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

profiles of trace elements and stable isotopes derived from giant long lived tridacna gigas bivalves potential applications in paleoclimate studies this study investigates the environmental and biological controls on trace element partitioning and stable isotope composition of modern giant long lived bivalves tridacna gigas with the aim to use these archives for paleoclimatic reconstructions firstly the intra shell variability is studied by measuring time equivalent profiles in the different shell layers characterised by different growth rates secondly the inter site variability is studied by comparing profiles derived from three modern specimens collected in sites across the indo pacific region characterised by different ranges of temperature and productivity these results show that 18o profiles are highly reproducible across the shell regardless of significantly different growth rates shell 18o is primarily controlled by water 18o and temperature comparison of intra shell mg ca profiles shows a clear and systematic partitioning where inner layer mg ca values are a least 2 3 times higher than outer layer and hinge areas inner layer mg ca shows seasonal oscillations but superimposed on an ontogenetic trend with increasing values and increasing amplitude mg ca oscillations with age the sr ca profiles do not show clear reproducible seasonal trends in the different shell zones it is concluded that mg ca and sr ca profiles appear to reflect a combination of biological and environmental controls that will need to be disentangled before using these proxies in paleoclimatic studies finally intra shell ba ca profiles are reproducible in great detail for all modern specimens studied inter site comparison shows that the amplitude and the timing of the ba ca peaks appear to reflect the timing and the amplitude of the chlorophyll peaks associated with phytoplankton blooms at each locality making this tracer a potential paleoproductivity indicator

strong biological controls on sr ca ratios in aragonitic marine bivalve shells 1 it is well known that skeletal remains of carbonate secreting organisms can provide a wealth of information about past environments sr ca ratios have been successfully used as a temperature proxy in corals and sclerosponges previous work on aragonitic bivalve shells has not been conclusive but suggests a major control of growth rate on sr ca ratios as many studies have used bivalve growth rates to determine temperature we tested if sr ca ratios could predict temperature through its relationship with growth rate shells from the two species of clams from the same family veneroidea studied here saxidomus giganteus and mercenaria mercenaria show vastly different seasonal sr ca profiles a strong relationship between average annual sr ca ratios and annual growth rate was found in s giganteus shells from both washington r2 0 87 and alaska r2 0 64 usa but not in m mercenaria shells from north carolina usa furthermore the sr ca growth rate relationship was also evident upon a more detailed inspection of subannual growth rates in s giganteus r2 0 73 although there were significant positive correlations between sr ca ratios and temperature in s giganteus shells the correlations were weak 0 09 r2 0 27 and thus sr ca ratios cannot be used as a reliable temperature proxy in these species of aragonitic bivalves it is clear from this study that sr ca ratios are not under thermodynamic control in either clam species since thermodynamics predict a negative correlation between sr ca ratios and temperature in aragonite this points toward dominance of biological processes in the regulation of sr2 this is also reflected by the largely differing sr ca partition coefficients dsr in these shells dsr 0 25 when compared to inorganic coral and sclerosponge studies dsr 1 all of which show a negative dependence of sr ca on temperature we suggest that caution be taken when using sr ca in any biogenic aragonite as a temperature proxy when the dsr greatly deviates from one as this indicates the dominance of biological controls on sr ca ratios

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

trace and minor element records in aragonitic bivalve shells as environmental proxies abstract investigation of the geochemical composition of bivalve shells can provide information on changes in the marine environment occurring during the lifespan of an organism three species locally abundant in the adriatic sea were chosen in this study namely glycymeris pilosa callista chione and venus verrucosa of these g pilosa has the longest lifespan exceeding 50 years and therefore presents a potential archive of decadal climate variability the other two species c chione and v verrucosa are commercially important samples were collected alive by scuba diving in the north adriatic sea near barbariga istria major growth increments in these shells form on an annual basis and are clearly visible in shell cross sections enabling correct temporal positioning of geochemical data obtained from different shell parts in this study we applied laser ablation inductively coupled plasma mass spectrometry la icp ms in line scan mode to determine the trace and minor element composition along the major growth axis in the hinge area of the shells objectives of this study were to compare signals within and between species and to relate results to available environmental parameters high resolution time series of na cashell mg cashell and sr cashell display characteristic cyclic variations na cashell minima and mg cashell and sr cashell maxima occur near shell annual growth lines in all three studied species results indicate that seawater temperature is not a main driver for incorporation of these elements ba cashell data vary between years and species and have noncyclic sharp peaks callista chione and v verrucosa specimens show higher peaks and background ba cashell values than g pilosa most pronounced peaks in ba cashell occurred at the beginning of the growing season corresponding to the end of year 2011 and beginning of year 2012 and coincided with unusual dense water formation and subsequent changes in the plankton communities in the adriatic sea implications of our study suggest that synchronous peaks in studied specimens are caused by external forcing which could occur on larger regional scales encouraging future studies covering wider area in the adriatic sea and other parts of the mediterranean

laboratory experiments on the influence of slope length on runoff percolation and rill development experiments to test relationships between slope length percolation and runoff were carried out in a laboratory flume under simulated rainfall at intensities from 24 2 26 mm h 1 a composite soil subject to sealing was mixed from a clay and a sand and tested on 2 39 m 7 18 m and 17 m slopes for a minimum of 200 min runoff discharge is not a simple function of rainfall excess and slope length but shows a complex pattern dominated by surface sealing rill development and headcut incision rill development by concentrated surface wash conformed to established threshold hydraulic conditions but subsequent headcut incision was necessary to breach the seal and significantly affect percolation runoff ratios headcut evolution is complex apparently reflecting hydraulic instability and possibly different stages in seal development headcut and rill incision shows a cyclic pattern interspersed with broad areas of sheetwash and colluvial deposition where percolation rates are very low

crystal fabrics and element impurities sr ca mg ca and ba ca in shells of arctica islandica implications for paleoclimate reconstructions sr ca mg ca and ba ca values are heterogeneously distributed in shells of arctica islandica these patterns are largely associated with crystal fabrics size habit and orientation of crystals or the processes controlling them the outer sublayer of the outer shell layer oosl homogenous and irregular simple prismatic crystal fabrics contained element ca values up to 62 higher than the inner sublayer iosl crossed acicular crossed lamellar fine crossed lamellar and irregular simple prismatic crystal fabrics a gradual decrease in sr ca and mg ca values was observed from the outer portions of the oosl toward the oosl iosl transition zone this chemical shift was accompanied by a gradual transition from homogenous crystal fabrics into crossed lamellar acicular crystal fabrics near annual growth lines irregular simple prisms i e during periods of slow growth sr and mg seemed to be deposited in equilibrium with the ambient environment because the sr ca and mg ca values approached values typical for seawater during the remainder of the growing season sr ca and mg ca remained far below values expected for thermodynamic equilibrium ba ca peaks however occurred erratically at different times of the year without any noticeable changes in crystal fabrics likely the environmental information contained in these peaks was less severely filtered by vital effects than in sr and mg the findings of the present study can help to develop new techniques with which extract environmental signals from the metal to calcium ratios of bivalve shells

implications of bank failures and fluvial erosion for gully development field observations and modeling 1 gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop field observations attribute the headward growth and widening of many gully systems to gravitational mass wasting processes of oversteepened sidewalls soil saturation groundwater sapping and tension crack development contribute to the instability recent landscape evolution models treat such mass failures as slope dependent continuous sediment transport processes sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient in this study we first present an explicit physically based theory for the stability analysis of gully heads and walls the theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face then we test the theory against field data collected in our field site in colorado and against other published data second the theory is implemented in a one dimensional hillslope profile development model and the three dimensional channel hillslope integrated landscape development child to study the effects of soil cohesion erosion thresholds and stochastic climate on the tempo of gully development and morphology preliminary results indicate that wider and shallower gullies develop and integrate forming wide valleys when soil cohesion is small as soil cohesion increases erosion slows down gullies become deeper with vertical walls and episodic mass failures occur differences in storm intensity duration characteristics and erosion thresholds are predicted to have a significant impact on gully development vertical gully walls develop rapidly and gullies enlarge by slab failures in a climate characterized by high intensity short duration storm pulses however under low intensity long duration storms gullies quickly stabilize and vertical walls are eliminated and rounded forming diffusion dominated hilltops erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction lowering the erosion threshold enhances gully widening by slab failures gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls

chemical labelling of oyster shells used for time calibrated high resolution mg ca ratios a tool for estimation of past seasonal temperature variations abstract the geochemical compositions of biogenic carbonates are increasingly used for palaeoenvironmental reconstructions the skeletal 18o temperature relationship is dependent on water salinity so many recent studies have focused on the mg ca and sr ca ratios because those ratios in water do not change significantly on short time scales thus those elemental ratios are considered to be good palaeotemperature proxies in many biominerals although their use remains ambiguous in bivalve shells here we present the high resolution mg ca ratios of two modern species of juvenile and adult oyster shells crassostrea gigas and ostrea edulis these specimens were grown in controlled conditions for over one year in two different locations in situ monthly mn marking of the shells has been used for day calibration the daily mg ca ratios in the shell have been measured with an electron microprobe the high frequency mg ca variation of all specimens displays good synchronism with lunar cycles suggesting that tides strongly influence the incorporation of mg ca into the shells highly significant correlation coefficients 0 70

space time variability of denudation rates at the catchment and hillslope scales on the tyrrhenian side of central italy studies on denudation rates can provide insight into the influence of climate change tectonics and human activities on landscape evolution research performed in central italy has shown considerable spatial variability of denudation rates in the major river basins these studies have focused mainly on the tyrrhenian side of the italian peninsula where plio pleistocene marine deposits filling nw se elongated sedimentary basins have been uplifted during the quaternary up to several hundreds of meters above present sea level small sub catchments developed on clays are affected by sharp and or rounded edged badlands i e calanchi and biancane representing denudation hot spots in the present day morphoclimatic framework in this paper we analyze the relationships between indirectly estimated denudation rates at the catchment scale and field monitoring data at the hillslope scale we attempt to better understand and quantify all hillslope processes that contribute to seasonal variability of denudation to help with predicting the net input from hot spots to the overall estimated sediment yield at the basin outlets at the hillslope scale we discuss in particular the variability of denudation rates at calanchi and biancane badlands as a function of their different morphoevolution

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

a numerical method for simulating one dimensional headcut migration and overtopping breaching in cohesive and zoned embankments 1 an erosion and force moments equilibrium based numerical method is proposed to simulate one dimensional headcut migration in cohesive embankment overtopping breaches and to simulate the one dimensional breach process of zoned embankment dams before they fully breach through at each time step of the simulation the depths and velocities of the breach outflow are calculated by solving the one dimensional shallow water equations three dimensional slope stability is checked and the eroded soil and instable soil block are removed the method was tested using data from a u s department of agriculture agricultural research service cohesive embankment and the yahekou zoned fuse plug test results showed that the proposed method predicts well the one dimensional headcut shape in the cohesive dam and the breach shape in the zoned embankment dam provided that the inputs are appropriately assigned sensitivity studies showed that the breach processes for a weakly cohesive dam under a small overtopping depth and a highly cohesive dam are controlled by erosion only they also showed that the breach processes for a weakly cohesive dam under a relatively large overtopping depth a noncohesive dam and a zoned embankment dam are all controlled by erosion and force moments equilibrium

sediment transport through self adjusting bedrock walled waterfall plunge pools many waterfalls have deep plunge pools that are often partially or fully filled with sediment sediment fill may control plunge pool bedrock erosion rates partially determine habitat availability for aquatic organisms and affect sediment routing and debris flow initiation currently there exists no mechanistic model to describe sediment transport through waterfall plunge pools here we develop an analytical model to predict steady state plunge pool depth and sediment transport capacity by combining existing jet theory with sediment transport mechanics our model predicts plunge pool sediment transport capacity increases with increasing river discharge flow velocity and waterfall drop height and decreases with increasing plunge pool depth radius and grain size we tested the model using flume experiments under varying waterfall and plunge pool geometries flow hydraulics and sediment size the model and experiments show that through morphodynamic feedbacks plunge pools aggrade to reach shallower equilibrium pool depths in response to increases in imposed sediment supply our theory for steady state pool depth matches the experiments with an r 2 value of 0 8 with discrepancies likely due to model simplifications of the hydraulics and sediment transport analysis of 75 waterfalls suggests that the water depths in natural plunge pools are strongly influenced by upstream sediment supply and our model provides a mass conserving framework to predict sediment and water storage in waterfall plunge pools for sediment routing habitat assessment and bedrock erosion modeling

climate records from a bivalved methuselah arctica islandica mollusca iceland we measured annual shell growth rates of a 374 year old radiometrically confirmed bivalve mollusk specimen of arctica islandica linnaeus this bivalve represents the oldest ever reported individual animal in addition we analyzed 1051 individual powder samples from ontogenetic years two to 40 80 of the entire shell height for isotope analyses oxygen and carbon isotopes exhibit clear intra annual cycles the highest measured carbon isotope values on average 2 73x occurred during summer i e during maximum primary productivity the d 18 oaragonite derived temperature time series cover the period of ad 1496 1533 temperatures calculated from oxygen isotopes ranged from 4 5 to 9 3 8c and exhibit a mean value of 6 2 8c the latter coincides well with the 1854 2003 mean value of 6 81 8c for sea surface temperature between february and september growing season of a islandica neither oxygen nor carbon isotopes exhibit age related unidirectional trends however d 13 caragonite and d 18 oaragonite values fluctuated at decadal periods of four six and eight to nine years nao type periods as well as 12 14 years which may represent teleconnections to cycles in the tropical atlantic annual shell growth is positively correlated to intra annual d 18 oaragonite minima i e warm summer temperatures r 2 0 34 and to intra annual d 13 caragonite minima higher food supply r 2 0 42 using a linear multiregression model 65 of the variation in annual shell growth can be explained by summer temperature and food supply the formation of extremely narrow annual increments coincides with major volcanic eruptions e g tambora 1815 a period of extremely variable growth occurred during the culmination of the little ice age in iceland between ca ad 1550 and 1620 shell growth during ad 1765 1780 however was characterized by very little year to year variability probably as the result of extremely mild climate near the end of the little ice age this study demonstrates that shells of a islandica provide

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

calibrating the element composition of donax deltoides shells as a palaeo salinity proxy abstract bivalve shell chemistries provide valuable insights into past environments if relationships between shell chemistry and environmental conditions are known this study assessed the suitability of goolwa cockle donax deltoides shell element composition as an environmental proxy for southern australia a region lacking high resolution environmental records cockles were collected along an environmental gradient and the element composition of corresponding water and shell samples were quantified using a combination of icp aes and icp ms for water samples and la icp ms for shells of the elements analysed ba ca li ca and mg ca in shells reflected ambient water chemistries shell ba ca concentrations showed a significant negative relationship with recorded salinities and a positive non significant trend was observed between mg ca and water temperature further assessment of environmental and biological controls on element incorporation are required however ba ca in d deltoides shells appears a suitable palaeo salinity tool and has the potential to provide an easily accessible and valuable proxy to gain insights into past environmental conditions over extensive time scales

neotectonic activity and parameters of seismotectonic deformations of seismic belts in northeast asia abstract the arctic asian and okhotsk chukotka seismic belts bordering the kolyma chukotka crustal plate are the subject of our study aimed at reconstructing the stress strain state of the crust and defining the types of seismotectonic deformation std in the region based on the degrees of activity of geodynamic processes the regional principles for ranking neotectonic structures were constrained and the corresponding classes of the discussed neotectonic structures were substantiated we analyzed the structural tectonic positions of the modern structures their deep structure parameters and the systems of active faults in the laptev kharaulakh koryak and chukotka segments and chersky seismotectonic zone as well as the tectonic stress fields revealed by tectonophysical analysis of the late cenozoic faults and folds from the earthquake focal mechanisms the average seismotectonic strain tensors were estimated using the geological geostructural geophysical and gps data and corresponding average tensors the directions of the principal stress axes were determined a regularity in the changes of tectonic settings in the northeast arctic was revealed

