Measurements of underwater noise at floating bridges and ferries

Abstract

In a hydroacoustic risk assessment context, acoustic noise has been recorded emanating from the floating Nordhordlands Bridge, from the diesel-electric ferry (Valestrand – Breistein), and from the hydrogen ferries across Bjørnafjorden - E39. Analysis of the recorded noise shows that the acoustic profiles from the ferries and from the floating bridge are distinctly different. The dominant source of acoustic noise around Nordhordlands bridge is related to boat traffic from ferries and fishing boats, not from traffic on the bridge itself. This causes the bridge to emit a comparatively small amount of noise, which cannot statistically be distinguished from background noise. The ambient noise has a daily average of approximately 100 dB re 1 Pa2, with a 5 dB variation between day and night. The noise from the ferry traffic across Bjørnafjorden is mainly related to engine and propulsion sounds, which can be described as broadband noise source with a strength of approximately 173 dB re 1 Pa2 @1 m. The source can be perceived by marine life at distances up to 14 km and can potentially influence the entire fjord system around Bjørnafjorden. The noise from the Bjørnafjorden ferries is predominantly within the frequency range 50 – 150 Hz, which corresponds to standard ship noise (RANDI 3.1C), showing that this particular model is well suited for further use in acoustic studies of ferry traffic in the region. In a life cycle perspective, ferry traffic causes a greater acoustic impact than floating bridges, except in the construction/decommissioning phases of the latter, which are characterized by noise from blasting, piling, construction traffic and machinery. A more detailed study on acoustic noise impact on the vulnerable ecosystems should be carried out, detailing timevarying source terms for both types of infrastructures. Generally, one should require that construction in the coastal zone is complemented by strategies for noise monitoring and mitigating measures, ensuring that marine biological diversity is safeguarded to the greatest extent possible.