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Sedimentation management in hydro reservoirs

Published on August 16, 2013 in Siltation

By S. Alam

Keynote speech by S. Alam

Water lndia – 4
Delhi, February 3 -4, 2004 .

Introduction

“Among the many sessions of the Third World Water Forum held in Kyoto, Japan in March 2003, there was one titled ‘Sedimentation Management Challenges for Reservoir Sustainability’. Two main messages emerged from that session:

– Whereas the last century was concerned with reservoir development, the 21 st century will need to focus on sediment management; the objective will be to convert today’s inventory of non-sustainable infrastructures for future generations

– The scientific community at large should work to create solutions for conserving existing water storage facilities in order to enable their functions to be delivered as long as possible, possibly in perpetuity.”

We must say that the above messages summarizes very well the challenge that all engineers involved in storage reservoir sedimentation management should keep in mind and work towards finding reasonable solutions.
For new projects this would mean:

– Having good knowledge of the watershed sediment yield and if necessary and/or possible propose long­term solutions which would either maintain the present sediment yield if considered satisfactory , if not, take actions which will gradually reduce the sediment yield from the watershed area.

– Confirm predicted sediment yield by actual field measurement of the total sediment load (sand, silt and clay) that is being transported by the river.

– Amongst other criteria used for determining the proposed dam and reservoir characteristics also include the impact of the expected sediment load on short and long-term evolutions of the project and possible remedial measures.

– Analyze reservoir sedimentation management strategies by using numerical model like RESCON proposed by the World Bank¹

– Incorporate in the dam design possibilities of future structural modifications or retro fitting of structural arrangements to alleviate the problems that might be created by a reservoir full of sediment.

For existing projects with severe reservoir sedimentation, it will be necessary:

– To develop solutions which will enable to stop its further degradation and carry out remedial measures which will enable restorations of ample power generation in a manner permanent.

– In this respect we think that the very small scale physical modelling technology developed for studying the diversion concepts of the bulk of the Mississippi River sediment load into the wetlands in its delta could eventually be used for:

* Reproducing the actual known historical sedimentation process of the reservoir

* Develop practical and viable alternative solutions of reservoir rehabilitation by flushing and
hydro suction.

* Verify the potential for optimization of water consumption for the combined flushing and
hydro-suction operation

* Limitations of flushing operation during the flood flows and hydro-suction during the average
or low flows.

We believe that such design procedures have great potential for reducing the initial construction cost, increase the project life and defer costs for structures not necessary at the project inception and eventually completely eliminate structures like the de-sanding structures for projects with a large reservoir. Where it is not required when the reservoir is free of sedimentation and with the reservoir full of sediment the concentrations during the flood flows are so high, that the de-sander is no longer capable of trapping sand particles adequately to prevent turbine abrasion, and plant shutdown becomes often necessary .

___________

1 Quoted from the foreword by lan Johnson, Vice President, Sustainable Development, World Bank for the World Bank Publication ‘Reservoir Conservation by Alessandro Palmieri, Farhad Shah, George W. Annandale and Ariel Dinar, June 2003.

1. Watershed sediment yield assessment and its management

During the project design phase thorough investigation of the project watershed regarding the parameters such as nature of the soil, intensive use of land for agriculture, pastures, systematic de-forestation, intensity of rainfall etc., should be carried out and documented. Based on the distribution of the above parameters over the entire watershed area its long-term annual sediment yield in tons/km²/year may be established. As general information we may indicate that the areas with highest sediment yields may produce more than 10,000t/km²/yr and the minimum value could be less than 50 t/km²/yr. The precise assessment of the sediment yield of a watershed is therefore not an easy task. Expert advice and in situ measurements may help to obtain more reliable indications.

Another way of estimating watershed sediment yield from a given particular geographical area will be to carry out regular and precise bathymetric survey by using multi-beam echo sounding technique in existing storage reservoirs and by collecting sediment samples at selected locations within the reservoir. This way it will be possible to have fairly complete information on the average annual or periodic volume and/or weight of sediment load transported by the river into the reservoir. At the same time it will be possible to obtain information such as: Sediment particle gradation, their mineralogical composition and sand particle form coefficient.

Based on the preliminary findings during the design stage it may either be concluded that the sediment yield from the watershed is compatible from the stand point of long term reservoir sedimentation rates and the project life or that in order to assure a reasonable project life it might be possible to create appropriate vegetal cover which will reduce the watershed erosion characteristics, but this may not be possible for some projects.

