Temporal variations in bedload transport rates: The effect of progressive bed armouring

Abstract The quantitative bedload transport data that are presently available confirm that the generalized bedload transport rate‐stream power relationship is applicable to natural streams. However, the bedload transport rate is not solely dependent upon hydraulic parameters, but also upon the inter‐relationship between bed material characteristics and flow properties. Segregation of the surficial bed material, as expressed through the development of an armour coat, limits the availability of transportable material. Under such circumstances observed bedload transport rates are less than the predicted values. The effect which the development of an armoured surface has upon the bedload transport rate is described with reference to bedload and bed material sampling in the Borgne d'Arolla, Valais, Switzerland. The data refer to two periods when the resumption of baseflow conditions following flood events which were of a sufficient magnitude to transport all but the coarsest (0·3–0·5 m) particles on the streambed, provided the opportunity for the bed to adjust to a comparatively stable flow regime. Observed and predicted bedload transport rate‐stream power relationships are compared. The theoretical relationship does not adequately describe conditions in some gravel‐bed channels, since it fails to take into account the effect which armouring may have upon the supply of transportable material.