Key concepts

• Visual observations are often the only source of wave information available to the engineer. Sometimes measurements made with instruments are available.

• Measurement techniques can be divided into in situ techniques (instruments deployed in the water) and remote-sensing techniques (instruments deployed at some distance above the water).

• The most common in situ instruments are wave buoys and wave poles. Other in situ instruments are inverted echo-sounders, pressure transducers and current meters. These instruments need to be mounted on some structure at sea.

• The most common remote-sensing technique is radar, which is based on actively irradiating the sea surface with electro-magnetic energy and detecting the corresponding reflection. Radar may be deployed from the coast (e.g., with a receiving station in the dunes), from fixed platforms (e.g., oil-production platforms) or from moving platforms at relatively low altitude (airplanes) or high altitude (satellites).

• Radar can be used to obtain images of the sea surface, but it can also be used as a distance meter or as a surface-roughness meter.

• Each measurement technique has its own peculiarities as regards operational performance, accuracy, maintenance, cost and reliability.

• The most common result of a wave measurement is a time record of the sea-surface elevation at a fixed (horizontal) location.

Introduction

Waves are not only observed by surfers, swimmers or tourists from the beach. Experienced crew members onboard voluntary observing ships (VOS; or voluntary observing fleet, VOF), too, observe the waves and report wave height, period and direction daily to meteorological institutions around the world. Scientists and engineers too are watching waves.