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.     

Prediction of concentrated flow width in ephemeral gully channels

Empirical prediction equations of the form W = aQ^b have been reported for rills and rivers, but not for ephemeral gullies. In this study six experimental data sets are used to establish a relationship between channel width (W, m) and flow discharge (Q, m³ s^?1) for ephemeral gullies formed on cropland. The resulting regression equation (W = 2·51 Q^0·412; R² = 0·72; n = 67) predicts observed channel width reasonably well. Owing to logistic limitations related to the respective experimental set ups, only relatively small runoff discharges (i.e. Q < 0·02 m³s^?1) were covered. Using field data, where measured ephemeral gully channel width was attributed to a calculated peak runoff discharge on sealed cropland, the application field of the regression equation was extended towards larger discharges (i.e. 5 × 10^?4m³s^?1 < Q < 0·1 m³s^?1). Comparing W–Q relationships for concentrated flow channels revealed that the discharge exponent (b) varies from 0·3 for rills over 0·4 for gullies to 0·5 for rivers. This shift in b may be the result of: (i) differences in flow shear stress distribution over the wetted perimeter between rills, gullies and rivers, (ii) a decrease in probability of a channel formed in soil material with uniform erosion resistance from rills over gullies to rivers and (iii) a decrease in average surface slope from rills over gullies to rivers. The proposed W–Q equation for ephemeral gullies is valid for (sealed) cropland with no significant change in erosion resistance with depth. Two examples illustrate limitations of the W–Q approach. In a first example, vertical erosion is hindered by a frozen subsoil. The second example relates to a typical summer situation where the soil moisture profile of an agricultural field makes the top 0·02 m five times more erodible than the underlying soil material. For both cases observed W values are larger than those predicted by the established channel width equation for concentrated flow on cropland. For the frozen soils the equation W = 3·17 Q^0·368 (R² = 0·78; n = 617) was established, but for the summer soils no equation could be established. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Soil erosion by surface water flow on a stony,semiarid hillslope

Soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows. This study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment. A field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface. The method allowed for measurements in this study of flow erosion at a much wider range of slopes (2·6 to 30·1 per cent) and unit discharge rates (0·0007 to 0·007 m2 s−1) than have been previously feasible. Flow velocities were correlated to discharge and hydraulic radius, but not to slope. The lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity. The detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected. Flow detachment rates were better correlated to a power function of either shear stress (r2 = 0·51) or stream power (r2 = 0·59). Published in 1999 by John Wiley & Sons, Ltd.     

Effects of microbiotic crusts under cropland in temperate environments on soil erodibility during concentrated flow

Several studies illustrate the wind and water erosion‐reducing potential of semi‐permanent microbiotic soil crusts in arid and semi‐arid desert environments. In contrast, little is hitherto known on these biological crusts on cropland soils in temperate environments where they are annually destroyed by tillage and quickly regenerate thereafter. This study attempts to fill the research gap through (a) a field survey assessing the occurrence of biological soil crusts on loess‐derived soils in central Belgium in space and time and (b) laboratory flume (2 m long) experiments simulating concentrated runoff on undisturbed topsoil samples (0.4 × 0.1 m²) quantifying the microbiotic crust effect on soil erosion rates. Three stages of microbiotic crust development on cropland soils are distinguished: (1) development of a non‐biological surface seal by raindrop impact, (2) colonization of the soil by algae and gradual development of a continuous algal mat and (3) establishment of a well‐developed microbiotic crust with moss plants as the dominant life‐form. As the silt loam soils in the study area seal quickly after tillage, microbiotic soil crusts are more or less present during a large part of the year under maize, sugar beet and wheat, representing the main cropland area. On average, the early‐successional algae‐dominated crusts of stage 2 reduce soil detachment rates by 37%, whereas the well‐developed moss mat of stage 3 causes an average reduction of 79%. Relative soil detachment rates of soil surfaces with microbiotic crusts compared with bare sealed soil surfaces are shown to decrease exponentially with increasing microbiotic cover (b = 0·024 for moss‐dominated and b = 0·006 for algae‐dominated crusts). In addition to ground surface cover by vegetation and crop residues, microbiotic crust occurrence can therefore not be neglected when modelling small‐scale spatial and temporal variations in soil loss by concentrated flow erosion on cropland soils in temperate environments. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of non‐uniformity of flow on velocity and turbulence intensities over a cobble‐bed

