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

channel bed steps and pool shapes along soda creek three sisters wilderness oregon abstract field study of bedrock step pool systems along the upper reaches of soda creek in the three sisters wilderness of oregon shows strong correlation between several form variables shape and channel slope although step height and step length showed no regular spacing and variable correlation with channel slope length to height ratios demonstrated strong negative correlations steep slopes 20 to 80 featured greater step height and shorter pool lengths than did flatter channel slopes correlations between step height to length ratios and channel slope varied between three lithologies explained variations ranged from 0 984 for the oldest channel steps developed in basalt to 0 982 for steps of intermediate age developed in andesite to 0 964 for the youngest steps developed in dacite sample size was 57 40 and 33 respectively the frequency of pool shape classes did not vary by lithology but specific shape classes developed under differing slope conditions by rock type all pool classes have adjusted developed their form to maximize resistance to flow h l s and they have done so in remarkably uniform fashion

nonlinear tide surge wave interaction at a shallow coast with large scale sequential harbor constructions abstract the southwestern part of bohai bay in northeastern china is a shallow water coast with a long fetch for water to pile up to produce large storm surge under strong wind action during the extreme weather condition in addition the coastline along the bay has experienced significant environmental impacts from large scale harbor constructions in the past two decades consequently there is a pressing need to better understand the effects of two local large scale sequential harbor constructions on storm surge and coastal vulnerability in the bay area the extreme cold front weather on 10 13th october 2003 caused a 1 31 billion yuan damage a two way coupled tide surge wave delft3d model is used to investigate the impact of the harbor constructions from 2003 to 2016 on the interaction of tide storm surge and wave during a storm like that the wind field by the newly released global reanalysis wind product era5 of european centre for medium range weather forecasts ecmwf has been improved by assimilation with field observations at a higher resolution than previous version and is used to drive the tide surge and wave modelling framework the model results indicate that the storm surge in the study area is dominated by the wind induced surge before and after the constructions whereas the maximum contribution from wave induced surge wave setup is increased from 5 15 to 8 20 by the harbor constructions between 2003 and 2016 it is found that semi diurnal modulation of storm surge is generated by nonlinear tide surge interaction which is mainly attributed to local tidal acceleration in the offshore zone and bottom stress in the nearshore zone respectively the harbor constructions affect the coastal storm surge mainly through nonlinear tide surge wave interaction in addition the quarter diurnal variation of storm surge induced by the harbor constructions is due to the influences of the harbor constructions on the local quarter diurnal tidal constituents and their modulation of wave momentum transfer to coastal circulation

bomb fallout137cs as a marker of geomorphic stability in dune sands and soils pinery provincial park ontario canada the anthropogenic radionuclide 137cs has been extensively utilized as a tracer of geomorphic processes in the northern hemisphere since its deposition during atmospheric testing of nuclear devices in the 1950s and 1960s the distribution of bomb fallout 137cs was measured on a sequence of coastal dune sands and soils at pinery provincial park on the coast of lake huron in southern ontario canada the depth distribution within the stabilized developed soils inland reflected the relationship between clay content and the adsorption and immobilization of the radionuclide however the influence of soil organic matter silt sized particles and vegetation cycling on the profile distribution could not be discounted within the geomorphically dynamic dune sands near the coast there was a significant activity of 137cs even though the sands were lacking in clay sized particles within a buried soil on the inland side of a large active dune blowout the distribution of 137cs with depth was useful as a stratigraphic marker of the rates of accumulation of sands at that position therefore 137cs may be a useful alternative to erosion pins sequential air photos and sediment traps in the monitoring of dune destabilization in coastal environments copyright 2001 john wiley sons ltd

flow resistance equation for rills in this paper a new flow resistance equation for rill flow was deduced applying dimensional analysis and self similarity theory at first the incomplete self similarity hypothesis was used for establishing the flow velocity distribution whose integration gives the theoretical expression of the darcy weisbach friction factor then the deduced theoretical resistance equation was tested by some measurements of flow velocity water depth cross section area wetted perimeter and bed slope carried out in 106 reaches of some rills modelled on an experimental plot a relationship between the velocity profile the channel slope and the flow froude number was also established the analysis showed that the darcy weisbach friction factor can be accurately estimated by the proposed theoretical approach based on a power velocity profile

analysis of changes in the step pool morphology and channel profile of a steep mountain stream following a large flood abstract the step pool morphology of steep mountain streams is reorganized during large floods by the removal of steps scouring of the channel bed and the formation of a new step pool sequence by the re deposition of step forming grains in this paper we document these changes caused by the largest recorded flood return period 50 years in the steep mountain stream erlenbach in central switzerland in 2007 by comparing pre flood and post flood surveys of a 650 m long study reach we show that the flood led to statistically significant changes in the distributions of step height and spacing most notably an increase in the variance of both variables organization at the river reach scale is not apparent and the correlations between step properties both pre and post flood are generally weak the flood led to widespread erosion of the river bed locally up to 2 3 m of vertical channel change was observed at least 60 of the steps moved even the largest ones however they remained constrained to the parts of the study reach where landslides on the adjacent hillslopes are most active detailed changes in bed structure were analyzed from the pre and post flood profiles with a statistical scaling approach which identifies the complexity of the spatial organization of sediment in the longitudinal profile multiscaling found in both step and pool sequences reflects the complex spatial structure which is the result of the superposition of steps formed by the history of flood events

