Rill Density Research Articles

An Experimental Study of the Morphological Evolution of Rills on Slopes under Rainfall Action

Accurately monitoring the morphology and spatiotemporal evolution characteristics of the entire process of slope erosion rill development is essential to circumvent the limitations inherent in traditional methods that rely on average flow velocity for hydrodynamic parameter calculations. This study employs an environmental chamber and a self-developed slope erosion test device to perform erosion tests on slopes with varying gradients and rainfall intensities. By integrating the structure-from-motion (SfM) method, fixed grid coordinate method, and continuous camera combined with the dye tracer technique, the morphological indexes and hydrodynamic parameters of the entire rill development process are precisely computed. The main conclusions are as follows: The entire process of slope rill development can be divided into three distinct stages. The initial stage is characterized by the appearance of tiny rills with mild erosion. The middle stage involves severe transverse spreading erosion and longitudinal undercutting, resulting in diverse rill morphologies. The final stage is marked by the stabilization of morphological characteristics. The peak slope soil loss is observed during the middle stage of rill development. The most effective parameters for characterizing slope soil loss from the beginning to the end are the Reynolds number and flow shear stress, the Froude number and flow shear stress, and the Froude number during different periods. Throughout the development of rills, the flow velocity initially decreases and then gradually increases until it stabilizes. The morphological indexes, including rill density, dissected degree, inclination, and complexity, generally show an increasing trend. However, in the middle stage, the rate of increase slows down, followed by a sharp rise at certain points. The optimal hydraulic parameters for evaluating rill density across different slope gradients, which were found to be the Darcy–Weisbach drag coefficient and real-time flow velocity, for assessing rill dissected degree, complexity, and inclination, were the Reynolds number and flow power. Under varying rainfall intensities, the most effective hydraulic and kinetic parameters for evaluating rill density, dissected degree, and inclination were flow shear stress and Reynolds number; for assessing rill complexity, the Reynolds number and flow power were used. The findings of this research enhance the accuracy of hydrodynamic parameter calculations in rill erosion tests, enable precise prediction of rill development trends on slopes, and offer innovative approaches for real-time dynamic monitoring of rill morphology and characteristics. These advancements are of significant importance for soil and water conservation and sustainability.

Characteristics and dynamic mechanism of rill erosion driven by extreme rainfall on karst plateau slopes, SW China

Driven by climate change and increased human activities, the frequency and intensity of extreme rainfall events are increasing, leading to increased risk of water erosion on slopes in karst rocky desertification (KRD) areas. However, the driving factors (rainfall intensity, slope gradient, and the degree of underground fissure pore), characteristics, and dynamic mechanisms of rill erosion on KRD slopes under extreme rainfall have received limited attention. A rainfall simulation experiment was performed to investigate the driving factors of rill erosion on KRD slopes under extreme rainfall events. The experiments were conducted using an erosion flume (4 m long, 1.5 m wide and 0.35 m deep) with five rainfall intensities (50, 105, 115, 125 and 145 mm·h−1), three-level slope gradients (5°, 15° and 25°), and three typical degrees of underground pore fissure (1 %, 3 % and 5 %). The results showed that extreme rainfall significantly stimulated the formation and development of rill erosion on KRD slopes. In particular, rainfall intensity and slope gradient significantly affected rill erosion (p < 0.05), whereas the degree of underground pore fissure did not have any distinct effect. Increasing rainfall intensity intensified rill erosion by significantly (p < 0.05) promoting the rill density, total rill surface area, and longest of rill length. With increasing slope gradient, the rill density, rill distribution density, and average rill depth significantly increased (p < 0.05). Moreover, extreme rainfall events exacerbated surface soil erosion by enhancing the hydrodynamic characteristics of slope runoff. Rill erosion was most sensitive to the shear stress of runoff, and rill density was found to be the most suitable rill index for describing surface soil erosion on the slope under extreme rainfall. The findings provide valuable insights into the characteristics and mechanisms of soil erosion in karst regions under future climate warming.

