Organic litter: dominance over stones as a source of interrill flow roughness on low‐gradient desert slopes at Fowlers Gap,arid western NSW,Australia

Thirty‐six runoff plot experiments provide data on flow depths, speeds, and Darcy–Weisbach friction coefficients (f) on bare soil surfaces, and surfaces to which were added sufficient extra plant litter or surface stones to provide projected cover of 5, 10 and 20 per cent. Precision flow depth data were derived with a computer‐controlled gantry and needle gauge for two different discharges for each plot treatment. Taking a fixed flow intensity (Reynolds number, Re = 150) for purposes of comparison shows means of f = 17·7 for bare soil surfaces, f = 11·4 for added stone treatments, and f = 23·8 for added litter treatments. Many individual values of f for stone treatments are lower than for the bare soil surface, but all litter treatments show increases in fcompared to bare soil. The lowering of f in stone treatments relates to the submerged volume that the stones occupied, and the associated concentration of flow onto a smaller part of the plot surface. This leads to locally higher flow intensities and lower frictional drag along threads of flow that the obstacles create. Litter causes higher frictional drag because the particles are smaller, and, for the same cover fraction, are 100 times more numerous and provide 20 times the edge or perimeter length. Along these edges, which in total exceed 2·5 m g^?1 (equivalent to 500 m m^?2 for a loading of 2 t ha^?1), surface tension draws up water from between the litter particles. This reduces flow depth there, and as a consequence of the lower flow intensity, frictional drag rises. Furthermore, no clear passage remains for the establishment of flow threads. These findings apply to shallow interrill flows in which litter is largely immobile. The key new result from these experiments is that under these conditions, a 20 per cent cover of organic litter can generate interrill frictional retardation that exceeds by nearly 41 per cent that of a bare soil surface, and twice that contributed by the same cover fraction of surface stones. Even greater dominance by litter can be anticipated at the many dryland sites where litter covers exceed those tested here. Copyright © 2002 John Wiley & Sons, Ltd.     

Determining friction coefficients for interrill flows: the significance of flow filaments and backwater effects

Friction coefficients in overland flows are customarily estimated from mean flow properties (depth, velocity, slope) that subsume spatial variations in flow arising from two major causes: microtopography and obstacles. This paper uses laboratory experiments in shallow flumes to examine the extent of non‐uniformity in flow conditions associated with each cause. Randomly placed emergent obstacles in a flume with a shallow axial channel generally yielded higher hydraulic roughness than the same pattern of obstacles on a planar flume, as well as greater variation in roughness as the obstacle locations were altered. In both flumes, hydraulic roughness fell with increasing Reynolds number for 10% obstacle cover, showed a flattening trend at 20% cover, and exhibited a convex‐downward trend at 30% obstacle cover. These results indicate the progressive onset of flow controls at narrow gaps in the obstacle field. In such flows, the use of mean flow properties conceals the existence of two main subdivisions of flow: flow filaments and backwater flows. In the experiments, flow filaments involved velocities more than twice the overall mean, whereas backwater flows were much slower than the mean. The existence of fast‐moving flow filaments may be significant in understanding soil transport in surface runoff, and backwater depths may modify splash detachment. Similarly, friction coefficients that fail to reflect these important non‐uniform flow components may not be optimal for hydraulic calculations or in erosion models. It is concluded that new approaches to observing and processing flow data may be required, in order to avoid the loss of important flow detail that is entailed in assuming uniform flow conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

