Nearshore Sediment Dynamics

Figure 1:Acoustic images of different bed states. Offshore is to the top of each panel. High amplitude reflections are yellow and red, weak signals dark blue. The different bed states are: (a) irregular ripples; (b) cross ripples; (c) linear transition ripples; and (d) flat bed. From Smyth et al. (2000).


Figure 2: Bed state and rms bottom roughness during SandyDuck97, determined from rotary fanbeam and pencilbeam acoustic images of the seabed: (a) bedstate (irregular ripples, cross ripples, linear transition ripples, and flat bed); (b) rms bottom roughness in the 3-10 cpm band, at instrument frames A, B, C, and D located respectively at 65 115 150 and 190 m distance from the shoreline. The bedstate data in (a) are from frame C. The results are from Hay and Mudge (2001, manuscript in preparation).


Figure 3: Coherent Doppler Profiler results showing, from top to bottom, horizontal velocity 20 cm above bottom, vertical velocity in the incident wave band as a function of height above bottom, vertical velocity in the turbulence band, and suspended sediment concentration. Irregular ripple bed state. (from Smyth et al., 2000).


Figure 4: Wave friction factors from 2 nearshore field experiments, Sandyduck97 and Queensland95, obtained from CDP-measured nearbed turbulence intensities, for different bed states (irregular ripples, I; cross ripples, X; linear transition ripples, LT; and flat bed, F) plotted as a function of grain-roughness Shields parameter (effectively wave energy, as the grain size was essentially the same). The vertical lines joining the solid and open diamonds indicate two different estimators of nearbed turbulence in the presence of waves. There are two such bars for each ripple type, one for each experiment, the one at higher Shields parameter being SandyDuck. The solid line corresponds to Tolman (1994). (from Smyth et al., 2000, and Smyth and Hay, 2001).


Figure 5: Nearbed maximum intensity of the vertical component of turbulence, 2w'rms (which has been shown in laboratory experiments to be approximately equal to u*) for different bed states during the Queensland95 and Sandyduck97 experiments. From Smyth and Hay, 2001.


Figure 6: Vertical profiles of the inertial subrange turbulence spectral slope for different bed states during Sandyduck97 in: (a) frequency domain; (b) wavenumber space. From Smyth and Hay, 2001.


Figure 7: Migration velocities (Mr) of linear transition ripples versus wave orbital velocity skewness. The solid red circles correspond to storm growth, when migration was negative offshore); the solid green triangles to storm decay, when migration was predominantly onshore (from Crawford and Hay, 2001a).


Figure 8: Migration velocities (Mr) of linear transition ripples versus the theoretical migration rate estimated from the Meyer-Peter and Müuller semi-empirical bedload transport formula. Bed stress estimated using grain roughness and (a) rms orbital velocity or (b) significant orbital velocity. Dashed line is best fit; dash-dot line 1:1. Symbols as in Figure 7. (From Crawford and Hay, 2001b).


Figure 9: Real parts of the bispectra of cross shore velocity during offshore ripple migration and storm growth (left panel), and onshore migration and storm decay (right panel). In each panel, the part below the diagonal is the bispectrum computed from the velocity measured 20 cm above the bed; the part above the diagonal the bispectrum computed from weakly non-linear wave theory (From Crawford and Hay, 2001b).


Figure 10: Energy spectra of nearbed cross shore velocity showing the swell and sea peaks (fI and fI') during storm growth (solid), and the swell and its harmonic (fII and 2fII) storm decay (dashed). From Crawford and Hay (2001a).


Figure 11: Comparison between CDP-measured and theoretical vertical structure of the velocity field above a flat sloping bed under waves. The bottom slope is 2o. Top panels are the magnitude and phase of the horizontal velocity; bottom panels are the magnitude and phase of the vertical velocity. The data are in blue; inviscid, horizontal bed theory in green; eddy viscosity, sloping bottom theory in blue. The plotted quantities are the velocity-surface elevation transfer function (from Zou and Hay, 2001).

This page last updated November 21, 2001.

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