THE EFFECT OF RAINFALL INTENSITY, FLOW AND PARTICLE FALL VELOCITIES ON RAINDROP INDUCED SALTATION

When particles fall back to the bed, as noted earlier, they are subjected to horizantal and vertical forces. The flow causes particles to accelerate horizantally towards the velocity of flow.

This figure shows the effect of flow velocity and rainfall intensity on the transport rate of 0.2mm sand induced by 2.7mm raindrops impacting 6mm deep flows. This experiment, together with other experiments controlling flow velocity, flow depth and rainfall intensity, shows that q_sR(p,D,d) varies linearly with flow velocity and rainfall intensity. Since

q_sR(p,D,d) = M_pDd [F_d x_pDd]

where M_pDd is the mass of p sized particles lifted by into the flow by each drop impact, and F_d is the frequency of the drop impacts within the distance x_pDd of the boundary, the experiments indicate that x_pDd varies linearly with flow velocity and the effect of rainfall intensity is consistent with the effect of rainfall intensity on F_d.

In still water, gravity causes particles to accelerate towards their terminal fall velocity in water. Kinnell (2001), Particle travel distances and bed and sediment compositions associated with rain-impacted flows, Earth Surface Proc. and Landforms 26, 749-758, measured particle travel distances of 0.46 mm sand and coal.

As can be seen from the figure above, x_pDd varied linearly with the inverse of the terminal velocity of fall of the particles. x_pDd in these experiments was about twice that expected had the particles fallen at their terminal velocities through the WHOLE depth of flow.

The effect of variations in x_pDd is to cause the sediment discharged from surfaces containing particles with varying p and D to be richer in the particles with lower values of p and D than the surface itself. Consider a surface made up of 40% 0.46 mm coal particles and 60% 0.46 mm sand particles. The proportion of coal particles in the discharge is given by

NEXT: Flow depth

More about previously detached particles

RD-RIFT, RD-FT and side-slope erosion

Detachment and transport systems

Erosion by rain impacted flow

Effect of previously detached particles (intro)

Rainfall intensity, flow and fall velocities

Powerpoint presentation on detachment and transport systems