Macroscale surface roughness and frictional resistance in overland flow

The hydraulics of overland flow on rough granular surfaces can be modelled and evaluated using the inundation ratio rather than the flow Reynolds number, as the primary dimensionless group determining the flow behaviour. The inundation ratio describes the average degree of submergence of the surface roughness and is used to distinguish three flow regimes representing partially inundated, marginally inundated and well-inundated surfaces. A heuristic physical model for the flow hydraulics in each regime demonstrates that the three states of flow are characterized by very different functional dependencies of frictional resistance on the scaled depth of flow. At partial inundation, flow resistance is associated with the drag force derived from individual roughness and therefore increases with depth and percentage cover. At marginal inundation, the size of the roughness elements relative to the depth of flow controls the degree of vertical mixing in the flow so that frictional resistance tends to decrease very rapidly with increasing depth of flow. Well-inundated flows are described using rough turbulent flow hydraulics previously developed for open channel flows. These flows exhibit a much more gradual decrease in frictional resistance with increasing depth than that observed during marginal inundation. A data set compiled from previously published studies of overland flow hydraulics is used to assess the functional dependence of frictional resistance on inundation ratio over a wide range of flow conditions. The data confirm the non-monotonic dependence predicted by the model and support the differentiation of three flow regimes based on the inundation ratio. Although the percentage cover and the surface slope may be of importance in addition to the inundation ratio in the partially and marginally inundated regimes, the Reynolds number appears to be of significance only in describing well-inundated flows at low to moderate Reynolds numbers. As these latter conditions are quite rare in natural environments, the inundation ratio rather than the Reynolds number should be used as the primary dimensionless group when evaluating the hydraulics of overland flow on rough surfaces. © 1997 by John Wiley & Sons, Ltd.     

Surface tension and friction coefficients in shallow,laminar overland flows through organic litter

This study investigates the contribution of surface tension forces to friction coefficients in shallow, laminar interrill flows. Friction coefficients in these flows are known to be increased greatly by organic litter and by stems. Fine litter provides extensive edges along which surface tension menisci can be drawn up, and evaluating the significance of this in the frictional retardation of flow was the primary objective of the experiments reported here. Using both standardized ‘litter particles’ (small wooden blocks of fixed dimensions) and natural plant litter, meniscus behaviour and the Darcy–Weisbach friction coefficient were evaluated in shallow flows on a laboratory sand board. For some tests, the surface tension of ordinary water was reduced by 40 per cent by the addition of a surfactant, and the friction coefficient redetermined. Results show that the presence of surface tension menisci flanking litter particles provides areas of deeper flow that are up to 7 mm in width and which can increase flow depths by 100–300 per cent. These zones support significantly higher flow speeds. Increased water depths within menisci are additionally associated with reduced depths beyond the menisci, so that an increase in the spatial variability of flow depths is a second consequence of meniscus formation. These modifications of flow depth by surface tension menisci are shown to reduce rather than increase the overall friction coefficient applicable to the flow. Consequently, additional frictional retardation does not arise within the menisci flanking litter particles and so cannot account for the greater drag arising from litter than from other surface features. Different factors, possibly the direct obstruction of flow paths, must therefore underlie the frictional drag. Copyright © 2002 John Wiley & Sons, Ltd.     

Effect of saltating sediment on flow resistance and bed roughness in overland flow

The acceleration of saltating grains by overland flow causes momentum to be transferred from the flow to the grains, thereby increasing flow resistance and bed roughness. To assess the impact of saltating sediment on overland flow hydraulics, velocity profiles in transitional and turbulent flows on a fixed sand-covered bed were measured using hot-film anemometry. Five discharges were studied. At each discharge, three flows were measured: one free of sediment, one with a relatively low sediment load, and one with a relatively high sediment load. In these flows from 83 to 90 per cent of the sediment was travelling by saltation. As a result, in the sediment-laden flows the near-bed velocities were smaller and the velocity profiles steeper than those in the equivalent sediment-free flows. Sediment loads ranged up to 87·0 per cent of transport capacity and accounted for as much as 20·8 per cent of flow resistance (measured by the friction factor) and 89·7 per cent of bed roughness (measured by the ratio of the roughness length to median grain diameter). It is concluded that saltating sediment has a considerable impact on overland flow hydraulics, at least on fixed granular beds. Saltation is likely to have a relatively smaller effect on overland flow on natural hillslopes and agricultural fields where form and wave resistance dominate. Still, saltation is generally of greater significance in overland flow than in river flow, and for this reason its effect on overland flow hydraulics is deserving of further study. © 1998 John Wiley & Sons, Ltd.     