jet and hydraulic jump near bed stresses below a horseshoe waterfall 1 horseshoe waterfalls are a common but unexplained feature of bedrock rivers as a step toward understanding their geomorphology a six component force torque sensor was used to evaluate clear water scour mechanisms in a 0 91 m broad crested horseshoe waterfall built into a 2 75 m wide flume to measure longitudinal lateral and temporal profiles of the near bed spatially averaged stress vector jet impact caused moderate near bed stress downstream of the impact point the stress vector shifted into a much more forceful upthrust with peak stress beyond the boil crest instantaneous fluctuations in near bed lift exceeded drag by 5 10 times and occurred over a wider range of frequencies suggesting a larger role for particle entrainment predictions of bed stress significantly underestimated actual conditions these results show that horseshoe waterfalls have an extended region of high scour along their centerline corresponding with the three dimensional convergent dynamics associated with both jet impact and jump enhanced lift fluctuations

gullying and erosion control at archaeological sites in grand canyon arizona gully erosion of cultural sites in grand canyon national park is an urgent management problem that has intensified in recent decades potentially related to the effects of glen canyon dam we studied 25 gullies at nine sites in grand canyon over the 2002 monsoon erosion season to better understand the geomorphology of the gully erosion and the effectiveness of erosion control structures ecs installed by the park under the direction of the zuni conservation program field results indicate that hortonian overland flow leads to concentrated flow in gullies and erosion focused at knickpoints along channels as well as at gully heads though groundcover type soil shear strength and permeability vary systemat ically across catchments gradient and to a lesser degree contributing drainage area seem to be the first order controls on gully extent location of new knickpoints and ecs damage the installed ecs do reduce erosion relative to reaches without them and initial data suggest woody checkdams are preferable to rock linings but maintenance is essential because damaged structures can exacerbate erosion topographic data from intensive field surveys and detailed photogrammetry provide slope contributing area data for gully heads that have a trend consistent with previous empirical and theoretical formulations from a variety of landscapes the same scaling holds below gully heads for knickpoint and ecs topographic data with threshold coefficients the lowest for gully heads slightly higher for knickpoints and notably higher for damaged ecs these topographic thresholds were used with 10 cm digital elevation models to create simple predictive models for gully extent and structure damage the model predictions accounted for the observed gullies but there are also many false positives purely topographical models are probably inadequate at this scale and application but models that also parameterize the variable soil properties across sites would be useful for predicting erosion problems and ecs failure copyright 2006 john wiley sons ltd

rapid coastal geomorphic change in the river murray estuary of australia abstract the river murray estuary a ramsar wetland site has experienced considerable rapid coastal change during the quaternary the interplay of aeolian processes river flows tidal oscillations wave action and variations in relative sea level due to global sea level changes and land subsidence provides the energy for the ongoing dynamism often accelerated by human impacts the estuary is the terminus of the murray darling catchment which covers 1 073 million km 2 of the australian continent terminal lakes alexandrina albert and the coorong lagoon are holocene features occupying tectonically subsiding quaternary interdune areas they formed in response to eustatic sea level rise following the last glacial maximum ca 20 ka the last interglacial shoreline 125 000 yr bp parallels the modern shoreline several kilometres inland dislocation of the last interglacial shoreline demonstrates ongoing tectonic subsidence as does historical seismic activity the northern half of hindmarsh island formed during last interglacial times when it was the sink for dominant longshore transport from the southeast which pushed the river murray westward partly explaining the large bend in the river murray at goolwa the modern coastal barriers of sir richard peninsula and younghusband peninsula formed from 7000 yr ago following glacier melt and sea level rise subsequently the barriers have migrated landward sporadically exposing lagoonal sediments on the ocean beaches differential loading of the soft lagoonal sediments by advancing dunes possibly in conjunction with seismic events has deformed and elevated them to up to 10 m above present sea level apsl during the mid holocene an extensive sand flat with associated dunes formed immediately inland of the coastal barrier at least six generations of late pleistocene dune systems occur in the region for example during last glacial times the climate was drier colder and windier than at present and a system of parallel west east trending yellow red desert dunes developed around the lakes aeolian processes remain important with occasionally up to 5000 tonnes of sand being in motion along 10 km of the modern shoreline during mouth migration dunes up to 2 m high have been formed and vegetated in 12 months directly inland from the mouth and replicating the formation of older dunes on hindmarsh island elsewhere sand blown directly from the barrier system infills channels barrage construction on the beach facies of the last interglacial shoreline transformed the estuary into freshwater lakes with permanently raised water levels and reduced the tidal prism by 90 increased deposition upstream and downstream accompanied barrage construction as have accelerated lakeshore erosion and the growth and consolidation of the flood tidal delta bird island these human accelerated changes provide rapidly formed analogues of older quaternary features and aid their interpretation the shape and location of the murray mouth is constantly changing migrating over 1 6 km since the 1830s migrations of up to 6 km over the past 3000 yr have influenced sedimentation on the landward shore of the back barrier lagoonal system

reconstructing the history of fluid flow at cold seep sites from ba ca ratios in vesicomyid clam shells hydrogen sulfide discharge at cold seep sites is recorded as enrichment in the barium to calcium ba ca ratio in shells of vesicomyid clams collected live from cold seeps in monterey canyon and the cascadia margin a direct relationship between increased ba fluxes from cold seeps and ba incorporation into shells was established for the cascadia margin site for the monterey canyon site a 2 yr episode of high fluid flow centered on 1992 was inferred from coherent changes in the ba ca profiles of threecalyptogena kilmerishells comparison with precipitation and d 18 o data indicates that this high flow period may have been driven by an increase in rainfall after the 1988 1990 california drought high resolution records preserved in clam shells are shown to be useful in elucidating characteristics history and possible mechanisms driving fluid discharge at continental margin seeps thus establishing their potential use as paleotracers of fluid seepage events an extensive set of geophysical and geochemical measurements is currently available to document the flow and expulsion of fluids at transform and convergent margins it is now recognized that such fluid discharge has far reaching implications that include hydrocarbon prospecting benthic ecosystem structure and function and groundwater research moore 1999 at present we are unable to incorporate geochemical data from cold seeps to models quantifying their effects on marine geochemical budgets because we lack a clear understanding of the spatial and temporal variability of these systems and of the mechanisms responsible for the advective transport however vent macrofauna may afford such an opportunity by recording in their shells the immediate environment in which they grow and thereby allowing us to reconstruct the variability in fluid discharge rates vesicomyid clams live in association with sites of sulfide 1

ba ca variations in the modern intertidal bean clam donax gouldii an upwelling proxy abstract the discovery and calibration of high resolution paleoceanographic proxies is necessary to extend historic climate records and to understand regional climate variability chemical variations of skeletal remains have emerged as an often reliable recorder of environmental conditions specifically ba ca ratios have been correlated to temperature salinity seawater ba ca and phytoplankton biomass although many of these relationships appear taxon and location specific to assess the sub weekly ba ca variations in the intertidal shallow burrowing bivalve donax gouldii specimens were collected from the southern california bight skeletal growth increments were cross dated based on tidal driven growth patterns and skeletal aragonite ba ca was determined using laser ablation inductively coupled plasma mass spectrometry cross dated growth among specimens revealed a simultaneous large and transient ba ca peak in all shells the timing of peak ba ca was compared to a suite of locally measured physical and biological data including temperature salinity density nitrate silicate chlorophyll diatom abundance dinoflagellate abundance and phytoplankton community composition based on cross dated chronologies ba cashell is significantly correlated with chl a from six and nine days prior and nutrients nitrate phosphate silicate and nitrite from three days prior in this system diatom abundance was not related to ba cashell transiently higher seawater ba ca resulting from upwelling may be reflected in peak ba cashell however the exact mechanisms leading to population wide ba ca peaks remains enigmatic

