Preventive management of undesired changes in alongshore sediment transport in the planning of a waterfront infrastructure

摘要

We show that undesired remote impacts of changes in the location of the waterfront may occur in fairly mild wave conditions. As an example, we analyse the consequences of a moderate reclamation action in the bayhead of Tallinn Bay, the Baltic Sea. The planned changes are fairly small: the waterfront will be shifted by a few tens of metres owing to the construction of a new major traffic junction. The potential impacts are established using generic concepts of the nature of coastal processes and a detailed reconstruction of the local wave climate. The main properties of closure depths and magnitude and direction of wave-driven alongshore transport in the entire bay are established using a triple-nested high-resolution version of the wave model WAM that is forced for 32 years by high-quality marine winds. It is demonstrated that the planned reclamation area currently serves as a convergence point of alongshore sediment transport. Even though the closure depth is only about 2 m, the presence of fine sand gives rise to a relatively wide equilibrium beach profile. The seaward end of this profile is currently close to the entrance to a major harbour about 0.5 km from the activity area. The pattern of sediment motions is such that even a minor shift in the coastline may lead to considerable increase in the transport of sand into the harbour entrance. We evaluate the time scale for this process based on laser scanning data about sand accumulation rates and the concept of equilibrium beach profile, and discuss the ways of preventive mitigation of the consequences of reclamation.