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C.J. Sloff (1997) Modelling reservoir sedimentation processes for sediment management studies.

Proc. conf. Hydropower into the next century , Portoroz, Slovenia, 15-17 sept. 1997, p. 513-524, Aqua Media Int., UK.

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M odel l ing reservoi r sedi mentation processes forsediment management studi es

Dr. C.J. SloffProject and Research EngineerDelft HydraulicsP.O.Box 1772600 MH DelftThe Netherlands

IntroductionHydropower reservoirs are loosing their capacity due to sedimentation processes, and are thereforeseriously threatened in their performance. The quiescent pool behind the dam generates favourableconditions for particle settling, such that important storage capacity is lost. Furthermore, significantchanges can occur in the stream basin due to the redistribution of sediments and discharges, notablydownstream. Without any mitigating measures the viability of many reservoirs is questionable, as theimpacts and losses are not balanced by the profits. It is apparent that for mastering thereservoir-sedimentation issues the use of strategies for controlling reservoir sedimentation becomesincreasingly important. Obviously a good prediction of the processes, and the endeavour to betterunderstanding of the reservoir behaviour is essential.

To illustrate the use of modelling techniques for predicting and reducing reservoir sedimentation, twotypes of applications are discussed in this paper: a sediment management study for the Tarbela reservoirin Pakistan, and a study to the development and release of turbidi ty currents in hydropower reservoirs ingeneral. Both studies aim at defining tools which enables us to reduce reservoir-sedimentation problems.For the Tarbela Hydropower Reservoir it is shown in this paper that deltaic deposits form a serious threatfor the performance of the reservoir. To mitigate the potential dangers for the dam operation, severalmanagement options should be studied and combined into an action plan for Tarbela. A systemcomputational framework is presented which comprises a sedimentation model for the reservoir, and awater-resources model for the entire river basin. The principles of sedimentation in this reservoir areil lustrated by means of exemplary computations with a one-dimensional morphological model. Differentto the formation of deltaic deposits at the head of most reservoirs like Tarbela, turbidity currents (density

currents caused by sediment) can bring significant amounts of fine sediment up to the dam where theycan be sluiced out without large losses. In this paper a general two-layer modelling technique developedby the author is presented, and the (for reservoir sedimentation and sluicing) most relevant propertiesand conditions of these currents are presented. Two examples are given to illustrate the modellingtechnique proposed for this type of flow.

1. Reservoi r-sedimentati on probl emsIt is well accepted that reservoir sedimentation poses a seri ous threat to available storage. The annual lossof storage in reservoirs is roughly 1% corresponding to a about 50 km3 world wide (Mahmood, 1987).Some reservoirs have a much higher storage loss, e.g., the Sanmenxia Reservoir in China looses about1.7% yearly. In the meantime significant transformations can occur in the stream basin due to theredistribution of sediments and discharges. Sloff (1991) reviewed these phenomena by means of a surveyof the scattered literature in order to find the remaining gaps in the applied theory. Theoretical

approaches are here desired to estimate the sedimentation threat and even to reconsider the design. In


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the past highly empirical models were used for this purpose, but often resulted (sometimes deliberately)in an underestimation of the actual sedimentation rate. This can be ascribed to failing theory as well as toa lack of data. For instance sedimentation rates of the Sefid-Rud reservoir in north-west Iran can be

estimated with a 60 years old highly empirical approach (Tolouie et al., 1993) to be about 35 106 m3/ a.

However, after construction (in 1962) the measured rate was about 45106 m3/ a causing a storage loss ofover 30% in 1980. The original predicted useful reservoir life of one century based on old data, was foundto be actually about 30 years (Pazwash, 1982). Not until 1980 flushing operations were started whichwere able to regain about 7% of the lost capacity.

When dealing w ith reservoir-sedimentation problems engineers are challenged by the dif ficul t questionsemerging. How to incorporate reservoir problems in feasibility studies (cost-benefit analyses) includingenvironmental and technical effects, limitations on benefit and possible measures? Or what is the impactof sedimentation on the reservoir performance, and what is the impact of the reservoir on stream-systemmorphology? Obviously a good prediction of the processes, and the endeavour to better understandingof the reservoir behaviour is essenti al to master the reservoir -sedimentation issues.

