Topographical Measurements of Water Waves at a Matrix of Measuring Points (PDF)

Inhaltsangabe:Abstract:
The measurement of the shape of a water wave is important for the study of a wide range of questions like the understanding of wave motions, the wave shapes dependence an the quality of the ground, the water depth or the surface tension which might be altered by pollution. The mixing of substances or gases at the surface of the sea is dependent an the waves shapes. The shape of a water wave is also interesting for understanding the transport mechanisms and the energy of waves. Knowledge about water wave shapes is essential for the design of an effective shore protection which is going to be more and more important when the sea level is rising. Furthermore offshore designers require knowledge about the shape of a wave during its interaction with installations for optimising the shape of offshore structures and for calculating the loading. This is a key requisite for the design and the construction of cost effective and save structures of any kind like buoys, lifeboats, ships, oil rigs or other offshore platforms. Measurements of wave shapes for example can help to understand the interference or upwelling caused by the legs of typical offffshore platform geometries which can enhance the wave amplitude and possible wave impact with the underside of the platform.
The probably simplest way of measuring the surface of a wave is to put a certain number of water depth gauges into the water, taking a photo of the wave when it is passing the gauges and then reading every single measure from the photograph. The wave shape can be reconstructed by interpolating the waves surface between the measuring points. The considerable distortion of the measuring object by the measuring instruments is disadvantageous.
An array of pressure sensors in a plain below the water surface can be used instead of the gauges to avoid the distortions. The pressure at every single sensor is a measure for the water height above the sensor. The disadvantage of this method is the sensitivity of the pressure sensors for the dynamic pressure of water flows caused by turbulences, in particular in shallow water, underneath breaking waves or close to constructions. Also disadvantageous is the necessity of mounting a part of the measuring system under the water.
To avoid this problems, photographic methods for the recording of surface waves have been developed:
At the beginning of the 20th century the first attempts have been made to collect elevation data of ocean [...]