a conceptual model for determining soil erosion by water current estimates of rates of soil erosion by water derived from plots are incompatible with estimates of long term lowering of large drainage basins traditional arguments to reconcile these two disparate rates are awed the ux of sediment leaving a speci ed area cannot be converted to a yield simply by dividing by the area because there is no simple relationship between ux and area here we develop an approach to the determination of erosion rates that is based upon the entrainment rates and travel distances of individual particles the limited available empirical data is consistent with the predictions of this approach parameterization of the equations to take account of such factors as gradient and sediment supply is required to proceed from the conceptual framework to quantitative measurements of erosion however our conceptual model solves the apparent paradox of the sediment delivery ratio resolves recent discussion about the validity of erosion rates made using usle erosion plots and potentially can reconcile erosion rates with known lifespans of continents our results imply that previous estimates of soil erosion are fallacious copyright 2004 john wiley sons ltd

comparison of rill flow velocity regimes between developing and stationary rills abstract flow velocity is fundamental in calculating related parameters of physical soil erosion models it has been well known that rill flow velocity varies with developing rill form which complicates and impedes the development of soil erosion models in filed rainfall erosion frequently occurs with pre existing rills not damaged by tillage practices however the rill flow velocity in this case has received little attention in this study we investigated rill flow velocity regimes for both developing and stationary rills using 5 m length plots in the laboratory the experiments included two consecutive rainfalls which had similar runoff rate the first rainfall had an intensity of 90 mm h 1 which generated well developed rills the second had an intensity of 60 mm h 1 and did not damage the rills formed by the first rainfall the results show that for a developing rill rill flow velocity is higher in the lower slope than that in the upper slope and rill flow velocity in the lower slope fluctuates much more over time as a result of the variation of rill morphology following rill development with rill development the rill flow velocity of the lower slope decreased responding to the stabilizing rill channel and exhibited a similar trend to that in upper slope for a stationary rill the rill flow velocities at both sections varied little over time demonstrating that rill flow velocity is primarily influenced by rill morphology hence the results suggested that the strength of fluctuation in rill flow velocity could be used to characterize the instantaneous intensity of soil erosion and active level of rill development the channel of the stationary rill simply acted as transport pathways of sediment supplied by inter rill erosion the physical models of soil erosion should take into account the micro topography on land surface related to the antecedently formed rills

geotectonic framework of the blueschist unit on anglesey lleyn uk and its role in the development of a neoproterozoic accretionary orogen abstract a 560 550 ma 5 km 25 km blueschist unit extends ne sw on the island of anglesey wales uk and continues to the southwest for more than 70 km along the northern coast of the lleyn peninsula it has the shape of a shallow dipping slab or sheet up to a few kilometres thick that was exhumed from the subduction zone and emplaced into the accretionary complex to the northwest today the top boundary of the slab is commonly a thrust that was originally a low angle normal fault at its top in the subduction zone above it is an accretionary complex and a unit of high grade gneisses the bottom boundary is today a thrust that was originally a thrust below which is a unit of low grade or unmetamorphosed rocks belonging to an olistostrome type accretionary complex this thrust was originally at the bottom of the slab when it was exhumed in the subduction zone the sandwiched assemblage was created by the tectonic insertion of the blueschist between the other units folding particularly during late doming of the tectonic sandwich rotated the top normal fault boundary into a thrust later high angle ne sw trending normal faults displaced the tectonic stratigraphy in horst and graben a 200 km long and 200 km wide 680 450 ma calc alkaline volcano plutonic belt extends southeastwards from the blueschist belt via northern wales to central england suggesting that the subduction of oceanic lithosphere was to the southeast and this is consistent with the extrusion of the thin high pressure slab into the accretionary complex to the northwest this took place on the western accretionary margin of avalonia finally we outline the key steps involved in unravelling the structure of a pacific type accretionary complex

