Brasileira de Recursos Hídricos (1996). Produção de sedimentos. II National Meeting on Sediments Engineering. Sediments Engineering Commission. Rio de Janeiro, RJ.

• ABRH, Associação Brasileira de Recursos Hídricos (1998). Assoreamento de reservatório e erosão à montante. III National Meeting on Sediments Engineering. Sediments Engineering Commission. Rio de Janeiro, RJ.

• AGRICULTURE, COMMERCE, DEFENSE, INTERIOR DEPARTMENTS, Independent Agencies Working Group: Work Group 3 on Sediment (1978). National handbook of recommended methods for water-data acquisition sediment. Washington, DC.

• ALMEIDA, Sérgio Barbosa, and CARVALHO, Newton de Oliveira (1993). Efeitos do assoreamento de reservatórios na geração de energia elétrica: análise da UHE Mascarenhas, ES. X Brazilian Symposium on Water Resources and I Symposium on Water Resources of the South Cone. Gramado, RS.

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• ANNANDALE, G. W. (1987). Reservoir sedimentation. Elsevier Science Publishers B. V. Amsterdam.

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• CARVALHO, Newton de Oliveira (1981). Cálculo da descarga sólida total pelo método de Colby. IV Brazilian Symposium on Hydrology and Water Resources. Fortaleza, CE.

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• CARVALHO, Newton de Oliveira (1984). Cálculo da descarga sólida total pelo método modificado de Einstein – adaptação ao sistema métrico. Unpublished. Rio de Janeiro.

• CARVALHO, Newton de Oliveira (1986). Aplicação do método modificado de Einstein para cálculo da descarga sólida total no sistema métrico – cálculo de Z’ segundo Lara. Unpublished. Rio de Janeiro.

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• CARVALHO, Newton de Oliveira (1991). Cálculo do assoreamento e da vida útil de um reservatório na fase de estudos de inventário. IX Brazilian Symposium on Water Resources and V Portuguese/Brazilian Symposium on Hydraulics and Water Resources. Rio de Janeiro, RJ.

• CARVALHO, Newton de Oliveira, and CATHARINO, Márcio Gomes, and PRODANOFF, Jorge Henrique Alves (1991). Curvas de transporte de sedimentos. IX Brazilian Symposium on Water Resources and V Portuguese/Brazilian Symposium on Hydraulics and Water Resources. Rio de Janeiro, RJ.

• CARVALHO, Newton de Oliveira, and CATHARINO, Márcio Gomes, and PRODANOFF, Jorge Henrique Alves (1991). Avaliação do assoreamento do reservatório da UHE Itaipu, PR, relatório preliminar. ELETROBRÁS. Unpublished. Rio de Janeiro, RJ.

• CARVALHO, Newton de Oliveira (1994). Erosão crescente na bacia do rio Doradas (Estado de Tachira, Venezuela). FURNAS/ELETROBRÁS/CADAFE. Rio de Janeiro, RJ

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• CARVALHO, Newton de O., GUILHON, Luiz G. e TRINDADE, Pedro A. (2000). O assoreamento de um pequeno reservatório - Itiquira, um estudo de caso. RBRH, Revista Brasileira de Recursos Hídricos, Volume 5, n. 1. Jan/Mar 2000, 68-79. Porto Alegre, RS.

• CARVALHO, N.O., GUILHON, L.G. e TRINDADE, P.A. (2000). O assoreamento de pequeno reservatório devido efeito de enchente extraordinária – Itiquira, um estudo de caso. I Symposium of Water Resources of the Middle West Region. ABRH, UnB, ANEEL et al. Brasilia, DF.

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• WILSON Júnior, Geraldo (1973). Transporte e dispersão de sedimentos. UFOP, Ouro Preto Federal University. Ouro Preto, MG.

• WILSON Júnior, Geraldo (1973). O estudo do transporte, da dispersão dos sedimentos e da acumulação de poluentes nos escoamentos à superfície livre. UFOP, Ouro Preto Federal University. Ouro Preto, MG.

• WMO, World Meteorological Organisation. Guide to Hydrological Practices. WMO-No. 168. Editions of 1981 and 1994. Geneva.

• WMO, World Meteorological Organisation (1980). Technical regulations. WMO No 555. Geneva, Switzerland

• YANG, Chih Ted (1996). Sediment transport - Theory and practice. The McGraw-Hill Companies, Inc. New York.

