fire and the evolution of steep soil mantled landscapes recent burns in the western united states attest to the signif icant geomorphic impact of fire in mountainous landscapes yet we lack the ability to predict and interpret fire related erosion over millennial time scales a diverse set of geomorphic processes is often invoked following fire the magnitude of postfire erosional processes coupled with temporal variations in fire frequency dic tate the extent to which fires affect sediment production and land scape evolution in the oregon coast range several models for long term rates of soil production and transport have been tested and calibrated although treatment of fire related processes has been limited following recent fires in the oregon coast range we observed extensive colluvial transport via dry ravel localized bed rock emergence due to excess transport and talus like accumula tion in adjacent low order valleys soils exhibited extreme but dis continuous hydrophobicity and no evidence for rilling or gullying was observed using a field based data set for fire induced dry rav el transport we calibrated a physically based transport model that indicates that soil flux varies nonlinearly with gradient the post fire critical gradient 1 03 which governs the slope at which flux increases rapidly is lower than the previously estimated long term value 1 27 reflecting the reduction of slope roughness from in cineration of vegetation by using a high resolution topographic data set generated via airborne laser swath mapping we modeled the spatial pattern of postfire and long term erosion rates postfire erosion rates exceed long term rates which average 0 1 mm yr 21 by a factor of six and subtle topographic variations generated local patches of rapid postfire erosion commonly 1 mm yr 21 our sim ulations indicate that fire related processes may account for 50 of temporally averaged sediment production on steep hillslopes our analysis provides a mechanistic explanation for the coincident early holocene timing of increased fire frequency and regional ag gradation in oregon coast range drainage basins given the sen sitivity of steep hillslopes to fire driven transport changes in cli mate and fire frequency may affect soil resources by perturbing the balance between soil transport and production

glassy dynamics of landscape evolution soil creeps imperceptibly downhill but also fails catastrophically to create landslides despite the importance of these processes as hazards and in sculpting landscapes there is no agreed upon model that captures the full range of behavior here we examine the granular origins of hillslope soil transport by discrete element method simulations and reanalysis of measurements in natural landscapes we find creep for slopes below a critical gradient where average particle velocity sediment flux increases exponentially with friction coefficient gradient at critical gradient there is a continuous transition to a dense granular flow rheology slow earthflows and landslides thus exhibit glassy dynamics characteristic of a wide range of disordered materials they are described by a two phase flux equation that emerges from grain scale friction alone this glassy model reproduces topographic profiles of natural hillslopes showing its promise for predicting hillslope evolution over geologic timescales

fitting degradation of shoreline scarps by a nonlinear diffusion model the diffusion model of degradation of topographic features is a promising means by which vertical offsets on holocene faults might be dated in order to calibrate the method we have examined present day profiles of wave cut shoreline scarps of late pleistocene lakes bonneville and lahontan it may be assumed that these scarps were initially at least as steep as the angle of repose offsets range from 1 to 12 m and present slope angles range from 9 to 29 a parameter called apparent diffusion age defined as half the mean square horizontal extent of the slope function of each profile is plotted as a function of scarp offset the points show a clear trend of apparent age increasing nearly linearly with offset if linear diffusion held and scarps were initially vertical apparent diffusion age would be the same for all the shoreline profiles the increasing trend can only partly be explained by nonvertical initial scarp slope and therefore the rate of transport of material downslope must increase significantly faster than a linear law in the range of slopes spanned by the data the transport law must become linear at small slope to reduce scatter between profiles with varying ambient slopes the transport law adopted for the purpose of dating is k0 1 5s2 where s is local slope the transport coefficient k0 is correlated inversely with fan slope suggesting that there is a dependence on the particle size distribution a table is included that allows easy application of the model to scarps with simple initial shape

sediment transport by dry ravel 1 dry ravel is a general term that describes the rolling bouncing and sliding of individual particles down a slope and is a dominant hillslope sediment transport process in steep arid and semiarid landscapes during fires particles can be mobilized by the collapse of sediment wedges that have accumulated behind vegetation on a daily basis particles may be mobilized by bioturbation and by small landslides experiments on a dry ravel flume indicate that a basic expression of the momentum equation predicts the distance traveled by particles propelled down a rough surface this equation is further elaborated to produce a nonlinear slope dependent transport equation for dry ravel that represents the rate at which sediment crosses a contour width of slope sediment traps installed on two hillslope transects near santa barbara california measured the flux from dry ravel initiated by bioturbation and the data support the form of the equation additionally a physical model based on the infinite slope stability analysis is proposed for the initiation of dry ravel by landsliding the analytical result from this model is supported by experiments and field data reported by others

