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Calanchi, a type of Italian badlands created by a combination of water erosion processes and environmental constraints controlling their development, is a striking example of long-term landscape evolution. Sediment connectivity can be defined as the degree to which a system facilitates the fluxes of sediments through itself. The goal of this research is testing the use of simple morphometric variables for assessing sediment connectivity of calanchi landforms distinguishing between dendritic and parallel systems. For detecting the morphological characteristics controlling the sediment connectivity of calanchi basins, literature data (146 calanchi basins) and measurements carried out in two areas located in Sicily (Italy) (119 calanchi basins) were examined. Application of Hack's law to the investigated calanchi basins, carried out distinguishing between landforms with dendritic and parallel systems, demonstrated that a calanco basin with a dendritic system has an exponent (k = 0.49) which is close to that (k = 0.55–0.60) typical of a river basin. The analysis also showed that Melton number assumes only for calanchi basins with a dendritic system a value (0.648) which is similar to that of the river case (0.694). For establishing calanchi hillslope connectivity, the hillslope sediment transport efficiency was also measured by the travel time, which is the ratio between the length and the square root of the hillslope steepness of each cell of the calanchi DEM, and the corresponding sediment delivery ratio. The concept of sediment delivery ratio of each hillslope cell SDRi was used as a measure of sediment connectivity since low values of this ratio imply a lack of connectivity (structural and/or hydrological). For each calanco the cumulative frequency distribution of the travel time allowed both to test the applicability of the exponential relationship used in SEdiment Delivery Distributed (SEDD) model to calculate SDRi and to estimate the coefficient β appearing in this relationship. Finally, the median travel time and the sediment delivery ratio SDRM of each calanco basin were calculated to evaluate the structural connectivity at calanchi basin scale. The analysis demonstrated that both a dendritic system has median travel time values less than those of a parallel system and that SDRM values of a dendritic system are higher than those of a parallel one. In conclusion, sediment connectivity of a dendritic system is higher than that of a parallel one.

The Wenchuan earthquake and subsequent strong rainfall triggered extensive mass wasting, which caused sufficient landslide sediment to be continuously transferred out of the alpine basin. Effective sediment management in earthquake areas requires a full understanding of how landslide sediment is transferred out of alpine basins. In this study, field investigations and remote sensing interpretations indicated that the bedrock or hillslope surface materials were transferred downstream of the hillslope through falls, slides, fall slides, slide falls, and slide flows. Subsequently, field investigations and box model tests showed that the loose landslide sediment was transferred to the channel mainly through three typical processes (erosion driven by torrents or debris flow, saturation, and surface runoff), which occurred simultaneously in the first several years after the Wenchuan earthquake. However, surface runoff erosion was the main transfer process in the following period. Using the number and area of effective landslides, the length and area of landslide sediment in channels, and the effective basin as indicators, we present observational evidence of the landslide sediment transport process based on a remote sensing image time series from 2005 to 2018. The trends of the indicators suggested that the sediment transfer process in the alpine basin was intense during the first 5 years after the Wenchuan earthquake; then, it gradually weakened without the influence of extreme rainfall, but it may last 20 years or longer before returning to the pre-earthquake level.

Aeolian and fluvial interactions (AFIs) are critical earth-surface processes in arid zones, especially in desert watersheds. The response of hyper-concentrated flows to seasonal alternate AFIs shows very high rates of sediment transport and has important environmental and ecological consequences from local to global scales. Here, we present the aeolian processes-induced hyper-concentrated (AHC) flows that occurred in the Sudalaer desert watershed upstream of the Yellow River, which primarily transport non-cohesive coarse aeolian sand (> 0.08 mm) and show a peak suspended sediment concentration (SSC) of 1.1 to 1.4 × 106 mg l− 3. Our field study and theoretical analysis indicate that non-cohesive coarse aeolian sand downstream in the channel can be entrained from the bed and can be suspended in the turbulent flow by the significant runoff generated upstream with a SSC γ0 value of 0.5 × 106 mg l− 3. Severe aeolian processes can provide an abundant coarse sediment supply in the channel, which, once entrained, can also trigger and promote the development of AHC flows. We define, for the first time, the ratio of the weight percentage of coarse sediment to fine sediment C> 0.08 mm/C

