scholarly journals Modelling soil detachment capacity by rill flow with hydraulic variables on a simulated steep loessial hillslope

2018 ◽  
Vol 50 (1) ◽  
pp. 85-98
Author(s):  
Nan Shen ◽  
Zhanli Wang ◽  
Qingwei Zhang ◽  
Hao Chen ◽  
Bing Wu
Keyword(s):  
Rill Erosion ◽  
Slope Gradient ◽  
Stream Power ◽  
Flow Discharge ◽  
Erosion Models ◽  
Physically Based ◽  
Critical Slope ◽  
Steep Slopes ◽  
Hydraulic Variables ◽  
Rill Flow

Abstract Modelling soil detachment capacity by rill flow with hydraulic variables is essential to understanding the rill erosion process and developing physically based rill erosion models. A rill flume experiment with non-erodible flume bed and small soil samples was conducted. Seven flow discharges and six steep slope gradients were combined to produce various flow hydraulics. The soil detachment capacity increases with the increase in slope gradient and flow discharge. The critical slope gradients of 21.26 and 26.79% cause the detachment capacity to increase at a slow pace. The soil detachment capacity can be defined by a power function of flow discharges and slopes. The contribution rates of slope gradient and flow discharge to soil detachment capacity are 42 and 54%, respectively. The soil detachment capacity increases with shear stress, stream power and unit stream power; the increase rates of these parameters are greater under gentle slopes than steep slopes. Stream power is the superior hydrodynamic parameter describing soil detachment capacity. The linear model equation of stream power is stable and reliable, which can accurately predict soil detachment capacity by rill flow on steep loessial hillslopes. This study can help to sufficiently clarify the dynamic mechanism of soil detachment and accurately predict soil detachment capacity for steep loessial hillslopes.

scholarly journals A field investigation on rill development and flow hydrodynamics under different upslope inflow and slope gradient conditions

2020 ◽  
Vol 51 (5) ◽  
pp. 1201-1220
Author(s):  
Pei Tian ◽  
Chengzhong Pan ◽  
Xinyi Xu ◽  
Tieniu Wu ◽  
Tiantian Yang ◽  
...  
Keyword(s):  
Soil Loss ◽  
Field Investigation ◽  
Rill Erosion ◽  
Slope Gradient ◽  
Power Functions ◽  
Stream Power ◽  
Erosion Processes ◽  
Initial Stage ◽  
Quantitative Impact ◽  
Inflow Discharge

Abstract Few studies focus on the quantitative impact of upslope inflow rate and slope gradient on rill development and erosion processes. Field plot experiments under varying inflow rates (6–36 L min−1m−1) and slope gradients (26, 42 and 57%) were conducted to address this issue. The results showed soil loss rates significantly demonstrated temporal variability in relevance to the rill developing process. Rill erosion and its contribution to soil loss increased with increasing inflow rates and slope gradients by power functions. There was a threshold inflow discharge (12–24 L min−1m−1), under which, rill erosion became the dominant erosion pattern. At the initial stage, downcutting of rill bottom and headward erosion were obvious, whereas rill broadening was significant at the actively rill developing period. Rill density increased with slope gradient increasing from 26% to 42%, and then decreased. For the 57% slope under high inflow rates (24–36 L min−1m−1), gravity caused an increase in the collapse of rills. Mean rill width increased with increasing inflow rates but decreased as slope gradients increased, while mean rill depth increased with increasing inflow rates and slope gradients. Stream power and rill flow velocity were the best hydrodynamic parameter to simulate rill erosion and rill morphology, respectively.

Slope threshold in rill flow resistance

2021 ◽  
Author(s):  
Alessio Nicosia ◽  
Vito Ferro
Keyword(s):  
Friction Factor ◽  
Flow Resistance ◽  
Sediment Load ◽  
Steep Slope ◽  
Limiting Factor ◽  
Rill Erosion ◽  
Hydraulic Characteristics ◽  
Transport Capacity ◽  
Steep Slopes ◽  
Rill Flow

