effect of soil stratification on the development and migration of headcuts in upland concentrated flows 1 experiments were conducted to examine the effect of vertical stratification in soil erodibility on the development and migration of steady state headcut scour holes in upland concentrated flows typical of agricultural fields packed soil beds with a preformed step were subjected to identical simulated rainstorms and clear water overland flow rates which resulted in predictable actively migrating headcut scour holes with nearly identical characteristics when an erosion resistant layer was incorporated into the packed soil bed at a depth that exceeded this expected plunge pool scour depth the erosion and hydraulic processes of the migrating headcuts remained unchanged when the erosion resistant layer was placed so as to intersect this potential headcut scour depth the erosivity of the reattached wall jet was unable to erode this layer and the depth of scour the nappe entry angle and sediment efflux all were reduced inversely proportional to the relative depth of the resistant layer these data were successfully predicted using modified jet impingement theory for headcut scour holes and demonstrate further the effects of soil management and tillage practices on total soil losses from agricultural fields

criterion delineating the mode of headcut migration two modes of headcut migration are generally recognized 1 rotating headcuts that tend to flatten as they migrate and 2 stepped headcuts that tend to retain nearly vertical faces a mathematical description of the sediment detachment potential immediately upstream and downstream of the headcut is used to delineate these modes of migration the delineating parameter is the ratio of the time required to erode the headcut face from above to the time required to undermine the headcut face from below this erosional time scale ratio is a dimensionless function of flow sediment and geometry parameters for the limiting case of homogeneous cohesive soils the time scale ratio is a simple function of a froude number and the aspect ratio of drop height to normal flow depth this relationship is calibrated using original laboratory experiments of headcut migration in initially vertical headcuts and verified by independent field experiments of headcuts propagating in four different homogeneous cohesive soils

stability of composite river banks the stability of a river bank depends on the balance of forces motive and resistive associated with the most critical mechanism of failure many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size geometry and structure of the bank the engineering properties of the bank material the hydraulics of flow in the adjacent channel and climatic conditions rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt clay bank erosion occurs by fluvial entrainment of material from the lower cohesionless bank at a much higher rate than material from the upper cohesive bank this leads to undermining that produces cantilevers of cohesive material upper bank retreat takes place predominantly by the failure of these cantilevers three mechanisms of failure have been identified shear beam and tensile failure the stability of a cantilever may be analysed using static equilibrium and beam theory and dimensionless charts for cantilever stability constructed application of the charts requires only a few simple measurements of cantilever geometry and soil properties in this analysis the effects of cracks and fissures in the soil must be taken into account these cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface following mechanical failure blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue hence the rate of bank retreat is fluvially controlled even though the mechanism of failure of the upper bank is not directly fluvial in nature this cycle of bank erosion undermining cantilever failure and fluvial scour of the toe operates over several flood events and has important implications for river engineering channel changes and the movement of sediment through fluvial systems

scour downstream of grade control structures a theoretical investigation of local scour downstream of gradecontrol structures based on twodimensional jet diffusion and particle stability is experimentally verified turbulent jet diffusion red

the cycle of instability stress release and fissure flow as controls on gully head retreat gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation sapping or piping the empirical evidence to substantiate these conceptual models is sparse however and often contradictory this paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology stability and elastic deformation of a marl gully complex in granada province south east spain the hydrological and slope stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head and that vertical or subvertical heads will be stable unless saturation is achieved owing to the high unsaturated strengths of marl measured in this research failure in unsaturated conditions is possible only where the gully head wall is significantly undercut head retreat thus requires the formation of either a tension crack or an undercut hollow finite element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base and a second stress anomaly just upslope of the head wall although tension cracks on gully heads have often been attributed to slope unloading this research provides strong evidence that the so called sapping hollow commonly found in the gully headwall base is also a function of stress release although further research is needed it seems possible that pop out failures in river channels may be caused by the same process the hydrological analysis shows that once a tension crack has developed throughflow velocity in the gully headwall will increase by an order of magnitude promoting piping and enlargement of this weakened area it is therefore possible to envisage a cycle of gully expansion in which erosion channel incision or human action unloads the slope below a gully head leading to stress patterns that account for the tension crack and a stress release hollow the tension crack promotes faster throughflow encouraging hollow enlargement and piping which undercut the gully head the tension crack permits the development of positive pore water pressures behind the gully head leading either to failure or contributing to toppling finally the debris may be eroded by fluvial action unloading a new section of slope and completing the cycle of gully head retreat copyright 2001 john wiley sons ltd