Figure 1 shows the principle processes involved with sedimentation in a storage reservoir as treated inSloff (1991 and 1997). The most important distribution principles of these sediments in the reservoir canbe subdiv ided into the following groups:

- Coarse sediment deltaic deposits: mainly the coarse sediment fractions are deposited in the head ofthe reservoir by backwater effects during high discharges, forming a delta. The delta proceeds into

the reservoir while the foreset slope can be considered as an area of instability and slumping.- Fine sediments in homogeneous flow: A large part of the fine sediments transported in suspension

or as washload are transported beyond the delta after which they settle out to form the bottomsetbed. They are more evenly spread than coarse sediment, but there distribution is highly dependenton reservoir circulation and stratification, for instance generated by river inflow and wind shear, orprecluded by an ice cover. Also for this type of deposition the quantification methods still yieldrough predictions.

- Turbidity currents: another important transport mode for fine sediments, i.e., sil t and clay, is theturbidi ty current. It is formed when the turbid river inf low plunges below the clear reservoir w aterand continues as a density underf low . Also other processes can generate them, such as underw aterslides (slumping of delta front) or coastal erosion. Turbidity currents are driven by an excess gravi tyforce (negative buoyancy) due to the presence of sediment-laden water in a clear ambient fluid.

These low velocity currents are capable of transporting large quantities of sediment over longdistances. They become more and more accepted as potential measure to reduce sedimentation

Figure 1 Schematic presentation of principle sedimentation processes in river-fed storage reservoirs.


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although their contribution is less than deltaic deposit processes (usually they create mud depositsnear the dam).

Methods to mi tigate the problems are usuall y based on reduce the inf low of sediment, the manipulationand control of the processes mentioned above, and the mechanical removal of deposits (e.g., see Sloff,1997, Fan and Morris, 1992). The most important motive to use strategies for controlling reservoirsedimentation still is the preservation of reservoir storage (especially if appropriate sites for replacementare unavailable), but impacts up- and downstream of the reservoir gain more consideration now.

For sake of fighting the reservoir-sedimentation problems a need exists for quantification of theprocesses. Prediction of capacity losses, impacts on the stream, and distribution of sediments, as well asthe efficiency of mitigating measures require modelling techniques which can be used to determineoperating rules and feasibility demands of the project. Above it is shown that various complex andcoupled mechanisms determine the issues. Furthermore we are dealing with large uncertainties inmeasured and forecasted data. For instance the sediment yield, which is the source of all sedimentationproblems can usually not be predicted accurately and with sufficient detail. Early modelling attemptshave often proved to be unreliable, which forced engineers to put a significant effort in inventing more

sophisticated approaches. In the following sections two specific studies are presented to illustrate themodelli ng techniques for reservoir-sedimentation management.

2. M odell ing sedi mentati on processes in Tarbela Reservoir, PakistanTo illustrate possible methodologies for modelling reservoir sedimentation processes and impacts, anexample is given of the Tarbela reservoir. The Tarbela dam and Reservoir project is a major waterresources and hydroelectr ic development project located in the Indus River about 100 km northw est f romIslamabad in Pakistan. For hydropower generation a capacity of 3,478 MW is installed. The 143 m highand 2743 m long dam earth-rockfill dam, and two auxiliary earth-rockfill embankment dams and theresulting reservoir were completed in 1974 (construction started in 1968). The reservoir had a capacity of14.3 km3 (less than one fifth of the total annual river runoff), and a length of approximately 85.5 km atnormal pool level. This volume included a dead storage of 2.59 km 3 below elevation 396 m (1300 ft ). Thelong shape of the reservoir is illustrated in Figure 2, in which a schematized plan of the reservoir ispresented based on the 1550 feet contour line (maximum water level in the reservoir).

