A COMBINED, SURFACE-SUBSURFACE, APPROACH TO BEDLOAD TRANSPORT CALCULATIONS IN GRAVEL BED STREAMS

摘要

The ranges of grain sizes as well as the quantity of «ach size present on a stream bed surface are among the important factors affecting the transport of sediments. Most of the bedload transport models are based on a single grain size parameter that represents either the surface (pavement) or the subsurface (sub-pavement) bed material. Recently, a two-parameter approach, one for the surface and the other for the subsurface sediments, for estimating bedload transport in gravel bed streams with unimodal sediments was proposed. This approach is pursued further here by considering fractional grain size analysis and similarity collapse techniques. The laboratory data obtained by Proffit (1980) is used to examine the validity of this new methodology. The developed combined, surface-subsurface, approach implicitly accounts for the variation of the make up of the surface bed material over a wide range of Shields stresses and reflects the resulting changes in the contributions made by the surface and subsurface layers to the bedload transport. It also reflects the increase in the mobility conditions resulting from the increase in the Shields stresses. The present approach describes bedload transport in gravel bed streams as a continuum gradually adjusting between two extremes, a fully developed pavement and selective particle entrainment on the lower end near threshold conditions and absence of vertical grain size stratification and equal mobility for very high Shields stresses and massive bedload transport.