Internal erosion of soil pipes: Sediment rating curves for soil pipes

The collapse of soil pipes due to internal erosion can result in fully mature gullies. Few studies have measured the rates of sediment detachment and transport through soil pipes in situ. The objectives of this work were to determine suspended sediment concentration (SSC) in soil pipes as a function of pipeflow rate to develop sediment rating curves (SRC) and measure the bedload transport as a function of cumulative flow per storm event. H-flumes were installed in seven discontinuous gullies formed by pipe collapse and instrumented for pipe discharge measurements and suspended sediment sampling. The typical response to pipeflow was an initial flush of high concentration of suspended sediment followed by a decrease as pipeflow increased (rising limb of hydrograph). Pipeflows were often so dynamic that it was difficult to consistently capture the initial flush of sediment, resulting in weak to non-existent SRCs. The falling limb of the hydrograph tended to have a relatively low SSC. Thus, soil pipe SRCs tended to be better represented by hysteretic SRCs, although relationships between SSC and flow rate were poorly represented by SRCs. A power law equation given by SSC = aQ^b was adopted to represent the SRC relationships. Fitting this equation to data showed a correlation between the offset, a, and the slope, b, with the slope decreasing as the offset increases. Both SRC parameters (a and b) were correlated to the contributing area of the individual pipe. Bedload appeared to be an important contributor to sediment transport, with bedload – expressed as an average event sediment concentration (mg l⁻¹) – decreasing as the volume of the event discharge (m³) increased. A significant portion (11–31%) of the bedload material was gravel and aggregates (>2 mm diameter material). While this work was the first to determine SRCs for soil pipes, refined sampling and measurement techniques are needed. © 2020 John Wiley & Sons, Ltd.     

Sediment transport capacity and erosion processes: model concepts and reality

Sediment transport capacity, T_c, defined as the maximum amount of sediment that a flow can carry, is the basic concept in determining detachment and deposition processes in current process-based erosion models. Although defined conceptually and used extensively in modelling erosion, T_c was rarely measured. Recently, a series of laboratory studies designed to quantify effects of surface hydrologic conditions on erosion processes produced data sets feasible to evaluate the concept of T_c. A dual-box system, consisting of 1·8 m long sediment feeder box and a 5 m long test box, was used. Depending on the relative magnitudes of sediment delivery from feeder and test boxes, five scenarios are proposed ranging from deposition-dominated to transport-dominated sediment regimes. Results showed that at 5 per cent slope under seepage or 10 per cent slope under drainage conditions, the runoff from the feeder box caused in the additional sediment transport in the test box, indicating a transport-dominated sediment regime. At 5 per cent slope under drainage conditions, deposition occurred at low rainfall intensities. Increases in slope steepness, rainfall intensity and soil erodibility shifted the dominant erosion process from deposition to transport. Erosion process concepts from the Meyer–Wishmeier, Foster–Meyer and Rose models were compared with the experimental data, and the Rose model was found to best describe processes occurring during rain. A process-based erosion model needs to have components that can represent surface conditions and physical processes and their dynamic interactions. Copyright © 1999 John Wiley & Sons, Ltd.     

A non‐equilibrium sediment transport model for rill erosion

Sediment transport in rill flows exhibits the characteristics of non‐equilibrium transport, and the sediment transport rate of rill flow gradually recovers along the flow direction by erosion. By employing the concept of partial equilibrium sediment transport from open channel hydraulics, a dynamic model of rill erosion on hillslopes was developed. In the model, a parameter, called the restoration coefficient of sediment transport capacity, was used to express the recovery process of sediment transport rate, which was analysed by dimensional analysis and determined from laboratory experimental data. The values of soil loss simulated by the model were in agreement with observed values. The model results showed that the length and gradient of the hillslope and rainfall intensity had different influences on rill erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Sediment transport by overland flow over an area of net deposition

