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Condit Dam Hydroelectric Project Final Supplemental EIS Action Post-Removal Management Management of Sediment and Woody Debris following Dam Removal Table 4.8-2 (Continued) Anticipated Noise Sources Sources Reference Noise Level Reference Distance Number On Site Blasting 122 dBPeak NA NA Drill rig 74 dBA 200 feet 1 Loader 85 dBA 50 feet 2 Excavator 82 dBA 50 feet 2 Scraper 89 dBA 50 feet 2 Truck 88 dBA 50 feet 10 Pickup 72 dBA 50 feet 2 Notes: a. dBPeak = Peak decibels (C-weighted) b. dBA = A-weighted decibels c. URS assumes the blast charges for the Proposed Action would be sized to produce peak acoustical overpressures below 122 dBPeak (C-weighted) at ¼ mile from the project site, i.e., the distance from Condit Dam to the nearest residential property (sensitive land use). Typically, blast charges are sized so as to produce peak acoustical overpressures below 122 dBPeak (the threshold of annoyance for blasts recommended by the U.S. Army) at any sensitive land uses. Predictive Modeling of Project Noise Levels Sound propagating outdoors through the atmosphere generally decreases in level with increasing distance between the noise source and receiver. In the vicinity of the Proposed Action, this attenuation is the result of several mechanisms, discussed in the following sections. Attenuation by Geometrical Divergence (A div ) Because sound energy spreads spherically as it radiates from a source, its apparent loudness also decreases. For a single point source, the sound level decreases at a rate of 6 dBA per doubling of the distance from the source due to geometrical divergence. Attenuation due to divergence of sound energy (A div ) is the same for all frequencies, and is independent of any weighting scale used. In the absence of hills or berms, distance is the primary mechanism for decreasing the noise from a site. Attenuation of noise levels generated by construction activities associated with the Proposed Action site due to geometrical divergence over specific distances from the sources was calculated using the following equation (Piercy and Daigle 1991): A div = 20 log 10 r + 10.9 – C where: r = distance from the noise source to the receiver in meters C = correction term (dependent on temperature and atmospheric pressure) Because A div depends on temperature and atmospheric pressure, attenuation may differ slightly between summer and winter months. According to the construction schedule included in the Project Description (PacifiCorp 2004), the Proposed Action would occur during both summer and winter months. Therefore, A div was calculated under both summer (i.e., 20°C, 1 atmosphere) and winter (i.e., 0°C, 1 atmosphere) atmospheric conditions. 4.8-5
Condit Dam Hydroelectric Project Final Supplemental EIS Attenuation Resulting From Air Absorption (A air ) Some of the energy in a sound wave is absorbed by the atmosphere. The amount of absorption depends on the frequency of the sound and the temperature and relative humidity of the atmosphere. Because of the more effective absorption at higher frequencies, atmospheric absorption would also tend to lower the pitch of noise generated at the site. This effect is small and ignored for short distances, but becomes significant as the distance between the source and the receiver increases. Attenuation of noise levels generated by construction activities associated with the Proposed Action due to air absorption (A air ) over specific distances from the sources was calculated using the following equation (Piercy and Daigle 1991): where: A air = α d 1,000 α = air attenuation coefficient (dependent on temperature and relative humidity) d = distance from the noise source to the receiver in meters Because A air depends on temperature and relative humidity, attenuation may differ slightly between summer and winter months. Therefore, A air was calculated under summer (i.e., 20°C, 30 percent relative humidity) and winter (i.e., 0°C, 90 percent relative humidity) atmospheric conditions. Attenuation by Foliage (A foliage ) Trees and bushes normally provide very little noise attenuation as a result of shielding. However, if the foliage is dense enough to completely obstruct the view and also intercepts the path of acoustic propagation (as in a dense forest), some attenuation can be quantified. A practical upper limit to noise attenuation by foliage (A foliage ) is reached at a path length through the foliage of approximately 200 meters (656 feet). In addition to distance, the magnitude of A foliage depends on the frequency of the sound. Attenuation of noise levels generated by construction activities associated with the Proposed Action due to foliage was calculated using the following equation (Piercy and Daigle 1991): A foliage = 0.04 dB / m where: dB = decibels m = length of sound propagation path through foliage in meters (maximum 200 meters) Noise Level Calculations After appropriate noise sources were identified, noise levels at 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, and one mile from the Proposed Action caused by construction activities were modeled considering the noise reductions caused by distance, topography, foliage, and atmospheric stability and absorption (Piercy and Daigle 1991), Mestre and Wooten 1980), as described in the previous sections. The key assumptions used for the modeling were as follows: 4.8-6
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Condit Dam Removal Final SEPA Suppl
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March 23, 2007 Dear interested part
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Condit Dam Hydroelectric Project Fi
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Page 105 and 106: Condit Dam Hydroelectric Project Fi
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Page 201: Appendix A Memorandum of Agreement
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Appendix C Supplemental Aquatic Res
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Agency and Organization Letters and
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Agency and Organization Letters Con
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Contents Public Meetings Response P
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Contents Individuals Individuals A-
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