Sediment transport is an important aspect of soil erosion, and sediment transport capacity (Tc) is a key to establishing process-based erosion models. A lot of studies exist that have determined Tc for overland flow, however, few studies have been conducted to determine Tc for loess sediments on steep slopes. Experimental data for this region are thus needed. The objectives of this study are to formulate new equations to describe Tc and evaluate the suitability of these equations for loess sediments on steep slopes. The slope gradients in this study ranged from 10.51% to 38.39%, and flow discharges per unit width varied from 1.11 × 10− 3 m2 s− 1 to 3.78 × 10− 3 m2 s− 1. Results showed that Tc increased as a power function with flow discharge and slope gradient, with R2 = 0.99 and Nash–Sutcliffe model efficiency (NSE) = 0.99. Tc was more sensitive to flow discharge than slope gradient. Tc increased as a power function with mean flow velocity, which was satisfied to predict Tc with R2 = 0.99 and NSE = 0.99. Shear stress (R2 = 0.89, NSE = 0.88) was also a good predictor of Tc, and stream power (R2 = 0.96, NSE = 0.96) was a better predictor of Tc than shear stress. However, unit stream power was not a good predictor to estimate Tc in our study, with R2 = 0.63 and NSE = 0.62. These findings offer a new approach for predicting Tc for loess sediments on steep slopes.

In the process of delta evolution, the flow regime and sediment transport change with the development of topography. Flow is the dynamic factor of shaping topography, while sediment scouring and silting is the direct cause of topography formation, and they are constrained and influenced by topographic changes simultaneously. In order to explore rules of water and sediment movement and topographic sedimentation of delta under microscale conditions, we studied the phenomena, rules and evolution mechanism of flow movement, sediment transport, particle sorting, topographic sedimentation, and flow path generation, merger and recombination under microscale conditions from the perspective of physical experiment. The influence of different external conditions on lacustrine shallow-water delta evolution and the self-organization of delta evolution are studied through comparative experiments under the condition of distinct sediment supplied rates or lake levels and repeatability experiments under the same conditions. Some rules of flow movement, sediment transport and topographic sedimentation such as characteristics of each stage during the process of evolution of lacustrine shallow-water delta, the periodic change of flow regime and the prediction model, the characteristics of sedimentary morphology under different conditions, and the influence of stochastic factors on delta evolution are summarized. The experiments suggest coupled linkage of interaction of water and sediment, self-organization of flow path evolution and the equilibrium tendency of delta structure under the action of coupling interaction of multiple factors of water, sediment and topographic sedimentation.

The loess region of the Central Saxon Hill Country is characterized by a relief of gently rolling hills, whose most distinctive landscape elements are well-defined shallow valleys, called dells. The aim of this research is to gather morphodynamic information on the dells in a qualitative and quantitative way by means of high-resolution field mapping and laboratory-based analyses. The intensity of sediment shifting has been modelled via sediment budgeting on a small catchment scale. Research objectives are to determine the character and magnitude of geomorphic processes for a Holocene time frame. Concerning the absence of currently obtainable dating, this study represents a reflection, which summarises all sediment shifting related to anthropogenic land use during the Holocene. As a specific parameter in terms of landscape development, the relief is considered to be strongly associated with varying process intensities. Agrarian land use of the loess region since the late Neolithic has resulted in severe soil erosion. Lower middle slope positions are the apparent spots were soil erosion processes take place in terms of rill erosion. A detailed survey of three dells provided a very heterogeneous pattern of the relocation of soil material. Dell 1 (Zehren) shows medial Holocene soil profile truncation of 1090 mm. In contrast, dell 3 (Lobschutz) shows soil loss of around 660 mm. All systems have similarly high slope gradients up to 12°. Actually, the deepening of the dell´s depth contours and therewith the extent of erosive slope lengths is varying. Thus, a higher longitudinal decline of the valley floors and a more intensive incision, lead to enlarged erosive slope lengths, which causes increasing sediment transport. Hence, land-use patterns trigger the process of material displacement, whereas relief modifies the intensity of geomorphic processes.