seismic modeling of gas chimneys we propose a simple acoustic model explaining the main features of gas chimneys the main elements of the model consist of gas diffusing from a connected fracture network and into the surrounding shale creating an inhomogeneous gas saturation the gas saturation results in an inhomogeneous fluctuating compressional velocity field that distorts seismic waves we model the fracture network by a randomwalk process constrained by maximum fracture length and angleofthefracturewithrespecttothevertical thegassaturation is computed from a simple analytical solution of the diffusion equation and pressure wave velocities are locally obtained assuming that mixing of shale and gas occurs on a scalemuchsmallerthanseismicwavelengths syntheticseismic sections are then computed using the resulting inhomogeneous velocity model and shown to give rise to similar deterioration in data quality as that found in data from real gas chimneys also synthetic common midpointcmpgathers show the same distorted and attenuated traveltime curves as those obtained from a real data set the model shows clearly thatthefeaturesofgaschimneyschangewithgeologicaltime a model parameter in our approach the deterioration of seismicwavesbeingsmallestjustafterthecreationofthegas chimney it seems likely that at least some of the features of gas chimneys can be explained by a simple elastic model in combinationwithgasdiffusionfromafracturenetwork

predicting head cut erosion and migration in concentrated flows typical of upland areas 1 soil erosion due to head cut development and migration can devastate agricultural lands yet current prediction technology does not address this important erosion process here an analytical model of this erosional phenomenon is presented realistic physically based approximations to the laws governing mass momentum and energy transfer in the neighborhood of the scour hole result in closed form predictive algorithms for the magnitude of the plunge pool erosion and the rate of head cut migration the model introduces a special treatment of nonventilated overfall conditions is limited to homogeneous unbounded soil layers and is validated by available experimental measurements

experiments and modeling of head cut migration in stratified soils 1 a two dimensional model is developed to predict head cut migration rate for the specific case of a relatively erosion resistant soil layer overlying a deep relatively erosive base soil for this geometry head cut migration can occur as a series of discrete mass failures of the surface layer caused by undercutting in a plunge pool formed immediately downstream the length scale of one failure episode is modeled as a cantilevered slab failing in bending the timescale for each failure episode is controlled by the rate and geometry of base layer scour which is modeled independently of cantilevered slab properties an integration of the two model components for the case of steady flow and uniform soil properties predicts linear upstream head cut migration with time occurring in discrete steps bounded by successive mass failure events the model is calibrated and tested in a series of flume experiments representing ideal conditions of different initial geometries and flow conditions head cut migration occurred by the proposed mechanism in a nearly linear rate average error between the observed and predicted retreat rate is 40 most of this error is attributable to prediction of the scour hole shape

in situ experiments on an excavation disturbed zone induced by mechanical excavation in neogene sedimentary rock at tono mine central japan abstract the excavation disturbed zone edz is defined as the rock zone where rock properties and conditions have been changed due to the processes induced by excavation such as fracturing stress redistribution and desaturation based on our results of the in situ blasting excavation disturbance experiment at the tono mines the rock mass around the drift behaved elastically and the excavation damage induced by blasting was considered to be a primary factor in causing a change in rock properties the excavation damage was considered to depend on the excavation method so we performed an excavation disturbance experiment with mechanical excavation using a boom header in the neogene sedimentary soft rock to compare the rock property change and width of the edz induced by blasting geological observation rock mass displacement and vibration measurement were performed during the drift excavation seismic refraction and tomographic surveys borehole expansion test and hydraulic test were performed before and after the drift excavation a low seismic velocity layer of the edz with a width of ca 0 3 m and a seismic velocity of 65 70 of the intact rock was detected by a seismic refraction survey along the drift wall deformability and hydraulic conductivity were not changed in the rock mass up to 0 3 m from the drift wall the change in rock properties and width of the edz was shown to depend on the excavation method in this geological setting the mechanical excavation method is more effective in limiting the extent of the edz and change in rock mass properties