Assessing the dye‐tracer correction factor for documenting the mean velocity of sheet flow over smooth and grassed surfaces

In the investigation of overland flow hydraulics, mean flow velocity (V) is frequently estimated using the measured surface flow velocity (Vs) multiplied by a correction factor, α. In total, 291 tests were performed in a flume with three beds [smooth glass (GL), sandpaper (SD), and plastic grass (GR)] to investigate α under submerged and non‐submerged flows, and Vs was observed using dye‐tracer method whilst V was calculated by the measured water depth and flow rate. For GL with 5.2% slope and 100 < Re < 5000 [Reynolds number (Re)], α ranged from 0.35 to 0.79, with an average of 0.54. For SD with slopes ranging from 2.6% to 25.9% and 300 < Re < 1200, α varied from 0.18 to 0.48 with an average of 0.32. Raindrop impacts decreased α for GL at 5.2% slope, but the effect diminished for SD as the slope increased. The α‐values less than the theoretical value of 0.67 in laminar flows may be attributed to the greater spatial variability in overland flow compared with channel flow. For GR with non‐submerged flows and Re < 4200, α varied inversely with sediment concentration (SC) at 5.2% slope but was only slightly related to SC at steep slopes of 15.6% and 25.9%. The α‐values were approximately 0.8 for turbulent flows and even greater than 1.0 under high flow discharges. This finding may relate to sheet flow disturbance and retarded surface velocity due to the protruding scattered grass stems. For each surface, α varied positively with Re; α was inversely related to slope for SD but positively related to slope for GR. There was a positive relation between h and α for GL and SD but a negative relation for GR, which highlights the importance of flow inundation status to α. The inundation ratio (h/Δ) is a promising indicator for predicting α; thus, further investigations using different submerged and non‐submerged surfaces are required to predict α effectively based on (h/Δ). Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of rock fragment size and cover on overland flow hydraulics,local turbulence and sediment yield on an erodible soil surface

The interactions between overland flow hydraulics and sediment yield were studied in flume experiments on erodible soil surfaces covered by rock fragments. The high erodibility of a non-cohesive fine sediment (D₅₀ + 0·09mm) permitted the effects of local turbulence and scour on sediment yield to be examined. Overland flow hydraulics and sediment yield were compared for experiments with pebble (D₅₀ + 1·5cm) and cobble (D₅₀ + 8·6cm) rock fragment covers. Cover percentages range from 0 to 99 per cent. Rock fragment size strongly affects the relations between flow hydraulics and rock fragment cover. For pebbles spatially-averaged hydraulic parameters (flow velocity, flow depth, effective flow width, unit discharge, total shear stress, Darcy-Weisbach friction factor, percentage grain friction and grain shear stress) vary most rapidly within cover percentages at low covers (power functions). In contrast, for cobbles these parameters vary most rapidly within cover percentages at high covers (exponential functions). As the type of the function that describes the relation between flow hydraulics and cover percentage can be deduced from the ratio of rock fragment height to flow depth, the continuity equation can be employed to determine the actual coefficients of the functions, provided the regression of one hydraulic parameter (e.g. flow velocity) with cover percentage is known and a good estimate exists for two values of another hydraulic variable for a low and a high cover percentage. The variation of sediment yield with cover percentage is also strongly dependent on rock fragment size, but neither the convex-upward relation for pebbles, nor the positive relation for cobbles can be solely attributed to the spatially averaged hydraulics of sheet-flow. Rock fragments induce local turbulence that leads to scour hole development on the stoss side of the rock fragments while deposition commonly occurs in the wake. This local scour and deposition substantially affects sediment yield. However, scour dimensions cannot be predicted by spatially averaged flow hydraulics. An adjustment of existing scour formulas that predict scour around bridge piers is suggested. Sediment yield from non-cohesive soils might then be estimated by a combination of sediment transport and scour formulas.     