Flume studies were conducted in order to evaluate the influence of slope, sediment size, discharge and inflow sediment concentration on sediment deposition by overland flow. Additionally, experiments were carried out to measure transport capacity of overland flow at low slopes, using a wide range of discharges. The experimental data show that the hydraulic conditions where net deposition occurs can be divided into two domains. The first domain is characterized by hydraulic conditions where transport capacity is not significant. In the second domain net deposition still occurs but transport capacity is significant. The size of the latter domain is dependent on the sediment size distribution, on the hydraulic roughness and on the inflow sediment concentration. The experiments clearly indicate the necessity of incorporating a threshold value in any deposition equation. These experiments demonstrate that shear stress is a valuable threshold for deposition modelling. Copyright © 1999 John Wiley & Sons, Ltd.     

Sediment transport at the network scale and its link to channel morphology in the braided Vjosa River system

In this article we apply the CASCADE network-scale sediment connectivity model to the Vjosa River in Albania. The Vjosa is one of the last unimpaired braided rivers in Europe and, at the same time, a data scarce environment, which limits our ability to model how this pristine river might respond to future human disturbance. To initialize the model, we use remotely sensed data and modeled hydrology from a regional model. We perform a reach-by-reach optimization of surface grain size distribution (GSD) and bedload transport capacity to ensure equilibrium conditions throughout the network. In order to account for the various sources of uncertainty in the calculation of transport capacity, we performed a global sensitivity analysis. The modeled GSD distributions generated by the sensitivity analysis generally match the six GSDs measured at different locations within the network. The modeled bedload sediment fluxes increase systematically downstream, and annual fluxes at the outlet of the Vjosa are well within an order of magnitude of fluxes derived from previous estimates of the annual suspended sediment load. We then use the modeled sediment fluxes as input to a set of theoretically derived functions that successfully discriminate between multi-thread and single-thread channel patterns. This finding provides additional validation of the model results by showing a clear connection between modeled sediment concentrations and observed river morphology. Finally, we observe that a reduction in sediment flux of about 50% (e.g., due to dams) would likely cause existing braided reaches to shift toward single thread morphology. The proposed method is widely applicable and opens a new avenue for application of network-scale sediment models that aid in the exploration of river stability to changes in water and sediment fluxes.     

Effect of bed topography on soil aggregates transport by rill flow

After 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 & Sons, Ltd.     

Availability and performance of sediment detachment and transport functions for overland flow conditions

Abstract

Soil erosion is a global environmental problem. To quantify water erosion rates at the field, hillslope or catchment scale, several spatially-distributed soil erosion models have been developed. The accuracy of those models depends largely on the sediment detachment and transport functions used, many of which were developed from empirical research. In this paper, the physical basis of the available sediment detachment and transport functions is reviewed, and their application boundaries determined. Well-known and widely-used sediment detachment and transport functions are discussed on the basis of composite force predictors, i.e. shear stress, stream power, unit stream power and effective stream power, and their suitability is elucidated based on information in the literature. It was found that only a few sediment detachment functions are available, and those have been poorly tested. Most erosion models ignore direct calculation of sediment detachment, but use the sediment transport capacity deficit approach to estimate detachment rate. Many more sediment transport functions are available that also tested better for overland flow conditions. However, our tests did not result in a single function that appeared to perform best under a range of experimental conditions. The unit stream power-based functions developed by Govers seem to be the most promising ones for water erosion modelling. It is therefore recommended to evaluate the performance of existing sediment transport functions with more detailed field and laboratory datasets.