a storm surge prediction model for the northern bay of bengal with application to the cyclone disaster in april 1991 abstract a numerical model for simulating and predicting tides and storm surges in regions that include areas of open sea combined with estuarine channels and intertidal banks is described the model makes use of modified depth averaged equations with a numerical scheme in which the solution of 1d equations for narrow channels 2d equations for the open sea and an approach to modeling inundation are combined within a unified framework this simplifies the construction of realistic models for complex coastal areas and deltas the model is applied to the northern shelf of the bay of bengal and the ganges delta including the coast of bangladesh devastated by a cyclone surge in april 1991 in which about 140 000 people died hindcast and forecast simulations of this event using forcing derived from a semianalytical cyclone model with data supplied by the joint typhoon warning center jtwc are described it is shown that the timing of cyclone landfall and its coincidence with high tide determine the area

the frictional resistance characteristics of a bedrock influenced river channel much of the published literature relating to reach scale total ow resistance concentrates on alluvial rivers and little mention is made of the more extreme resistance found in bedrock in uenced river systems this paper presents the results of a detailed investigation of total ow resistance variation for ve channel types on the sabie river south africa cohesive mixed anastomosed uncohesive mixed anastomosed mixed pool rapid alluvial braided and alluvial single thread over a range of discharges manning s n and darcy weisbach f resistance coef cients are calculated using barnes methodology h h barnes us geological survey water supply paper 1849 1967 these data are analysed at the scales of channel type and morphologic unit and the resultant resistance values are compared with the literature generally the low ow friction values estimated for the bedrock in uenced channel types are higher than any reported flow resistance quanti cation of the alluvial channel type study sites on the sabie river produced values slightly higher than earlier studies of similar morphologies in situations where signi cant areas of irregular bedrock are exposed cohesive mixed anastomosed and mixed pool rapid channel types the high ow resistance coef cients at low discharges probably re ect energy dissipation due to hydraulic jumps and internal distortions however these values fall as discharge increases to values comparable to those recorded for step pool rivers it is suggested that the slightly elevated resistance coef cients recorded in the alluvial channel types are the result of the vegetation roughness component operating when the macro channel bar features are inundated copyright 2004 john wiley sons ltd

comment on effects of different tillage practices on the hydraulic resistance of concentrated flow on the loess plateau in china by j sun et al abstract for concentrated flows which are characterized by small water depth and steep sloping beds hydraulic conditions different from those typical of streams and rivers occur in this study a new theoretically deduced flow resistance equation was tested using the experimental data by sun et al for three different tilled surfaces manual dibbling md manual hoeing mh and contour drilling cd at first the profile parameter relationship which is the relationship between the velocity profile parameter the channel slope and the flow froude number was calibrated using rill flow data by di stefano et al then the applicability of this relationship was tested by the measurements of sun et al the calculated darcy weisbach friction factor values were generally lower than those measured by sun et al and this result was justified taking into account that the rill measurements used for calibrating the profile parameter relationship simply correspond to grain resistance conditions for taking into account both grain resistance and tillage induced surface roughness the profile parameter relationship was calibrated by the measurements by sun et al this calibration was carried out for each tillage practice md mh and cd and using all the measurements carried out during each 20 min experimental run this study was also developed using exclusively the data concerning the beginning of the experiment 0 4 min in order to state the effect of the initial roughness and those regarding the end of the experiment 16 20 min to estimate the influence of the temporal evolution of roughness on the friction factor the study proved that the darcy weisbach friction factor due to grain resistance and tillage induced roughness can be accurately estimated using the proposed theoretical approach a temporal variation of the darcy weisbach friction factor was also recognized finally the data were supportive of the slope independence hypothesis of flow velocity stated by govers

interaction between bed roughness and flow hydraulics in eroding rills govers 1992 showed that the flow velocity in rills eroding loose nonlayered materials could be predicted from knowledge of discharge only without significant slope or soil effect the objective of this paper is to investigate to what extent the observed slope independence of flow velocity in eroding rills can be explained by the interaction between rill bed roughness and flow hydraulics in a laboratory study two situations were compared 1 rills which can freely erode a uniform soil layer and 2 rills with a fixed bed geometry during the experiments rill discharge and flow velocity were recorded after each experiment a detailed topographic survey of the rill bed was carried out using a laser scanner from these data the main hydraulic variables mean values of flow depth wetted perimeter and hydraulic radius were estimated the experiments confirmed the slope independence of rill flow velocities on mobile beds when the bed is fixed the flow velocity in rills is clearly slope dependent the slope independence of flow velocity on mobile beds is due to a feedback between rill bed morphology and flow conditions the roughness amplitude which was assessed from the standard deviation of corrected height values and the frequency of macroroughness elements which was assessed by counting the number of peaks per unit length in a previously simplified longitudinal profiles both increase and tend to counteract the effect of the increase in slope gradient on rill flow velocity the final rill flow velocity appears to be characterized by a constant average froude number

