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The available measurements of the acoustic attenuation coefficient, α, in aqueous suspensions of glass beads and sand are investigated for 10 -3 1 ϕ+B 2 ϕ 2 , with ka-dependent coefficients, provide a prediction that is within a factor of 2 for low and intermediate values of ka.

Noise from turbidity currents flowing down the face of a fjord delta was measured underwater at frequencies from 1 to 1200 kHz. The noise spectra are consistent with sound generation by collisions among sand-sized particles. The spectra from the leading head extend to higher frequencies than those from the trailing body, indicating that collisions were between finer-grained particles in the head and coarser-grained particles in the body. Noise intensity increased 100-fold for a two-fold increase in head speed, consistent with the expected collision rate for granular materials in the high-flow gas-like phase and highly turbulent particle-laden flows.

Measurements of the phase velocity of compressional sound waves in water-saturated granular materials are reported for the 1.0–2.0 MHz frequency range. The sound speed estimates are based on travel times through granular layer thicknesses ranging from 8 to 17 mm. Three types of granular media were used: 336 μm median diameter glass beads and two natural sands with median diameters of 219 and 406 μm. These grain sizes and frequency range correspond to 0.5 < k a < 1.2 , where k is the wavenumber and a the grain radius. To remove trapped air, the samples were boiled under pressure before transfer to the measurement tank. The results are compared to previously reported experimental results and to the Schwartz and Plona [J. Appl. Phys. 55(11), 3971–3977 (1984)] multiple scattering prediction, confirming negative dispersion for ka > 0.5. Scaling the data by a factor depending on porosity and grain density reduces the spread among the available phase speed estimates by nearly a factor of 2, from 12.5% to 6.9%. [ABSTRACT FROM AUTHOR]

Sound scattering by solid particles suspended in a turbulent jet is investigated. Measurements of the scattered amplitude were made in a bistatic geometry at frequencies between 1.5 and 4.0 MHz, and at scattering angles from 95° to 165° relative to the forward direction. Two types of particle were used: nearly spherical lead-glass beads and aspherical natural sand grains. For each particle type, experiments were carried out using ∼200 and ∼500 μm median diameter grain sizes, corresponding to 0.7≤=ka≤=4. The sphericity of the sand grains, defined as the ratio of projected perimeter size to projected area size, was 1.08. The lead-glass bead results are consistent with an elastic sphere model. A rigid movable sphere model provides the best fit to the sand data, and the best-fit diameter is within 4% of the equivalent volume size. However, the scattering pattern for sand is systematically smoother than predicted: that is, the undulations in the angular scattering pattern predicted by spherical scatterer theory are present, but muted. This observed departure from spherical scatterer theory is attributed to disruption of the interference among creeping waves by the irregular surfaces of natural sand grains. [ABSTRACT FROM AUTHOR]

Wave friction factors are estimated from vertical profiles of near-bed turbulence and horizontal velocity spanning the wave bottom boundary layer. Measured values are partitioned by bed state, which ranged from irregular ripples to flat bed, and are examined as a function of two traditionally selected parameters—physical bed roughness and outer flow Reynolds number. The measurements are from two field experiments in very different nearshore environments: a relatively protected unbarred pocket beach and a linear barred beach exposed to the open shelf (Duck). Measured wave friction factors are remarkably similar for the two beaches and are highest for low-energy rippled beds and lowest for the high-energy flat bed conditions. The reduction in the friction factor for high-energy conditions corresponds to a decrease in the physical roughness of the bed as wave energy increases. As a function of relative roughness, measured friction factors are generally consistent with previous laboratory results and theoretical results for the high-energy cases, but the predicted values for the low-energy rippled beds show some significant differences. A new expression is derived for the bed roughness and is found to have a stronger dependence on ripple steepness than previously suggested laboratory-based relationships. Estimated friction factors exhibit a power-law dependence on Reynolds number and occupy a narrow band within the rough turbulent and transition regions. [ABSTRACT FROM AUTHOR]

Field observations are presented of the morphological evolution of beach cusps on a 1:10 slope, megatidal (8–12 m range), mixed sand-gravel beach at the head of the Bay of Fundy, Nova Scotia. Cusps had mean wavelengths of 3–6 m and displayed pronounced horn/bay sediment size segregation, with sand-sized material in the bays and gravel-sized material in the horns. Cusp occurrence was limited to the upper third of the beach face. Shoreline position during the tidal cycle was estimated at three minute intervals from time-averaged video imagery. Three cusp events are examined in detail, two exhibiting pronounced topographic relief, and the third demonstrating sensitivity of the rate of cusp evolution to the beach surface grain size distribution. Forcing conditions were weak, with significant offshore wave heights of 10–20 cm and peak periods of 4–7 s. Relict cusp morphology was inundated with the rising tide and destroyed or reworked during high tide. New cusps formed during the falling tide through a combination of accretion at the horns and erosion in the embayments. Timescales of growth and decay were short, ranging from 10 to 30 min. The location and dynamics of cusp horns appeared to depend on the high water line and its location relative to any pre-existing cusp morphology. The apparent sensitivity of cusp formation timescales to the local grain size distribution suggests that size segregation is intrinsic to the process of mixed sand-gravel cusp evolution. • Timescales of initial beach cusp emergence were of the order of 10 min. • The very broad grain-size distribution affects cusp evolution timescale. • Positions of emergent cusps were influenced by pre-existing morphology. • Cusp evolution was modulated by the 10-m range tide. [ABSTRACT FROM AUTHOR]