Padma Multipurpose Bridge Project

the pile driving activities in this project. However, no fish killings or adverse impacts on fishes werereported near the bridge site during construction activities.Noise and Vibration Impacts due to Pile Driving16. Pile driving activities generate very high under water noise levels. These noise levels areexpressed in (i) peak sound pressure level (peak), which is maximum pressure level generated by asingle strike, (ii) accumulated sound exposure level (SEL), which is a sum of cumulative squared overthe time of entire single pile driving event, and (iii) root mean square level (RMS), which is the squareroot of the mean square of the single pile driving event. Units of the sound levels are expressed indecibels (dB) over a range of frequencies which the level is measured (1 μP a 2 /Hz).17. Piles are usually driven into the substrata using one of two types of hammers – impact hammersand vibratory hammers. Impact hammers consist of a heavy weight that is repeatedly dropped ontothe top of the pile, driving it into the substrata. Vibratory hammers utilize a combination of a stationary,heavy weight and vibration, in the plane perpendicular to the long axis of the pile, to force the pile intothe substrate. The type of hammer used depends on a variety of factors, including pile material andsubstrate type. While impact hammers are able to drive piles into most substrates including hardpan,vibratory hammers are limited to softer, unconsolidated substrates (e.g., sand, mud, gravel).18. Driving hollow steel piles with impact hammers produce intense, sharp spikes of sound whichcan easily reach levels that injure fish. Vibratory hammers, on the other hand, produce sounds oflower intensity, with a rapid repetition rate. A key difference between the sounds produced by impacthammers and those produced by vibratory hammers is the responses they evoke in fish. Whenexposed to sounds which are similar to those of a vibratory hammer, fish consistently displayed anavoidance response and did not habituate to the sound, even after repeated exposure 1 . Fishes mayrespond to the first few strikes of an impact hammer with a "startle" response. After these initialstrikes, the startle response wanes and the fishes may remain within the field of a potentially-harmfulsound. The differential responses to these sounds are due to the differences in the duration andfrequency of the sounds. Thus, impact hammers may be more harmful than vibratory hammers fortwo reasons: first they produce more intense pressure waves, and second, the sounds produced donot elicit an avoidance response in fishes, which will expose them for longer periods to those harmfulpressures.19. Several studies have been carried out throughout the world to assess the impact of soundassociated with pile driving. Popper et all (2006) 2 presented a review of all these documents andpresented a scientific assessment of pile driving exposures which are injurious to fish, andrecommended a criteria for pile driving (SEL 187 dB and Peak 208 dB at 10 m from source).20. The noise criteria of pile driving on the impact of fish as given by NOAA (National Oceanic andAtmosphere Administration, USA) are given in Table 7-5.Table 7-5: NOAA Criteria for Pile Driving and its Impact on FishEffect Parameter Fish Mass ThresholdOnset of physical injury Peak Pressure N/A 206 dB (re: 1 µPa)Accumulated Sound> 2 g 187 dB (re: 1µPa2*sec)AdverseeffectsbehavioralExposure Level (SEL) < 2 g 183 dB (re: 1µPa2•sec)Root Mean SquareN/A150 dB (re: 1 µPa)Pressure (RMS)1 The National Marine Fisheries Service, NOAA - Summary of Potential Impacts To Fish From Pile Driving -http://www.fakr.noaa.gov/habitat/letters/2003/dec/valdezharbordredge.pdf2 AN Popper, TJ Carlson, AD Hawkins, BL Southall and RL Gentry (2006) Interim Criteria for Injury of FishExposed to Pile Driving Operations: A White Paper - http://www.wsdot.wa.gov/NR/rdonlyres/84A6313A-9297-42C9-BFA6-750A691E1DB3/0/BA_PileDrivingInterimCriteria.pdf7-9