Testing a theoretical resistance law for overland flow on a stony hillslope

Overland flow, sediments, and nutrients transported in runoff are important processes involved in soil erosion and water pollution. Modelling transport of sediments and chemicals requires accurate estimates of hydraulic resistance, which is one of the key variables characterizing runoff water depth and velocity. In this paper, a new theoretical power–velocity profile, originally deduced neglecting the impact effect of rainfall, was initially modified for taking into account the effect of rainfall intensity. Then a theoretical flow resistance law was obtained by integration of the new flow velocity distribution. This flow resistance law was tested using field measurements by Nearing for the condition of overland flow under simulated rainfall. Measurements of the Darcy–Weisbach friction factor, corresponding to flow Reynolds number ranging from 48 to 194, were obtained for simulated rainfall with two different rainfall intensity values (59 and 178 mm hr⁻¹). The database, including measurements of flow velocity, water depth, cross-sectional area, wetted perimeter, and bed slope, allowed for calibration of the relationship between the velocity profile parameter Γ, the slope steepness s, and the flow Froude number F, taking also into account the influence of rainfall intensity i. Results yielded the following conclusions: (a) The proposed theoretical flow resistance equation accurately estimated the Darcy–Weisbach friction factor for overland flow under simulated rainfall, (b) the flow resistance increased with rainfall intensity for laminar overland flow, and (c) the mean flow velocity was quasi-independent of the slope gradient.     

Hydraulic properties affected by litter and stem cover under overland flow

Vegetation stems and litter cover have different effects on sediment transport capacity under the same experimental conditions, which in essence, may be due to differences in their hydraulic properties, but the availability of comparative studies is limited. This study aimed to compare the hydraulic properties affected by litter and stem cover, compare differences in the drag forces exerted by litter and stems on overland flow, and develop new Manning's n and flow velocity equations for litter cover. Two series of flume experiments were conducted with the same slope gradients (8.8%, 17.6%, 26.8%) and flow discharge rates (0.5, 1.0 × 10⁻³ m³ s⁻¹). Artificial Gramineae stems with a 0%–30% cover level and Pinus tabulaeformis litter with a 0%–70% cover level were used in series 1 and series 2, respectively. The flow velocity and depth were measured. The results showed that the Froude number and flow velocity affected by stem cover were much lower than those affected by litter cover, while the opposite trend was observed in the relative magnitude of the Reynolds number, flow depth and shear stress. The form resistance caused by stems was 22–57 times greater than that caused by litter for the same cover level, which suggests that stem cover contributes more than litter cover to increasing the flow resistance and reducing the flow's ability for sediment detachment and transport. Two new equations for calculating Manning's n and flow velocity under the influence of litter cover were developed, with R² and NSE values of 0.96. The results of this study contribute to revealing the mechanisms of the differences of the effects of stem and litter cover on soil erosion.     

Overland runoff erosion dynamics on steep slopes with forages under field simulated rainfall and inflow

Although numerous studies have acknowledged that vegetation can reduce erosion, few process-based studies have examined how vegetation cover affect runoff hydraulics and erosion processes. We present field observations of overland flow hydraulics using rainfall simulations in a typical semiarid area in China. Field plots (5 × 2 m²) were constructed on a loess hillslope (25°), including bare soil plot as control and three plots with planted forage species as treatments—Astragalus adsurgens, Medicago sativa and Cosmos bipinnatus. Both simulated rainfall and simulated rainfall + inflow were applied. Forages reduced soil loss by 55–85% and decreased overland flow rate by 12–37%. Forages significantly increased flow hydraulic resistance expressed by Darcy–Weisbach friction factor by 188–202% and expressed by Manning's friction factor by 66–75%; and decreased overland flow velocity by 28–30%. The upslope inflow significantly increased overland flow velocity by 67% and stream power by 449%, resulting in increased sediment yield rate by 108%. Erosion rate exhibited a significant linear relationship with stream power. M. sativa exhibited the best in reducing soil loss which probably resulted from its role in reducing stream power. Forages on the downslope performed better at reducing sediment yield than upslope due to decreased rill formation and stream power. The findings contribute to an improved understanding of using vegetation to control water and soil loss and land degradation in semiarid environments.     

