Coastal hydrodynamics I

We simulate numerically the interaction of river influxes with tidal propagation in regions of varying complexity for the Scheldt basin (river, tributaries and estuary) and the European continental shelf. While the computational domain for rivers is one-dimensional and extends up to the limit of tidal influence, the estuary and shelf region are dealt by two-dimensional equations. Within this region, a hydraulic structure in one of the tributaries is implemented to simulate the bidirectional flow of the tide. The coupled model was first calibrated for a quiet period and then extensively validated using available measurements for simulations in the month of January (2021) that involves storm surges. Moreover, the shelf region of the model was compared with the existing literature for the harmonic analysis of the dominant M2 tide. The simulated sea level elevation exhibits an RMS error smaller than 0.3 meters.

Laboratory experiments will be conducted to measure the mean flow and turbulence over fixed concrete low and intermediate-angle estuarine bedforms with the use of an Acoustic-Doppler Velocimeter (ADV). Three sets of experiments are planned with modelled shapes resembling that of the estuarine dunes observed in the Weser Estuary, Germany. Furthermore, measurements with reversed flow direction will also be carried out for each experimental case. Mean flow and turbulence structures will be measured through the two component instantaneous velocity profiles. Specifically, horizontal and vertical velocities will be used to define the (intermittent) flow separation zone, the turbulent kinetic energy will be used to define the turbulent wake structure, and the vertical gradient of the horizontal velocity to define the region of shear layer. The results from this study can be used to characterise in detail the flow dynamics over low-angle estuarine bedforms.

Transient rip currents transport material between the surf zone and the inner shelf. During directionally spread wave conditions, waves break in finite-length regions, generating vorticity near crest ends. Energy associated with the injected vorticity may be transferred to larger scale eddies that eject offshore as transient rip currents. However, our understanding of these processes is limited, partially due to the challenges of isolating and measuring them in the field. To overcome this, we conducted large-scale wave basin experiments. We found that increasing the directional spread results in more crest ends that inject vorticity. Also, the along-crest gradient in breaking force varies from crest to crest and with the wave conditions, as assessed using remotely sensed water surface elevation maps. Lastly, by employing optically derived surface velocities, the surfzone flow fields for directionally spread waves are consistent with an inverse energy cascade from small to larger scale eddies.

Low-crested detached breakwaters, vital for coastal protection, face scour challenges due to wave-structure-seabed interactions. This study advances the understanding of scour near detached rubble-mound breakwaters. Using a 3D mobile bed physical model at the University of Porto, various hydrodynamic conditions and breakwater geometries were explored. Scour predominantly occurred in the trunk section, with a distinct pattern in the seaward roundhead quadrant. Notably, the scour-accretion boundary was dynamic, dictated by prevailing maritime conditions. Peak wave period significantly influenced scour depths, emphasizing the need to incorporate realistic sea conditions into existing scour depth formulations for coastal protection structure design. Interestingly, plunging breaker phenomena were absent, challenging conventional assumptions and underscoring the complexity of scour dynamics in detached breakwaters. This research contributes to better-informed coastal protection strategies and highlights the importance of considering wave characteristics in scour depth assessment and design.