estimates of the rate of regolith production using and from an alpine hillslope abstract the production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material we use concentrations of the cosmogenic radionuclides crns 10 be and 26 al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the wind river range wy these calculations are based on a theoretical model which we develop here this model shows that it is important to consider dissolution of regolith in regolith production and in basin averaged erosion rate studies rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 be and 26 al are 14 3 4 0 and 13 0 4 0 m ma 1 respectively rates of production of regolith deduced from 10 be concentrations in regolith mantled bedrock support the rates deduced from regolith concentrations in the alpine environment examined here the rate of production of regolith beneath 90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces which small et al small e e anderson r s repka j l finkel r 1997 erosion rates of alpine bedrock summit surfaces deduced from in situ 10 be and 26 al earth and planetary science letters 150 413 425 previously documented rock mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope the regolith volume flux must be proportional to the local slope of the hillside therefore our results are consistent with gilbert s gilbert g k 1909 the convexity of hilltops journal of geology 17 344 350 steady state hillslope hypothesis if tor height and the difference between rates of weathering on bare and regolith mantled rock provide a fair estimate of the age of summit flats steady state hillslope conditions have been attained in less than several million years

fluvial sediment transport in response to moderate storm flows following chaparral wildfire ventura county southern california chaparral wildfire has a profound effect on erosion and sedimentation in southern california the wheeler fire in july 1985 burned the entire basin of a tributary drainage area 2 14 km 2 of the north fork of matilija creek near ventura california after the fire fine gravel was delivered to the channel by the process of dry ravel dry particle by particle sliding of debris under the force of gravity at a rate of 0 29 m 3 km 2 month the first winter flow 2 1 m 3 s following the fire deposited 550 m 3 of fine gravel in the 270 m study reach near the mouth of the tributary at least 90 of this fine gravel was derived from colluvium delivered by dry ravel processes from hillslopes near the channel the second winter flow 2 5 m 3 s eroded the channel to the pre fire thalweg a reduction in particle size and critical shear stress associated with deposition of small gravel following the fire allowed these moderate magnitude flows to transport large volumes of sediment deposits of two debris flows were identified in the tributary basin radiocarbon dating of these deposits gives dates of 1045 95 yr b p for the older deposit and between 295 35 and 385 84 yr b p for the younger deposit these dates indicate that the recurrence interval of large debris flows in the study basin is at least an order of magnitude greater than the recurrence interval of fire in the area

illuminating wildfire erosion and deposition patterns with repeat terrestrial lidar erosion following a wildfire is much greater than background erosion in forests because of wildfire induced changes to soil erodibility and water infiltration while many previous studies have documented post wildfire erosion with point and small plot scale measurements the spatial distribution of post fire erosion patterns at the watershed scale remains largely unexplored in this study lidar surveys were collected periodically in a small first order drainage basin over a period of 2 years following a wildfire the study site was relatively steep with slopes ranging from 17 to 30 during the study period several different types of rain storms occurred on the site including low intensity frontal storms 2 4 mm h 1 and high intensity convective thunderstorms 79 mm h 1 these storms were the dominant drivers of erosion erosion resulting from dry ravel and debris flows was notably absent at the site successive lidar surveys were subtracted from one another to obtain digital maps of topographic change between surveys the results show an evolution in geomorphic response such that the erosional response after rain storms was strongly influenced by the previous erosional events and pre fire site morphology hillslope and channel roughness increased over time and the watershed armored as coarse cobbles and boulders were exposed the erosional response was spatially nonuniform shallow erosion from hillslopes 87 of the study area contributed 3 times more sediment volume than erosion from convergent areas 13 of the study area however the total normalized erosion depth volume area was highest in convergent areas from a detailed understanding of the spatial locations of erosion we made inferences regarding the processes driving erosion it appears that hillslope erosion is controlled by rain splash for detachment and overland flow for transport and quasi channelized erosion with the sites of highest erosion corresponding to locations with the lowest roughness by contrast in convergent areas we found erosion caused by overland flow soil erosion was locally interrupted by immobile objects such as boulders bedrock or tree trunks resulting in a patchy erosion network with increasing roughness over time

quantification of soil production and downslope creep rates from cosmogenic 10be accumulations on a hillslope profile average soil transport rates over a period of 3500 yr on a convex soil mantled hillslope have been quantified using a mass balance model that incorporates the soil concentration of the cosmogenic isotope 10 be the 10 be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient the predicted diffusion coefficient is 360 55 cm 3 yr 1 cm 1 contour length and the average rate of soil production is 0 026 0 007 cm yr within the uncertainty of this technique the data do not reject g k gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate however the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil production rate law