hydraulic conditions for rill incision under simulated rainfall a laboratory experiment a series of controlled laboratory experiments were conducted in order to obtain precise data on the hydraulic and sediment transport conditions during rill formation tests were carried out using a crusting prone binary mixed soil in a 15 m long flume at an average slope of 0 087 under simulated rainfall rainfall intensities varied from 30 35 mm h 1 and developed about 70 per cent of the kinetic energy of natural rainfall of similar intensity runoff and sediment discharge measured at the downstream weir were strongly influenced by rill forming processes essentially rill incision reduced runoff discharge as a result of increased percolation in rill channels but increased sediment discharge secondary entrainment processes such as bank collapse also increased sediment discharge at the weir knickpoint bifurcation and colluvial deposition however decreased sediment discharge rills always developed through the formation of a knickpoint the critical condition for knickpoint initiation was the development of supercritical flow and waves which mould and incise the bed prior smoothing of the soil surface by entrainment and redistribution of sediment facilitated supercritical flow statistical analysis showed that hydraulic and sediment transport conditions differed significantly in rilled and unrilled flows the relationship between sediment discharge rill erosion and flow hydraulics was found to be nonlinear conforming to a standard power function in the form y axb rills were also associated with significantly increased sediment transport capacities however rill initiation was not clearly defined by any specific hydraulic threshold instead rilled and unrilled flows were separated by zones of transition within which both types of flow occur

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

the influence of bedrock geology on knickpoint development and channel bed degradation along downcutting streams in south central indiana geologic controls on knickpoint development and the mechanics of channel bed degradation were eluci dated by comparison of downcutting streams in south central indiana developed in lithologically homoge neous carbonate units and layered siliciclastic strata knickpoints formed in a single lithologic unit are stepped each step consists of a knickpoint face and tread the height of each step is equal to and controlled by bed thickness spatially knickpoints in homogeneous terrain are limited to jointed strata that dip 1 upstream or 2 downstream at angles less than the channel gradient knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units total knickpoint relief is influenced by thickness of the weaker strata knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision knickpoint distribution is also influenced by joint

barium and molybdenum records in bivalve shells geochemical proxies for phytoplankton dynamics in coastal environments barium calcium and molybdenum calcium ratios were investigated in shells of the tropical scallop comptopallium radula three juvenile specimens were harvested alive in the southwest lagoon of new caledonia after a 1 year hydrological survey calcite samples representing a few hours of biomineralization were laser ablated along the maximal growth axis and analyzed for ba and mo content using an inductively coupled plasma mass spectrometer absolute dates of shell precipitation assigned on the basis of periodic formation of shell growth patterns led to the accurate reconstruction of ontogenetic variations of elemental ratios with subweekly resolution inter individual variability of ba ca and mo ca time series was low indicating an environmental control on the incorporation of these elements within shells both profiles were characterized by a background level punctuated by sharp peaks the ingestion of diatoms enriched in ba adsorbed on iron oxyhydroxides associated with the frustules is the most likely cause of the formation of ba ca peaks some contribution of diatom associated barite is also possible in every instance ba ca would possibly be a proxy for the timing and magnitude of diatom blooms among all the theories that could be advanced to explain the occurrence of mo ca peaks the most plausible appears to be the ingestion of phytoplankton cells grown upon no3 and therefore containing high levels of mo required for the activity of nitrate reductase if this is so then mo ca could be a new proxy for nitrate uptake by phytoplankton in coastal ecosystems helping to reconstruct the balance between new and regenerated production in paleoenvironments

sea surface water temperatures over the period 1884 1983 reconstructed from oxygen isotope ratios of a bivalve mollusk shell arctica islandica southern north sea although climate system modeling cannot be imagined without long sea surface temperature sst records observational data is spatiotemporally incomplete and inhomogeneous prior to about 1950 moreover almost no high resolution sst proxies exist from temperate and boreal marine settings here we present a temporally equidistant high resolution sst proxy record of the southern north sea during the period 1884 1983 calculated from oxygen isotope ratios 18oaragonite of arctica islandica l a bivalve mollusk shell arctica islandica produces daily growth increments which enable precise intraannual dating the growing season of specimens from about 25 m water depth upper well mixed layer of the ocean started in february and ended in september observational coads etc and 18oaragonite derived sst data during february through september compare well to each other especially after 1950 apart from similar overall sst trends we found north atlantic oscillation type cycles of 7 9 years in our proxy sst record and in various different instrumentally determined sst chronologies no vital effects exist so that 18oaragonite ratios determined across the shell i e throughout lifetime of the studied specimen reflect ambient water temperatures that occurred during shell growth results of our study demonstrate that a islandica provides long independent and high resolution sst proxy chronologies from higher latitudes such records can complement and further validate observational sst data and help improving climate system models

mg ca sr ca and stable isotope δ18o and δ13c ratio profiles from the fan mussel pinna nobilis seasonal records and temperature relationships 1 we present new annually resolved 18o 13c mg ca and sr ca ratio records for two shells of the fast growing mediterranean fan mussel pinna nobilis collected from proximal spanish coast sea grass meadows the relationship between the potential geochemical proxies and ontogenetic and environmental controlling factors is investigated specifically the use of shell mg ca and sr ca ratios as potential calcification temperature proxies the latter calculated from measured shell 18o values has been assessed the 18o cycles along the growth axis indicate that our p nobilis specimens are 10 5 and 4 5 years old shell sr ca ratios do not exhibit any consistent interannual cyclicity and are not correlated to temperature a subtle ontogenetic effect on shell mg ca ratios was observed during the first 4 5 years of recorded growth but was highly evident during the organism s later growth years in p nobilis shells different mechanisms influence ontogenetic variation in shell mg ca and 18o records shell mg ca ratios from the first 4 5 years of growth correlate significantly to temperature in a best fit relationship described by the equation mg ca 17 16 1 95 exp 0 022 0 004 t p nobilis shell mg ca records therefore are a valid temperature proxy only during an early growth phase for the same range of temperatures shell mg ca ratios in p nobilis are approximately 1 3 lower than those reported for inorganic calcite but 3 to 4 times higher than in another bivalve species mytilus trossulus and 4 to 16 times higher than in foraminifera we suggest these offsets are due to a higher degree of similarity between seawater and calcification fluid composition in p nobilis than in other bivalves and foraminifera the observed shell mg ca ratio change per c of 2 2 also is lower than that observed for inorganic and other biogenic calcites our findings strongly support taxon and species specific mg ca temperature relationships for bivalves and other calcifying organisms an appreciation of the physiology and calcification mechanisms of any biogenic carbonate archive therefore is paramount prior to the application of stable isotope and element ca ratio proxies for paleotemperature reconstructions

submerged and unsubmerged natural hydraulic jumps in a bedrock step pool mountain channel high resolution digital elevation models dems for natural submerged and unsubmerged jump regions were tested against the classical hydraulic jump chj and engineering analogues that have dominated previous geomorphic and engineering research dems were compared for two discharge conditions the data showed bed and water surface features of the natural jump regions differ significantly from chj conditions and engineering analogues with respect to boundary conditions roughness elements were highly irregular in frequency spacing size and orientation and were comprised of form obliquity and bed scour in boulder bed and pure bedrock conditions the data also showed bed and water surface features of natural jump regions have similarities and deviations from chj conditions and engineering analogues with respect to hydraulic characteristics several variations were observed in the hydraulic characteristics between the ballistic and sloping jets revised conceptual models based on idealized corollaries were developed for the natural jump regions based on upstream energy head relative to the downstream tailwater depth and step geometry the dems highlight the importance of recognizing the interrelationship between spatial transcritical flow structures and localized topographic heterogeneities in bedrock channels