apportioning contributions of individual rill erosion processes and their interactions on loessial hillslopes abstract channels rill ephemeral gully gully and river channel exhibit a continuum of sizes and flow magnitudes rill erosion as the initial stage of channel erosion accounts for 80 of total eroded sediments on sloping farming lands in many parts of the world previous researches often regard rill erosion as an entirety or focus on individual rill erosion process separately few attentions have been paid to the contributions of individual processes including rill headcut advance bed incision sidewall expansion and their interactions to the over all rill erosion thus simulated upslope inflow experiments were designed to investigate the impacts of individual processes involved and their interactions in rill erosion under four inflow rates 1 0 2 0 3 0 and 4 0 l min 1 and two slope gradients 15 and 20 photogrammetry and manual sampling were used to measure hillslope morphology variation and sediment delivery repectively the results show that headcut advance bed incision and sidewall expansion interact with each other and exhibit both independent and dependent features across spatial and temporal scales headcut advance interacts with bed incision and sidewall expansion before rill head advancing to a critical slope length initial rill depth and width are determined by initial headcut morphology bed incision and sidewall expansion dominate rill erosion before and after the non erodible layer is exposed to concentrated flow respectively headcut advance contributed the largest amount of rill erosion 44 68 followed by bed incision 27 44 and sidewall expansion 3 8 12 headcut advance contributed more 63 83 to total rill width increment while bed incision contributed larger percentage 51 65 to total rill depth increment prediction equations for length width and depth of a single rill on a loessial hillslope and empirical equations of rill erosion for the individual erosion processes were fitted and validated quantification and understanding of the contributions of individual rill erosion processes provides the necessary scientific basis for the development of process based rill erosion models and then for preventions of soil losses and land degradation

excavation induced damage studies at the underground research laboratory the underground research laboratory url is situated in the lac du bonnet granite batholith in southeastern manitoba canada the url was developed to study issues related to the deep geologic disposal of used fuel from nuclear reactors and has developed into an international atomic energy agency recognized geotechnical center of excellence extensive rock mechanics research has been conducted including work to understand the character and extent of excavation damage experience at the url has shown that damage exists around underground openings and that the damage develops from the energy imparted to the rock by the excavation method and by redistribution of the in situ stress field around the excavated openings subsequent near by excavations removal of loose material from the existing tunnel thermal loads and pore pressure changes will all influence the development and extent of rock damage as does the rock type and its fabric studies at the url have shown that in highly stressed rock damage will develop around underground openings even when a low energy excavation method is used and that properly designing the excavation geometry and orientation plays a major role in constructing stable openings two zones of excavation affected rock have been shown to exist from studies at the url 1 a zone of irreversibly damaged rock surrounding the excavation which may include failed zones inner and outer damage zones and 2 a zone of excavation disturbance where the in situ stresses are influence by the excavation no damage is measurable measurements of the properties of the damaged rock ultrasonic velocity transmissivity using a variety of instruments have shown that a less intensively damaged outer zone surrounds a more highly damaged inner zone the inner damage zone may have zones of failed rock depending on stress concentrations relative to rock strength and damage imparted by excavation method the inner damage zone is attributed to the effects of the excavation method and stress redistribution and the outer damage zone is attributed to the effects of stress redistribution alone in lower stressed rock the extent of the outer zone is much less than for an opening in highly stressed rock with the same excavation shape and orientation relative to in situ stresses the excavation disturbed zone describes the volume of rock surrounding the damage zone where the stress redistribution is too small to permanently change rock properties unless some other stress increase such as thermal loading is added and it is also characterized non permanent but potentially long term changes in hydraulic head

patterns and controls of gully growth along the shoreline of lake huron an empirical approach was used to examine the morphology and behaviour of gullies along the eastern shoreline of lake huron canada gully and watershed dimensions and percentage vegetation cover of a sample of 44 gullies were measured from aerial photographs between 1930 and 1992 gullies with larger watersheds had higher area growth rates larger gullies continue to expand over time while small gullies are more likely to become stable growth rates increased between 1955 and 1978 because of increased snowfall extreme flow events the extension of municipal drains and the use of subsurface drainage after 1970 efforts to rehabilitate gullies using drain pipes and check dams contributed to a decrease in gully growth rates 1997 john wiley sons ltd