reconstruction of ocean plate stratigraphy in the gwna group nw wales implications for the subduction accretion process of a latest proterozoic trench forearc abstract the gwna group in anglesey island and lleyn peninsula wales consists of a latest proterozoic volcano sedimentary trench melange which has a complicated accretionary structure and is poorly constrained by isotopic ages the melange contains oceanic trench rocks including pillow basalts cherts mudstones and sandstones which have not previously been interpreted as ocean plate stratigraphy ops we reconstructed imbricated ops at 5 localities in the coastal lleyn peninsula in order to constrain the depositional u pb age of the upper clastic sediments detrital zircons separated from 9 clastic sediments were analyzed with a nu attom single collector inductively coupled plasma mass spectrometer the ages indicate that there are two gwna groups maximum depositional ages of 1 at 608 601 ma and 2 at 564 539 ma that were deposited between the late neoproterozoic and the middle cambrian contemporaneously with dated calc alkaline arc magmatism and regional metamorphism in the anglesey lleyn complex the age spectra of the detrital zircons show a prominent peak at ca 650 600 ma and several proterozoic and archean ages to account for the older ages we integrated our new isotopic data with published radiometric and fossil ages and conclude that the clastic sediments at the top of the ops were deposited in a trench on the western active margin of avalonia when it was close to the amazonian craton and that the gwna group ops began to be incorporated into an accretionary wedge in an active subduction zone in the latest proterozoic

morphometric and hydraulic geometry assessment of a gully in sw spain abstract gully erosion represents one of the most significant types of land degradation in the mediterranean areas giving place to important on and off site effects in this paper a second order gully located in sw spain is analyzed along the gully 28 cross sections were established and measured with a leica tcrm1102 laser total station approximately every 6 months from 2001 to 2007 the sections were located at variable distance placing them in areas where active erosion was evident in total 13 field measurements were carried out and the geometric characteristics of 28 cross sections were obtained morphometric analyses were carried out in both the main gully and a tributary reach by applying an empirical relationship between channel length and eroded volume morphometric variables of the gully sections were combined into two dimensionless groups and a morphological similarity between different linear erosion landforms rills ephemeral and permanent gullies was obtained then the coefficient of variation of the calculated volumes was used to compare the instability between the main gully and the tributary reach finally the hydraulic geometry of the gully was assessed by calibrating three empirical power equations which relate bankfull discharge with mean flow velocity cross sectional depth and width the hydraulic characterization of the main gully and the tributary reach was investigated for each field survey and a different behavior was detected the hydraulic analysis also demonstrated that higher values of discharge provide better predictions of flow velocity the size of the main and tributary gullies affects the discharge width relationship and that gully depth is the variable which can be predicted with the highest accuracy

dye tracer technique for rill flows by velocity profile measurements abstract water flow on hillslope soil surface supplies energy which is required to detach soil particles to transport and deposit sediments therefore flow velocity is a key variable related to hillslope hydrodinamics of soil erosion processes among the different methods available for measuring velocity of shallow interrill and rill flow the trace technique is widely used trace technique is applied by adding a material salt magnetic material water isotope floating object and then measuring the speed of the material to travel a known distance from the injection point when flow velocity is measured using a dye tracing method the mean velocity is calculated by multiplying the measured surface velocity of the leading edge of the tracer plume by a correction factor which was generally empirically deduced the main uncertainty of the dye tracing technique stands in the estimate of this correction factor which is the ratio between the mean flow velocity and the surface velocity the main goal of this paper is establishing a theoretical relationship for calculating the correction factor by using a power velocity profile at first the developed analysis demonstrated that the correction factor only depends on the exponent of the power velocity distribution eq 5 then this theoretical expression of the correction factor was applied using the velocity profiles measured by baiamonte and ferro for the condition of sediment free flow in motion on a rough bed and by coleman for a sediment laden flow moving on a smooth flume using the correction factor values calculated by the velocity measurements carried out for the sediment free flow in motion on a rough bed the relationship between the correction factor and roughness height was established in agreement with a previous study by ali et al for a sediment free flow the correction factor increases with roughness height finally the velocity profile measurements carried out for a sediment laden flow in motion on a smooth bed were used to state the effect of sediment load on the correction factor this last analysis allowed to conclude in agreement with li and abrahams and zhang et al that the correction factor decreases when the sediment load increases

responses of interrill runoff and erosion rates to vegetation change in southern arizona abstract vegetation change fi om grassland to shrubland has occurred over much of the sonoran and chihuahuan deserts during the past century the effect of this vegetation change on interrill runoff and erosion was examined by conducting rainfall simulation experiments on large runoff plots on contemporary grassland and shrubland hillslopes these experiments show that compared to the grassland the interrill portions of shrubland hillslopes 1 have higher runoff rates 2 experience equilibrium runoff conditions much more frequently 3 exhibit higher overland flow velocities and 4 are subject to greater rates of erosion the environmental change that has led to the vegetation change has been relatively minor but its geomorphic impact has been substantial

