Guang-Hui Zhang

Detachment of Undisturbed Soil by Shallow Flow

portance of the roles played by raindrop impact and overland flow (Gilley and Finkner, 1985; Bradford et Quantification of soil detachment rates is necessary to establish a al., 1987), the effect of flow depth and sediment load basic understanding of soil erosion processes and to develop fundamental-based erosion models. Many studies have been conducted on on splash (Hirschi and Barfield, 1988; Kemper et al., the detachment rates of disturbed soils, but very little has been done 1985), and transport capacity (Guy et al., 1987; Kinnell, to quantify the rates of detachment for natural soil conditions. This 1993) have been simulated and analyzed. The relationstudy was conducted to evaluate the influence of flow discharge, slope ship between soil detachment by raindrop impact, raingradient, flow velocity, shear stress, stream power, and unit stream drop size and mass, drop velocity, kinetic energy, soil power on detachment rates of natural, undisturbed, mixed mesic typistrength, water drop impact angle, and surface sealing cal Udorthent soil. Flow rates ranged from 0.25 to 2.0 L s 1 and slope have also been investigated (Nearing and Bradford, gradient ranged from 8.8 to 46.6%. This study was compared with a 1985; Bradford et al., 1987; Sharma et al., 1991; Sharma previous study that used disturbed soil prepared by static compression. et al., 1993; Cruse et al., 2000). These experiments have The results indicated that the detachment rates of disturbed soil were contributed to the better understanding of the mecha1 to 23 times greater than the ones of natural undisturbed soil. It was necessary to use natural undisturbed soil samples to simulate the nism of soil detachment by raindrop impact and prodetachment process and to evaluate the influence of hydraulic paramevided a basis for models for interrill areas (Gilley and ter on detachment rate. Along with flow rate increasing, detachment Finkner, 1985; Sharma et al., 1991; Sharma et al., 1995). rate increased as a linear function. Detachment rate also increased Detachment of cohesive soils by shallow clear-water with slope gradient, but the functional relationship between the two flow under laboratory conditions has received less attenvariables depended on flow rate. Stepwise regression analysis indition (Nearing et al., 1991). Detachment by overland cated that detachment rate could be well predicted by a power function flow occurs when the stress or energy applied by the of flow rate and slope gradient (R2 0.96). Mean flow velocity was overland flow is great enough to pull the soil particles closely correlated to detachment rate (r2 0.91). Flow detachment away from the bulk material. Shear stress ( ), stream rate was better correlated to a power function of stream power (r2 power ( ), and unit stream power (P) are normally 0.95) than to functions of either shear stress or unit stream power. used hydraulic parameters to simulate detachment rate in rills, which given the functions as follow: S erosion has been defined as the process of deghS [1] tachment and transportation of soil material by erowhere (Pa) is shear stress, (kg m 3) is water mass sive agents (Ellison, 1947). Soil detachment is the subdensity, g (m s 2) is the gravity constant, h (m) is the process of dislodgment of soil particles from the soil depth of flow, and S (fraction) is the tangent value of mass at a particular location on the soil surface. The bed slope degree. dislodgment is caused by the forces applied on the soil particles by the erosive agents, which are mainly rainV ghSV [2] drops and overland flow (Owoputi and Stolte, 1995). In process-based soil erosion models, the sediment source where (kg m 3) is stream power, V (m s 1) is mean is conceptually separated into that from interrill and rill flow velocity. areas. In interrill areas, dominant processes are detachP VS [3] ment by raindrop impact and transport by raindropimpacted shallow flow. In rills, dominant processes are where P (m s 1) is unit stream power. It is clear that detachment and transport by concentrated flow (Huang shear stress, stream power, and unit stream power are et al., 1996). Therefore, understanding of the detachfunctions of flow depth, velocity, and slope gradient. ment mechanisms for both interrill and rill areas is necTherefore, through combinations of different slope graessary for the development of process-based erosion dients, flow rates, and flow depths, the relationship bemodel. tween soil detachment rate and these hydraulic parameDetachment by raindrop impact has been studied in ters can be derived based on the data from hydraulic detail during the past several decades. The relative imflume studies. Lyle and Smerdon (1965) were among the first to use a hydraulic flume to investigate the relaG. Zhang, B. Liu, and X. He, Dep. of Resources and Environmental tionship between soil erosion and flow shear stress unSciences, Beijing Normal Univ., Beijing, 100875, China; M.A. Nearing, der constant slope. The results revealed a unique relaUSDA-ARS Southwest Watershed Research Center, Tucson, AZ tionship for a given soil type. 85719; G. Liu, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resource, Yangling, Nearing et al. (1991) conducted a series of experiShaanxi, 712100, China. G. Zhang, currently at: CREST, Japan Science ments in a hydraulic flume with varying bed slope to and Technology Corp., Japan. Received 6 Sept. 2002. *Corresponding investigate the relationship between soil detachment by author (zhang@urban.env.nagoya-u.ac.jp). shallow flow, flow depth, bed slope, and mean weight diameter of the aggregates with small, statically comPublished in Soil Sci. Soc. Am. J. 67:713–719 (2003).

