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sound at a short range from the source, such as by a factor of 4, then the biological response would SPLpeak at 10 m, it is possible to obtain an estimate increase by 1 Mild injury (totaling 4 Mild injuries), of the SPLpeak of the sound impulse at greater ranges or by 1 Moderate injury (totaling 2 Moderate from the source (Au and Hastings, 2008). injuries). The non-linearity of biological response, The barotrauma risk to a fish is a function of the RWI, with increasing severity of exposure is evi- probability of its location in the sound field and the dent in Figure 8. energy in the incident sound on the fish (for each Conversely, for this example, as range from the strike impulse and cumulative over all impulses) pile doubles from 10 m to 20 m, SELss would while it is in that location. Because of the absence decrease to 177.75 dB from 180 dB re 1 Pa2s, of behavior-based exposure models, it is usually affected volume would increase from 314H m3 to assumed that the fishes of interest are stationary in the 1,257 H m3, and the expected biological response sound field over the duration of exposure, which is would decrease to an RWI of approximately 1, the typically defined as the time required to drive a pile. equivalent of 1 Mild injury. It is interesting to consider the risk of baro- trauma to fishes from particular exposure criteria, Derivation of Exposure Criteria considering the characteristics of a propagating sound field generated by pile driving. Assume that Terms used to express exposure criteria relate an RWI value of 2 was selected as the maximum directly to the pile driving activity and the maximum acceptable physiological exposure response of juve- acceptable impulsive sound exposure at the identi- nile Chinook salmon to pile driving sound. Refer- fied monitoring location in an operating permit. The ring to Figure 8, it can be seen that if SELss was entity performing the pile diving is typically expected limited to 180 dB re 1 Pa2s and the number of to monitor the generated sound and to mitigate their strikes required to drive the pile was less than 960, the activities as needed to avoid exceedance of exposure SELcum received by the fish would be less than 210 dB criteria. Possible mitigating actions for underwater re 1 Pa2s. The exposure criteria by permit would sound could include management of the applied then be SELss 180 dB re 1 Pa2s and SELcum 210 energy such as bubble curtains, which might reduce dB re 1 Pa2s. The SPLpeak corresponding to the SELss the amount of energy that propagates away from the criteria can be estimated, using Equation 4.1, to be immediate vicinity of the pile. 207 dB re 1 Pa. If permitting and monitoring con- The research results reported here permit deriva- vention were followed, this would be the criteria at a tion of exposure criteria by starting with a selected range of 10 m from the pile at mid-depth. level of biological response that protects the individ- As the range to the pile decreased from the 10 m uals in an exposed area from injuries that affect per- monitoring location, two factors would change. formance and/or energetics. The selected biological First, the amount of energy in the sound exposure response level and the results of this study can be would increase and second, the affected region used to identify the level of exposure that should not around the pile would decrease. For example, mov- be exceeded, i.e., exposure criteria, to assure pro- ing from a range of 10 m to a range of 5 m, the energy tection of the fish of concern. The results of this in a single impulse, SELss, would increase by 2.25 dB research are specific to juvenile Chinook salmon. (a doubling in energy would be 3 dB), from 180 to Extension of juvenile Chinook salmon's biological 182.25 dB re 1 Pa2s, assuming cylindrical spread- responses to other species and size groups may be ing. Additionally, the region of volume affected possible with consideration of potential differences in would decrease in proportion to the square of the biological responses resulting from species' physio- distance by a factor of 4, assuming no water depth logical differences. change, from approximately 314H m3 to 78 H m3, Two cases for derivation of exposure criteria are where H is water depth. At a 10 m range and 180 dB considered. The first case is for examples where the re 1 Pa2s sound exposure, the expected biological pile driving duration is short and/or the number of response would be an RWI of 3. An RWI of 3 strikes required to drive a pile are less than 960. This would be 3 Mild injuries or 1 Moderate injury. is the most likely scenario for the majority of piles Given the example criteria, as the distance to the driven. The second case is for occasions where the pile decreases from 10 m to 5 m, sound exposure number of strikes required to drive a pile is more increases by 2.25 dB, the volume affected decreases than 960 but less than 1,920. 20