Obstacles in fluvial environments cause flow separation and the emergence of three-dimensional flow fields that can lead to scour and deposition, even when no general sediment transport at the bed occurs. Resulting forms are commonly denoted as ‘fluvial obstacle marks’. The morphology and dynamics of these forms is depended on obstacle-, flow- and sediment characteristics. As no generally approved approach for analysis of these forms exists yet, a process-based method is developed that relates certain dependent morphometric variables to an adapted obstacle Reynolds number. The novel approach was applied by conducting experiments in a laboratory flume and validated against other laboratory and field data. The results of this work have shown a significant relationship between the morphometry of fluvial obstacle marks and obstacle Reynolds number, especially when morphometric variables were combined. Further validation, calibration and extension and of this approach will help to adequately asses the influence of obstacles in the fluvial environment on micro- and meso-scale processes of sediment transport.

Bedforms and channel deposits architecture were studied in the ephemeral streams of the structural basin of Kobo in northern Ethiopia. The Froude numbers, calculated at bankfull flow for a few of the study reaches, are in agreement with the extensive occurrence of horizontal, planar lamination. Small bedforms, not previously reported in the literature, are found and described as produced mainly by scouring rather than accretion processes. Equations to predict particle entrainment proved that large, individual boulders standing scattered on the stream bed can be transported at bankfull flow for long distances. Direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of divisions with specific characteristics. This model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models in contrast with recent theories of flat asymmetrical bed waves progradation at Froude numbers close to unity.

Treating the dynamics of sediment transport as two-dimensional on interrill-areas and as one-dimensional in rill sections, areally averaged sheet sediment transport equations are developed. The two-dimensional sheet sediment transport equation is averaged over an individual interrill-area width and then along the interrill-area length to obtain local-scale areally averaged interrill-area sheet sediment transport equation (local-scale areal averaging). Similarly, the cross-sectionally-averaged rill sediment transport equation is averaged along an individual rill length to obtain local-scale areally averaged rill sediment transport equation (local-scale areal averaging). In order to minimize computational effort and economize on the number of model parameters, the local-scale areally averaged equations are then averaged over a whole hillslope section (large-scale areal averaging). These equations constitute the areally averaged model. The expectations of the terms containing more than one variable are obtained by the method of regular perturbation. In the large-scale areal averaging it is assumed that all the randomness in the state variable is due to the randomness in the parameters of the process. Comparison of the results obtained from the areally averaged model with those of the point-scale model indicates that the areally averaged model uses far less data and yet it performs as well as the point-scale model. The results of the developed model indicate that on a rilled-surface most of the sediment loads comes from rill sections. The developed model is successfully tested against experimental data obtained from a bare rilled hillslope. It predicted measured runoff and sediment rates with mean absolute errors of 11.07 l/min and 0.382 kg/s, respectively.

The effects of the February 2000 extreme flood (6000–7000 m3 s−1, 200 year return period flow) on channel morphology are examined over 108 km of the semiarid bedrock-influenced Sabie River, South Africa. The river is incised into bedrock forming a macrochannel with a planform that is fixed over the timescale of change being investigated. Within the confines of the macrochannel, the river may be zoned into distinct channel types (pool rapid, bedrock anastomosing, etc.) between 1 and 20 km long each characterised by a distinctive assemblage of smaller morphologic units ranging from large lateral bars filling most of the macrochannel to metre-scale lee bars and sand sheets. Changes in morphologic unit composition and spatial coverage are presented for each channel type, along with reconstructed flood hydraulics. This appraisal is based on 1:10,000 scale aerial photographs taken 5 months before and 7 months after the event, the latter verified by post-flood field observations. Some channel types (e.g., cohesive mixed anastomosing) displayed widespread sedimentary stripping, while others (e.g., bedrock anastomosing) remained essentially unchanged. At the morphologic unit scale, large cohesive lateral deposits were largely unaffected by the flood, whereas smaller active channel unconsolidated and consolidated features associated with perennially flowing sub-channels were stripped. At the scale of channel type, change appears to be controlled by a complex combination of factors including (i) spatial and temporal flow energy level change affecting sediment transport and (ii) tributary location. Susceptibility to erosion at the scale of the morphologic unit appears to be linked to elevation above the low flow active channel with features at lower elevations in the channel being stripped. Field observations suggest that unconsolidated, active channel units are re-forming at stripped locations.

