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

duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

the chamberlin 1910 balanced section context contribution and critical reassessment abstract chamberlin 1910 was the first to quantitatively predict the shape and position of a basal detachment from detailed surface observations his predicted detachment beneath the appalachian valley and ridge fold thrust belt of central pennsylvania is substantially deeper and differs in geometry from the current interpretation the modern profile by faill and nickelsen 1999 across the same area shows a much shallower planar lower detachment and the presence of a duplex below the surface folds a significant conceptual difference from the chamberlin profile two assumptions prove to be critical in causing the discrepancies the width of the segments used as the unit of analysis and the assumption of constant bed length the segmentation scheme led to an erroneous conjugate fault detachment geometry the depth discrepancy is primarily due to ignoring the layer parallel strain the duplex is of the coupled roof style and not responsible for the differences a result supported by an experimental model we use the modern profile and the area depth relationship to quantify the effects of uncertainty on profile geometry and estimates of orogenic shortening small differences in the assumptions inference error lead to significant differences in displacement detachment location and layer parallel strain our best area balanced interpretation is a slightly modified version of the faill and nickelsen profile

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

near vent chemical processes in a hydrothermal plume insights from an integrated study of the endeavour segment support from onc for sample collection is gratefully acknowledge as is the analytical expertise of jody spence bruno keiffer and maureen soon who helped generate the data set reported here funding for sample analysis came from an nserc discovery grant to l a c ocean networks canada is supported through the canadian foundation for innovation all data are provided in the supporting information or through onc at http dmas uvic ca datasearch

a unified model for subaqueous bed form dynamics 1 bed form evolution remains dynamic even in the special case of steady uniform flow data from the sandy braided north loup river nebraska show that roughness features on the channel bottom display a statistical steady state and robust scaling that are maintained through the collective interactions of transient short lived bed forms motivated by such field data and laboratory observations of bed form growth we develop a nonlinear stochastic surface evolution model for the topography of bed load dominated sandy rivers in which instantaneous sediment flux explicitly depends on local elevation and slope this model quantitatively reproduces laboratory observations of initial growth and saturation of bed forms from a flat surface and also generates long term dynamical behavior characteristic of natural systems we argue that the variability in geometry and kinematics of bed forms in steady flow and the existence of roughness at all wavelengths up to the largest dunes are a consequence of the nonlinear relationship between sediment flux and topography subject to noise

a 1 d morphodynamic model for rill erosion 1 the key objective of this paper is to develop a 1 d hydrodynamic and sediment transport model namely rill1d that handles transcritical flows over abrupt changes e g formation of pool crests and width change in a single rill and also predicts changes in rill bed elevation replication of rill conditions in terms of flow and bed evolution changes necessitated the use of an enhanced tvd maccormack scheme with implementation of tseng s surface gradient method to provide an oscillation free solution over formed pool crests the model at the end of each time step calculates the flow depth velocity and bed shear stress and provides changes in bed elevation and grain size distribution within a cell the hydrodynamic performance of rill1d is evaluated by comparing its results with either analytical solutions or experimental observations from various benchmark tests with rigid bed conditions that include 1 formation of a hydraulic jump in a rectangular channel 2 steady subcritical and transcritical flow without a shock over a hump 3 formation of a hydraulic jump in a converging diverging frictionless channel and 4 flows over step pools it is shown that the enhanced tvd maccormack scheme adequately simulates transcritical flows by preserving the mass conservation and reducing the artificial numerical error the scheme also approximates satisfactorily depth and velocity for a single rill with a fixed bed consisting of steps and pools except near the pool headwalls where a backroller forms with negative velocities the sediment component of rill1d is evaluated by two field investigations of single rills with mobile beds with the field data the code is tested for its ability to reproduce measured values of sediment transport rates a sensitivity analysis is performed to assess the effects of cell size and critical erosional strength in the predictive ability of the model rill1d performs reasonably well in these simulations in terms of sediment prediction rates and fared adequately in terms of replicating rill bed morphology