assessment of crack initiation and propagation in rock from explosion induced stress waves and gas expansion by cross hole seismometry and fem dem method abstract single hole blast induced damage in a granitic outcrop has been assessed through both controlled experiments and numerical simulations with a combined finite discrete element method fem dem damage tomographies from decoupled short and long explosive charges in flooded boreholes were obtained through a high resolution cross hole system by measuring pre and post blast seismic velocities around the blastholes damage was assessed through crack density which was calculated by inversion of p wave velocity measurements through an effective medium theory emt method the resulting damage was found to be highly asymmetrical around each blast hole both along the vertical and horizontal planes despite the apparent isotropy and homogeneity of the granitic rock mass experimental results combined with numerical simulations carried out to assess damage from stress waves alone showed that most damage from experiments was caused by the expansion of gases while its magnitude and extension were strongly dependent on confining conditions along the blasthole the difficulty of quantifying the relative contribution of stress waves and gas expansion and the problems inherent in prediction of blast induced damage are described

effect of soil texture tailwater height and pore water pressure on the morphodynamics of migrating headcuts in upland concentrated flows rill and gully erosion in upland and agricultural areas can result in significant soil degradation worldwide and headcuts are the primary mechanism by which this landscape dissection occurs experiments were conducted to further examine the morphodynamic behavior of actively migrating headcuts in upland concentrated flows with varying boundary conditions the effect of soil texture greatly modified the erodibility of the select soils and headcut migration rates scour depths and sediment discharges were correlated to the critical shear stress and erodibility of the select soils the effect of increasing tailwater height greatly reduced the nappe entry angle at the brink of the headcut completely arresting the soil erosion processes altering subsurface pore water pressures markedly changed the erodibility coefficients of the select soil which caused greater headcut migration rates and sediment discharges yet shallower scour holes in the presence of a subsurface water table current analytic formulations based on jet impingement theory successfully predicted these experimental observations further demonstrating the utility of such analytic formulations in advancing watershed management technology copyright 2009 john wiley sons ltd

estimating the excavation disturbed zone in the permanent shiplock slopes of the three gorges project china abstract the stability and deformation of the permanent shiplock slopes are among the key issues in the design and construction of the three gorges project the permanent shiplock slopes formed by deeply and steeply cutting into weathered and fresh granites are 1607 m in length and 50 170 m in height the disturbed zone in the permanent cut slopes induced by excavation is therefore one of the most important aspects in slope stability and deformation evaluation a comprehensive investigation including non linear finite element analysis in situ testing instrumentation and monitoring back analysis and rock mass quality rating has been carried out for the identification and evaluation of the excavation disturbed zone in the permanent cut slopes the results of the investigation confirmed the existence of such an excavation disturbed zone in the permanent cut slopes this zone is characterized by a considerable weakening in the mechanical properties of the rock mass from the cut surface to the deeper region of the permanent cut slopes the excavation disturbed zone can be further divided into a damaged zone an affected zone and a slightly affected zone according to the extent of weakening in the rock mass the damaged zone and the affected zone have thicknesses of 5 10 m and 10 20 m respectively the exact pattern of the excavation disturbed zone is variable in different parts of the permanent cut slopes and are generally similar to that of the plastic zones estimated by using the non linear finite element analysis the present investigation has provided both factual data and insights for the stability and deformation evaluation of the permanent shiplock slopes the approach and methodology developed in the paper can be used to assess similar excavation disturbed zones in other large cut slopes

