Effects of rock fragments incorporated in the soil matrix on concentrated flow hydraulics and erosion

Rock fragments can act as a controlling factor for erosional rates and patterns in the landscape. Thus, the objective of this study is to better understand the role that rock fragments incorporated into the soil matrix play in concentrated flow hydraulics and erosion. Laboratory flume experiments were conducted with soil material that was mixed with rock fragments. Rock fragment content ranged from 0 to 40 per cent by volume. Other treatments were slope (7 and 14%) and flow discharge (5·7 and 11·4 l min^?1). An increase in rock fragment content resulted in lower sediment yield, and broader width of flow. Rock fragment cover at the soil surface, i.e. surface armour, increased with time in experiments with rock fragments. Flow energy was largely dissipated by rock fragment cover. For more turbulent flow conditions, when roughness elements were submerged in the flow, hydraulic roughness was similar for different rock fragment contents. In experiments with few or no rock fragments a narrow rill incised. Flow energy was dissipated by headcuts. Total sediment yield was much larger than for experiments with rock fragments in the soil. Adding just a small number of rock fragments in the soil matrix resulted in a significant reduction of sediment yield. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of rock fragment cover on hydrological processes under rainfall simulation in a semi‐arid region of China

Rock fragment cover has long been an important agricultural crop production technique on the Loess Plateau, China. Although this approach plays an important role in controlling hydrological processes and preventing soil erosion, inconsistent results have been recovered in this field. In this study, we investigated the effects of rock fragment cover on infiltration, run‐off, soil erosion, and hydraulic parameters using rainfall simulation in the field in a semi‐arid region of China. Two field plots encompassing 6 rock fragment coverages (0%, 10%, 20%, 25%, 30%, and 40%), as well as 2 rock fragment positions and sizes were exposed to rainfall at a particular intensity (60 mm h^?1). The results of this study showed that increasing the rock fragment coverage with rock fragments resting on the soil surface increased infiltration but decreased run‐off generation and sediment yield. A contrasting result was found, however, when rock fragments were partially embedded into the soil surface; in this case, a positive relationship between rock fragment coverage and run‐off rate as well as a nonmonotonic relationship with respect to soil loss rate was recovered. The size of rock fragments also exerted a positive effect on run‐off generation and sediment yield but had a negative effect on infiltration. At the same time, both mean flow velocity and Froude number decreased with increasing rock fragment coverage regardless of rock fragment position and size, whereas both Manning roughness and Darcy–Weisbach friction factor were positively correlated. Results show that stream power is the most sensitive hydraulic parameter affecting soil loss. Combined with variance analysis, we concluded that the order of significance of rock fragment cover variables was position followed by coverage and then size. We also quantitatively incorporated the effects of rock fragment cover on soil loss via the C and K factors in the Revised Universal Soil Loss Equation. Overall, this study will enable the development of more accurate modelling approaches and lead to a better understanding of hydrological processes under rock fragment cover conditions.     

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 grain-size distribution and shape on sediment bed stability,near-bed flow and bed microstructure

Different studies investigating the stability of mixed sediment have found that the fine fraction can either stabilize or mobilize the bed. This study aims to find where the transition between these two modes occurs for sandy sediment and to identify the underlying (grain-scale) processes. Flume experiments with bimodal sediment were used to investigate near-bed processes of a non-cohesive sediment bed, and in particular how the grain shape and the ratio of different grain sizes influence bed mobility. Medium sand (D_50,c ≈ 400 μm) was mixed with 40 % fine material of different diameters (D_50,f = 53; 111; 193 μm) and subjected to increasing flow velocities (U = 1.3–22.2 cm s^-1). The bed mobility (i.e. the change of the bed level over time), turbidity and near-bed hydrodynamics were analysed. Selected results were compared with similar previous experiments with spherical glass beads. The findings indicate that, due to the complex grain shapes of natural sediment, a sand bed is more stable than a bed composed of glass beads. The grain-size ratio RD = D_c /D_f between the coarse and fine grain diameters controls whether the mixed bed is stabilized or mobilized by the presence of fines, with the transition between the modes occurring at RD = 4–5.5. Mixed beds with a very low RD < 2 behave like a unimodal bed. The results suggest that RD and grain shape influence bed roughness, near-bed flow, bed microstructure and the flow into and through the upper bed layers, which subsequently governs bed mobility. The interplay between all these processes can explain the transition between the stabilizing effect (high RD, small pore space) and the mobilizing effect (low RD, large pore space) of a fine fraction in a grain-size mixture. © 2018 John Wiley & Sons, Ltd.     

