Overland Flow Sediment Research Articles

Sediment transport capacity equation for soils from the Loess Plateau and northeast China

Sediment transport capacity (Tc) is a relevant component in physical-process-based soil erosion models. At present, the sediment transport capacity prediction model for soils with a low median grain diameter is not fully understood. In this study, the Tc values of three different soils (median grain diameter d50 = 0.015, 0.040, and 0.095 mm) were investigated by conducting indoor experiments at unit flow discharges of 0.19 – 1.89 × 10-3 m2·s−1 and slopes of 0.07–0.21. In addition, data describing the overland flow sediment transport capacity of black soil (d50 = 0.01 mm) were obtained from Yu et al. (2018). First, the performance of seven shear stress, unit stream power, stream power, and mean flow velocity concept-based Tc equations were evaluated based on the measurements. The results confirmed that none of the existing formulas performed well over the entire experimental range. Tc equations derived using soils with a large particle size generally underestimate the Tc of soils containing small particles. The Zhang equation, when calibrated based on the data in this study, can predict overland flow Tc with a PoP of 66 % (PoP refers to the percentage of predictions in which the discrepancy ratio falls between 0.5 and 2.0, whilst the discrepancy ratio is the ratio of the predicted and measured Tc). Based on the general flow parameter concept, a more accurate Tc equation was developed for the four types of soils, which was characterized by a PoP of 83 %. When these equations are beyond the scope of their development conditions, they should be verified further.

Impact of anthropogenic disturbances on carbon cycle changes in terrestrial‐aquatic‐estuarine continuum by using an advanced process‐based model

AbstractRecent research has shown that inland water—including rivers, lakes, and groundwater—may play some role in carbon cycling, although the extent of its contribution has remained uncertain due to the limited amount of reliable data available. To evaluate global changes in the carbon cycle due to anthropogenic factors, such as application of fertilizer and manure, in major rivers including 130 tidal estuaries over an 18‐year period (1998–2015), the present study used an advanced process‐based model derived by coupling a process‐based national integrated catchment‐based eco‐hydrology (NICE) model with various biogeochemical cycle models (NICE‐BGC). Generally, total nitrogen and phosphorus transports in overland flow were found to increase. In contrast, total suspended sediment in overland flow decreased in some regions because the vegetation was able to expand to cover the ground, resulting in less erosion. NICE‐BGC simulated the difference in carbon budget in major rivers with and without nutrient application. Generally, CO2 degassing across the water‐air interface decreased and particulate organic carbon (POC) increased in most rivers through variations in carbon budget, where excess nutrients might stimulate gross primary production of carbon‐rich algal biomass. The simulated result also showed that the estuarine carbon cycle was sensitive to intense anthropogenic disturbances reflected by nutrient load, seawater temperature, increases in sea level, and ocean acidification. Extension of previous studies only by categorizing MARgins and CATchments Segmentation (MARCATS) segment numbers showed that the estimated total CO2 flux from the world's estuaries was 0.14 Pg C/year. Generally, the simulation showed that incorporation of the nutrient cycle into the terrestrial‐aquatic‐estuarine continuum improved estimates of net land flux and carbon budget in inland waters, thus emphasizing that the effect of estuarine inland water should be explicitly included in the global carbon model to minimize the range of uncertainty.

Estimating sediment transport capacity for overland flow

Reliable estimation of sediment transport capacity is essential for soil erosion modeling. The objective of this study was to evaluate the performance of twelve widely used transport capacity functions with a wide range of hydraulic conditions and soil properties. The comparisons with observed data indicated that none of the selected twelve functions gave satisfactory results for transport capacity prediction over all overland flow datasets in this study, and all of them gave very poor predictions for loess soil. Therefore, a new equation was developed for sediment transport capacity estimation. Among these twelve functions, the Ali equation gave the best fit for the whole dataset, but calibration was necessary when using it for large sediment grains (>2 mm) or loess soil. The Yalin equation and the simplified Yalin equation worked better for soils than sands, and predictions for smaller sands (∼0.2 mm) were acceptable. The Govers equation had relatively uniform performance for all datasets. The performance of empirical equations changed dramatically for different datasets. A new function developed in this study was generated based on dimensional analysis and gave good predictions within the range of hydraulic conditions and particle sizes in the datasets considered in this study. The results provide a summary of the transport capacity functions evaluated and suggest an improved function for modeling overland flow sediment transport capacity.