hillslope evolution by nonlinear creep and landsliding an experimental study landscape evolution models are widely used to explore links between tectonics climate and hillslope morphology yet mecha nisms of hillslope erosion remain poorly understood here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion in our experimental hillslope disturbance driven sediment transport rates increase nonlinearly with slope due to dilation driven gran ular creep and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding we use spectral analysis to quantify the variability of sediment flux and estimate the slope dependent transition from creep to landsliding the power spec trum of sediment flux steepens with hillslope gradient exhibiting fractal 1 f scaling just below the creep landsliding transition by evolving the experimental hillslope under fixed base level boundary conditions we demonstrate how disturbance driven transport gen erates hillslope convexity the transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep dominated transport

yin yang grid an overset grid in spherical geometry 1 a new kind of overset grid named yin yang grid for spherical geometry is proposed the yin yang grid is composed of two identical component grids that are combined in a complemental way to cover a spherical surface with partial overlap on their boundaries each component grid is a low latitude part of the latitude longitude grid therefore the grid spacing is quasi uniform and the metric tensors are simple and analytically known one can directly apply mathematical and numerical resources that have been written in the spherical polar coordinates or latitude longitude grid the complemental combination of the two identical component grids enables us to make efficient and concise programs simulation codes for geodynamo and mantle convection simulations using finite difference scheme based on the yin yang grid are developed and tested the yin yang grid is suitable for massively parallel computers

a morphometric analysis of gullies scoured by post fire progressively bulked debris flows in southwest montana usa in the fall of 2001 an intense thunderstorm in southwest montana triggered many debris flows in the burned area of sleeping child creek in most instances the debris flows cut deep gullies into previously unchannelized colluvial hollows and deposited large volumes of sediment onto the valley floor the presence of rill networks above the gullies as well as the absence of landslide features indicate that the gullies were scoured by progressively bulked debris flows a process in which dilute surface runoff becomes increasingly more laden with sediment until it transforms into a debris flow in this contribution we present a morphometric analysis of six of the gullies to better understand this relatively understudied process we find that the locations of the rill heads and gully heads conform to slope area thresholds that are characteristic of erosion by overland flow our data also suggest that the volumes of the debris flows increase exponentially with normalized drainage area thus lending support to an assumption used in a recently proposed debris flow incision law finally the debris flow fans have been relatively unaltered since deposition suggesting that the valley may be currently aggrading while the hillslopes are being denuded

the effects of bioturbation on soil processes and sediment transport abstract plants and animals exploit the soil for food and shelter and in the process affect it in many different ways for example uprooted trees may break up bedrock transport soil downslope increase the heterogeneity of soil respiration rates and inhibit soil horizonation in this contribution we review previously published papers that provide insights into the process of bioturbation we focus particularly on studies that allow us to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production using geometrical relationships and data from others work we derive simple sediment flux equations for tree throw and root growth and decay

effect of storms during drought on post wildfire recovery of channel sediment dynamics and habitat in the southern california chaparral usa author s florsheim joan l chin anne kinoshita alicia m nourbakhshbeidokhti samira abstract current global warming projections suggest a possible increase in wildfire and drought augmenting the need to understand how drought following wildfire affects the recovery of stream channels in relation to sediment dynamics we investigated post wildfire geomorphic responses caused by storms during a prolonged drought following the 2013 springs fire in southern california usa using multi temporal terrestrial laser scanning and detailed field measurements after the fire a dry season dry ravel sediment pulse contributed sand and small gravel to hillslope channel margins in big sycamore creek and its tributaries a small storm in wy 2014 generated sufficient flow to mobilize a portion of the sediment derived from the dry ravel pulse and deposited the fine sediment in the channel totaling 0 60 m3 m of volume per unit length of channel the sediment deposit buried step pool habitat structure and reduced roughness by over 90 these changes altered sediment transport characteristics of the bed material present before and after the storm the ratio of available to critical shear stress t o t c increased by five times storms during wy 2015 contributed additional fine sediment from tributaries and lower hillslopes and hyperconcentrated flow transported and deposited additional sediment in the channel together these sources delivered sediment on the order of six times that in 2014 further increasing t o t c these storms during multi year drought following wildfire transformed channel dynamics the increased sediment transport capacity persisted during the drought period characterized by the longer residence time of relatively fine grained post fire channel sedimentation this contrasts with wetter years when post fire sediment is transported from the fluvial system during the same season as the post fire sediment pulse results of this short term study highlight the complex and substantial effects of multi year drought on geomorphic responses following wildfire these responses influence pool habitat that is critical to longer term post wildfire riparian ecosystem recovery