Characteristics of concentrated flow hydraulics for rangeland ecosystems: implications for hydrologic modeling

Concentrated flow is often the dominant source of water erosion following disturbance on rangelands. Because of the lack of studies that explain the hydraulics of concentrated flow on rangelands, cropland‐based equations have typically been used for rangeland hydrology and erosion modeling, leading to less accurate predictions due to different soil and vegetation cover characteristics. This study investigates the hydraulics of concentrated flow using unconfined field experimental data over diverse rangeland landscapes within the Great Basin Region, United States. The results imply that the overall hydraulics of concentrated flow on rangelands differ significantly from those of cropland rills. Concentrated flow hydraulics on rangelands are largely controlled by the amount of cover or bare soil and hillslope angle. New predictive equations for concentrated flow velocity (R² = 0·47), hydraulic friction (R² = 0·52), and width (R² = 0·4) representing a diverse set of rangeland environments were developed. The resulting equations are applicable across a wide span of ecological sites, soils, slopes, and vegetation and ground cover conditions and can be used by physically‐based rangeland hydrology and erosion models to estimate rangeland concentrated flow hydraulic parameters. Published in 2011. This article is a US Government work and is in the public domain in the USA.     

Mining soil databases for landscape‐scale patterns in the abundance and size distribution of hillslope rock fragments

Landscape‐scale variation in rock fragments on soil‐mantled hillslopes is poorly understood, despite the potential importance of rock fragments in soil weathering and coarse sediment supply to river networks. We explored the utility of soil survey databases for data mining, with the goals of identifying landscape‐scale patterns in the abundance and size distribution of rock fragments (diameter D > 2 mm) and potential controls on grain size production. We focus on data from three regions: the Hawaiian Islands, and the Sierra Nevada and Cascade Mountains, where elevation transects span a range of environmental conditions. We selected pedons from pits dug on hillslopes with active soil production and transport. For the 27 pedons selected, we constructed depth‐averaged grain size distributions and calculated the mass fraction of rock fragments (F_RF) and the median rock fragment grain size (D_50RF). We also categorized as bimodal, size distributions with a clear ‘breakpoint’ between fine and coarse modes. Several strong patterns emerge from the data. We find rock fragments in 85% of the pedons, primarily in distinct coarse modes within bimodal size distributions. Values of F_RF and D_50RF are strongly correlated, although the best‐fit power law scaling between F_RF and D_50RF differs between the warmer Hawaiian, and colder Sierra Nevada and Cascade Mountain sites. We also find a regional contrast in the variation in F_RF with elevation; F_RF declines with elevation in Hawaii, but increases in the mainland sites. Although this contrast could be an artifact of variable lithology, precipitation may influence many patterns in the data. Lower mean‐annual precipitation correlates with higher F_RF, dominantly bimodal distributions and surface enrichment in the vertical distribution of rock fragments. These observations may be useful in refining models of coarse sediment supply to rivers, and suggest opportunities for future work to test mechanistic hypotheses for rock fragment production on soil‐mantled hillslopes. Copyright © 2011 John Wiley & Sons, Ltd.     

Rapid assessment of beta dose variation inside cobbles,and implications for rock luminescence dating

Variation in beta dose rate within rocks may impact the results of rock surface luminescence dating, for both the burial age of cobbles and exposure age of rock surfaces. Current methods of rock surface luminescence dating assume that radionuclides are homogeneously distributed inside rocks. In this study, two rapid methods based on beta counting and on a portable XRF instrument were developed to measure the radioactivity of rock slices. These methods were applied to rock slices from four glaciofluvial granite cobbles that had previously been used for equivalent dose determination to test whether beta dose variation could be observed. Results from beta counting and K content from XRF show similar patterns and both vary along the depth profiles, but the magnitude of this variability is very different amongst the four cobbles. In rocks where the dose rate is highly variable, bleaching may not be the only source of variation of L_n/T_n or equivalent dose (D_e) along the luminescence-depth profile of cobbles, and it may be necessary to measure the beta dose rate for every single slice to determine whether multiple bleaching events are recorded or variations in D_e are due to dose rate heterogeneity.     

