Abstract. Observations of the temporal evolution of the geometric properties and migration of wave-formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO-15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to nonorbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near-bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave-to-current velocities.

Abstract. Field observations were made in 3-4 m water depth of linear transition ripple geometry and migration using a high-resolution laser-video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long-crested, low-steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high wave energies just below the fiatbed threshold, with wavelengths of 8.5 4-0.5 cm and heights of 0.3 + 0.1 cm. The maximum observed migration rate was 0.7 cm/min. Migration was offshore during storm growth and onshore during storm decay. The observed ripple migration velocities were highly correlated (r 2 > 0.7) with nearbed wave orbital velocity skewhess in both cross--shore directions. During storm growth the incident wave spectrum was bimodal and the orbital velocity skewhess was negative. During storm decay the xvave spectrum was unimodal and the velocity skewhess was positive. Bispectral analysis shows that the main contribution to negative velocity ske•vness during storm growth was due to a difference interaction between the two principal (sea and swell) components of the bimodal velocity spectrum. Positive skewhess during storm decay was due to selfself interaction of the narrowband residual swell. The negative velocity skewhess observed during storm growth is consistent with prediction by a two-frequency second-order wave theory.

Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three‐frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4–6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5‐ to 10‐cm height, over time scales ranging from 6.5 min to 4–6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5–20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time‐averaged concentration profiles exhibit an exponential decrease with height above an O(10)‐cm‐thick near‐bottom region of nonexpo‐nential decrease. In contrast, the time‐averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.

Multifrequency acoustic scattering experiments were carried out in the laboratory using a free, turbulent water jet carrying solid particles in suspension. A crossed-beam geometry was used, in which the axis of the jet was perpendicular, or nearly so, to the direction of the incident sound. The suspended particles were either natural sand grains or lead-glass beads, 100-500 tim in diameter. The results, based on measurements made simultaneously at 1, 2.25, and 5 MHz, include backscatter and attenuation as a function of suspended sediment concentration, measurements of the total scattering and backscattering cross sections of natural sand grains and glass beads as a function of frequency and particle size, and two-point correlation measurements of the fluctuations in apparent suspended sediment concentration. The leadglass bead cross sections exhibit both the resonance and diffraction extrema expected of solid spherical scatterers; the sand cross sections do not and, for acoustic wavelengths comparable to or less than the particle circumference, are larger than spherical scatterer theory would predict. The two-point correlation estimates of jet velocity and measurements of the timeaveraged jet width as a function of particle size are related to the mean and turbulent structure of two-phase jets and illustrate the potential of acoustic methods for noninvasive investigations of two-phase turbulent flow.

[1] Results are presented from 70+ days of nearly continuous in situ acoustic imagery of the nearshore sandy seabed in $3-m mean water depth, at two locations separated by 40-m cross-shore distance. The bottom sediments were 150 mm median diameter sand, with nearly identical size distributions at the two locations. Five principal bed states were observed: irregular ripples, cross ripples, linear transition ripples, lunate megaripples, and flat bed. The linear transition and flat bed states were the most frequent, together accounting for 68% of the total time. Bed state occurrence was a strong function of incident wave energy, each bed state occurring within a relatively narrow range of seaand-swell energies. During the 12 major storm events spanned by the record, the bed response was characterized by a repeatable bed state storm cycle, involving four of the five principal states (lunate megaripples did not appear repeatedly, and thus may be a special case), with no obvious dependence of bed state occurrence on prior bed state, or on thirdmoment measures of wave nonlinearity. Radial spectra from the rotary acoustic images indicate pronounced differences in the anisotropy of spatial scales for the different bed states, and exhibit onshore-offshore differences which are likely related to ripple migration.Citation: Hay, A. E., and T. Mudge (2005), Principal bed states during SandyDuck97: Occurrence, spectral anisotropy, and the bed state storm cycle,