slope grain size and roughness controls on dry sediment transport and storage on steep hillslopes existing hillslope sediment transport models developed for low relief soil mantled landscapes are poorly suited to explain the coupling between steep rocky hillslopes and headwater channels here we address this knowledge gap using a series of field and numerical experiments to inform a particle based model of sediment transport by dry ravel a mechanism of granular transport characteristic of steep hillslopes we find that particle travel distance increases as a function of the ratio of particle diameter to fine scale 1 m topographic variability associated with rocky landscapes applying a 2 d dry ravel routing model to lidar derived surface topography we show how spatial patterns of local and nonlocal transport control connectivity between hillslopes and steep headwater channels that generate debris flows through failure of ravel filled channels following wildfire our results corroborate field observations of a patchy transition from soil mantled to bedrock landscapes and suggest that there is a dynamic interplay between sediment storage roughness grain sorting and transport even on hillslopes that well exceed the angle of repose

the malvinas current at the confluence with the brazil current inferences from 25 years of mercator ocean reanalysis twenty five years of high resolution 1 12 ocean reanalysis are used to examine the confluence of the malvinas current mc with the brazil current bc from synoptic to interannual time scales the model transports of the mc 38 0 sv 7 4 sv 57 at 41 s and the bc 23 0 sv 11 sv at 36 s agree with observations the model shows the branching of the mc near the confluence with an offshore branch returning south and an inner branch sinking below the bc and managing to continue northward along the continental slope northward velocities associated with the subsurface inner branch peak at 40 cm s at 36 s at 700 m depth the model documents the migrations of the subantarctic saf and subtropical fronts stf at the confluence the saf and stf positions vary over a large range at synoptic 800 km and interannual scale 300 and 200 km respectively compared to the rather small seasonal migrations of the stf 150 km and saf 50 km while trends in the mc are small over the 25 years of the reanalysis the bc becomes more intense 12 5 cm s saltier 0 37 psu and warmer 2 5 c in the upper 1 000 m these trends are accompanied with a southward displacement of the stf and the saf of 150 and 50 km

landslides on coastal sage scrub and grassland hillslopes in a severe el nino winter the effects of vegetation conversion on sediment delivery during the 1997 1998 el nino record rainfall triggered 150 shallow landslides within a 9 5 km2 area near santa barbara cal ifornia they were studied to analyze the sediment delivery to val ley floors from landslides in coastal sage scrub and converted grasslands the conversion of coastal sage to grasslands primarily to provide pasturage for cattle is common in the region and the landscape s response may affect water quality reservoir infilling and debris flow hazards we explore the relationship between lateral root reinforcement and landslide volume by developing a slope stability analysis that incorporates root cohesion along the sides of the failure the stability analysis correctly predicts an inverse re lationship between landslide volume and hillslope angle in the sage the volumes of failures in the grasslands do not vary systemati cally with slope and are generally smaller than those in the sage from aerial photograph analysis and field mapping we find that there are 22 9 failures per square kilometer in the grasslands com pared to 13 2 failures per square kilometer in the sage despite the lower failure density in the coastal sage greater failure volumes and longer transport distances delivered more sediment to valley floors with a specific volumetric flux of 2 8 3 10 2 m 3 m 1 for this el nino compared to 1 73 10 2 m 3 m 1 in the grasslands we con clude that the conversion from vegetation with stronger and deeper roots coastal sage to vegetation with weaker and shallower roots grass has caused a pulse of increased landsliding in the grasslands because the soils are currently too thick for the prevailing root reinforcement we suggest that over time soils in the grassland hollows will become thinner as the evacuation by landslides is re peated until the landsliding rate declines to balance the soil sup plied from local colluvium production and diffusive processes upslope

stochastic forcing of sediment supply to channel networks from landsliding and debris flow sediment influx to channel networks is stochastically driven by rainstorms and other perturbations which are discrete in time and space and which occur on a landscape with its own spatial variability in topography colluvium properties and state of recovery from previous disturbances the resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin the regime varies systematically with basin area we describe how the stochastic sediment supply is generated by climatic topographic geotechnical and biotic controls that vary between regions the general principle is illustrated through application to a landscape where sediment is supplied by mass wasting and the forcing variables are deterministic thickening of colluvium random sequences of root destroying wildfires and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength landslides stop in channels or convert to scouring debris flows depending on the nature of the low order channel network sediment accumulates within these channels for centuries before being transferred downstream by debris flows time series of sediment supply transport and storage vary with basin scale for any combination of climatic topographic and geotechnical controls in a companion paper benda and dunne this issue we use simulations of timing volumes and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks

short term post wildfire dry ravel processes in a chaparral fluvial system abstract dry ravel the transport of sediment by gravity transfers material from steep hillslopes to valley bottoms during dry conditions following wildfire dry ravel greatly increases in the absence of vegetation on hillslopes thereby contributing to sediment supply at the landscape scale dry ravel has been documented as a dominant hillslope erosion mechanism following wildfire in chaparral environments in southern california however alteration after initial deposition is not well understood making prediction of post fire flood hazards challenging the majority of big sycamore canyon burned during the may 2013 springs fire leaving ash and a charred layer that covered hillslopes and ephemeral channels dry ravel processes following the fire produced numerous deposits in the hillslope channel transition zone field data focus on 1 deposition from an initial post wildfire dry ravel pulse and 2 subsequent alteration of dry ravel deposits over a seven month period between september 2013 and april 2014 we quantify geomorphic responses in dry ravel deposits including responses during the one small winter storm that generated runoff following the fire field measurements document volumetric changes after initial post wildfire deposition of sediment derived from dry ravel erosion and deposition mechanisms that occurred within dry ravel deposits situated in the hillslope channel transition zone included 1 mobilization and transport of a portion or the entire deposit by fluvial erosion 2 rilling on the surface of the unconsolidated deposits 3 deposition on deposits via continued hillslope sediment supply and 4 mass wasting that transfers sediment within deposits where surface profiles are near the angle of repose terrestrial lidar scanning point clouds were analyzed to generate profiles quantifying depth of sediment erosion or deposition over remaining dry ravel deposits after the first storm season this study contributes to the understanding of potential effects of wildfire on fine sediment delivery to fluvial systems in chaparral ecosystems

impact of coarse sediment supply from hillslopes to the channel in runoff dominated dryland fluvial systems sediment supply from hillslopes to channels is an important control on basin functioning and evolution however current theoretical frameworks do not adequately consider processes of runoff driven hillslope sediment supply which affect river channels spatially and temporally mountainous dryland basins exhibit an important manifestation of these processes because their debris mantled hillslopes produce coarse sediment and because rainfall is delivered as infrequent high intensity short duration rainstorms this paper combines field measurements and modeling to explore runoff driven coarse sediment supply from hillslopes to the channel and assesses a range of plausible storms on the longitudinal patterns of sediment load and its caliber over a dryland basin reach our results show that modeled sediment load and its grain size distribution are determined by the nonlinear interaction between rainfall characteristics and hillslope attributes resulting in longitudinal fluctuations in sediment supply the relative magnitude and location of which varies between storms results suggest that long hillslopes are most sensitive to rainfall and they exhibit large variations in supplied sediment load and grain size for different runoff characteristics short and steep hillslopes are less sensitive to rainfall variations as gradient effects dominate over the role of length in modulating runoff accumulation furthermore the signal of the median fraction d50 of modeled sediment supplied by the hillslope is preserved in the coarse fraction of the measured in channel grain sizes d90 finally we propose a simple index which provides new insights into the effectiveness of different rainstorms in terms of the impact of hillslope sediment supply on the channel

particle transport over rough hillslope surfaces by dry ravel experiments and simulations with implications for nonlocal sediment flux 1 past studies of hillslope evolution have typically assumed that soil creep processes are governed by a linear relationship between local hillslope angle and transport distance the assumption of linear diffusion has fallen out of favor because when coupled with an expression of mass continuity it yields unrealistic hillslope profiles as a consequence a better understanding of the mechanics of sediment transport is needed here we report results from a series of flume experiments performed to investigate sediment transport by dry ravel a common soil creep process in arid and semiarid environments we find that at gentle slopes transport distances follow distributions characteristic of local transport as gradients steepen a fraction of the particles begins to exhibit nonlocal transport and that fraction increases rapidly with slope a stochastic discrete element model that couples an effective friction term with a shock term reproduces the results from the flume experiments suggesting that it can be used to explore the nature of particle transport on rough surfaces the model predicts that exponential distributions of transport distances on gentle slopes evolve into quasi uniform distributions on steep slopes and the transition occurs as slopes approach the angle of repose our results support previous findings that the angle of repose represents a threshold between friction and inertial regimes in addition we propose that the angle of repose represents a fuzzy boundary between local and nonlocal transport