Non‐uniform flows encompassing both accelerating and decelerating flows over a cobble‐bed flume have been experimentally investigated in a flume at a scale of intermediate relative submergence. Measurements of mean longitudinal flow velocity u, and determinations of turbulence intensities u′, v′, w′, and Reynolds shear stress ?u_fw_f have been made. The longitudinal velocity distribution was divided into the inner zone close to the bed and the outer zone far from the bed. In the inner zone of the boundary layer (near the bed) the velocity profile closely followed the ‘Log Law’; however, in the outer zone the velocity distribution deviated from the Log Law consistently for both accelerating and decelerating flows and the changes in bed slopes ranging from ?2% to + 2% had no considerable effect on the outer zone. For a constant bed slope (S = ±0·015), the larger the flow rate, the smaller the turbulence intensities. However, no detectable pattern has been observed for u′, v′ and w′ distributions near the bed. Likewise, for a constant flow rate (Q = 0·040 m³/s), with variation in bed slope the longitudinal turbulent intensity profile in the longitudinal direction remained concave for both accelerating and decelerating flows; whereas vertical turbulent intensity (w′) profile presented no specific form. The results reveal that the positions of maximum values of turbulence intensities and the Reynolds shear stress depend not only on the flow structure (accelerating or decelerating) but also on the intermediate relative submergence scale. Copyright © 2009 John Wiley & Sons, Ltd.     

Controls on at‐a‐station hydraulic geometry in steep headwater streams,Colorado, USA

Detailed hydraulic measurements were made in nine step‐pool, five cascade and one plane‐bed reach in Fraser Experimental Forest, Colorado to better understand at‐a‐station hydraulic geometry (AHG) relations in these channel types. Average values for AHG exponents, m (0·49), f (0·39), and b (0·16), were well within the range found by other researchers working in steep gradient channels. A principal component analysis (PCA) was used to compare the combined variations in all three exponents against five potential control variables: wood, D₈₄, grain‐size distribution (σ), coefficient of variation of pool volume, average roughness‐area (projected wetted area) and bed gradient. The gradient and average roughness‐area were found to be significantly related to the PCA axis scores, indicating that both driving and resisting forces influence the rates of change of velocity, depth and width with discharge. Further analysis of the exponents showed that reaches with m > b + f are most likely dominated by grain resistance and reaches below this value (m < b + f) are dominated by form resistance. Copyright © 2010 John Wiley & Sons, Ltd.     

Measurements and estimation of flow velocity in mobile bed rills

Rills are primary sediment sources and hillslope water/sediment runoff transport channels. Water flow velocities in rills are easily affected by bed condition over eroding and non-eroding slopes, which is an important hydrodynamic process in soil erosion research. This research is done to demonstrate the poorly understood “feedback mechanism” related to slope independence of flow velocity to slope gradient. A series of experiments were done on silt loam soil slopes to measure water flow velocity...     

Rill erosion along the thalweg of a hillslope hollow: A case study from the cotswold hills,Central England

Rill erosion is an important erosional form on agricultural soils in England, causing large losses of soil, particularly on cultivated slopes. This paper describes a rill system that developed in a small agricultural catchment in north Oxfordshire during the winter of 1992–93. The rill system comprised two components: a system of ‘feeder rills’ along the valley-side slopes, which were the result of flow concentration and erosion along wheelings, and a thalweg rill, which formed along a dry valley bottom as a result of surface runoff concentration from the feeder rills. Total volumetric soil loss from the rill system was 32·28 m³, equivalent to 3·01 m³, ha^?1 for the rill catchment area, or 3·91 t ha^?1. Mean discharge for the thalweg rill and feeder rills, calculated during a storm event, was 31·101s^?1 and 1·171s^?1, respectively. All flows were fully turbulent and supercritical. We emphasize the need for a spatially distributed approach to the study of runoff and erosion at the catchment scale.     