<p>Rills are small, steep sloping and ephemeral channels, shaped in soils, in which shallow flows move. Rill erosion strictly depends on hydraulic characteristics of the rill flow and for this reason flow discharge <em>Q</em>, rill width <em>w</em>, water depth <em>h</em>, mean flow velocity <em>V</em>, and friction factor are required to model the rill erosion process.</p><p>Erosive phenomena strictly depend on the attitude of the soil particles to be detached (<em>detachability</em>) and to be transported (<em>transportability</em>). These properties are affected by soil texture and influence the sediment load <em>G</em> to be transported by flow. The actual sediment load depends on the transport capacity <em>T<sub>c</sub></em> of the flow, which is the maximum amount of sediment, with given sizes and specific weight, that can be transported by a flow of known hydraulic characteristics.</p><p>According to Jiang et al. (2018) the hydraulic mechanisms of soil erosion for steep slopes are different from those for gentle slopes. Recent research on <em>T<sub>c </sub></em>equations exploring slopes steeper than 18% (Ali et al., 2013; Zhang et al., 2009; Wu et al., 2016) established that <em>T<sub>c</sub></em> relationships designed for gentle slopes (<18%) are unsuitable to be applied to steep slopes (17–47%). Also Peng et al. (2015) noticed that <<<em>there has been little research concerning rill flow on steep slopes (e.g. slope gradients higher than 10°)</em>>>. In other words, the slope of 18% could be used to distinguish between the “gentle slope” and the “steep slope” case for the recognized difference in hydraulic and sediment transport variables.</p><p>The applicability of a theoretical rill flow resistance equation, based on the integration of a power velocity distribution (Barenblatt, 1979; 1987), was tested using measurements carried out in mobile rills shaped on plots having different slopes (9, 14, 15, 18, 22, 24, 25 and 26%) and soil textures (clay fractions ranging from 32.7% to 73% and silt of 19.9% – 30.9%), and measurements available in literature (Jiang et al. (2018), Huang et al. (2020) and Yang et al. (2020)).</p><p>The Darcy-Weisbach friction factor resulted dependent on slope, Froude number, Reynolds number and <em>CLAY</em> and <em>SILT</em> percentages, which represent soil transportability and detachability, respectively. This theoretical approach was applied to two different databases distinguished by the slope threshold of 18%. The results showed that, for gentle slopes (< 18%), the Darcy-Weisbach friction factor increases with slope, <em>CLAY</em> and <em>SILT</em> content. Taking into account that for gentle slopes the hydraulic characteristics limit the transport capacity, for this condition <em>T<sub>c</sub></em> and the sediment load <em>G</em> are both limiting factors.</p><p>For steep slopes (> 18%), the flow resistance increases with slope and the ratio between <em>SILT</em> and <em>CLAY</em> percentage. Steep slopes determine high values of the transport capacity, which is consequently not a limiting factor. Thus, in this condition the actual sediment load is determined exclusively by the ratio between <em>SILT</em> and <em>CLAY</em> percentage. In other words, the only limiting factor for a steep slope condition is the sediment which can be transported (i.e. the sediment load <em>G</em>), affected by its soil detachability and transportability.</p>

scholarly journals Experimental validation of some basic assumptions used in physically based soil erosion models

2011 ◽  
Vol 8 (1) ◽  
pp. 1247-1286 ◽  
Author(s):  
S. Wirtz ◽  
M. Seeger ◽  
J.-F. Wagner ◽  
J. B. Ries
Keyword(s):  
Soil Erosion ◽  
Experimental Validation ◽  
Sediment Concentration ◽  
External Factors ◽  
Fallow Land ◽  
Rill Erosion ◽  
Basic Assumptions ◽  
Erosion Models ◽  
Physically Based ◽  
Process Based Models

Abstract. In spring 2009, four rill experiments were accomplished on a fallow land. Most external factors as well as discharge quantity (9 L min-1) were held constant or at least in the same range. Following most process based soil erosion models, detachment or runoff values should therefore be similar, but the experimental results show clear differences in sediment concentration, runoff and other measured and calculated values. This fact underlines the problems of process based models: concerning rill erosion, different processes take part and the process described by the models is only responsible for a part of the eroded material.

A morphologically-consistent expression for the transition from stream-power-law regime to a debris-flow regime

2020 ◽  
Author(s):  
Odin Marc ◽  
Hussain Alqattan ◽  
Sean Willett
Keyword(s):  
Steady State ◽  
Debris Flow ◽  
Landscape Evolution ◽  
Drainage Area ◽  
Slope Gradient ◽  
Stream Power ◽  
Denudation Rates ◽  
Fluvial Incision ◽  
Critical Slope