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

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

Sediment particle selectivity and its response to overland flow hydraulics within grass strips

Particle selectivity plays an important role in clarifying sediment transport processes in vegetative filter strips (VFS). 10-m long grass strips at slopes of 5° and 15° were subjected to a series of sediment-laden inflows experiments with different particle sizes to investigate the sediment transport and its response to overland flow hydraulics. The inflow sediments came from local soil, river-bed sand, and mixed, with median particle size d₅₀ of 39.9, 207.9 and 77.4 μm, respectively. Three independent repeated experiments were carried for each treatment. The results show that when the sediment trapping lasted for a certain length of time, the re-entrainment of some small-sized particles was greater than the deposition; that is, net loss occurred, which was not erosion of the original soil. Net loss of particles is mainly determined by the particle diameter. The coarser the inflow sediment particles and/or the steeper the slope, the coarser the particles can be net lost. Deposited sediment causes the VFS bed surface to become smooth and hydraulic resistance decrease exponentially. Unit stream power P is more suitable than shear stress τ of overland flow to be used to describe the process of sediment particle transport in VFS. The relationship between P and d₅₀ of outflow sediment is very consistent with the form of power function with a constant term. These results are helpful to understand the physical process of sediment transport on vegetation hillslopes.     

Comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel‐bed rivers

Several methods were employed in the Ardennian rivers (Belgium) to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events. The use of scour chains has shown that the depth of the active layer is systematically less than the b‐axis of the average particle size (D₅₀) of the elements which compose the surface layer of the riffles. This indicates that only a partial transport exists during low magnitude floods. The bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers. Quantities of sediment transported during frequent floods are relatively low (0·02 t km^–2) due to the armour layer which protects the subsurface material. These low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with PIT (passive integrated transponder)‐tags (diameter > 20 mm), whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles. The break‐up of the armour layer was observed only once, for a decennial discharge. During this event, the bedload discharge increased considerably (2 t km^–2). The use of sediment traps, data from dredging and a Helley–Smith sampler confirm the low bedload transport in Ardennian rivers in comparison to the bedload transport in other geomorphological contexts. This difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow. Finally, the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries. In the Lembrée River, the river‐bed contains slag elements up to a depth of about 50 cm, indicating that exceptional floods may rework the bed to a considerable depth. Copyright © 2012 John Wiley & Sons, Ltd.     

Influences of coupled wind–water processes on suspended sediment grain size: an example from tributaries of the Yellow River / Influences de processus couplés éoliens-hydriques sur la granulométrie des matières en suspension: exemple d'affluents du Fleuve Jaune

Abstract

Some unique coupled wind–water erosion processes exist in the desert-loess transitional zone in the middle Yellow River basin. Based on data from 40 stations on 29 rivers, a study was made on the influence of such processes on suspended sediment grain-size characteristics of the tributaries of the Yellow River. Results show that the percentage of >0.05-mm grain size decreases with the increased annual mean precipitation, but increases with the increase in the annual mean number of sand-dust storm days. The percentage of <0.01-mm grain size increases with the increase in the annual mean precipitation, but decreases with the increase in the annual number of sand-dust storm days. Based on annual mean data from 40 stations, multiple regression equations were established between the percentages of >0.05-mm grain size (r _>0.05) and <0.01-mm grain size (r _<0.01), annual mean precipitation (P _m) and annual mean number of sand-dust storm days (D _ss). On this basis, the relative contributions of the variations in D _ss and P _m to the variation in r _>0.05 and r _<0.01 were estimated. The results indicate that the variation in sand-dust storm frequency exerts greater influences on the variation in grain-size characteristics of suspended load than does the variation in annual mean precipitation. With the increase in the coupled wind–water processes index, expressed by P _m/D _ss, the percentage of >0.05-mm grain size in suspended sediment decreases and the percentage of <0.01-mm grain size increases. With the variation in P _m/D _ss, different combinations of r _>0.05 with r _<0.01 appear, which have some influence on the formation of hyperconcentrated flows. There exist some optimal ratios of coarse to fine fractions in suspended sediment that make sediment concentrations of hyperconcentrated flows the highest. The optimal r _>0.05/r _<0.01 value is related to some range of the index P _m/D _ss. When the P _m/D _ss index falls in this range, the optimum combination of relative coarse with fine sediments in the suspended load appears, and thus results in the peak values of sediment concentration.     