failure mechanisms and pore water pressure conditions analysis of a riverbank along the arno river central italy abstract mechanisms of failure occurring in two portions of a riverbank along the arno river central italy are investigated in detail starting by a series of periodic field observations and bank profile measurements two dominant mechanisms involving the silty sand portion of the bank have been observed a alcove shaped failure in the middle portion of the bank b slab failure involving the middle upper bank a portion of the riverbank was subject to laboratory grain size analysis phase relationship analysis triaxial tests and in situ tests borehole shear tests bsts to characterise the geotechnical properties of the overbank deposits two different procedures of bank stability analysis have been performed 1 a complete analysis coupling seepage analysis with the limit equilibrium method 2 two simplified analyses through the limit equilibrium method with simple assumptions on pore water pressures distribution for the complete analysis saturated unsaturated flow within the riverbank was modelled by finite element seepage analysis in transient conditions using as boundary conditions eight hydrographs with increasing water stage riverbank stability analyses have been conducted by the morgenstern price rigorous method dividing each of the eight hydrographs in 21 time steps and calculating the safety factor for each step the analysis revealed the occurrence of two possible mechanisms of failure slab type and alcove shaped sliding failures according to the field observations related to different river stages and pore water pressures within the riverbank alcove failures are likely to occur with moderate flow events while slab failures are favoured by flow events with higher peak river stage a first type of simplified analysis representing critical conditions reached during a rapid flow event was based on the main hypothesis of the occurrence of a zero pore water pressures zone within the portion of the bank between the low water stage and the peak stage reached a second type of simplified analysis was applied in order to represent rapid drawdown conditions following a prolonged flow event worst case with the main assumption of total saturation of the material up to the same elevation of the peak river stage the first simplified analysis has given similar results to the complete seepage stability analysis confirming slab type and alcove shaped failure as the two main mechanisms of instability while the second type of simplified analysis has conducted to too conservative results compared to the other previous analyses field observations regarding different characteristic bank geometries in adjacent sub reaches have been summarised in a conceptual cyclic sketch that include all the possible paths of bank evolution depending on the succession of river stages reached during flow events and related pore pressure conditions

main types of soil mass failure and characteristics of their impact factors in the yuanmou valley china abstract gully erosion is a dominant process causing soil loss in the yuanmou valley of yunnan province china soil mass failure is a common process involved in gully development the development of the gully and the occurrence of mass failure are interdependent and mutually promotive but mass failure types have not been very well understood in this study we identify the main types of soil mass failure and the characteristics of their impact factors in the yuanmou valley a detailed field survey was conducted in may 2010 in a representative area of the studied region and selected soil parameters were measured in the field and laboratory results showed that there are four different soil mass failure types falling sliding exfoliating and toppling each having unique characteristics of geology topography soil and climate examination of these characteristics indicates that the occurrence of mass failure is controlled by four factors 1 the typical geological structure of the yuanmou group represented by alternating layers of sand and clay 2 the different terrain and landforms 3 the different gully development stages and 4 the particular soil properties under special weather conditions including a dry hot climate with distinct wet and dry seasons

development of gullies on the landscape a model of headcut retreat resulting from plunge pool erosion 1 head advance due to plunge pool erosion is a common process in gullies incising resistant soils a model of headcut retreat resulting from plunge pool erosion is developed and implemented in the channel hillslope integrated landscape development child model an existing three dimensional landscape evolution modeling framework the model estimates horizontal headcut retreat as a function of discharge height of the headcut upstream slope and relevant land surface and soil properties for soil erosion we analyze the sensitivity of headcut retreat to flow discharge upstream slope and surface roughness and headcut height child simulations indicate that headcut retreat is most significant in zones with either gentle slopes or large headcut heights model parameters have contrasting effects on the retreat rates depending on the size and depth of the pool beneath the headcut and upstream flow hydraulics making the process difficult to predict as a function of topographic thresholds and simple geomorphic transport laws

experiments on headcut growth and migration in concentrated flows typical of upland areas experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas in a laboratory channel packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow the rainfall produced a well developed surface seal that minimized surface soil detachment during overland flow soil erosion occurred exclusively at the headcut and after a short period of time a steady state condition was reached where the headcut migrated at a constant rate the scour hole morphology remained unchanged and sediment yield remained constant a fourfold increase in flow discharge resulted in larger scour holes yet aspect ratio was conserved a sediment bed was deposited downstream of the migrating headcut and its slope depended weakly on flow discharge