Controls on effective settling velocities of mud flocs in the Changjiang Estuary,China

Theoretical and experimental studies were undertaken to gain insight into physical parameters controlling the flocculation and settling properties of mud flocs in the Changjiang Estuary, China. The Rouse equation is applied to vertical profiles of suspended sediment concentration to determine the bulk mean settling velocity (w_s) of sediment suspended in the Changjiang Estuary. Both in situ point‐sampled and acoustically measured profiles of suspended mud concentrations were fit selectively. The calculated settling velocities w_s mainly ranged from 0·4 to 4·1 mm s^?1 for the point‐sampled data set, and from 1·0 to 3·0 mm s^?1 for the acoustically measured data set. Furthermore, the settling velocities of mud flocs increased with mean concentration (C?) of mud flocs in suspension and were proportional to increasing bottom shear stress (τ_b) of tidal flow. The best equation for the field settling velocity of mud flocs in the Changjiang Estuary can be expressed by the power law: w_s = mC?ⁿ (m, 1·14–2·37; n, 0·84–1·03). It is suggested that C? and τ_b were the dominant physical parameters controlling the flocculation and w_s of mud flocs in suspension. Copyright © 2004 John Wiley & Sons, Ltd.     

Estimating the mean speed of laminar overland flow using dye injection‐uncertainty on rough surfaces

A common method for estimating mean flow speeds in studies of surface runoff is to time the travel of a dye cloud across a measured flow path. Motion of the dye front reflects the surface flow speed, and a correction must be employed to derive a value for the profile mean speed, which is always lower. Whilst laminar flow conditions are widespread in the interrill zone, few data are available with which to establish the relationship linking surface and profile mean speeds, and there are virtually none for the flow range 100 < Re < 500 (Re = Reynolds number) which is studied here. In laboratory experiments on a glued sand board, mean flow speeds were estimated from both dye speeds and the volumetric flow relation v = Q/ wd with d measured using a computer‐controlled needle gauge at 64 points. In order to simulate conditions applicable to many dryland soils, the board was also roughened with plant litter and with ceramic tiles (to simulate surface stone cover). Results demonstrate that in the range 100 < Re < 500, there is no consistent relation between surface flow speeds and the profile mean. The mean relationship is v = 0·56 v _surf, which departs significantly from the theoretical smooth‐surface relation v = 0·67 v _surf, and exhibits a considerable scatter of values that show a dependence on flow depth. Given the inapplicability of any fixed conversion factor, and the dependence on flow depth, it is suggested that the use of dye timing as a method for estimating v be abandoned in favour of precision depth measurement and the use of the relation v = Q/ wd , at least within the laminar flow range tested. Copyright © 2001 John Wiley & Sons, Ltd.     

Empirical predictors of annual bed load travel distance,and implications for salmonid habitat restoration and protection

Measurements of annual travel distance (L_b) of bed load sediment at 16 locations in Alaska, the intermountain USA, west coast USA and Scotland are strongly correlated with bankfull channel width (r² = 0·86, p < 0·001). Travel distance of particles is probably limited by trapping in bars, which have a longitudinal spacing proportional to channel width. Increased abundance of woody debris reduces bar spacing and may reduce L_b. Longer cumulative duration of bed load transporting flows in a year appears to increase L_b. Other predictors of annual travel distance such as stream power per unit length, drainage area and bankfull discharge were less well correlated with L_b (r² ranging from 0·27 to 0·51). Stream power per unit bed area, basal shear stress and slope were not significantly related to L_b (r² < 0·05). Most correlations were improved when regressions were limited to data from the west coast USA. Travel distance estimates can be used to help identify reaches that may take longer to recover from large, short‐term increases in sediment supply. Published in 2001 by John Wiley & Sons, Ltd.     

Relationship between discharge,velocity and flow area for rills eroding loose,non-layered materials

A relationship between discharge, flow velocity and flow area in rills is established using data from four field and laboratory studies. The proposed relationship is shown to predict successfully flow velocities measured in six other studies. Although slopes range from 0.035 to 0.45 and soil materials range from stony sands over silt loams to vertisols, mean flow velocity can be well predicted from discharge alone. Thus, there is no important influence of slope and/or soil material characteristics on flow velocities in rills. The proposed relationship may be used to improve performance of deterministic flow routing models when applied to rilled catchments. Furthermore, it allows the calculation of unit stream power, which has been shown to be related to the transporting capacity of overland flow, in terms of slope and discharge.     

