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 (0.343 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.

Partial entrainment of gravel bars during floods

“…Under such a condition, bed material is transported indiscriminately with respect to particle size such that the bed surface is unlikely to armor or otherwise form stable structures [Little and Mayer, 1976;Garde et al, 1977;Gomez, 1983aGomez, , 1983bShen and Lu, 1983;Kuhnle and Southard, 1988;Hassan and Reid, 1990;Chin et al, 1992]. In contrast to small and intermediate floods, large floods maintain the spatial independence of bed material entrainment by transporting particles without regard to their size and rearranging much of the bed surface.…”

Exploring the sensitivity on a soil area-slope-grading relationship to changes in process parameters using a pedogenesis model

“…If the energy of the transport medium increases, the existing armour can be disrupted, and a newer stable armour with coarser material can be formed . Armouring in river beds has been widely understood and studied extensively for mostly streams and rivers (Gessler, 1970;Gomez, 1983;Lisle and Madej, 1992;Little and Mayer, 1976;Parker and Klingeman, 1982).…”

The effects of floods and sockeye salmon on streambed morphology.

“…During periods o f high flow, the armour and sub armour layers are fully mobilized, and all the materials are entrained (full mobilization), until such time as the flow decreases and the armour layer is once again re-established (Parker et al 1982, Andrews 1983, Gomez 1983). …”