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

an improved method for shallow water flow velocity measurement with practical electrolyte inputs summary the measurement of shallow water flow velocity is of interest to hill slope hydrology and soil erosion mechanics lei et al lei t xia w zhao j 2005 method for measuring velocity of shallow water flow for soil erosion with an electrolyte tracer journal of hydrology 301 139 145 proposed an electrolyte pulse method for the purpose but found that the velocity measured at short distances from the salt injector was not as well estimated as those further away from the injector though it is desirable to estimate velocity at short distances for the purpose of portable device development in this study the system was improved over the previous development an additional sensor was added at the salt injection location to register the input signal as the actual boundary condition function instead of using the pulse assumption two function forms a normal distribution function normal model and a sine function sine model were suggested to approximate the measured boundary conditions these two determined boundary models were used to fit the experimentally obtained data using the least square method to determine the model parameters of the boundary conditions the determined normal model and the sine model were both used to solve the transport processes as the convolution integrals of the pulse response and the actually determined boundary functions so as to determine the shallow water flow velocity laboratory experiments were conducted to get data to verify the proposed methods and computational procedures the experiment device included a flume 4 m long and 15 cm wide a solute injector a data logger for control and data acquisition and a computer with specially designed software for data measurement and velocity estimation the experiments involved three flow rates 12 24 and 48 l min 1 and three slope gradients 4 8 and 12 six sensors were used to measure the solute transport processes at 5 30 60 90 120 and 150 cm from the location where the salt solute was injected into the water flow the solutions of both the normal model and sine model fitted the measured data very well for all the experimental conditions the velocities at different distances from the salt solution injector and under different flow rates and slope gradients as computed by the two models were almost identical which indicated the feasibility of the method and the computational procedures as well as the two suggested models for velocity estimation the velocity values estimated by the normal and sine models agreed also with those measured by using the floating objects and the volumetric method these demonstrated the rationality of the improved methods for shallow water flow velocity measurement the results will be useful to design a practical system for the measurement of shallow water flow velocity

comment on rill erosion processes on steep colluvial deposit slope under heavy rainfall in flume experiments with artificial rain by f jiang et al abstract since rill flows are characterized by small water depths and steeply sloping channels the corresponding hydraulic conditions are very different to those which are typically found in channels of streams and rivers furthermore limited information is currently available on the effect of rainfall on flow resistance the objective of this comment was to investigate the applicability of a recently theoretically deduced rill flow resistance equation based on a power velocity profile using measurements carried out by jiang et al for both different slope steepness conditions and rainfall intensity the relationship between the velocity profile parameter the channel slope and the flow froude number was calibrated using the data by jiang et al the theoretical approach and the measurements carried out in the investigated conditions allowed to state that a the darcy weisbach friction factor can be accurately estimated using the proposed theoretical approach b the data were supportive of the slope independence hypothesis of rill velocity stated by govers and c the darcy weisbach friction factor varies with rainfall intensity

glaucophanic amphibole in a monian shear zone on the mainland of n wales glaucophanic amphibole occurs within recrystallized basic volcanic rocks derived from the gwna group at penrhyn nefyn in the lleyn peninsula the glaucophanic rocks are restricted to the penmynydd zone of metamorphism which at nefyn is interpreted as a narrow blastomylonite belt produced by the shearing of the gwna group against a body of tonalite the blue amphibole coexists with blue green amphibole and grades into actinolite greenschist away from the focus of shearing fragments of gwna group dated as lower cambrian and blastomylonitic penmynydd lithologies lie in the cambro ordovician sediments of the welsh basin therefore the metamorphism of parts of the gwna group during the penmynydd subduction zone shearing is interpreted as having occurred in cambrian times

rb sr whole rock isochron ages δ18o values and geochemical data for the sarn igneous complex and the parwyd gneisses of the mona complex of llŷn n wales the mona complex exposed on the ll n peninsula includes the sarn complex which is a suite of variably deformed plutonic igneous rocks originally mapped partly as gneisses ranging from gabbro to granite and an inlier of high grade paragneisses geochemical data for samples from the sarn complex may be divided into two distinct populations the rb sr data for samples from these geochemical groups define significantly different ages the older age 549 19 ma initial 87 sr 86 sr 0 7096 3 is identified as the age of intrusion in late precambrian to cambrian times while a younger age of 458 16 ma may approximate to the age of a resetting event the parwyd paragneisses yield an rb sr whole rock isochron age of 542 17 ma concordant with the age of intrusion of the sarn complex with an initial 87 sr 86 sr of 0 7069 4 for the sarn complex 18 o values range from 9 5 to 12 0 per mil smow and in the parwyd metasediments from 9 4 to 13 3 per mil we conclude that the earliest magmatism and metamorphism in the mona complex of ll n occurred during late precambrian to cambrian times broadly contemporaneous with similar events in anglesey the high initial 87 sr 86 sr and high 18 o values for the sarn complex are most simply interpreted as reflecting either a crustal source region or incorporation of a significant proportion of crustal material such as the parwyd gneisses into a mantle derived magma

hydraulics of interrill overland flow on a semi arid hillslope southern arizona parsons a j abrahams a d and luk s h 1990 hydraulics of interrill overland flow on a semi arid hillslope southern arizona j hydrol 117 255 273 simulated rainfall experiments were conducted on an 18 m wide by 35 m long runoff plot located in southern arizona by using a wide plot that extended from the divide to beyond the rill heads it was possible to examine the full range of lateral variability and downslope changes in interrill overland flow on a shrub covered semi arid piedmont hillslope the mean depth d mean velocity v and reynolds number re of overland flow were computed for two measured sections situated 12 5 and 21 m from the top of the plot at a section d vs re and v vs re relations show that increases in re are accomodated largely by increases in d these hydraulic relations are principally the result of resistance to flow increasing with re as overland flow spreads into new areas inasmuch as the maximum equilibrium discharge simulated in these experiments has a return period of about 7 yr increasing resistance to flow as re increases may be considered typical of most overland flow events on shrub covered piedmont hillslopes in southern arizona downslope hydraulic relations differ strikingly from at a section relations under equilibrium runoff conditions resistance to flow decreases rapidly as re increases permitting increases in re to be accomodated entirely by increases in v the decrease in resistance to flow is due to progressive downslope concentration of flow into fewer deeper threads under non equilibrium conditions downslope hydraulic relations may be quite different from those obtaining at equilibrium but resistance to flow always decreases downslope this is the result of low flows following pathways formed by higher flows that concentrate downslope analyses of overland flow hydraulics at the scale of interrill areas produce results that are quite different from those obtained from small plots a knowledge of the hydraulics at this scale is essential if models of interrill overland flow are to become more realistic and hence better predictors of runoff hydrographs and sediment yield

rill erosion in natural and disturbed forests 1 measurements 1 rill erosion can be a large portion of the total erosion in disturbed forests but measurements of the runoff and erosion at the rill scale are uncommon simulated rill erosion experiments were conducted in two forested areas in the northwestern united states on slopes ranging from 18 to 79 we compared runoff rates runoff velocities and sediment flux rates from natural undisturbed forests and in forests either burned at low soil burn severity 10 months or 2 weeks post fire high soil burn severity or subject to skidding of felled logs the runoff rates and velocities in the natural sites 2 7 l min 1 and 0 016 m s 1 respectively were lower than those in all the disturbed sites 12 to 21 l min 1 and 0 19 to 0 31 m s 1 respectively except for the 10 month old low soil burn severity site where the velocity 0 073 m s 1 was indistinguishable from the natural sites the mean sediment flux rate in the natural sites also was very small 1 3 10 5 kg s 1 as compared to the rates in the disturbed areas 2 5 10 4 to 0 011 kg s 1 the hillslope gradient did not affect the runoff or sediment responses the sediment flux rates generally were greater in the initial stage of each inflow period than in the steady state condition but there was no corresponding transient effect in runoff rates rill erosion modeling implications based on these data are presented in part 2 of this study

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

rill hydraulics on a semiarid hillslope southern arizona seventy field experiments were conducted in seven rills located on a semiarid rangeland hillslope underlain by gravelly soils at walnut gulch arizona the rills which are characterized by wide shallow cross sections and gravel covered beds have mean at a station hydraulic geometry exponents of b 0 33 f 0 34 and m 0 33 although the differences between these values and typical values of b 0 30 f 0 40 and m 0 30 for cropland rills are not statistically significant they are thought to be real as cropland rills often have more rectangular cross sections and steeper sides than the rangeland rills under study for rills formed in silty loamy soils govers developed an empirical relation between mean flow velocity and discharge emphasizing the generality of this relation he suggested that it may be used as a simple means of routing runoff through rills he also noted that this relation appeared to be unaffected by either slope or soil materials the present data represent rills underlain by coarser and somewhat more varied gravel rich soils these data do not conform to govers relation and a multiple regression analysis reveals that slope and soil materials either directly or indirectly through bed roughness exert almost as much influence on flow velocity as does discharge three alternative methods are developed for predicting flow velocity in the rills under study all three methods give good results with the largest root mean square deviation being 3 115 cm s 1

