Offshore dredger sound: source levels, sound maps and risk assessment (abstract)

The Port of Rotterdam is expanding to meet the growing demand to accommodate large cargo vessels. One of the licensing conditions was the monitoring of the underwater sound produced during its construction, with an emphasis on the establishment of acoustic source levels of the Trailing Suction Hopper Dredgers during their various activities: dredging, transport and discharge of sediment. Initial background noise measurements were performed in the absence of dredging in September 2008. Source level and background noise measurements were made while dredging activities were underway in September 2009. Possible effects of underwater sound on marine fauna were considered, for scenarios with and without dredgers. The measured source levels for the dredgers, estimated source levels for other shipping and time dependent position data from a vessel tracking system were used as input for propagation modelling in the area, to generate dynamic maps of the broadband underwater sound pressure levels of the dredgers and regular shipping. The possible effects of the dredging activities on the marine fauna are assessed, specifically for porpoise, seal and fish. Quantitative criteria are selected in the form of risk thresholds for permanent and temporary hearing threshold shift. Two scenarios are considered: a stationary animal exposed to sound from a transiting dredger; and an animal swimming past a stationary dredger. Finally, a 24-hour dynamic scenario is constructed, enabling the comparison of cumulative exposures for stationary and swimming animals with risk thresholds from available criteria.