Sediment carried by the Indus is deposited in the reservoir at an annual rate of about 202 million short

tons, corresponding to about nearly 98 % of the sediment inflow. This results in large deltaic deposits.The composit ion of the deposits varies betw een coarse sand and fine sil t or clay fractions. Most of the

Figure 2 Tarbela reservoir in Pakistan (schematic)


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sediment enters the reservoir during the months June to A ugust when the river discharges are maximaldue to summer snow melt in the Himalayas with peaks up to 8,500 m3/ s. In the intermediate periods theflow and sediment discharges are low. At the beginning of the flood season the reservoir level ismaintained at minimum pool (M ay and June). The fi rst high inf lows (increasing f rom 1500 to about 5000m3/ s) are used to rework the and flush part of sediment which is deposited in the year before. Thesedeposits are laid down in the upper reaches of the reservoir when the reservoir level rapidly r ises in Junewhen f il ling starts. The reservoir is fi ll ed to el. 1550 ft (472.4 m).

During minimum pool level the incoming floods erode a flushing channel in the deltaic deposits. Theflushing channel gradually increases in width by bank-erosion processes during this period. For instancechannel widths increasing from some 400 m to 1400 m are reported in 1981 by discharges up to 5000m3/ s. Highly erosive flows with suspended-sediment concentrations of about 20 times the inf lowconcentration, are moving the upstream deposits to the delta front. For instance in 1981 the top of theforeset slope advanced three miles during the flushing period.

In November 1996 the delta extended from about mile 44 (km 71) to about mile 6 (9.7 km) from the dam,which impl ies that nearly 44% of the inactive storage and 15 % of the active storage is now occupied by

the sediment delta. Depending on the reservoir operation the average rate of advance of the delta to thedam is about 600 m a year. To slow the rate of propagation of the delta the minimum pool level wasraised in stages starting from 1991, raising it from a level 1320 ft to a level of 1374.7 ft in 1995. As aconsequence the downstream movement of the main delta was practically arrested, but a new small'piggy-back' delta develop on top of the main delta. The observed development of the delta is illustratedin Figure 3.

The rate of which sedimentation proceeds is considered as threat for the reservoir performance. Theproblems were put forward by WAPDA (Water and Power Development Authority Pakistan) in 1996.The present location of the delta front imposes the danger of clogging of the five low-level tunnels of thedam when the fine-sediment foreset slope slumps, for instance due to liquefaction under a moderateearthquake. On the other hand, if the sedimentation front practically reaches the dam, highconcentrations of sand will pass through the turbines and outlets, causing scour and abrasion, severelydiminishing the economic li fe of the project.

Possible and proposed countermeasuresThe arrestment of further advance of the delta and the mitigation of potential dangers for the damoperation have become very urgent topics for Tarbela. To reduce or counteract the amount of deposition

in the reservoir three groups of commonly applied measures can be distinguished:1. Methods to reduce the inflow of sediment into the reservoir, e.g. by erosion control in the catchment

Figure 3 Observed average bed levels in Tarbela reservoir.


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or bypassing of sediment-laden flows. This approach is usually very effective, but the dangersalready present in the reservoir are not mit igated.

2. Methods using the hydrauli cs of flow to reduce accumulation of sediments, or to induce erosion ofaccumulated material (sluicing and flushing). The aim of these methods is to reduce the trapeff iciency of the reservoir. The method is already practised in Tarbela at the begin of the high-flowseason by the draw down of the pond level. It is very effective due to the long-narrow shape of thereservoir. N evertheless the flushing period in Tarbela is to short for removing a sufficient amount ofsediment from the reservoir. Effectively the deposits are eroded from the upper reach andredeposited at the front set of the delta, increasing the annual advance speed of the delta. Flushingmethods exercise serious restraints on the reservoir operation and the reservoir y ield . Therefore thechief disadvantage of sluicing options is that all require sluicing during the initial period of the highflow season and imply curtailment of power during this period of increasing power demand.

3. Methods based on hydraulic dredging and mechanical excavation. This is an often used eff icientalternative but a very costly one.

Specifically for Tarbela, additional to these approaches several options have been put forward in the pastto eliminate the direct dangers of the deltaic deposit ion for the dam, for instance:- conserv ing the present posit ion of the delta front or to reduce its progress. This is achieved already

by modifying reservoir operation by raising minimum level in stages. Although it prevents furtherprogress of the delta front, it results in a further reduction of active storage by deposit ion at the headof the reservoir.