a model for knickpoint migration in first and second order streams we introduce a new theoretical framework for knickpoint migration in unventilated flows prevalent in channelized semicohesive first and second order streams characterized by a sequence of constrictions and expansions in the channel cross section a key feature of the framework is the inclusion of shear induced fluvial erosion due to flow suction at the nappe as the key mechanism driving migration channel surveys water stage time lapse photography and laser scans from mud creek iowa usa confirm this unventilated erosive mechanism and reveal a dual advective diffusive mode of retreat we treat the fluvial bed shear stress as being similar to the flow on the lee side of a submerged obstacle and derive a governing equation which is a generalized burgers equation the equation is solved to successfully simulate knickpoints monitored in two sites iowa and mississippi and a modified peclet number is used to analyze the advective diffusive nature of the knickpoint migration

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

influence of detachment layer thickness on style of thin skinned shortening abstract there are several parameters which may determine whether detachment folds or thrust faults develop in thin skinned compressional systems a fundamental but little publicised parameter is apparent from consideration of detachment fold growth as a detachment fold amplifies its core must be filled by redistribution of ductile material if the mobile material were insufficient to supply the volume required by the fold core through redistribution fold growth would be inhibited and eventually faulting would occur the volume of a detachment fold core is compared here with the volume of available underlying mobile material for various amounts of shortening and sizes of fold the occurrence of detachment folds versus thrusts in several compressional belts is successfully predicted by this relationship

field mapping and digital elevation modelling of submerged and unsubmerged hydraulic jump regions in a bedrock step pool channel high resolution tacheometric field surveying was integrated with computer assisted drafting to visualize and contrast three dimensional bed and water surface digital elevation models dems for submerged and unsubmerged hydraulic jump regions in a bedrock step pool channel measurements were conducted for two discharge conditions since previous applica tions of three dimensional field mapping and digital elevation modelling of stream channels have been limited to smoothly contiguous gravel bedded systems surveying was optimized by topographic setting and scaled to localized bed and water surface discontinuities traces and visualizations of the jump regions indicated that dichotomous decimetre shifts in water surface topography occurred for both jump regions from lower to higher discharges systematic removal of the survey points and dem differencing indicated that point densities of ten points per square metre in conjunction with a survey structure targeting grade breaks of 0 3 0 5 m were required to capture decimetre form variations of the natural jump regions the dems highlight the importance of recognizing the relationship between transcritical flow structures and localized topographic heterogeneities in bedrock channels copyright 2006 john wiley sons ltd

cleavage duplexes in the marcellus shale of the appalachian foreland abstract cleavage duplexes are zones of platy spaced cleavage that either parallel bedding or ramp through shale sections examples are 2 60 m thick the cleavage within the zones is generally subperpendicular to the zone boundaries but sigmoidally dragged against floor roof and internal thrusts between the well developed floor thrust and the more obscure roof thrusts of the duplexes the shale has been shortened 30 perpendicular to cleavage the thrusts are sharp strain discontinuities because both overlying and underlying shale is uncleaved and less deformed

hillslope and channel evolution in a marine terraced landscape santa cruz california a flight of marine terraces along the central california coastline provides a unique setting for the study of topographic evolution wavecut platforms mantled by 2 6 m of marine terrace cover deposits are separated by 10 50 m tall decaying sea cliffs paleocliff edges become more rounded with age yet the details of the profiles and frequent bedrock exposure on the upper slopes imply weathering limited transport five bedrock stream channels etched through the marine terrace sequence display one to three distinct convexities in their longitudinal profiles detailed hand level surveys of the hillslopes and of the stream channel longitudinal profiles constrain hillslope evolution and channel incision components of a numerical model of landscape evolution we account for regolith production as a function of regolith depth in accord with the field observation that hillslope processes are presently dominated by the activities of burrowing rodents the transport process is taken to be diffusive stream incision is assumed to be controlled by stream power for which we use the surrogate of local drainage area slope product best fits of the numerical model to field data imply hillslope diffusivity is 10 m2 kyr 1 regolith production rate on bare bedrock is 0 3 m kyr 1 and falls off rapidly with regolith cover and the constant controlling the efficiency of stream incision is 5 to 7 10 7m 1 kyr1