Field evidence for the control of grain size and sediment supply on steady‐state bedrock river channel slopes in a tectonically active setting

We exploit a natural experiment in Boulder Creek, a ~ 30 km² drainage in the Santa Cruz mountains, CA, USA to explore how an abrupt increase in the caliber of bedload sediment along a bedrock channel influences channel morphology in an actively uplifting landscape. Boulder Creek's bedrock channel, which is entirely developed on weak sedimentary rock, has a high flow shear stress that is about 3.5 times greater where it transports coarse (~ 22 cm D₅₀) diorite in the lower reaches in comparison with the upstream section of the creek that transports only relatively finer bedload (~2 cm D₅₀) derived from weak sedimentary rocks. In addition, Boulder Creek's channel abruptly widens and shallows downstream and transitions from partial to nearly continuous alluvial cover where it begins transporting coarse diorite. Boulder Creek's tributary channels are also about three times steeper where they transport diorite bedload, and within the Santa Cruz mountains channels in sedimentary bedrock are systematically steeper when >50% of their catchment area is within crystalline basement rocks. Despite this clear control of coarse sediment size on channel slopes, the threshold of motion stress for bedload, alone, does not appear to control channel profile slopes here. Upper Boulder Creek, which is starved of coarse sediment, maintains high flow shear stresses well in excess of the threshold for motion. In contrast, lower Boulder Creek, with a greater coarse sediment supply, exerts high flow stresses much closer to the threshold for motion. We speculate that upper Boulder Creek has evolved to sustain partial alluvial cover and transfer greater energy to the bed via bedload impacts to compensate for its low coarse sediment supply. Thus bedload supply, bedrock erosion efficiency, and grain size all appear to influence channel slopes here. Copyright © 2017 John Wiley & Sons, Ltd.     

Displacement and transport of marked pebbles,cobbles and boulders during floods in a steep mountain stream

In a small experimental catchment of the Dolomites (Rio Cordon, 5 km²) field observations have been carried out on the movement of various sized bed material particles. Displacement length of 860 marked pebbles, cobbles and boulders (0·032 < D < 0·512 m) has been measured along the river bed during individual snowmelt and flood events in the periods 1993–1994 and 1996–1998. Floods were grouped into two categories. The first includes ‘ordinary’ events, which are characterized by peak discharges with a return period of 1–5 years and by an hourly bedload rate not exceeding 20 m³ h^?1. The second refers to ‘exceptional’ events with a return period of 50–60 years. A flood of this latter type occurred on 14 September 1994, with a peak discharge of 10·4 m³ s^?1 and average hourly bedload rate of 324 m³ h^?1. The variation according to grain size of total displacement length L_i depends on the degree of mobilization of the individual fractions of the bed surface: L_i is independent of D_i for smaller, fully mobile grain sizes and decreases rapidly with D_i for larger fractions in a state of partial transport. Sustained selective transport without a supply of sediment from upstream leads to the development of a stable coarse armoured surface through progressive winnowing of finer material from the bed surface. With supply unlimited conditions for transport, both the occurrence of extreme events and the duration of a sequences of ‘ordinary’ floods play an important role in the degree of mobilization of the individual fractions of the bed. Copyright © 2004 John Wiley & Sons, Ltd.     

EXPERIMENTAL INVESTIGATION AND ANALYSIS OF HEC-6 RIVER MORPHOLOGICAL MODEL

Only comparatively few experimental studies have been carried out to investigate the performance of the HEC-6 river morphological model. The model was developed by the Hydrologic Engineering Center of the US Army Corps of Engineers. In this study, experiments were carried out in a 20 m long concrete flume 0.6 m wide with varying rectangular cross-sections. The channel bed is paved with uniform sand of D₅₀ = 0.9 mm and D₉₀ = 1.2 mm within the test reach of 12 m. Two types of experiments were carried out with sediment transport, one under steady uniform flow and another under steady non-uniform flow conditions. Nine steady uniform flow experiments were carried out to compare the measured equilibrium relationship of flow and sediment transport rate with two bedload formulae, namely, Du Boys and Meyer–Peter and Muller, and with three total load formulae, namely, Toffaleti, Laursen and Yang. It was found that even though the sediment transport consists of a certain portion of bedload, the total load formulae give satisfactory results and better agreement than the two bedload formulae. Five steady non-uniform flow experiments were carried out under various conditions of varying bed profile and channel width and also with sediment addition and withdrawal. The measured transient water surface and bed profiles are compared with the computed results from the HEC-6 model. It was found that the Toffaleti and Yang total load formulae used in the HEC-6 model give the most satisfactory prediction of actual bed profiles under various conditions of non-uniform flow and sediment transport. The effects of Manning's n, variations of sediment inflow, various sediment transport formulae, sediment grain size and the model numerical parameters, i.e. distance interval Δx and numerical weighting factor, on the computed water surface and bed profiles were determined. It was found that the selection of the sediment transport formulae has the most significant effect on the computed results. It can be concluded that the HEC-6 model can predict satisfactorily a long-term average pattern of local scour and deposition along a channel with either a small abrupt change in geometry or gradually varying cross-sections. However, the accuracy of the model prediction is reduced in the regions where highly non-uniform flow occurs.     

