A Review on Rill Erosion Process and Its Influencing Factors

Rills are frequently observed on slope farmlands and rill erosion significantly contributes to sediment yields. This paper focuses on reviewing the various factors affecting rill erosion processes and the threshold conditions of rill initiation. Six factors, including rainfall, runoff, soil, topography, vegetation and tillage system, are discussed. Rill initiation and network are explored. Runoff erosivity and soil erodibility are recognized as two direct factors affecting rill erosion and other types of factors may have indirect influences on rill erosion through increasing or decreasing the effects of the direct factors. Certain conditions are necessary for rill initiation and the critical conditions are different with different factors. Future studies should be focused on 1) the dynamic changes of rill networks; 2) the combined effect of multiple factors; and 3) the relationships of threshold values with other related factors.

Rainfall and inflow effects on soil erosion for hillslopes dominated by sheet erosion or rill erosion in the Chinese Mollisol region

Erosion agents and patterns profoundly affect hillslope soil loss characteristics. However, few attempts have been made to analyze the effects of rainfall and inflow on soil erosion for hillslopes dominated by sheet erosion or rill erosion in the Chinese Mollisol region. The objective of this study was to discuss the erosive agent(rainfall or inflow), hillslope erosion pattern(sheet erosion or rill erosion) and slope gradient effects on runoff and soil losses. Two soil pans(2.0 m long, 0.5 m wide and 0.5 m deep) with 5° and 10° slopes were subjected to rainfall(0 and 70 mm h–1) and inflow(0 and 70 mm h–1) experiments. Three experimental combinations of rainfall intensity(RI) and inflow rate(IR) were tested using the same water supply of 70 mm by controllingthe run time. A flat soil surface and a soil bed with a straight initial rill were prepared manually, and represented hillslopes dominated by sheet erosion and rill erosion, respectively. The results showed that soil losses had greater differences among treatments than total runoff. Soil losses decreased in the order of RI70+IR70 > RI70+IR0 > RI0+IR70. Additionally, soil losses for hillslopes dominated by rill erosion were 1.7-2.2 times greater at 5° and 2.5-6.9 times greater at 10° than those for hillslopes dominated by sheet erosion. The loss of <0.25 mm soil particles and aggregates varying from 47.72%-99.60% of the total soil loss played a dominant role in the sediment. Compared with sheet erosion hillslopes, rill erosion hillslopes selectively transported more microaggregates under a relatively stable rill development stage, but rills transported increasinglymore macroaggregates under an active rill development stage. In conclusion, eliminating raindrop impact on relatively gentle hillslopes and preventing rill development on relatively steep hillslopes would be useful measures to decrease soil erosion and soil degradation in the Mollisol region of northeastern China.

Assessment of Rill Erosion Development during Erosive Storms at Angereb Watershed, Lake Tana Sub-basin in Ethiopia

Application of simple and locally based erosion assessment methods that fit to the local condition is necessary to improve the performance and efficiency of soil conservation practices. In this study, rill erosion formation and development was investigated on the topo-sequence of three catchments （300-500 m slope length）; and on agricultural fields （6 m and 24 m slope lengths） with different crop-tillage surfaces during erosive storms. Rill density and rill erosion rates were measured using rill cross section survey and close range digital photogrammetry. Rill formation and development was commonly observed on conditions where there is wider terrace spacing, concave slope shapes and unstable stone terraces on steep slopes. At field plot level, rill development was controlled by the distribution and abrupt change in the soil surface roughness and extent of slope length. At catchment scale, however, rill formation and development was controlled by landscape structures, and concavity and convexity of the slope. Greater rill cross sections and many small local rills were associated to the rougher soil surfaces. For instance, relative comparison of crop tillage practices have showed that faba-bean tillage management was more susceptible to seasonal rill erosion followed by Teff and wheat tillage surfaces under no cover condition. Surface roughness and landscape structures played a net decreasing effect on the parallel rill network development. This implies that spatial and temporal variability of the rill prone areas was strongly associated with the nature and initial size of surface micro-topography or tillage roughness. Thus, it is necessary to account land management practices, detail micro-topographic surfaces and landscape structures for improved prediction of rill prone areas under complex topographic conditions. Application of both direct rill cross section survey and close range digital photogrammetric techniques could enhance field erosion assessment for practical soil conservation improvement.