post fire geomorphic response in steep forested landscapes oregon coast range usa abstract the role of fire in shaping steep forested landscapes depends on a suite of hydrologic biologic and geological characteristics including the propensity for hydrophobic soil layers to promote runoff erosion during subsequent rainfall events in the oregon coast range several studies postulate that fire primarily modulates sediment production via root reinforcement and shallow landslide susceptibility although few studies have documented post fire geomorphic response here we describe field observations and topographic analyses for three sites in the central oregon coast range that burned in 1999 2002 and 2003 the fires generated strongly hydrophobic soil layers that did not promote runoff erosion because the continuity of the layers was interrupted by pervasive discontinuities that facilitated rapid infiltration at each of our sites fire generated significant colluvial transport via dry ravel consistent with other field based studies in the western united states fire driven dry ravel accumulation in low order valleys of our sulphur creek site equated to a slope averaged landscape lowering of 2 5 mm given holocene estimates of fire frequency these results suggest that fire may contribute 10 20 of total denudation across steep dissected portions of the oregon coast range in addition we documented more rapid decline of root strength at our sites than has been observed after timber harvest suggesting that root strength was compromised prior to fire or that intense heat damaged roots in the shallow subsurface given that fire frequencies in the pacific northwest are predicted to increase with continued climate change our findings highlight the importance of fire induced dry ravel and post fire debris flow activity in controlling sediment delivery to channels

post fire thin debris flows sediment transport and numerical modelling the creation of a hydrophobic layer in the soil during res in semi arid environments inhibits the in ltration of rainfall this leads to increased rates of runoff and associated sediment transport when the hydrophobic layer is deposited beneath the soil surface a perched water table develops which may cause thin 1 2 cm hillslope failures that are distinguishable from features caused by rilling and sheet ow evidence for these failures was observed after a re near santa barbara california the amount of sediment eroded from some hillslopes was substantial with 290 kg of sediment per metre width of hillslope delivered to the valley oor the mechanics of these failures are examined with a numerical model that incorporates a stability analysis with subsurface ow routing along a typical hillslope pro le the model correctly predicts the location of the failures as well as the rainfall amount necessary to trigger them copyright 2003 john wiley sons ltd

sediment storage by vegetation in steep bedrock landscapes theory experiments and implications for postfire sediment yield 1 mechanistic models for sediment transport on hillslopes are needed for applications ranging from landscape evolution to debris flow hazards progress has been made for soil mantled landscapes however little is known about sediment production and transport in bedrock landscapes that often maintain a patchy soil mantle even though slopes exceed the angle of repose herein we investigate the hypothesis that patchy soil cover is stable on steep slopes due to local roughness such as vegetation dams that trap sediment upslope to quantify local sediment storage we developed a new theory and tested it against tilt table experiments results show that trapped sediment volume scales with the cube of dam width where the dam width is less than about fifty grain diameters particle force chains appear to enhance stability resulting in greater trapped volumes and sediment pile slopes that exceed the angle of repose trapped volumes are greatest for hillslopes that just exceed the friction slope and are independent of hillslope gradient for gradients greater than about twice the friction slope for neighboring dams spaced less than about five grain diameters apart grain bridging results in a single sediment pile that is larger than the sum of individual piles this work provides a mass conserving framework for quantifying sediment storage and nonlocal transport in bedrock landscapes results may explain the rapid increase in sediment yield following wildfire in steep terrain in the absence of rainfall as sediment dams are incinerated particles become gravitationally unstable and move rapidly downslope as dry ravel

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient

a model for fire induced sediment yield by dry ravel in steep landscapes sediment flux from hillslopes to channels commonly increases following wildfires with implications for the carbon cycle river habitats and debris flow hazards although much of this material is transported via dry ravel existing ravel models are not applicable to hillslopes with gradients greater than the angle of repose which can constitute the majority of mountainous terrain to fill this knowledge gap we develop a continuity model for sediment storage by vegetation dams on steep hillslopes to predict sediment yields following wildfire the maximum volume of sediment stored prior to wildfire is set to be a function of vegetation density the capacity of plants to impound sediment and the contributing hillslope area time is required after fire to establish vegetation and replenish hillslope sediment storage which introduces vegetation regrowth rate soil production rate and fire recurrence interval as important variables that affect ravel yield model results for the san gabriel mountains california predict that sediment yield can increase by several orders of magnitude following fire these results are consistent with field data of ravel yield 30 mm per contributing area of hillslope in 5 months we collected following the 2009 station fire as well as postfire sediment flux recorded by 93 debris basins in contrast to previous work our model shows that heightened postfire sediment yields can be explained by a change in hillslope sediment storage independent of major changes in the soil production rate and landscape form over geomorphic timescales

nonlocal sediment transport on steep lateral moraines eastern sierra nevada california usa available for download is the author s version of the paper the final publisher s version is available at https dx doi org 10 1002 2017jf004325 the dataset doi is https doi org 10 15695 vudata cee 1

