Erosion process and characteristics of different specifications of fish-scale pit slope

This study explored the characteristics of fish-scale pit slope erosion under different rainfall intensities. Two specifications of fish-scale pits (0.6 m×0.4 m×0.1 m and 0.4 m×0.3 m×0.1 m) were adopted in the simulated rainfall experiment including three rainfall intensities (40, 70 and 100 mm/h). Runoff and sediment samples were collected to calculate the benefits of runoff and sediment reduction, and the widths and depths of rills were measured to describe the rill morphology. Runoff generation time and fish-scale pit damage time were also observed and recorded during the rainfall process. Moreover, runoff rate and sediment transport rate were calculated and the relationships between runoff and sediment yield and rill morphology were analyzed. The results showed that when the rainfall lasted for one hour, the morphology of fish-scale pits were intact under the 40 mm/h rainfall intensity, but the fish-scale pits were damaged under the 70 and 100 mm/h rainfall intensities. The fish-scale pit had an obvious effect on runoff and sediment yields and the threshold existed. Based on the fish-scale pit damage time, the fish-scale pit slope erosion was divided into two processes. The runoff generation time increased with decrease of the rainfall intensity. The two specifications of fish-scale pits delayed the runoff generation time, which was 27.79%-59.12% and 72.42%-116.85% higher than smooth slope. The small fish-scale pits were easier to be damaged than the big fish-scale pits under the same rainfall intensity. The benefits of runoff and sediment reduction were 66.02%-82.20% and 85.77%-91.91%, and the mean value of runoff rate and sediment transport rate were 18.87%-33.21% and 10.04%-15.38% of the smooth slope before the fish-scale pits damage. After fish-scale pits damage, slope measures would lose the regulatory function and would have the negative effects on soil and water reduction. The runoff rate and sediment transport rate were suddenly increased and the mean value of runoff rate and sediment transport rate were 4.60-7.46 and 13.70-16.75 times that before fish-scale pits damage. The mean runoff rate and sediment transport rate of the smooth slope were 51.50 mm/h and 3.04 kg/(m2·h) under the 70 mm/h rainfall intensity, and the mean runoff rate and sediment transport rate were 78.61 mm/h and 5.20 kg/(m2·h) under the 100 mm/h rainfall intensity. The flow-sediment relationship under 100 mm/h rainfall intensity showed that the sediment was greatly different before and after fish-scale pits damage. The sediment yield could be decreased by fish-scale pit measures application. The slopes of linear regression equations for fitting the relationship between runoff and sediment were 0.020-0.035 before fish-scale pit damage and they were increased to 0.205-0.231 after fish-scale pit damage. The slopes of linear regression equations for fitting the relationship between runoff and sediment on smooth slope was 0.038 under 100 mm/h rainfall intensity. The width of rill had significant correlation with peak sediment yield (P<0.01) and the depth of rill had significant relationship with total sediment yield and peak runoff yield (P<0.01). The overflow time of fish-scale pit was an important indicator of the regulation threshold of runoff and sediment, and slope erosion characteristic and rill morphology showed obvious difference before and after fish-scale pits damage. This study will provide valuable information for allocation and management of regional soil and water conservation measures.