Effects of sediment load on hydraulics of overland flow on steep slopes

Eroded sediment may have significant effects on the hydraulics of overland flow, but few studies have been performed to quantify these effects on steep slopes. This study investigated the potential effects of sediment load on Reynolds number, Froude number, flow depth, mean velocity, Darcy–Weisbach friction coefficient, shear stress, stream power, and unit stream power of overland flow in a sand‐glued hydraulic flume under a wide range of hydraulic conditions and sediment loads. Slope gradients were varied from 8·7 to 34·2%, unit flow rates from 0·66 to 5·26×10^?3 m² s^?1, and sediment loads from 0 to 6·95 kg m^?1 s^?1. Both Reynolds number (Re) and Froude number (Fr) decreased as sediment load increased, implying a decrease in flow turbulence. This inverse relationship should be considered in modeling soil erosion processes. Flow depth increased as sediment load increased with a mean value of 1·227 mm, caused by an increase in volume of sediment‐laden flow (contribution 62·4%) and a decrease in mean flow velocity (contribution 37·6%). The mean flow velocity decreased by up to 0·071 m s^?1 as sediment load increased. The Darcy–Weisbach friction coefficient (f) increased with sediment load, showing that the total energy consumption increased with sediment load. The effects of sediment load on f depended on flow discharge: as flow discharge increased, the influence of sediment load on f decreased due to increased flow depth and reduced relative roughness. Flow shear stress and stream power increased with sediment load, on average, by 80·5% and 60·2%, respectively; however, unit stream power decreased by an average of 11·1% as sediment load increased. Further studies are needed to extend and apply the insights obtained under these controlled conditions to real‐world overland flow conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Terrestrial laser scanning soil surfaces: a field methodology to examine soil surface roughness and overland flow hydraulics

Conventional roughness–resistance relationships developed for pipe and open‐channel flows cannot accurately describe shallow overland flows over natural rough surfaces. This paper develops a new field methodology combining terrestrial laser scanning (TLS) and overland flow simulation to provide a high‐resolution dataset of surface roughness and overland flow hydraulics as simulated on natural bare soil surfaces. This method permits a close examination of the factors controlling flow velocity and a re‐evaluation of the relationship between surface roughness and flow resistance. The aggregate effect of flow dynamics, infiltration and depression storage on retarding the passage of water over a surface is important where runoff‐generating areas are distant from well‐defined channels. Experiments to separate these effects show that this ‘effective resistance’ is dominated by surface roughness. Eight measurements of surface roughness are found to be related to flow resistance: standard deviation of elevations, inundation ratio, pit density (measured both perpendicular and parallel to the flow direction), slope, median depth, skewness of the depth distribution and frontal area. Hillslope position is found to affect the significant roughness measures. In contrast, infiltration rate has little effect on the velocity of water fronts advancing over the soil surfaces examined here and the effect of depression storage is limited. Overland flow resistance is depth dependent where complex microtopographic structures are progressively inundated. Copyright © 2010 John Wiley & Sons, Ltd.     

The errors in surface runoff prediction by neglecting the relationship between infiltration rate and overland flow depth

Many simplifications are used in modeling surface runoff over a uniform slope. A very common simplification is to determine the infiltration rate independent of the overland flow depth and to combine it afterward with the kinematic-wave equation to determine the overland flow depth. Another simplication is to replace the spatially variable infiltration rates along the slope i(x, t) due to the water depth variations h(x,t) with an infiltration rate that is determined at a certain location along the slope. The aim of this study is to evaluate the errors induced by these simplications on predicted infiltration rates, overland flow depths, and total runoff volume. The error analysis is accomplished by comparing a simplified model with a model where the interaction between the overland flow depth and infiltration rate is counted. In this model, the infiltration rate is assumed to vary along the slope with the overland flow depth, even for homogeneous soil profiles. The kinematic-wave equation with interactive infiltration rate, calculated along the slopy by Richard's equation, are then solved by a finite difference scheme for a 100-m-long uniform slope. In the first error analysis, we study the effect of combining an ‘exact’ and ‘approximate’ one-dimensional infiltration rate with the kinematic-wave equation for three different soil surface roughness coefficients. The terms ‘exact’ and ‘approximate’ stand for the solution of Richard's equation with and without using the overland flow depth in the boundary condition, respectively. The simulations showed that higher infiltration rates and lower overland flow depths are obtained during the rising stage of the hydrograph when overland flow depth is used in the upper boundary condition of the one-dimensional Richard's equation. During the recession period, the simplified model predicts lower infiltration rates and higher overland flow depths. The absolute relative errors between the ‘exact’ and ‘approximate’ solutions are positively correlated to the overland flow depths which increase with the soil surface roughness coefficient. For this error analysis, the relative errors in surface runoff volume per unit slope width throughout the storm are much smaller than the relative errors in momentary overland flow depths and discharges due to the alternate signs of the deviations along the rising and falling stages. In the second error analysis, when the spatially variable infiltration rate along the slope i(x, t) is replaced in the kinematic-wave equation by i(t), calculated at the slope outlet, the overland flow depth is underestimated during the rising stage of the hydrograph and overestimated during the falling stage. The deviations during the rising stage are much smaller than the deviations during the falling stage, but they are of a longer duration. This occurs because the solution with i(x, t) recognizes that part of the slope becomes dry after rainfall stops, while overland flow still exists with i(t) determined at the slope outlet. As obtained for the first error analysis, the relative errors in surface runoff volume per unit slope width are also much smaller than the relative errors in momentary overland flow depths and discharges. The relation between the errors in overland flow depth and discharge to different mathematical simplifications enables to evaluate whether certain simplifications are justified or more computational efforts should be used.     