geomorphic significance of postglacial bedrock scarps on normal fault footwalls the existence of well preserved holocene bedrock fault scarps along active normal faults in the mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate we test and quantify this interpretation using a case study in the italian central apennines holocene rates are derived from measurements of weathering pit depth along the magnola scarp where previous cosmogenic cl 36 analyses constrain exposure history to estimate the average hillslope erosion rate over similar to 10 5 years we introduce a simple geometric model of normal fault footwall slope evolution the model predicts that the gradient of a weathering limited footwall hillslope is set by fault dip angle and the ratio of slip rate to erosion rate if either slip or erosion rate is known the other can be derived applying this model to the magnola fault yields an estimated average weathering rate on the order of 0 2 0 4 mm yr more than 10 times higher than either the holocene scarp weathering rate or modern regional limestone weathering rates a numerical model of footwall growth and erosion in which erosion rate tracks the oxygen isotope curve reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles we conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking

vegetation and wildfire controls on sediment yield in bedrock landscapes steep rocky landscapes commonly exhibit high sediment yields and are especially sensitive to climate tectonics and wildfire predicting landscape response to these perturbations demands a quantitative understanding of erosion processes however existing models for hillslope sediment production and transport do not apply to landscapes with patchy soil and slopes that exceed the angle for sediment stability here we present field measurements in southern california usa which indicate that sediment storage on steep slopes is enabled by vegetation that traps sediment upslope we find that the storage capacity of unburned vegetation dams follows a geometric scaling model with a cubic dependence on effective plant width and an inverse dependence on local slope measured sediment volumes behind burned vegetation dams indicate a loss of at least 75 relative to unburned dams and when expanded to the catchment scale our measurements match records of postfire sediment yield from nearby retention basins contrary to existing models our observations indicate that wildfire induced sediment yield is driven by transient storage and release of sediment by vegetation dams rather than increased bedrock to soil conversion rates without a feedback between soil production and wildfire fire may play little role in long term landscape evolution and increasing fire frequency in response to climate change may not result in heightened sedimentation hazards due to supply limitations

macroscale dynamics of experimental landscapes abstract we review results from laboratory scale modelling of erosion and relief dynamics under variable uplift and rainfall rates under constant values of these forcing parameters an experimental landscape typically evolves towards a steady state between uplift and erosion and we show how the geometry of the steady state landscape adjusts to the rates of uplift and rainfall the comparison between these laboratory scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions laboratory scale modelling in geomorphology is therefore only experimental not analogue despite these limitations experimental models may be used to provide physical tests for numerical models and they give insights into first order behaviours and directions for future research

bedrock erosion by root fracture and tree throw a coupled biogeomorphic model to explore the humped soil production function and the persistence of hillslope soils 1 in 1877 g k gilbert reasoned that bedrock erosion is maximized under an intermediate soil thickness and declines as soils become thinner or thicker subsequent analyses of this humped functional relationship proposed that thin soils are unstable and that perturbations in soil thickness would lead to runaway thinning or thickening of the soil to explore this issue we developed a numerical model that simulates the physical weathering of bedrock by root fracture and tree throw the coupled biogeomorphic model combines data on conifer population dynamics rootwad volumes tree throw frequency and soil creep from the pacific northwest usa although not hardwired into the model a humped relationship emerges between bedrock erosion and soil thickness the magnitudes of the predicted bedrock erosion rates and their functional dependency on soil thickness are consistent with independent field measurements from a coniferous landscape in the region imposed perturbations of soil erosion during model runs demonstrate that where bedrock weathering is episodic and localized hillslope soils do not exhibit runaway thinning or thickening the pit and mound topography created by tree throw produces an uneven distribution of soil thicknesses across a hillslope thus although episodes of increased erosion can lead to temporary soil thinning and even the exposure of bedrock patches local areas of thick soils remain these soil patches provide habitat for trees and serve as nucleation points for renewed bedrock erosion and soil production model results also suggest that where tree throw is a dominant weathering process the initial mantling of bedrock is not only a vertical process but also a lateral process soil mounds created by tree throw flatten over time spreading soil over bedrock surfaces