Editor Z.W. Kundzewicz     

A rational sediment transport scaling relation based on dimensionless stream power

The concept of stream channel grade – according to which a stream channel reach will adjust its gradient, S, in order to transport the imposed sediment load having magnitude Q_b and characteristic grain size D_b, with the available discharge Q (Mackin, 1948 , Geological Society of America Bulletin 59 : 463–512; Lane, 1955 , American Society of Civil Engineers, Proceedings 81 : 1–17) is one of the most influential ideas in fluvial geomorphology. Herein, we derive a scaling relation that describes how externally imposed changes in either Q_b or Q can be accommodated by changes in the channel configuration, described by the energy gradient, mean flow depth, characteristic grain size and a parameter describing the effect of bed surface structures on grain entrainment. One version of this scaling relation is based on the dimensionless bed material transport parameter (W*) presented by Parker and Klingeman ( 1982 , Water Resources Research 18 : 1409–1423). An equivalent version is based on a new dimensionless transport parameter (E*) using dimensionless unit stream power. This version is nearly identical to the relation based on W*, except that it is independent of flow resistance. Both versions of the scaling relation are directly comparable to Lane's original relation. In order to generate this stream power‐based scaling relation, we derived an empirical transport function relation relating E* to dimensionless stream power using data from a wide range of stable, bed load‐dominated channels: the form of that transport function is based on the understanding that, while grain entrainment is related to the forces acting on the bed (described by dimensionless shear stress), sediment transport rate is related to the transfer of momentum from the fluid to the bed material (described by dimensionless stream power). Copyright © 2010 John Wiley & Sons, Ltd.     

Sediment transport measurements with tracers in very low‐energy beaches

This study investigates sediment transport at a very low‐energy backbarrier beach in southern Portugal, from a spring‐to‐neap tide period, during fair‐weather conditions. Rates and directions of transport were determined based on the application of fluorescent tracer techniques. Wind and currents were collected locally, whereas the dominant small and short‐period wind waves were characterized using a morphodynamic modelling system coupling a circulation model, a spectral wave model, and a bottom evolution model, well validated over the study area. For the recorded conditions sediment transport was small and ebb oriented, with daily transport rates below 0.02 m³ day^‐1. Tidal currents (mainly ebb velocities) were found to be the main causative forcing controlling sediment displacements. Transport rates were higher during spring tides, tending towards very small values at neap tides. Results herein reported points towards the distinction between tracer advection and tracer dispersion in this type of environment. Transport by advection was low as a consequence of the prevailing hydrodynamic conditions (H_s < 0.1 m, and max. current velocity of 0.5 m s^‐1) and the tracer adjustment to the transport layer, whereas dispersion was relatively high (few metres per day). Tracer techniques allowed distinguishing the broad picture of transport, but revealed the need for refinement in this type of environments (bi‐directional forcing by ebb and flood cycles). Copyright © 2012 John Wiley & Sons, Ltd.     

Distributed evaluation of the contribution of soil erosion to the sediment yield from a watershed

The correct determination of the sediment yield from a basin is of paramount importance in several hydraulic and environmental applications, such as the evaluation of the storage reduction of artificial reservoirs. However, due to the highly episodic nature of sediment supply and transport in many environments and to the extreme complexity of the processes involved, the evaluation of the sediment load in a river is still highly uncertain. When the time scale of interest is sufficiently long, and when the primary sediment source comes from distributed erosion in the watershed, the problem can be tackled in an indirect fashion, by computing the contribution to the annual suspended yield from soil erosion. In order to accomplish this task, we propose a distributed application of the widely used USLE formula. The formula is automatically applied along drainage networks derived from a digital elevation model and properly modified to take into account the presence of deposition zones in the watershed. © 1997 John Wiley & Sons, Ltd.     

