Rill Bed Research Articles

Rill development and its change rate: a field experiment under constant rainfall intensity

Rill erosion is a widespread form of soil erosion on loess slope surfaces. The rill formation, evolutionary mechanism, and the quantitative expression of rill erosion dynamic changes in a micro-topographic level are currently lacking, primarily owing to lack of finer-scale measurements. This work presented the rill erosion evolutionary process and their dynamic changes at different slope gradients, (12°, 18°) in a millimeter-scale analysis. We achieved it by constructing an artificial excavation in the field and performed a soil pan with dimensions of 8 m in length and 2 m in width, which were subjected to a constant rainfall intensity of 100 mm h−1. The multiphase micro-geomorphologic changes during each rainfall phase were monitored using high-resolution terrestrial laser scanner (TLS). We noticed that the larger overland flow in the rill corresponded to more pronounced fluctuations in the cross-section lines of the rill bed. Also, with more the number of rills, the more easily the overland flow was dispersed, which required a larger slope gradient to resume the erosion and fill processes. Erosion rate and fill rate were highly correlated in general, and they increased abruptly at 70–90°. Due to the presence of swelling minerals, the loess surface expanded rapidly after encountering water in the early stage of rainfall. Therefore, the amount and rate of erosion on the slopes were underestimated. Understanding the detailed evolution process of rill erosion from a micro-perspective can provide a valuable reference for the development law of the initial stage of slope erosion in the loess region where erosion is extremely serious.

Impacts of soil properties on flow velocity under rainfall events: Evidence from soils across the Loess Plateau

Flow velocity is one of the most significant hydraulic parameters directly connected to sediment transport-deposition processes. Four soils were collected from north to south of the Loess Plateau, namely Sandy Loess (SL), Loessial Soil (LS), Heilu Soil (HS) and Anthrosol Soil (AS), to examine the impacts of soil property on mean flow velocity in both interrill and rill flows under different simulated rainfall experiments. The mean velocity of interrill flow (MVIF) followed the order of LS > HS ≈ AS > SL at 90 mm h−1 rainfall intensity and LS > HS > AS > SL at 120 mm h−1 rainfall intensity. The mean velocity of rill flow (MVRF) decreased as LS ≈ HS > AS at 90 mm h−1 and LS ≈ HS > AS > SL at 120 mm h−1. The order of MVIF and MVRF on four soil slopes is determined by the relations of runoff discharge, Darcy-Weisbach friction factor in interrill area (fI) and rills (fR), which are closely related to soil properties. Soil properties also changed the effects of rainfall intensity on flow velocities in different erosion stages, resulting in the increasing trend of MVIF on SL, HS and AS slopes and the decreasing trend of MVIF on LS slope with the increase of rainfall intensity. Moreover, soil properties may change the variations of MVIF and MVRF with the increase of slope gradient, by altering the relations of sealing progress and slope effect. The slope effect determined the increasing trend of MVIF with the increase of slope gradients. However, the sealing progress may offset the slope effect and cause the decrease of MVIF on the critical slopes, and the critical slope decreased from the north (20° and 25°) to the south (15°). The equal roles of rill bed roughness and slope effect caused the unchanged of MVRF on LS and HS slopes, while rill bed roughness dominated the fluctuations of MVRF on SL and AS slopes.

Comparing flow resistance law for fixed and mobile bed rills

AbstractRills caused by run‐off concentration on erodible hillslopes have very irregular profiles and cross‐section shapes. Rill erosion directly depends on the hydraulics of flow in the rills, which may differ greatly from hydraulics of flow in larger and regular channels. In this paper, a recently theoretically deduced rill flow resistance equation, based on a power–velocity profile, was tested experimentally on plots of varying slopes (ranging from 9% to 26%) in which mobile and fixed bed rills were incised. Initially, measurements of flow velocity, water depth, cross‐section area, wetted perimeter, and bed slope, carried out in 320 reaches of mobile bed rills and in 165 reaches of fixed rills, were used for calibrating the theoretical flow resistance equation. Then the relationship between the velocity profile parameter Γ, the channel slope, and the flow Froude number was separately calibrated for the mobile bed rills and for the fixed ones. The measurements carried out in both conditions (fixed and mobile bed rills) confirmed that the Darcy–Weisbach friction factor can be accurately estimated using the proposed theoretical approach. For mobile bed rills, the data were supportive of the slope independence hypothesis of velocity, due to the feedback mechanism, stated by Govers. The feedback mechanism was able to produce quasicritical flow conditions. For fixed bed rills, obtained by fixing the rill channel, by a glue, at the end of the experimental run with a mobile bed rill, the slope independence of the flow velocity measurements was also detected. Therefore, an experimental run carried out by a rill bed fixed after modelling flow action is useful to detect the feedback mechanism. Finally, the analysis showed that, for the investigated conditions, the effect of sediment transport on the flow resistance law can be considered negligible respect to the grain roughness effect.

