University of Twente Student Theses


The effect of wind and waves on the hydrodynamic and morphodynamic properties of sand waves

Singh, A. (2021) The effect of wind and waves on the hydrodynamic and morphodynamic properties of sand waves.

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Abstract:The seabed of shallow continental shelf consists of rhythmic bed patterns of varying spatial and temporal scales. It consists of as small as (mega)ripples to as large as tidal sand banks. Among them lies the intermediate dynamic bed forms, sand waves. They are formed when bottom perturbations are subjected to tidal flow. They may migrate up to several tens of meters per year and grow up to one-third of the local water depth. These physical features may severely influence offshore human activities, such as navigation, construction of wind farms, and dredging. Therefore, it is essential to understand the physical processes that affect sand wave morphology to ensure safety in shipping routes and optimize the strategies of expensive dredging work. In general, the effect of several physical processes and factors have already been studied. For example, previous recorded observations and results of a simplified model by Campmans et al. (2018) demonstrated the significant effect of severe wind and waves on sand waves. However, few values of model results, such as migration rate or sand wave height, were overestimated compared to field observations. In particular, the model made certain physical processes simplified by using constant coefficient viscosity, linear wave-current interaction and excluding suspended load. Including these non-linear effects will add more insight and improve the previous findings related to the combined effect of wind and waves on sand wave properties. Thus, this study presents a 2DV process-based model using Delft3D that investigates the effect of wind and waves on sand waves. The model includes advancements in physical processes such as variable eddy viscosity, non-linear wave-current interaction, and suspended load. The study examines both hydrodynamic and morphodynamic properties. All simulations related to hydrodynamic properties are performed on predefined four beds of the same wavelength (216 m), increasing amplitude (0-3 m), and varying asymmetricity. These are short-term runs for two tidal cycles on a domain length of 45 km. The morphodynamic properties related simulations are performed on an initial bed: 0.25 m symmetrical sand wave of wavelength 216 m when wind and waves are present for the complete duration. At the same time, morphodynamic simulations for intermittent storms are performed on a randomly perturbed initial bed. These are long-term simulations for 100 years on a similar domain length. The hydrodynamic results show an increase in average viscosity and turbulence with the addition of wind and waves. It distorts circulation cells formed due to symmetrical tide and leads to the asymmetrical distribution of bed shear stress over sand waves. In addition, it causes near-bed wind-driven residual in wind direction, including a small reverse drift above it opposite to the wind direction in tidally average horizontal flow. An increase in reverse drift with depth reverses near-bed mean velocity at 30 m or greater mean water depth. The effect of severe wind and waves is more significant than intermediate wind and waves on sand waves. The morphodynamics results show that waves alone cause flattening of the crest but do not induce any significant migration. However, together with wind, waves intensify the migration, reduce sand wave height and increase wavelength. Our work also identifies that suspended load increases during storm conditions. This increase also intensifies the reduction The effect of wind and waves on the hydrodynamic and morphodynamic properties of sand waves Author: Singh,A. Page 3 in sand wave height when compared to tide-only conditions. The change in hydrodynamic properties is reflected in morphodynamic properties as well. A steeply reducing migration rate is observed at 30 m or greater depth due to reverse drift observed in tidally average flow. The above results concern steady-state complete duration storm conditions. In actuality, storms are intermittent, and this study considered short-duration storm conditions of frequency three months per year. In their presence, sand wave fields show a migration rate of 0.5 – 1 m/year. The storm conditions cause a height reduction of 33% when present for the complete duration and 5% – 8% when present intermittent, compared to no storm conditions. The results of this research study related to the sand wave properties are comparable to field data. Additionally, it shows that the effect of intermittent storms depends on the wind speed, duration and the frequency between the calm and storm periods. Therefore, high speed and long duration wind conditions can add substantial value to migration. It will modify the safety factors while designing the foundation of offshore wind farms, the base layer of cables and pipelines. It will also help in refining the dredging work frequency required in navigation channels. Further, it is recommended to include residual current with intermittent storms followed by the 2-D Fourier analysis of sand waves bed level as tides are not symmetrical in reality. Also, the calm period between intermittent storms is recovering the symmetrical behaviour of sand waves lost during storms due to symmetrical tide. Further, the suitability of the present lateral boundary conditions (Riemann boundary) should be checked with the periodic boundaries as few properties such as mean vertical velocity showed phase differences at the lateral boundaries. Unfortunately, this suitability could not be checked in this study as implementing periodic boundaries was not possible in the present version of Delft3D.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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