Abstract: Surface layers of gravel bed of wide gradation are usually covered by armoring layer with different coarsening degrees. Most of the existing bed-load transport formulas could just reflect the characteristics of sediment transport in a certain coarsening degree, hence are of poor applicability. Theoretical analysis and experiment results show that bed-load transport rate decreases as the coarsening degree of armoring layer increases under the same incoming flow and sediment condition. Introducing the parameter of coarsening degree into the calculation could solve this problem. The stability of armoring layer plays an important role in bed-load transport in gravel bed. Bed-load transport rate increases sharply when the armoring layer is broken. The calculation results of the existing formulas are obviously small. Therefore, the sharp increase of sediment transport rate caused by the failure of armoring layer should be considered in calculating the bed-load transport rate.

Key words: gravel bed, bed-load transport rate, coarsening degree, stability of armoring layer, critical conditions