sr ca profile of long lived tridacna gigas bivalves from south china sea a new high resolution sst proxy abstract high resolution 18o and sr ca ratio profiles of a modern live caught and a fossil dead collected giant bivalve shell tridacna gigas from northern south china sea were analyzed the high resolution 18o profiles as documented in most of previous studies showed pronounced seasonal cycles and recorded the combined influence of seawater 18o and sea surface temperature the sr ca ratio profiles derived from icp oes had well defined annual cycles and significant and negative correlation with sea surface temperature while the sr ca profiles derived from la icp ms did not reveal clear annual cycles this difference was likely caused by the organics contents in t gigas shells which had a chemical composition different than caco3 crystals and a heterogeneous distribution across the shell the sampling methods and the experimental errors two calibration equations between sr ca ratios determined by icp oes of modern t gigas shell and instrumental sst were calculated based on all monthly and extremum sst data and the equation from extremum data produced better sst predictions these results suggested that the high resolution icp oes derived sr ca ratios of t gigas in the south china sea had the potential of providing a good proxy of historical sst

do deterministic sediment detachment and transport equations adequately represent the process interactions in eroding rills an experimental field study abstract this paper tackles two main questions by linking observations determination of hydraulic parameters and measurement of sediment transport with the formulae used in soil erosion models first do constant shear stress values in different rills with constant soil parameters result in the same soil detachment values secondly deterministic soil erosion models make the assumption that there is a relationship often further assumed to be linear between shear stress and soil detachment is this suitable for representing real erosion process combinations in natural rills following most process based deterministic soil erosion models derived hydraulic and erosion parameters should be similar however the results from the different experiments showed clear differences in sediment concentration transport rates and other measured as well as calculated values in contrast to our experimental results a model simulation would produce erosion parameters with low variations represented by the relative measurement error and the empirical variation coefficient this reveals the general problems of using process based deterministic models for erosion in shallow rills while soil erosion models simulate the processes resulting from the shear forces of flowing water on the soil surface other processes like side wall failure headcut retreat and plunge pool dynamics are not taken into account our results suggested that these other processes may contribute substantially to rill erosion processes the results of this study strongly suggested that the model concept of most physical based soil erosion models is inadequate for modeling rill erosion processes measured total rill erosion rates were the sum of erosion rates caused by a combination of different soil erosion processes with different spatial and temporal distribution this combination cannot be described by a single equation

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

short term environmental change in a jurassic lagoon deduced from geochemical trends in aragonite bivalve shells the bathonian age inverbrora member of the brora coal formation northern scotland was deposited in a predominantly fresh water coastal lagoon setting however two prominent shell beds within the mostly argillaceous succession have been interpreted as resulting from temporary inundation of marine water and establishment of brackish conditions the ensuing lagoon hydrology and paleoenvironment were investigated by measuring stable isotopic 18o and 13c and trace element mg sr mn ba variations within five exceptionally well preserved aragonite shells of the bivalve isognomon murchisoni transects were sampled across the visible growth banding with a resolution of 0 25 0 15 mm for stable isotopes and 0 05 mm for trace elements oxygen and carbon isotope data define a broadly covariant trend 18o between 0 4 and 4 8 peedee belemnite pdb 13c between 2 2 and 0 7 pdb that supports a variably brackish water setting with slight seasonal evaporation absolute trace element values vary from shell to shell but show consistent patterns of covariation profiles of mg ca across shell layers display multiple sharp episodic minima that are coincident with peaks in sr ca and ba ca this is interpreted as documenting inundation of the restricted brackish water lagoon by relatively cooler seawater leading to a temporary increase in metabolic activity of the bivalves less frequent minima in mg ca sr ca 18o and 13c record fresh water influx and dramatic salinity reduction in the lagoon bivalve metabolic efficiency was decreased and in some cases breaks in shell growth occurred this study shows that in hydrodynamically complex settings trace element variation in bivalve shells can provide a detailed record of short term environmental change

fluvial terrace formation and controls in the lower river danube se romania abstract the black sea climatically driven level change is the main control of the lower danube incision and aggradation in the long term subsiding setting of the eastern romanian plain the interglacial episodes of aggradation suggest a decreasing fluvial accommodation while t4 terrace fill partially buried the cut terrace t5 in the upstream part of the studied area lower danube arges d mbovi a younger deposition did not compensate the previous incisions black sea level fallings during glaciations were followed by lower danube incisions which propagated upward through knickpoints loess deposits can be correlated with these periods five episodes of incision were recognized based on terrace levels t5 to t1 their ages and of the covering loess were obtained through optically stimulated luminescence osl and were correlated with mis 6 mis 3 and classic glacial chronological events latest rissian middle wurmian the maximum limiting age of danube oldest terrace t5 in the studied area is 141 25 kyr while of the youngest t1 is 50 6 kyr the equivalent terraces on tributaries are younger the time delays between terrace incisions on the lower danube system were estimated the 32 7 15 9 kyr danube floodplain deposits and the 5 kyr forests buried up to 7 m under arges floodplain suggest that younger than 44 40 kyr terraces may be buried

water table control on rill initiation and implications for erosional response on sloping sandy agricultural soil sites near toronto canada summer storms adequate to generate runoff occur frequently but rill development occurs mainly in spring when snowmelt or rainfall induced runoff occurs above frozen subsoil this suggests that on low and moderate slopes on these soils rill initiation is controlled primarily by hydraulic impedance close to the surface rather than critical hydraulic conditions in runoff laboratory flume experiments were carried out on 10 m slopes at 1 5 5 and 9 with loamy sand clay composite soil sample to test this hypothesis runoff with hydraulic conditions adequate for rill initiation occurred rapidly in most tests but on 1 5 and 5 slopes little knickpoint scour or sediment transport occurred before water table development this coincided with reduced surface soil strength knickpoint scour and marked increase in sediment discharge particularly on 5 slopes where increase was 20 to 30 fold further increase in sediment discharge occurred when water tables reached the surface on 9 slopes runoff occurred more quickly with higher hydraulic values significant rill incision and sediment discharge occurred well before water table development and ultimately reached much higher values than on lower slopes results show that soil erodibility can change dramatically over short time periods during storms due to soil moisture conditions and that the presence of a hydraulic impedance close to the surface which causes a perched water table to develop can strongly influence rill incision and sediment transport the influence is unlikely to be marked on soils which are very erodible regardless of moisture conditions or on extremely resistant soils it will also be limited on very gentle or steep sites but can be a significant factor in rill development on intermediate slopes

modeling energy dissipation and hydraulic jump regime responses to channel nonuniformity at river steps 1 the river step is an important driver for geomorphic evolution in bedrock rivers but the effect that variations in channel geometry upstream and downstream of a river step have on hydraulic jump regime and energy dissipation has not previously been investigated the associated hydraulic jump is inherent to a river step and its regime is a primary control on step morphodynamics in turn the hydraulic jump regime is controlled by several variables as detailed in a new conceptual framework herein also in this study a parsimonious semianalytical numerical model of step hydraulics is developed to quantify energy dissipation and delineate hydraulic jump regimes accounting for discharge jump submergence and nonuniform channel geometry through a step despite remaining limitations in step theory the model simulates how natural steps respond to a wide range of conditions the model shows that hydraulic jump regime and energy dissipation exhibit greater sensitivity to channel nonuniformity as discharge increases and or step height decreases also channel conditions that create greater jump submergence lead to decreased energy dissipation regardless of the discharge regime the model also reinforces the common observation about gully erosion that downstream channel widening enhances upstream knickpoint migration the new algorithm may be used to aid river engineering involving steps and could be useful for landscape evolution modeling