erodibility of cohesive streambeds in the loess area of the midwestern usa excess stress parameters critical shear stress c and erodibility coefficient k d for degrading channels in the loess areas of the midwestern usa are presented based on in situ jet testing measurements critical shear stress and k d are used to define the erosion resistance of the streambed the jet testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters streams tested were primarily silt bedded in texture with low densities which is typical of loess soils results indicate that there is a wide variation in the erosion resistance of streambeds spanning six orders of magnitude for c and four orders of magnitude for k d erosion resistance was observed to vary within a streambed from streambed to streambed and from region to region an example of the diversity of materials within a river system is the yalobusha river basin in mississippi the median value of c for the two primary bed materials naheola and porters creek clay formations was 1 31 and 256 pa respectively streambeds composed of the naheola formation are readily eroded over the entire range of shear stresses whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the porters creek clay formation therefore assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams

considerations and observations of stress induced and construction induced excavation damage zone in crystalline rock abstract bsed on the recent observations from two underground laboratories the construction induced excavation damage zone edz ci caused by explosion or by mechanical excavation and stress induced excavation damage zone edz si are distinguished the key location for in situ testing of edz and method development has been the tbm tunnel in aspo hard rock laboratory in sweden since the 1990s the research and development has been continued between 2010 and 2012 with in situ stress measurement campaign and in 2013 with ground penetrating radar investigations with new gpr edz method the new gpr edz method based on frequency analysis of the reflected signal indicates mean depth for edz ci of 120 mm the same zone is indicated also by various other geophysical and laboratory methods and is distinguished as construction induced and continuous around the tunnel perimeter fracture mechanics modelling suggests a lowered hydraulic conductivity of 1 8 10 13 m s in edz si and 4 0 10 11 m s in edz ci biaxial testing suggests a zone in which young s modulus values are lowered and which is not continuous around the tunnel this zone is distinguished as an observable part of stress induced edz si

effect of slope on the growth and migration of headcuts in rills abstract experiments were conducted to examine soil erosion by headcut development and upstream migration in rills typical of upland areas soil material simulated rain overland flow discharge and initial headcut height were held constant in each experiment but initial slope of the bed varied from 1 to 10 air dried crushed and sieved sandy loam to sandy clay loam soil was incrementally packed into a laboratory channel 2 m long and 0 165 m wide to a depth of 0 25 m soil bulk density was 1425 kg m 3 in each experiment a pre formed headcut 25 mm high was constructed 1 5 m downstream of the entrance of the flume simulated rain applied at 21 mm h 1 for 4 h produced a well developed surface seal that minimized the detachment of the surface soil following the rainfall overland flow at a rate of 52 l min 1 was released onto the bed soil erosion occurred at the pre formed headcut overfall and a scour hole developed enlarged and migrated upstream the rate of headcut migration was constant within each experiment but higher slopes of the bed generally resulted in lower rates of migration at slopes on the bed of 2 and smaller the overfall nappe at the headcut brinkpoint remained submerged and a steady state condition was achieved sediment yield and geometry of the scour hole remained constant as the headcut migrated upstream for slopes on the bed of 3 and greater the overfall nappe became aerated and as the headcut migrated upstream the depth of scour increased higher slopes on the bed resulted in deeper scour holes mechanisms of soil erosion included the formation of tension cracks and seal removal at the headcut brinkpoint soil washout along the aerated headcut face and plunge pool scour the slope of the sediment deposit downstream of the migrating headcut was 2 2 for all experiments and suggests that flow discharge and not the initial slope of the bed controlled transport capacity and downstream adjustment of the bed

development of gullies and sediment production in the black soil region of northeastern china abstract gully erosion is an important environmental hazard in the black soil region of northeastern china it is a primary sediment source in the region which needs appropriate soil conservation practices gully incision in rolling hills typical of this region was monitored using real time kinematic gps to assess the rates of gully development and the resultant sediment production from 2002 to 2005 gully heads in the study area retreated between 15 4 and 33 5 m giving an average retreat rate of 8 4 m yr 1 field measurements showed that total sediment production due to gully erosion during the three years ranged between 257 and 1854 m3 yr 1 which is equivalent to 326 to 2355 t yr 1 with gully head retreat accounting for 0 to 21 7 4 4 in average the sediment delivery ratio was especially high during the summer rainy season 56 in average sediment production by ephemeral gullies and permanent gullies was 1 5 times greater than that from surface erosion gully heads retreated faster in the spring freeze thaw period than in the summer the stage of gully development could be identified based on short term changes in the gully erosion rate