2.Sediment sampling

Generally the gauging stations used for discharge rating and sediment sampling are located in the same area where the river flows are fairly uniformly distributed and the reference channel section is constant over time. Discharge measurement and sediment sampling are generally carried out at regular intervals, often every 15 days for mobilization and cost reasons.

In the past we have discussed the difficulty of assessing sediment data sufficiently accurately by using the conventional depth integrated sediment sampling method at periodic intervals. Regular sampling intervals combined with a fairly small number of verticals (Fig. 1) may in our opinion induce some error in the assessment of total sediment load. Perhaps this is one of the reasons for which the actual reservoir sedimentation rates are often much higher than that predicted during the initial evaluation of the reservoir sedimentation rates and the ultimate Project life.

Fig.1 – Typical depth integrated sediment sampling verticals in a river cross section 

 

Figure 2 shows a typical correlation between the river discharge variations in a fairly large river like the Mississippi River at Old River Control and the corresponding total sediment load and sand load variations based on daily sediment sampling. This figure shows clearly that if the sediment sampling is carried out every 15 days interval that is say December 1 and 15 the average sediment load estimated will be very different than if the samplings were carried out on December 10 and 25.

To avoid such uncertainty a sediment sampling arrangement has been developed at Old River Controlon the Lower Mississippi River where samples are taken twice a day and at three different structures: Sidney A. Murray Hydroelectric Station and two US Army Corps of Engineers flow diversion structures. In the past a paper has been published to give full details of this system.²

We will therefore only briefly describe the main concept ofthe system and the need for knowing the daily variations of the sediment concentrations as shown in Figure 2 and its importance in assessing correctly the total sediment load.

Mixture of water and sediment is pumped from the highly turbulent flow areas such as: From the top of the turbine runner chamber and the energy dissipation areas of the flow diversion structures and hand samples are collected twice a day from easily accessible installations adjacent to the structures. Experience has shown that the vertical mixing of the total sediment load is complete and samples collected represented the total sediment load including the coarsest sand particles found on the river bead.

Fig. 2 – Correlation between the river discharge and the sediment load variations

These sampling stations have now been in operation for 13 years and have proven that their operation is simple safe and very reliable. Only one technician is required to collect and analyze all the samples and carrying out sediment analysis using standard United States Geological Survey (USGS) laboratory procedures.

It is also very important ta know the mineralogical composition of the sand particles. Sediments with very high content of Quartz sand (85% or more) are very detrimental to the turbines. Himalayan rivers often transport sand with high Quartz content, so particular attention must be given in the design of the structural arrangements, which will reduce the risks as much as possible entrainment of such material into the turbine flow. This aspect is of prime importance if the project designers intend to use de-sanding structures for this purpose.

It would be of interest to mention that at the Jhimruk Run-of river Hydroelectric Station in Nepal, due to the very high concentration of Quartz sand in the river flow (maximum sediment concentration recorded 23,760 PPM or 23.76 kg/m3) has produced very severe abrasion of turbine cover, guide vanes and blades short time after starting the project operation.³ ln this case the de-sander was designed to retain sand particle sizes equal or greater than 0.09 mm. This would tend to indicate that the use of de-sander in certain cases is not a viable solution. The project designers should therefore weigh carefully the need and the cost benefit ratio of such structures before retaining them as necessary project component.

For information it is interesting to know the average annual sediment discharge of the 10 major rivers of the world4

“Geology, slope, climate, drainage density, and patterns of human disturbance all affect sediment yield, and no single parameter or simple combination of parameters explain the wide variability in global yields.”

3. Sediment management practice for various types of dam and reservoirs

Depending on the project characteristics we may divide the dams and reservoirs into the following main categories:

3.1 Run-of river dams with no or relatively small reservoirs
3.2 Run-river-dams in series with excavated power discharge conveyance channels
3.3 Large dams and reservoirs with minimum pool level variations
3.4 Large dams with long reservoirs and significant variation in pool levels
3.5 Reservoirs filled with sediment

3.1 Run-of-river dams with no or relatively small reservoirs

In the absence of reservoir water is supplied directly to the turbine water intake through a de-sanding structure. The power plant capacity is based on average flow of the stream and often combined with a relatively high head.

