spatial organization in the step pool structure of a steep mountain stream vogelbach switzerland 1 spatial organization in the step pool structure of a 1 58 km long section of the vogelbach a steep mountain stream average channel bed slope 0 17 m m in the alpthal basin switzerland is analyzed to evaluate the extent of randomness in the occurrence of steps and to isolate the potential hydraulic and topographic controls on step placement statistics of the observed step sequence are compared with those of random sequences generated by permutation from the observed step height distribution hydraulic and topographic controls on step placement are analyzed from step length and steepness distributions and relations between mean step properties and step height and channel gradient for both observed and random sequences results show that 1 observed step length distributions are statistically significantly different from randomly generated sequences 2 step steepness is significantly different in observed data because of a positive correlation between mean step length and height and it remains fairly constant for all step sizes 3 spatial organization in steps does not extend far beyond the nearest step and 4 the influence of channel gradient on step properties is insignificant and highly variable indicating that hydraulic rather than topographic controls are dominant for step placement in this stream although the vogelbach is a steep stream on the boundary between a cascading and step pool morphology where we would expect randomness to dominate spatial organization was nevertheless detected in many important aspects of the step pool geometry

investigating feedbacks in human landscape systems lessons following a wildfire in colorado usa abstract as human interactions with earth systems continue to intensify understanding the complex relationships among human activity landscape change and societal responses to those changes becomes increasingly important interdisciplinary research centered on the theme of feedbacks in human landscape systems serves as a promising focus for unraveling these interactions this paper examines the specific case of the 2012 waldo canyon fire of colorado where human responses after the fire to perceived threats of hydro geomorphological hazards included construction of tall fences at the base of a burned watershed these actions prompted feedbacks that promoted further landscape change that ultimately increased those hazards rather than dampening the hydro geomorphological effects of fire geomorphic analysis showed that the fences trapped particles that would naturally move through the system by flows with recurrence intervals greater than 3 3 years with the particles blocked by the fences the channel downstream became erosive because it was devoid of large particles that produce substantial hydraulic resistance channel incision prompted a second human response to pave the eroding channel which led to further incision downstream this cycle of positive feedbacks between human decision making and landscape change eventually led to a complete channelization of the stream channel downstream of the fences the explanation for the transformation of the post fire landscape therefore lies in the interacting human impacts and feedbacks rather than the expected post fire hydro geomorphological adjustments an initial agent based model capable of integrating social and hydro geomorphological data simulates these interacting impacts and feedbacks further refinement with more complete data input especially pertaining to human decision making at individual or local levels is required to fully demonstrate the utility and promise of this tool for application to geomorphic analysis

influence of survey strategy and interpolation model on dem quality accurate characterisation of morphology is critical to many studies in the field of geomorphology particularly those dealing with changes over time digital elevation models dems are commonly used to represent morphology in three dimensions the quality of the dem is largely a function of the accuracy of individual survey points field survey strategy and the method of interpolation recommendations concerning field survey strategy and appropriate methods of interpolation are currently lacking furthermore the majority of studies to date consider error to be uniform across a surface this study quantifies survey strategy and interpolation error for a gravel bar on the river nent blagill cumbria uk five sampling strategies were compared i cross section ii bar outline only iii bar and chute outline iv bar and chute outline with spot heights and v aerial lidar equivalent derived from degraded terrestrial laser scan tls data digital elevation models were then produced using five different common interpolation algorithms each resultant dem was differentiated from a terrestrial laser scan of the gravel bar surface in order to define the spatial distribution of vertical and volumetric error overall triangulation with linear interpolation tin or point kriging appeared to provide the best interpolators for the bar surface lowest error on average was found for the simulated aerial lidar survey strategy regardless of interpolation technique however comparably low errors were also found for the bar chute spot sampling strategy when tins or point kriging was used as the interpolator the magnitude of the errors between survey strategy exceeded those found between interpolation technique for a specific survey strategy strong relationships between local surface topographic variation as defined by the standard deviation of vertical elevations in a 0 2 m diameter moving window and dem errors were also found with much greater errors found at slope breaks such as bank edges a series of curves are presented that demonstrate these relationships for each interpolation and survey strategy the simulated aerial lidar data set displayed the lowest errors across the flatter surfaces however sharp slope breaks are better modelled by the morphologically based survey strategy the curves presented have general application to spatially distributed data of river beds and may be applied to standard deviation grids to predict spatial error within a surface depending upon sampling strategy and interpolation algorithm

