In nature, bed forms are found in three environments of greatly differing 

 characteristics: 



• Rivers - unidirectional and channelized; large variety of grain sizes. 



• Sandy coastal bays - semi-channelized, unsteady, reversing (tidal) flows. 



• Continental shelves - deep, unchannelized; dominated by geostrophic 

 flows, storms, tidal currents, wave-generated currents. 



Because of the diverse natural settings and the differing disciplines of 

 researchers who have studied sedimentology, the classification and nomencla- 

 ture of bed forms has been confusing and contradictory. The following classi- 

 fication scheme, proposed by the Society for Sedimentary Geology (SEPM) 

 Bedforms and Bedding Structures Research Group in 1987 (Ashley 1990) is 

 suitable for all subaqueous bed forms: 



a. Ripples. These are small bed forms with crest-to-crest spacing less than 

 about 0.6 m and height less than about 0.03 m. It is generally agreed 

 that ripples occur as assemblages of individuals similar in shape and 

 scale. On the basis of crestline trace, Allen (1968) distinguished five 

 basic patterns of ripples: straight, sinuous, catenary, linguoid, and 

 lunate (Figure 22). The straight and sinuous forms may be symmetrical 

 in cross section if subject to primarily oscillatory motion (waves) or 

 may be asymmetrical if influenced by unidirectional flow (rivers or tidal 

 currents). Ripples form a population distinct from larger-scale dunes, 

 although the two forms share a similar geometry. The division between 

 the two populations is caused by the interaction of ripple morphology 

 and bed shear stress. At low shear stresses, ripples are formed. As 

 shear stress increases above a certain threshold (which varies with grain 

 size, fluid density, and other properties) a "jump" in behavior occurs, 

 resulting in the appearance of the larger dunes (Allen 1968). 



b. Dunes. Dunes are flow-transverse bed forms with spacings from under 

 1 m to over 1,000 m that develop on a sediment bed under unidirec- 

 tional currents. These large bed forms are ubiquitous in sandy environ- 

 ments where water depths are greater than about 1 m, sand size coarser 

 than 0.15 mm (very fine sand), and current velocities greater than about 

 0.4 m/sec. In nature, these flow-transverse forms exist as a continuum 

 of sizes without natural breaks or groupings (Ashley 1990). For this 

 reason, "dune" replaces terms such as megaripple or sand wave, which 

 were defined on the basis of arbitrary or perceived size distributions. 

 For descriptive purposes, dunes can be subdivided as small (0.6 - 5 m), 

 medium (5 - 10 m), large (10 - 100 m), and very large (> 100 m). In 

 addition, the variation in pattern across the flow must be specified. If 

 the flow pattern is relatively unchanged perpendicular to its overall 

 direction and there are no eddies or vortices, the resulting bed form will 



Chapter 3 Field Data Collection and Observation 



51 



