Managing Dredging Noise During the Construction of the World's Longest Immersed Tunnel in the Fehmarnbelt Between Denmark and Germany
WODCON XXIII - Dredging is changing - The Practice. The Science. The Business.
G. Nehls, M. Schmiing, S. Bräger. T. Folegot, E. Hemon, M. Bellmann, S. Gerlach, R. Matuschek, J. Flamme
"Underwater noise immissions from construction vessels has been subject to intensive discussions in the planning phase of the Fehmarnbelt Fixed Link Project, because of a lack of national or international standards to measure and assess noise from shipping and dredging. The Fehmarnbelt - though one of the busiest waterways in Europe - holds important numbers of the protected harbor porpoise, and the Immersed Tunnel will cross a protected Natura 2000 area established for this endangered population. The German Plan Approval decision limits Project-related underwater noise (mostly vessel-based construction noise) to facilitate porpoise migration through the Fehmarnbelt and to limit disturbance in the Natura 2000 area. Furthermore, Project-related noise immissions must be monitored to meet strict thresholds throughout the construction phase. Monitoring Project-related underwater noise is challenging as the soundscape of the Fehmarnbelt is dominated by commercial shipping noise which usually exceeds noise emitted by the construction vessels of the Project. Therefore, underwater noise monitoring cannot be performed solely by measurements. The approach chosen is an underwater noise model supported by parallel underwater noise measurements. The real-time model predicts the natural background noise as well as vessel-based noise. Based on source levels assigned to vessels as well as the relevant parameters influencing sound propagation, the model calculates the sound spread by every vessel in the area based on their positions derived from AIS data. This allows modeling ambient noise (i.e., the sum of all non-Project-related noise) and Project-related sounds separately in real time. Sound immissions along the construction sites are continuously measured with hydrophones and the noise recordings are used to evaluate model predictions and to calibrate the model. In this paper, first experiences with this twin approach of modelling and measuring from the Fehmarnbelt construction site are outlined."
Keywords: Dredging, monitoring, near real-time sound modelling, threshold, underwater noise