- guarantee clear operati on of the outlets and turbines. For instance the construction of a submergedretention structure (under water dike) is proposed to form an arc in front of the intakes. It isdesigned to prevent clogging of the outlets by sediments descending from the delta. It should bestrong enough to withstand the impact of the collapsing foreset slope caused by l iquefaction duringan earthquake. Also construction of bypass tunnel/ channel w ith various sluicing options is apossible option to conserve the operation of the outlets and turbines.

The solutions to the problems in Tarbela should be looked for in a combination of the measuresmentioned above. To recommend an action plan for these sediment management options, a study mustbe carried out accounting for costs, power and irrigation benefits, and risks and failures of the project.Clearly a system modelling approach is needed because of the effects of lost storage on system output(i.e., energy generation and irrigation water releases) and also because of the impact of possible changesin reservoir operation policy at Tarbela on system output. Also other projects planned in the vicinityshould be taken into account. For instance the Ghazi Barotha Hydropower Project (1,450 MW power) atabout 7 km downstream of Tarbela, which will be on line by 2001. Or the planned Kalabagh DamHydropower Project which is 193 km downstream of Tarbela, and the aimed Basha Dam Project which is274 km upstream of Tarbela.

System modelling approachIn the following a principle way of modelling is presented which can be applied for this type ofreservoir-sedimentation studies. In this presentation we have tried to reflect these techniques for theexisting situation in the Tarbela Reservoir. On basis of the identified state of sedimentation and possible

measures, a system computational f ramework wi ll be set up comprising the follow ing two elements:- a sedimentation model for Tarbela reservoir , regarding the dynamic simulation of physical

processes in the reservoi r.- a water resources system model for the ri ver basin comprising the Tarbela reservoir, the Basha

project, the Ghazi-Barotha project and the Kalabagh project. For this type of modelling the DelftHydrauli cs' generic river-basin simulation model RIBASIM can be applied. The main elementscomprised in the model are ill ustrated i n the scheme in f igure 4.


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There must be a close interaction between the two models but not necessarily an on-line link. We proposean exchange of data between the models as fol low s:- From water-resources system model to sedimentation model: the inf low and outf low of the Tarbela

reservoir and its resulting water level are the main output data per time step of the system model to

be used for sedimentation calculations.- From sedimentation model to water-resources system model: The resul ts of a sedimentation

calculation can for instance be used in the water-resources model in the form of a'level-area-volume' curve.

In principle there are several simulation options for the combined water-resources andreservoir-sedimentation model. For instance a simulation with a maximum reality could be used, whichimplies a continuous adaptation over time of the reservoir sedimentation and related reductions inhydropower and irrigation water allocations. Although this seems to be a very 'natural' option, such amethod is less attractive when the influence and sensitivity for various parameters must bedistinguished. To get a proper insight in for instance the effects of sedimentation on water allocationreductions, it is better to make separate simulations with well-defined conditions in which only a few

parameters are varied at a time. Only in that way it wil l be possible to draw conclusions from simulationsand to determine the effects of recommended options for reductions of sedimentation.

The water-resources model is especially worthwhile for optimalisation of reservoir managementtechniques based on influencing the operation rules (e.g., the stage-wise raise of the minimum reservoirlevel). Reservoir operation, c.q. management, is reflected in the 'ru le curves' of the reservoir . Rule curvesdetermine how the reservoir is operated in a simulation. With the target reservoir rule it will be possibleto influence the sediment delta at the entrance of the Tarbela reservoir. Different scenarios for ir rigationand firm hydropower can be tested. If for example such a model is composed in the river-basinsimulation model RIBASIM, detailed information on reservoir levels, irrigation-supply shortages (andcrop damages), hydropower generation, and shortages in f irm energy can be provided.

The sedimentation model for Tarbela must be able to simulate all relevant processes occurring in the

Reservoir, such as the characteristic dynamic cycle of sediment accumulation and redistribution. Thisprocess is a result of the fluctuation of the reservoir level in combination with seasonally changingvolume of inflow and corresponding sediment load. To simulate these unsteady processes incombination w ith the water-resources model it is reasonable to apply a one-dimensional morphologicalmodel. The use of a one-dimensional model enables simulation of a wide range of scenarios andproposed alternatives for the entire reservoir. It allows for the deposition processes in the upper reachduring high reservoir level, followed by the reworking of the deposits during the subsequent draw-downperiod. Furthermore it provides information on the time scales of delta development and sluicingprocesses, necessary to support decisions regarding operational strategies. here the flexibility of a one-dimensional approach is much more beneficial than a two-dimensional or three-dimensionalcomputational models.