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

reconstructing el nino southern oscillation activity and ocean temperature seasonality from short lived marine mollusk shells from peru abstract a critical need exists for quantitative reconstructions of long term el nino southern oscillation enso variability in the eastern tropical pacific presented here is a method to quantitatively estimate past changes 1 in the seasonal amplitude of sea surface temperature sst in the peruvian coastal upwelling system and 2 in the amplitude of enso related interannual variability in the eastern tropical pacific the seasonal amplitude of sst t along the length of the peruvian coast is strongly correlated with the nino1 2 index we show that the frequency distribution of t values provided by a modern sample of 13 mesodesma donacium shells faithfully reflects modern enso variability at the regional scale including the range of anomalies from la nina to moderate el nino events but excludes extreme warm anomalies because of high shell mortality we propose to use the frequency distribution of enso anomalies in paleoclimate studies for comparisons between shell records coral records and gcm simulations reconstruction uncertainties can be quantified using monte carlo simulations the method presented here opens new perspectives for quantitative paleo enso reconstructions in the eastern pacific since it may be applied with any mollusk species from peru provided at least one annual cycle of sst is faithfully recorded by shell 18 o

assessment of mg ca in saccostrea glomerata the sydney rock oyster shell as a potential temperature record bivalve shell mg ca records can be important potential tracers of paleo temperature but evidence has shown that the temperature dependence of trace metal incorporation rates can vary significantly between different species and even within the same species this study assesses one such bivalve specimen as a potential temperature proxy for the east coast of australia specimens of the sydney rock oyster saccostrea glomerata were cultured for approximately one year at two locations on the east coast of australia moreton bay queensland a semi enclosed bay with consistently marine conditions and pambula lake new south wales a tidal lake which can undergo significant variations in salinity as a result of rainfall and river discharge transects of the oysters growth axis were analysed via la icp ms mg ca temperature relationships for the oysters show a robust temperature dependence but there is also a clear discrepancy between the two sites however this discrepancy was not evident when the temperature dependence of mg partition coefficients dmg was determined indicating that the difference seen in the direct mg cashell temperature relationships can be accounted for by variation in salinity at the estuarine site of pambula lake this has implications for the use of mg ca as a paleo temperature tracer within estuarine fauna as knowledge of the water chemistry is required to determine dmg values however s glomerata mg ca remains a promising paleo temperature tracer as the direct mg ca temperature relationships can still be applied to samples from environments that experienced little variation in salinity combined analysis of mg ca coupled with 18oshell records has the potential to enable the deconvolution of the combined salinity temperature signal in the 18oshell and mg cashell records

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

hydraulics and erosion in eroding rills rills often act as sediment sources and the dominant sediment and water transport mechanism for hillslopes six experiments were conducted on two soils and a uniform sand using three experimental methodologies the results of this study challenge the assumption often used in hydrologic and erosion models that relationships derived for sheet flow or larger channel flow are applicable to actively eroding rills velocity did not vary with slope and reynolds number was not a consistent predictor of hydraulic friction this result was due to interactions of slope gradient flow rate erosion and the formation of rill roughness bed structures and head cuts a relationship for rill flow velocities was proposed stream power was found to be a consistent and appropriate predictor for unit sediment load for the entire data set while other hydraulic variables were not the data for stream power and sediment load fit the form of a logistic curve r2 0 93 which is promising relative to recently proposed erosion models which are based on probabilistic particle threshold theory

analysis and modeling of gully headcut dynamics north american high plains gullies are dynamic fluvial features that can be the primary driver for landscape dissection and sediment production in many settings this research exploits a well constrained field area near west bijou creek colorado u s in order to develop a natural experiment in which we explore gully headcut erosion rates the controls on gully headcut height and the morphology of gully longitudinal profiles analysis of headcut retreat using aerial photography and airborne lidar imagery indicates that headcut retreat rates correlate with the square root of drainage area approximately we investigate how a drainage area control on headcut retreat translates into the longitudinal profile morphology over time using a simple numerical model the model combines fluvial erosion deposition and headcut retreat to identify the necessary and sufficient conditions needed to reproduce longitudinal profiles observed in the field field profiles are typically concave upward predominantly aggradational channel profiles with retreating headcuts whose height varies with catchment position systematic variation of environmental parameters in the model showed that the most successful model was achieved when highly resistant vegetation is applied throughout the channel excluding a bare soil zone downstream of the headcut this model scenario maintained an abrupt headcut over hundreds of model years and produced a realistic longitudinal profile that aggrades downstream of the headcut over time the vegetation pattern used in the best model fit is observed at the field site where easily erodible sparsely vegetated soil downstream of the headcut grades into a more resistant grassy channel downstream

the use of oxygen isotope variation in shells of estuarine mollusks as a quantitative record of seasonal and annual colorado river discharge abstract we describe a new method for the calculation of river flow that uses the oxygen isotope composition of bivalve mollusk shells that grew in the river water seawater mixing zone of the colorado river estuary sclerochronological techniques are used to identify tidally induced fortnight scale bundles of daily growth increments within shell cross sections these fortnightly markers are used to establish a chronology for samples taken for 18 o analysis a composite seasonal 18 o profile derived from five shells that grew in the absence of river water flow is used as a baseline against which profiles of river influenced shells are compared because this comparison is between matched fortnights within a year the temperature of shell growth is likely to be very similar the difference in 18 o between the river influenced shell and the no flow composite shell therefore represents the change in the 18 o of the water due to the presence of river water in the mixing zone the river water end member is also determined within a fortnightly context so that the change in the 18 o of mixing zone water can be used to calculate the relative proportions of seawater and fresh water the fresh water end member is calculated from the 18 o of bivalves alive prior to the emplacement of dams and water diversions on the colorado river the marine end member is based on direct measurements of the 18 o of northern gulf of california water during times of no colorado river flow the system has been calibrated to absolute flow amounts using recent releases of known volume and rate

longitudinal profile development into bedrock an analysis of hawaiian channels analysis of topographic maps of rivers incised into dated hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power field observations suggest that two processes may control hawaiian channel downcutting 1 stream power dependent erosion including abrasion of the channel bed by transported particles and 2 step wise lowering caused by knickpoint propagation modeling results indicate that a simple stream power dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of kauai channels but is insufficient to predict two other characteristic features the upslope propagation of knickpoints and the straight 5 8 channel slopes below the knickpoints thus more than this single transport law is apparently required to model bedrock channel incision field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders we propose that

a mechanistic model of waterfall plunge pool erosion into bedrock landscapes often respond to changes in climate and tectonics through the formation and upstream propagation of knickzones composed of waterfalls little work has been done on the mechanics of waterfall erosion and instead most landscape scale models neglect waterfalls or use rules for river erosion such as stream power that may not be applicable to waterfalls here we develop a physically based model to predict waterfall plunge pool erosion into rock by abrasion from particle impacts and test the model against flume experiments both the model and experiments show that evolving plunge pools have initially high vertical erosion rates due to energetic particle impacts and erosion slows and eventually ceases as pools deepen and deposition protects the pool floor from further erosion lateral erosion can continue after deposition on the pool floor but it occurs at slow rates that become negligible as pools widen our work points to the importance of vertical drilling of successive plunge pools to drive upstream knickzone propagation in homogenous rock rather than the classic mechanism of headwall undercutting for a series of vertically drilling waterfalls we find that upstream knickzone propagation is faster under higher combined water and sediment fluxes and for knickzones composed of many waterfalls that are closely spaced our model differs significantly from stream power based erosion rules in that steeper knickzones can retreat faster or more slowly depending on the number and spacing of waterfalls within a knickzone which has implications for interpreting climatic and tectonic history through analysis of river longitudinal profiles

scour on alluvial bed downstream of grade control structures this paper describes an approach for predicting local scour downstream of grade control structures the developed analysis applies the incomplete self similarity iss theory for deducing some physically based dimensionless groups controlling the geometrical pattern of the scour profile the scour measurements available in the literature in conjunction with numerous unpublished data allow a multiregressive calibration of the iss relationships the experimental sample includes different bed grain size distributions and scales of the erosive phenomenon the results prove that the ratio between the upstream water head and the weir height is able to explain the measurements of scour depth carried out in both small and large scale installations this estimation of maximum scour depth was improved introducing variables representative of both the jet contraction and the bed particle grain size distribution variables related to the longitudinal sizes of the scour profile tend to be predicted with more accuracy than those related to the scour depth and appear more influenced by the coarsest component of the alluvial bed