Impact of Rock Fragment Shapes and Soil Cohesion on Runoff Generation and Sediment Yield of Steep Cut Slopes under Heavy Rainfall Conditions

The erodibility and erosion resistance of Quaternary sediments play a pivotal role in both the hydrologic and erosion processes of soil cut slopes. To investigate the runoff and sediment yield of soil cut slopes along the Pai-Mo road, we performed indoor simulated rainfall experiments under a 50° steep slope and high rainfall intensity (120 mm/h), based on the area’s climatic characteristics and Quaternary sediment properties. The experiments included various rock fragment contents (30%, 40%, and 50%), different levels of rock fragment roundness (i.e., rounded gravel, angular gravel), and varying soil cohesion. The results indicated that the average infiltration rate of the uncemented rounded gravel soil slope (URGSS) was higher than that of the uncemented angular gravel soil slope (UAGSS), resulting in less runoff and a delayed initial runoff time. The runoff shear stress, runoff power, drag coefficient, and Reynolds number of the URGSS were smaller than those of the UAGSS. In contrast, the Froude number and flow velocity of the URGSS were larger, resulting in a stronger runoff erosion capacity. The sediment yield of the URGSS was approximately two times that of the UAGSS, and the cumulative sediment yield was about 20% higher. The cemented angular gravel soil slope (CAGSS) had a larger runoff rate, runoff shear stress, runoff power, and flow velocity than those of the UAGSS, leading to less sediment yield. Overall, a more rounded shape and a larger radius of curvature of the spherical particles resulted in stronger erosion, due to local turbulence. Therefore, the rill density and cumulative sediment yield of the steep alluvial cut slope were greater than that of the steep colluvial cut slope under heavy rainfall. Moreover, due to its strong cohesion, only raindrop splash erosion and inter-rill erosion occurred on the steep moraine cut slopes under heavy rainfall.

Slope gradient impacts on rill morphological characteristics: Using indoor simulation experiment on loamy clay under certain rainfall intensity

In the present study, the rill morphological parameters and their spatial distributions were investigated using indoor rainfall simulation experiments in combination with 3D terrestrial laser scanning technique. Moreover, the optimal rill morphological indicators for rill erosion evaluation were selected based on the relationship of rill morphological parameters and erosion amount. The results showed that total rill length (L), rill density (ρ), maximum width (Wmax), the degree of contour line departure (μ) and shape ratio (η) increased with the increase of slope gradient, whilst the mean rill depth (Dmean), mean rill width (Wmean) and width/depth ratio (W/D) decreased with the increase of slope gradient. The spatial distribution pattern of each rill morphology parameter was also changed with slope gradient, showing differences in upper slope, central slope and lower slope. Most of rill morphological parameters were highly correlated with rill erosion amount, except the mean rill width (Wmean), degree of contour line departure (μ), and rill density (ρ). The plane area of rill (Parea) or the degree of rill dissection (δ) was selected as the optimal morphological parameter to predict rill erosion amount, followed by the maximum area of cross section (CSmax), the maximum rill depth (Dmax), and the mean rill depth (Dmean). In particular, the parameters of CSmax and Dmax are recommended for rill erosion estimation based on the convenience and ease of data acquisition.

Characterizing the rill erosion process from eroded morphology and sediment connectivity on purple soil slope with upslope earthen dike terraces

