The sediment perching phenomenon, in our view, is similar to the decision making proposed by Arrow et al: one waits until the stock is down to a certain value before replenishing the stock.Literature has focussed on the debate of whether a dam should have “design life” or whether it should be run sustainably by using life cycle management strategy. Intergenerational equity requires that a dam either be run sustainably or the generation that benefit from the dam pay for its decomissioning cost . Palmieri et al discussed about a method to generate such fund in a reservoir in China. In addition to these studies, Keohane et al proposed a SFQ model which they suggested could be used in the context of reservoir management. In their model, stock and flow both must be controlled to promote the quality, which in the context of reservoir management problems requires the control of both sediment flow and sediment stock to maintain the quality of the reservoir and reservoir products.Their result implied that if the dam operator has the choice of both sediment removal and restoration, then the threshold that triggers restoration in the absence of choice regarding sediment removal would be lower than the case in which planner has the option to remove sediment. On the other hand, the feasibility of restoration will reduce the optimal sedimentation removal at each level. The author seem to treat restoration as if the asset being restored is renewable. However, we believe that is clearly not the case in reservoir management. However, the issue is similar to much studied machine replacement problem in finance and economics. The major study in the literature was due to Rust,grow bucket who studied the decision of an administrator making decision on repair or replacement of GMC bus engines.
A dam administrator is in a way similar to Harold Zurcher, the bus administrator: making a decision on repair or decomissioning, but most likely, without the option of replacement. Furthermore, with dam, the concept of sustainably running it is more important, where as with the bus, it is not even considered. This class of technique include the investment in erosion control upstream so that the river doesn’t carry a lot of sediment into the reservoir. This method is mainly focussed in rehabilitation of degraded soil and watershed upstream. Literature in sedimentation management emphasize that such management strategies be carried out with the help of landowners upstream as their noncooperation result in the failure of erosion control programs. Sediment management and erosion control techniques may use methods ranging from basic land use changes to the complicated high fixed cost structural methods such as construction of terraces, diversion channels, grassed waterways, check dams. Nonstructural methods include agronomic measures which rely on the regenerative properties of vegetables. Other methods in use include operational measures such as scheduling construction to minimize the area of exposed soil. Land use changes doesn’t involve fixed cost, and may not result in reduced sedimentation yield immediately downstream. Faulkner and McIntyre reported that there were no change in sediment yield even 20 years after the transition to less erosive land use. There are several basic agricultural engineering techniques in erosion control for a detailed study on it. In the United States, Best Management Practices are recommended for erosion control. From the economic point of view, these methods can be divided into two classes: structural methods are fixed cost method with low annual maintance cost and nonstructural methods have no fixed cost, but have relatively higher annual maintanance cost. They also differ in their effcacy: it is recognized that the nonstructural methods can never lead to zero sedimentation yield downstream. Erosion control is also topography dependent.
In countries like Nepal, which is situated in the tectonically active Himalayas, erosion control in the watershed is not considered technically feasible in several possible reservoir sites. This is the same case in Tarbela, the reservoir about which we study in detail later. Excavation are costly options and most of the time, they are the only options once sediments are firmly deposited in the reservoir. Excavation option often depend on sediment volume, grain size, geometry of deposit, available disposal and reuse options and water level and environmental criterion. Dredging is an operation in which sediment is lifted from the bottom of the surface of a waterbody and is deposited elsewhere. In the United States, 500Mm3 sediment is dredged every year. Dry excavation involves completely emptying the reservoir, desiccating the surface and deposits and using earth moving equipment to remove the silt from the surface. Hydraulic excavation will require dewatering dredge slurry after it has been removed from the water surface, so that it can be removed in conventional hauling equipments to dump elsewhere. In small ponds in the united states, there have been some use of explosives to excavate sediments, but such use is rare among the large ponds. Dredging as a long term strategy for reservoir management is possible only if a good dumping site can be found. Although in many mountainous regions, the river downstream is considered the natural target for dumping dredged materials, such dumping is considered environmentally undesirable. There is a related method called Hydrosuction removal system that uses the hydrostatic head at the dam to provide energy for sediment removal. HSRS is of interest because there has been one major economic study of this method in detail. This method is similar to dredging, but it applies the hydraulic head available at the dam as the energy for dredging and is considered cheaper than dredging.