Efficiency of sediment management in such projects depends on the performance of the de-sanding structure. In mountain streams during high discharge the sediment source is often from land slides and bank failures, causing sudden rise in the sediment concentration. For this reason it is found that concentrations in certain Himalayan streams could be very high as much as 40,000 – 80,000 PPM or40 to80kg/m3. It has also been observed that performance of de-sanding structures for sediment concentrations in excess of 2,000 – 5,000 PPM are not satisfactory and often it is necessary to shutdown the plant. However, it is often too late and sediments have already reached the turbines long before the decision to shutdown the plant has been taken. This is confirmed very often by reports of serious equipment abrasion problems from such plants. In such cases a real time and continuous sediment concentration detector could be very useful for timely warning and plant shutdown.

For run-of-river projects with small reservoir it is necessary to equip the diversion dam with adequate gate size and number which will allow establishment of pre-dam stage discharge conditions at the dam site. This will allow efficient flushing of the sediments accumulated at the upstream end of the upper pool under normal operating conditions. Such flushing is necessary otherwise permanent aggradation of the river bed upstream of the diversion dam will occur.

Fig.3 – Marsyangsi Hydropower Station (Nepal) upper pool sedimentation profile

The sediment build up on the upstream of the diversion dam of the Marsyangdi Hydropower Station5 is a good example where a permanent loss of upper pool storage volume has occurred. Figure 3 shows the rise in the river bed by about 14 m out of a total depth of 21 m at the dam. This situation is responsible for producing at times extremely high sediment concentration in the flow entering the settling basin (up to 80,000 PPM or 80 kg/m3 ) reducing its trap efficiency and causing severe equipment abrasion. This is often the case with reservoirs full of sediment, at the onset of flood the saturation sediment concentration is attained for which no de-sanding structure can be designed.

3.2 Run-of-river dams in series with excavated water conveyance channel to the plant

On the Rhône River in France6 there is a series of low head water diversion dams where the plant discharge is supplied through excavated channels almost parallel to the river. The diversion dams are spaced in such a manner that the backwater from the downstream dam reaches the tail water level of the plant upstream. During the flood flows when the total river discharge is much in excess of the plant discharge, sediment is flushed out of the upper pool to the lower pool through the gated diversion dam with a combination of flushing and dredging. However, these projects are in relatively low sediment yield area, with sediments containing very little or no quartz sand and the total head in the range between 20 to 25 m, hence no abrasion problem. Average discharge is around 2000 m3/s. So we may conclude that where the average river discharge is high, sediment yield not excessive, low head run-of-river projects may be a good solution for sediment management.

3.3 Large dam and reservoir with minimum pool level variation

A large number of world hydro dams are in this category. The total sediment inflow in this case is stored in the reservoir and the dam height and volume of storage is designed to assure fairly long project life, 100 to 150 years. In such cases the correct assessment of the average annual sediment load that is going to be transported into the reservoir is very important. After putting the project into operation regular reservoir sedimentation survey should be carried to compare the actual sedimentation rate with the predicted volumes. Also the nature of the watershed soil erosion protection should be regularly evaluated and if necessary adequate measures should be taken to prevent its de gradation and if possible work towards its further enhancement.

The reservoir sedimentation process in this case will create a fairly fiat top-set bed slope of coarser material close to the pool surface, with a steeper tore-set slope, with tirne the foreset slope slowly and gradually encroach upon the earlier deposits of the finer materials (silt and clay) as shown in Figure 4.

Fig. 4 – Schematic representation of the reservoir sedimentation process with minimum pool level variation

If the bulk of the sediment load is very fine, silt and clay, then the possibility of its evacuation with the power fiow should be attempted. If suitable the extraction of the coarser material from the upstream end of the reservoir for construction purposes is also a possible solution. Diversion of sediment from the upstream end of the reservoir to the downstream of the dam will reduce reservoir sedimentation, and may be economically justified if the reservoir is not too long. In this regard we might site the example of the 86 m high Asahi Arch Dam in Japan7 where a bypass tunnel of about 2.350 m long and diameter 3.80 m has been used for diverting the sediment load from the upstream end of the reservoir to the downstream of the dam. According to the available information the system has produced very satisfactory sediment diversion and has reduced the reservoir sedimentation by about 83,000 m3 after the first year of operation. The sediment material diverted contained coarse sand and gravels and this has produced about 550 m3 of tunnel invert erosion with an average depth) of 0.62m.