Rills are critical venues for the transport of eroded sediments along hillslopes. The sediment transport efficiency and connectivity within hillslopes are affected by the spatiotemporal evolution of rill erosion and morphology. However, the effect of upslope sediment-laden inflow on rill erosion and connectivity remains unclear. This study investigated the variation in rill erosion from the eroded morphology and sediment connectivity using flume scouring experiments. Upslope sediment-laden inflow was simulated considering the upslope terrace areas of 0.15, 0.30, and 0.45 m2 and an upslope inflow of 6 L min−1. The quantity and cross-sectional depth of rills gradually decreased with increasing upslope terrace area. The cross-sectional morphology of rills changed from being V-shaped to U-shaped in the rill erosion process. All of the mean values of the morphological parameters gradually decreased with increasing upslope terrace area, in contrast to the width-depth ratio (Rw/d) and rill density (ρ), which both initially increased and then decreased. The average length, width, and depth of rills were smaller under an upslope terrace area of 0.45 m2 than those under an upslope terrace area of 0.15 m2; they decreased by 2.78 %, 20.67 %, and 33.68 %, respectively. Soil and water loss induced by rill erosion decreased with increasing upslope terrace area. Rills, as major venues for sediment transport on hillslopes, exhibited a higher sediment connectivity (IC) than that observed in interrill areas under the different upslope terrace areas. Rill development resulted in higher erosion between the upslope and downslope parts within rill channels. The variations in Rw/d and ρ were significantly correlated with runoff and eroded sediment yield, which could be used to estimate the rill erosion process under different upslope terrace areas.

Effect of Rill Development on Slope Erosion and Sediment Yield Based on Stereophotogrammetry Technology

Rill erosion is an important kind of slope erosion and the main source of sediment. Through simulated rainfall tests, the morphological characteristics of rill were quantified by stereophotogrammetry technology, and the relationship between rill development and sediment yield was studied. The results show that there was a positive correlation between sediment yield and slope and rainfall intensities. With the increase in rainfall duration, sediment yield first increased sharply and then decreased gradually after reaching the peak value, until it reached dynamic stability. With the increase in rainfall intensity and slope, the length, width, and number of rills increased significantly, with a maximum length of 2.58 m and a maximum width and depth of 9.7 and 2.2 cm. The rill density (RD) increased from 16.67% to 62.65%; rill fragmentation degree (RFD) increased from 16.67% to 100.00%; rill complexity (RC) increased from 10.62% to 30.84%, and rill width–depth ratio (RWDR) decreased from 15.82% to 56.28% with the increase in slope from 6° to 15° and rainfall intensity from 2.0 to 3.0 mm/min. There was a good nonlinear relationship between sediment yield and RC and RWDR (R2 = 0.89, NSE = 0.85, n = 10). This study could provide help for the quantification research of rill erosion mechanisms and provide reference for the measurement and scale transformations of soil erosion at different scales.

Linking rill development characteristics to sediment production on different coarse-textured granite topsoils

The sheet and rill erosion phases dominate the erosion process of the catchment area and up-slope of Benggang (a special gully) area, and its evolution has a vital impact on erosion characteristics in the erosion feedback loop. Here, the rill development processes, morphological characteristics and corresponding soil loss were investigated by laser scanning on the four coarse-textured granite topsoils defined as WH, TC, GX, and CT for the counties of Wuhua, Tongcheng, Ganxian, and Changting in China, respectively. A soil flume (3 × 0.8 m) at a slope gradient of 16.7% was treated with rainfall-inflow at the upslope inflow rate of 2 L min−1 and rainfall intensity of 90 mm h−1. Results showed that soil erosion rate increased with erosion evolution (sheet erosion, rill advance, and rill maturity phases). A comparison of the four different soils exhibited that the soil erosion rate showed a downtrend with the increase of soil clay and gravel contents. Specifically, the CT soil with high clay and gravel content showed the minimal soil erosion rate (0.024 kg m−2 min−1), rill density (0.95 m m−2), headward erosion rate (0.41 cm min−1), and surface evolution rate. The maximal soil erosion rate and rill depth were observed on the WH slope, with the rill headward erosion rate reaching the maximum of 20.59 cm min−1. Despite no significant difference in the geomorphologic comentropy of the four soils, their variation trend and rate could reflect dynamic changes in erosion. The total sediment transport efficiency was improved by rill formation and development, with rill erosion contributing to about 70% of total soil loss for the four soils. This study can enrich our understanding of the rill erosion process and surface morphology of coarse-textured topsoils.

