Write a short note on Continuity Equation for sediment
Answers
Rivers move sediment in a variety of forms. At very steep slopes sediment may be
moved in the form of debris flows. These flows consist of slurries for which the solid
fraction by weight is of the same order of magnitude as the water fraction. Here,
however, the sediment transport typical of rivers is considered.
In rivers the ratio of annual sediment discharge to annual water discharge is in general a
very small number. That is, typically rivers move much more water than they do
sediment. Sediment can be moved as either wash load or bed material load. Wash load
consists of material that moves down the system without interacting with the channel bed.
For example, in most sand bed streams the majority of the load is in the range of medium
silt to clay, sizes that are present in the bed in only negligible quantities. In the case of
gravel bed streams, sand may move through as a type of washload known as throughput
load; the sand may deposit in the interstices of the gravel, but otherwise does not
determine bed morphology.
These notes are concerned with bed material load. This is the portion of the load that is
found on the bed of the stream and actively undergoes exchange between the bed and the
water column. Bed material load may move as bedload or suspended load. Bedload
consists of grains that slide, roll or hop (saltate) over the bed, with saltation being the
most important mechanism. Turbulence plays an auxiliary role in the mechanics of
bedload transport, which are largely governed by water drag and the role of bed collisions
in converting streamwise particle momentum to upward particle momentum, so
maintaining the saltation. Particles participating in bedload typically hug the bed as they
move. The primary hydraulic factor determining the rate of bedload transport is the
boundary shear stress τb. Bedforms such as dunes offer form drag that increase overall
resistance without helping to transport sediment. With this in mind, it is that component
of τb associated with skin friction, τbs that determines the bedload transport rate. Bedload
tends to respond rather quickly to changes in boundary shear stress.
Particles participating in suspended load feel the turbulence, and can be wafted high into
the water column by the action of the eddies. Only the rate of entrainment of such
particles from the bed is determined by boundary shear stress. The transport rate itself is
generally not locally in phase with variations in boundary shear stress.