In order to get a better simulation of engineering accumulation in indoor experiment

image analysis and classical statistics methods were used to generalize the distribution and content of gravel with diameter > 1 cm on 368 engineering accumulation slopes

which were located in six water erosion regions. The generalized content intervals of gravel was obtained on different slope positions. Results showed as follows: Most gravel of different particle diameters obeyed normal distribution and there was a significant differency in the contents of gravel in mountain areas and in plain-hilly areas. The gravity separation effects of gravelwere obvious

and the bigger the gravel was

the greater the effects were. In mountain areas

it was appropriate to set the total amount of gravel between 50%~60% when designing the spoil underlying surface for laboratory rainfall simulation; the content of fine gravel

medium gravel

coarse and macrosome gravel were set in 20%~30%

10%~15%

and 10% respectively; the content of gravel on top

middle and bottom slope of engineering accumulation are suggested to be 1∶1.3∶1.5. In hilly and plain areas

it was appropriate to set the total amount of gravel between 15%~20%

in while the fine gravel

medium gravel

coarse gravel and macrosome gravel accounted for 5%～10%

3%～7%

2%～5% and 2%～3% respectively. The optimized proportion of gravel on top

middle and bottom slope of engineering accumulation was 1∶1.5∶2.5.