Sediment routing hypothesis for pool‐riffle maintenance

This paper provides comprehensive evidence that sediment routing around pools is a key mechanism for pool‐riffle maintenance in sinuous upland gravel‐bed streams. The findings suggest that pools do not require a reversal in energy for them to scour out any accumulated sediments, if little or no sediments are fed into them. A combination of clast tracing using passive integrated transponder (PIT) tagging and bedload traps (positioned along the thalweg on the upstream riffle, pool entrance, pool exit and downstream riffle) are used to provide information on clast pathways and sediment sorting through a single pool‐riffle unit. Computational fluid dynamics (CFD) is also used to explore hydraulic variability and flow pathways. Clast tracing results provide a strong indication that clasts are not fed through pools, rather they are transported across point bar surfaces, or around bar edges (depending upon previous clast position, clast size, and event magnitude). Spatial variations in bedload transport were found throughout the pool‐riffle unit. The pool entrance bedload trap was often found to be empty, when the others had filled, further supporting the notion that little or no sediment was fed into the pool. The pool exit slope trap would occasionally fill with sediment, thought to be sourced from the eroding outer bank. CFD results demonstrate higher pool shear stresses (τ ≈ 140 N m^–2) in a localized zone adjacent to an eroding outer bank, compared to the upstream and downstream riffles (τ ≈ 60 N m^–2) at flows of 6 · 2 m³ s^–1 (≈ 60% of the bankfull discharge) and above. There was marginal evidence for near‐bed velocity reversal. Near‐bed streamlines, produced from velocity vectors indicate that flow paths are diverted over the bar top rather than being fed through the thalweg. Some streamlines appear to brush the outer edge of the pool for the 4 · 9 m³ s^–1 to 7 · 8 m³ s^–1 (between 50 and 80% of the bankfull discharge) simulations, however complete avoidance was found for discharges greater than this. Copyright © 2013 John Wiley & Sons, Ltd.     

The influence of pool length on local turbulence production and energy slope: a flume experiment

The in?uence of pool length on the strength of turbulence generated by vortex shedding was investigated in a 6 m long recirculating ?ume. The experiment utilized a 38% constriction of ?ow and an average channel‐bed slope of 0·007. The base geometry for the intermediate‐length pool experiment originated from a highly simpli?ed, 0·10 scale model of a forced pool from North Saint Vrain Creek, Colorado. Discharge in the ?ume was 31·6 l/s, which corresponds to a discharge in the prototype channel of 10 m³/s. Three shorter and four longer pool lengths also were created with a ?xed bed to determine changes in turbulence intensities and energy slope with pool elongation. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 31–40 different 0·6‐depth and near‐bed locations downstream of the rectangular constriction. The average velocity and root mean square (RMS) of the absolute magnitude of velocity at both depths are signi?cantly related to the distance from the constriction in most pool locations downstream of the constriction. In many locations, pool elongation results in a non‐linear change in turbulence intensities and average velocity. Based on the overall ?ow pattern, the strongest turbulence occurs in the center of the pool along the shear zone between the jet and recirculating eddy. The lateral location of this shear zone is sensitive to changes in pool length. Energy slope also was sensitive to pool length due to a combination of greater length of the pool and greater head loss with shorter pools. The results indicate some form of hydraulic optimization is possible with pools adjusting their length to adjust the location and strength of turbulent intensities in the center of pools, and lower their rate of energy dissipation. Copyright © 2004 John Wiley & Sons, Ltd.     

Assessment of hydrodynamic simulation results for eco‐hydraulic and eco‐hydrological applications: a spatial semivariance approach

Output from a three‐dimensional numerical flow model (SSIIM) is used in conjunction with high‐resolution topographic and velocity data to assess such models for eco‐hydraulic applications in river channel design and habitat appraisal. A new methodology for the comparison between field measurement and model output is detailed. This involves a comparison between conventional goodness‐of‐fit approaches applied to a spatially structured (riffle and pool) sample of model and field data, and a ‘relaxation’ method based upon the spatial semivariance of model/field departures. Conventional assessment indicates that the model predicts point‐by‐point velocity characteristics on a 0·45 m mesh to within ±0·1 m s⁻¹ over 80% of the channel area at low flow, and 50% of the area at high in‐bank flow. When a relative criterion of model fit is used, however, the model appears to perform less well: 60–70% of channel area has predicted velocities that depart from observed velocities by more than 10%. Regression analysis of observed and predicted velocities gives more cause for optimism, but all of these conventional indicators of goodness of fit neglect important spatial characteristics of model performance. Spatial semivariance is a means of supplementing model appraisal in this respect. In particular, using the relaxation approach, results are greatly improved: at a high in‐bank flow, the model results match field measurements to within 0·1 m s⁻¹ for more than 95% of the total channel area, provided that model and field comparisons are allowed within a radius of approximately 1 m from the original point of measurement. It is suggested that this revised form of model assessment is of particular relevance to eco‐hydraulic applications, where some degree of spatial and temporal dynamism (or uncertainty) is a characteristic. The approach may also be generalized to other environmental science modelling applications where the spatial attributes of model fits are of interest. Copyright © 2005 John Wiley & Sons, Ltd.     

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde,Sachsen‐Anhalt,Germany)

Riffle‐pool sequences are a common feature of gravel‐bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing cross‐sectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage‐dependant morphometric changes to riffle and pool maintaining processes. In accordance with previous authors, pool cross‐sections of the Mulde River are narrow and riffle cross‐sections are wide suggesting that they should rather be addressed as two general types of channel cross‐sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Q_bf (40% bankfull discharge). An index for the longitudinal amplitude of riffle‐pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes. According to the observed dynamics of bed amplitudes, maintenance of riffle‐pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post‐flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility. The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload. Copyright © 2011 John Wiley & Sons, Ltd.     

Morphology of riffle–pool sequences in the River Severn,England

Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero‐crossing’, the ‘spectral analysis’ and the ‘control‐point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ _p) and channel width ( W ), λ _p = 3 W , applies to the River Severn. Reach‐average riffle height ( H ) is a constant proportion of bankfull depth ( h ); typically H ≅ 0·16 h . Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed‐level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L _r) to water depth, i.e. L _r = α2π h, wherein α ≅ 1 for steep near‐equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent‐flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self‐similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd.     

Spatial variations in surface sediment structure in riffle–pool sequences: a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)

Riffle–pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in situ grain‐scale field measurements from a riffle–pool sequence to parameterize a physically‐based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilizes pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a two‐dimensional computational fluid dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle–pool sequences. This finding has implications for the prediction of sediment transport and the morphological evolution of gravel‐bed rivers. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic model for constriction scour caused by large woody debris

Large woody debris (LWD) can have a significant impact upon local channel morphology by creating scour pools and zones of reduced shear stress in which sediment is deposited. It is important to predict scour depths associated with LWD, as it is becoming increasingly common for debris to be added into river channels to improve sediment retention and create pools for aquatic habitat. Engineered log‐jams should therefore be designed using factor of safety engineering analysis, which includes estimates of associated scour and deposition rates. However, the rate and total depth of scour associated with LWD have not been modelled comprehensively, with authors resorting to the use of generic local and constriction scour models to predict scour depths. Also, constriction scour models presented, to date, do not calculate the rate of scour development. In this paper a model is presented for predicting the rate and total depth of scour associated with a channel constriction. The model is one‐dimensional and is based upon the sediment continuity equation, the calculation of specific head changes through the constricted reach and also allows for a variable free surface elevation above the bed at the constriction. This model could be applied to any channel constriction problem but here is used to determine scour rates and depths associated with deflector‐type LWD jams. Deflector jams are one category of jam type presented in a debris jam classification scheme, in which jam type is a function of the ratio of average riparian tree height to average channel width. Deflector jams, as the name implies, partially block the flow and therefore act as a channel constriction, which results in constriction scour. Copyright © 2002 John Wiley & Sons, Ltd.     