If it is regularly necessary to open the spillways during flood flows then some fme sediment deposited earlier may also be added to the outgoing flow with the help of panning, dredging and/or hydro- suction. The combined effect of the aforementioned activities could help to increase significantly the project life.

3.4 Large dam and long reservoir with important pool level variation

A large number of multipurpose dams (hydro, irrigation, urban water supply and navigation) are in this category. In this case also the total sediment inflow is stored into the reservoir and the dam height and volume of storage is designed to assure long project life. However, due to the important pool level variations the top-set bed slope will be steeper and the fore-set bed slope will move much faster (Fig. 5) towards the dam because at the onset of the flood flows the pool level will be low and the flow velocities over the steeper top-set slope will be high and create massive erosion and transport of sediment towards the dam each year. Such annual operation may create sand transport into the power and irrigation intakes long before the whole reservoir is full of sediment. Because of the high storage dam and the very long reservoir length it is practically impossible to achieve worthwhile results in removing sediment by flushing. The eventual problems related to sediment removal in such projects should be considered at the site selection stage and also during the design phase. Amongst possible solutions alternatives such as diversion of the sediments at a convenient location along the upper part of the reservoir to an adjacent valley or tributary and/or use of multiple smaller diversion dams that will serve as sediment excluder and supply relatively sediment free water to the main reservoir for power generation and irrigation. 8

Fig. 5 – Schematic representation of the reservoir sedimentation process with important pool level variation

3.5 Rehabilitation of reservoirs filled with sediment

There are many dams and reservoirs around the world where long before the predicted project life the reservoir sedimentation has reached such a stage that adequate power generation, or irrigation and urban water supply is no longer possible. Remedial measures which would eventually restore at least partially their initial power generation and irrigation and urban water supply capacity is worth trying.

Type of solution will depend on the individual project characteristics and the potential environmental impacts the remedial measures might cause to the river downstream. Use of equipment such as: drag line, dredging and hydro-suction on smaller scales to solve the problem excessive sedimentation, on relatively smaller reservoirs have been often used with success. But for large dams and reservoirs such solutions will require considerable mobilization, time and cost. So before undertaking such an operation it will certainly be convenient for the decision makers to have a good understanding of the feasibility of the proposed solutions. There is nothing better than a reliable physical model simulation of the actual process of sediment removal and its impact on long-term behaviour of the sediment movement and deposition over the entire reservoir length .and also the down stream movement of the removed sediments during flood flows or during low flows. For large reservoirs this might mean reproduction of combined river and reservoir lengths of 20, 50 or 100 km.

If we use conventional movable bed physical hydraulic models for such purpose the size of the model will be huge, its cost prohibitive, and the testing time and conditions will simply be unmanageable.

Recently we have developed a very small scale physical model of the Mississippi River delta over a length of 100 km for studying large scale water and sediment diversion in to the marshes for recreating land. Because of the existing Mississippi River levee system annually about 210,000,000 tons of sediment is getting lost in the Gulf of Mexico and at the same time Louisiana is loosing about 80km²/year. The geometrical and sedimentation scales are such that it can reproduce 100 years evolution in 50 minutes. This renders the models testing conditions very easy to manage. The model was initially proposed as a highly qualitative model, but in the light of fairly accurate reproduction of the river sediment transport and distribution patterns in the various passes and distributaries, and also the total sediment balance after 100 year operation we can say that it is may be considered also as almost quantitative in certain respect.

A similar model could be used for simulating firstly the actual known historical reservoir sedimentation process, i.e., satisfactory reproduction of the progression of the top-set bed slope and the fore-set bed slope through the reservoirs over the actual number of years of operation and then study sediment removal project concepts and procedures for partial rehabilitation of the reservoir.

This would mean that it will be possible ta recover sufficient area and volume of the reservoir allowing normal power generation without the risks of abrasive sand getting into the power intake and adequate supply of irrigation water year round.

As in most cases the maximum percentage of sediment is transported during the onset of flood flows, detailed information regarding the major flood hydrographs and the estimate of corresponding total sediment load deposited in the reservoir should be gathered for accurate calibration of the model.

To explain the basic concepts of reservoir rehabilitation design study we have considered the well known case of Salal Dam9 reservoir with a total storage volume of 96 million cubic yard. Figure 6 shows a schematic representation of the project layout. The reservoir is full of sediment, to the water surface level at the upstream end and to the spillway crest level at the downstream end.

