The causes of bedload pulses in a gravel channel: the implications of bedload grain-size distributions

摘要

Unsteady bedload transport was measured in two c. 5 m wide anabranches of a gravel-bed braided stream draining the Haut Glacier d'Arolla, Switzerland, during the 1998 and 1999 melt seasons. Bedload was directly sampled using 152 mm square Helley-Smith type samplers deployed from a portable measuringbridge, and independent transport rate estimates for the coarser size fractions were obtained from the dispersion of magnetically tagged tracer pebbles. Bedload transport time series show pulsing behaviour under both marginal (1998) and partial (1999) transport regimes. There are generally weak correlations between transport rates and shear stresses determined from velocity data recorded at the measuring bridge. Characteristic parameters of the bedload grain-size distributions (D_(50), D_(84)) are weakly correlated with transport rates. Analysis of full bedload grain-size distributions reveals greater structure, with a tendency for transport to become less size selective at higher transport rates. The bedload time series show autoregressive behaviour but are difficult to distinguish by this method. State-space plots, and associated measures of time-series separation, reveal the structure of the time series more clearly. The measured pulses have distinctly different time-series characteristics from those modelled using a one-dimensional sediment routing model in which bed shear stress and grain size are varied randomly. These results suggest a mechanism of pulse generation based on irregular low-amplitude bedforms, that may be generated in-channel or may represent the advection of material supplied by bank erosion events.