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

flow resistance in steep streams an experimental study 1 frequently an assessment of the mean water velocity in a stream is necessary to estimate the discharge associated with a particular flow depth or conversely the mean depth associated with a particular discharge in the absence of a direct measurement of flow velocity a flow resistance approach which establishes the relation between depth and velocity can be applied two approaches have been used in the past traditional approaches based on the use of a resistance coefficient e g darcy weisbach or dimensionless hydraulic geometry approaches to examine if one approach is more appropriate for steep streams data from 31 flume experiments conducted to examine flow resistance in self formed cascade channels were analyzed a dimensionless hydraulic geometry approach developed using at a station data to characterize the q exponent and between site data to characterize the exponent on the channel slope term was more accurate than more traditional approaches the developed relation was similar to the established rational relation v g0 2q0 6s 0 4s0 2 suggesting that the rational relation has merit the approach does not utilize a flow partitioning approach since in steep streams with small relative depths the grains themselves generate form and spill resistance the observation that a single dimensionless hydraulic geometry flow resistance relation can describe measurements across a range of grain sizes and bed slopes 3 21 suggests that steep streams may follow a single scaling relation similar to the regime equations associated with lowland rivers

quantifying the time scale of elevated geomorphic response following wildfires using multi temporal lidar data an example from the las conchas fire jemez mountains new mexico abstract sediment yields commonly increase following wildfires in mountainous landscapes quantifying the magnitude and duration of elevated sediment yields following wildfires is important for quantifying the relative importance of wildfire affected versus non wildfire affected erosion in the long term evolution of landscapes and for understanding the processes of post wildfire erosion in this study we use bi annual terrestrial laser scanning tls surveys conducted over a 2 yr period together with repeat airborne laser scanning als surveys conducted over a 1 yr period to estimate the dependence of sediment yield on time following the high severity las conchas fire of 2011 within two small approximately 1 3 km 2 watersheds the dependence of sediment yield on time since the wildfire was quantified using dems of difference dods of the upland watersheds to measure erosion and of the adjacent piedmonts to measure the deposition of eroded sediment over the 2 yr study period sediment volume decreased linearly with time and sediment yield decreased exponentially with time in one watershed while the other watershed showed a more complex relationship with time influenced by the specific sequence of rainstorms that impacted that watershed both watersheds had a recovery time of approximately 1 yr suggesting that vegetation regrowth and the stabilization of incised valley bottoms were primarily responsible for the decline in sediment yield versus time following the wildfire multi temporal dods of the piedmonts show how they responded to the delivery of elevated sediment yields from the upland watersheds i e by 1 incision of the proximal portion of the piedmont triggering distal deposition 2 infilling of the incised channel and avulsion and 3 incision of the distal portion creating an incised channel connecting the proximal and distal ends of the piedmont the dynamic nature of incision aggradation and channel shifting on piedmonts following wildfires has important implications for post wildfire flood hazard assessments

identification of steps and pools from stream longitudinal profile data abstract field research on step pool channels has largely focused on the dimensions and sequence of steps and pools and how these features vary with slope grain sizes and other governing variables measurements by different investigators are frequently compared yet no means to identify steps and pools objectively have been used automated surveying instruments record the morphology of streams in unprecedented detail making it possible to objectively identify steps and pools provided an appropriate classification procedure can be developed to achieve objective identification of steps and pools from long profile survey data we applied a series of scale free geometric rules that include minimum step length 2 25 of bankfull width w b minimum pool length 10 of w b minimum residual depth 0 23 of w b minimum drop height 3 3 of w b and minimum step slope 10 greater than the mean slope the rules perform as well as the mean response of 11 step pool researchers who were asked to classify four long profiles and the results correspond well with the channel morphologies identified during the field surveys from which the long profiles were generated the method outperforms four other techniques that have been proposed sensitivity analysis shows that the method is most sensitive to the choice of minimum pool length and minimum drop height detailed bed scans of a step pool channel created in a flume reveal that a single long profile with a fixed sampling interval poorly characterizes the steps and pools five or more long profiles spread across the channel are required if a fixed sampling interval is used and the data suggest that survey points should be located more frequently than the diameter of the step forming material a single long profile collected by a surveyor who chooses breaks in slope and representative survey points was found to adequately characterize the mean bed profile