In soil erosion by overland flow, sediment discharge is driven by discharge and gradient against the resistance of the soil and vegetation. The form of the relationship, which is taken as a sum of power law terms, may then be inferred from several data sources: (1) ephemeral gully head location, (2) permanent channel head location, (3) slope profile form, (4) consistency with evolutionary models and (5) erosion plot data. The paper focuses on (1) to (4), in order to understand how far these data constrain the form of the dependence on discharge and gradient. Field data from mapping a set of ephemeral gully heads after storms show a weak inverse relationship between channel head gradient and catchment area, which flattens towards an upper threshold gradient of about 25%. These relationships are compared with those theoretically obtained for a range of sediment transport laws which have been previously used, and which are analysed to forecast both overall slope profile form and the location of stream heads. The theory predicts that the location of ephemeral gully heads produced in a single storm will differ from the long-term average stream head location. There is a fairly good relationship between theoretical and observed locations, associated with the form of the sediment transport law. For a single event, the location depends on storm magnitude which cannot be well defined from the data sets, creating a spread of data with an envelope curve defined for the largest events. The empirically acceptable sediment transport equations have also been used to analyse families of slope profiles in a small area, and used within a landscape evolution model to generate landscapes which are compatible with those observed in the field, and provide confirmation of the long-term viability of the expressions obtained to forecast the direction, style and rates of landscape evolution in response to land use or climate drivers.

High resolution data on spatial and temporal variability in flow hydraulics and sediment transport within riffle–pool sequences are required to improve understanding of how fluvial processes maintain these meso-scale bedforms. This paper addresses this issue by providing velocity and boundary shear stress data over a range of discharges from base flow (0.07 m3 s−1) to just over bankfull (8.52 m3 s−1), from a sequence of four pools and three riffles in the River Rede, Northumberland. The data supports the reversal hypothesis of Keller [Geol. Soc. Am. Bull. 87 (1971) 753.] as the primary explanation for the maintenance of the riffle–pool sequence, although they also indicate that spatial variability in tractive force is highly stage dependent and complex. Section-averaged velocity data indicate reversal to be evident at four out of six riffle–pool units. An equalisation in velocity was found for the other two riffle–pool units close to bankfull stage. The spatial patterns of tractive force exhibited in the study reach as a result of increased discharge demonstrate that riffle–pool units operate independently of one another. Shear stress reversals were observed in individual riffle–pool units at different river stages during a flood hydrograph, and in some instances, two occurred in the same riffle–pool unit during a single flow event. Pools were characterised by coarser bed sediments and narrower channel widths in comparison to riffles, increasing the likelihood of tractive force reversal in the River Rede. Areas of predicted bed sediment entrainment obtained from τo−τc, matched observed channel changes in the upper part of the study reach, but over-estimated change in the middle portion of the reach.

Gullies in the Guadalentin catchment, SE Spain, are formed in a variety of lithologies, ranging from schists to marls. There are clear differences in valley and channel morphology between these two types. Gully erosion in schists produces large amounts of clastic material that restricts sediment transport, leading to a relatively smooth transition from gullies to hillslopes, and to broad alluvial valley bottoms with laterally migrating channels. Gullies in marls contain only limited gravels, so gully extension leads to sharply incised gully heads. A two-dimensional model has been developed to investigate and simulate the distribution of gully heads in the Guadalentin. The model runs on a smooth initial surface with an added fractal perturbation. Without some perturbation of the initial surface or material properties, symmetry prevents the formation of any channels. Fractal perturbation was used here to provide initial irregularities at all scales. This paper explores the dependence of model response on the form of the sediment transport law. Initially, sediment transport was considered to be transport-limited and so depends on the relationship between wash and creep/splash. This was modified by (i) altering the exponents for gradient and area to conform with experimental results, (ii) adding a critical tractive power for the initiation of rillwash, (iii) including a more explicit treatment of mass movements, (iv) including terms to reduce the dependence of the model on the computational grid size, (v) providing a basis for explicitly integrating from individual storms to average long-term behaviour and, most critically (vi) introducing the `effective bedload fraction' (ebf) as a simplifying concept to allow for the selective transportation of eroded sediment. This modification allows the model to represent removal of fines in marl-rich catchments, without the need to introduce a fully explicit travel distance formulation. The steep gully heads and vertical side walls developed in marl catchments in the Guadalentin basin are qualitatively well-represented by the model. Preliminary tests showed that steep-walled gullies are generated where the ebf falls to below about 0.5. If a tractive power threshold is also introduced, gentler headcuts are produced. Selective transportation is thus seen as the critical determinant of steep headcuts in gully systems.