In‐situ assessment of the spatial arrangement of step–pool units on eroded rills

AbstractRills are generated on homogeneous hillslopes by the action of different discharges and evolve morphologically over short timescales due to a strong interaction between the flow and bed morphology. Such an interaction generates a reconfiguration of the bed geometry. Previous works suggest that bed geometry is often characterized by alternation between pools and flat reaches (steps). Each step–pool unit may contribute to hydraulic resistance and affects flow behaviour. The objectives of this work are (i) to assess different (innovative) techniques for the in‐situ assessment of rill bed geometry, (ii) to use these techniques to assess the geometry of eroded rills in situ in order to determine the spatial arrangement in the bed macro‐scale roughness and (iii) finally to analyse the role of slope and discharge as driving factors associated with the development of these macroforms.Roughly rectilinear, long rills were formed in the field as a result of combining different slope and discharges. Photogrammetry provided detailed digital elevation models (DEMs) before and after the experiments. The rills were morphologically characterized from the DEMs. In each rill, the presence of step–pools was identified from long profiles according mainly to morphological criteria published elsewhere, but with ad hoc critical threshold values more appropriate for small eroded channels.The minimum slope required for the development of step–pool units seems to be somewhere between 5 and 15%. Discharge seems to affect pool size or roughness amplitude.There does not seem to be a clear step–pool periodicity. However, external factors could have affected the normal growth and alternation of these structures. Identification of steps and pools from longitudinal elevation profiles can be objectively accomplished using a series of geometric rules originally proposed for rivers and large channels, and adapted to rills. © 2019 John Wiley & Sons, Ltd.

A simulation of rill bed incision processes in upland concentrated flows

Quantifying rill bed incision provides fundamental information for process-based erosion modeling; while the morphodynamic and hydrodynamic mechanism in bed incision processes are still unclear. Thus, experiments were conducted to examine rill bed incision processes in upland concentrated flows. DEMs (2 mm × 2 mm resolution) obtained by photogrammetry were used for rill bed morphology analysis. Rill channel (2.0 m-long, 0.08 m-wide and 0.15 m-deep) with two slope gradients (15° and 20°) were subjected to four overland flow rates (1.0, 2.0, 3.0 and 4.0 L min−1). The results showed that sediment delivery, rill bed incision rate and average rill depth increased with inflow rate and bed slope. Sediment delivery increased from 0.060 to 0.226 kg min−1 per 1 L min−1 inflow increment and from 0.043 to 0.207 kg min−1 when bed slope increased from 15° to 20°. In a well-developed rill channel, rill bed incision could be divided into three phases: pre-headcut formation (dominated by rill flow shear stress), headcut incision (dominated by headcut advancing) and post-headcut incision (dominated by rill flow shear stress). Headcut incision phase, which only accounted for <15% of total experimental time, produced >65% of rill bed sediment. In the pre-headcut formation phase, rill flow velocity, shear stress and stream power increased with increases of inflow rate and slope gradient. Conversely, flow velocity showed no evident trend with increased inflow rate and bed slope during headcut incision phase. Initial headcut advancing rate could be predicted by a non-linear function based upon soil characteristics, rill flow shear stress and headcut height. Sediment delivery showed a power function with the product of inflow rate and squared bed slope. Because rill bed incision is dominated by headcut advancement and incision, practices for controlling headcut initiation should be implemented to decrease hillslope soil loss.

