AN EXPERIMENT ON THE DISTRIBUTION OF BEDFORM-SIZE IN THE TRANSITION FROM CURRENT RIPPLES TO DUNES IN FINE SAND

It is well known that there is a break in size between current ripples and dunes (Ashley, 1990), and each of them has a stability field in the flow depth - velocity - grain size diagram (e.g., Southard and Boguchwal, 1990). There have been several experimental studies on the wavelength of current ripples. Yalin (1985) examined ripple geometry using experimental data and derived the following relation: L ~ 1000 D (Where L is wavelength, and D is grain size). Recently, Baas (1999) showed that equilibrium ripple wavelengths are constant for given grain size and not related to flow velocity. Baas (1999) also showed, however, that the equilibrium ripple wavelength is proportional to flow velocity near the stability field of dunes.

In this study, we examined the distribution of the wavelength of current ripples in a transition field in which the average wavelength increases with flow velocity. The experiments were performed in a recirculating flume, 9 m long, 16 cm wide, using well-sorted fine sand (D₅₀=0.237 mm). The water depth was 10 - 12 cm. The 10-degree-equivalent, depth-averaged flow velocity ranged from 21.3 to 53.6 cm/sec, including the stability fields of ripples and dunes. The bedform geometry was measured after the bedform reached equilibrium state, and 300 to 400 bedwaves were measured for each condition. The result showed that the average wavelength increases with flow velocity, but the histograms of the wavelength showed that it did not mean a single dominant mode increased its average value. Instead, the histograms became skewed and the tails of the histograms became long on the sides increasing their average values. This means that large bedwaves like dunes existed even under the lower flow velocity, about 35 cm/sec. In this transition field, both bedwaves, such as relatively large ripples and dunes exist under a single condition. The large ripples have about 40-60% longer wavelengths compare with the regular ripples in their stability field. As flow velocity increased, the frequency of the existence of dunes increased. Finally the large ripples disappeared, to be replaced by dunes in the stability field of dunes.