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

evaluation of a gully headcut retreat model using multitemporal aerial photographs and digital elevation models 1 a large fraction of soil erosion in temperate climate systems proceeds from gully headcut growth processes nevertheless headcut retreat is not well understood few erosion models include gully headcut growth processes and none of the existing headcut retreat models have been tested against long term retreat rate estimates in this work the headcut retreat resulting from plunge pool erosion in the channel hillslope integrated landscape development child model is calibrated and compared to long term evolution measurements of six gullies at the bardenas reales northeast spain the headcut retreat module of child was calibrated by adjusting the shape factor parameter to fit the observed retreat and volumetric soil loss of one gully during a 36 year period using reported and collected field data to parameterize the rest of the model to test the calibrated model estimates by child were compared to observations of headcut retreat from five other neighboring gullies the differences in volumetric soil loss rates between the simulations and observations were less than 0 05 m3 yr 1 on average with standard deviations smaller than 0 35 m3 yr 1 these results are the first evaluation of the headcut retreat module implemented in child with a field data set these results also show the usefulness of the model as a tool for simulating long term volumetric gully evolution due to plunge pool erosion

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

exploring the calcium isotope signature of arctica islandica as an environmental proxy using laboratory and field cultured specimens abstract the calcium isotope composition 44 40ca of the aragonitic bivalve arctica islandica grown in laboratory and field cultures was investigated in terms of environmental and biological controls to explore its potential as a palaeoceanographic proxy while we found no significant effect of salinity on ca isotope fractionation the relationship between 44ca 40ca ratios of a islandica and temperature was best explained by a generalised additive model which describes the temperature versus ca isotope relationship as a nonlinear second degree polynomial with an optimum temperature of 15 c since 44ca 40ca ratios clearly increase with increasing temperature up to 15 c the temperature versus ca isotope relation can be approximated through a linear fit up to 15 c the slope of this linear temperature dependent ca isotope fractionation resembles previous calibrations of inorganic and biogenic aragonites providing insights into fractionation processes during bivalve biomineralization our findings suggest that the observed biologically induced ca isotope fractionation occurs when ca is transported from seawater to the site of mineralization most likely during the passage through the epithelial mantle the observed evidence for strong vital effects indicates limited use of ca isotopes in a islandica as a temperature proxy

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

new research on the development of high resolution palaeoenvironmental proxies from geochemical properties of biogenic carbonates abstract geochemical signatures from biogenic carbonates are being increasingly employed as palaeoenvironmental proxies in turn many of these proxy archives including mollusc shells corals and otoliths have periodic growth structures which allow the reconstruction of chronologically constrained records of palaeoenvironmental variability at unparalleled high temporal resolution studying the growth and chemistry of these periodic growth structures is known as sclerochronology biogenic hard parts accumulate in geological or archaeological deposits and can be directly dated using radiometric and racemisation methods they therefore offer the opportunity for high resolution palaeoenvironmental reconstructions across many time intervals all over the globe such data are important for several reasons 1 understanding past climate and environmental change provides a means of contextualising current and future climate change and ecological disturbance 2 high resolution palaeoenvironmental records are essential for constraining testing and validating global and regional numerical climate models 3 palaeoenvironmental records from biogenic carbonates can provide an environmental framework from which to understand the behavioural changes and interactions of peoples with their environment however inter and intra species differences in growth rate physiology and environmental response can cause variations in the chemical profiles of biogenic carbonates before geochemical data is employed for palaeoenvironmental reconstructions it is thus necessary to examine modern specimens of the target species or related taxa to understand how geochemical variations are influenced by local environmental conditions kinetic and vital effects this allows the generation of quantitative and more reliable proxy records of environmental change this special issue brings together the latest research on palaeoenvironmental proxy development and validation in biogenic carbonates it includes studies on marine freshwater and estuarine organisms molluscs corals and echinoderms and on traditional as well as novel geochemical proxies the papers presented here include in situ field calibration studies laboratory growth experiments as well as methodological studies into the effects of sampling and pre treatment the geographical scope is broad encompassing both the northern and southern hemispheres including south africa south america australia asia the mediterranean sea and the north atlantic

mg ca sr ca and stable isotopes in modern and holocene protothaca staminea shells from a northern california coastal upwelling region this study explores the potential of intertidal protothaca staminea shells as high resolution geochemical archives of environmental change in a coastal upwelling region mg ca and sr ca ratios were analyzed by excimer laser ablation inductively coupled plasma mass spectrometry la icp ms at sub weekly temporal resolution in shells growing 1 mm per month growth patterns of a modern p staminea shell from humboldt bay california collected in december 1999 made it possible to infer a lifespan from 1993 to 1998 growth hiatuses in the shell may have excluded records of extreme events mg ca ratios appeared to be partly controlled by water temperature the correlation coefficient between temperature and mg ca was r 0 71 in one of four growth increments significant year to year differences in the sensitivity of mg ca to temperature in p staminea could not be explained however sr ca ratios appeared to be more closely related to shell growth rate oxygen isotopes measured at 2 week temporal resolution in the same shell did not show a clear relation to local temperature in summer possibly because temperatures were higher and less variable at the king salmon mudflat where the shell was collected than in the main channel of humboldt bay where water properties were monitored negative shell 13c values 0 5 marked spring and summer coastal upwelling events the mg contents of p staminea midden shells dated to 3 ka and 9 ka were significantly lower than in the modern shell this may have resulted from degradation of a mg rich shell organic matrix and precluded quantitative interpretation of the older high resolution records elevated 13c values in the 3 ka shell suggested that the individual grew in highly productive or stratified environment such as a shallow coastal embayment or lagoon

an evaluation of mg ca sr ca and ba ca ratios as environmental proxies in aragonite bivalve shells the influence of salinity and water chemistry on mg ca sr ca and ba ca ratios in the aragonitic shells of the manila clam was investigated clams were reared at constant temperature 20 c under different controlled conditions of salinity commonly encountered in their natural habitat clams were held in three tanks with a constant salinity of 35 for the first 35days and then two tanks were changed to lower salinities 20 and 28 for the next 29days individual shell mg ca sr ca and ba ca ratios were studied through time despite stable conditions temperature salinity and mg cawater for clams reared at salinity 35 during the experiment mg ca shell ratios increased through the time moreover the salinity decrease at t35 slowed the increase of mg ca shell ratios at salinity 28 and resulted in an mg cashell decrease at salinity 20 despite similar mg cawater ratios in the different salinity treatments microprobe analyses illustrate that mg varies along contemporaneous growth lines the variable shell mg ca ratios suggest that incorporation of magnesium into shell carbonate is strongly regulated by the organism and not by environmental conditions interestingly microprobe analyses illustrated that mg was not associated with shell sulfur as other studies have suggested sr ca shell ratios of clams reared at salinity 35 and under constant conditions were also not as constant as expected if sr ca ratios were an environmental proxy there was an inverse correlation between shell sr ca and salinity despite a slight positive correlation between salinity and sr ca ratios of the water indicating that sr ca ratios do not reflect environmental conditions a strong inverse correlation between salinity and ba ca shell ratios and a positive correlation between ba ca shell and ba ca water was observed therefore ba ca shell ratios seem to be a promising proxy of high resolution 1day salinity variations in estuarine waters via the relationship between ba ca water and salinity this study clearly illustrates that both mg ca and sr ca ratios in aragonite shells are not under environmental control and that ba ca ratios are with the later tracking high resolution water ba ca ratios and hence estuarine salinity variations

unraveling environmental histories from skeletal diaries advances in sclerochronology high resolution proxy archives from aquatic settings are essential to better understand processes and mechanisms of global change during the last decade it has become increasingly evident that calcified tissues of bivalve mollusks and cold water corals in particular can significantly increase our knowledge of seasonal to multi decadal paleoclimate and paleoenvironmental variability in the extratropical oceans and coastal marine settings daily tidal fortnightly and annual growth patterns of periodically formed skeletal hard parts provide a means to place the proxy record in a precise temporal context their extreme longevity coupled with the running similarity between growth increment time series of contemporaneous bivalves opens the possibility of constructing stacked chronologies covering centuries to millennia which provide information on decadal climate and environmental variability and magnitudes of climate variability through time a current major research focus is on how to translate geochemical properties of the skeletons into quantifiable environmental proxy data and how to eliminate the adverse influence of vital effects on the proxy records this special issue presents the most recent advances in the field of sclerochronology specifically calcified tissues of bivalve mollusks cold water corals and tropical shallow water corals the majority of the contributions deal with the potential reconstruction of temperature ph and productivity through the analysis of variable growth rates sr ca mg ca ba ca u ca li ca 13c 18o and 44 40ca four papers present composite chronologies of two long lived bivalve species and discuss inherent decadal climate variability and one study demonstrates that sclerochronology can shed light on socioeconomic patterns of past human populations as well as resource use and resource management by people

