A Simple Instrument for the Measurement of the Slope and Height Distributions of Small Scale Wind-Driven Water Waves

A new optical wave gauge was built to measure simultaneously two statistical parameters of wind induced water surface waves, namely the surface slope and surface amplitude probability distribution functions. The new instrument was tested in a linear wind wave flume with a water depth of 10 cm. The surface slope is determined using the refraction of light at the water surface. The wave amplitude is measured using the absorption of infrared light in the water column. The wave gauge consists of a point-like dichromatic light source which is positioned under the flume (Ulbricht sphere with high-power LEDs, (RED = 632nm and IR = 850nm) and a camera above the flume that looks vertically through the water into the light source. No other optical components are needed. The light source is pulsed and the camera is triggered. The images show light speckles whose positions are a measure for the wave slope. The relative intensities are a measure for the wave amplitude. These quantities are derived from the digital images by means of image processing and simple geometrical considerations. The influence of different analysis methods on the wave slope and amplitude distribution was studied. For the characterization of the new linear wind wave flume in the Institut für Umweltphysik (IUP), Heidelberg, mean square slope and root mean square wave height were measured for a wind speed up to 6.7 m/s and a fetch between 0.80m and 2.40m. The findings agree with comparable measurements in other linear wind wave flumes.