Changes in morphologic, hydraulic, and hydrodynamic properties of rill erosion due to surface inoculation of endemic soil cyanobacteria

Rill erosion is one of the most important causes of land degradation, and it is caused by the flow concentration on the surface of the hillslopes. It, therefore, results in several times increase in the amount of soil loss and sediment yield. However, the biological control of the rill erosion using soil microorganisms has not been adequately endeavored. In this respect, soil microorganisms have been used and accepted as one of the latest methods for managing surface erosion. Following that, we evaluated the efficacy of soil cyanobacteria inoculation in controlling rill erosion components in the current study. To this end, inoculation of cyanobacteria in mid-sized plots with dimensions of 6 × 1 × 0.5 m, at a slope of 30%, was performed on marl soil and prone to rill erosion from Marzanabad Region, northern Iran. An artificial rainfall of 50 ± 7 mm h−1 was simulated after 40 days of cyanobacteria inoculation, along with a supplementary runoff of 2 ± 0.32 L min−1. Many different outflows and soil loss and runoff hydraulic and hydrodynamic properties were then measured to assess the effects of soil cyanobacteria inoculation compared to untreated conditions. The results showed that inoculation of cyanobacteria reduced runoff volume, runoff coefficient, soil loss, and sediment concentration by 42, 64, 82, and 62%, respectively. It also increased infiltration volume by 74% showing high water retention capacity of the treated soil. Also, the hydraulic and hydrodynamic components of the flow, including velocity, depth, Froude number, and Reynolds number, were significantly reduced (P < 0.05), in contrast, the shear stress and friction coefficient did not change significantly in treated plots. The statistical analyses further showed that soil surface cyanobacterisation considerably affected morphological factors of rill depth, rill mean width, rill maximum length, and rill density by 44.34, 54.25, 40.75, and 38.21%, respectively. Overall, the findings revealed that using cyanobacteria to mitigate rill erosion is a practical and effective method for the bio-conservation of soil and water resources in most arid and semiarid areas.

Characterizing erosion processes on a convex slope based on 3D reconstruction method

Rill erosion is one of the main forms of soil erosion on hillslopes in hilly and gully regions of the Loess Plateau. For simplicity, many studies assume hillslopes with a uniform sloping profile. However, the most loess hillslopes are convex shape with variable degrees in this area. Estimating soil losses and planning soil conservation practices on such slopes have required much local judgment. The objective of this study was to clarify soil erosion evolution qualitatively and quantitatively on a convex slope based on a three-dimensional (3D) reconstruction technique and geographic information system (GIS) in simulated rainfall experiments. The results showed that: (1) The erosion processes of the convex slope could be reasonably described with the following stages: splash and sheet erosion – drop pits – head-cut erosion – intermittent rill – continuous rill – rill network development – transition from rill to ephemeral gully. (2) Soil loss rate from the convex slope varied from 0.127 Kg m−2 min−1 in the sheet erosion stage to 0.342 Kg m−2 min−1 in rill network development stage, which was much greater compared with the results of uniform slope erosion reported by others in this area. Once rill erosion evolved into the dominant erosion pattern (after 30 mm of rainfall), rill length, width, and depth enlarged noticeably with headward and lateral erosion. (3) The contribution of sediment from rill and inter-rill erosion to total erosion was accurately depicted, and rill erosion contributed up to 60% of the total sediment amount. (4) Soil erosion rate was significantly correlated with rill morphological parameters—fractal dimension, rill density, and degree of rill dissection (correlation coefficients of 0.978, 0.989, and 0.980, respectively). These results are helpful for the qualitative and quantitative understanding of the erosion processes of hillslopes in Chinese Loess Plateau, and have an important reference for the rational arrangement of erosion control measures.

