Subproject P1 will study groundwater flow and transport patterns as a function of hydro- and morphodynamics. We hypothesize that the state-of-the-art concept of groundwater flow and salinity distribution in STEs under tidal influence does not hold for high-energy beaches. Instead, we expect highly transient flow conditions, with high frequencies and amplitudes of change in salinities, flow directions and residence times. P1 will survey beach and foreshore morphology using camera imaging, laser-scanning and drones. Field observations will be combined with numerical models to derive typical adaptation time- and length scales of the beach morphology. Geophysical methods will enable a fast and non-invasive way of identifying subsurface salinity patterns and their change over time. Environmental tracer data will be used to estimate groundwater residence times. Morphological, hydro(geo)logical and geophysical data from the STE Online Observatory will be utilized in numerical groundwater flow and transport modelling to provide a comprehensive picture of transient flow and transport patterns within the STE. This knowledge on the distribution of water bodies, flow velocities, residence times and extent of mixing will provide the basis for biogeochemical investigations carried out in subprojects P2-P6.