Response of soil erosion and sediment sorting to the transport mechanism on a steep rocky slope

A deeper understanding of the sediment characteristics associated with rock fragment content can improve our knowledge of the erosional processes and transport mechanisms of sediments on steep rocky slopes. This research used simulated rainfall experiments lasting for 1 h at a rate of 90 mm h⁻¹ and employed 5 × 1 × 0.4 m parallel troughs filled with purple soils with different rock fragment volumetric contents (0, 5, 10, 20, 30 and 40%) on a 15° slope gradient. For each simulated event, runoff and sediment were sampled at 1- and 3-min intervals, respectively, to study, in detail, the temporal changes in the size distributions of the eroded sediments. The results show that sediment concentrations, soil erosion rates and soil loss ratios significantly decreased as rock fragment content increased for rock fragment contents from 0 to 40% in purple soils. During the transportation process, clay particles often formed aggregates and were then transported as larger particles. Silt particles were more likely to be transported as primary particles with a low degree of sediment aggregation. Sand-sized particles, which constituted a greater proportion of the original soil than the eroded sediments, were formed from other fine particles and transported as aggregates rather than as primary particles. Suspension-saltation, which mainly transports fine particles of 0.02–0.05 mm and coarse particles larger than 0.5 mm in size, was the most important transport mechanism on steep rocky slopes. The results of this study can help to explain the inherent laws of erosional processes on steep rocky slopes and can provide a foundation for improving physical models of soil erosion. © 2019 John Wiley & Sons, Ltd.     

Using additional roughness to characterize erosion control treatment effectiveness in roadside ditch lines

Forest roadside ditch lines capture and redirect road runoff and typically have erosion control treatments installed therein. Existing methods used to determine the effectiveness of roadside ditch line erosion control treatments estimate fixed fractional reductions in sediment yield. However, fixed fractional reductions do not describe dependence on any measurable physical property of treatment, climate, and the environment. Here, we use additional flow roughness induced by erosion control treatments as a metric that can be used as the basis of estimating treatment effectiveness in varying contexts. We investigate its utility in small-scale field experiments in western Washington. We measured the physical characteristics of each ditch (e.g., shape, soil texture, and slope) and flow velocities and sediment concentrations for each treatment under multiple experimental discharges. We then used the concept of shear stress partitioning to relate sediment yield from the ditch line erosion treatments to grain shear stress, which is a function of flow roughness (Manning's n) of the respective treatment. We found that (1) a given erosion control treatment produced consistent Manning's n values across multiple replications and sites, with a bare ditch (no treatment) yielding the lowest roughness (n = 0.05) and a densely wattled ditch yielding the highest roughness (n = 0.75); (2) sediment load and calculated grain shear stress data yielded a single positive relationship when data from each experiment were combined, which suggests the effect of additional roughness on grain shear stress is a main driver in the reduction of ditch line sediment load; and (3) in our dataset, fractional erosion reduction had a variable and nonlinear sensitivity to low flow rates (99% of observed flows) for lower roughnesses. Our results demonstrate how additional flow roughness can be used as a general metric to help evaluate the effectiveness of ditch line erosion control treatments for a variety of physical conditions.     

Sediment transport,redistribution and storage on logged forest hillslopes in south‐eastern Australia

Post‐logging changes in catchment sediment yield have traditionally been attributed to increases in hillslope erosion and delivery rates as a result of forest harvesting activities. Linking hillslope erosion to catchment yield in forestry environments remains difficult, however, primarily because of the scarcity of data on the nature of hillslope sediment storage and delivery processes. A large rainfall simulator (350 m²) was used to apply rainstorms to a logged hillslope containing a snig track (skid trail) and a general logging or harvesting area (GHA) on 10 forest compartments in south‐eastern Australia. The experiments confirmed that the compacted, disturbed surfaces, such as roads and tracks, are the dominant sources of sediment in forestry areas. Sediment transport rates were limited by available sediment supply on both the snig track and the GHA, introducing important implications for the modelling of these surfaces using sediment transport capacity theories. Sediment delivery from the snig track to the adjacent GHA, via a cross‐bank (drainage diversion), was strongly influenced by the percentage fine fraction in the eroded sediment. Preferential deposition of coarse aggregates was measured at erosion control structures and on the adjacent GHA. Over 50% of fine‐grained material were deposited on the hillslope over a relatively short, flow path length of <5 m, highlighting the effectiveness of runoff diversion as a practice in reducing sediment flux. The transfer of water and sediment from disturbed to less disturbed parts of the landscape, and the associated potential for sediment storage, needs to be considered as part of any catchment impact assessment. Copyright © 1999 John Wiley & Sons, Ltd.     

