Figure 3: Schematic of the uplift of soil material into the air
and into the flow by a drop impact.
It is well known that not all materials detached by raindrop impact
become airborne. Figure 3 shows a schematic of the uplift of soil
material into the air and into the flow by a drop impact. Theoretically,
the mass lifted into the air by a drop impact is given by
MM.A = a MM (2)
and the mass that remains in the flow
MM.F = b MM (3)
where a + b = 1.0. It has been well demonstrated by Moss
and Green (1983) that the ratio of material splashed and material
transported by raindrop induced saltation decreases greatly as flow
depth increases. Although a tends towards 1.0 as flow depth
decreases. its value is not known to be 1.0 in the shallow flows that
occur in the Zhang et al. (2020) experiments. Also, when rain falls and
produces runoff on any inclined plane, the value of a will vary
spatially as flow depths vary down along the plane. Consequently, the
amount of material splashed into the slot is not a reliable measure of
the amount of material detached by the raindrops that impact the flow
during the Zhang et al experiments.