trouble with diffusion reassessing hillslope erosion laws with a particle based model 1 many geomorphic systems involve a broad distribution of grain motion length scales ranging from a few particle diameters to the length of an entire hillslope or stream studies of analogous physical systems have revealed that such broad motion distributions can have a significant impact on macroscale dynamics and can violate the assumptions behind standard local gradient flux laws here a simple particle based model of sediment transport on a hillslope is used to study the relationship between grain motion statistics and macroscopic landform evolution surface grains are dislodged by random disturbance events with probabilities and distances that depend on local microtopography despite its simplicity the particle model reproduces a surprisingly broad range of slope forms including asymmetric degrading scarps and cinder cone profiles at low slope angles the dynamics are diffusion like with a short range thin tailed hop length distribution a parabolic convex upward equilibrium slope form and a linear relationship between transport rate and gradient as slope angle steepens the characteristic grain motion length scale begins to approach the length of the slope leading to planar equilibrium forms that show a strongly nonlinear correlation between transport rate and gradient these high probability long distance motions violate the locality assumption embedded in many common gradient based geomorphic transport laws the example of a degrading scarp illustrates the potential for grain motion dynamics to vary in space and time as topography evolves this characteristic renders models based on independent stationary statistics inapplicable an accompanying analytical framework based on treating grain motion as a survival process is briefly outlined

gopher bioturbation field evidence for non linear hillslope diffusion it has generally been assumed that diffusive sediment transport on soil mantled hillslopes is linearly dependent on hillslope gradient fieldwork was done near santa barbara california to develop a sediment transport equation for bioturbation by the pocket gopher thomomys bottae and to determine whether it supports linear diffusion the route taken by the sediment is divided into two parts a subsurface path followed by a surface path the first is the transport of soil through the burrow to the burrow opening the second is the discharge of sediment from the burrow opening onto the hillslope surface the total volumetric sediment flux as a function of hillslope gradient is found to be qs cm3 cm 1 a 1 176 dz dx 3 189 dz dx 2 68 dz dx 34 dz dx 0 4 this result does not support the use of linear diffusion for hillslopes where gopher bioturbation is the dominant mode of sediment transport a one dimensional hillslope evolution program was used to evolve hillslope profiles according to non linear and linear diffusion and to compare them to a typical hillslope the non linear case more closely resembles the actual profile with a convex cap at the divide leading into a straight midslope section copyright 2000 john wiley sons ltd

prediction of spatially explicit rainfall intensity duration thresholds for post fire debris flow generation in the western united states abstract early warning of post fire debris flow occurrence during intense rainfall has traditionally relied upon a library of regionally specific empirical rainfall intensity duration thresholds development of this library and the calculation of rainfall intensity duration thresholds often require several years of monitoring local rainfall and hydrologic response to rainstorms a time consuming approach where results are often only applicable to the specific region where data were collected here we present a new fully predictive approach that utilizes rainfall hydrologic response and readily available geospatial data to predict rainfall intensity duration thresholds for debris flow generation in recently burned locations in the western united states unlike the traditional approach to defining regional thresholds from historical data the proposed methodology permits the direct calculation of rainfall intensity duration thresholds for areas where no such data exist the thresholds calculated by this method are demonstrated to provide predictions that are of similar accuracy and in some cases outperform previously published regional intensity duration thresholds the method also provides improved predictions of debris flow likelihood which can be incorporated into existing approaches for post fire debris flow hazard assessment our results also provide guidance for the operational expansion of post fire debris flow early warning systems in areas where empirically defined regional rainfall intensity duration thresholds do not currently exist

characterizing the primary material sources and dominant erosional processes for post fire debris flow initiation in a headwater basin using multi temporal terrestrial laser scanning data abstract wildfire dramatically alters the hydrologic response of a watershed such that even modest rainstorms can produce hazardous debris flows relative to shallow landslides the primary sources of material and dominant erosional processes that contribute to post fire debris flow initiation are poorly constrained improving our understanding of how and where material is eroded from a watershed during a post fire debris flow requires 1 precise measurements of topographic change to calculate volumetric measurements of erosion and deposition and 2 the identification of relevant morphometrically defined process domains to spatially constrain these measurements of erosion and deposition in this study we combine the morphometric analysis of a steep small 0 01 km 2 headwater drainage basin with measurements of topographic change using high resolution 2 5 cm multi temporal terrestrial laser scanning data made before and after a post fire debris flow the results of the morphometric analysis are used to define four process domains hillslope divergent hillslope convergent transitional and channelized incision we determine that hillslope divergent and hillslope convergent process domains represent the primary sources of material over the period of analysis in the study basin from these results we conclude that raindrop impact induced erosion ravel surface wash and rilling are the primary erosional processes contributing to post fire debris flow initiation in the small steep headwater basin further work is needed to determine 1 how these results vary with increasing drainage basin size 2 how these data might scale upward for use with coarser resolution measurements of topography and 3 how these results change with evolving sediment supply conditions and vegetation recovery