Bedforms and Associated Sediment Dynamics on the Inner Shelves at Different Spatio-temporal Scales

Book excerpt: This Thesis conducts the study of bedform development and dynamics in the inner shelf integrating observations at different spatial and temporal scales. An initial assumption is that different scales of sedimentary processes strongly interact between them and small-scale sedimentary processes intended to foster those of larger scale. The potential contribution of small-scale bedforms to the sediment transport is particularly addressed. The study includes the monitoring and analysis of sand ridges, ripples and near-bottom suspended sediment variations at the Ebro Delta (NW Mediterranean Sea) and Perranporth (Atlantic Ocean) inner shelves. A sand ridge field with maximum ridges heights of 2.5 m and 400 m spaced is located over a retreating lobe in the Ebro Delta. Ridges are mostly symmetric arranging obliquely to the shoreline. The change of the main Ebro River channel led to the progressive abandonment of the former river mouth and to the severe coastal retreatment, providing large amount of sediment available in the coastal zone. NW winds induce strong near-bottom currents flowing towards the SE, which are able to transport sediment and produce ridge formation and migration towards the SE at ̃10 m/y. The characteristics of the Ebro sand ridges match well with those of shoreface-connected sand ridges and, particularly, with the initial stages of sand ridge development on storm-dominated continental shelves. Time-scales related to their genesis can be within a few decades. The presence of ripples on the different inner shelves is the most usual situation under low- to moderate-energetic conditions. In the wave-dominated and tideless coast (Ebro Delta) ripples were observed superimposed on sand ridges. Four types were identified: small undulations that were the precursor ripples, 2D wave-ripples, current-dominated 2D-3D-ripples, and combined wave-current 3D-ripples. The wave-ripples were static while the current-dominated ripples migrated. In a macro-tidal sandy beach exposed to high-energetic Atlantic storms (Perranporth), only wave-ripples were developed arranging orthogonal to wave approach and they were static. The size of ripples changed from larger (developed in equilibrium conditions) to smaller ripples, the latter interpreted as degraded ripples in wash-out conditions. The ripple prediction did not well-adjust to neither ripple appearance nor dimension. However, if the observed thresholds of seabed states are applied the model improves ripple appearance forecast. The near-bottom suspended sediment variability from seconds to months in the Ebro Delta is described distinguishing between waves, currents, and combined wave-currents conditions. In general, waves dominated the sediment resuspensions although strong currents also have an important contribution in the suspended sediment concentration (SSC) increases. The time-averaged SSC usually showed redundant structure by forming three layers with different patterns. In the lower and intermediate layers the SSC oscillated at gravity and infragravity wave frequencies. Time-varying and instantaneous profiles exhibit sediment patterns potentially related to ripples. The potential role of ripple migration as an additional long-term mechanism of sediment transport was analysed. In Perranporth, wave ripples are stationary and did not contribute to beach recovery because of wave orbital symmetry. The sediment transport is alongshore during low-energetic regimes and cross-shore during more energetic regimes when ripples are washed-out. In the Ebro Delta, 3D-ripples migration agrees with sand ridges migration direction towards the SE. The respective migration rates and their differences in size, support that a subordinate part of sand ridge migration can be the result of ripple migration contribution under low-, moderate-regimes, suggesting that in specific environments, dynamics of small-scale bedforms can play a subordinate but not negligible role in the evolution of larger bedforms.