Temporal variations in bedload transport rates associated with the migration of bedforms

Abstract

Temporal variations in bedload transport rates that occur at a variety of timescales, even under steady flow conditions, are accepted as an inherent component of the bedload transport process. Rarely, however, has the cause of such variations been explained clearly. We consider three data sets, obtained from laboratory experiments, that refer to measurements of bedload transport made with continuously recording bedload traps. Each data set is characterized by a predominant lowfrequency oscillation, on which additional higher-frequency oscillations generally are superimposed. The period of these oscillations, as isolated through the use of spectral analysis, ranged between 0.47 and 168 minutes, and was associated unequivocally with the migration of bedforms such as ripples, dunes, and bars. The extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time, with respect to the period of a given bedform.Several theoretical probability distribution functions have been developed to describe the frequency distributions of (relative) bedload transport rates that are associated with the migration of bedforms (Einstein, 1937b; Hamamori, 1962;Carey and Hubbell, 1986). These distribution functions were derived without reference to a sampling interval. We present a modification of Hamamori's (1962) probability distribution function, generated by Monte Carlo simulation, which permits one to specify the sampling interval, in relation to the length of a bedform. Comparisons between the simulated and observed frequency distributions, that were undertaken on the basis of the data described herein, are good (significant at the 90 per cent confidence level). Finally, the implications that temporal variability, which is associated with the migration of bedforms, have for the accurate determination of bedload transport rates are considered.

Spatio-temporal variability of bedload transport rate: analysis and 2D modelling approach

“…Temporal and lateral variations in bedload transport have been observed by Mühlhofer (1933), Hinrich (1972), Jones & Seitz (1980), Leopold & Emmett (1977), Carey (1985), and Powell et al (1998) [11,12,13,14,15,16,17], among others.…”

A Comparison of Coarse Bedload Transport Measured with Bedload Traps and Helley-Smith Samplers

“…One criterion was that the sampler can collect a wide range of gravel and cobble particle sizes over transport rates ranging from one small gravel particle per hour to hundreds of gravel particles per minute. Due to the typically skewed distribution of short-term transport rates with frequent small and infrequent large rates (e.g., [36,37,[54][55][56][57][58][59][60]), bedload traps had to be capable of long sampling times to minimize oversampling at low and undersampling at high transport events [61]. Due to lateral variability of transport over a stream channel cross-section and its change over time, bedload traps had to be able to collect samples at several locations within the cross-section simultaneously.…”

Fréquence et importance du charriage dans les rivières du Massif ardennais

“…Elles portent notamment sur les différentes techniques de marquage (Ergenzinger et al, 1989 ;Hassan et Ergenzinger, 2003 ;Lamarre et al, 2005), sur la tendance qu'a la charge de fond à se mouvoir sous forme de vagues (Gomez et al, 1989 ;Hassan et al, 1992 ;Hoey, 1992 ;Ferguson et al, 2002), sur les critères d'entraî-nement basés sur les débits unitaires, les tensions de cisaillement et les puissances spécifiques (Bathurst et al, 1979 ;Ferguson, 1994 ;Martin, 2003 ;, sur l'influence des effets de saillie et de masquage (Reid et Frostick, 1984 ;Richards, 1988Richards, , 1990, et sur les variations des tensions de cisaillement au sein même du lit qui peuvent influencer localement les quantités charriées (Powell et Ashworth, 1995).…”