Hydrodynamic characteristics of rill flow on steep slopes

Rill erosion is a dominant sediment source on sloping lands. However, the amount of soil loss from rills on steep slopes is vastly more than that on gentle slopes because of differences in rill shape and hydraulic patterns. The aims of this paper are to determine the hydrodynamic characteristics of rills and the friction coefficients in steep slope conditions and to propose modifications of some hydraulic parameters used in soil loss prediction models. A series of inflow experiments was conducted on loess slopes. The results show that the geometric and hydraulic properties of rill on the steep loess slopes, which are characterized by the mean width of cross sections, mean velocity and mean depth of flow, are related to discharge and slope gradient in power functions. However, the related exponents to discharge are 0.26, 0.48 and 0.26, respectively, which are different from the exponents derived in previous studies, which were conducted on gentle slopes. The Manning roughness coefficient ranged from 0.035 to 0.071, with an average of 0.0536, and the Darcy–Weisbach friction coefficients varied from 0.4 to 1.9. The roughness coefficients are closely related to the Reynolds numbers and flow volumes; however, the correlations vary with slope gradient. The roughness coefficients are directly proportional to the Reynolds number and the flow volume on steep slopes, in contrast with the roughness coefficients found on gentle slopes, which decrease as the Reynolds number and flow volume increase. This difference is caused by the interactions among the hydraulics of the flow, the shape of the rills and the sediment concentrations on steep slopes. The results indicate that parameters used in models to predict rill erosion have to be modified according to slope gradient. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrological processes controlling ground and surface water flow from a hypermaritime forest–peatland complex,Diana Lake Provincial Park,British Columbia,Canada

The proposed harvesting of previously undeveloped forests in north coastal British Columbia requires an understanding of hydrological responses. Hydrometric and isotopic techniques were used to examine the hydrological linkages between meteoric inputs to the surface‐groundwater system and runoff response patterns of a forest‐peatland complex. Quickflow accounted for 72–91% of peak storm discharge. The runoff ratio was lowest for open peatland areas with thick organic horizons (0·02–0·05) due to low topographic gradients and many surface depressions capable of retaining surface water. Runoff ratio increased comparatively for ephemeral surface seep flows (0·06–0·40) and was greatest in steeply sloping forest communities with more permeable soils (0·33–0·69). The dominant mechanism for runoff generation was saturated shallow subsurface flow. Groundwater fluxes from the organic horizon of seeps (1·70–1·72 m³ day^?1 m^?1) were an important component of quickflow. The homogeneous δ²H? δ¹⁸O composition of groundwater indicated attenuation of the seasonal rainfall signal by mixing during recharge. The positive correlation (r² = 0·64 and 0·38, α = 0·05) between slope index and δ¹⁸O values in groundwater suggests that the spatial pattern in the δ¹⁸O composition along the forest‐peatland complex is influenced by topography and provides evidence that topographic indices may be used to predict groundwater residence time. Copyright © 2006 John Wiley & Sons, Ltd.     

An upland soil erosion model derived from the basic principles

Starting from the basic erosion principles, an upland soil erosion model to predict soil loss by overland flow from individual storms on forested hillslopes can be derived in the form where Q_s is total soil loss for a storm event, n is roughness coefficient, x is down slope distance, K_f is soil erodibility factor, S is slope, α is slope exponent and Q is runoff. Values of n and α are to be determined for different environments and are 0·58 and 2·1 for a mixed pine forest ecosystem. A significant correlation (r = 0·933, n = 96) fits between the observed and predicted values using this expression, and the model fitting is good.     