The simulation of delta development in Tarbela reservoir requires a correct physical simulation ofdeposition processes and erosion processes. Account ing for the measured longitudinal sort ing of

Figure 4 Scheme of the water-resources system for Tarbela.


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sediment grains in transport and deposits (downstream fining) the simulation must therefore be carriedout with a graded-sediment module. Such a module must be able to simulate the periodic fining andcoarsening of the bed material, the sorting effects observed in vertical direction, and the hiding andexposure effects of sediment grains of different sizes during erosion. Furthermore the model mustaccount for the flushing channel developing in the deposits during reservoir drawdown. The entrenchingeffect of this channel can be studied in a semi-empirical way using state-of-the-art validated relations. Asimilar but not yet comprehensive model was proposed by Chao and Ahmed (1985). The stability andimpacts of liquefaction of the foreset slope can be studied independently on basis of the outcomes of thesedimentation study.

To il lustrate the principle of such a one-dimensional model appl ication some simulations are carr ied outusing the one-dimensional modell ing system SOBEK developed by Delf t H ydraulics. For simplicity sake,and for the lack of sufficiently detailed data, the calculations have been carried out for generalizedconditions and simplified schematization. The model is not calibrated, as the calculations are only aimingat presentation of the principles. We have concentrated on the simulation of delta front and grain -sizevariation, without accurately accounting for channel formation.

In Figure 5 the bed levels computed w ith SOBEK are plotted, showing the gradual progress of the deltaas shown in Figure 3. In the next figure, Figure 6, the computed variation of the mean grain-size diameter

is plotted. It illustrates the longitudinal sorting effects of the sediment characteristic for these type ofsituations during the development of the delta. Coarse fractions are deposited within the delta, leaving

Figure 5 Computed delta growth with the uncalibrated model (dotted line is initial state in 1974, drawnline is delta position in 1978).

Figure 6 Mean grain size Dm computed with the uncalibrated model (dashed line is situation in 1975,drawn line is situation in 1978).


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the finer fractions downstream at the bottom set. A very sharp transition in grain size is visible at thepivot point of the delta.

The high quantities of fine sediment during the early periods of reservoir filling provide favourableconditions for the occurrence of turbidity currents. These density currents,, other than caused by theslumping of sediment on the foreset slope, can be considered in relation with sediment sluicing. Amodelling approach for this type of phenomena is subject of the next section.

3. Turbi di ty currentsAs coarse sediments usually are deposited in the head of the reservoir forming a delta, the finersediments can be transported over significantly larger distances up to the dam. The most efficienttransport of fine sediments into the reservoir is generated by turbidity currents, i.e. density-currentunderflows of fine sediment such as silt and clay. For instance, if during reasonable high inflows withadequate sediment concentrations the inflow plunges below the clear reservoir water, a density current isformed which is driven by the excess density of the turbid fluid, and maintains its suspension by theturbulence generated by bottom friction. If the turbidity current reaches the dam, it is possible to sluice itthrough bottom outlets efficiently without significant losses (without water-level draw down). Sediments

carried by turbidity currents are generally transported much further than sediments in normalsuspension.

As part of his PhD study, Sloff (1997) studied the modelling techniques which can be used to simulatethis type of underflow. Simulations of these currents must aim at determining the conditions at whichthese currents can exist, and how they develop after originating. For this reason Sloff proposed a two-layer mathematical model (depth-averaged layers) and studied the behaviour of these currents inreservoirs. In this section a short overview is given of the main results of this study, and an outlook topractical appl ication.