Bed material transport estimated from the virtual velocity of sediment

This study evaluates the possibility of determining bed material transport using the virtual rate of travel of individual particles, dimensions of the active layer of the streambed, and porosity and density of streambed material. Magnetically tagged stones and scour indicators were employed in Carnation Creek, British Columbia, to quantify transport rates. Observations cover flows up to 36 m³ s⁻¹ (τ* = 0·081). Transport rates, ranging from 0·090 to 9·7 kg s⁻¹ (0·12–13·2 m³ h⁻¹), display a relatively sensitive trend with maximum stream power, as expected. Error analysis indicates that uncertainty in virtual velocity covers the majority of sample variance. An evaluation of the two measurement techniques used to delineate active layer dimensions, magnetically tagged stones and scour indicators, indicates that they yield comparable depths, widths and transport rates over the range of flows observed. Issues for further study are discussed. © 1998 John Wiley & Sons, Ltd.     

The challenge of modeling pool–riffle morphologies in channels with different densities of large woody debris and boulders

Studies on pool morphologies include reports of over 80% or 90% of pools being associated with structural controls and large obstructions that include boulders, bedrock outcrops and large woody debris (LWD). A Monte Carlo simulation approach and developmental computer model was created to predict pool formation, spacing and the percentage length covered by pools, riffles, scour holes and runs based on input data that include channel slope, width, the number of small and large boulders, and the number of 10–30 cm, 30–60 cm and >60 cm pieces of wood. The statistical‐empirical model is founded on the idea that boulders, bedrock outcrops and large woody debris provide a physical framework that then controls local water‐surface slopes, velocity patterns and the locations of pools and riffles. The spacing values of individual types and sizes of obstructions are modeled as log‐normal distributions with separate distributions for each obstruction type. Pools are assigned different probabilities of development depending on the obstruction type. Pool and riffle lengths used to create the subsequent morphology follow their own slope‐dependent, log‐normal trends. A minimum distance develops between successive pools because of the backwater and turbulent conditions needed for pool formation. The total number and spacing of pools, riffles and scour holes thus reflects the number and locations of obstructions and characteristics of the pool–riffle couplet. The simulation model accurately captures the number of pools in the modeled data range at 65% of all the verification field sites, and 86% of the verification field sites with a more limited range of width and slope characteristics. Lower levels of prediction capabilities are associated with modeled numbers of scour holes and log jams. The model accurately mimics some statistical attributes of pool spacing, and future versions of the model could be developed to improve overall predictive capabilities. Copyright © 2011 John Wiley & Sons, Ltd.     

Step–pool evolution in the Rio Cordon,northeastern Italy

The principle that formative events, punctuated by periods of evolution, recovery or temporary periods of steady‐state conditions, control the development of the step–pool morphology, has been applied to the evolution of the Rio Cordon stream bed. The Rio Cordon is a small catchment (5 km²) within the Dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded. Detailed field surveys of the step–pool structures carried out before and after the September 1994 and October 1998 floods have served to illustrate the control on step–pool changes by these floods. Floods were grouped into two categories. The first includes ‘ordinary’ events which are characterized by peak discharges with a return time of one to five years (1·8–5·15 m³ s^?1) and by an hourly bedload rate not exceeding 20 m³ h^?1. The second refers to ‘exceptional’ events with a return time of 30–50 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. Step–pool features were characterized primarily by a steepness parameter c = (H/L_s)/S. The evolution of the steepness parameter was measured in the field from 1992 to 1998. The results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events. During this period, bed armouring dominate the sediment transport response. However, following an extraordinary flood and unlimited sediment supply conditions, the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing. The analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Longitudinal velocity patterns and bed morphology interaction in a rill