This study is a comparison of the strength, stability and resistance to abrasion of monospecific sand crusts formed by three species of free-living fungi: Aureobasidium pullulans, Trichoderma harzianum, and Absidia corymbifera. Compared to three photoautotrophs examined in a previous study, the fungal crusts were thicker and stronger in flexure by a factor of two or more. In addition to the morphological properties of these fungi, the organic substrate required for growth contributes to the total crust strength. Wind tunnel tests, in which the crusts were subjected to particle impact from an upwind sediment source (nominally 0.014 kg m−1 s−1) and varied levels of freestream velocity (u∞), showed that crusts comprised of Trichoderma and Absidia were stable at wind velocities under 10 m s−1. Crusts of Aureobasidium generally ruptured at velocities well below this level and to a much greater extent. The distinctive morphologies and metabolic rates of the three fungi appear to explain these differences. As compared to the wind tunnel experiments, independent tests of crust strength in flexure, which were intended to simulate crust flaking, were found to be less sensitive indicators of relative crust stability in aeolian settings. Analysis of the time rate of expansion of the perimeter of crust loss indicates that this relation is approximately exponential, with the exception of low wind velocities near the threshold for grain ejection where the momentum of the saltators at impact appears to be of prime importance.

The paper illustrates the results of 10 years of research conducted on the Rio Cordon catchment (5 km2), where an experimental station has been operating for the integral and continuous automatic recording of water discharge and coarse sediment transport. The device operates by separating bedload from water discharge and fine sediment and subsequently measuring the two solid components. The recorded floods are grouped in two distinct categories according to the bedload transport rates: `ordinary' and `exceptional' floods (with return times exceeding 30–50 years). Based on data collected and field studies, the aspects characterising threshold conditions of bedload transport at flood initiation and cessation are illustrated. Then, a description is given of the conditions which are necessary for two distinct mechanisms of incipient motion of bed particles to occur: selective transport and equal mobility of stream bed material.

Despite a thin regolith layer, measured sediment yields indicate denudation rates of more than 10 to 20 mm yr−1 on strongly denuded slopes in black marl (terres noires), near Draix, Alpes de Haute-Provence. Overland-flow and rainfall-simulation experiments were conducted on three slopes in the Draix Erosional Research Basin to determine the nature and intensity of hillslope erosion processes. The experiments showed that during a single storm, sediment transport includes water–solid mixtures with a wide range of concentrations, from highly charged rill flow (up to 400 g l−1) to hyperconcentrated flow (up to 800 g l−1) to debris flows (up to 1400 g l−1). Miniature debris flows (MDFs), strongly controlled sediment transport as long as the sediment store was not depleted. Slope surfaces were completely reworked in the winter and the regolith layer was rebuilt. Temperature records from nine recent winter periods indicate that conditions are favourable for soil frost processes.

Terracettes are common features on steep slopes between 1700 and 2700 m a.s.l. in the central Pyrenees. These microforms can be understood as the result of the interaction between the growth of bunch grass (Festuca eskia) and geomorphic processes; the first element provides a discontinuity of the physical properties of the slope, and the second element modifies the growing pattern and affords the characteristic microprofile of the slope. In order to analyse the former relationship, assuming that the sediment transport is only caused by a surface process, a computer simulation experiment has been performed. The first purpose of this model is the construction and verification of a set of hypotheses, and the production of a guide for field investigations. This model may then be used to investigate several questions concerning the geoecology of these features such as the role of geomorphic processes and vegetation behaviour, whether they represent a steady-state or an aggradation or degradation succession, and their efficiency as soil stabilisation features. The first results, obtained during sensitivity analysis, show that it is easy to simulate the formation and evolution of individual forms although the biological model parameters for the generation of a continuous trend of terracettes are rather narrow, due to the risk of demographic instabilities or explosions. Once a parameter set is achieved, simulated terracettes are self reproducing in a dynamic equilibrium condition. Simulated sediment transfer is lowered when a good terracette pattern is obtained, as a consequence not only of the vegetation cover but also of the topographical organisation. The main questions posed are the role of reproduction from seeds and the fact that the model routes sediment but not water. The model is unable therefore to produce features related to concentrated runoff, indirectly simulating the role of montane processes which disturb the organisation of incipient channel forms.

