ABSTRACT

Gravel dunes are rarely reported owing to comparative rarity of occurrence and generic confusion with antidunes and classes of gravel bars. A compilation of data from the literature for sediments with a median size greater than 2 mm shows that dunes have been developed in the laboratory in median grain sizes up to 28.6 mm. For field conditions, there are data for grain sizes up to 60 mm. Equilibrium and non-equilibrium gravel dunes range in length from less than 0.6 m to greater than 100 m. Heights range from less than 0.1 m to 16 m. Height and length data for the steepest three-dimensional (3-D) gravel dunes are consistent with the H:L function reported by Ashley (1990) for equilibrium dunes developed in sand. For a broad range of dune length, 3-D gravel dunes are steeper than two-dimensional (2-D) dunes, but for L > 100 m and H > 8 m there are too few data to draw any conclusion. Ashley's H:L function does not apply to a broad range of 2-D gravel dunes. A separate well defined function is proposed to describe the H:L relationship for 2-D gravel dunes of maximum steepness.

There are few data on the hydrodynamic development of gravel dunes. As a guide, gravel dunes develop for a range of Froude numbers up to 0.75 given that the nondimensional mean bed shear stress () exceeds circa 0.1. Dunes reach their maximum height at = 0.25 and have reduced in height when = 0.3. Froude numbers of 0.84 and above lead to crestal flattening, dune diminution, and eventual replacement by antidunes or upper-stage plane beds. However, owing to lag effects, transitional bedforms have been recorded for Froude numbers near the critical value of 1. The development of gravel dune steepness from lower-stage plane bed through to upper-stage plane bed is well described in only one case, by Dinehart (1992a). Fredsoe's (1975) function for equilibrium dune steepness applies reasonably well to Dinehart's steepest dunes. Allen's (1984) bedform existence field based on and a characteristic grain size of the bed sediment has been extended from 5 mm to include bedstocks as coarse as 33 mm.