ENERGY DISSIPATION FROM INTERNAL SOLITARY WAVES SHOALING ON FLAT AND MILDLY SLOPING BEDS

Abstract

Abstract Boundary layer separation-driven near-bed instability beneath internal solitary waves (ISWs) of depression may be a significant source of wave-energy dissipation and drive localized mixing and resuspension in coastal regions. In chapter 2, the details of estimates of energy flux by internal waves to quantify the dissipation of ISWs as they propagate over a flat bottom. Wave flume experiments were carried out to measure the dissipation of turbulent kinetic energy in the boundary layer beneath shoaling ISWs. Wave dissipative length scale of O(100) wavelengths for unstable waves; which is in agreement with limited field observations. Stable waves propagated significantly further (>1000 wavelengths). Internal solitary wave energetics were used to model wave dissipation lengthscales in terms of the wavelength and momentum thickness Reynolds number L^*=100λ+2.5×10^10 λ〖Re〗_ISW^(-3.7). In chapter 3, laboratory experiments were performed to measure dissipation within the turbulent bottom boundary layer beneath an ISW. Velocity vector maps were measured using Particle Image Velocimetry. Different methods of estimating dissipation from spatial gradients of the vector maps were compared, based on measured components of the dissipation tensor. Estimates based on assumptions of isotropy are typically larger than those based on methods using available velocity gradients with least number of assumptions (e.g., direct method). All the methods reproduce the same order-of-magnitude estimates for the dissipation rate (ε~10^(-7)-10^(-6) W kg^(-1)) but the differences varied from 5% to 80% than the corresponding direct estimates. Chapter 4 describes an experimental investigation on the dissipative phase of ISWs as they propagate over uniformly sloping topography (S = 0.04) in a 20 m flume. Velocity profiles were recorded with three acoustic Doppler velocimeters. As each wave propagated up the slope, the wave of depression steepened and developed into an elevation-like wave before finally dissipating through forming a discrete vortex of dense fluid (bolus) which propagated for some distance up the slope. The dissipation length scale was parameterized in terms of the internal Iribarren number (ξ=S⁄√(a⁄λ)) as L^*=(302.7×ξ-10.4)×a and requires only a knowledge of incident ISW amplitude a and wavelength λ.