Measuring Wave Impact on Coastal Structures with High Spatial and Temporal Resolution - Tactile Pressure Sensors a Novel Approach

Wave impact forces on coastal structures have been measured and studied both in the laboratory and in the field since the last century. Traditionally, pressure measurements in laboratory studies are performed by using an array or few arrays of pressure transducers placed in the middle of the structure, where the maximum pressures are commonly assumed to occur. The impact forces are then calculated by spatially integrating the point measurements of pressures. Thus, the existing knowledge on the vertical distribution of impact pressures on vertical/sloped walls is quite well established. However, for breaking wave impact on coastal structures producing impulsive loads with very short duration, the pressure distribution is not well known. This is because the impact pressures resulting from breaking waves are highly variable both temporally and spatially, and often far from being two-dimensional. As a result, current predictions of resultant forces from one dimensional array may considerably vary from the actual loads and therefore give conservative or under predicted loads. Hence, there is a strong need for a pressure measurement system with very high spatial and temporal resolution, which could be met by tactile pressure sensors. In a collaboration between the University of Southampton, UK and Forschungszentrum Kuste (FZK), Germany within the framework of the HYDRALAB IV project the wave impact pressure distribution on a sea dike of 1:3 slope was mapped by use of a tactile sensor over an area of 42 cm x 48 cm. Preliminary experiments were carried out in the Large Wave Flume (Grosser Wellenkanal, GWK) in Hannover. This paper describes the details of this novel measurement technique and its application in laboratory experiments. A calibration procedure for dynamic loads, which is an indispensable matter as the sensors response differently on static and dynamic loads, has been developed. Challenging issues of employing the sensor in large scale experiments such as removing the entrapped air or making the system water proof have also been explored.