equilibrium pool dimensions and sediment sorting patterns in coarse grained new england channels abstract pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat an understanding of this morphology is necessary for the successful design of a reconstructed or restored channel to better understand pool geometry and sorting characteristics of new england channels characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions characterizations of channel bed sediments were also performed at 34 of these locations data from 120 pools were used in least squares multiple regression analysis with 19 different independent variables to develop prediction equations for pool depth length and exit slope gradient these equations were then tested using the 25 remaining pools eighteen different geometric variables were then used to predict sediment 50 values in pool centers pool exit slopes and riffles for the subset of 34 samples the regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit slope width constriction gradient constriction width drainage area upstream channel width and the exit slope expansion ratio with an r 2 value of 0 53 similarly pool length is influenced by channel gradient location of the channel constriction pool width drainage area and constriction length with an r 2 value of 0 72 the gradient of pool exit slope is influenced by drainage area channel gradient and constriction gradient with an r 2 value of 0 34 however r 2 values for exit slope increased to 0 65 when sediment d 50 data were utilized using the resulting regression equations to predict dimensions for the 25 verification pools yields average residuals near zero although moderate variations do exist a tukey kramer hsd comparison of the means for sediment 50 values indicates pool sediments are significantly coarser than pool exit slope and riffle sediments for the subset of 34 pools regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with r 2 values that range from 0 45 to 0 62 the results suggest that pool dimensions are related to basin wide variability and local hydraulic conditions the findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout the pool riffle morphology

field measurements of three dimensional hydraulics in a step pool channel we investigated the effects of morphologic position and discharge on flow structure in a steep 0 10m m mountain channel by collecting three dimensional measurements of time averaged and turbulent velocity components with a sontek flowtracker handheld adv acoustic doppler velocimeter on a 30 m reach of a step pool channel in the colorado rockies velocity profiles were measured at morphologic positions characteristic of steep channels above steps step lips base of steps pools cascades runs and at five different discharges a marked three dimensionality of flow structure was documented in east st louis creek velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes cross stream and vertical components contributed averages of 20 and 15 respectively to overall vector magnitudes turbulence intensities were especially multi dimensional however with large contributions to turbulent kinetic energy from the vertical component of velocity analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities with substantially higher velocities upstream from steps than in pools discharge and morphology effects on cross stream and vertical velocity components however were not significant discharge and morphologic position also significantly affected turbulence intensities for all flow components with the greatest turbulence intensities occurring in pools and at high discharges these results illustrate the strong discharge dependence of hydraulics in step pool channels where relative submergence of bedforms changes rapidly with discharge and the substantial spatial variation in hydraulics created by step pool sequences

bed load transport flume experiments on steep slopes experiments were conducted over uniform gravel bed materials to obtain 143 friction factor values under bed load equilibrium flow conditions in an attempt to add to the scarce data available on slopes between 1 and 9 for shields numbers between 0 08 and 0 29 analyses showed that when only flows over flat beds are considered a distinction must be made between flows with and without bed load more particularly fitting flow resistance equations indicated that the roughness parameter increases by a factor of 2 5 from clear water flow to intense bed load transport between these two states the flow resistance can be approximated by a constant for a given slope

field investigation of rill and ephemeral gully erosion in the sparacia experimental area south italy abstract this paper reports the results of a field investigation aiming at characterizing the morphology and hydraulics of both rills and ephemeral gullies egs monitored at sparacia experimental area sicily italy the comparison between rill and interrill erosion measurements showed that sediment delivery processes occurred in some erosive events and that the rill erosion rate was dominant in many cases the measurements were used for testing both the empirical relationships between the channel rill egs and gully length and its eroded volume and among the morphological variables length width depth and volume describing the channelized process finally the rill and egs hydraulic geometry was modelled by three well known power equations relating the discharge with the mean flow velocity with the flow depth and with the width of each channel segment respectively the rill measurements also showed that the flow velocity was affected by the rill segment slope while the flow depth and width were controlled by the plot slope therefore three equations taking into account these slope effects were proposed to estimate the hydraulic characteristics of the rills monitored at the sparacia area

bed load effects on hydrodynamics of rough bed open channel flows the extent to which turbulent structure is affected by bed load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed sized granular sediment with a d50 of 1 95 mm three different sizes of sediment d50 0 77 1 99 and 3 96 mm were fed into the flow at two different rates 0 003 and 0 006 kg m s and subsequently transported as bed load particle image velocimetry piv was used to determine the turbulence characteristics over the fixed bed during clear water and sediment feed cases mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases the exception was with the transport of fine grains at the higher feed rate in this case longitudinal mean flow velocities increased compared to the clear water condition the coarse grains tended to augment bed roughness but fine grains saturated the troughs and interstices in the bed topography effectively causing the influence of bed irregularities to be smoothed the piv technique permitted examination of both temporal and spatial fluctuations in flow variables therefore many results are presented in terms of double averaged quantities in temporal and spatial domains in particular the form induced stress which arises from spatially averaging the reynolds averaged navier stokes equations and is analogous to the reynolds turbulent stress contributed between 15 and 35 of the total measured shear stress in the roughness layer flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production suggesting that this process is highly intermittent near rough beds

