Near Real-Time Underwater Sound Modeling of Dredging Noise to Meet Regulatory Noise Thresholds

The Fehmarnbelt Fixed Link consists of an immersed tunnel connecting Denmark and Germany in the western Baltic Sea. Marine sediments are excavated to create the tunnel trench. The importance of the Fehmarnbelt for the harbor porpoise is recognized and managed in German waters through a Natura 2000 area. The authorities defined two indicators to safeguard its function. The first threshold demands that not more than 20% of the cross-section between Lolland and Fehmarn shall be exposed to construction Sound Pressure Level exceeding 144 dB ref. 1 μPa. The second threshold states that not more than 1% of the Natura 2000 area shall be exposed to construction Sound Pressure Level exceeding 140 dB ref. 1 μPa from June to September. Measurement of dredging sound does not allow to assess the compliance with these thresholds since public shipping sound regularly exceeds them already. Therefore, a hybrid approach combining real-time modeling and continuous measurements has been developed, using measurements for continuous calibration of the model. Results of the first and half construction year revealed an average deviation between the model and measurements less than 1 dB. Compliance with the thresholds is assessed with confidence and reliability despite the interference of sounds introduced by other maritime activities.