A laboratory study on rill network development and morphological characteristics on loessial hillslope

Rills are basic pathways for runoff, sediment, and pollutant transport at hillslopes within agricultural watershed. The objectives of this study were to investigate the development processes of rill network and morphological characteristics and to examine their affecting factors. A soil box (10 m long, 1.5 m wide, and 0.5 m deep) was subjected to four successive simulated rains under rainfall intensity of 90 mm h−1 with slope gradients of 15° and 25°. Digital elevation models (5 mm resolution) were created from the terrestrial laser scanning measurements. Total soil loss was 46.3 and 61.0 kg m−2 at the 15° and 25° slope gradients, and rill erosion occupied over 75% of the total soil loss. Soil loss and rill erosion were expressed as power equations to the product of slope gradient and accumulated rainfall. Rill networks evolved in a converging way and reached maturity in the fourth rain. Main rill length and rill width, depth, and degree of contour line departure increased with increased rains, while rill width/depth ratio showed the opposite trend. Secondary rill length and rill density increased in the first two rains, and then both decreased in the latter two rains. Scour effect of lateral interfluve flow and meander cutoffs of rill flow were two sub-processes of rill piracy. Rill length and density decreased due to rill piracy specific in merging of secondary rills into main rills. Plow pan and secondary headcuts played key roles in main rill bed incision and sidewall expansion processes, while both had little impact on secondary rills. Results of this study can improve the understanding of how plow pan, rill piracy, and secondary headcut affect rill network and morphologies and provide fundamental knowledge for designing rill prevention practices.

A 1‐D morphodynamic model for rill erosion

The key objective of this paper is to develop a 1‐D hydrodynamic and sediment transport model, namely, RILL1D, that handles transcritical flows over abrupt changes (e.g., formation of pool crests and width change) in a single rill and also predicts changes in rill bed elevation. Replication of rill conditions in terms of flow and bed evolution changes necessitated the use of an enhanced TVD‐MacCormack scheme with implementation of Tseng's surface gradient method, to provide an oscillation free solution over formed pool crests. The model at the end of each time step calculates the flow depth, velocity, and bed shear stress and provides changes in bed elevation and grain size distribution within a cell. The hydrodynamic performance of RILL1D is evaluated by comparing its results with either analytical solutions or experimental observations from various benchmark tests with rigid bed conditions that include (1) formation of a hydraulic jump in a rectangular channel, (2) steady subcritical and transcritical flow (without a shock) over a hump, (3) formation of a hydraulic jump in a converging‐diverging frictionless channel, and (4) flows over step‐pools. It is shown that the enhanced TVD‐MacCormack scheme adequately simulates transcritical flows by preserving the mass conservation and reducing the artificial numerical error. The scheme also approximates satisfactorily depth and velocity for a single rill, with a fixed bed consisting of steps and pools, except near the pool headwalls where a backroller forms with negative velocities. The sediment component of RILL1D is evaluated by two field investigations of single rills with mobile beds. With the field data the code is tested for its ability to reproduce measured values of sediment transport rates. A sensitivity analysis is performed to assess the effects of cell size and critical erosional strength in the predictive ability of the model. RILL1D performs reasonably well in these simulations in terms of sediment prediction rates and fared adequately in terms of replicating rill bed morphology.

Effect of bed topography on soil aggregates transport by rill flow

AbstractAfter its formation, a rill may remain in the field for months, often receiving lower flow rates than the formative discharge. The objective of this work was to evaluate the rill flow transport capacity of soil aggregates at discharges unable to erode the rill, and to analyse the influence of the rill macro‐roughness on this transport process. A non‐erodible rill was built in which roughness was reproduced in detail. In order to assess only the rill macro‐roughness, a flat channel with a similar micro‐roughness to that in the rill replica was built. Rill and channel experiments were carried out at a slope of 8 and at six discharges (8·3 × 10−5 to 5·2 × 10−4 m3 s−1) in the rill, and eight discharges (1·6 × 10−5 to 5·2 × 10−4 m3 s−1) in the channel. Non‐erodible aggregates of three sizes (1–2, 3–5 and 5–10 mm) were released at the inlet of the rill/channel. The number of aggregates received at the outlet was registered. The number and position of the remaining aggregates along the rill/channel were also determined. The rill flow was a major sediment transport mechanism only during the formation of the rill, as during that period the power of the flow was great enough to overcome the influence of the macro‐roughness of the rill bed. At lower discharges the transport capacity in the previously formed rill was significantly less than that in the flat channel under similar slope and discharge. This was determined to be due to local slowing of flow velocities at the exit of rill pools. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Longitudinal velocity patterns and bed morphology interaction in a rill