flow resistance estimation in mountain rivers examination of the flow resistance of high gradient gravel and boulder bed rivers using data collected in british mountain rivers with slopes of 0 4 4 shows that there are differences in resistance variation between mountain and lowland rivers and that between site variations do not necessarily reflect at a site variations comparison of data with the familiar resistance equation relating the dracy weisbach friction factor to the logarithm of relative submergence shows that the equation tends to overestimate the resistance in uniform flow the equation also tends to underestimate the rate of change of resistance at a site as discharge varies with high gradients the influences of nonuniform channel profile sediment size distribution channel slope and sediment transport are reviewed but the data do not allow any quantification of these effects instead an empirical approach based on the available data is presented allowing the friction factor to be calculated from the relative submergence with an error of up to n25 to n35 a summary of the field data is included

the role of morphology on the displacement of particles in a step pool river system abstract although studies of sediment transport in steep and coarse grained channels have been more numerous in recent years the dynamics of sediment transport in step pool river systems remain poorly understood this paper investigates displacements of individual clasts through spruce creek quebec canada a classic step pool channel and the effects of the channel morphology on the path length of the clasts passive integrated transponder tags pit were used to track the displacement of 196 individual particles over a range of discharges including the bankfull stage clasts were tracked after five sequences of flood events the results showed that the distance distributions match a two parameter gamma model equal mobility transport occurs for the particle size investigated during each sequence of flood events mean travel distance of the clasts can be estimated from excess stream power and the mobility of the clasts is more than an order of magnitude less than the model reported in riffle pool channels the dominant morphological length scale of the bed also controls the path length of the clasts these results confirm some preliminary observations on sediment transport in step pool channels

influence of coarse lag formation on the mechanics of sediment pulse dispersion in a mountain stream squire creek north cascades washington united states 1 mountain channels closely coupled to landslide prone hillslopes often exhibit bed surface grain sizes coarser than transportable by annual high flows coarse particles within poorly sorted sediment delivered to channels by mass wasting processes may not be readily transported as bed load and can consequently form lag deposits that influence the morphology hydraulic roughness and sediment storage within mountain channel networks a tracer study and comparison of supply and bed grain size distributions from a valley spanning landslide in the north cascades of washington state were used to derive relations between shear stress and the probability of particle entrainment and erosion from the sediment pulse rapid bed surface armoring formed a relatively immobile lag deposit within 2 years covering of 20 of the bed by lag boulders with 5 probability of entrainment was sufficient to retard vertical incision and force considerable channel widening during a flood with an 8 to 152 year recurrence discharge on locally gauged streams our results imply that numerical models of sediment pulse evolution that do not explicitly incorporate the influence of lag formation may substantially overestimate long term dispersion rates the grain size distribution and lithology of a sediment input relative to the flow competence of the receiving channel are important factors influencing the rates and mechanisms of sediment pulse dispersion and the sediment capacitance provided by coarse grained sediment pulses in mountain drainage basins

comparing flow resistance law for fixed and mobile bed rills rills caused by run off concentration on erodible hillslopes have very irregular profiles and cross section shapes rill erosion directly depends on the hydraulics of flow in the rills which may differ greatly from hydraulics of flow in larger and regular channels in this paper a recently theoretically deduced rill flow resistance equation based on a power velocity profile was tested experimentally on plots of varying slopes ranging from 9 to 26 in which mobile and fixed bed rills were incised initially measurements of flow velocity water depth cross section area wetted perimeter and bed slope carried out in 320 reaches of mobile bed rills and in 165 reaches of fixed rills were used for calibrating the theoretical flow resistance equation then the relationship between the velocity profile parameter the channel slope and the flow froude number was separately calibrated for the mobile bed rills and for the fixed ones the measurements carried out in both conditions fixed and mobile bed rills confirmed that the darcy weisbach friction factor can be accurately estimated using the proposed theoretical approach for mobile bed rills the data were supportive of the slope independence hypothesis of velocity due to the feedback mechanism stated by govers the feedback mechanism was able to produce quasicritical flow conditions for fixed bed rills obtained by fixing the rill channel by a glue at the end of the experimental run with a mobile bed rill the slope independence of the flow velocity measurements was also detected therefore an experimental run carried out by a rill bed fixed after modelling flow action is useful to detect the feedback mechanism finally the analysis showed that for the investigated conditions the effect of sediment transport on the flow resistance law can be considered negligible respect to the grain roughness effect

channel units in small high gradient streams on vancouver island british columbia descriptions and a classification are presented of channel units in very high gradient first order mountain channels bedrock cascades boulder cascades rapids chutes riffles glides and pools had distinct mean bed gradients of 0 49 0 45 0 20 0 12 0 09 0 06 and 0 03 respectively multiple pairwise comparisons showed that these gradients are significantly different except bedrock cascades were not different from boulder cascades and glides were not different from chutes riffles or pools channel width and entrenchment do not discriminate channel units in any consistent way but characteristic substrates occur constituent substrates of bedrock and boulder cascades are nominally apparent while chutes also are bedrock bound mixtures of boulders and cobbles constitute rapids while mixtures of cobbles and gravel constitute riffles glides and pools generally are composed of boulders cobbles and gravel in various proportions log steps sometimes delimit units particularly pools but wood is not a dominant element in these channels gradients of channel units in these small first order dominantly nonalluvial channels are significantly higher than correspondingly identified units in larger channels farther down the drainage system the reason for this distinction is supposed to be related to channel scale hence potential stream power