Variable and conflicting shear stress estimates inside a boundary layer with sediment transport

This paper presents a comparison between two methods for estimating shear stress in an atmospheric internal boundary layer over a beach surface under optimum conditions, using wind velocities measured synchronously at 13 heights over a 1.7 m vertical array using ultrasonic anemometry. The Reynolds decomposition technique determines at‐a‐point shear stresses at each measurement height, while the Law‐of‐the‐Wall yields a single boundary layer estimate based on fitting a logarithmic velocity profile through the array data. Analysis reveals significant inconsistencies between estimates derived from the two methods, on both a whole‐event basis and as time‐series. Despite a near‐perfect fit of the Law‐of‐the‐Wall, the point estimates of Reynolds shear stress vary greatly between heights, calling into question the assumed presence of a constant stress layer. A comparison with simultaneously measured sediment transport finds no relationship between transport activity and the discrepancies in shear stress estimates. Results do show, however, that Reynolds shear stress measured nearer the bed exhibits slightly better correlation with sand transport rate. The findings serve as a major cautionary message to the interpretation and application of single‐height measurements of Reynolds shear stress and their equivalence to Law‐of‐the‐Wall derived estimates, and these concerns apply widely to boundary layer flows in general. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Insight into sediment transport processes on saline rangeland hillslopes using three‐dimensional soil microtopography changes

In arid and semi‐arid rangeland environments, an accurate understanding of runoff generation and sediment transport processes is key to developing effective management actions and addressing ecosystem response to changes. Yet, many primary processes (namely sheet and splash and concentrated flow erosion, as well as deposition) are still poorly understood due to a historic lack of measurement techniques capable of parsing total soil loss into these primary processes. Current knowledge gaps can be addressed by combining traditional erosion and runoff measurement techniques with image‐based three‐dimensional (3D) soil surface reconstructions. In this study, data (hydrology, erosion and high‐resolution surface microtopography changes) from rainfall simulation experiments on 24 plots in saline rangelands communities of the Upper Colorado River Basin were used to improve understanding on various sediment transport processes. A series of surface change metrics were developed to quantify and characterize various erosion and transport processes (e.g. plot‐wide versus concentrated flow detachment and deposition) and were related to hydrology and biotic and abiotic land surface characteristics. In general, erosivity controlled detachment and transport processes while factors modulating surface roughness such as vegetation controlled deposition. The extent of the channel network was a positive function of slope, discharge and vegetation. Vegetation may deflect runoff in many flow paths but promoted deposition. From a management perspective, this study suggests that effective runoff soil and salt load reduction strategies should aim to promote deposition of transported sediments rather than reducing detachment which might not be feasible in these resource‐limited environments. Copyright © 2016 John Wiley & Sons, Ltd.     

Feedbacks between erosion and sediment transport in experimental bedrock channels

Natural bedrock rivers flow in self‐formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. To explore the mechanics of river incision, we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate (sand and cement), a single sediment size primarily transported as bedload, a single erosion mechanism (abrasion) and sediment‐starved transport conditions. We find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport and bottom roughness. Abrasion only occurs where sediment impacts the bed, and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows. However, the spatial focusing of erosion results in tortuous flow paths and erosional forms (inner channels, scoops, potholes), which dissipate flow energy. This energy dissipation is a negative feedback that reduces sediment transport capacity, inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load. Copyright © 2007 John Wiley & Sons, Ltd.     