Sediment deposition and overland flow hydraulics in simulated vegetative filter strips under varying vegetation covers

Vegetative filter strips (VFSs) can effectively trap sediment in overland flow, but little information is available on its performance in controlling high-concentration sediment and the runoff hydraulics in VFS. Flume experiments were conducted to investigate the sediment deposition, hydraulics of overland flow and their relationships in simulating VFS under a great range of sediment concentrations with four levels of vegetation cover (bare slope and 4%, 11% and 17%) and two flow rates (15 and 30 L min−1). Sediment concentrations varied from 30 to 400 kg m−3 and slope gradient was 9°. Both the deposited sediment load and deposition efficiency in VFS increased as the vegetation cover increased. Sediment concentration had a positive effect on the deposited load but no effect on deposition efficiency. A lower flow rate corresponded to greater deposition efficiency but had little effect on deposited load. Flow velocities decreased as vegetation cover increased. Sediment concentration had a negative effect on the mean velocity but no effect on surface velocity. Hydraulic resistance increased as the vegetation cover and sediment concentration increased. Sediment deposition efficiency had a much more pronounced relationship with overland flow hydraulics compared with deposited load, especially with the mean flow velocity, and there was a power relationship between them. Flow regime also affected the sediment deposition efficiency, and the efficiency was much higher under subcritical than supercritical flow. The results will be useful for the design of VFS and the control of sediment flowing into rivers in areas with serious soil erosion. Copyright © 2015 John Wiley & Sons, Ltd.

Low losses of a P and sediment in overland flow suggest that this tillage site and others like it may not be high risk for phosphorous pollution

This trial examined phosphorus (P) loss in overland flow (OLF) at a tillage farm in the southeast of Ireland. The soil was a grey-brown podzolic, and was representative of land covering 16% of the total agricultural area, with tillage practised on approximately one-third of this area. The object of the trial was to establish whether this site is a P-pollution risk to surface water. The productive tillage field was divided into three plots, each of approximately 1ha in size. Plots had low earth dams and open lined drains on the lower boundaries. A flow meter sampler with V-notch tank was connected to each of the three drains. Macro-pore flow (MPF) samplers were installed in the plots. Average depth of overland flow across the three plots was 5.4, 17.5 and 18.1mm y−1 for 2005, 2006 and 2007, respectively. Phosphorus concentration was 1.01, 0.64 and 0.6mg l−1, respectively, and P load was 0.04, 0.11 and 0.18kg ha−1 y−1. The median P concentration in MPF was 1.59mg l−1. Sediment yield (95kg ha−1 y−1) was low compared to that achieved in other studies. No pollution parameter measured represented a serious pollution threat. The results of this study suggest that a large proportion of tillage land in Ireland might be classified as low risk for P loss through the overland flow pathway.

Low losses of a P and sediment in overland flow suggest that this tillage site and others like it may not be high risk for phosphorous pollution

This trial examined phosphorus (P) loss in overland flow (OLF) at a tillage farm in the southeast of Ireland. The soil was a grey-brown podzolic, and was representative of land covering 16% of the total agricultural area, with tillage practised on approximately one-third of this area. The object of the trial was to establish whether this site is a P-pollution risk to surface water. The productive tillage field was divided into three plots, each of approximately 1ha in size. Plots had low earth dams and open lined drains on the lower boundaries. A flow meter sampler with V-notch tank was connected to each of the three drains. Macro-pore flow (MPF) samplers were installed in the plots. Average depth of overland flow across the three plots was 5.4, 17.5 and 18.1mm y 1 for 2005, 2006 and 2007, respectively. Phosphorus concentration was 1.01, 0.64 and 0.6mg l , respectively, and P load was 0.04, 0.11 and 0.18kg ha 1 y . The median P concentration in MPF was 1.59mg l . Sediment yield (95kg ha 1 y ) was low compared to that achieved in other studies. No pollution parameter measured represented a serious pollution threat. The results of this study suggest that a large proportion of tillage land in Ireland might be classified as low risk for P loss through the overland flow pathway.