Experiment on morphological and hydraulic adjustments of step-pool unit to flow increase

Exceptional flood events with a return period of about 50 years can be destructive to step-pool channel segments. However, field investigations and flume experiments have not examined the hydraulic and morphological feedbacks of step-pool morphology during unsteady hydrographs of exceptional flood events. We performed a series of flume experiments with a manually constructed step model, perturbed with three hydrographs that varied in the rate of water supply change. The bed texture, topography, flow regimes, surface flow field and water depth were characterized and measured as the flow rate was increased during the experiments. A distinct pool feature emerged downstream of the manually constructed step when the flow rate exceeded the threshold scaled to the peaks of ordinary flood events in well-graded mountain streams. The pool feature was modified in several different ways with flow rate increase. The bed surface steadily coarsened, micro-bedforms developed and became more pronounced, the bed topography became more spatially complex based on analysis using the Hurst exponent, and last, pool depth steadily increased. Pool modification was also linked to the flow regime: the impinging jet regime led to grain size segmentation in the pool while the jump regime contributed to decelerating flow velocity. The steeper rising limb of hydrograph led to a less developed pool feature, with smaller sized micro-bedforms in the pool bottom to outlet, and higher discharge threshold for distinct coarsening and scouring in the pool. The estimated energy dissipation within the step-pool unit decreased as a power function from low to high flow, quantified as the ratio h_c/H_S, where h_c is the critical water depth and H_S is scour depth. Our results highlight the interaction between morphology, hydraulics, and energy dissipation of step-pool unit and the crucial role of hydrograph shape on the interaction during flow increase © 2019 John Wiley & Sons, Ltd.     

The differential impact of some soil loss factors on flow,runoff creep and rainwash

The Hortonian model of runoff flow which had been thought to be applicable in arid areas has previously been shown not to be valid, notably in Israel, where inverse relations have been observed between slope angle, and runoff discharge and slope erosion. The paper discusses laboratory experiments on simulated slope conditions in a rather arid environment. It is shown by rain simulation on granite grus that infiltration capacity is a function of rainfall intensity, slope angle and runoff discharge. The infiltration capacity f can equal the rainfall intensity beyond a critical distance x(m) so that discharge becomes constant. Debris covers affect runoff hydraulics, especially on poorly cohesive soils, and both slow downslope and upslope movements which correspond to the process of so-called runoff creep can occur. Coarse debris and grass covers, as roughness factors, induce hydraulic discontinuities and activate local turbulent flow and slope erosion. Instead of being merely protective elements these factors tend to catalyze the slope wash, in comparison with naked surfaces, if the Reynolds number of the flow exceeds a certain critical value.     

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.     

Coarse bed load transport in an alluvial gravel bed stream,Dupuyer Creek,Montana

Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank‐full, and bed load was sampled at flows 0·7–1·7 times bank‐full. A large aperture bed load sampler (1 m by 0·45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D₂₅ of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0·85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D₇₀ and larger (88–155 mm), while particles in the range D₃₀–D₇₀ (35–88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under‐represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0·75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright © 2007 John Wiley & Sons, Ltd.     

The hydrological response of soil surfaces to rainfall as affected by cover and position of rock fragments in the top layer

Rainfall experiments have been conducted in the laboratory in order to assess the hydrological response of top soils very susceptible to surface sealing and containing rock fragments in different positions with respect to the soil surface. For a given cover level, rock fragment position in the top soil has an ambivalent effect on water intake and runoff generation. Compared to a bare soil surface rock fragments increase water intake rates as well as time of runoff concentration and decrease runoff volume if they rest on the soil surface. For the same cover level, rock fragments reduce infiltration rate and enhance runoff generation if they are well embedded in the top layer. The effects of rock fragment position on infiltration rate and runoff generation are proportional to cover percentage. Micromorphological analysis and measurements of the saturated hydraulic conductivity of bare top soils and of the top layer underneath rock fragments resting on the soil surface reveal significant differences supporting the mechanism proposed by Poesen (1986): i.e. runoff generated as rock flow or as Horton overland flow can (partly) infiltrate into the unsealed soil surface under the rock fragments, provided that they are not completely embedded in the top layer. Hence, rock fragment position, beside other rock fragment properties, should be taken into account when assessing the hydrological response of soils susceptible to surface sealing and containing rock fragments in their surface layers. A simple model, based on the proportions of bare soil surface, soil surface occupied by embedded rock fragments, and soil surface covered with rock fragments resting on the soil surface, describes the runoff coefficient data relatively well.     

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.