A multivariate statistical methodology consisting of R- and Q-mode cluster analysis and Q-mode factor analysis is employed to examine the interrelationships between landforms, the particle size properties of the regolith, and the genetic geomorphic processes in the Linthorpe catchment in the Darling Downs district of south-east Queensland. Cluster analysis of randomly sampled regolith materials on the basis of their detailed particle size properties shows that the materials form both statistical and spatial clusterings (called here “textural groups”). Textural groups with geomorphic significance are recognizable at two hierarchical levels in the Q-mode cluster dendrograms, and indicate that the spatial variability of particle size can be envisaged as existing at two levels of scale. The three dominant geomorphic processes responsible for the spatial variability of particle size are identified from the results of Q-mode factor analysis of the same particle size data. The three principal factors are associated with suspended sediment transport and deposition, weathering, and bedload transport and deposition respectively. The precise nature of the downslope changes in particle size and in the relative amounts of the three inferred processes is revealed by the position, size and composition of the cluster analysis textural groups in the three factor space. Process-response models displaying these relationships at two levels of scale are constructed on the basis of the classifications derived at different hierarchical levels in the Q-mode cluster dendrograms. At the higher hierarchical level, zones upslope and downslope of the pediment head are dominated by weathering and surface transport factors respectively. At the lower level, four zones with distinctive process-response characteristics are recognised. In descending elevation from the drainage divide, a valley-side slope with all zones present, consists of (i) a residual zone, (ii) an erosional zone, (iii) an accumulation zone of fine textured colluvial and alluvial deposits, and (iv) a second accumulation zone of coarse textured infilled channel deposits.

The south-east reach of Castiglione della Pescaia beach, Italy, has been studied to demonstrate the effect of the breakwaters on sediment characteristics as regards as the cuspate shape of the shoreline. Several samples have been collected to investigate the longshore distribution of sediment parameters such as mean size, sorting and skewness. In addition, comparison with data from natural cuspate beaches has been made. It shows an opposite sediment size distribution while sorting and skewness are almost the same for cusps and bays. That is a consequence of different hydrodynamic processes evenly resulting in a similar shoreline morphology. The grain size analysis has also allowed to state that the sediment of the protected beach is substantially of the same quality of that where natural conditions prevail and that the influence of breakwaters can be neglected in terms of skewness sign. Statistical tests, using mean size, confirmed the main longshore direction of sediment transport, calculated theoretically. Core samples have been taken in order to examine the subsurface sediment characteristics in the breakwater protection area. The core sediments show a predominant positive skewness while the grab and the berm samples are more frequently negatively skewed. The analysis of core stratigraphy taken in different periods shows a sea-bed scour of at least 1 m in the aggradational protected area during ordinary sea-storms.

Sheetwash sediment transport (Q s ) is often expressed by Q S =kQ m S N where S is slope gradient and Q water discharge. For practical reasons the length-rainfall intensity product (LI) is often used in this equation as a surrogate for discharge. The estimation of the exponents m and n is biased whenever complex hillslope shapes are used and when water discharge is not linearly related to LI. An experiment was designed to simultaneously measure water and sediment discharges on short straight hillslopes of similar material, sloping at 5°, 11° and 27°. Natural rainfalls were used throughout this study. The values obtained for the exponents m and n using all the available measurements are 1.5 and 0.9 respectively. The value of m lies between the boundaries proposed in literature and the exponent n is slightly under the lower limit. This experiment showed that water discharge is not linearly related to the length-rainfall intensity product and that the value of m calibrated with Q increases with slope gradient. This result is explained by the return flow occuring at the base of steeper slopes and by the contribution of rainsplash to particle detachment. Consequently, three points will be discussed in detail: (i) the relevancy of using LI rather than Q to estimate m and n or to facilitate measurement and mainipulation in hillslope simulation models; (ii) the implications on erosion processes and simulation of hillslope evolution of the systematic effect of gradient on water and sediment discharges; and (iii) the problem of scale in the calibration of sediment transport equations.