HSRS consists of a barge that controls the flow in the suction and discharge pipe and can be used to move the suction end of the pipe around. The pipe’s upstream end is located at the sediment level in the reservoir and the downstream end is draped over the dam to discharge sediment to downstream. Because of this, its applicability is limited to shorter reservoir. This method is normally considered energy conserving,and environmentally friendly. Public’s perception of dam as a clean source of energy has undergone some changes recently. In particular, the role of a dam as an emitter of green house gas has been asserted by researchers such as Ruud et al and Duchemin et al . Duchemin et al studied methane and carbon dioxide emission in two hydroelectric reservoirs in northern Quebec for two years and found “above average emission fluxes”. Their result showed the emission flux to be five to eight times less than what Ruud et al found out. Though Duchemin et al found the emission was on a much smaller scale than conventional thermal power plants equivalent amounts of energy, studies done in Brazil’s Balbina reservoir , Irion et al showed that the reservoir produces more greenhouse gas than coal fired equivalent due to the vegetation inundated by the reservoir. Such results have made it diffcult for large reservoirs to qualify for carbon credit in carbon markets, even though the small hydropower with no forest inundation often qualify for it. If large dams are sources of substantial emission, then their actual cost to the society is likely to be uncertain for long,dutch bucket for tomatoes since there is significant uncertainty related to the damage function: damage to the society due to GHG induced increase in temperature. Hence the dam operator may know the cost of decomission at any moment, but the cost in the future is uncertain. This calls for the modification in assumption of Palmieri et al that the salvage value of the dam is fixed and constant. This also provides motivation to learn how sediment removal rate will be changed under such scenario. There are two main reasons why a reservoir is decomissioned: the owners may find it economically infeasible or the regulatory agencies may demand that the reservoir is decomissioned. In the United States, Federal Energy Regulatory Commission stated in its statement that it has the right to decomission a project when considering its relicensing request. When a dam is decommissioned, there are three major issues: what should be done regarding the dam? what should be done regarding the sediment deposited in reservoir? How should environmental restoration be carried out? The dam could be left as it is, partially breached or completely removed.
The sediments could be left as it is if dam is left as it is. The other choices regarding sediment management are to allow natural erosion, construction of a channel through the deposits while leaving off chanel sediment as it is, and removal by mechanical excavation or hydraulic dredging.Some agencies may demand that the dam operator restore early fluvial condition. In such case, the dam operator may incur extra costs, apart from sediment management and infrastructure removal.It is reasonable to assume that the change in the cost related to and are relatively known and deterministic, but the change in the salvage cost related to will be uncertain. Such uncertainties also point to the need to study dams in stochastic settings. As we noted earlier, global warming implies higher erosion. Higher erosion increases the sedimentation arrival rate at the reservoir and this leads to the change in the value of reservoir. Our model shows that increase in the sedimentation rate decreases the value of the reservoir, in particular at the lower storage level. This is because increased sedimentation implies increased cost of removal of sediment.The cost of removing the storage is high at the lower level and therefore, increase in sediment is likely to decrease the value of the reservoir. Moreover, as Figure shows, the increased sediment arrival implies increased sediment removal at all level where sediment removal is optimal. Discount rate features in our model in two important ways. The first is that discount rate has its traditional meaning regarding the patience of the society.For example, it is expected that higher discount rate encourages individuals or society to consume more today. It also enters our model in a different way . If the society faces uncertainty about the future of the reservoir, its decision making , under some assumption about the nature of such risk, is akin to increased discount rate. Figure implies that increased discount rate increases sedimentation removal at the lower level water storage. Impatience in this case doesn’t mean the policymaker will lessen the sedimentation removal. At all levels of water storage, increased impatience also increases the value of reservoir by a small amount. It is possible that uncertainty about the future makes people value the reservoir more. Both of these results imply that increased discount rate will not lead to social planner scrambling to abandon the reservoir by decreasing sedimentation removal. The impact of increase in price is also reflected in the increase of value of the reservoir in the entire domain except at the end points. Figure shows this expected result. Figure shows that the impact of increase in implicit price on sediment removal. Higher price led to the increased sediment removal, as water is now more valuable. The results above were all conditional upon several things: that the cost functions were of a particular form, that the social planner was risk neutral and that the sedimentation arrival rate followed a certain temporal path. The debates underlying large reservoirs are often hard to address particularly because in most of the cases most of these functions are also less understood. In deed, the reservoir management literature is only recently trying to understand various aspects of reservoir managements. For example, there are very few works that explain the role of different factors in contributing erosion in the reservoir.Pacific Southwest Interagency Committeeís watershed inventory method is often used in predicting sediment yield from watershed condition but it is a very speculative method. Similarly, few literature exists that explain the precise nature of cost function for removing sediments from the reservoir. As WCD report made clear, the systematic study of reservoirs have recently begun, and hence there is still a lot of scope for identification of different parts of a reservoirís economic system to make a precise and integrated statement about the system. Sustainability of dam is a topic of interest when talking about the consumption of natural resources.