characteristics of log and clast bed steps in step pool streams of northwestern montana usa the role of log and clast steps was studied in 53 step pool reaches in 32 streams in northwestern montana in each reach 20 consecutive steps were sampled drainage areas ranged from 0 5 to 11 5 km2 mean step spacing was found to be proportional to channel gradient width of the bankfull channel and drainage area mean step height was proportional to the width of the channel no significant difference exists between clast and log steps with regard to step spacing and step height this suggested either that 1 woody debris is mobile in even the smallest channels in the study area or 2 channel flows arrange clast steps around immobile woody debris calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels the presence of immobile log steps composed of living roots in the upstream reaches an increase in the percentage of log steps with gradient and a decrease in this percentage with the width of the bankfull channel and drainage area imply that woody debris is mobile primarily in the downstream reaches calculations of flow resistance suggest that most steps serve to maximize flow resistance these results support the second hypothesis that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics

development of channel organization and roughness following sediment pulses in single thread gravel bed rivers large episodic inputs of coarse sediment sediment pulses in forested mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness a conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels channels exhibited self organizing behavior to various degrees based on channel gradient presence of large in channel wood or other forcing elements the size of the sediment pulse and the number of bed mobilizing flows since disturbance typical channel changes following a sediment pulse were initial decreases in water depth in variability of bed elevations and in the regularity of bed form spacing trajectories of change subsequently showed increased average water depth more variable and complex bed topography and increased uniformity of bed form spacing bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale two to five channel widths than in streams without such forcing mechanisms five to 10 channel widths channel roughness increased as bed forms developed

dimensions of standing waves at steps in mountain rivers 1 oscillations of the water surface are common features of flows around critical conditions fr 1 and of both natural and structure induced undular hydraulic jumps the paper describes experimental tests and field data on the dimensions of standing waves created by artificial drops in steep gravel bed rivers the purpose of the research is to analyze what parameters affect the phenomenon to provide semiempirical formulations for wave dimensions and to assess whether previous models for undular hydraulic jumps and antidunes are reasonably predictive the dimensionless wavelength and wave amplitude were correlated to the froude number at the sill the first wavelength appears linearly related to the froude parameter whereas the amplitude of the first wave shows a less defined behavior possibly indicating that a maximum value occurs for sill froude numbers around 1 6 1 7 the corresponding wave steepness i e wave amplitude length results between 0 15 0 16 and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum probably related to hydrodynamic conditions just before wave breaking the transition between roller and undular jumps at drops is then argued to likely represent a critical stage in step pool streams and the hydraulic and morphological implications of this finding are also discussed with particular emphasis on step stability and step wavelength

measuring rill erosion using structure from motion a plot experiment abstract in this paper the results of a plot experiment on rill erosion are reported the rill network manually incised on the soil and further shaped by a clear inflow discharge was surveyed using the three dimensional photo reconstruction 3d pr technique which allows to obtain a digital terrain model dtm by a large series of oblique images of the channel from consumer un calibrated and non metric cameras the three dimensional 3d dtm and the quasi tridimensional 2 5d model were generated by agisoft photoscan software for a single rill channel the reliability of the 3d image based ground measurements of morphological and hydraulic variables was positively tested by the corresponding measurements carried out on the rill gypsum cast moreover the morphological and hydraulic variables were measured at 11 transects of the plot by a profilometer p the 2 5d and 3d models using the 3d model as reference the analysis showed that the reliability of the measurements by p and 2 5d methods was comparable and the error was generally lower than 15 the rill measurements carried out by the three methods agreed with the available literature measurements and supported the applicability of both the empirical relationship between rill length and its eroded volume and the theoretical dimensionless relationship among the morphological variables describing the channelized erosion process the analysis also showed that the effect of the distance between two consecutive cross sections on the rill volume measurement by 3d model is negligible finally the shaping effect of different flow discharges on three straight rills incised into the plot was tested in particular three increasing flow discharges 0 15 0 35 and 0 5 l s 1 were used sequentially in two rills while the maximum discharge of the sequence 0 5 l s 1 was used into the third rill both the grain size distribution of the eroded sediment and the total amount of soil loss were affected by the discharge sequence

a synchronously coupled tide wave surge model of the yellow sea abstract a coupled wave tide surge model has been established in this study in order to investigate the effect of tides storm surges and wind waves interactions during a winter monsoon on november 1983 in the yellow sea the coupled model is based on the synchronous dynamic coupling of a third generation wave model wam cycle 4 and the two dimensional tide surge model the surface stress generated by interactions between wind and waves is calculated using the wam cycle 4 directly based on an analytical approximation of the results obtained from the quasi linear theory of wave generation the changes of bottom friction factor generated by waves and current interactions are calculated by using simplified bottom boundary layer model the model simulations showed that bottom velocity and effective bottom drag coefficient induced by combination of wave and current were increased in shallow waters of up to 50 m in the yellow sea during the wintertime strong storm conditions

incomplete self similarity and flow velocity in gravel bed channels velocity measurements previously carried out using both a miniature current flowmeter and an acoustic doppler velocimeter are used to verify the applicability of the incomplete self similarity theory to deduce the velocity profile in a gravel bed channel then for the velocity profiles having the maximum value below the free surface and for the s shaped profiles the power velocity distribution is corrected using a new divergence function for each value of the depth sediment ratio the nondimensional friction factor parameter is calculated by integration of the measured velocity distributions in the different verticals of the cross section finally a semilogarithmic flow resistance equation is empirically deduced

time and the rivers flowing fluvial geomorphology since 1960 fluvial geomorphology has been the largest single subdiscipline within geomorphology for many decades fluvial geomorphic expertise is integral to understanding and managing rivers and to developing strategies for sustainable development this paper provides an overview of some of the significant advances in fluvial geomorphology between 1960 and 2010 with respect to conceptual models fluvial features and environments being studied tools used by fluvial geomorphologists geomorphic specialty groups within professional societies journals in which fluvial geomorphic research is published and textbooks of fluvial geomorphology during this half century fluvial geomorphology broadened considerably in scope from a focus primarily on physical principles underlying process and form in lower gradient channels with limited grain size range to a more integrative view of rivers as ecosystems with nonlinear behavior and great diversity of gradient substrate composition and grain size the array of tools for making basic observations analyzing data and disseminating research results also expanded considerably during this period as did the diversity of the fluvial geomorphic community

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

modelling the spatial variation of processes in the redistribution of soil digital terrain models and 137cs in southwest niger abstract an interpolated grid based digital elevation model dem was created from survey data to derive two compound topographic indices a stream power index to provide a surrogate measure of water erosion and an index of susceptibility to aeolian processes maps of both indices were compared with a map of interpolated percentage 137 cs residuals used as a surrogate for net soil redistribution over the last ca 30 years results suggest that wind erosion is pervasive throughout the study area whereas water erosion is concentrated on the steeper slopes areas of potential aeolian deposition were identified on the lee side of obstacles but were validated in only a few cases by the map of 137 cs residuals this anomaly probably results from considerable reworking of aeolian material deposited in the lee of obstacles on slopes highly susceptible to water erosion although the digital terrain models dtms could not explain the pattern of 137 cs residuals in some locations the inclusion of the percentage of vegetation cover further elucidated the patterns of soil redistribution vegetation cover was found to explain a considerable amount of variation in the patterns of soil redistribution over the medium time scale further research into the modelling of these processes is necessary to elucidate the pattern of vegetation cover removal and its link with soil redistribution

riffle pool maintenance and flow convergence routing observed on a large gravel bed river abstract geomorphologists have studied and debated over the processes responsible for natural riffle pool maintenance for decades most studies have focused on small wadable rivers but they lack much description of overbank flood conditions or a spatially explicit characterization of morphodynamics in this study 1 m horizontal resolution digital elevation models were collected from a riffle pool run sequence before and after an overbank flood with a 7 7 year recurrence interval on the relatively large gravel bed lower yuba river california digital elevation model differencing was used to quantify the magnitude and pattern of flood induced morphodynamic change cross section based analysis and two dimensional hydrodynamic modeling of flows ranging from 0 147 to 7 63 times bankful discharge were completed to evaluate the hydraulic mechanisms responsible for the observed topographic changes one key finding was that riffle pool relief increased by 0 42 m confirming the occurrence of natural hydrogeomorphic maintenance spatially complex patterns of scour and deposition exceeding 0 15 m at the scale of subwidth morphological units were reasonably predicted by the two dimensional mechanistic model that accounts for convective acceleration the one dimensional cross section based method underperformed the two dimensional model significantly consequently multiple scales of channel non uniformity and a dynamic flow regime caused the observed maintenance of the pool riffle morphology through the mechanism of flow convergence routing proposed by macwilliams et al macwilliams m l wheaton j m pasternack g b kitanidis p k street r l 2006 the flow convergence routing hypothesis for riffle pool maintenance in alluvial rivers water resources research 42 w10427 doi 10 1029 2005wr004391

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

on the wave current interaction during the passage of a tropical cyclone in the bay of bengal abstract the impact of waves on currents and vice versa is an integral part of the complex coastal dynamics the wave current interaction is prominent in the coastal shelf slope regions particularly during the passage of a tropical cyclone in the present study we describe the importance of wave current interaction in the northern bay of bengal bob using a fully coupled three dimensional coupled ocean atmosphere wave sediment transport coawst modelling system the coawst model is applied to the case of a very severe cyclonic storm phailin that passed over the bob basin during 10 15 october 2013 to represent the interaction between waves and currents we utilized the newly implemented vortex force method in the coupled model numerical experiments with different coupling configuration within the coawst modelling framework were performed to separately identify the effects of wind tide and wave current interaction process a comparison of model results with the buoy observations of water elevations currents and wave measurements shows a good agreement between model and observed data various terms of the momentum balance were calculated from the model simulated and diagnosed parameters a comparison of the horizontal momentum balance term identifies the wave breaking induced acceleration as one of the leading terms along the storm affected the east coast of india further an increase in the apparent bed roughness caused by waves found to affect the values and distribution of the bottom shear stress the pressure gradient term showed significant changes to the pure tidal case the study highlights that the changes in the momentum balance caused by waves includes contributions from the variations in the water level and currents the most relevant effect on the coastal hydrodynamics was in the form of a wave induced setup near the location of landfall of the cyclone

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

soil erosion by surface water flow on a stony semiarid hillslope soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows this study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment a field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface the method allowed for measurements in this study of flow erosion at a much wider range of slopes 2 6 to 30 1 per cent and unit discharge rates 0 0007 to 0 007 m2 s 1 than have been previously feasible flow velocities were correlated to discharge and hydraulic radius but not to slope the lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity the detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected flow detachment rates were better correlated to a power function of either shear stress r2 0 51 or stream power r2 0 59 published in 1999 by john wiley sons ltd

step pool evolution in the rio cordon northeastern italy the principle that formative events punctuated by periods of evolution recovery or temporary periods of steady state conditions control the development of the step pool morphology has been applied to the evolution of the rio cordon stream bed the rio cordon is a small catchment 5 km2 within the dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded detailed field surveys of the step pool structures carried out before and after the september 1994 and october 1998 floods have served to illustrate the control on step pool changes by these floods floods were grouped into two categories the first includes ordinary events which are characterized by peak discharges with a return time of one to five years 1 8 5 15 m3 s 1 and by an hourly bedload rate not exceeding 20 m3 h 1 the second refers to exceptional events with a return time of 30 50 years a flood of this latter type occurred on 14 september 1994 with a peak discharge of 10 4 m3 s 1 and average hourly bedload rate of 324 m3 h 1 step pool features were characterized primarily by a steepness parameter c h ls s the evolution of the steepness parameter was measured in the field from 1992 to 1998 the results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events during this period bed armouring dominate the sediment transport response however following an extraordinary flood and unlimited sediment supply conditions the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing the analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed copyright 2001 john wiley sons ltd

detailed recording of gully morphology in 3d through image based modelling abstract the ability to understand gully erosion development is closely related to our ability to quantify the morphology of gullies at present various technologies are at hand to collect data at increasing levels of detail however many of the developed technologies are time consuming difficult to apply or expensive as an alternative image based modelling offers a cost efficient flexible and rapid method to quantify gully morphology from photographs taken in the field in this study the use of image based modelling was tested and fine tuned to quantify the morphology of four gully heads in contrasting biophysical environments prone to gully erosion two bank gullies in central belgium and two permanent gullies in northern ethiopia ground photographs n 88 235 were taken with a reflex canon eos 450d camera having a 20 mm wide angle lens with a fixed focal length the data collection occurred during days of 30 100 cloud cover and after removing excessive vegetation in the gullies processing of the photographs occurred in photoscan 1 0 2 software using the semi automated structure from motion multi view stereo sfm mvs workflow and allowed to produce 3d digital elevation models with accuracies that range from millimetres to centimetres in addition for the same surface 2 5d models were created in arcgis gully morphological properties were derived and included cross sections total volume and volume of undercut walls and soil pipe inlets for the volume calculation optocat software was used cross sections were also quantified by tape meter measurements when compared to 3d models cross sections quantified from tape meter measurements and from 2 5d models underestimate the cross sectional area by

effect of saltating sediment load on the determination of the mean velocity of overland flow where overland flow velocity is measured using dye or salt tracing the mean velocity is often determined by multiplying the velocity of the leading edge of the tracer plume by a correction factor flume experiments show that a varies with both reynolds number and sediment load for sediment free flow over a sand covered bed is less than the theoretical value of 0 67 in laminar flow and increases rapidly with reynolds number in transitional flow and more slowly with reynolds number in turbulent flow for sediment laden flow decreases as sediment load increases in transitional and turbulent flows saltating sediment extracts momentum from the flow causing velocities near the bed to decrease and to decrease a multiple regression equation is developed which can be used to predict the value of from reynolds number and sediment load in transitional and turbulent overland flows

longitudinal velocity patterns and bed morphology interaction in a rill present day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills the latter are dif cult to collect with traditional techniques due to the very low water depths and the relatively high ow velocities in rills the objectives of this paper were to investigate the feasibility of miniaturized acoustic doppler velocimeter madv measurements in rill ow and to explore longitudinal variations in ow velocities and their relationship with rill bed morphology detailed data on longitudinal ow velocity were required to achieve these objectives a 1 8 m long rill was formed freely in a ume at 5 slope and 0 001 m3 s 1 discharge rill topography was characterized by an alternation of steps and pools the ume surface was then xed to preserve rill roughness a topographical scanning of the entire ume surface was made velocity was measured with a madv along the rill and at different depths flow depth in a longitudinal direction was also measured using an elevation gauge a strong relationship exists between rill topography and ow hydraulics over steps ow was unidirectional and rapidly accelerating until a threshold froude number fn value between 1 3 and 1 7 was reached and a hydraulic jump occurred leading to the formation of a pool in the pool the ow pattern was multidirectional and complex the ow was subcritical when leaving the pool and accelerated over the next step until the threshold froude number value was again reached energy loss in the rill was concentrated in the pools mainly due to the action of a hydraulic jump this mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution copyright 2004 john wiley sons ltd

