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Formation and evolution of relict trochoidal sediment waves in the Irish Sea throughout their post-glacial history

Rademaker, Marcus (2023) Formation and evolution of relict trochoidal sediment waves in the Irish Sea throughout their post-glacial history.

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Abstract:Trochoidal sediment waves are large-scale rhythmic bedforms in a non-cohesive, shallow and tidedominated marine environment and are observed at several locations which experienced glaciations, e.g. the Irish Sea. The abnormal heights of trochoidal sediment waves do not scale with the present-day water depths and current hydrodynamics are unable to form these bedforms. Therefore, the hypothesis states that the coarse cores of trochoidal sediment waves are relicts and their generation is linked to palaeo-tidal conditions during the marine transgression after the Last Glacial Maximum, i.e. 26, 000 years before present. Over time, the Irish Sea encountered extreme sea-level rise and significantly changing tidal dynamics before conditions stabilised around 6, 000 years ago. Nowadays, a sand veneer on the flanks of the present-day core is mobile during high-energy events. These relicts can comprise a nuisance to offshore projects and affect hydrodynamic processes as macroscale roughness to the bed due to their abnormal heights, thereby impacting the entire ecosystem. Processes influencing the genesis of trochoidal sediment waves however remain poorly understood. Process-based models can be used to simulate present-day sand wave behaviour. The contemporary model set-up however needs significant adjustments due to the coarse nature, larger water depths and a substantial time-scale. Nevertheless, these models may give insights into the genesis of relict trochoidal sediment waves. The aim of this research is therefore to explain the formation and evolution of trochoidal sediment waves throughout their post-glacial period using an existing process-based sand wave model in Delft3D. First, reconstructed palaeo-tidal conditions and the present-day morphological characteristics of trochoidal sediment waves in the Irish Sea are analysed. The first indicates a clear trend of intensifying peak bed shear stresses and significant sea-level rise. Bathymetry data from the Amplifies Sediment wave Irish Sea project subsequently showed site-specific characteristics with wave heights and wavelengths ranging between 11 to 35 m and 340 to 560 m, respectively. Furthermore, grab samples obtained along trochoidal sediment waves mainly presented multimodal sediment mixtures with the coarsest sediments found in the troughs. Subsequently, the model is adjusted according to the Irish Sea. A sensitivity analysis is conducted to find a suitable parameter setting. Through a 2DV case study sediment wave formation and evolution on a millennial time-scale is modelled following the trend obtained from the palaeo-tidal reconstruction. Sea level rise and enhanced hydrodynamics are simulated with depth increments and increasing bed roughness and tidal conditions, respectively. The model results show that sediment waves grow in height over time and that longer wavelengths prevail at larger depths. This resulted in wave heights and wavelengths comparable to those observed in the field. At last, a thin sand layer is added at the bed to the 2DV case study model through bed stratification to determine the initial morphodynamic response. This layer of sand represents the observed mobile sand veneer. Model simulations show that the sand covers the flanks and troughs of the coarse core over time thereby decreasing its original height and tending towards a new equilibrium. Observations however imply that the coarsest sediments are found in the troughs. Furthermore, the sand layer is shown to be mobile dependent on the tidal current during the simulations. In comparison to previous sand wave modelling studies, this research can be considered as a starting point of modelling bedforms with a coarser nature which at located at significantly larger water depths. Through a case study model with an idealized set-up the research shows that it is also possible to model over a significant time-scale despite the associated large uncertainties. Moreover, this study indicates that observed trochoidal sediment waves in the Irish Sea formed and evolved between 12,000 and 6,000 years before present thereby strengthening the geological hypothesis. This model set-up is recommended for modelling on a substantial time-scale to qualitatively support hypotheses. Recommended research includes the application of the case study model for other study areas which experienced or face sea level rise. Collecting more grab sample data and further exploration of including processes within the set-up could potentially help to further understand the formation and evolution of these relict bedforms.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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