Sediment sources and transport pathways in a rural catchment,Herefordshire, UK

The relative significance of primary and secondary suspended sediment sources has been identified in a small (15 km²) agricultural catchment through the application of a linear unmixing model using mineral magnetic, geochemical and radionuclide signatures of fine sediments. Roads appear to be important as a secondary source of suspended sediment and in the conveyance of topsoils to the river channel. Roads also alter the source signatures of sediment in transport, especially that derived from agricultural topsoils. Modelling suggests that c. 30% of the suspended sediment collected from suspended sediment traps in the River Leadon were derived from, or transported via, roads. Relative contributions from primary sources of c. 43% and c. 27% were estimated from a combined (subsoil and channel bank) subsurface source, and a combined (arable and grassland) topsoil source, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

Sediment mobility and bedload transport conditions in an alpine stream

The mobility conditions of bedload transport in an alpine high‐gradient step–pool stream (Rio Cordon) are analysed. Since 1986, a device system at the downstream end section of the stream has been operating in order to monitor the water discharge, suspended sediment and bedload transport. Sediment distribution of bedload transported by various floods has been analysed, and equal‐mobility evidence is recognized only for the high‐magnitude flows ever recorded (RI > 50 years). The thresholds for size‐selective and equal‐mobility transport conditions are identified and quantified by using both data provided by the fractional transport rate and by length displacements of marked particles. Size‐selective bedload transport seems to dominate when the critical shear stress of the size fractions τ_ci considered is exceeded, whereas the equal‐mobility condition is approached as levels of excess shear stress become higher (τ_eqi = 1·45τ_ci). Copyright © 2006 John Wiley & Sons, Ltd.     

The effect of soil type,meteorological forcing and slope gradient on the simulation of internal erosion processes at the local scale

Numerical simulation experiments of water erosion at the local scale (20 × 5 m) using a process‐based model [Plot Soil Erosion Model_2D (PSEM_2D)] were carried out to test the effects of various environmental factors (soil type, meteorological forcing and slope gradient) on the runoff and erosion response and to determine the dominant processes that control the sediment yield at various slope lengths. The selected environmental factors corresponded to conditions for which the model had been fully tested beforehand. The use of a Green and Ampt model for infiltration explained the dominant role played by rainfall intensity in the runoff response. Sediment yield at the outlet of the simulated area was correlated positively with rainfall intensity and slope gradient, but was less sensitive to soil type. The relationship between sediment yield (soil loss per unit area) and slope length was greatly influenced by all environmental factors, but there was a general tendency towards higher sediment yield when the slope was longer. Contribution of rainfall erosion to gross erosion was dominant for all surfaces with slope lengths ranging from 4 to 20 m. The highest sediment yields corresponded to cases where flow erosion was activated. An increase in slope gradient resulted in flow detachment starting upstream. Sediment exported at the outlet of the simulated area came predominantly from the zone located near the outlet. The microrelief helped in the development of a rill network that controlled both the ratio between rainfall and flow erosion and the relationship between sediment yield and slope length. Copyright © 2010 John Wiley & Sons, Ltd.     

Sediment transport in an alpine river before and after a dambreak flood event

Fluvial sediment transport in the high mountain Partnach River (Reintal Valley, Bavarian Alps) was investigated during a 10‐year observation period (2001–2010). During this period, the downstream reach of the Partnach River was decoupled from upstream sediment throughput by a rockslide deposit until 2005. In August 2005, the dam was partially breached during a flood event resulting in renewed sediment coupling between the upstream and downstream reaches. A comparison of pre‐ and post‐dambreak river sediment load data showed that the dissolved load dominated sediment transport prior to August 2005 with a switch to the dominance of bedload transport, post‐dambreak. The higher post‐dambreak bedload rates were particularly evident during the first years after the dam failure due to significant coarse material coupling between active sediment sources (undercut banks/talus cones) and the Partnach River. In the last years of the observation period (2009 and 2010) the dominance of dissolved load transport was re‐established. Copyright © 2011 John Wiley & Sons, Ltd.