Rill erosion and controlling factors on highway side-slopes in the permafrost region

Soil erosion on highway side-slope has been recognized as a cause of environmental damage and a potential threat to road embankments in the high-altitude permafrost regions.To assess the risk to roads and to protect them effectively,it is crucial to clarify the mechanisms governing roadside erosion.However,the cold climate and extremely vulnerable environment under permafrost conditions may result in a unique process of roadside erosion,which differs from the results of current studies conducted at lower altitudes.In this study,a field survey was conducted to investigate side-slope rill erosion along the permafrost section of a highway on the Qinghai‒Tibet Plateau of China.Variations in erosion rates have been revealed,and intense erosion risks(with an average erosion rate of 13.05 kg/m^(2)/a)have been identified on the northern side of the Tanggula Mountains.In the case of individual rills,the detailed rill morphology data indicate that the rill heads are generally close to the slope top and that erosion predominantly occurs in the upper parts of highway slopes,as they are affected by road surface runoff.In the road segment scale,the Pearson correlation and principal component analysis results revealed that the protective effect of vegetation,which was influenced by precipitation,was greater than the erosive effect of precipitation on roadside erosion.A random forest model was then adopted to quantify the importance of influencing factors,and the slope gradient was identified as the most significant factor,with a value of 0.474.Accordingly,the integrated slope and slope length index(L0.5S2)proved to be a reliable predictor,and a comprehensive model was built for highway side-slope rill erosion prediction(model efficiency=0.802).These results could be helpful for highway side-slope conservation and ecological risk prediction in alpine permafrost areas.

Quantifying spatial distribution of interrill and rill erosion in a loess at different slopes using structure from motion(SfM)photogrammetry

The spatial distribution of interrill and rill erosion is essential for unravelling soil erosion principles and the application of soil and water conservation practices.To quantify interrill and rill erosion and their spatial development,four 30-min rainfalls at 90 mm h^(-1)intensity were consecutively simulated on runoff plots packed with a loess at six slopes of 10°,15°,20°,25°,30°and 35°.The soil surface was measured using the structure from motion(SfM)photogrammetry upon each simulation run,and the runoff and sediment samples were collected and measured at every 10 min.Rills did not develop until the third simulation run.During the initial two runs,the lower third section was more severely eroded than the upper and middle thirds along the slope direction,yet the interrill erosion was statistically uniform from left to right.Rills tended to emerge by both sidewalls and in the lower portion in the third run.The corresponding rill erosion increased with slope from 10°to 20°and then decreased for the slopes steeper,which was consistent with the slope trend of the sediment yield directly measured.The rills expanded substantially primarily via head retreat and to a lesser extent via sideward erosion after receiving another 30-min rainfall.Rill erosion contributed 69.3%of the total erosion loss,and shifted the critical slope corresponding to the maximum loss from 20°to 25°.These findings demonstrate the significance of rill erosion not only in total soil loss but also in its relation to slope,as well as the effectiveness of SfM photogrammetry in quantifying interrill and rill erosion.

Finite element method for one-dimensional rill erosion simulation on a curved slope

Rill erosion models are important to hillslope soil erosion prediction and to land use planning.The development of rill erosion models and their use has become increasingly of great concern.The purpose of this research was to develop mathematic models with computer simulation procedures to simulate and predict rill erosion.The finite element method is known as an efficient tool in many other applications than in rill soil erosion.In this study,the hydrodynamic and sediment continuity model equations for a rill erosion system were solved by the Galerkin finite element method and Visual Cþþprocedures.The simulated results are compared with the data for spatially and temporally measured processes for rill erosion under different conditions.The results indicate that the one-dimensional linear finite element method produced excellent predictions of rill erosion processes.Therefore,this study supplies a tool for further development of a dynamic soil erosion prediction model.

