Time-lag of dunes in unsteady flows: An analysis of nasner's data from the R. Weser,Germany

In order to contribute towards the development of a sound actualistic basis for the interpretation of dune structures, phase diagrams and related graphs are presented for six-year time-series of daily mean dune wavelength, daily mean dune height, and daily freshwater discharge obtained by Nasner from four fixed sites on the R. Weser. Dune dimensions at these sites vary out of phase with the freshwater discharge, in a year of typical discharges by approximately π/2 rad in the case of wavelength, and by roughly 3 π/2 rad in terms of height. The mean annual wavelength (daily wavelength averaged over a hydrological year, equal to one flow cycle) is independent of the mean annual discharge, but the relative range of wavelength over the cycle increases with this discharge. The relative range of dune height also increases with the mean annual discharge. However, the mean annual dune height is inversely proportional to the mean annual discharge. Related modes of behaviour may be expected of dunes in other environments where unsteady flows prevail, but their recognition will depend on sampling carried out at sufficiently small time intervals over protracted periods. The analysis of Nasner's data suggests that although in natural environments the dimensions of dunes are related in defineable ways to hydraulic conditions, the relationships are likely to prove complex and multi-valued, rather than simple as has commonly been supposed by sedimentologists in the past.