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

a simulation of rill bed incision processes in upland concentrated flows abstract quantifying rill bed incision provides fundamental information for process based erosion modeling while the morphodynamic and hydrodynamic mechanism in bed incision processes are still unclear thus experiments were conducted to examine rill bed incision processes in upland concentrated flows dems 2 mm 2 mm resolution obtained by photogrammetry were used for rill bed morphology analysis rill channel 2 0 m long 0 08 m wide and 0 15 m deep with two slope gradients 15 and 20 were subjected to four overland flow rates 1 0 2 0 3 0 and 4 0 l min 1 the results showed that sediment delivery rill bed incision rate and average rill depth increased with inflow rate and bed slope sediment delivery increased from 0 060 to 0 226 kg min 1 per 1 l min 1 inflow increment and from 0 043 to 0 207 kg min 1 when bed slope increased from 15 to 20 in a well developed rill channel rill bed incision could be divided into three phases pre headcut formation dominated by rill flow shear stress headcut incision dominated by headcut advancing and post headcut incision dominated by rill flow shear stress headcut incision phase which only accounted for 65 of rill bed sediment in the pre headcut formation phase rill flow velocity shear stress and stream power increased with increases of inflow rate and slope gradient conversely flow velocity showed no evident trend with increased inflow rate and bed slope during headcut incision phase initial headcut advancing rate could be predicted by a non linear function based upon soil characteristics rill flow shear stress and headcut height sediment delivery showed a power function with the product of inflow rate and squared bed slope because rill bed incision is dominated by headcut advancement and incision practices for controlling headcut initiation should be implemented to decrease hillslope soil loss

estimating seepage and hydraulic potentials near incised ditches in a homogeneous isotropic aquifer soil erosion on upland areas is a highly complex phenomenon involving many component processes one of the least explored processes is the role of subsurface flow rain or irrigation water entering the soil profile either accrues to groundwater or exits at some point downslope as seepage in the latter case the positive water pressure may lead to detachment of soil particles aggregates or bulk soil that often is a prelude to incipient rilling or gullying this article concerns the analysis of seepage to ditches with a water level lower than the adjacent field water table and the hydraulic pressures near the ditches in order to assess their contributing role in gully development the approach taken is to analyze the subsurface flow regime under steady state conditions using the theory of conformal transformations first seepage to a fully filled circular drain in a homogeneous isotropic layer overlaying an impermeable layer was analyzed subsequently the case of drainage to a ditch partially filled with water and the effect of a buffer strip adjacent to the ditch on seepage reduction were studied using the same methodology two approaches were considered to approximate the water level in the buffer strip 1 a confined boundary represented by a straight line between the ditch water level and the field surface water level and 2 a free boundary in which the groundwater level was approximated by the pressure potential relationship for flow from a ponded surface area to a sink placed at the surface at a distance equal to the width of the buffer strip the latter analysis yielded explicit expressions of seepage and the groundwater hydraulic and stream potential functions in terms of the spatial coordinates this relationship should facilitate the analysis of the role of the subsurface water regime on seepage soil erosion and gully development published in 2009 by john wiley sons ltd

clams as recorders of ocean ridge volcanism and hydrothermal vent field activity the clam calyptogena magnifica lives at abyssal depths in association with hydrothermal venting on midocean ridges analysis of strontium calcium ratios in c magnifica shells provides a temperature proxy with submonthly time resolution a 21 year strontium calcium record of two clams from 9 50 n on the east pacific rise captures the known 1991 and 1992 eruptive events documents several additional events between 1992 and 1996 and demonstrates the absence of major hydrothermal episodes during the period 1974 to 1991 these clam archives can increase our understanding of the thermal and chemical history of midocean ridge hydrothermal and volcanic activity on decadal time scales

convergent hydraulics at horseshoe steps in bedrock rivers abstract horseshoe waterfalls are a common feature of steep bedrock rivers as a first step toward understanding their geomorphology a detailed study of the fluid mechanics at a 0 91 m vertical drop horseshoe waterfall was performed in a 2 75 m wide flume five non dimensional upstream energy levels each with 3 5 non dimensional downstream tailwater depths 21 runs total were assessed for water surface topography via digital elevation modeling flow dynamics via digital videography and overall energy dissipation via an energy and momentum conservation model regardless of tail depth the horseshoe waterfall was found to have three distinct zones beyond the step brink 1 a nappe whose degree of convergence depends on upstream energy and brink configuration 2 a convergence zone whose features vary strongly with upstream energy brink configuration and tail depth and 3 a downstream tailwater region whose dynamics primarily depend on tail depth the centerline nappe profile and brink velocity were reasonably predicted using rouse s jet trajectory equations when h p h 2 peripheral profiles were not predictable using existing equations for any arbitrary broad crested step brink configuration maximum energy dissipation was found to occur when no jump was present and downstream tail depth was exactly critical rather than providing maximal energy dissipation hydraulic jumps below steps provide efficient conversion of kinetic energy to potential energy

modulation of headcut soil erosion in rills due to upstream sediment loads 1 headcut erosion can severely accelerate soil loss in upland concentrated flows and lead to significant soil degradation in agricultural areas previous experimental work has demonstrated that actively migrating headcuts display systematic morphodynamic behavior and that impinging jet theory can provide an excellent analytic foundation for predicting these erosional phenomena this research sought to examine systematically the effect of an upstream sediment load on the morphodynamics of actively migrating headcuts in upland concentrated flows using a specially designed experimental facility actively migrating headcuts in packed soil beds were allowed to develop freely when subjected to concentrated clear water flows similar to rills crop furrows and ephemeral gullies after such headcut development and without altering the boundary conditions of the experiment a sediment load composed of relatively coarser grained sand was introduced into the flow well upstream of the erosional feature at relatively low upstream sediment loadings the morphodynamics and sediment discharge of the actively migrating headcuts remained unchanged in time and space as upstream sediment loadings increased the size and migration rate of the headcuts decreased markedly yet sediment discharge was not strongly affected as its sediment texture shifted from headcut controlled flux to sediment feed flux the headcut erosion process was completely arrested when the upstream sediment feed rate exceeded a specific magnitude analytic arguments are used to explain the reaction time and morphodynamic response of an actively migrating headcut to this upstream sediment loading this research shows that headcut erosion can be strongly modulated by an upstream sediment load further complicating the prediction of soil erosion on upland areas dissected by sediment laden concentrated flows

an empirical investigation of gully widening rates in upland concentrated flows abstract soil erosion in its myriad forms devastates arable land and infrastructure as an integral landscape feature gully erosion is a complex system as its evolution is controlled by upstream migration of a gully head headcut face incision of the gully bed plunge pool and gravitational mass movement widening on gully channel sidewalls bed incision is often limited by the presence of a less erodible soil layer when erosion reaches such a layer the gully typically widens creating a wide shallow cross section once a gully is initiated transport and deposition of the eroded soil and widening of the channel further govern its evolution our knowledge of these processes within agricultural regions however is still quite limited with theory largely borrowed from river hydraulics experiments were conducted to examine gully expansion widening as a function of channel slope and overland flow discharge packed soil beds were subjected to simulated rainstorms followed by clear water overland flow as bed slope and discharge increased channel widening increased an empirical equation based upon measured channel width from constant discharge experiments was developed in terms of slope and discharge by further quantifying key aspects of gully erosion processes results from this experimental campaign will significantly improve soil erosion prediction technology for use in agricultural regions

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