Dynamic processes of soil erosion by runoff on engineered landforms derived from expressway construction: A case study of typical steep spoil heap

• The steep slope flow over spoil deposit was hyper-concentrated flow.
• Gravitational effect was important for sediment concentration variations on steep slope.
• Hydraulics of steep slope flow over spoil heap from road construction was explored.
• The steep slope flow over the spoil heap belonged to supercritical flow.
• Stream power showed more appropriate in describing soil detachment dynamics.

Soil erosion on newly engineered landforms severely threatens the ecological security of construction sites and their surrounding areas. A field scouring-erosion experiment was conducted on a typical spoil heap along the Shenmu–Fugu Expressway in China to investigate the processes of soil erosion by simulated runoff. A 2.5 × 12 m2 runoff plot with 72.7% gradient was established on the spoil heap. Clean water was applied at the top of the plot with five different inflow rates of 0.167 × 10− 3, 0.25 × 10− 3, 0.33 × 10− 3, 0.417 × 10− 3 and 0.5 × 10− 3 m3/s to simulate surface runoff processes. The variation of sediment concentration was greatly influenced by gravitational erosion which occurred at the critical inflow rates ranging from approximately 0.33 × 10− 3 to 0.417 × 10− 3 m3/s. The process of sediment yield on the spoil heap, in terms of sediment transport rate, was characterized by three stages: abruption, fluctuation and stabilization. The spatial distribution of sediment yield in different slope segments at different inflow rates showed two trends: steady decreasing and drastically fluctuated decreasing. The flow over the spoil heap belonged to supercritical flow and the critical inflow rate controlling the transformation from transitional flow to turbulent flow for rill flow was between 0.33 × 10− 3 and 0.417 × 10− 3 m3/s. Statistically, power, exponential and logarithmic-linear equations can be used to describe the interrelations of main hydraulic parameters including flow velocity, Reynolds Number, Froude Number and Darcy–Weisbach roughness coefficient. The three forms of equations were also appropriate for describing sediment concentration in relation to the hydraulic parameters mentioned. Slope length was more significant than inflow rate in affecting hydraulic parameters, which showed an exponential relation with the increase in distance from the top of the plot for all the parameters except Reynolds Number. Stream power was the best hydrodynamic parameter to simulate the changing trend of soil detachment rate. As for different control equations, mechanic parameters and energy parameters had some relative advantages in depicting the dynamic processes of sheet erosion and rill erosion, respectively.