Sediment Dynamics in Estuarine Tidal Flats in Transition

Abstract

Intertidal ecosystems are at the boundary between land and sea, ranging from seagrass meadows, mangroves, and salt marshes to tidal flats. These habitats offer essential ecosystem services, such as coastal protection, carbon sequestration, and food supply, crucial for the well-being of the growing coastal communities. Intertidal ecosystems serve as natural barriers, absorbing the energy from waves and tides, thus mitigating the impacts of sea level rise and increased storm events. Further, intertidal ecosystems are important as foraging areas for migratory and resident birds. Tidal flats are found globally and are influenced by local and regional geology, hydrology, and climate differences. These areas are dynamic, and the bed level is constantly changing due to the action of waves, currents, the activity of benthic animals, and (indirect) human influences such as dredging or coastal constructions. These dynamic conditions challenge studying tidal flats on a short-term, local scale and over global, decadal timelines. A combination of technologies is essential to monitor tidal flats to understand the response to changing environmental conditions such as climate change. Analysis of satellite data spanning several decades has revealed that tidal flats worldwide require a minimum amount of sediment dependent on the tidal range, which is essential for adapting to rising sea levels. A tidal flat will become vulnerable and potentially disappear if sediment supply is hindered due to human or natural causes. In such cases, large-scale technical interventions are necessary to preserve tidal flats. An effective measure in the Eastern Scheldt, the Netherlands, is sand nourishment to preserve the foraging habitats of birds in tidal flats for the near future. Research into the daily fluctuations in surface elevation is vital for assessing tidal flats' ecological roles, such as supporting seedling growth and providing habitats for benthic communities. In this thesis, new techniques and statistical methods have been developed to monitor and describe the daily dynamics of tidal flats. Surface Elevation Dynamic (SED) sensors, developed by NIOZ, measure local changes in surface elevation with high frequency. Multiple signatures have been established to interpret the bed level fluctuations, which focus on the magnitude, frequency and driving environmental forces (e.g. wind or tide) behind surface elevation changes, offering a comprehensive insight into the factors influencing tidal flat dynamics. The newly developed morphodynamic signatures quantify the sensitivity of the daily bed level changes to environmental conditions. We found that the daily changes in bed level are often correlated to variations in wind force and direction. However, the morphology of a tidal flat is more a product of the tidal range. Indices of the surface elevation dynamics, magnitude and frequency may be comparable at a local scale. However, the study highlighted that the short-term responses to environmental conditions are more heterogeneous in space, indicating the need for high-frequency local monitoring to understand the tidal flats in an estuary and individual responses of tidal flats to climate change. Therefore, local monitoring of short-term dynamics complements existing regional and global models, supporting insight into adaptive management for preserving tidal flats.