AbstractPresent‐day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills. The latter are difficult to collect with traditional techniques due to the very low water depths and the relatively high flow velocities in rills. The objectives of this paper were to investigate the feasibility of miniaturized acoustic Doppler velocimeter (mADV) measurements in rill flow and to explore longitudinal variations in flow velocities and their relationship with rill bed morphology. Detailed data on longitudinal flow velocity were required to achieve these objectives.A 1·8 m long rill was formed freely in a flume at 5° slope and 0·001 m3 s−1 discharge. Rill topography was characterized by an alternation of steps and pools. The flume surface was then fixed to preserve rill roughness. A topographical scanning of the entire flume surface was made. Velocity was measured with a mADV along the rill, and at different depths. Flow depth in a longitudinal direction was also measured using an elevation gauge.A strong relationship exists between rill topography and flow hydraulics. Over steps, flow was unidirectional and rapidly accelerating until a threshold Froude number (Fn) value between 1·3 and 1·7 was reached and a hydraulic jump occurred leading to the formation of a pool. In the pool, the flow pattern was multidirectional and complex. The flow was subcritical when leaving the pool and accelerated over the next step until the threshold Froude number value was again reached. Energy loss in the rill was concentrated in the pools, mainly due to the action of a hydraulic jump. This mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Interaction between bed roughness and flow hydraulics in eroding rills

Govers [1992] showed that the flow velocity in rills eroding loose, nonlayered materials could be predicted from knowledge of discharge only (without significant slope or soil effect). The objective of this paper is to investigate to what extent the observed slope independence of flow velocity in eroding rills can be explained by the interaction between rill bed roughness and flow hydraulics. In a laboratory study, two situations were compared: (1) rills which can freely erode a uniform soil layer and (2) rills with a fixed bed geometry. During the experiments, rill discharge and flow velocity were recorded. After each experiment, a detailed topographic survey of the rill bed was carried out using a laser scanner. From these data the main hydraulic variables (mean values of flow depth, wetted perimeter, and hydraulic radius) were estimated. The experiments confirmed the slope independence of rill flow velocities on mobile beds. When the bed is fixed, the flow velocity in rills is clearly slope‐dependent. The slope independence of flow velocity on mobile beds is due to a feedback between rill bed morphology and flow conditions. The roughness amplitude (which was assessed from the standard deviation of corrected height values) and the frequency of macroroughness elements (which was assessed by counting the number of peaks per unit length in a previously simplified longitudinal profiles) both increase and tend to counteract the effect of the increase in slope gradient on rill flow velocity. The final rill flow velocity appears to be characterized by a constant average Froude number.

Mechanisms governing incipient motion of ellipsoidal rock fragments in concentrated overland flow

AbstractA modified Shields criterion for incipient motion of ellipsoidal rock fragments in concentrated overland flow was previously defined based on laboratory data. In order to apply this new criterion, an estimate of the angle of repose for the considered rock fragment is needed. Therefore, a set of laboratory experiments was conducted in order to develop an algorithm for this estimation. The main result of these experiments shows that each rock fragment lying on a rill bed of a given roughness is characterized by a normally distributed set of angles of repose. This distribution reflects the different local slopes that a rock fragment can experience. The new criterion has been successfully validated for single rock fragments lying on an erodible rill bed. Application of this criterion to field conditions, however, did not explain all variation in the observed data. In fact, rock fragments having sizes larger than the predicted flow competence were removed during a moderate rainfall event. A consideration of several mechanisms reveals that collisions between rock fragments seems likely to be a significant mechanism explaining the observed discrepancy.

Flow resistance in concentrated overland flow on rough slope surfaces

AbstractField experiments have been done on simulated flows over different rough pre‐rill and rill bed surfaces. The purpose of the experiments was to evaluate the hydraulic roughness properties of these bed surfaces. Conventional equations describing the relation of the resistance coefficient to the Reynolds number for flow in closed conduits or open channels can also be applied to shallow flows provided the boundary values of the Reynolds number, which discriminate between the various flow regimes, are adapted. When the Reynolds number is less than 100 the flow regime is laminar. Above this figure the flow is either transitional or turbulent, depending on the roughness of the bed surface. The degree of turbulence can be deduced from a morphological description of the pre‐rill and rill bed surface roughness.