Engineering piles is a special man-made geomorphic unit and has been found much more serious in soil erosion. The studying of hydrodynamic process in the engineering piles is of great necessity

for the engineering piles with three-dimensional is quite different from the traditional slope. In this paper

a self-made platform was used to simulate the three-dimensional pyramidal engineering piles and an indoor artificially simulated rainfall experiment was carried out under different rainfall intensities (1.0

1.5

2.0

2.5 mm/min) to study the hydrodynamic process and shape characteristics of rills of engineering piles with different mass percentage of gravel (0

10%

20%

30%

40%). The results showed that: (1) Flow velocity and runoff intensity increased with the increase of rain intensities and decreased with the increase of gravel contents. Both the rainfall intensity and gravel contents had the significant influences on flow velocity and runoff intensity

and the influence of rainfall intensity was greater. (2) The runoff was in the laminar and subcritical flow state

and Reynolds number and Froude number increased with the increase of rain intensities and decreased with the increase of gravel contents. (3) Erosion detachment rate increased exponentially with the increase of rainfall intensities

and decreased linearly with the increase of gravel contents

there were significant power function relationships between erosion detachment rate and shear stress

stream power

unit flow power and unit energy of water-carrying

among which stream power had the best correlationship

indicating it was the optimal factor to describe soil erosion. (4) The rill occurring time advanced and the length

width

depth of rill increased with rainfall intensities increasing

while the rill occurring time was delayed

the depth and width of rill decreased gradually with the increase of gravel contents under the same rainfall intensity.