<p> Many long-term landscape evolution models are currently combining equations describing the evolution of the surface under fluvial incision (using the so-called stream-power incision model) and hillslope transport (often modeled as linear diffusion). Some models combine these two terms (e.g., Fastscape) and implicitly contain a transition from hillslope to fluvial processes dependent on the ratio of the diffusive and fluvial erosional parameters, D and K respectively (Perron et al., 2009). Other models require as input a hillslope-fluvial transition length (e.g., DAC) and apply hillslope erosion from the ridge-top to this lengthscale and fluvial incision only downstream of it. Still, in both cases the influence of non-linear processes such as landslide and debris-flow on this transition are not accounted.</p><p>We have analyzed the scaling between slope gradient and drainage areas in LIDAR-derived high-resolution DEM for >30 catchments, with apparent steady-state morphology, and where long-term denudation estimates, E, were estimated from cosmogenic nuclides . The catchments span different lithology, climate and denudation rates from ~0.05 to ~3 mm/yr but show a consistent pattern where substantial portion of upstream channels exhibit slope gradient roughly constant with drainage area, and transition towards a negative scaling between slope and area (characteristic of fluvial processes) after a critical drainage area, A<sub>c.</sub> Previous work (Stock and Dietrich, 2003) suggested the portion with constant slope may be dominated by erosion due to debris-flow processes, maintaining the channel at a critical slope, S<sub>df</sub>.</p><p>Here we show that both S<sub>df</sub>, and A<sub>c</sub>, are strongly correlated to the long-term denudation, E. Further, we find that S<sub>df</sub> seems to saturate at a critical slope angle, S<sub>c</sub> , near 40° when denudation rates reach about 1mm/yr consistent with predictions for the slope of a non-linear diffusive hillsllopes (Roering et al., 2007). Combining this expression with the empirical model for the steady-state slope of Stock and Dietrich, 2003, and enforcing the consistency with a stream-power-law downstream we find that the steady state values for S<sub>df</sub> and A<sub>c</sub> can be fully expressed as analytical functions of E, K, D and S<sub>c</sub>. We assess the validity of these expressions with independent estimate of K and D extracted from local channel steepness and hilltop curvature. </p><p>As the impact of debris flow on landscape morphology seems ubiquitous on landscape with more than 0.1 mm/yr of erosion, the classical landscape evolution formulation may need to be upgraded to correctly represent steady-state morphology of the upstream part of catchment (<span>i.e.</span>, <1km<sup>2</sup>). Even if it still lack physical basis, we propose a formulation that adequately represent the steady state morphology from ridge to large drainage area. We show that it yield a new definition of Chi that may be better match the morphology of channel approaching ridges and we also discuss how to implement this new-steady state formulation in landscape evolution models.</p>

scholarly journals Spatial heterogeneity of surface roughness on tilled loess slopes in erosion stages

2018 ◽  
Vol 13 (No. 2) ◽  
pp. 90-97
Author(s):  
Q. Zhang ◽  
J. Wang ◽  
F. Wu
Keyword(s):  
Surface Roughness ◽  
Spatial Heterogeneity ◽  
Loess Plateau ◽  
Chinese Loess Plateau ◽  
Strengthening Effect ◽  
Methodological Framework ◽  
Rill Erosion ◽  
Slope Gradient ◽  
Chinese Loess ◽  
Relief Effect

The main soil erosion areas of the Chinese Loess Plateau are tilled slopes. The knowledge of their spatial heterogeneity will contribute to the understanding of erosion mechanisms on a microtopographic scale. In this study, the spatial heterogeneity of four conventionally tilled slopes was examined under simulated rainfall conditions using a semivariogram-based methodological framework. Results show that all tilled slopes have a relatively stable spatial structure and the erosion stages of all tilled slopes have a similar spatial variability. The rainfall in the splash, sheet, and rill erosion stages has a degree of relief effect, strengthening effect, and relief effect on the surface roughness, respectively. However, the effects of tillage practices and slope gradient on the spatial heterogeneity are much larger than those of the rainfall. The spatial heterogeneity decreases with increasing slope gradient. The general autocorrelation scale of the tilled slopes is 3.15 m and their fractal dimension ranges from 1.59 to 1.85. The tilled slopes have certain anisotropy with respect to the slope direction from 10° to 22.5° while they show isotropy or weaker anisotropy in other directions. In this work, a semivariogram-based methodological framework was established for the spatial heterogeneity of microtopographic-scale slopes. The results also provide a theoretical foundation for future tillage measures on sloping fields of the Loess Plateau.

Estimating rill erosion and sediment transport processes along a saturated purple soil slope

2021 ◽  
Author(s):  
Han Zhen ◽  
Xiaoyan Chen ◽  
Yanhai Li ◽  
Shiqi Chen ◽  
Xiaojie Gu ◽  
...  
Keyword(s):  
Sediment Transport ◽  
Sediment Concentration ◽  
Transport Processes ◽  
Purple Soil ◽  
Rill Erosion ◽  
Soil Slope ◽  
Sediment Transportation ◽  
Erosion Models ◽  
Increasing Trend ◽  
Soil Slopes