experimental study of knickpoint and longitudinal profile evolution in cohesive homogeneous material a series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal profile evolution knickpoints are created by successive drops in base level which simulate intermittent uplift channel morphology and flow characteristics vary systematically along a knickpoint reach as the knickpoint lip is approached width decreases but depth velocity and bottom shear stress increase as a result knickpoints are rapidly destroyed through the process of 1 knickpoint replacement and 2 subsequent knickpoint inclination knickpoint replacement consists of two morphologically distinct erosional reaches the knickpoint face below the knickpoint lip which decreases in slope and an incising reach above the knickpoint lip which increases in slope knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock within experimental limits this study supports the notion that knickpoints do not undergo parallel head ward retreat in homogeneous bedrock given intermittent uplift except in those possible cases of extensive and pervasive bedrock jointing only the erosional sediment transport discontinuity above the knickpoint is developed in cohesive sediments below the knickpoint a steep bedrock channel slope low sediment load and rapid decrease in grain size of sediment load preclude development of the aggradational sediment transport discontinuity knickpoint evolution can be characterized by three general models parallel retreat replacement and inclination these models are based on the following criteria within the knickpoint reach 1 relationship between bottom shear stress 0 and critical shear stress needed to initiate motion c 2 nature of the bed load transport discontinuities and 3 spatial variability of bedrock resistance to fluvial erosion

stability analysis of waterfall cliff face at niagara falls an implication to erosional mechanism of waterfall although recession of waterfalls or knickpoints in bedrock rivers is a common geomorphological process detailed mechanics of waterfall recession has only been examined in a few cases caprock recession model at niagara falls in which gravitational collapse of caprock induced by undercutting notch plays a significant role has been one of the well known models describing the waterfall erosion but the validity of the model has hardly been examined in a quantitative context here we assess the stability of the cliff of waterfall face of niagara falls in terms of the strength of bedrock and the length of undercutting notch the result of a cantilever model analysis shows that the caprock remains stable until the undercut reaches tens to over a hundred meters however the actual length of undercutting notch of waterfall face is up to 10 m and such a long notch to cause gravitational collapse of the caprock can hardly be formed the recession of the waterfall could therefore be caused by gradual detachment of the rock of the waterfall face induced by fluvial erosion of surface water flow rather than by elongation of undercutting notch and episodic gravitational collapses of the caprock

assessment of sidewall erosion in large gullies using multi temporal dems and logistic regression analysis although in the last decades gully erosion has been a thriving research field few studies have specifically addressed the contribution and location of sidewall erosion processes in gullies in this paper sidewall erosion in some large gullies in a mediterranean area anoia penedes ne spain is mapped and assessed for two time intervals 1975 1995 and 1995 2002 using detailed digital elevation models derived from aerial photographs at a scale of 1 5000 to 1 7000 logistic regression analysis is applied to compute the probability of occurrence of gully sidewall erosion from terrain variables the results confirm the complex nature of sidewall processes whose intensity is most probably related to rainfall characteristics prolonged wet soil conditions in the period 1995 2002 together with the large and high intensity rainfall of an extreme event occurred on 10th june 2000 help to explain the different sediment production rates 16 0 4 mg ha 1 year 1 in 1975 1995 and 83 6 3 mg ha 1 year 1 in 1995 2002 the logistic regression analysis revealed that gully wall slope angle was the main factor controlling gully sidewall failure in gully walls with high slope angles tension crack development is the main process promoting wall collapse the application of the logistic regression model showed a high overall accuracy 87 but over 50 of commission and omission errors for the class of interest sidewall erosion in agreement with the variance explained by the model

weir flows and waterfalls two dimensional free surface flows which are uniform far upstream in a channel of finite depth that ends suddenly are computed numerically the ending is in the form of a vertical wall which may force the flow upward before it falls down forever as a jet under the effect of gravity both subcritical and supercritical solutions are presented the subcritical solutions are a one parameter family of solutions the single parameter being the ratio between the height of the wall and the height of the uniform flow far upstream on the other hand the supercritical solutions are a two parameter family of solutions the second parameter being the froude number moreover for some combinations of the parameters it is shown that the solution is not unique