• YUQIAN, Long (1989). Manual on operational methods for the measurement of sediment transport. WMO, World Meteorological Organisation. Geneva, Switzerland.

• ZHIDE, Zhou (1998). Assoreamento de reservatório e erosão à montante. Text book for the course during the III ENES/ABRH. Belo Horizonte.



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GLOSSARY OF TERMS, SYMBOLS AND UNITS

The following definitions are provided in order to assist in the understanding of the terms used in this Guide. They were mainly obtained from the International Committee on Large Dams Guide (ICOLD, 1989) and publications from USGS. • AGGRADATION –Geologic process wherein streambeds, floodplains, sandbars,

and the bottom of water bodies are raised in elevation by the deposition of sediment; the opposite of degradation.

• ALLUVIAL – Pertaining to deposits of alluvia by either a water stream or runoff. • ALLUVIAL RIVER or ALLUVIAL STREAM – a stream in which the bed channel

is made up by significant amounts of sediments transported by the runoff, and in which changes on the bed shape due to changes on the runoff usually occur.

• ARGYLE – particles of sediments smaller than 0,004mm, according to AGU classification. According to ABNT, argyle is particles with granulometry lower than 0,005mm.

• AVERAGE DIAMETER– the sediment size where half material is composed by particles greater than the average diameter, and the other half is made up by smaller particles.

• BED or BOTTOM – the bed or bottom of a stream, reservoir or lake. • BED FRONT LAYERS DEPOSIT – a layer of sediment deposit on the top of a

delta surface. • BED LOAD –Sediment that moves by jumping, rolling or sliding along or nearby

the streambed • BED-LOAD SEDIMENT DISCHARGE SAMPLER – equipment for directly

measuring bed-load sediment discharge, for part or all the stream width. • BED MATERIAL- material composing the riverbed, usually made of fragmented

rocks. • BED MATERIAL DISCHARGE– the quantity of sediment passing through a cross-

section corresponding to bed material particles in movement, both suspended and at the bed.

• BED MATERIAL SAMPLER – equipment for collecting a sampling of the sediment that composes the bed.

• BED SEDIMENT DISCHARGE (usually called as entrainment sediment discharge) – the quantity of bed sediment passing through a cross-section in a time unit.

• BED UPPER LAYERS DEPOSIT – sloped layers of sandy material settled along a higher slope. That layer progressively covers an overbank and, on its turn, is covered by a front layer.

• CHANNEL – generic term for any natural or artificial water runoff having free surface.

• COHESIVE SEDIMENTS – sediments in which the initial resistance to movement or erosion is greatly affected by cohesion chain among particles.



97

• COMPOSITE SAMPLE – a sample made up by the combination of all individual samples, or sub-samples, concerning a suspended sediment measurement performed by the process of equal width increment or equal discharge increment.

• DEGRADATION – the geologic process wherein riverbeds, plain areas susceptible to floods and other water bodies bed are lowered due to the removal of material. Is the opposite of sedimentation.

• DELTA – deposit of sediment made up where there is water in movement (such as a river outfall).

• DENSITY– the mass of a substance by volume unit, ρ in kg/l or t/m3. • DENSITY OF SEDIMENT-WATER MIXTURE – mass by volume unit, including

water and sediment. • DENSITY STREAM – a stream reporting high turbidity and relative density that

usually moves along the bed of a still water body. • DEPOSITION – the mechanical or chemical process through which the sediment is

settled in a still water site. • DRAINAGE AREA– The area that drains to a particular point on a river or stream. • DROPPING VELOCITY – the rate of fall or deposition of a particle in liquid

means. • EROSION – the wearing away of ground by displacement of soil and rock

fragments, due to the action of water movement and other geological agents. • FINE MATERIAL– particles reporting granulometry finer than particles present in

significant quantities of bed material; usually are silts and argyles (particles finer than 0,062mm, according to AGU).

• FINE MATERIAL LOAD or WASH LOAD –part of the total sediment load made up of granulometry not significant in terms of quantities at the bed sediment, and consisting of material finer than the bed material. Usually, the fine material load is made up by particles smaller than 0,062mm; nevertheless, it is a function of the load being transported by the river.

• FLUVIAL SEDIMENT – all solid materials transported by river water and reporting an average density close to the one for fragmented rocks: 2,65.