Mean and turbulent flow fields in a simulated ice‐covered channel with a gravel bed: some laboratory observations

Northern rivers experience freeze‐up over the winter, creating asymmetric under‐ice flows. Field and laboratory measurements of under‐ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside. In this study, flume experiments of flows under a simulated ice cover are presented. Open water conditions and simulated rough ice‐covered flows are discussed. Mean flow and turbulent flow statistics were obtained from an Acoustic Doppler Velocimeter (ADV) above a gravel‐bed surface. A central region of faster flow develops in the middle portion of the flow with the addition of a rough cover. The turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used. Two distinct boundary layers (near the bed and in the vicinity of the ice cover, near the water surface) are clearly identified, each being characterized by high turbulent intensity levels. Detailed profile measurements of Reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics. In general, for y/d > 0·4 (where y is height above bed and d is local flow depth), the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and Reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover. The high negative Reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate. Copyright © 2012 John Wiley & Sons, Ltd.     

CORRECTION FACTORS IN THE DETERMINATION OF MEAN VELOCITY OF OVERLAND FLOW

The velocity of overland flow has been conventionally measured using tracers, but it is difficult to measure the mean flow velocity directly because the centroid of the tracer plume is not easily identified. Consequently, previous investigators have measured the velocity of the leading edge of the plume and multiplied it by a correction factor α to obtain an estimate of mean velocity. An alternative method is to measure the velocity of the peak concentration in the tracer plume and multiply this velocity by another correction factor β to estimate mean velocity. To investigate the controls of α and β and develop predictive models for these correction factors, 40 experiments were performed in a flume with a mobile sand bed. Multiple regression analyses reveal that both α and β vary inversely with slope and directly with Reynolds number. The derived regression equations may be used to calculate the mean velocity of other shallow overland flows, at least within the range of slope and Reynolds number for which the equations were developed. In the experiments, slope ranged from 2.7;° to 10° and Reynolds number from 1900 to 12 600.     

Modeling depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation

This paper presents an approach to modeling the depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation. The depth-averaged equation of vegetated compound channel flow is given by considering the drag force and the blockage effect of vegetation, based on the Shiono and Knight method (1991) [40]. The analytical solution to the transverse variation of depth-averaged velocity is presented, including the effects of bed friction, lateral momentum transfer, secondary flows and drag force due to vegetation. The model is then applied to compound channels with completely vegetated floodplains and with one-line vegetation along the floodplain edge. The modeled results agree well with the available experimental data, indicating that the proposed model is capable of accurately predicting the lateral distributions of depth-averaged velocity and bed shear stress in vegetated compound channels with secondary flows. The secondary flow parameter and dimensionless eddy viscosity are also discussed and analyzed. The study shows that the sign of the secondary flow parameter is determined by the rotational direction of secondary current cells and its value is dependent on the flow depth. In the application of the model, ignoring the secondary flow leads to a large computational error, especially in the non-vegetated main channel.     