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

a field investigation of the influence of sediment supply on step pool morphology and stability abstract the effect of the sediment supply on step pool morphology and stability was studied through the investigation of 42 step pool reaches eight in the vosges and 34 in the alps france step pool geometry step wavelength l and height h was measured for each reach and the data set split into two distinct groups comprising 25 connected to a sediment source step pool reaches and 17 disconnected step pool reaches the geometry analysis showed differences between the two groups with the shape factors k l hr and the h hr ratios hr is the residual pool depth being lower for disconnected step pools this was interpreted to result from progressively changing geometry over time with disconnected step pool geometry defining the probable asymptotic characteristic associated with this long term process the measured step pool dimensions were in good agreement with equations based on maximum flow resistance abrahams et al 1995 energy of a falling jet comiti et al 2005 and channel spanning deposition chartrand et al 2011 however the average shape factor for the comiti et al equation was k 17 instead of 8 as the authors had proposed the data set was extended to 255 values with data from the literature which confirmed this average value for k field evidence moss cover and stone imbrication indicate that connected step pools are mobile whereas disconnected step pools are stable this was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are 20 50 years for connected step pools and even less when considering scouring effects and greater than 50 years for disconnected step pools this difference in step mobility was ascribed to the fact that with sediment supply the channel can freely adjust its local slope allowing a relatively efficient transfer downstream of large boulders by scouring effects

the influence of weathering and soil organic matter on zn isotopes in soils zinc is an essential micronutrient that is ultimately released during mineral weathering in soils organic matter plays a key role in influencing zn partitioning and therefore on zn biogeochemical cycling soil organic matter is partitioned between carbon that is more readily available for decomposition by microorganisms and more stable carbon transiently preserved from decomposition the role of the stable pool of soil organic matter on zn biogeochemical cycling remains poorly understood the pool of stable carbon is controlled by combination with mineral constituents or is material that is intrinsically resistant to decomposition the zn stable isotopes are fractionated by interactions between zn and soil mineral and organic constituents this study reports the zn isotope composition of five icelandic soil profiles derived from the same parent basalt and characterized by contrasting degrees of weathering and organic matter content 66zn 0 10 0 05 to 0 35 0 02 the distribution of reactive mineral constituents available to form associations with soil organic matter and the amount of stable organic carbon throughout these soils the 66zn isotope variations are little influenced by mineral constituents but rather by soil organic matter content these data suggest that a combination of organic matter accumulation and zn loss by leaching is required to explain the observed decrease in zn concentration in soils and lighter soil 66zn with increasing organic carbon content these results suggest that the presence of stable organic carbon in soils provides a pool of light zn attributed to the zn isotope signature of organic matter partially preserved from decomposition crucially this stable organic carbon pool may also contribute to the formation of the light zn isotope sink reported in organic rich marine sediments a key output required to explain the oceanic mass balance of zn isotopes

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

step pool formation models and associated step spacing step pool bedforms develop under conditions of high flow rates and high sediment influx and transport rates step setting events are capable of mobilizing and rearranging the large step forming grain sizes thus observations of the processes of step formation are limited to flume experiments the results of laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps are presented over 350 step formations were observed and documented creating a large data set for detailing and evaluating step formation processes existing step formation models focus on step development from antidunes particle clusters transverse ribs or depositional berms or from a hydraulic regime that includes cascade flow and hydraulic jumps these models were developed using field measurements of artificial grade control structures or stable step pool systems during low flow conditions or degradational flume experiments where flows elucidated the step pool bedform in the channel profile but did not create equilibrium transport of the large step forming clasts this research evaluates the pre existing formation models and proposes three new models the rough bed exhumation and dune models the rough bed model was dominant throughout the experiments followed by the exhumation model and then the dune model a single model was not responsible for all step formations the rough bed exhumation and dune models all influenced step development and every step sequence was created through a mixture of formation processes this research generated a large data set where each step spacing is paired with the corresponding step formation model making it possible to identify a distinct exclusion zone length for each model thus given knowledge of the step formation model the minimum distance to the next step downstream can be predicted from the final spacing of a step sequence it may be possible to deduce the likely formation mechanisms active during a step resetting event copyright 2007 john wiley sons ltd