Mathematical modelMost models for turbid underflows are in some way analogous to models for conservative saline and

thermal density currents for which a lot of references exists. By contrast to the latter the sediment in aturbidity current is in general a non-conservative contaminant. Sediments can be entrained and depositedat the bed, thus changing the total amount of sediments in suspension. For a swift turbidity flow on asteep slope the net pick-up of sediments increases its negative buoyancy through which it accelerates andpicks-up more sediments. Although this is a credible phenomenon, most turbidity currents in reservoirsare of a net depositing nature with relatively low velocities and low densities. Contrary to self-accelerating turbidity currents are those which loose their transport energy and eventually die out bysettling of particles. Clearly the dynamic interactions between sediment exchange, sediment suspension,mean flow and turbulence are very delicate, hence turbidity-current modelling is much more difficultthan modelling of open-channel flow or conservative density currents. To quantify the underflow it isimportant to realize that it is originating and determined by the integral behaviour of the fluid andsediment mixture. A different behaviour than individual particles is due to their aggregation and theresulting mutual interactions between the part icles.

Whereas 1-D layer models are still the most commonly used there are some extensions to 2-DVapproaches (transversally averaged). Also for these models the delicate flow-sediment interaction and thelack of data delays the progress, and still impede a justified extension to 3-D. Similarity in modellingturbid flow in sedimentation basins for sewer systems can be used as a reference for further research.Considering the existing models it can be observed that there is still need for improvement. Most layermodels are still limited to 1-D and one layer (turbidity current entering a infinitely deep ambient fluid),or they do not account for shocks. Although their simplicity compared to a fully 2-DV or 3-D model isobvious, the price to be paid for simplicity is the requirement of empirical closure relations to describephysical processes which are still rather obscure. On the other hand, the much more advanced turbulencemodels for 2-DV and 3-D approaches are only in a very early stage of development (with respect tosediment-fluid interaction) and at the time (in 1991) their superiority over layer models could not beproved. For reservoir sedimentation we have finally chosen, in agreement with observed stratification, todevelop a two-layer model w ith a clear quiescent upper layer and a turbulent dense lower layer which is


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free to exchange sediment with the bed. The derivation of this model and the required closure relations istreated in Sloff (1994).

Key elements in the two-layer model are the formulation of the fluid-sediment interaction and thetreatment of fronts and jumps in the turbid underflow. The derivation of the model is based on smalldensity differences (e.g., concentrations lees than 5%), and the flow is assumed to be fully turbulent. Infigure X a definition sketch of the approach is given. Here u represents flow velocity, a is depth, c isconcentration, wie is entrainment (mixing), and is shear stress.

The resulting mathematical model is two-dimensional in plan and allows for unsteady turbidity-current

development on a mobile (alluvial) bed. The important effect of a finite reservoir depth is accounted for.The primary closure of the model consists of relations for velocity and sediment profiles, boundary shearstresses, entrainment velocities (interfacial mixing), bed-load transport, and sediment fall velocity. The 3-D adaptation effects of suspended-sediment concentration profiles are included by means of a semi-theoretical approach.

Properties of turbidity-current developmentThe two-dimensional model is analyzed by means of analytical solutions in simplified situations.Furthermore a one-dimensional numerical model is derived from the equations to verify the approach.The observations and computational results indicate that dur ing the development of the turbidi ty current(directly after plunging below the reservoir water) the flow is characterised by the propagation of agravity-current front, followed by a relatively uniform underflow in which shocks and jumps are easily

developing. Internal jumps in the underf low are usually caused by unsteadiness of inf low and changes inreservoir geometry. If the current losses its momentum (e.g., due to an obstruction) it will break down,and sediment rapidly deposits.

Usually the plunge point of the turbidity current is located near the pivot point of the delta topset slope.Depending on the following reservoir geometry the following underflow often spreads over the reservoirbottom. However, the results of the analyses prove that spreading of the current is unfavourable for thepropagation and sluicing of the current. In Figure 8 two analytical solutions for respectively anunderflow in a flume (upper figure) and a sector tank (lower figure) are illustrated. On the right side ofthis figure is shown how the current appears in nature. Both currents are originating from identicalinflow conditions.

Figure 7 Definition sketch.


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In both situations vortices (Kelvin-Helmholtz instabilities) develop behind the gravity-current head.These vortices are responsible for entrainment of clear water from the ambient fluid. In a prismaticchannel or flume these vortices break up to form a stable interfacial mixing layer. However, in aspreading current the intensity of rotational motion is increased by vortex stretching, and a significantamount of clear water is entrained. The rim computed at the front of the spreading current can beassociated to the large leading edge vortex often observed in nature. The remaining vortices (visible asmultiple fronts) in the following current practically take up the entire underflow depth, only leaving asmall layer of dense fluid near the bottom. Calculations with entrainment in these currents indicate thatthe dilution is severe to such an extent that this type of current can rapidly break up, and deposit itssediment.

