Cross‐ripple patterns and wave directional spectra

Abstract

[1] Rotary sonar images of the seafloor during SandyDuck97 are used to investigate the geometry of the cross-rippled bed state relative to the directional properties of the incident waves. The sonar imagery indicates that the cross-ripple pattern is a quasi-regular diamond-shaped lattice constructed of variable length ladder-like tiles, each with shorter-wavelength ripples residing within the troughs of the longer-wavelength component. The longer-wavelength crests were oriented at approximately ±30°with respect to the incident wave direction. Most wave directional spectra were not bimodal in direction and, for the small number (17%) which were, the distribution of the angular separations of the two peaks was very broad and indicated no preferred value. Thus, contrary to some previous suggestions in the literature, these results indicate not only that cross ripples do not require waves propagating from two different directions for their formation but also that for most of the occurrences during this experiment, cross ripples formed when the incident wave field was unimodal in direction. Finally, the orientation angles of the long-crested ripple components exhibit no dependence on mean current velocity.

Occurrence and orientation of anorbital ripples in near‐shore sands

“…These spectra were corrected for refraction using Snell's law, and shoaled to the frame locations assuming linear wave theory and constant cross‐shore energy flux. The wave height spectrum was then transformed to near‐bottom velocities using linear wave theory as described by Cheel and Hay [2008]. The resulting wave direction, α p , corresponds to the direction of maximum energy in the shoaled and transformed directional spectrum.…”

Geometric bed roughness and the bed state storm cycle

“…That cross ripple steepness should be anomalously low relative to other ripple types may simply be indicative of their three‐dimensional character and the fact that the ripple crests are not orthogonal to the incident wave direction. (Note that, on the basis of the results reported by Cheel and Hay [2008], the pencil beam profiles would have intersected the cross ripple crests at an angle of about 50° on average. The associated steepness correction would be a 55% increase, which does significantly affect the location of the cross ripple points in Figure 8).…”

“…For LTRs, Maier and Hay [2009] obtained λ ○ = 7.6 ± 0.06 cm. For cross ripples, Cheel and Hay [2008] obtained mean wavelengths of 44 and 55 cm for the long‐wavelength component at the two frames, and 9.6 to 9.8 cm for the short‐wavelength component (these are the values from the radial spectra). Hay and Mudge [2005] obtained wavelengths of 7.7 to 20 cm for irregular ripples.…”

“…Thus, irregular ripples and cross ripples are suborbital‐type ripples in that sense. However, it should be born in mind that cross ripples involve 2 distinct wavelengths and orientations, and the ripple crests are not orthogonal to the incident wave direction [ Cheel and Hay , 2008]. Thus, it is unclear either how or if cross ripples should be represented in the parameter space of Figure 7.…”

Acoustic inversions for measuring boundary layer suspended sediment processes

“…Acoustics has the capability to measure collocated and simultaneously the three components of this dynamic sediment triad. Multifrequency acoustic backscattering systems, ABS, are used to measure profiles of suspended particle size and concentration (Crawford and Hay, 1993;Thorne and Hardcastle, 1997), coherent acoustic Doppler velocity profilers, ADVP, are used to measure the three orthogonal components of flow Betteridge et al, 2006;Hurther and Lemmin 2008) and high resolution acoustic ripple profilers, ARP, are used to measure detailed changes in small scale bedforms (Williams et al, 2004;Traykovski, 2007;Cheel and Hay, 2008). The combined application of these acoustic technologies has made sound a valuable tool in the study of fundamental sediment transport processes .…”