Investigating source areas of eroded sediments transported in concentrated overland flow using rare earth element tracers

Rare earth element oxides (REOs) have excellent potential for use as tracers in erosion studies. Using laboratory and field experiments we aimed to develop and test a simple application method for spreading REOs and to use REOs to determine the source of sediment to concentrated overland flow paths. Tracks left by farm machinery (tramlines) in fields act as concentrated flow paths, delivering a large amount of surface runoff and entrained sediment to the field boundary however little is known regarding where this sediment is sourced from. Two field experiments were conducted to investigate this, one looking at whether sediment is entrained from different points along the length of the tramline and one looking at the area over which sediment is entrained from either side of a tramline. Results from leaching tests and rainfall simulations clearly demonstrated the potential of this method for tracing sediment, there was very little movement of REOs through the soil profile and they were transported with sediment in overland flow. In the field experiment a large proportion of the sediment transported in the tramline originated within the first metre either side of it. However, the total area over which eroded sediment collects and flows into the tramline is potentially large with small amounts of sediment eroded from the between 4 and 6 m being transported in the tramline. REOs have an excellent potential for use as a tracer of soil erosion. They provide a quick and cheap method of identifying sources of eroded sediments and have good potential for use in determining erosion rates.

Simulating Watershed Outlet Sediment Concentration using the ANSWERS Model by applying Two Sediment Transport Capacity Equations

Soil erosion is one of the major sources of environmental deterioration and has many impacts on soil and water resources quality. Scientists generally use computational models to simulate the amount of sediment yield from watersheds. One of the leading equations used in these models is the overland flow sediment transport capacity. Owing to the lack of such equations, the streamflow equations have been used widely instead. Some of these equations relatively resulted good estimations of runoff flow sediment concentration. In this study the Areal Non point Source Watershed Environment Response Simulation (ANSWERS) model code was changed and the original equation of the model replaced with a new equation. Although the new equation underestimates sediment concentration, the original model resulted in closer agreement between observed and simulated sediment concentration in different rainfall events. Results of this study suggested that adding some components considering fine particles of soil such as silt and clay to the new and original equations, may improve the accuracy of prediction of sediment concentration by the ANSWERS model. Although, both equations revealed that tend to underestimate sediment concentration; however, the original equation overestimated sediment concentration whenever the runoff coefficient exceeded 0·3 under relatively moderate rainfall intensity. Furthermore, the results showed that soil moisture conditions, rainfall depth and rainfall intensity affect underestimation or overestimation of the model; and initial soil moisture is a key factor in simulation of sediment concentration. Wet and dry soil conditions caused overestimation and underestimation of sediment concentration for the original model, respectively. Nevertheless, no specific erosion model is available now which can simulate sediment yield accurately.