Impacts of sediment load on Manning coefficient in supercritical shallow flow on steep slopes

The Manning equation is one of the most widely used formulae for calculating the velocity of shallow overland flow in hydrological and erosion models. Precise estimation of the Manning's friction coefficient (n) is critical to determining overland flow and soil erosion processes. Few studies have been conducted to quantify the effects of sediment load on Manning's n on steep slopes. This study was conducted to investigate the potential effects of sediment load on Manning's n in a flume with a fixed bed, under wide ranges of hydraulics and sediment loads. Slope gradient varied from 8·7 to 34·2%, unit flow rate from 0·66 to 5·26 × 10^?3 m² s^?1, and sediment load from 0 to 6·95 kg m^?1 s^?1. The Reynolds number ranged from 350 to 5899. Results showed that Manning's n varied in both sediment‐free and sediment‐laden flows ranging from 0·012 to 0·055. The apparent Manning's coefficients of sediment‐laden flow were much greater than those of sediment‐free flow. The mean Manning coefficient of sediment‐laden flow was 51·27% greater than the mean value of sediment‐free flow. For sediment‐laden flow, Manning's n could be estimated with a power function of unit flow discharge and sediment content. Further studies are needed to quantify the potential effects of sediment load on the Manning's n on erodible beds and in fields. Copyright © 2010 John Wiley & Sons, Ltd.     

A field study of mean and turbulent flow characteristics upwind,over and downwind of barchan dunes

Field‐measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes. Turbulence was sampled using a high frequency sonic anemometer, deriving near‐surface Reynolds shear and normal stresses. Measurements upwind of and over a crest‐brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe. The mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress. This field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements. Downwind, the effect of different dune profiles on flow re‐attachment and recovery was apparent. With transverse incident flow, a re‐attachment length between 2·3 and 5·0h (h is dune brink height) existed for a crest‐brink separated dune and 6·5 to 8·6h for a crest‐brink coincident dune. The lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes, and a decrease in turbulence with distance characterized flow recovery. Recovery of mean velocity for the crest‐brink separated dune occurred over a distance 6·5h shorter than the crest‐brink coincident form. As the application of sonic anemometers in aeolian geomorphology is relatively new, there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle. This paper demonstrates the effect on Reynolds stresses of mathematically correcting data to the local streamline over varying dune slope. Where the streamline angle was closely related to the surface (windward slope), time‐averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines. In the close lee, however, the angle of downwardly projected (separated) flow was not aligned with the flat ground surface. Here, shear stress appeared to be underestimated by streamline correction, and corrected shear stress values were less than half of those uncorrected. Copyright © 2011 John Wiley & Sons, Ltd.     

Interrill erosion on cultivated Greek soils: modelling sediment delivery

For interrill erosion, raindrop‐induced detachment and transport of sediment by rainfall‐disturbed sheet flow are the predominant processes, while detachment by sheet flow and transport by raindrop impact are negligible. In general, interrill subprocesses are inter‐actively affected by rainfall, soil and surface properties. The objective of this work was to study the relationships among interrill runoff and sediment loss and some selected para‐meters, for cultivated soils in central Greece, and also the development of a formula for predicting single storm sediment delivery. Runoff and soil loss measurement field experiments have been conducted for a 3·5‐year period, under natural storms. The soils studied were developed on Tertiary calcareous materials and Quaternary alluvial deposits and were textured from sandy loam to clay. The second group of soils showed greater susceptibility to sealing and erosion than the first group. Single storm sediment loss was mainly affected by rain and runoff erosivity, being significantly correlated with rain kinetic energy (r = 0·64***), its maximum 30‐minute intensity (r = 0·64***) and runoff amount (r = 0·56***). Runoff had the greatest correlation with rain kinetic energy (r = 0·64***). A complementary effect on soil loss was detected between rain kinetic energy and its maximum 30‐minute intensity. The same was true for rain kinetic energy and topsoil aggregate instability, on surface seal formation and thus on infiltration characteristics and overland flow rate. Empirical analysis showed that the following formula can be used for the successful prediction of sediment delivery (D_i): D_i = 0·638βEI₃₀tan(θ) (R² = 0·893***), where β is a topsoil aggregate instability index, E the rain kinetic energy, I₃₀ the maximum 30‐minute rain intensity and θ the slope angle. It describes soil erodibility using a topsoil aggregate instability index, which can be determined easily by a simple laboratory technique, and runoff through the product of this index and rain kinetic energy. Copyright © 2006 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.