Moored elastic sheets under the action of nonlinear waves and current

This study is concerned with the interaction between nonlinear water waves and uniform current with moored, floating elastic sheets, resembling floating solar panels, floating airports, tunnels and bridges, and floating energy systems. The Green-Naghdi theory is applied for the nonlinear wave-current motion, the thin plate theory is used to determine the deformations of the elastic sheet and Hooke’s law defines the effect of the mooring lines. The horizontal displacement of the floating sheet is determined by substituting the forces induced by the fluid flow and the tensions generated in the mooring lines into the equations of motion of the floating body. The resulting governing equations, boundary and matching conditions are solved in two dimensions with a finite-difference technique. The results are compared with the available numerical data. Overall, very good agreement is observed. In the model developed here, the sheet is allowed to drift due to the wave-current impact, and hence the mooring lines partially restrict both deformation and the horizontal motions of the sheet. The influence of the mooring lines on the dynamics of the floating sheet is assessed in terms of wave- and current-induced elastic deformations and surge movements of the sheet. It is demonstrated that the mooring lines attached to the leading and trailing edges of the sheet can be highly effective in mitigating the horizontal oscillations and vertical elastic deformations of the floating sheet subjected to the wave and current actions. Special attention is given to the horizontal periodic motions of the sheet, which are analysed by use of a Fourier transform technique. It is shown that the moored elastic sheet can oscillate at a frequency different from its exciting frequency as a result of restoring forces from the mooring lines, exciting resonance when both frequencies meet. Extensive study in a broad range of sheet parameters, mooring stiffnesses and wave-current conditions established the location of resonant regimes of different configurations of the moored systems. Analysis of wave reflection and transmission coefficient revealed that mooring lines of increasing stiffness intensify the wave reflection and, consequently, result in smaller energy transformation downwave

Solitary and cnoidal wave scattering by a submerged horizontal plate in shallow water

Solitary and cnoidal wave transformation over a submerged, fixed, horizontal rigid plate is studied by use of the nonlinear, shallow-water Level I Green-Naghdi (GN) equations. Reflection and transmission coefficients are defined for cnoidal and solitary waves to quantify the nonlinear wave scattering. Results of the GN equations are compared with the laboratory experiments and other theoretical solutions for linear and nonlinear waves in intermediate and deep waters. The GN equations are then used to study the nonlinear wave scattering by a plate in shallow water. It is shown that in deep and intermediate depths, the wave-scattering varies nonlinearly by both the wavelength over the plate length ratio, and the submergence depth. In shallow water, however, and for long-waves, only the submergence depth appear to play a significant role on wave scattering. It is possible to define the plate submergence depth and length such that certain wave conditions are optimized above, below, or downwave of the plate for different applications. A submerged plate in shallow water can be used as a means to attenuate energy, such as in wave breakers, or used for energy focusing, and in wave energy devices

Morphodynamics of ebb tidal delta sandbanks in a meso to macro tidal environment; Teignmouth, UK.

Merged with duplicate record (10026.1/2039) on 03.01.2017 by CS (TIS)This contribution utilises a multifaceted approach to investigate the physical processes responsible for the onshore migration of ebb tidal sandbanks at an estuarine inlet dominated by low energy waves. A coastal video system was used to obtain two-weekly quantitative measurements of the position, plan form and crest depth of the landward migrating sandbanks over a five year period which encompassed four consecutive events. These data were supplemented with a 35 year photographic record of the inlet's development. Onshore sandbank migration was characterised by mean and maximum rates of I to 2 m. day"1 and 5 m. day" 1 respectively. The migration rate was found to be highly correlated (R2 = 0.7) with, and linearly related to the ratio of the incident wave height to the crest depth particularly prior to shore-attachment. The plan form, area and crest depth of the sandbanks are a function of the sediment availability. In the inlet's current morphologic mode two plan form geometries are typically observed. These are crescentic and elongate forms (high aspect ratios) where the latter have their major axis transversely-orientated with respect to the coast. Which of these forms develops is dependent on the chronology of wave energy and the crest depth in relation to the tidal water level variation. Elongate transverse morphologies are associated with low relief sandbanks which are synonymous with periods of relatively low sediment availability. Conversely the crescentic morphologies typically occur when sediment availability is higher and the depth to the sandbank crests is shallower. A unique set of in-situ measurements of waves, currents and sediment transport were obtained from four positions on a sandbank in the mid term of its onshore migration. Analysis of the data revealed that the sandbank was dominated by onshore directed sediment transport in the shallow surf zone where current maxima occurred. The gradients in transport were highest on the flood tide. An energetics analysis of the data revealed that short wave stirring and wave driven mean flows were the physical processes responsible for morphological change. The mean flows are in effect longshore currents since they are generated by strong refraction and focussing of the incident waves by the morphologies. These generate a zone of wave convergence over the sandbanks when in the submerged state and very oblique wave breaking along the flanks when the features are exposed by the tide. A numerical model (MIKE 21) was subsequently applied in order to simulate the wave driven patterns of erosion and deposition over the sandbanks using both idealised and measured boundary conditions. The numerical experiments determined that there exists a dichotomy in the patterns of erosion and deposition which are laterally constrained in the submerged state and divergent in the exposed state. Morphological evolution was therefore governed by the variable residence times of the causative wave driven processes at different tidal elevations. It was found that low energy waves when combined with neap tides promoted shoreward elongation through the dominance of the patterns of deposition in the shallow submerged state. Higher energy conditions were predicted to promote a degree of broadening in the longshore dimension and increased crest elevation. This was caused by the patterns of deposition being dominated by both the laterally constrained (high tide) and divergent (low tide) patterns due to their longer residence times