Present‐day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills. The latter are dif?cult to collect with traditional techniques due to the very low water depths and the relatively high ?ow velocities in rills. The objectives of this paper were to investigate the feasibility of miniaturized acoustic Doppler velocimeter (mADV) measurements in rill ?ow and to explore longitudinal variations in ?ow velocities and their relationship with rill bed morphology. Detailed data on longitudinal ?ow velocity were required to achieve these objectives. A 1·8 m long rill was formed freely in a ?ume at 5° slope and 0·001 m³ s^?1 discharge. Rill topography was characterized by an alternation of steps and pools. The ?ume surface was then ?xed to preserve rill roughness. A topographical scanning of the entire ?ume surface was made. Velocity was measured with a mADV along the rill, and at different depths. Flow depth in a longitudinal direction was also measured using an elevation gauge. A strong relationship exists between rill topography and ?ow hydraulics. Over steps, ?ow was unidirectional and rapidly accelerating until a threshold Froude number (Fn) value between 1·3 and 1·7 was reached and a hydraulic jump occurred leading to the formation of a pool. In the pool, the ?ow pattern was multidirectional and complex. The ?ow was subcritical when leaving the pool and accelerated over the next step until the threshold Froude number value was again reached. Energy loss in the rill was concentrated in the pools, mainly due to the action of a hydraulic jump. This mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution. Copyright © 2004 John Wiley & Sons, Ltd.     

Suspended sediment discharge in subsurface flow from the head hollow of a small forested watershed,northern Japan

Surface flow and suspended sediment discharge from the head hollow of the Jozankei Experimental Watershed in Hokkaido, northern Japan, were measured to clarify the implications of subsurface hydrology for soil movement. Subsurface discharges during the extremely large storms of 1993 to 1994 were measured in a V-notch weir installed at a natural spring near the bottom of the head hollow, and shallow groundwater levels were observed in the wells excavated in the hollow. Sediment samples whose particle size range from 0·001 to 0·1 mm were manually and automatically collected at 15 to 60 min intervals, by use of 1 or 21 polyethylene bottles. Maximum concentration and flux of suspended sediment during the storms preceded the peak discharge of subsurface flow by several hours. Neither the changes in concentration (mg l⁻¹) nor flux (mg s⁻¹) of suspended sediment coincided with those in subsurface discharge (l s⁻¹). Furthermore, sediment concentration was poorly correlated with the rate of change in subsurface discharge (l s⁻²) during the rising limb of the hydrograph. Suspended sediment flux during the acceleratory limb, however, was closely correlated with the rate of change in subsurface discharge. The relationship between suspended sediment flux and rate of change in subsurface discharge were in inverse proportion to initial subsurface discharge before the storm runoff and they represented rare seasonal variation. Subsurface hydraulic erosion and transport of suspended sediment resulting from changes in rate of change in subsurface discharge actively occur during the acceleratory rising limb of the hydrograph. Accordingly, subsurface hydraulic erosion during the acceleratory rising limb of the hydrograph can be physically understood by analysing suspended sediment flux associated with rate of change in subsurface discharge and initial subsurface discharge. © 1997 John Wiley & Sons, Ltd.     

The role of vortex shedding in the scour of pools

Flume experiments were conducted in a 6-m flume to determine the role of turbulence in the scour of pools. Paired results from constricted-flow experiments with and without a wake zone formed behind obstructions to flow show that pools are deeper and shorter when vertical free-shear layers are present. Although non-streamlined obstructions initially present more resistance to flow, channel-bed scour develops a pool morphology that lowers mechanical energy losses to levels below those in pools with streamlined obstructions. Scour primarily enlarges the cross-sectional area in the constricted section. Feedback between pool geometry and localized turbulence production play a secondary role in total scour but still exert a major influence on final energy slopes. The experiment shows that pools with wake zones evolve to reduce longitudinal energy expenditure with an associated reduction in total turbulence production associated with the obstruction.     

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.     

Hillslope–channel sediment transfer in a slope failure event: Wet Swine Gill,Lake District,northern England

