Pore pressure monitoring of surficial coastal sediments on wave-resolving scale with outlook on liquefaction risk prediction at offshore wind and marine hydrokinetic energy converters

Offshore and ocean renewable energy gain increasingly in importance with the global race to explore and exploit new green energy options. They also represent a novel way to bring energy independence and reliability to remote places which are off the grid and currently depend on fossil fuel generators. Yakutat, Alaska, is one of the currently investigated sites with regard to the harvesting of wave energy. However, many offshore and ocean renewable energy projects are still facing a number of different challenges. One of these challenges can be the sediment specific pore pressure response to wave action, structure vibration and earthquake shaking, which governs the risk for higher sediment erodibility and liquefaction. In this study a set of two pressure sensors complemented by an acoustic Doppler current profiler (ADCP) and an underwater camera were tested to investigate pore pressure evolution on wave- and tide-resolving scale in the lower intertidal zone of Cannon Beach, Yakutat, Alaska. The following observations were made. Groundwater dynamics were noticeable at a sediment depth of 20cm before and during the passing of the swash zone. Wave characteristics of the surf and swash zone were reflected and deviations between the signals recorded at the different sediment depths noticed. Wave damping of the surf with increasing sediment depth was observed, as well as single pore pressure peaks which were often associated with wave irregularity. Sustaining of pore pressure and incomplete pore pressure relief in wave troughs was noticed and increased with wave irregularity. The results led to the development of a concept of a novel pore pressure lance with options for complementary instruments such as a commercial Cone Penetration Test (CPT) tip and/or an ADCP that would contribute to the understanding of pore pressure evolution at beaches and in the vicinity of coastal structures, and especially, near offshore and ocean renewable energy converters in coastal areas and the nearshore zone.