Observations of Bed Load Transport and Channel Bed Changes in a Proglacial Mountain Stream

Abstract

Bed load transport and channel change were measured during May to July 1987 in the proglacial stream of Bas Glacier d'Arolla, Switzerland. Over sampling periods of several hours, variations in bed load transport show little apparent relationship with discharge; however, over the 67-d study period, cross section and long profile surveys suggest that steady-state transport is maintained, except during floods. Bed load transport occurs over approximately two-thirds of the channel width, but the majority of the load is transported in bands of narrow width corresponding loosely to the maximum thread of flow. Bed load movement in narrow bands causes transport rates to vary by up to an order of magnitude within the space of 1 m (in a cross channel direction). Large errors associated with bed load sampling make it very difficult to estimate the true variability in the sediment transport process. Short-term daily changes in channel sediment storage are minor during periods of low to moderate discharge, but during floods the channel shifts dramatically from a slightly aggrading state to a state of considerable aggradation or degradation. On the basis of field observations and sediment transport measurements, a three-phase model of bed load transport is proposed for steep coarse-bed streams where bed material is highly heterogeneous. Phase one generally involves the flushing out of fines deposited in the channel during low flows; phase two transport is associated with the break-up of the gravel-bed surface layer; and phase three transport results in the destruction of the step/pool topography during flood.