Factors affecting connectivity and sediment yields following wildfire and post‐fire salvage logging in California's Sierra Nevada

AbstractSediment delivery following post‐fire logging is a concern relative to water quality. While studies have assessed the effect of post‐fire logging on sediment yields at different spatial scales, none have explicitly identified sediment sources. Our goal was to quantify post‐fire and post‐salvage logging sediment yields and use rill patterns to identify sediment sources. We measured the extent and type of logging disturbance, length of rills per unit area or “rill density”, ground cover, and sediment yields in nine logged and five control small catchments or “swales”, 0.09 to 0.81 ha, for 5 years after the 2013 Rim Fire in California's Sierra Nevada. The logged swales had a mean ground disturbance of 31%. After the first wet season following logging, there was no difference in either mean rill density (0.071 and 0.088 m m−2, respectively) or mean transformed, normalized sediment yields between the control and logged swales. Untransformed mean sediment yields across three sites ranged from 0.11–11.8 and 1.1–3.2 Mg ha−1 for the controls and salvage‐logged swales, respectively. Rill density was strongly related to sediment yield and increased significantly with the amount of high‐traffic skid trail disturbance in logged swales. Rill density was not significantly related to the amount of bare soil despite a significant relationship between sediment yields and bare soil. Rills usually initiated in bare soil and frequently connected high traffic skid trails to the drainage network after being diverted by waterbars. Rill connectivity and sediment yields decreased in control and logged swales where vegetation or other surface cover was high, suggesting this cover disconnected rills from the drainage network. Increasing ground cover on skid trails and between areas disturbed by post‐fire logging and stream channels may reduce sediment yields as well as the hydrologic connectivity between hillslopes and the drainage network.

Integrating land‐water‐people connectivity concepts across disciplines for co‐design of soil erosion solutions

AbstractSoil resources in East Africa are being rapidly depleted by erosion, threatening food, water and livelihood security in the region. Here we demonstrate how the integration of evidence from natural and social sciences has supported a community‐led change in land management in an agro‐pastoral community in northern Tanzania. Geospatial analysis of erosion risk and extent (based on a drone survey across a 3.6 km2 sub‐catchment) revealed that recently converted land had ca 12‐times greater rill density than established slow‐forming terraced plots (987 ± 840 m2 ha−1 vs. 79 ± 110 m2 ha−1). Slope length and connectivity between plots were key factors in the development of rill networks rather than slope per se wherein slope length was augmented by weak boundaries between newly formed plots. Erosion evidence, supported by communication of 'process' and 'structural' hydrological connectivity, was integrated with local environmental knowledge within participatory community workshops. Demonstration of the critical time window of hillslope‐scale rill erosion risk during early phases of slow‐forming terrace development catalysed a community‐led tree planting and grass seed sowing programme to mitigate soil erosion by water. This was grounded in an implicit farmer understanding of the need for effective governance mechanisms at both community and District levels, to enable community‐led actions to be implemented effectively. The study demonstrates the wide‐reaching impact of integrated and interdisciplinary 'upslope‐downslope' thinking to tackle global soil erosion challenges.

The impact of concentrated flow and slope on unpaved loess‐road erosion on the Chinese Loess Plateau

AbstractLoess‐roads (LR) still experience severe soil erosion due to concentrated flow during rainstorms in watersheds on the Chinese Loess Plateau, where soil erosion in sloping farmland, abandoned land, and grassland has been controlled. However, road erosion is commonly overlooked because unpaved roads represent a small part of the catchment area. In situ scouring‐erosion experiments were performed on LR surfaces in the Liuaogou watershed on the Loess Plateau. The erosion rate of the LR was 14.1–234.6 g m−2 min−1, which is 10.8–57.5‐times higher than that on an adjacent grassy hillslope. In most experiments, the LR erosion rate generally increased linearly with increasing concentrated flow and slope gradient (R2 &gt; .85; significance p &lt; .05). Concentrated flow had a greater impact on road erosion than the road surface slope. Rill erosion occurred in our road plots, with rill area densities of 0.03–0.38 m2 m−2. The total amount of road erosion was significantly correlated linearly with the rill area density (R2 = .61, p &lt; .01), indicating that high rill density led to high total erosion on the LR. A road erosion equation based on concentrated flow and slope gradient was established and validated. The R2, p, and NSE results were .91, &lt;.01 and 0.93, respectively, implying that the fitted equation can be considered acceptable. Our findings can provide references for the prevention of road erosion and the establishment of road erosion models.