• GRANULOMETRIC DISTRIBUTION – the frequency distribution of the relative amount of particles in a sampling, comprised by a granulometric band, or the accumulated frequency distribution for a given amount of particles thicker or finer than a given size. These amounts are expressed as percentage by mass.

• GRAVEL – particles of sediment ranging from 64 to 2mm according to AGU classification. According to ABNT, argyles have granulometry ranging from 76 to 4,8mm.

• INTEGRATOR-TYPE SEDIMENT SAMPLER – a sampler capable of iso-kinetically collecting a water-sediment mixture while its beak is moved across the flow.

• NET DISCHARGE or DISCHARGE – the amount of water passing through a stream cross-section in a given time.

• NON-COHESIVE SEDIMENTS – sediments made up by isolated particles. • NON-MEASURED SEDIMENT DISCHARGE – the quantity of sediment load that

the sampler could not sample.



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• NON-SAMPLED ZONE – distance from the sampler bill to the bottom of the river, in a sampling vertical, and that is not sampled; part of the cross-section that is not covered by the sediment sampling.

• OVERBANK – fine material, usually silts and argyles, deposited on the reservoir bed and that later may become covered by upper and front layers.

• RESERVOIR– an artificial lake, basin or pan where a huge amount of water may be stored.

• PARTICLE DIAMETER or SIZE – linear dimension used for characterizing the size of a given particle. The diameter may be determined by any of the several techniques, including sedimentation, siftering, micrometric measures or direct measurements.

• SAMPLED ZONE – the part of the cross-section that is represented by sediment samplings.

• SAND – sediment particles with granulometry ranging from 0,062 to 2,0mm according to AGU classification. According to ABNT, they are particles with granulometry ranging from 0,05 to 4,8mm.

• SCOUR – the enlargement of a section by material removal due to the action of a fluid in movement.

• SEDIMENT – a) particles deriving from rocky or biological materials that are transported through a fluid; b) suspended material or material settled on bed.

• SEDIMENTATION – a comprehensive term that comprises the five basic processes for building sedimentary rocks: a) intemperism; b) detachment; c) transportation, d) deposition (sedimentation) and, e) diagenesis; sedimentation is also defined as the gravitational deposition of suspended particles that are heavier than water.

(b)sediment deposit on a bed river or reservoir that jeopardizes the water resource management.

• SEDIMENT CONCENTRATION – the quantity of sediment in relation to the transported amount of water or water-sediment mixture. The dry weight of solids contained in the water-sediment mixture in relation to the volume of the mixture (mg/l) or in relation to the mixture weight (ppm).

• SEDIMENT DISCHARGE– Rate at which sediment passes a stream cross-section in a given period of time. The sediment discharge may be limited or refer to some sediment granulometry, as well as be considered at a specific part of the cross-section, due to either bed or a section segment suspended sediment.

• SEDIMENT LOAD– the sediment being transported by a stream (load refers to the material itself and not to the quantity being transported).

• SEDIMENT SPECIFIC WEIGHT - dry weight by sediment volume unit or dry weight of the sediment in relation to volume.

• SEDIMENT YIELD – the total amount of tributary sediment in a hydrograph basin or drainage area in a reference point and during a specific period of time. It is equivalent to the sediment discharge in relation to the drainage area.

• SHIELDING– The formation of a resistant layer made up by particles relatively greater, resulting from the removal of fine particles by erosion.

• SILT – sediment particles reporting granulometry between argyle and sand (0,004 to 0,062mm according to AGU or 0,005 to 0,05mm according to ABNT).



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• STATION or FLUVIOMETRIC GAGING STATION – a river channel cross-section where one or more variables are measured, either continuously or periodically.

• STONE – particles of sediment ranging from 256 to 64mm according to AGU classification.

• SUSPENDED SEDIMENT or SUSPENDED LOAD – sediment that is transported by ascending components of turbulent streams, and that remains suspended for a considerable time period.

• SUSPENDED SEDIMENT DISCHARGE – the quantity of sediment passing through a stream cross-section in a time unit.

• THALWEG – Deepest part of a river. • TOTAL LOAD – the total sediment being carried along a stream. • TOTAL SEDIMENT DISCHARGE – the total sediment discharge for a stream. It

includes the measured suspended discharged, the non-measured suspended discharge and the bed discharge.

• TRANSIT or ROUTE VELOCITY – velocity in which the sediment load sampler is submerged into a vertical integration sampling.