Fig. 6 – Schematic layout of the Salai Dam reservoir completely filled up with sediment

Average annual sediment load is approximately 30,000,000m3 of which about 25% is sand and during the flood flows the concentrations are very high, most probably much more than 10,000 PPM or 10 kg/m3. So the spillway and the power intake are continuously passing sand causing severe abrasion damage to the spillway concrete structure and the turbine equipment.

Fig. 7 – Schematic longitudinal section of Salal Dam upper pool

As fist approximation a storage volume equivalent to at least 4 times the annual sand transport volume, i.e., about 30 million m3 may have to be created in the upper pool by using a combination of means such as: extraction, flushing, dredging, panning (putting bed sediments into suspension during flood), and hydro-suction ,etc. Removing sediments buried many years within the reservoir will also need especial care in disposal method, to avoid negative environmental impact for the water users downstream. Thus it might be necessary to carry out the rehabilitation work over several water years.

Assuming that during the low flow period the sediment volume removed from the reservoir may be stored in the river bed between immediately downstream of the spillway and the powerhouse tailrace tunnel outlet, in this way it will then be progressively carried away by the spillway discharge with reasonable concentration.

During the flood period when the flow velocities are high it might be possible to remove bulk of the sediment volume from the reservoir by panning. Sheet-pile groins judiciously located (to be determined by model studies), will also create turbulence and remove and direct the heavier sediment particles towards the spillway maintaining the power intake area reasonably free of sand.

– The small scale physical model will confirm the feasibility of the project and then allow optimization of the essential features such as:

– Configuration, dimensions and the volume of the reservoir excavation that will be able to trap efficiently the total sand load transported by the river during the flood flows, for annual flood, ten year flood and 20 year flood frequencies.

– What are the best maintenance procedures for removing sediment during the low flow period and
average flows and flood flows

– Best use of the river flow velocities to transport and remove the bulk of the sediment load from the reservoir .

– Best way to keep the flow to the power intake, free of sand under all river discharge conditions.

– Optimization of cost/benefit ratio of such operation.

4. General conclusion

Reservoir sediment management, especially in sediment rich areas of India and other countries around the world is becoming more and more a major problem for the hydro projects. It is therefore important that aspects related to improved reservoir sedimentation management is better understood and practiced. We would strongly recommend that:

– Concern about reservoir sedimentation becomes an integral part of design standards, so that hydro and storage dams in sediment rich areas become as sustainable as possible.

– Project operators and designers should try to use known technologies and also when possible advance the state of the art in sediment management by using innovative ideas and new technology.

– Due attention must be given to all the parameters related to sediment source, its transportation and deposition patterns in the reservoir.

– Try to develop reservoir storage volume maintenance methods adapted to local conditions

Reference:

1. Alessandro Palmieri – Farhed Shah-George W. Annandale. Ariel Dinar ., Reservoir Conservation – The World Bank, Jillle 2003
2. Sultan Alam, Cecil Soileau and Ralph L. Laukhuff ., Sediment Transport Assessment in the Old River Control Area of the Lower Mississippi River -Waterpower ’93 Proceedings of the International Conference on Hydropower
3. Shailendra Basnyat ., Proceedings, Optimum use of Run-of-river hydropower schemes, Seminar in
Trondheim, Norway, June 1999.
4. Gregory L. Morris, Jiahua Fan ., Reservoir Sedimentation HandJook, 1997
5. Gyanendra Prasad Kayastha ., Proceedings, optimum use of Run-of-river hydropower schemes, Seminar in Tronfdheim, Norway, June 1999.
6. Valérie Chabrier, Alain Comtet, Jacques Lovenq ., Evaluation of 50 year development on the Rhône valley, Rehabilitation of the old river at Pierre Bénite., ICOLD, Beijing Congress, 2000
7. Minoru Harada, Hiroshi Morimoto, Tetsuya Kokubo ., Operational results and effects of sediment bypass system. lCOLD Beijing Congres s, 2000
8. S. Alam ., Improving sedimentation management using multiple dams and reservoirs; The
International Journal on Hydropower and Dams, volume nine, Issue 1,2002
9. V.K. Vanna., Virendra Johri ., P roceedings, Removal of silt through hydro-suction at Salal Dam, India.lnternational Conference HYDRO 2003


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