This paper describes and analyses a hillslope–channel slope failure event that occurred at Wet Swine Gill, Lake District, northern England. This comprised a hillslope slide (180 m³, c. 203 ± 36 t), which coupled with the adjacent stream, resulting in a channelized debris flow and fluvial flood. The timing of the event is constrained between January and March 2002. The hillslope failure occurred in response to a rainfall/snowmelt trigger, on ground recently disturbed by a heather moorland fire and modified by artificial drainage. Slide and flow dynamics are estimated using reconstructed velocity and discharge values along the sediment transfer path. There is a rapid downstream reduction in both maximum velocity, from 9·8 to 1·3 m s^?1; and maximum discharge, ranging from 33·5 to 2·4 m³ s^?1. A volumetric sediment budget quantified a high degree of coupling between the hillslope and immediate channel (～92%: 167 m³), but virtually all of the sediment was retained in the first‐order tributary channel. Approximately 44% (81 m³) of the slide volume was retained in the run‐up deposit, and termination of the debris flow prior to the main river meant that the remainder did not discharge into the fluvial system downstream. These results suggest poor transmission of sediment to the main river at the time of the event, but importantly an increase in available material for post‐event sediment transfer processes within the small upland tributary. Copyright © 2007 John Wiley & Sons, Ltd.     

Water flux measurement and prediction in young cashew trees using sap flow data

Measurements of sap flow, meteorological parameters, soil water content and tension were made for 4 months in a young cashew (Anacardium occidentale L.) plantation during the 2002 rainy season in Ejura, Ghana. This experiment was part of a sustainable water management project in West Africa. The Granier system was used to measure half‐hourly whole‐tree sap flow. Weather variables were observed with an automatic weather station, whereas soil moisture and tension were measured with a Delta‐T profile probe and tensiometers respectively. Clearness index (CI), a measure of the sky condition, was significantly correlated with tree transpiration (r² = 0·73) and potential evaporation (r² = 0·86). Both diurnal and daily stomata conductance were poorly correlated with the climatic variables. Estimated daily canopy conductance g_c ranged from 4·0 to 21·2 mm s⁻¹, with a mean value of 8·0 ± 3·3 mm s⁻¹. Water flux variation was related to a range of environmental variables: soil water content, air temperature, solar radiation, relative humidity and vapour pressure deficit. Linear and non‐linear regression models, as well as a modified Priestley–Taylor formula, were fitted with transpiration, and the well‐correlated variables, using half‐hourly measurements. Measured and predicted transpiration using these regression models were in good agreement, with r² ranging from 0·71 to 0·84. The computed measure of accuracy δ indicated that a non‐linear model is better than its corresponding linear one. Furthermore, solar radiation, CI, clouds and rain were found to influence tree water flux. Copyright © 2005 John Wiley & Sons, Ltd.     

Particle travel distances and bed and sediment compositions associated with rain‐impacted flows

Laboratory experiments were performed with rain of uniform drop size (2·7 mm, 5·1 mm) impacting flows over non‐cohesive beds of uniform sized sand (0·11–0·9 mm) and coal (0·2–0·9 mm) particles with flow velocities (20 mm s^?1, 40 mm s^?1) that were insufficient for the flow to entrain the particles without the aid of raindrop impact. Measurement of particle travel distance under rain made up of 2·7 mm drops confirmed a theoretical relationship between settling velocity and the distance particles travel after being disturbed by drop impact. Although, in theory, a relationship between settling velocity and particle travel distance exists, settling velocity by itself was unable to account for the effect of changes in both particle size and density on sediment discharge from beds of uniform non‐cohesive material. Particle density was also a factor. Further study of how particle characteristics influence sediment discharge will aid modelling of the impact of the soil in process‐based models of erosion by rain‐impacted flow. Copyright © 2001 John Wiley & Sons, Ltd.     

Sediment-transport competence of rain-impacted interrill overland flow

Laboratory experiments to determine the maximum size of sediment transported in shallow, rain-impacted flow were conducted in a recirculating flume 4·80 m long and 0·50 m wide. Rainfall intensities were varied between 51 and 138 mm h⁻¹, flow was introduced from a header tank into the flume at rates ranging from 0 to 0·64 l s⁻¹, and experiments were conducted on gradients between 3·5 and 10°. The following equation was developed: ML = (REFE)^1·6363 in which M is particle mass, L is distance moved in unit time (cm min⁻¹), RE is rainfall energy (J m⁻² s⁻¹) and FE is flow energy (J m⁻² s⁻¹). This equation can be used to predict sediment-transport competence of interrill overland flow. The equation is limited in its utility insofar as it has been developed using quartz grains and takes no account of variations in absorption of rain energy by natural ground surfaces. © 1998 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.