Sand wave

Langhorne, D.N. (1977) Consideration of meteorological conditions when determining the navigational water depth over a sand wave field. Inti. Hydrogr. Rev. LIV, 17-30. 

In recent subtidal sands cemented layers commonly occur below a cover of loose sand. Wave and current reworking of the uppermost sand layer inhibits lithification, because the sand must be stable to permit cementation. Such a stable situation exists when deposition occurs only by periodic processes such as storms (Ginsburg, 1953), or when the surface layer is biogenically stabilized by sea grass meadows or algal mats (Davies Kinsey, 1973 Harris, 1978 Dravis, 1979). Otherwise, in places where sand is continually moved by waves or tidal currents, cementation takes place at a few... 

Successive cross sections of one-large sand wave are shown in Fig. 

Fig. 12. Observed displacement of a large sand wave on the Mattituck sill over 148 days.

When the sand content of the sediment rises above 90%, sand waves appear on the Sound floor (Bokuniewicz et al., 1977). Sand waves that cover Mattituck Sill are composed of fine sand with a mean grain diameter of about 0.3 mm. Divers have observed sand grains moving over the bottom in this area and have found ripples superimposed upon the sand waves. The rate of migration of the sand waves has been documented at two locations on the sill by Bokuniewicz et al. (1977) and Karen Zim-... 

It was found that the sand-mud transition zone could be adequately represented by allowing the sedimentary processes to proceed at constant rates. When this was done, the calculated sand flux agrees well with the sand flux that has been measured over the sand-wave field in the eastern Sound. Since the tides and the estuarine circulation control the sand fluxes, the resulting distribution of sand approaches a steady state very quickly. It is likely that the sand-mud transition was established soon after the Sound became an arm of the sea and has persisted unchanged to the present day. As a corollary to this hypothesis, it would seem that Mattituck Sill has been accreting over the lifetime of the Sound. Small variations in the sand content preserved in short cores are probably due to the perturbation of the sand distribution by a series of winter storms rather than individual storm events. 

Ludwick, J. C. (1972). Migration of tidal sand waves in the Chesapeake Bay entrance. In Shelf Sediment Transport (D. J. P. Swift, D. B, Duane, and O. H. Pilkey, eds.), pp. 377-410. Dowden, Hutchinson Ross, Inc., Stroudsburg, Pennsylvania. 

Stability existence and susceptibUity to slumping, sliding, turbidity currents, sand waves, and cratering... 

McCave, I.N. 1971. Sand waves in the North Sea off the coast of Holland. Marine Geology, 10 199-225. 

National Conf Hydraulic engineering. 311-316, E.V. Richardson, ed. ASCE New York. Nordin, C.F., Algert, J.H. (1966). Spectral analysis of sand waves. Journal of the Hydraulics... 

Depending on the aim of the bathymetric survey, the equipment and line density is chosen. For an overall bathymetric survey that is carried out for example on the North Sea, a line density of one hne every 25 or 50 metres using a single beam echo sounder can be sufficient. On the other hand, in case of monitoring the transport behaviour of so-called sand waves much more detail is needed and hence the use of multibeam echo sounding to obtain a full coverage of the sea bottom is advised. 

The surface roughness represented by Equation 12.2 may not be realistic for natural sediment beds. The values of A are representative of engineered solid surface and are very small in magnitude compared to sediment bed-forms. Thibodeaux et al. (1980) and Christy and Thibodeaux (1982) found that organic liquids used as tracers positioned in the sand wave valley have MTCs lower than the flat sediment surfaces. The in-valley MTC correlation for A = 5 cm and 15 cm sand waves were described by the following equation ... 

Re < 1600 with A in meters. In this case, rc = [1 - - 9.6A / ] . Based on the parameter values used in Table 12.3, the wave valley = 0.25. This correction factor is less than unity and is significantly lower than those presented in Table 12.3. Recent studies by the authors support the finding that local gypsum-measured MTCs in valleys are lower than those on the crests of sand waves. This point warrants further study. 

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