The characteristics of rill development and their effects on runoff and sediment yield under different slope gradients

Rill formation is the predominant erosion process in slope land in the Loess Plateau, China. This study was conducted to investigate rill erosion characteristics and their effects on runoff and sediment yielding processes under different slope gradients at a rate of 10°, 15°, 20° and 25° with rainfall intensity of 1.5 mm min-1 in a laboratory setting. Results revealed that mean rill depth and rill density has a positive interrelation to the slope gradient. To the contrary, width-depth ratio and distance of the longest rill to the top of the slope negatively related to slope gradient. All these suggested that increasing slope steepness could enhance rill headward erosion, vertical erosion and the fragmentation of the slope surface. Furthermore,total erosion tended to approach a stable maximum value with increasing slope, which implied that there is probably a threshold slope gradient where soil erosion begins to weaken. At the same time, the correlation analysis showed that there was a close connection between slope gradient and the variousindices of soil erosion: the correlation coefficients of slope gradient with maximal rill depth, number of rills and the distance of the longest rill from the top of the slope were 0.98, 0.97 and-0.98, respectively,indicating that slope gradient is the major factor of affecting the development of rills. Furthermore,runoff was not sensitive to slope gradient and rill formation in this study. Sediment concentration,however, is positively related to slope gradient and rill formation, the sediment concentrations increased rapidly after rill initiation, especially. These results may be essential for soil loss prediction.

Rill flow velocity affected by the subsurface water flow depth of purple soil in Southwest China

Subsurface water flow above the weakly permeable soil layer commonly occurs on purple soil slopes.However,it remains difficult to quantify the effect of subsurface water flow on the surface flow velocity.Laboratory experiments were performed to measure the rill flow velocity on purple soil slopes containing a subsurface water flow layer with the electrolyte tracer method considering 3 subsurface water flow depths(SWFDs:5,10,and 15 cm),3 flow rates(FRs:2,4,and 8 L min^(-1)),and 4 slope gradients(SGs:5°,10°,15°,and 20°).As a result,the pulse boundary model fit the electrolyte transport processes very well under the different SWFDs.The measured rill flow velocities were 0.202 to 0.610 m s^(-1) under the various SWFDs.Stepwise regression results indicated a positive dependence of the flow velocity on the FR and SG but a negative dependence on the SWFD.The SWFD had notable effects on the rill flow velocity.Decreasing the SWFD from 15 to 5 cm increased the flow velocity.Moreover,the flow velocities under the 10-and 15-cm SWFDs were 89%and 86%,respectively,of that under the 5-cm SWFD.The flow velocity under the 5-,10-and 15-cm SWFDs was decreased to 89%,80%,and 77%,respectively,of that on saturated soil slopes.The results will enhance the understanding of rill flow hydrological processes under SWFD impact.

Monitoring of Soil Loss from Erosion Using Geoinformatics and Geotechnical Engineering Methods

In this study, the position of all major rill and gully erosion sites were located using hand held GPS （Global Positioning System） receiver during reconnaissance surveys. Based on severity rating and geopolitical considerations, six of the erosion gully sites were selected for monitoring. Control points were established around each of the gully sites using three Leica 500 dual frequency GPS receivers by method of DGPS （differential GPS） surveys. Detailed topographical survey of the gully sites was carried out using total stations. With the aid of SPOT satellite imageries in combination with total station data and GIS （geographic information system） location maps, contoured maps along with DEM （digital elevation model） were generated using ARCGIS 9.2 software. The morphological parameters of the gullies including depth, width, length and area of the gullies were determined. Volumetric estimate of the amount of soil loss from gully erosion was also carried out. Soil samples were recovered from the gully sites to determine their erodibility and other parameters to be used for soil loss modeling. The result of the studies was used as an indicator for determining the gully initiation point. Slope-area relationship and threshold of gully initiation was established. The minimum volume of soil loss occurred in gully No. 2 （Queen Ede）. The minimum AS^2 value was 345 while the maximum was 3,267.

几种土壤的细沟侵蚀过程及其影响因素

通过间隔为11h的二次人工模拟降雨和冲蚀槽试验研究了10种土壤的侵蚀过程,发现在第2次降雨中大多数土壤出现了细沟侵蚀形式。采样微形态观察发现,在第1次降雨中形成结皮的土壤在第2次降雨中出现了细沟侵蚀;产流产沙观测发现所有出现细沟侵蚀的土壤在第1次降雨中径流含沙量逐渐减少,而第2次降雨中有一个先剧增后减少的过程。土壤理化分析及多元逐步回归分析结果表明,土壤颗粒组成、有机质含量、铁铝含量等因素对细沟侵蚀的影响表现不明显,而与土壤团聚体有关结构指标、细沟侵蚀、产流产沙速率及总量呈显著相关关系。分析表明,团聚体分散度、崩解速率与渗透系数之比两个指标能较好地预测细沟侵蚀发生的可能性,同时也能很好地预测侵蚀产沙量。

