Observations of wave orbital scale ripples and a nonequilibrium time‐dependent model

Abstract

[1] Measurements of seafloor ripples under wave-dominated conditions from the LEO15 site and the Martha's Vineyard coastal observatory were used to develop a time-dependent model for ripple geometry. The measurements consisted of backscatter imagery from rotary side-scan sonars, centimeter resolution bathymetric maps from a two-axis rotary pencil-beam sonar, and forcing hydrodynamics. During moderate energy conditions the ripple wavelength typically scaled with wave orbital diameter. In more energetic conditions the ripples reached a maximum wavelength of 0.8 to 1.2 m and did not continue to increase in wavelength or decrease in height. The observations showed that the relict ripples left after storms typically had wavelengths close to the maximum wavelength. The time-dependent model is based on an equilibrium model that allows the ripples to maintain wavelength proportional to wave orbital diameter until a suspension threshold determined by wave velocity and grain size is reached. The time-dependent model allows the ripple spectra to follow the equilibrium solution with a temporal delay that is based on the ratio of the ripple cross-sectional area to the sediment transport rate. The data was compared to the equilibrium model, a simplified version of the time-dependent model (where the ripples were assumed to follow the equilibrium model only when the bed stress was sufficient to move sediment), and the complete time-dependent model. It was found that only the complete time-dependent model was able to correctly predict the long wavelength relict ripples and that the other approaches underpredicted relict ripple wavelengths.

Comparison of measurements and models of bed stress, bedforms and suspended sediments under combined currents and waves

“…Bedforms also impact on the mechanism of sediment entrainment, from diffusive to convective vortex processes and depending on the type of mechanism different shape functions and reference concentrations arise (Ribberink and Al-Salem, 1994;van der Werf et al, 2006;Thorne et al, 2009). Owing to the impact bedforms are deemed to have on the hydrodynamics and sediment processes, there have been many studies to measure and predict ripple formation, type and dimensions (Nielsen, 1992;Williams et al, 2005;Davis et al, 2004;Soulsby and Whitehouse, 2005;Traykovski, 2007;Grant and Madsen, 1982;Pedochi and Garcia, 2009). However, there is still a high degree of scatter in the presented data sets, particularly when associated with field observations, and predictions are variable, depending on the formulation chosen.…”

Effect of roughness formulation on the performance of a coupled wave, hydrodynamic, and sediment transport model

“…2). Ripple dimensions were initialized with height g r = 0.1 m and wavelength k r = 0.6 m on the coarse patches, and g r = 0.01 m and k r = 0.1 m over the fine patches, based on diver observations and previous studies (Goff et al, 2005;Traykovski, 2007). Transport during our three-week model simulations produced mixed sediment in computational cells at the coarse-fine boundaries, but did not alter the boundaries.…”

A Method to Quantify Bedform Height and Asymmetry from a Low-Mounted Sidescan Sonar

“…On tidal megaripples, at the reversal of flow, incipient current ripples (l ' 1000d 50 , or ;20 cm) form on top of larger megaripples (Yalin 1964;Venditti et al 2005;Traykovski 2012). On wave-orbital ripples, smaller (;20 cm) ripples can form in the troughs of meter-scale relict ripples left from previous wave events (Traykovski 2007). When there are superimposed bedforms, the secondary bedforms cast small shadows that otherwise would have been a bright return from the larger bedforms.…”

“…The location is dominated by waves with little-to-no influence of tidal currents. The data were collected in 2007 using a quadpod equipped with both a rotary sidescan sonar and a two-axis pencil beam sonar similar to a dataset collected in 2005 and documented in Traykovski (2007) that was also located at MVCO in 12-m water depth. The sidescan sonar, deployed 1.67 m from the seabed, captured the backscatter intensity over a radius of 20 m around the quadpod, and the pencil beam sonar obtained bathymetric data over a 2 m 3 2 m region directly beneath the quadpod (Fig.…”