A semi-empirical model to assess uncertainty of spatial patterns of erosion

Distributed erosion models are potentially good tools for locating soil sediment sources and guiding efficient Soil and Water Conservation (SWC) planning, but the uncertainty of model predictions may be high. In this study, the distribution of erosion within a catchment was predicted with a semi-empirical erosion model that combined a semi-distributed hydrological model with the Morgan, Morgan and Finney (MMF) empirical erosion model. The model was tested in a small catchment of the West Usambara Mountains (Kwalei catchment, Tanzania). Soil detachability rates measured in splash cups (0.48–1.16 g J − 1 ) were close to model simulations (0.30–0.35 g J − 1 ). Net erosion rates measured in Gerlach troughs (0.01–1.05 kg m − 2 per event) were used to calibrate the sediment transport capacity of overland flow. Uncertainties of model simulations due to parameterisation of overland flow sediment transport capacity were assessed with the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. The quality of the spatial predictions was assessed by comparing the simulated erosion pattern with the field-observed erosion pattern, measuring the agreement with the weighted Kappa coefficient of the contingency table. Behavioural parameter sets (weighted Kappa > 0.50) were those with short reinfiltration length (< 1.5 m) and ratio of overland flow power α to local topography power γ close to 0.5. In the dynamic Hortonian hydrologic regime and the dissected terrain of Kwalei catchment, topography controlled the distribution of erosion more than overland flow. Simulated erosion rates varied from − 4 to + 2 kg m − 2 per season. The model simulated correctly around 75% of erosion pattern. The uncertainty of model predictions due to sediment transport capacity was high; around 10% of the fields were attributed to either slight or severe erosion. The difficult characterisation of catchment-scale effective sediment transport capacity parameters poses a major limit to distributed erosion modelling predicting capabilities.

Sediment Movement and Filtration in a Riparian Meadow following Cattle Use

Improper livestock grazing practices in western U.S. riparian areas may reduce the nutrient and pollutant removal function of riparian communities, resulting in degradation of surface water quality. Short duration-high intensity cattle use in 3 x 10 m plots was evaluated in a montane riparian meadow in northern Colorado to quantify livestock effects on sediment movement and filtration under simulated rainfall (approximately equal to 100 mm hour(-1)) plus overland flow (approximately equal to 25 mm hour(-1)) conditions. Four treatments: 1) control, 2) mowed to 10 cm stubble height, 3) trampled by cattle, and 4) cattle grazed plus trampled (grazed) were evaluated. Sixty kg of sediment was introduced to overland flow in each plot. Sediment movement was evaluated using sediment traps positioned in microchannels and on vegetation islands at 5 distances downslope from the upper end of the plots and by sediment front advancement. Most sediment deposition occurred within the first meter downslope from application. About 90% of the applied sediment was filtered from runoff within 10 m in the control and mowed treatments, while approximately 84 and 77% of the applied sediment was trapped in the trampled and grazed treatment plots, respectively. The primary variables that influenced sediment filtration were stem density and surface random roughness. Stem density was the most influential variable that affected sediment filtration. Cattle grazing reduced the stem density by 40%. Monitoring of stem density should aid land managers in regulating cattle use of riparian communities and facilitate the protection of surface water quality from sediment in overland flow. DOI:10.2458/azu_jrm_v55i4_mceldowney

Sediment movement and filtration in a riparian meadow following cattle use

Improper livestock grazing practices in western U.S. riparian areas may reduce the nutrient and pollutant removal function of riparian communities, resulting in degradation of surface water quality. Short duration-high intensity cattle use in 3 x 10 m plots was evaluated in a montane riparian meadow in northern Colorado to quantify livestock effects on sediment movement and filtration under simulated rainfall (approximately equal to 100 mm hour(-1)) plus overland flow (approximately equal to 25 mm hour(-1)) conditions. Four treatments: 1) control, 2) mowed to 10 cm stubble height, 3) trampled by cattle, and 4) cattle grazed plus trampled (grazed) were evaluated. Sixty kg of sediment was introduced to overland flow in each plot. Sediment movement was evaluated using sediment traps positioned in microchannels and on vegetation islands at 5 distances downslope from the upper end of the plots and by sediment front advancement. Most sediment deposition occurred within the first meter downslope from application. About 90% of the applied sediment was filtered from runoff within 10 m in the control and mowed treatments, while approximately 84 and 77% of the applied sediment was trapped in the trampled and grazed treatment plots, respectively. The primary variables that influenced sediment filtration were stem density and surface random roughness. Stem density was the most influential variable that affected sediment filtration. Cattle grazing reduced the stem density by 40%. Monitoring of stem density should aid land managers in regulating cattle use of riparian communities and facilitate the protection of surface water quality from sediment in overland flow.

