Measurement and analysis of wave loading on a full scale coastal structure

• Main Author: Crawford, Adam Randolph
• Published: University of Plymouth 1999
• Subjects: 624; Breakwater design
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.300692

The aim of this investigation was to measure and analyse wave loading on a full scale coastal structure in order to validate current breakwater design methods and to improve understanding of the physical processes involved. A range of new, robust field measurement instrumentation was developed and deployed at the chosen field site, the Alderney Breakwater in the Channel Islands. The instrumentation deployed in this particularly harsh wave loading environment included an array of wave loading pressure sensors together with co-located void fraction gauges, which were used to measure the percentage air entrained within the seawater. Wave data was measured by means of a sea bed array of six pressure sensors which were logged using an underwater data logger. Data from the instrumentation mounted on the breakwater wall was logged with a high specification remote data logger. Both the instrumentation and the data acquisition equipment were developed and adapted specifically for this investigation and as a result over 150 high quality data sets were recorded at very high logging rates, which allowed field data analysis at an unprecedented level. New calibration and data processing methods were developed for the analysis of this novel set of data records. Due to the meticulous planning, instrument development, data acquisition development, and deployment the data collected is, to the best of the Author's knowledge, the highest quality wave loading field data collected to date. The wave conditions measured at the site were used as inputs to three commonly used design methods for vertical coastal structures, which were used to estimate the maximum wave loading pressures over the height of the structure. The pressures and forces predicted by the models were contrasted with measured values and it was found that the Goda method (1985) predicted the events with a high degree of accuracy provided that the waves were not breaking directly onto the structure. When waves did break onto the structure high magnitude, short duration pressures were frequently measured which sometimes also acted over a very small spatial area. There was a large degree of temporal and spatial variability in the high magnitude breaking wave pressures and they were not accurately predicted by any of the models. The relationship between wave momentum flux and wave loading impulse was investigated both on a record by record basis and using a wave by wave analysis. For the Alderney field site a consistent relationship was found between the wave momentum flux and wave loading impulse, which could be used to estimate the wave loading impulse and duration for known wave input conditions. Features of interest were also identified from temporal comparisons of individual co-located pressure and aeration traces, including negative pressures and a negative correlation between air content and pressure over short time scales.