avalonian magmatism and terrane linkage new isotopic data from the precambrian of north wales new u pb isotope data from the precambrian sarn igneous complex of north wales establishes that calc alkaline plutonic rocks of the same age occur on either side of the menai strait fault system zircon fractions from the sarn igneous complex gabbro give a near concordant u pb age of 615 2 ma which is interpreted as the first accurate age of igneous crystallization and supersedes previous dating attempts using the rb sr method the new u pb age is the same within error as recent zircon dates obtained from the arfon group ignimbrites along strike from the sarn igneous complex and from the coedana complex granite on anglesey these data emphasize the importance of what appears to have been the major phase of calc alkaline magmatism at 630 600 ma recognizable along the avalonian arc in britain newfoundland and elsewhere despite the similar ages and tectonic setting of the coedana and sarn plutonic rocks however they remain petrogenetically and geochemically distinct and are not directly linked as part of the same igneous complex instead the coedana complex granite is interpreted as having been generated in the avalonian arc during a widespread magmatic event that is also recorded in britain not only by the sarn igneous complex and arfon group but by volcanic and plutonic rocks in central england investigation of granite clasts in the gwna group melange monian supergroup on anglesey failed to produce zircons adequate for dating purposes but revealed rb sr whole rock geochemical and petrographic characteristics similar to the nearby coedana complex and suggest linkage between fault bounded 9monian9 terranes on anglesey in the light of these new data rocks on either side of the menai strait terrane boundary are interpreted as belonging to the same arc system that was dismembered dispersed and laterally duplicated by transcurrent faulting after late precambrian magmatism

the late aeronian graptolite sedgwickii event associated positive carbon isotope excursion and facies changes in the prague synform barrandian area bohemia study of the lower silurian black shale succession of the prague synform has enabled detailed insight into graptolite faunal dynamics and diversity trends from the mid aeronian diversity maximum through to the late aeronian crisis graptolite diversity decreased from 33 taxa in the lituigraptus convolutus biozone to 17 taxa in the upper part of the stimulograptus sedgwickii biozone and newly erected lituigraptus rastrum biozone the graptolite assemblages of the latter biozones exhibit low species richness along with high dominance many graptolite species that became extinct in the early part of the sedgwickii zone were promptly replaced by new forms in the later part of the sedgwickii zone however replacement of extinct species by new forms considerably decelerated the increased rate of graptolite extinction recorded in the convolutus sedgwickii biozone boundary beds coincided with subtle changes in black shale lithologies and a positive shift in 13 c org of 2 2 compared to baseline values sea level drawdown can be inferred from siltstones and or temporary nondeposition in the middle sedgwickii zone this level also sees total organic carbon toc fluctuations and a strong positive 13 c org excursion with a peak shift of at least 7 the sedgwickii event exhibits substantial reorganization of the graptolite fauna its taxonomic impoverishment and concomitant increase in species dominance rather than a sudden collapse of the pre extinction assemblage associated changes in lithology toc and the pronounced 13 c org excursion suggest a relatively extended and probably multi phase period of stressed conditions that affected the pelagic realm inhabited by graptolites in the course of the late aeronian interval

3d modelling of the effect of thermal elastic stress on grain boundary opening in quartz grain aggregates abstract during the exhumation of rocks and the associated temperature and pressure decrease the anisotropic thermoelastic properties of minerals lead to internal stresses on the grain scale which in turn cause fracturing and opening of grain and phase boundaries to gain deeper insight into the onset of grain boundary cracking and the subsequent evolution of grain boundary fracture networks after crack initiation 3d grain scale numerical modelling has been performed in detail the fracturing and opening of quartz grain boundaries during exhumation of quartzite with millimeter scale grains and randomly oriented crystallographic axes accompanied by cooling from 300 to 25 c and decompression from 300 to 22 mpa have been modelled with contact mechanics and the finite element method model grains have an anisotropic linear elastic rheology and an anisotropic thermal expansion tensor grain boundaries are modelled as contact surfaces with a non linear strain softening rheology in tension and a mohr coulomb rheology in shear grain boundary fractures nucleate at grain vertices and triple lines in agreement with observations in nature and experiment comparison with grain boundary opening data from natural quartzite indicates that the tensile yield strength y of quartz grain boundaries has an upper bound of 75 mpa with best fit results for y 25 mpa moreover the best fit model implies that quartz grain boundary opening initiates after a temperature and pressure decrease of around 80 c and 80 mpa