Modeling Soil Erosion With Evolving Rills on Hillslopes

AbstractWhereas current erosion models are successful in quantitative estimates of soil erosion by water flow, modeling the coevolution of geomorphological features, particularly rill network properties and soil erosion on hillslopes, is still a major challenge. In this study, we propose a rill evolution modeling approach and combine it with a rainfall‐runoff and soil erosion model to simulate the feedback loop of hillslope geomorphic development and soil erosion processes. Rill evolution is mainly characterized by three rill network attributes, rill density, orientation angle, and rill width, all modeled with physical equations. The entire rainfall‐runoff‐erosion and rill evolution model is tested against a set of rill network evolution and soil erosion data from an experimental hillslope subjected to successive rainfall events. The simulated spatial and temporal variations of rill network characteristics and soil erosion agree well with the measured data. The results demonstrate that the three rill network characteristics continually alter the partitioning of interrill and rill flows and affect the interrill and rill flow erosivity and soil erosion, which in turn modify the rill geometry and rill network planform. Comparatively, existing approaches such as Water Erosion Prediction Project (WEPP) that ignore the rill evolution processes largely underestimate the hillslope soil erosion when using time independent model parameters. Moreover, a sensitivity analysis indicates that both the rill evolution and soil erosion processes are sensitive to the rill evolution parameters, rainfall intensity, and slope angle. These results can inform the development of general geomorphic evolution and soil erosion models on evolving rilled hillslopes.

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

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.

Erosion processes and features for a coarse-textured soil with different horizons: a laboratory simulation

Severe water erosion of coarse-textured soils, thereby exposing subsurface horizons at the surface, is a serious problem in southern China. Currently, there are few systematic studies on erosion processes and features as the various coarse-textured subsurface horizons are exposed. The main objective in this study was to investigate the effects of rainfall intensity and inflow rate on the runoff, soil loss, and erosion features of three horizons of a typical soil developed from granite (topsoil horizon, illuvial horizon, and parent material horizon) in the steeply sloped region. A series of laboratory simulation tests were conducted in a 0.8-m by 3-m flume using two combined rainfall and upslope inflow condition (90 mm h−1 plus 2 L min−1 and 120 mm h−1 plus 2.66 L min−1, respectively). Runoff coefficient and soil loss increased with the evolution of erosion phase from sheet erosion to rill advancement to rill maturity phase. Runoff generation increased as soil horizons with higher bulk density were exposed on the surface and as the rainfall-inflow rate increased. Additionally, rainfall-inflow rate and soil horizon had significant effects on soil loss rate, headward erosion rate, and rill density (p < 0.01). The hillslope roughness was dependent on the interaction of soil properties (clay content) and rainfall characteristics (rainfall intensity and duration). Despite the different hillslope roughness levels exhibited as result under the different rainfall-inflow treatments, the topsoil horizon always exhibited low soil loss. Moreover, as rainfall duration increased, coarse grains (quartz) were gradually enriched on the hillslope surface, which worked as “armoring” to protect the soil surface of interrill areas from further erosion, especially for the topsoil horizon. Due to the surface armoring, rill erosion was considered the main sediment source during the rill maturity phase. These results suggested that coarse grains could be an effective on-site soil conservation practice controlling soil erosion, which should be considered in future erosion prediction and bare land restoration in coarse-textured soils.

