Environmental and Social Impact Assessment - Gibe III

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Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 7 IMPACT ASSESSMENT AND MITIGATION ACTIONS 7.1 General In order to assess the potential environmental impacts caused by the construction and operation of Gibe III scheme have been identified considering: Physical, biological, socio-economic environmental data obtained form direct field observations; Information gathered from the available scientific publications and information derived by the study of similar projects; and the activities that may produce impact, evaluated in the description of the Gibe III Hydroelectric Project. Adverse impacts of project construction and operation and the mitigation and benefit enhancement measures are discussed below, where also a specific section about downstream effects is presented. Beside potential effects on environment due to the project, mitigation actions, representing the ways to reduce the impacts on a specified target, allowing finding out ways to guarantee basic health and safety requirements, are introduced. 7.2 Impacts on Physical Environment 7.2.1 Climate Hydropower plants and relevant reservoirs may have significant impacts on global and local climate. 7.2.1.1 Global Climate Effects The present knowledge on the positive or negative effects on global climate changes related to large reservoirs has not yet reached a detailed conclusion. In general terms, according to the International Hydropower Association (IHA), we can say that hydropower has a very low or even positive impact on climate change because reservoirs sequester large amounts of carbon. The important question is whether reservoirs are important sinks for anthropogenic carbon. IHA uses an estimate indicating that reservoirs sequester 2.5% of global CO2 emissions. For several reasons this estimate gives a misleading indication of the climate impact of reservoirs. In some cases reservoirs will only be temporary sinks for carbon due to measures to mitigate reservoir sedimentation and dam decommissioning. Where reservoirs are permanent sinks their ability to sequester carbon will end once their storage capacity is filled with sediments. However because of the very large reservoir capacity when compared to the sediment transport volumes this risk can be considered very low. 7.2.1.2 Greenhouse Gases Emissions The major greenhouse gases are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). These gases are emitted from both natural systems and from anthropogenic sources. CESI SpA - Mid-Day International Consulting Engineers Page 189
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Carbon dioxide from the burning of fossil fuels is the largest single source of greenhouse gas emissions from human activities. The supply and use of fossil fuels accounts for about 80 percent of mankind’s carbon dioxide (CO2) emissions, one fifth of the methane (CH4), and a significant quantity of nitrous oxide (N2O). It also produces nitrogen oxides (NOx), hydrocarbons (HCs), and carbon monoxide (CO), which, though not greenhouse gases themselves, influence chemical cycles in the atmosphere that creates or destroy other greenhouse gases, such as tropospheric ozone. Meanwhile, fuel-related releases of sulphate aerosols are temporarily masking part of the warming effect of greenhouse gases. Hydropower is a very efficient way to produce electricity, in terms of greenhouse gases (GHGs), showing emission factors between one and two orders of magnitude lower than the thermal alternatives. Nevertheless, hydroelectric dams can produce significant amounts of carbon dioxide and methane. Carbon emissions vary from dam to dam. Emissions of GHG are related to the decomposition of flooded “soft” organic matter. The Production of Carbon Dioxide and Methane in the Reservoir Environment Methane and Carbon dioxide are the two major greenhouse gases (GHG’s) associated with hydroelectric dams. Both affect the global climate, but methane is significantly more powerful than carbon dioxide in terms of its efficiency in trapping heat. However, methane is shorter lived in the atmosphere, possibly decreasing its influence on global warming. The source of the carbon dioxide and methane in reservoirs is rotting and decaying vegetation. When land is flooded to create a reservoir, the vegetation dies and is no longer able to absorb carbon dioxide from the atmosphere via photosynthesis. Instead, those plants decay and the stored organic carbon is converted into methane and carbon dioxide. This conversion of the stored organic carbon into methane and carbon dioxide occurs by two distinct mechanisms depending on the oxygen level of the water: In an oxic environment, more toward the surface of the reservoir, aerobic decomposition of organic matter produces carbon dioxide and methane. In an anoxic environment, methanogenesis produces carbon dioxide and methane. Methanogenesis is the pathway in which the products carbon dioxide and methane are produced by a biological process involving methanogen bacteria. In either case, methane and carbon dioxide are the product of the decomposition of vegetation in the reservoir environment. Once methane and carbon dioxide are produced, they are not immediately released into the atmosphere. The gases are soluble in the water of the reservoir until a chemical event occurs that causes the gases to be released. There are several ways by which this release can occur, depending on the characteristics of the reservoir. The gases can be released from the surface of the reservoir by rapid diffusion directly into the atmosphere, or, the gas can form bubbles at the bottom of the reservoir and rise to the surface. These events are random and unpredictable, however, making them very difficult to predict and quantify. In addition, there is no consistent proportionality between the amount of bubbling and the amount of diffusion from a particular reservoir, making it even more difficult to measure. CESI SpA - Mid-Day International Consulting Engineers Page 190
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