Author(s)

JAN-VICTOR BJÖRKQVIST, ILKKA VÄHÄAHO, KIMMO KAHMA

Abstract

Wave reflection at a steep shore radically affects the proper building height and must therefore be taken into account in coastal planning. The reflection of the waves can be partially mitigated by building e.g. wave damping chambers. While there is a great deal of laboratory data available even for irregular waves, field data verifying the effectiveness of different existing structures are sparse. An extensive wave measurement campaign was conducted in the coastal waters of Helsinki in the Baltic Sea, financed by the city. The region has a characteristic archipelago, which affects the wave properties. The reflection of waves was studied by comparing simultaneous measurements from two devices. A wave buoy was moored ca 800m from a steep wall, while the wave height was also measured from the shore using a capacitive wave gauge. The coastal structure is equipped with caisson chambers built to damp the waves. Each of the chambers were perforated with two 0.7m wide and 3.2m high openings. The inside of the chambers are 5.9m deep and 4.6m wide. By comparing the wave buoy and wave staff spectra we observed that the waves were reflected up to frequencies of about 0.4–0.5 Hz, with the longest waves of about 0.2 Hz being fully reflected. Waves shorter than 0.45 Hz (i.e. of order 10 m) were not reflected, from which we can draw the conclusion that they were damped by the chambers. The reflection coefficient for the significant wave height was 1.5 for the total significant wave height, 1.7 for waves below 0.45 Hz and 0.9 for waves above 0.45 Hz. We conclude that a damping chamber of this particular type and dimension is not sufficient for attenuating waves longer than approximately 10m.

Keywords

wave damping, wave reflection, archipelago, wave run-up, coastal construction