Optimizing breakwater configuration for vessel wakes and wind waves

Abstract

The introduction of strongly powered ships has introduced a new component of hydrodynamic activity in semi-sheltered seas, with properties (wave height, period and dominant direction) largely differing from the usual wind waves. We address the properties of the incoming vessel waves at the Noblessner Port, a small marina in Tallinn Bay, the Baltic Sea. The basic properties of wake waves (the maximum height and period of the largest waves), variability in their properties at the entrance to the port and their attenuation rate during propagation through the port is analyzed. Surface elevation at the port entrance is tracked using an ultrasonic echo-sounder. Properties of oscillations in the port interior are evaluated using a submerged wave recorder. The measured data are used for setting up a high-resolution version of the COULWAVE model for the optimization of the configuration of breakwaters. Dangerously high wake waves appear at the entrance to Noblessner Port about ten times a day. The 80%ile of the highest ship wakes at the entrance of the port was 1.24 m. The periods of the highest wake waves were 6.5 s, that is, considerably larger than typical periods of wind waves in Tallinn Bay. The waves arrive from the North-East, from where high and long wind waves never approach. The wave height in the port interior is, in average, about 40 % of their height at the port entrance. Interestingly, surface oscillations with periods much longer (>15 s) than periods of the incoming waves and with a height of up to 0.35 m appear inside the port.