Effects of site preparation on runoff, erosion, and nutrient losses from Pinus plantations established on the coastal lowlands of south-east Queensland, Australia

Site preparation practices used during establishment of second rotation Pinus plantations on the coastal lowlands of south-east Queensland became more conservative during the 1990s. Traditionally, site preparation included stick raking of residue into windrows, followed by burning. Areas subject to periodic saturation were then cultivated to form continuous high mounds, while better drained sites were contour strip cultivated. These practices were replaced by retention of residue biomass on hillslopes. Similarly, intensive site disturbance was increasingly replaced by tillage of the immediate planting zone only and, as far as possible, avoidance of continuous furrows, major drainage structures, and high mounding works. In the study reported here, simulated rainfall and overland flows were used to assess the impacts of the earlier, less-conservative site preparation practices on hillslope soil loss, nutrient loss, and the potential for off-site nutrient movement.Where residue from clearfell harvesting of the first rotation was not removed, runoff, erosion, and nutrient loss were negligible. Harvesting tended to distribute residue widely across hillslopes, and, together with naturally occurring understorey biomass, it formed a largely continuous groundcover. Highest concentrations of sediment in overland flow were recorded for hillslopes in the newly stick-raked condition. Sediment concentrations in overland flows applied to properly designed and constructed high mounds were low, even for large flows simulating relatively long furrow lengths. However, sediment in rainfall-induced runoff from mounded areas was observed to be nutrient enriched, indicating potential for an off-site nutrient pollution risk, at least prior to establishment of grass cover along furrows. Nutrient enrichment was correlated with enrichment of finer size classes of sediment in runoff.The work reported in this paper vindicated the move away from traditional residue removal and site preparation practices to an approach based on residue retention, minimum site disturbance, and, wherever possible, ‘spot’ rather than continuous mounding/cultivation.

The effect of rainfall intensity on soil erosion and particulate phosphorus transfer from arable soils

Soil erosion, in the form of transported suspended sediment in overland flow, is often associated with high rates of particulate phosphorus (PP) (total P&amp;gt;0.45 μm) transfer from land to watercourses. Particulate P may provide a long-term source of P for aquatic biota. Twenty-two sites for winter overland flow monitoring were selected in south-west England within fields ranging from 0.2–3.8 ha on conventionally-managed arable land. Fields were situated on highly porous, light textured soils, lacking impermeable horizons and often overlying major aquifers. Long arable use and modern cultivation methods result in these soils capping under rain impact. Overland flow was observed when rainfall intensity approached the modest rate of 0.8 mm hr−1 on land at or near to field capacity. Low intensity rainfall (&amp;lt;2 mm hr−1) produced mean suspended sediment losses of 14 kg ha−1 hr−1, with associated PP transfer rates of 16 g ha−1 hr−1. In high intensity rainfall (&amp;gt;9 mm hr−1) mean PP losses of 319 g ha−1 hr−1 leaving the field were observed. As might be expected, there was a good relationship between PP and suspended sediment transfer in overland flow leaving the sites. The capacity of light soils to cap when in arable use, combined with heavy or prolonged rainfall, resulted in substantial discharges, soil erosion and associated PP transfer. Storms with heavy rain, typically of only a few hours duration, were characterised by considerable losses of PP. Such events, with return periods of once or twice a winter, may account for a significant proportion of total annual P transfer from agricultural soils under arable crops. However, contributions from less intense rain with much longer duration (around 100 hours per winter in many arable districts of the UK) are also demonstrated here.

