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 accurnulated 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 funher 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 fore-set 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 fiow 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 ofthe 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 poollevel variation
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