Bubbles, formed by breaking waves, play an important role in the transfer of gases between the Earth’s oceans and atmosphere and have been shown to increase the flux of gases during periods of heightened sea state. Having been formed, these bubble clouds evolve through the effects of buoyancy, gas exsolution and dissolution, and the fragmentation and coalescence of bubbles. A number of experimenters have successfully measured sub-surface bubble clouds using a variety of acoustic and optical techniques, although data over a wider range of bubble radii are required for fuller comparison with models of how these clouds evolve and contribute to air-sea transfers of mass, momentum and energy. This study details the design of an acoustic system deployed on an 11 metre spar buoy during two sea trials in the Atlantic Ocean. Through the measurement of the additional attenuation due to bubbles, bubble size distributions were inferred over the broadest range of bubble radii ever measured using active acoustics in the open ocean. The volumetric backscatter strength from the bubble clouds were also measured to gain a profile of these bubble populations. A gas transfer model was then developed, with the measured data used as an input to calculate the associated fluxes. With this method, bubble-mediated transfer velocities and equilibrium supersaturations were found for the first time based on experimental work. These parameters aid the characterisation of air-sea gas transfer and therefore help improve the accuracy of existing climate models.

The results of laboratory experiments are described, relating to aspects of hydrodynamics and sediment dynamics under second-order Stokes type waves (or flows), of varying degrees of asymmetry. The majority of the measurements related to laminar and/or transitional flow conditions and were made using an oscillating trolley apparatus. The transition to turbulence over smooth beds has been reported previously in terms of a (single) critical flow amplitude Reynolds number, Recrit=U∞a/ν. On the basis of observations undertaken using sinusoidal flows (Li, 1954) and during the present study, this is found to be the case for wave periods of T>3.5s, where mean Recrit=1.66×105. However, for T<3.5s, it is shown that Recrit decreases in proportion to T. On the basis of the observations made by Li (1954), Manohar (1955) and during the present study, transition over rough (granular) beds is described by Recrit=c(a/D), where c is a coefficient that, for relatively fine sediment (D<275μm), is a linear function of T; for relatively coarse sediment (D>421μm), it is a linear function of D. At large values of Recrit, corresponding to longer wave periods together with relatively small bed roughness length-scales, the observed values deviate from the rough-bed relationship and tend towards the smooth-bed limiting value. Flow asymmetry acts to stabilise the boundary layer, increasing either the critical boundary Reynolds number RE 2ν /ω ν crit c =U (in the case of smooth beds), or Recrit (in the case of rough beds), following a non-linear relationship. Regulating mechanisms are proposed by which the transition to turbulence is governed over (relatively) smooth and/or rough beds. Of principle importance is the balance between the stabilising effect of fluid acceleration and the destabilising effects of vertical gradients in the horizontal velocity (thought to be important in regulating transition over a smooth-bed) and localised eddy formation around individual grains on the bed (similarly over rough beds). The threshold of motion for non-cohesive, sand-sized sediment is expressed typically as a critical bed shear stress amplitude, τo, relative to the resistant properties of individual grains (due to gravity). On this basis, numerous critical shear stress (e.g. the well known approach of Shields, 1936) and velocity amplitude relationships have been presented elsewhere. Previously, Voulgaris et al. (1995) have identified that a higher τo is required to cause threshold at smaller wave periods. On the basis of a large number of observations undertaken (elsewhere, and as part of the present study) using similar equipment, a negative linear relationship has been established between T and τo; this becomes progressively more significant, for threshold occurring under larger values of Re (into the transitional regime). Flow asymmetry has the effect of increasing τo crit; however, the critical orbital diameter for given conditions remains approximately constant, irrespective of the asymmetry. Using these data, in combination with detailed observations of the phase of the onset and the subsequent duration of sediment motion, it is suggested that (especially under (near) laminar flows) the threshold of motion is in response to a ‘time-‘ or ‘phase-mean’ shear stress, corresponding to some form of cumulative force. In addition, under turbulent or partially turbulent flow conditions, the stochastic distribution of the instantaneous shear stress is broader under waves of larger T and/or smaller R; this permits similarity in the occurrence of high-shear events, over a range of conditions. However, the mean τ0 crit decreases. Hence, an artefact or anomalous decrease is included, at longer wave periods, in the (time-mean) peak value of τo crit used to represent such flows.

The inversion of compressional wave properties is presently emerging as a technique for determining the geotechnical properties of marine sediments. However, the relationships required to perform such an inversion are still under debate, with further research required to resolve the dependence of compressional wave properties on both frequency and geotechnical properties. Though the use of in situ probes provides the most promising manner of examining these relationships, previous work in this field has encountered a number of experimental difficulties. This work presents a series of well-constrained in situ transmission experiments. These were undertaken on inter-tidal sediments using a purpose built in situ device, the Sediment Probing Acoustic Detection Equipment (SPADE). Compressional wave properties were measured from 16 to 100 kHz in a range of sediment types (medium to fine sands and medium to fine silts), with several closely spaced locations examined at each general site to assess the local variability in compressional wave properties. Spreading losses, which were adjusted for sediment type, were incorporated into the data processing. Also included were a thorough error analysis and an examination of the repeatability of both the acoustic wave emitted by the source and the coupling between the probes and the sediment. The results indicate that sands possess greater group velocities, greater effective attenuation coefficients and lower quality factors than silts, while the low velocities measured in silts imply that the bulk moduli of the silt sites examined are lower than expected owing to a considerable fraction of organic matter. Significant variations were observed in compressional wave properties, which were more reliably related to variations in geotechnical properties in sands than in silts. Group velocities were observed to be independent of frequency in sands within 95 % confidence limits, with no reliable frequency-dependence being determined in silts owing to variability in the measured values. Effective attenuation coefficients were proportional to frequency within 95 % confidence limits for the majority of the sand and silt locations examined. Results indicate that compressional wave properties can be used to determine porosity, bulk density and sand fraction, while the reliable determination of mean grain diameter from compressional wave properties in inhibited by the scatter in the data. The results from this study were also used to assess the effectiveness of Biot Theory to predict the compressional wave properties of these sediment types. In sands, the Biot phase velocities agreed with measured group velocities, while Biot absorption coefficients were less than measured effective attenuation coefficients, owing to scattering or squirt flow not accounted for in the Biot Theory. In silts, Biot phase velocities are greater than measured group velocities, while Biot absorption coefficients generally agree with or are greater than measured effective attenuation coefficients. In silts, predicted velocities are greater than those measured, while absorption coefficients generally agree with or are greater than measured attenuation coefficients. The discrepancy between the measured attenuation coefficients and predicted absorption coefficients can be explained through the over-estimation of in situ porosities by the geotechnical measurement techniques adopted.