Micro-topographic assessment of rill morphology highlights the shortcomings of current protective measures in loess landscapes

Due to urban expansion and the rapid development of infrastructure in the loess area of China, artificial earth-fill embankments and excavated slopes are increasingly widespread in recent years. Erosion is typical in such loess slopes; however, quantitative statistical analyses of various counter-measures that affects rill erosion are still lacking. Here, we quantified rill morphology and rill erosion development in two newly constructed slopes with different engineering protection measures. We used high-resolution digital surface models (DSMs) acquired using an Unmanned Aerial Vehicle (UAV) to analyze the case areas during two-time periods. Our results from centimeter accuracy differential DSMs demonstrated that rapid rill erosion is prevalent in the study area, expressed as rill density varying between 2.03 km−2 and 8.81 km−2 at different slope surfaces (viz., erosion protected slopes [EPS], landslide protected slopes [LPS], and unprotected slopes [US]). The slope gradient responsible for rill erosion of the EPS, LPS and US are obviously different, and such information is essential for planning preventive measures in each slope type. At the EPS, the severity of erosion is maximum at the top of the ridges, whereas the gap between reinforced concrete lattice and loess deposits are of serious concern at the LPS. The current engineering measures employed in the study area are thus found ineffective for protection against rill erosion. We therefore propose an improved design by implementing an intercepting drain to the existing design for preventing further erosion.

Effects of rainfall intensity and topography on rill development and rill characteristics on loessial hillslopes in China

Rill development is a major soil erosion process that causes severe soil degradation. This study examined the effects of representative rainfall intensities (50 and 75 mm h−1), slope gradients (10° and 15°), and slope lengths (7.5 and 10.0 m) on rill development and rill characteristics on loessial hillslopes in China. Loessial soil was collected from the cropland of Ansai Town, Yan’an City, Shaanxi Province. The soil with 28.3% sand, 58.1% silt, and 13.6% clay was packed into a soil pan to conduct rainfall simulations in 2012. The results showed that the time of the knickpoint occurrence (5–16 min), the rill headcut extension (9–33 min), and the mean headward erosion rate (1.7–5.0 cm min–1) were better representative indicators for reflecting the changes in the rill development than other indicators used in this study. For a quick evaluation of the rill erosion severity, the rill coverage ratio (1%–12%, generally increasing with an increase in the rainfall intensity) was better than the other indicators for treatments with different rainfall intensities, and the rill width-depth ratio (1.56–2.27, generally decreasing with an increase in the slope gradient) was better than the other indicators for treatments with different slope gradients. Furthermore, the rill inclination angle (8.2°–19.1°, significantly increasing with an increase in the slope length) and rill density (0.19–1.34 m · m−2, generally increasing with an increase in the slope length) were more suitable for evaluating the rill erosion severity on hillslopes with different slope lengths. Therefore, the representative indicators could reflect the differences in the rill development and rill characteristics under different rainfall and topographic situations. The study greatly improved the evaluation of rill erosion severity and the prediction of the development of rills for loessial hillslopes.

Distribution, morphology and influencing factors of rills under extreme rainfall conditions in main land uses on the Loess Plateau of China

Rill erosion is one of the main forms of soil erosion of the hillslopes on the Loess Plateau of China. Few studies have evaluated the effects of land use on the distribution and morphology of rills under extreme rainstorms. A total of 50 rill survey sites were selected in a typical watershed of the Loess Plateau of China to investigate the rill distribution and morphology of three land uses (fallow land, FL; slope farmland, SF; and annual grassland, AG) under extreme rainstorm conditions (called “7·26” rainstorm). Seven parameters (rill width, rill depth, width-depth ratio, cross-section area, rill density, degree of rill dissection and rill erosion modulus) were used to characterize rill morphology, and the basic soil and vegetation properties (bulk density, particle size distribution, and vegetation coverage) and topographic factors (slope length, gradient, direction) were measured. It was found that rills developed on three land uses, and those with widths ranging from 5 to 20 cm accounted for >80%. Rills developed on FL were deeper than those on SF and AG. Almost all rills on FL had a depth of 5–20 cm (90.3%), while the depth of rills on SF and AG were mainly smaller than 15 cm (91.5% and 91.9%, respectively). In addition to width, rill depth and cross-section area of SF and AG were also significantly less than those of FL. SF and FL had a relatively high width-depth ratio and dissection degree. However, land use had no significant effect on rill density. Crop plantations (SF) and grass restoration (AG) significantly reduced rill erosion modulus (28.70% and 45.07% reduction, respectively). Land use, topography, soil, and vegetation factors significantly affected rill morphology (p = 0.001) based on redundancy analysis (RDA) and can explain 48.0% of the total variability of rill morphology, with slope length, vegetation coverage and slope gradient having the greatest influences. Most of the rill morphological parameters significantly decreased as the vegetation coverage increased but they significantly increased with increasing slope length and slope gradient. This study clearly illustrates the severe rill erosion caused by this rainstorm, and indirectly indicates that crop harvesting (FL) aggravates rill erosion and that revegetation (AG) can effectively contain rill erosion. Therefore, rehabilitation efforts should be focused on revegetation (grassland and/or woodland) on steep slopes to control soil erosion.

