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 ofcombined river and reservoir lengths of 20, 50 or 100 km.
If we use conventional movable bed physical hydraulic models for such purpose the size ofthe 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 offairly 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 historicalreservoir sedimentation process, i.e., satisfactory reproduction ofthe 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 ofthe 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 confinu the feasibility of the project and then allow optimization of the essential features such as:
 Configuration, dimensions and the volume ofthe 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 mailtenance 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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