Objective  Slope length is an important factor affecting the formation of slope runoff and soil erosion processes. This study focused on the sediment concentration at different slope lengths in the loess area of western Shanxi Province of northern China, aiming to explore the variation of sediment concentration in slope runoff with slope length and elucidate the influence of slope length factors on slope erosion process.

  Method  In order to reveal the variation law of runoff sediment concentration with slope length and to clarify the slope erosion process, the runoff sediment concentration and soil detachment rate at different locations within 0−5 m slope length were measured using 7.5 and 10.0 L/(min·m) unit width discharge on the 5° and 20° slopes, respectively.

  Result  (1) The runoff sediment concentration increased gradually with the increase of slope length, and increased faster under the larger slope gradient and flow discharge. In addition, the increase of slope gradient and flow discharge will increase the runoff sediment concentration. (2) The soil detachment rate first increased and then decreased with the increase of slope length, and the peak position moved toward the slope top with the increase of flow discharge and slope gradient. The soil detachment rate increased with the increase of slope gradient and flow discharge. (3) The S-shaped curve can well fit the variation of runoff sediment concentration with slope length under various conditions, and the coefficient of determination R² ≥ 0.97. The three parameters in the curve model can well reflect the characteristics of slope erosion, namely, the maximum sediment carrying force (a) increased with the increase of slope gradient and flow discharge, the position with the fastest erosion rate (x_c) decreased with the increase of slope gradient and flow discharge, and the changing rate of sediment concentration (k) increased with the increase of slope gradient and flow discharge. (4) Based on the S-curve process theory and experimental calculation value, if the rainfall intensity of 60 mm/h was taken as the prevention object, it was recommended that the distance between vegetation engineering slope protection measures and slope top under 5° conditions should not exceed 426 cm, and the distance between vegetation engineering slope protection measures and slope top under 20° conditions should not exceed 313 cm. If the rainfall intensity of 90 mm/h was taken as the object of prevention, it was recommended that the distance between the vegetation engineering slope protection measures and the slope top under 5° conditions should not exceed 366 cm, and the distance between the vegetation engineering slope protection measures and the slope top under 20° conditions should not exceed 283 cm.

  Conclusion  Based on the theoretical process of slope erosion and the results of erosion tests, this study proposes and simulates an S-shaped curve growth process of slow increase, rapid increase, and slow increase to saturation of sediment content with slope length. Based on the characteristics of the curve, the layout method of slope protection measures in the loess area of western Shanxi Province is proposed. This helps to deeply understand the mechanism of slope erosion and provide a basis for estimating the parameters of slope erosion models and laying out soil and water conservation measures in the loess area of western Shanxi Province.