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

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

storm rainfall conditions for floods and debris flows from recently burned areas in southwestern colorado and southern california abstract debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the western u s field evidence indicate that unlike landslide triggered debris flows these events have no identifiable initiation source and can occur with little or no antecedent moisture using rain gage and response data from five fires in colorado and southern california we document the rainfall conditions that have triggered post fire debris flows and develop empirical rainfall intensity duration thresholds for the occurrence of debris flows and floods following wildfires in these settings this information can provide guidance for warning systems and planning for emergency response in similar settings debris flows were produced from 25 recently burned basins in colorado in response to 13 short duration high intensity convective storms debris flows were triggered after as little as six to 10 min of storm rainfall about 80 of the storms that generated debris flows lasted less than 3 h with most of the rain falling in less than 1 h the storms triggering debris flows ranged in average intensity between 1 0 and 32 0 mm h and had recurrence intervals of two years or less threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south central and southwestern colorado are defined by i 6 5 d 0 7 and i 9 5 d 0 7 respectively where i rainfall intensity in mm h and d duration in hours debris flows were generated from 68 recently burned areas in southern california in response to long duration frontal storms the flows occurred after as little as two hours and up to 16 h of low intensity 2 10 mm h rainfall the storms lasted between 5 5 and 33 h with average intensities between 1 3 and 20 4 mm h and had recurrence intervals of two years or less threshold rainfall conditions for life and property threatening floods and debris flows during the first winter season following fires in ventura county and in the san bernardino san gabriel and san jacinto mountains of southern california are defined by i 12 5 d 0 4 and i 7 2 d 0 4 respectively a threshold defined for flood and debris flow conditions following a year of vegetative recovery and sediment removal for the san bernardino san gabriel and san jacinto mountains of i 14 0 d 0 5 is approximately 25 mm h higher than that developed for the first year following fires the thresholds defined here are significantly lower than most identified for unburned settings perhaps because of the difference between extremely rapid runoff dominated processes acting in burned areas and longer term infiltration dominated processes on unburned hillslopes

flow regimes bed morphology and flow resistance in self formed step pool channels received 3 july 2008 revised 28 december 2008 accepted 13 january 2009 published 25 april 2009 1 we used a mobile bed flume with scaled grain size distribution channel geometry and flow to examine morphology and hydraulics of stepped channels we hypothesized that 1 step geometry and flow resistance differs significantly as a function of the range of grain sizes present 2 a transition from nappe to skimming flow occurs in stepped channels with mobile beds for conditions similar to stepped spillways and 3 the partitioning of flow resistance changes significantly when flow passes from nappe to skimming conditions results support each of these hypotheses and help to illuminate the complexity of v q relationships in stepped channels in which a dramatic decrease in flow resistance and increase in velocity accompany the transition from nappe to skimming flow near step forming events therefore a single flow resistance equation applicable to both ordinary and large floods may not be ideal in stepped channels nonetheless models based on dimensionless velocity and unit discharge appear more robust compared to those based on the darcy weisbach friction factor

functional and structural connectivity within a recently burned drainage basin abstract studies examining post wildfire sediment transport have often focused on changes to individual landscape compartments planar slopes rills gullies channels or alluvial fans or have captured coarse scale hydrologic and sediment transport events at the drainage basin scale we advance the understanding of functional and structural connectivity by quantifying changes of the morphodynamics of and sediment transport along seven rill gully threads rgts after two low intensity rainstorms in a burned basin from the 2008 gap fire near goleta ca usa tls surveys conducted within two months of the initial fire and three days after the rainfall events provide point clouds for high resolution digital terrain models dtms dtm differencing techniques and morphological sediment budgets from the rgts showed discontinuous sediment transport along the extent of these two landscape compartments immediately after the rainfall surface runoff was unable to remove dry ravel deposits within the rgts and implied a high degree of structural disconnectivity there dry ravel and runoff erosion from the contributing areas to the rgts indicated functional and structural connectivity at this scale of analysis the results provide clear evidence that small amounts of rainfall and gravity induced erosion are interacting at different scales within the recently burned watershed to produce structural and functional disconnectivity along the rgts while the current system was transport limited during the analyzed event higher magnitude rainstorms may produce enhanced connectivity resulting in the ability of surface runoff to remove the stored sediments and perhaps produce debris flows

stream bed stabilization using boulder check dams that mimic step pool morphology features in northern italy during the last 10 years the check dams made of boulders have been fully employed for torrent management particularly for high gradient stream stabilization the analysis of this typology is shown in relation to three aspects building features field of use and design according to geomorphologic principles the maso di spinelle torrent located in the province of trento italy north italian alps is an example where a sequence of low check dams made of boulders have been used for bed stabilization the design criteria are taken from the step pool morphology features and the results are encouraging for further applications the artificial step pool grade control structures in the maso di spinelle torrent have been successfully tested by floods events with return periods of about 7 10 and 20 25 years

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