Boundary condition control of fluvial obstacle mark formation – framework from a geoscientific perspective

Obstacle marks are sedimentary bedforms, typically composed of an upstream local scour hole and a downstream sediment accumulation in the vicinity of an obstruction that is exposed to a current. However, specific morphologies are variable in fluvial, coastal and submarine environments. Although obstacle marks and the phenomenon of local scouring are subject to different scientific disciplines, the objectives of investigations are rather incoherent and no systematic framework for analysing and evaluating boundary condition control exists yet, especially concerning limited knowledge of the cause and effect relationship of obstacle mark formation at instream boulders or vegetation elements in variable environmental conditions. Thus, a parameter framework is developed which identifies a spectrum of extrinsic and intrinsic boundary conditions that control the major process dynamics of obstacle mark formation. The framework is composed of dimensionless control parameters that are separated by a hierarchical order regarding their significance for obstacle mark formation. Primary control parameters determine the geometrical scale of flow field at the obstacle, and therefore control the potential maximum size of the obstacle. Secondary control parameters affect the dynamics of the flow field in geometrical scale and limit the potential maximum size of the emerging sedimentary structure if thresholds are crossed. The framework is supposed to be a foundation for subsequent quantification and determination of thresholds by systematic laboratory studies. To elucidate this, flume-based research is presented, evaluating the influence of different flow levels at boulder-like obstacles of different shapes. The results show that obstacle mark dimensions were maximized at shallow flow depths compared to obstacle dimensions, while deep flows at submerged boulder-like obstructions caused considerably smaller obstacle marks. In interdependency with a rounded and more streamlined obstacle shape, deep flows even cause a deviation of morphology if the flow depth above an obstacle exceeds 1.6 times the obstacle's dimensions. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Predicting sediment transport by interrill overland flow on rough surfaces

Modelling soil erosion requires an equation for predicting the sediment transport capacity by interrill overland flow on rough surfaces. The conventional practice of partitioning total shear stress into grain and form shear stress and predicting transport capacity using grain shear stress lacks rigour and is prone to underestimation. This study therefore explores the possibility that inasmuch as surface roughness affects flow hydraulic variables which, in turn, determine transport capacity, there may be one or more hydraulic variables which capture the effect of surface roughness on transport capacity suffciently well for good predictions of transport capacity to be achieved from data on these variables alone. To investigate this possibility, regression analyses were performed on data from 1506 flume experiments in which discharge, slope, water temperature, rainfall intensity, and roughness size, shape and concentration were varied. The analyses reveal that 89·8 per cent of the variance in transport capacity can be accounted for by excess flow power and flow depth. Including roughness size and concentration in the regression improves that explained variance by only 3·5 per cent. Evidently, flow depth, when used in combination with excess flow power, largely captures the effect of surface roughness on transport capacity. This finding promises to simplify greatly the task of developing a general sediment equation for interrill overland flow on rough surfaces. Copyright © 1998 John Wiley & Sons, Ltd.     

A distributed TOPMODEL for modelling impacts of land‐cover change on river flow in upland peatland catchments

There is global concern about headwater management and associated impacts on river flow. In many wet temperate zones peatlands can be found covering headwater catchments. In the UK there is major concern about how environmental change, driven by human interventions, has altered the surface cover of headwater blanket peatlands. However, the impact of such land‐cover changes on river flow is poorly understood. In particular, there is poor understanding of the impacts of different spatial configurations of bare peat or well‐vegetated, restored peat on river flow peaks in upland catchments. In this paper, a physically based, distributed and continuous catchment hydrological model was developed to explore such impacts. The original TOPMODEL, with its process representation being suitable for blanket peat catchments, was utilized as a prototype acting as the basis for the new model. The equations were downscaled from the catchment level to the cell level. The runoff produced by each cell is divided into subsurface flow and saturation‐excess overland flow before an overland flow calculation takes place. A new overland flow module with a set of detailed stochastic algorithms representing overland flow routing and re‐infiltration mechanisms was created to simulate saturation‐excess overland flow movement. The new model was tested in the Trout Beck catchment of the North Pennines of England and found to work well in this catchment. The influence of land cover on surface roughness could be explicitly represented in the model and the model was found to be sensitive to land cover. Copyright © 2014 John Wiley & Sons, Ltd.