From these resul ts can be concluded that turbidi ty currents are more likely to reach the dam in a narrowreservoir, or if they can travel through a channel. Obviously such a channel may be formed by drawdown flushing operations, or by dredging. It is therefore worthwhile to study turbidity-current sluicingin combination wi th d raw-down f lushing. Sluicing turbidity currents requires low-level outlets near thedam. As it is diff icult to predict the actual path of the current i t is necessary to provide these outlets over

a sufficient width. Nevertheless, for draw-down flushing operations a similar argument is valid. Inpractice flushing and sluicing operations are most efficient i f w ide low pitched outlets are installed.

ExamplesDetailed and accurate field data from turbidity currents is hardly available. To verify the computedphysical properties and development of such currents, we therefore applied data from laboratoryexperiments. These applications were presented in Sloff (1997). For instance we applied the model to aflume experiment carried out in St. Anthony Falls Hydraulic Laboratory by Garcia (1993). A sediment-fluid mixture with a volumetric concentration of about 0.00133 of silt particles with D50 of 9 m wasdischarged (0.0025 m/ s) into an 11.6 m long and 0.3 m wide f lume. The latter facili ty consists of an

inclined bed (4.6) followed by an horizontal section. In the resulting equilibrium state (after at least 20min.) it can be shown that on the sloping floor the flow remains internally supercritical (densimetricFroude numbers larger than unity), while on the horizontal floor the flow becomes internally subcritical.The agreement between measured and computed depth in the final equilibrium state is illustrated in

Figure 8 Comparison of gravity currents in a prismatic channel and a sector tank using identicalinf low parameters


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Figure 9. Similarly a good agreement in depth-averaged velocity and sediment concentration in theunderf low was found (Sloff, 1997).

Although high-quality field data is hardly available, one occurrence of turbidity currents in the field ispresented here for verification of the model. In 1987 extensive field measurements were obtained byChikita et al. (1991) of a turbidity current in the glacier-fed Peyto Lake. The typical geometry of this lake

is il lustrated in Figure 10.

Figure 9 Computed and measured depths for experiment DAPER2 (turbidity).

Figure 10 Bathymetric map of Peyto Lake (from Chikita et al., 1991) used for the two-layer model.


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Figure 11 Computed development of the underflow in Peyto Lake, and measured and computedprofi les at station D, 14 July 1987.

The turbidity current plunges near the foreset slope of the lake delta (left in Fig. 10) and advancesnorthward to the central part of the lake where it meets a sub-aqueous sill. Measurements at stations C, Dand E in this figure clearly revealed the presence of turbidity currents which were able to pass the sill.Simulations with the two-layer model as presented in Figure 11 show how, before crossing the narrowelevation, the underflow builds up in front of it while reducing its velocity, and finally passes it.Although the concentrations and velocities during this event (computed with a semi 2-D approach and

compared with data in Figure 11) remain small, stil l about 200103 kg sediment enters the lake in a periodof 12 hours. The total contribution of underflows to yearly sedimentation in this lake is about 61%, whi leanother 32% is due to delta propagation.

4. Conclusions

The loss of reservoir storage due to reservoir sedimentation can be considered as a serious threat toreservoir performance. To master these issues it is necessary to develop modelling techniques which areable to give a good prediction and a better understanding of the reservoir behaviour . Part of this paper isbased on the PhD study on reservoir sedimentation presented in Sloff (1997). The following conclusionshave been drawn in this paper:- Reservoir sedimentation processes are determined by di fferent transport and deposition modes of

sediment fractions. The coarsest fractions result in deltaic deposits at the head of the reservoir, whilefiner sediments are transported further into the reservoir. If turbidity currents occur these finesediments can be transported up to the dam.

- For reducing the sedimentation rate some effective methods exist based on adaptation of thereservoir operation rules. Flushing of deltaic deposits by periodic draw down of the water level, andsluicing of turbidi ty currents through bottom outlets are valid options.