The effect of rainfall intensity on soil erosion and particulate phosphorus transfer from arable soils

Soil erosion, in the fonn of transported suspended sediment in overland flow, is often associated with high rates of particulate phosphorus (PP) (total P >0.45μm) transfer from land to watercourses. Particulate P may provide a long-term source of P for aquatic biota. Twenty-two sites for winter overland flow monitoring were selected in south-west England within fields ranging from 0.2–3.8 ha on conventionally-managed arable land. Fields were situated on highly porous, light textured soils, lacking impermeable horizons and often overlying major aquifers. Long arable use and modem cultivation methods result in these soils capping under rain impact. Overland flow was observed when rainfall intensity approached the modest rate of 0.8 mm hr−1 on land at or near to field capacity. Low intensity rainfall (<2 mm hr−1) produced mean suspended sediment losses of 14 kg ha−1 hr−1, with associated PP transfer rates of 16 g ha−1 hr−1. In high intensity rainfall (>9 mm hr−1) mean PP losses of 319 g ha−1 hr−1 leaving the field were observed. As might be expected, there was a good relationship between PP and suspended sediment transfer in overland flow leaving the sites. The capacity of light soils to cap when in arable use, combined with heavy or prolonged rainfall, resulted in substantial discharges, soil erosion and associated PP transfer. Storms with heavy rain, typically of only a few hours duration, were characterised by considerable losses of PP. Such events, with return periods of once or twice a winter, may account for a significant proportion of total annual P transfer from agricultural soils under arable crops. However, contributions from less intense rain with much longer duration (around 100 hours per winter in many arable districts of the UK) are also demonstrated here.

Evaluating overland flow sediment transport capacity

An equation for evaluating the sediment transport capacity of overland flow is a necessary part of a physically based soil erosion model describing sediment detachment and transport as distributed processes. At first, for the hydraulic conditions of small-scale and large-scale roughness, the sediment transport capacity relationship used in the WEPP model is calibrated by Yalin and Govers' equation. The analysis shows that the transport coefficient Kt depends on the Shields parameter, Y, according to a semi-logarithmic (Yalin) or a linear (Govers) equation. The reliability of the semi-logarithmic equation is verified by Smart's, and Aziz and Scott's experimental data. Then the Low's formula, whose applicability is also proved by Smart's, and Aziz and Scott's data, is transformed as a stream power equation in which a stream power coefficient, KSP, depending on Shields parameter, slope, sediment and water-specific weight, appears. A relationship between transport capacity and effective stream power is also proposed. Finally, the influence of rainfall on sediment transport capacity and the prediction of critical shear stress corresponding to overland flow are examined. © 1998 John Wiley & Sons, Ltd.

Geomorphic considerations for erosion prediction

Current soil-erosion prediction technology addresses processes of rainsplash, overland-flow sediment transport, and rill erosion in small watersheds. The effects of factors determining sediment yield from larger-scale drainage basins, in which sediment movement is controlled by the combined small-scale processes and a complex set of channel and other basin-scale sediment-delivery processes, such as soil creep, bioturbation, and accelerated erosion due to denudation of vegetation, have been poorly evaluated. General suggestions are provided for the development of erosion-prediction technology at the geomorphic or drainage-basin scale based on the separation of sediment-yield data for channel and geomorphic processes from those of field-scale soil loss. An emerging technology must consider: (1) the effects on sediment yield of climate, geology and soils, topography, biotic interactions with other soil processes, and land-use practices; (2) all processes of sediment delivery to a channel system; and (3) the general tendency in most drainage basins for progressively greater sediment storage in the downstream direction.

Initiation of motion in overland flow

ABSTRACTThis article reports the results of an experimental study concerning the initial movement of sediment in overland flow. Experiments were carried out in a small flume using water and quartz materials of various grain sizes.The comparison of our experimental results with classic criteria showed that the Shields‐curve and the criterion of Yang can also be used for turbulent overland flow. Generally, higher shear stresses are necessary to provoke bed‐instability under laminar flow conditions, especially at higher shear‐velocity particle Reynolds numbers. A relative roughness effect also exists in laminar flow. Contrary to the observations by Savat on fixed beds, laminar flow does not show an ‘uncommon selectivity’, when loose, homogeneous beds are considered.