17-09-2012, 02:53 PM
DAM DECOMMISSIONING
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ABSTRACT
A dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a reservoir, lake or impoundment. dams are built all over the world for various purpose.
Intended purposes of dams include providing water for irrigation or town or city water supply, improving navigation, creating a reservoir of water to supply industrial uses, generating hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and containing effluent from industrial sites such as mines or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one.
As per statistics in America about one dam is build every day after their independence. construction of dams at this rate resulted in many ecological problems. More than half of the world’s large rivers have been dammed, regulating and flooding approximately 400,000 square kilometers of land worldwide. These diversions have an effect on diverse ecosystems and habitats around the globe, replacing them with uniform structures and reservoirs and ultimately changing the way otherwise balanced, stable ecosystems function.
INTRODUCTION
The purpose of this research is to introduce the issues associated with the decommissioning of dams. Decommissioning is defined as the full or partial removal of an existing dam and its associated facilities or significant changes to the operations. Dam decommissioning has not received much attention until recently, but is rapidly gaining momentum as societal priorities change. This research is intended to create an awareness of the concept and stimulate thinking when dam decommissioning is being considered.
Once revered as temples of engineering prowess, dams are now viewed more critically. Dams devastate river ecosystems and undermine the rights and livelihoods of affected communities. Increased international recognition of the high environmental and social costs of dams, along with numerous river restoration successes, are inspiring dam removal campaigns worldwide.
Numerous dams are now slated or proposed for removal. Many have simply outlived their purpose or sit abandoned, posing a danger to public safety. Other dams continue to operate, though with significant environmental and social consequences. Over a dam’s lifespan, costs borne by damaged ecosystems and communities may outweigh other project benefits. With dam removal already outpacing dam construction in the US, decommissioning has significant implications for global river management.
ECOLOGICAL IMPACT OF DAMS
Large dams permanently alter the natural physical processes that sustain river structure, and this has a catastrophic effect on the plants and animals that depend on those processes. There are three physical processes most affected by dams:
Water quantity this is the most obvious effect of dams. Water quantity is diminished downstream of the dam, reducing the water available to aquatic animals and riparian plants.
Peak flows dams reduce or eliminate the seasonal flooding that naturally occurs during storm events and spring snow melt. It is these high-intensity pulses of water that are crucial for moving sediment downstream, scouring river channels, and creating the complex structure of boulders, logs, gravel bars, riffles, and pools that distinguish healthy river systems.
Sediment transport large reservoirs capture almost all of the sediment carried by a river. Silt, sand, gravel, boulders, and organic debris are trapped behind the dam instead of being carried down the river. This starves the river of the material needed to replenish gravel bars, rebuild the stream bed, and renew floodplain soils. Of the three processes listed above, sediment transport has proven most difficult for dam engineers to recreate. While many dam operators have made some progress in simulating natural water levels and peak flows, there has been no effective way to engineer passage of sediments downstream.
NEED FOR DAM REMOVAL
There has been a growing movement to remove dams where the costs - including environmental, safety, and socio-cultural impacts - outweigh the benefits - including hydropower, flood control, irrigation, or recreation - or where the dam no longer serves any useful purpose. The goal of removal can be multi-faceted, including restoring flows for fish and wildlife, reinstating the natural sediment and nutrient flow, eliminating safety risks, restoring opportunities for whitewater recreation, and saving taxpayer money.
Structural safety
While dams have multiple benefits, their failure or misoperation could threaten public safety, local and regional economies, and the environment. Risk has two primary components: (1) the damage and deaths associated with dam failure or misoperation, and (2) the likelihood of such a failure. as dam structures age and weaken, some dams become unsafe to operate. The first step toward rectifying dam-safety issues is to identify safety deficiencies. Such deficiencies are often identified by engineers during informal inspections, or during formal inspections conducted by senior engineers. Formal inspections are generally conducted after major seismic or weather events, and on a periodic basis. As described below, the frequency and type of periodic inspections varies across agencies. Safety and safe run period of the dam is decided by many factors like siltation. seismic effects, intensity and frequency of floodingetc.
Reservoir siltation :
siltation will reduce the dam's ability to store water and produce electricity. large reservoirs capture almost all of the sediment carried by a river. Silt, sand, gravel, boulders, and organic debris are trapped behind the dam instead of being carried down the river. This starves the river of the material needed to replenish gravel bars, rebuild the stream bed, and renew floodplain soils. Of the three processes listed above, sediment transport has proven most difficult for dam engineers to recreate. While many dam operators have made some progress in simulating natural water levels and peak flows, there has been no effective way to engineer passage of sediments downstream.
CONCLUSION
Decommissioning of dams is a reality that engineers and dam owners will be facing more and more in the next few decades. It is time to gather and begin to exchange ideas now. Costs associated with decommissioning can be many times the cost of repairs and upgrades. Much can be learned from reviewing the decision process from decommissioning case studies. it is high time that the engineers be made aware of the Importance and significance of dam decommissioning. More research must be done to make the process more economical and easy. This seminar report gives an overview about the factors to be considered and the after effects of dam decommissioning.