simulation of cracking near a large underground cavern in a discontinuous rock mass using the expanded distinct element method abstract the rock masses in a construction site of underground cavern are generally not continuous due to the presence of discontinuities such as bedding joints faults and fractures the performance of an underground cavern is principally ruled by the mechanical behaviors of the discontinuities in the vicinity of the cavern a number of experimental and numerical investigations have demonstrated the significant influences of discontinuities on the mechanical thermal and hydraulic behaviors of discontinuous rock masses indicating that the deformation mechanism and stability of rock structures in the discontinuous rock masses depend not only on the existing discontinuities but also on the new cracks generated and thereafter keep propagating due mainly to the stress redistribution induced by excavation in this study an expanded distinct element method edem was developed for simulating the crack generation and propagation due to the shear and tension failures in the matrix rock blocks using this method excavation simulations of deep underground caverns have been carried out on the models with differing depths of cavern and differing geometrical distributions of the existing discontinuities model experiments by using the base friction test apparatus were conducted to verify the proposed numerical approach furthermore the support effects of rock bolts on controlling the deformations of the rock mass surrounding a cavern and movements of key blocks were evaluated by means of the edem approach

combined finite discrete element modelling of rock fracture and fragmentation induced by contour blasting during tunnelling with high horizontal in situ stress a combined finite discrete element method fdem parallelized on the basis of gpgpu is implemented to model the rock fracture and fragmentation process and the resultant excavation damaged zone edz development induced by the controlled contour blasting which was conducted in the tasq tunnel with high in situ stresses in the aspo hard rock laboratory in sweden the combination of in situ stress field equation of state based blast loading fracturing in tension and shear with gas flow loading of fractures enables the modelling of complex dynamic interactions from multiple blast rounds for the contour blasting under high horizontal in situ stresses blasting induced fractures initially propagate horizontally even though the holes are decoupled later these fractures coalesce into larger cracks which prevents the formation of smooth tunnel walls and increases the edz smoother surfaces are created at the crown and invert by the propagation of long fractures in the horizontal direction due to the combined effect of the free surfaces provided by the adjacent blast holes and the maximum principal stress induced by in situ stresses the fractures at the lower part of the tunnel sidewalls have the tendency to propagate upwards with a diagonal direction about 60 relative to vertical in accordance with the field test results removing the in situ stresses results in smoother sidewall fracturing with more damage in the crown and invert increase in rock heterogeneity above a threshold induces more fractures increasing the detonation timing between blast holes induces more damage into the rock mass and fragmentation in the burden outcomes of this study show that the gpgpu parallelized y hfdem ide provides a powerful tool to replicate the mechanisms of rock fracture and fragmentation induced by blasting

experimental study on the development of collapse of overhanging layers of gully in yuanmou valley china the collapse of overhanging layers of the heads and sidewalls of gullies accelerates gully development thus the development features of the collapse of overhanging layers must be understood in this study the process of collapse of overhanging layers in man made soil bodies containing different soil layers was investigated by flow scouring experiments a soil body composed of a lower sandy layer and an upper layer of clay soil was evaluated and a field soil profile was modelled in a test tank with an open side the results revealed that the occurrence of the collapse of overhanging layers could be attributed to the development of a scour hole in the open side the greater the flow rate the lower the proportion of on wall runoff on wall runoff played a more important role in the development of scour holes than off wall runoff the shape of scour holes formed under different flow conditions was similar also the scour hole is not apparently related to the flow rate implicating that the collapse is more dependent on rainfall runoff time rather than rainfall runoff intensity in the studied region this is substantially different from other soil erosion types that are closely related to rainfall intensity moreover the maximum length s l and height s h of the scour holes increased gradually with accumulated flow and scouring time in addition the height of the hanging soil body h d gradually decreased and a linear relationship between sl and hd was observed h d 0 359 s l 117 3 r 2 0 861 p 0 003 after the seventh scour test the scour hole triggered the collapse of the upper clay layer the cantilever model was modified to predict the stability of the overhanging soil body it provided a satisfactory explanation of the static balance of the weight of the soil as well as the tensile strength after the seventh scour test the measured and predicted threshold values of h d s l and t c were nearly equal and a stable balance state was attained however when a 10 cm tension crack appeared in the overhanging soil body a stable balance state was obtained during the sixth scour test c 2013 elsevier b v all rights reserved