Plot investigation on rill flow resistance due to path tortuosity

The path tortuosity t is an indicator of rill morphology accounting for the deviation of the thalweg from a straight alignment.The effect of t on flow resistance has been little investigated for rills.This paper reports the results of a plot investigation aimed to establish the suitable accuracy of the rill thalweg measurement to determine the tortuosity parameter and to test the reliability of a theoretical flow resistance law.Four rills were incised in clay soil(CS)and clay-loam soil(LS)and shaped by a clear flow discharge.The three-dimensional Digital Terrain Models were created by the Structure from Motion technique.For rills on LS,an approximate thalweg was tracked by photo-interpretation,and a specific calculation routine was applied to identify the cross sections with a constant spacing d.The actual rill thalweg was obtained as the line joining the lowest points of these cross-sections.Among the different tested d values,d=0.075 m was chosen to determine t.For both CS and LS,the Darcy-Weisbach friction factorffeatured a non-monotonic relation with t,which was explained as the result of three additive components due to bed roughness,sediment transport,and localized energy losses due to curves.The effect of the former two components onff contrasts that of the third,resulting in a linearly decreasing f-t relationship and constant flow velocity for the three lowest tortuosity values,and an increased friction factor and reduced flow velocity for the highest tortuosity value.The flow resistance law was positively tested,and the predicted friction factor was dependent on t.

Laboratory Testing of Magnetic Tracers for Soil Erosion Measurement

Soil erosion, which includes soil detachment, transport, and deposition, is one of the important dynamic land surface processes. The magnetic tracer method is a useful method for studying soil erosion processes. In this study, five types of magnetic tracers were made with fine soil, fly ash, cement, bentonite, and magnetic powder （reduced iron powder） using the method of disk granulation. The tracers were uniformly mixed with soil and tested in the laboratory using simulated rainfall and inflow experiments to simulate the interrill and rill components of soil erosion, in order to select one or more tracers which could be used to study detachment and deposition by the erosive forces of raindrops and surface flow of water on a slope. The results showed that the five types of magnetic tracers with high magnetic susceptibility and a wide range of sizes had a range of 0.99-1.29 gcm-3 in bulk density. In the interrill and rill experiments, the tracers FC1 and FC2 which consisted of fly ash and cement at ratios of 1：1 and 2：1, respectively, were transported in phase with soil particles since the magnetic susceptibility of sediment approximated that of the soil which was uneroded and the slopes of the regression equations between the detachment of sediment and magnetic tracers FC1 and FC2 were very close to the expected value of 20, which was the original soil/tracer ratio. The detachment and deposition on slopes could be accurately reflected by the magnetic susceptibility differences. The change in magnetic susceptibility depended on whether deposition or detachment occurred. However, the tracer FS which consisted of fine soil and the tracers FB1 and FB2 which consisted of fly ash and bentonite at ratios of 1：1 and 2：1, respectively, were all unsuitable for soil erosion study since there was no consistent relationship between sediment and tracer detachment for increasing amounts of runoff. Therefore, the tracers FC1 and FC2 could be used to study soil erosion by water.

Determination of rill erodibility and critical shear stress of saturated purple soil slopes

The hydrological conditions near the soil surface influence the soil erosion process,as determined by the soil erodibility and critical shear stress.The soil erodibility and critical shear stress of saturated purple soil slopes were computed and compared with those of unsaturated purple soil slopes.The detachment capacities computed through the numerical method(NM),modified numerical method(MNM)and analytical method(AM),from rill erosion experiments on saturated purple soil slopes at different flow rates(2,4,and 8 L min-1)and slope gradients(5,10,15,and 20°),were used to comparatively compute the soil erodibility and critical shear stress.The computed soil erodibilities and critical shear stresses were also compared with those of unsaturated purple soil slopes.At the different slope gradients ranging from 5°to 20°,there were no significant differences in the soil erodibilities of the saturated purple soil and also in those of the unsaturated purple soil.The critical shear stresses slightly varied with the slope gradients.The saturated purple soil was relatively significantly more susceptible to erosion.The NM overestimated the soil erodibility of both saturated and unsaturated soils by 31%and underestimated the critical shear stress.The MNM yielded the same soil erodibility and critical shear stress values as the AM.The results of this study supply parameters for modeling rill erosion of saturated purple soil slope.