assessing uncertainties in balanced cross sections abstract balanced structural cross sections are models that are fit to incomplete data the models are under constrained with respect to any particular two dimensional line length model but enough data generally exists to yield a well constrained area balance solution furthermore the area balance encompasses all possible line length solutions therefore where the primary objective of section balancing is the determination of horizontal shortening magnitude area balancing provides an analytical solution we use this analytical solution to develop a comprehensive robust analysis of the uncertainty in shortening estimates resulting from cross section balancing the analytical solution allows us to propagate errors formally on the input parameters stratigraphic thicknesses depth to decollement eroded hanging wall cutoffs through the equations and produce the resulting uncertainty on the magnitude of shortening balanced cross sections from the subandean belt of the central andes are used to demonstrate the relative importance of stratigraphy and eroded hanging wall cutoffs in the contribution to the overall error

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

hydraulic erosion of cohesive riverbanks abstract this study identifies and assesses the controls on hydraulic erosion of cohesive riverbanks on a 600 m reach of an urban ephemeral stream with active bank erosion we examined hydraulic bank erosion by separating estimated bank shear stress into four properties magnitude duration event peak and variability the values of these independent variables were used as a bank erosion context at three transects stepwise regression showed that the event peak maximum peak of excess shear stress best predicts cohesive bank erosion at the two transects with moderate critical shear stresses 1 93 4 08 n m 2 while the variability all peaks of excess shear stress best predicts erosion at the transect with low critical shear stress 0 95 n m 2 these results suggest that the amount of hydraulic erosion of cohesive riverbanks is dictated by flow peak intensities finally the results of this study were combined with results from previous bank erosion studies to produce a conceptual model for estimating bank erosion rates based on their silt clay content

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

development of longitudinal profiles of alluvial channels in response to base level lowering degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0 01 cm cm degradation was initiated by lowering base level to a fixed position and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat diffusion equation it is based on the equation of sediment continuity combined with an assumption regarding sediment transport namely that sediment discharge is linearly proportional to the channel slope in accordance with the boundary and initial conditions imposed by the experimental setup and procedure the basic equation is amenable to an analytical solution which defines bed elevation at any distance and time as a function of the amount of base level lowering and a diffusion coefficient additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model the results show that in homogeneous alluvial sediments not subject to armouring the ultimate result of base level lowering by a certain amount is degradation all along the channel by the same amount the main impact of erosion is felt in the early stages after initiation of the process and mainly near the mouth the rate of degradation at any station along the channel reaches a peak and then slowly decreases with time and the peak rate is attenuated with distance from the outlet the model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times

modeling of knickpoint retreat on the roan plateau western colorado 1 the roan plateau in western colorado constitutes a natural experiment for studying landscape response to a drop in base level late cenozoic incision of the upper colorado river led to elevational isolation of the plateau and initiation of a wave of incision into its southern edge knickpoints oversteepened reaches that contain waterfalls 60 110 m in height mark the upstream extent of this headward propagating wave that this incision has occurred in a laterally extensive well stratified and essentially flat lying bedrock and in an area with relatively uniform climate implies that it should serve as a good test of existing knickpoint propagation models we predict the locations of knickpoints by using a stream power based celerity model in which knickpoint recession rate is a power function of drainage area and is proportional to rock susceptibility to erosion models of the parachute and roan drainages 17 and 16 knickpoints respectively show expected rapid initial knickpoint propagation rates which decline as drainage area decreases stepwise at tributary junctions the modeled positions of knickpoints match well with observed features using a single combination of parameters to model retreat in both drainage basins we compare our celerity model results with past studies and explore how longitudinal profile analysis may be used to derive independently the exponent on drainage area in the celerity model

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

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex