Large-Scale Simulation of a Shipping Speed Limitation Measure in the Western Mediterranean Sea: Effects on Underwater Noise

Underwater noise from shipping activity can impact marine ecosystems in the long term and at large scale. Speed limitation has been considered to reduce noise emission levels. In this article, the effects of speed limitation on shipping noise levels are investigated at high spatial resolution (5 arc-min) in the Western Mediterranean Sea. Scenarios of maximum speed limits of 10 kt and 15 kn are computed. The impact of a speed reduction is time-dependent and tends to redistribute sources of noise temporally, smoothing the contrasts existing in marine traffic at the daily scale. The effectiveness of the measure is evaluated over short successive time windows (6 h), allowing for capture of the dynamic of the effect of speed reduction. Several metrics are proposed to evaluate the effectiveness of speed reduction as a mitigation measure according to its temporal stability. This study illustrates complex phenomena related to (1) the increased vessel density in the speed limitation area due to longer navigation time and (2) deep-water and shallow-water propagations. The bathymetry and the local distribution of traffic are two elements of importance with respect to the effectiveness and the stability of the measure, whereas the traffic properties seem to impact the stability of the effect in particular, and deep waters seem to increase the effectiveness. This research shows the areas in which the proposed measure would be the most effective.