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Both cases were developed using an RWI of 2 as In both Equations 5.1 and 5.2, Strike means the num- an acceptable level. The research team's findings ber of strikes. Given two of the four variables in these during this study and those of subsequent and ongo- equations, the other two variables can be found. ing research (Carlson et al., 2011) permit us to con- For a pile driving project, the variables most clude that 2 Mild barotrauma injuries do not reduce likely to be known a priori will be the RWI and SELss. fish performance and are a conservative threshold The reason is that acceptable biological responses for barotrauma injury from exposure to impulsive will be the initial selected variable in consultation sound generated by pile driving. between regulatory agencies and those wanting to drive piles. In addition, the type of pile, hammer, and Case 1: Number of Strikes 960, RWI = 2 pile driving conditions will likely be determined dur- ing project planning, and available information will The lower bound for exposures in this study at permit estimation of the expected SELss value. the 960 strike level was a SELss of 174 dB re 1 Pa2s Given SELss and RWI, it is most likely accurate with corresponding SELcum of 203 dB re 1 Pa2s to use Figure 8 to estimate SELcum and the number (see Table 2). Exposures with fewer strikes were not of strikes available to drive a pile. In practice, the considered, and extrapolation of RWI to exposures expectations intended for fish protection are implicit with less than 960 strikes using the function rela- in the selection of an RWI value and the implication tionships of Figure 8 is not advised. However, for for underwater pile driving sound generation that pile driving durations less than 960 strikes the rela- will require negotiable tradeoffs using Figure 8 tionships between RWI, SELss, and SELcum shown and/or Equations 5.1 and 5.2. in Figure 8 can be used to obtain conservative expo- For example, if the selected RWI is 2 and the sure criteria. expected SELss is greater than 181 dB re 1 Pa2s, a The exposure criteria would be the SELss and solution for SELcum and number of strikes is not pos- SELcum shown in Figure 8 where the RWI curve sible within the bounds of the data for this study. intersects the horizontal 960 strike line. For an RWI Either the RWI value would need to be increased or of 2, these values would be a SELss of approximately the pile driver would need to identify strategies that 180.25 dB re 1 Pa2s and a SELcum of 210 dB re would reduce SELss. Alternatively, the negotiating 1 Pa2 s. These criteria would be conservative parties might elect to extrapolate the RWI curves because if the SELss is not exceeded, the SELcum outside the range of experimental data to obtain would always be less than 210 dB re 1 Pa2s. For an solutions. exposure maximum of 960 strikes at an RWI of 2, Other strategies for exposure criteria derivation more precise estimates of SELss and SELcum can be could be to permit higher SELss outside of the solution found using the equations presented in Figure 8. range of experimental data, or limiting SELcum and RWI within the solution range. An example would be permitting SELss of 182 dB re 1 Pa2s but limiting Case 2: Number of Strikes between 960 and 1,920, SELcum to 208 dB re 1 Pa2s. Such strategies would RWI = 2 most likely pose a high risk of exceedance of RWI; If the expected number of strikes to drive a pile however, an SELcum of 208 would seem adequate to is greater than 960 and the selected biological prevent exceedance of an RWI of 2 (Figure 8). The response threshold is an RWI of 2, then the equa- risk of exceeding the RWI threshold would be partic- tions in Figure 8 may be used to identify the SELss ularly high if the majority of barotrauma injuries at a and SELcum exposure criteria. The relationships be- particular SELss occurred during the initial stages of tween number of strikes, SELss, and SELcum pre- exposure when the number of strikes was low. sented in Figure 8 are shown below as equations 5.1 and 5.2. CHAPTER 6 FUTURE RESEARCH SEL cum = SEL ss + 10 Log ( Strike ) Equation 5.1 This study was designed to explore the relation- ships between the number of pile strikes, SELss, and SELcum, with particular reference to Chinook salmon. RWI = (exp( -30.050 + 0.149 g SEL cum The results provide a highly quantified view of the - 0.000171 g (Strike))) - 1 Equation 5.2 most important variables in pile driving and their 21
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effects on fish. Yet, this is just the first critical step scope of the study did not permit investigation in the goal of fully quantifying the effects on fishes into how injuries accrued during exposure. from impulsive pile driving sounds and developing It is possible that injury accumulation was a full array of criteria to protect fish. However, to almost complete within the initial exposure- fully understand effects of pile driving, it is impera- strikes and that the increase in injury accumu- tive to get quantitative data on other variables asso- lation rate and their severity was not uniform ciated with pile driving. These range from impulsive over the course of exposure. Understanding sounds generated by different pile types to responses the growth of effects is essential to more accu- by different species from impulsive sounds. Future rately estimate biological response. studies should also be directed at refining knowledge · The use of the results from studies such as the and extrapolating to other sounds, other species, and one described here must deal with the ques- fish of different sizes. The following are a number of tion of applicability across fish species and such studies that the research team believes are the sizes within species. The most conservative most critical next steps: approach is not to extrapolate the results of a · Temporary threshold shifts in hearing sensi- study to other species and size groups. How- tivity are called TTS, and are a temporary loss ever, the reality is that data are few and quite of some hearing. Both TTS and barotrauma difficult to obtain. Thus, in most situations, it tissue injury have been used to assess the is better to use available data to inform a deci- response of fish to sound exposures. However, sion. Indeed, and as discussed in Chapters 4 and 5, it is possible to generalize the results their respective positioning along a biological of this study with caution. While the most response continuum has not been determined thorough approach would be to do the same for any species. It would be useful to learn if detailed studies as done here on many other the biological responses for each assay lies species, the time and expense of such studies along a continuum for impulsive sound expo- make that approach prohibitive. However, it sure. If the responses fall on a continuum, then would be possible, and of great value, to do a a useful comparison would be TTS and baro- limited set of studies using a defined array of trauma sensitivities modeled as measures for exposures across a range of carefully selected the onset of effect and/or injury in fish from species to represent the diversity of fish struc- exposure to impulsive sound. ture and physiology, as well as fish under the · Studies on the effects of decompression on fish most regulatory-concern. This, combined with have shown that the magnitude of the ratio of the extensive results reported here, would per- pressure to which fish are acclimated and the mit a broader understanding of the compre- pressure at which fish are exposed is propor- hensive effects, which could serve as a guide tional to the severity of barotrauma injury. If for extrapolation to a wider range of species. this ratio extends to pile driving and seismic · The least significant class of barotrauma impulsive sounds, it would introduce depth as injury defined in this study (Mild) includes a variable into the assessment of the effects those injuries that are not likely to impose of these sounds. The result would be a rapid a physiological burden that will affect the decrease in the severity of exposure and health of a fish. This assessment is the basis response from relatively small changes in for selection of a RWI value of 2 as an accept- depth, given that the static pressure in water able response threshold for derivation of expo- increases by about 100k Pa per 10 m of depth. sure criteria to impulsive sound. However, it is Research is needed to determine the rela- also possible that higher levels of injury would tionship between acclimation- and exposure- not pose physiological risk to fish health. pressures, and if response severity is the same Through experiments that assess the fitness of for impulsive sound exposure as it is for fish with Moderate injuries, it would be possi- decompression. ble to estimate the level of injury fish can expe- · The results of this study show that severity of rience without impact on overall health. In the responses increases with increased exposure to absence of such data, regulation to protect fish impulsive pile driving sounds. However, the must be very conservative to assure protection 22