A plough pan with reduced permeability always accumulates infiltrated water along slopes then saturates the cultivated layer under continuous rain. Topsoil saturation is a frequent phenomenon and an important process of the special soil slopes. A methodology and device system was used in this study to keep cultivated purple soil saturated. Strands of scouring tests were developed to quantify the rill erosion and sediment transport processes along a saturated purple soil slope at four experiment slopes (5°, 10°, 15°, and 20°) and three flow discharges (2, 4 and 8 L min−1). The experimental results indicated that the sediment transport capacity on a saturated purple soil slope ranged from 0.03 to 1.56 kg s−1 m−1 with the increasing trend along the slope gradient and flow discharge, and the increasing trend could be well matched by a nonlinear multivariable equation. The sediment concentration of the saturated purple soil slope exponentially increased with rill length and decreased with the increment rate and the maximum sediment concentrations observed in this study in different hydraulic events ranged from 108.13 to 1174.20 kg m-3. Saturated and non-saturated purple soil slopes erode differently with the maximum sediment concentration of saturated purple soil slope recorded at approximately 1.42-2.10 times the values for non-saturated purple soil slope. The findings of this research help illustrate the sediment transportation and erosion behaviors of a saturated purple soil slope, and serve as the basis for determining the parameters in the erosion models of the purple soil slope.

scholarly journals Effect of hydraulic parameters on sediment transport capacity in overland flow over erodible beds

2012 ◽  
Vol 16 (2) ◽  
pp. 591-601 ◽  
Author(s):  
M. Ali ◽  
G. Sterk ◽  
M. Seeger ◽  
M. Boersema ◽  
P. Peters
Keyword(s):  
Shear Stress ◽  
Sediment Transport ◽  
Hydraulic Parameters ◽  
Transport Capacity ◽  
Slope Gradient ◽  
Mean Flow ◽  
Stream Power ◽  
Unit Discharge ◽  
Sediment Transport Capacity ◽  
Mean Flow Velocity

Abstract. Sediment transport is an important component of the soil erosion process, which depends on several hydraulic parameters like unit discharge, mean flow velocity, and slope gradient. In most of the previous studies, the impact of these hydraulic parameters on transport capacity was studied for non-erodible bed conditions. Hence, this study aimed to examine the influence of unit discharge, mean flow velocity and slope gradient on sediment transport capacity for erodible beds and also to investigate the relationship between transport capacity and composite force predictors, i.e. shear stress, stream power, unit stream power and effective stream power. In order to accomplish the objectives, experiments were carried out in a 3.0 m long and 0.5 m wide flume using four well sorted sands (0.230, 0.536, 0.719, 1.022 mm). Unit discharges ranging from 0.07 to 2.07 × 10−3 m2 s−1 were simulated inside the flume at four slopes (5.2, 8.7, 13.2 and 17.6%) to analyze their impact on sediment transport rate. The sediment transport rate measured at the bottom end of the flume by taking water and sediment samples was considered equal to sediment transport capacity, because the selected flume length of 3.0 m was found sufficient to reach the transport capacity. The experimental result reveals that the slope gradient has a stronger impact on transport capacity than unit discharge and mean flow velocity due to the fact that the tangential component of gravity force increases with slope gradient. Our results show that unit stream power is an optimal composite force predictor for estimating transport capacity. Stream power and effective stream power can also be successfully related to the transport capacity, however the relations are strongly dependent on grain size. Shear stress showed poor performance, because part of shear stress is dissipated by bed irregularities, bed form evolution and sediment detachment. An empirical transport capacity equation was derived, which illustrates that transport capacity can be predicted from median grain size, total discharge and slope gradient.

scholarly journals Understanding the factors influencing rill erosion on roadcuts in the south eastern region of South Africa

Solid Earth ◽  
2015 ◽  
Vol 6 (2) ◽  
pp. 633-641 ◽  
Author(s):  
K. E. Seutloali ◽  
H. R. Beckedahl
Keyword(s):  
South Africa ◽  
Vegetation Cover ◽  
Site Effects ◽  
Eastern Region ◽  
Rill Erosion ◽  
Economic Costs ◽  
Slope Gradient ◽  
Factors Influencing ◽  
Relationship Of ◽  
The Relationship

Abstract. Erosion on roadcuts is a concern due to the potential of causing environmental degradation, which has significant economic costs. It is therefore critical to understand the relationship between roadcut characteristics and soil erosion for designing roadcuts that are less vulnerable to erosion and to help road rehabilitation works. This study investigated the characteristics (i.e. gradient, length, percentage of vegetation cover and soil texture) of degraded (i.e. with rills) and non-degraded roadcuts (i.e. without rills) and explored the relationship of the roadcut characteristics with the dimensions (widths and depths) of the rills. Degraded roadcuts were steep (52.21°), long (10.70 m) and had a low percentage of vegetation cover (24.12) when compared to non-degraded roadcuts which had a gradient of 28.24°, length of 6.38 m and 91.7% of vegetation cover. Moreover, the gradient and percentage of vegetation cover of the roadcut significantly determine the rill dimensions. The widths and depths of the rills increase with the increase in slope gradient and decrease with an increase in percentage of vegetation cover. Moreover, the widths and depths of the rills decreased downslope of the roadcuts. Based on these results, re-vegetation of roadcuts as well as construction of gentle gradients could minimise rill erosion and hence the negative on-site and off-site effects.

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