- Modell ing techniques related to threats imposed by deltaic deposits are il lustrated by means of an

example for the Tarbela Dam and Reservoir Project in Pakistan. A system computational frameworkconsisting of a sedimentation model (regarding dynamic simulation of all relevant physicalprocesses) and a water-resources model (regarding the entire river-basin) is presented. Theapproach allows for an optimalisation of reservoir-management options, accounting for irrigationwater releases, hydropower generation and impacts on other projects in the vicini ty.

- The sedimentation model as part of the system modelling approach should account for thelongitudinal sorting of sediment transport and deposits (physics of graded-sediment), and channelformation during f lushing. For Tarbela reservoir a one-dimensional model can be appl ied.

- A modell ing technique for turbidity-current simulation is presented, which can support sluicingoperations design. The two-layer computational model is capable of computing the type of turbiditycurrents which are common in reservoirs. The properties of turbidity currents derived from themodel and from observations show that for optimal use of these currents as transport medium,

unbounded spreading of the current must be prevented. Narrow reservoirs, or flushing channelsare favourable conditions for turbidi ty current sluicing.


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- Detailed and accurate field data from turbidity currents is hardly available. Verif ication of themodel is therefore carried out by means of laboratory data and data from a glacier lake. Twoexamples of these verif ications show a good agreement between computed and observed data.

5. ReferencesChao, P.C. and S. Ahmed 1985) A mathematical model for reservoir sedimentation planning. Water

Power & Dam Construction, Vol. 37, No. 1, Jan. p.45-52.Chikita, K., N. Yonemitsu and M. Yoshida (1991) Dynamic sedimentation processes in a glacier-fed lake,

Peyto Lake, Alberta, Canada. Japanese. J. of Limnology, Vol.52, No.1, p.27-43.Fan, J. and G.L. Morris (1992) Reservoir Sedimentation II: Reservoir desiltation and long-term storage

capacity, J. Hydr. Engrg., Vol. 118, No.3, p.370-384.Garcia, M.H. (1993) Hydraulic jumps in sediment-driven bottom currents. J. Hydr. Engrg., ASCE,

Vol.119, No.10, p.1094-1117.Mahmood, K. (1987) Reservoir sedimentation: Impact, extent, and mitigation. Techn. Paper No.71, The

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Tolouie, E., J.R. West, and J. Bill am (1993) Sedimentation and desil tation in the Sefid-Rud Reservoir , Iran.In: J. McManus and R.W. Duck (eds.) Geomorphology and sedimentology of lakes and reservoirs.J. Wi ley & Sons, England, Chapt. 9, p.125-138.

Sloff, C.J. (1991) Reservoir Sedimentation: a literature survey. Comm. on hydr. and geotechn. engrg.,Report No. 91-2, Delft Univ. of Technology, The Netherlands, 126 pp.

Sloff, C.J. (1994) Modelling turbidity currents in reservoirs. Comm. on hydr. and geotechn. engrg., ReportNo. 94-5, Delft Univ. of Technology, The Netherlands, 142 pp.

Sloff, C.J. (1997) Sedimentation in Reservoirs, Doctoral Thesis, Delft University of Technology, 270 pp.(also published as: Comm. on hydr. and geotechn. engrg., Report No. 9712, Delft Univ. ofTechnology, The Netherlands).

6. Biographi cal detai ls of the author

M r. C.J. Sloff graduated in Civil Engineering at the Delft University of Technology in 1990. Aftergraduation Mr. Sloff was employed at the Hydraulic and Geotechnical Engineering Division of theFaculty of Civil Engineering of the Delft University of Civil Engineering as a research assistant to studyreservoir sedimentation under supervision of Prof.Dr. M. de Vr ies, and obtained his Ph.D degree in 1997.This study was carried out as part of a research project on sedimentation in reservoirs, a joint cooperationbetween Delft University of Technology and Delft Hydraulics. In 1995, Mr. Sloff joined Delft Hydraulicsin the function of project engineer in the field of river hydraulics and morphology. Since then, he hasbeen involved in projects with a strong river-engineering background, with particular emphasis on one-and two-dimensional mathematical modelling approaches.

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