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RWI of 2 were selected as the maximum acceptable The specified exposure criteria are typically con- physiological response, then pile strikes that can servative or precautionary in nature. Pile strikes gen- generate impulsive sound with SELss values up to erate a sound field that is seldom known or modeled. 180 dB re 1 Pa2s could be used, as long as the num- Furthermore, the presence, distribution, and behav- ber of strikes required to drive the pile was less than ior of fishes of concern, particularly species within 960. If the sound generated by each strike contained regions at risk of high level sound exposures, are less SELss energy, then the number of strikes avail- also unknown. Over time, information from monitor- able to drive the pile could be increased, as long as ing activities has accumulated, and it is now possible the combination of SELss and number of strikes were to review available reports and peer reviewed publi- to the left of the RWI 2 contour in Figure 8. The data cations. These reports can be used to provide initial from this study do not permit the RWI curves to be estimates of the observed level of sound likely to be extrapolated beyond the 960 and 1,920 strike bounds found at various monitoring ranges for a variety of of this study. pile types, hammer types, and environmental settings. It should be noted that the current west coast The same cannot be said for observations of the interim criteria use a dual approach for decisions on behavioral effects on fishes from pile driving expo- potential onset of physiological effects, either a sure. Because of the lack of behavioral information, specific SELcum or a single-strike peak level of sound regulators tend to preferentially permit operations pressure (SPLpeak). Under those criteria, if the SELcum during times when species of concern are unlikely is reached over multiple strikes, or any one strike to be present or only present in small numbers. Pile exceeds the SPLpeak, mitigation occurs (Popper et al., drivers seem to have accepted restrictive schedules 2006; Carlson et al., 2007; Stadler and Woodbury, in consideration of the expected high cost and com- 2009). Furthermore, Carlson and Weiland, (2007) plexities of assessing the exposure and impacts to found that SELss and SPLpeak are highly correlated. fishes during times when the species of concern may In their study of pile driving sounds, they deter- be present. mined that SPLpeak could be estimated given SELss While exposure criteria are defined for a point using Equation 4.1. The linear fit of SPLpeak to SELss in the volume of water ensonified by pile driving had an r2 of 0.85. Using equation 4.1, all of the sounds, both the generated sound field and the risks impulsive sound metrics, used in previous criteria, to exposed fishes are four-dimensional (time and can be defined for operating criteria obtained from a three-dimensional location in the ensonified vol- selected RWI value and Figure 8. ume). The energy in the generated sound field is continuously variable from the pile to the range at which the energy falls below the ambient noise. The SPL peak = 18.02 + 1.05 SEL ss Equation 4.1 amount of energy at any point in the field depends upon many factors, and it is transient because of the intermittent nature of pile driving with one pile CHAPTER 5 APPLICATION strike every 1.5 seconds or so. TO PILE DRIVING PROJECTS A fish would experience sound exposure on the Background order of 0.01 seconds in every 1.5 seconds, with the amount of exposure energy highly dependent Exposure criteria for fishes exposed to pile driv- on the location of the fish in the water column rel- ing sound are almost always given as one or two ative to the pile being driven. The instantaneous sound metrics such as SELss, SELcum, SPLpeak, or amount of energy a fish may be exposed to SPLrms. During pile driving operations, monitoring decreases rapidly with distance from the pile. In is conducted at one or two points in the sound field. sound propagation models, the decrease in energy is Over time, a convention has evolved to monitor the referred to as transmission loss. Typical models for sound field at 10 m from the pile being driven at one transmission loss, in decibels relative to a Pa in or two depths in the water column, with one of the deep and shallow water, are 20 log R and 15 log R depths being near the midpoint of water depth. As respectively, where R is the range (i.e., distance) from long as the regulatory criteria are not exceeded, the the sound source and log is the logarithm base 10. pile driving activity is determined to be in compli- However, the majority of pile driving projects take ance with operating permits. place in shallow water. Yet, given a measure of the 19