Geotechnical site investigation in energetic nearshore zones: opportunities and challenges

Nina Stark


Coastal erosion and scour around structures in the nearshore zone represent major societal challenges with regard to coastline conservation, the protection of coastal communities and eco-systems, as well as the development of coastal structures or renewable energy projects. Despite rapidly advancing sediment erosion, scour and morphodynamics prediction tools, the models still struggle to correctly simulate the impact of severe storm events and storm event clusters, particularly regarding long-term projections considering sea level rise and climate change. Scour prediction models still struggle to accurately predict the depth and extent of the scour around a structure, and often rely on significant overpredictions which impact the cost-efficiency of the structural foundation design. A review of common erosion and scour prediction models reveals that particularly sediment characteristics appear underrepresented. This results from challenges to derive this information in the field. Areas of active sediment remobilization processes, such as the nearshore zone, are characterized by energetic hydrodynamics (waves, tides and currents), and morphodynamics (migrating bars, etc.) representing challenges and risks to people, vessels and instrumentation. Most geotechnical field instrumentation to-date are not designed or suitable for measurements in such conditions, and new devices are needed to fill this gap. This paper presents results (i) using a portable free fall penetrometer of projectile-like shape to investigate in situ characteristics and stratification of sediment surface sediments in the nearshore zone under hydrodynamic forcing, and (ii) preliminary data using embedded pressure sensors to investigate the pore pressure response to irregular wave forcing in the nearshore zone and its potential impact on sediment erosion. The devices proved to be suitable for the deployment in energetic nearshore conditions. The data emphasize the potential regarding deriving novel information about in situ sediment characteristics, such as changes in sediment strength under the active sediment dynamics, as well as an increase of erodibility through the development of excess pore pressures on different time scales. However, the data also reveal challenges related to calibration of the instrumentation and data processing, particularly with limited additional information about the sediment.