Soil Detachment Capacity by Overland Flow for Soils of the Beijing Region

Abstract Soil type may influence soil detachment process by overland flow, but few studies have quantified the effect fully and systematically. This study was undertaken to quantify the effects of soil type on soil detachment capacity using undisturbed soil samples collected from 11 soil types from the Beijing Region. The soil samples were placed in a 5.0-m long and 0.38-m wide hydraulic flume and eroded by overland flow under three slope gradients (17.4%–34.2%) and three unit discharges (1.32–5.26 × 10−3 m2 sec−1). The results showed that soil detachment capacities were significantly affected by soil types. Aeolian Sandy Soil was the most easily detached, whereas Wet Meadow Soil was the hardest. Soil detachment capacity was significantly affected by soil properties. For soil texture, loamy sand had the largest detachment capacity, followed by sandy loam, loam, and silt loam. Furthermore, detachment capacity was negatively correlated to clay and silt content and positively related to sand content and median diameter of soil particles. Both shear strength and organic matter content were negatively correlated to detachment capacity. Detachment capacity could be predicted reasonably well by stream power, clay content, and organic matter content (r2 = 0.743) with a coefficient of Nash-Sutcliffe efficiency of 0.644. An equation was developed to estimate detachment capacity based on stream power and soil properties (r2 = 0.704; Nash-Sutcliffe efficiency, 0.702). Rill erodibility ranged from 0.73 to 85.22 × 10−3 sec m−1, and the critical shear stress changed from 0.683 to 7.978 Pa. The results are helpful to understand the mechanism of soil detachment process under different soil types and to develop the process-based erosion models.

Impacts of headcut height on flow energy, sediment yield and surface landform during bank gully erosion processes in the Yuanmou Dry-hot Valley region, southwest China: Changes in flow energy, sediment yield and surface landform

To quantify the changes in flow energy, sediment yield and surface landform impacted by headcut height during bank gully erosion, five experimental platforms were constructed with different headcut heights ranging from 25 to 125cm within an in situ active bank gully head. A series of scouring experiments were conducted under concentrated flow and the changes in flow energy, sediment yield and surface landform were observed. The results showed that great energy consumption occurred at gully head compared to the upstream area and gully bed. The flow energy consumption at gully heads and their contribution rates increased significantly with headcut height. Gully headcuts also contributed more sediment yield than the upstream area. The mean sediment concentrations at the outlet of plots were 2.3 to 7.3 times greater than those at the end of upstream area. Soil loss volume at gully heads and their contribution rates also increased with headcut height significantly. Furthermore, as headcut height increased, the retreat distance of gully heads increased, which was 1.7 to 8.9 times and 1.1 to 3.2 times greater than the incision depth of upstream area and gully beds. Positive correlations were found between energy consumption and soil loss, indicating that energy consumption could be used to estimate soil loss of headcut erosion. Headcut height had a significant impact on flow energy consumption, and thus influenced the changes in sediment yield and landform during the process of gully headcut erosion. Headcut height was one of the important factors for gully erosion control in this region. Further studies are needed to identify the role of headcut height under a wide condition. Copyright (c) 2018 John Wiley & Sons, Ltd.