Prediction of rainfall runoff‐induced sediment load from bare land surfaces by generalized regression neural network and empirical model

AbstractBased on three rainfall run‐off‐induced sediment transport data for bare surface experimental plots, the generalized regression neural network (GRNN) and empirical models were developed to predict sediment load. Rainfall intensity, slope, rainfall duration, soil particle median diameter, clay content of the soil, rill density and soil particle mass density constituted the input variables of the models while sediment load was the target output. The GRNN model was trained and tested. The GRNN model was found successful in predicting sediment load. Sensitivity analysis by the GRNN model revealed that slope and rainfall duration were the most sensitive parameters. In addition to the GRNN model, two empirical models were proposed: (1) in the first empirical model, all the input variables were related to the sediment load, and (2) in the second empirical model, only rainfall intensity, slope and rainfall duration were related to the sediment load. The empirical models were calibrated and validated. At the calibration stage, the coefficients and the exponents of the empirical models were obtained using the genetic algorithm optimization method. The validated empirical models were also applied to two more experimental data sets: (1) one data set was from a field experiment, and (2) one set was from a laboratory experiment. The results indicated the success of the empirical models in predicting sediment load from bare land surfaces.

Rill erosion processes on a steep colluvial deposit slope under heavy rainfall in flume experiments with artificial rain

Understanding rill erosion processes is important in the prediction of soil erosion and the prevention of soil loss. However, limited information is available concerning the impacts of rainfall on rill erosion on steep slopes. Colluvial deposits with steep slopes make up the packed material underlying the collapsing walls in benggang, which collapse due to hydraulic pressure and gravity. They contain loose materials and large amounts of coarse particles. The objectives of this study were to investigate the impacts of rainfall intensity and slope gradient on the rill erosion process, rill development and rill flow dynamic mechanisms on the steep slopes of colluvial deposits. The colluvial soils were subjected to simulated rainfall in a 5-m2 (5-m by 1-m) flume at heavy rainfall intensities (100, 120, and 140 mm h−1) and on five steep slopes (20, 25, 30, 35, and 40°). Rill erosion contributed significantly to colluvial slope erosion; on average, rills accounted for 61% of the soil loss, and the effects of slope gradient were greater than those of the rainfall intensity. After rill development, rill density, rill length, width, and depth all significantly increased. Correspondingly, the soil loss rate sharply raised and irregularly fluctuated. Moreover, the collapse of rill heads or sidewalls tended to increase the relative contribution to rill erosion and rill development. The rill flow was characterized by transitional and subcritical flow regimes. The rill flow velocity was the most sensitive hydraulic parameter, and the unit stream power provided the optimal hydrodynamic parameter to characterize the dynamic mechanisms of rill erosion on colluvial deposits. The collapse of rill heads or sidewalls could result in negative values for critical shear stress, critical stream power, and critical unit stream power of rill erosion, which were −19 Pa, −5.3 N m−1 s−1, and −0.09 m s−1, respectively. These results provide a better understanding of the mechanism of rill erosion on steep slopes.