• VERTICAL INTEGRATION SAMPLE – water-sediment mixture that is continuously accumulated in a sampler that moves vertically in an almost constant transit rate, between surface and a point a few centimeters immediately above the bed. The mixture enters in a velocity almost equivalent to the stream instantaneous velocity at each point in vertical. Since the sampler bill remains above the bottom, there is a zone that is not sampled, few centimeters in depth, right above the riverbed (see non- sampled zone).

• VERTICAL or DEPTH INTEGRATION – sampling method for obtaining a representative sample of water-sediment discharge for the whole vertical, except for the non-sampled zone nearby the bed.

• VERTICAL FOR SAMPLING or just VERTICAL – a line approximately vertical, from the water surface to the bed, where samplings are taken in order to define the sediment concentration or granulometry.



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Symbols and units as recommended for studying sediments transportation in streams

(WMO, 1980)

Element Symbol Unit Note Acceleration due to gravity g m s-2 ISO Area (cross-section) A m2 ISO Area (drainage area) A km2 ISO (there is also in use) Chézy coefficient [v(RhS)1/2] C m1/2s-1 ISO Conveyance (coefficient) K m3 s-1 ISO Density ρ kg m-3 ISO Depth, diameter, Thickness

D m cm

ISO

Discharge (river runoff) (by unit of area Q A-1 , or partial)

Q q

m3 s-1

m3 s-1 km-2 l s-1 km-2

ISO ISO

Kinematics viscosity υ m2 s-1 ISO Length L cm

m km

ISO

Manning Coefficient = Rh

2/3S1/2v-1 n s m-1/3 ISO

Mass M

kg g

ISO

Sediment concentration Cs mg l-1 kg m-3

Or ppm Also used g m-3

Sediment discharge (or sediment) Qs t d-1 Shearing tension τ Pa ISO Slope (hydraulics, basin) S Non-dimension

Number ISO

Temperature θ t

oC ISO

Total dissolved solids Md mg l-1 (for diluted solution) ppm is also used

Velocity (water) v m s-1 ISO Volume V m3 ISO Wet perimeter Pw m Width (cross-section, Basin)

B

m km

ISO



101

Sediment classification according to granulometry by AGU, American Geophysical Union (Wentworth Classification)

Granulometric Classification

Millimeter Micron Feet Tyler Standard

US Standard

(mm) (μ) (in) (sifter diameter)

(sifter diameter)

Very big cobblestone 4096 – 2048 160 - 80 Big cobblestone 2048 – 1024 80 - 40 Medium cobblestone 1024 - 512 40 - 20 Small cobblestone 512 - 256 20 - 10 Big stone 256 – 128 10 - 5 Small stone 128 - 64 5 - 2.5 Very thick gravel 64 – 32 2.5 - 1.3 Thick gravel 32 – 16 1.3 - 0.6 Medium gravel 16 - 8 0.6 - 0.3 2 - ½ Fine gravel 8 – 4 0.3 – 0.16 5 5 Very fine gravel 4 – 2 0.16 - 0.08 9 10 Very thick sand 2.000 - 1.000 2000 - 1000 16 18 Thick sand 1.000 - 0.500 1000 - 500 32 35 Medium sand 0.500 - 0.250 500 - 250 60 60 Fine sand 0.250 - 0.125 250 - 125 115 120 Very fine sand 0.125 - 0.062 125 - 62 250 230 Thick silt 0.062 - 0.031 62 - 31 Medium silt 0.031 - 0.016 31 - 16 Fine silt 0.016 - 0.008 16 - 8 Very fine silt 0.008 - 0.004 8 - 4 Thick argyle 0.004 - 0.0020 4 - 2 Medium argyle 0.0020 - 0.0010 2 - 1 Fine argyle 0.0010 - 0.0005 1 - 0.5 Very fine argyle 0.0005 - 0.00024 0.5 - 0.24 Colloid < 0.00024 < 0.24

Notes: 1) For some countries, including Brazil, the following classification is adopted

by ABNT (Atterberg Classification) - Gravel: 76 - 4.8 mm Sand: 4.8 - 0.05 mm Silt: 0.05 - 0.005 mm Argyle: < 0.005 mm

2) AGU classification is used in these works due to the use of formulas and software developed in Britannic measurement units.

Documents

RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE · RESERVOIR SEDIMENTATION . ASSESSMENT GUIDELINE . Newton de Oliveira Carvalho . Naziano Pantoja Filizola Júnior . Paulo Marcos Coutinho