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

hydrodynamic characteristics of rill flow on steep slopes rill erosion is a dominant sediment source on sloping lands however the amount of soil loss from rills on steep slopes is vastly more than that on gentle slopes because of differences in rill shape and hydraulic patterns the aims of this paper are to determine the hydrodynamic characteristics of rills and the friction coefficients in steep slope conditions and to propose modifications of some hydraulic parameters used in soil loss prediction models a series of inflow experiments was conducted on loess slopes the results show that the geometric and hydraulic properties of rill on the steep loess slopes which are characterized by the mean width of cross sections mean velocity and mean depth of flow are related to discharge and slope gradient in power functions however the related exponents to discharge are 0 26 0 48 and 0 26 respectively which are different from the exponents derived in previous studies which were conducted on gentle slopes the manning roughness coefficient ranged from 0 035 to 0 071 with an average of 0 0536 and the darcy weisbach friction coefficients varied from 0 4 to 1 9 the roughness coefficients are closely related to the reynolds numbers and flow volumes however the correlations vary with slope gradient the roughness coefficients are directly proportional to the reynolds number and the flow volume on steep slopes in contrast with the roughness coefficients found on gentle slopes which decrease as the reynolds number and flow volume increase this difference is caused by the interactions among the hydraulics of the flow the shape of the rills and the sediment concentrations on steep slopes the results indicate that parameters used in models to predict rill erosion have to be modified according to slope gradient copyright 2015 john wiley sons ltd

assessing dye tracer technique for rill flow velocity measurements abstract rill erosion is considered one of the most important processes affecting soil because of the large amount of soil loss the rill network acts as sediment source and is able to transport both rill flow detached particles and those delivered from the interrill areas small flow depth in a rill and steep slope values of its bed affect significantly flow hydraulics when rill flow velocity is measured using a dye tracing method the mean velocity is calculated by multiplying the measured surface velocity of the leading edge of the tracer plume by a correction factor the main uncertainty of the dye tracing technique stands in the relationship between mean and surface flow velocity in this paper this relationship was firstly tested using the measured data pairs available from literature and then the influence of the adopted relationship on the estimate of the darcy weisbach friction factor was examined the developed analysis showed that the applied estimate criteria of the correction factor do not affect the estimate performances of the theoretical flow resistance equation finally a new flow resistance equation for rill flows which can be directly calibrated by surface velocity measurements was deduced the proposed procedure for estimating the friction factor was calibrated by rill data available from literature and was positively tested by the rill velocity measurements carried out in this investigation

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

modelling gravel bed topography from sediment transport a simple queueing model which generates bed topography consistent with the mechanics of gravel motion is presented the equations on which it is based are derived from the application of simple theoretical ideas and an analysis of published flume data meland and norrman 1966 francis 1973 abbott and francis 1977 the simulation produces an extremely variable bed topography in which at least two scales of bedform may be identified features of 5 10 grain wavelengths similar to pebble clusters are superimposed on other forms whose wavelength is up to 30 40 grains and which resemble step pools systems and antidunes sediment transport shows many of the characteristics found in the field and the model is therefore thought to provide a useful starting point from which to examine the interaction between flow sediment transport and bedforms in gravel bed and cobble streams

dissipative analogies of step pool features from rills to mountain streams abstract in this paper the dissipative similarity of step pool units at different spatial scales ranging from rills to streams is analyzed this investigation benefits from the latest theoretical advances in open channel flow resistance high resolution topography from close range photogrammetry applied to rill erosion and the availability of published data from literature on step pool streams at first the integration of a power velocity distribution allowed to obtain a theoretically based expression of darcy weisbach friction factor in which function and exponent of the velocity profile are included then this theoretically deduced flow resistance relationship is calibrated and tested by flow measurements carried out in rill reaches where the step pool units occurred in particular the proposed function is calibrated using the measurements corresponding to 88 rill reaches shaped on a plot having a slope equal to 14 22 and 24 and the calibrated equation is also positively tested with measurements carried out in 48 rill reaches shaped on a 26 sloping plot for the rill flow the developed analysis stated that the friction factor is characterized by estimate errors which are less than or equal to 15 for 86 of cases and less than or equal to 10 for 70 of cases using measurements of flow velocity water depth width and bed slope measurements carried out in 109 reaches of step pool streams this investigation demonstrates that the theoretical flow resistance equation can be applicable to step pool streams carrying out a specific calibration of the function the comparison between the darcy weisbach friction factor values measured in streams and step pool rills demonstrates that in the stream features the friction factor values are on average higher than those related to rills with step pool sequences in conclusion the comparison between rills and streams with step pool units highlights that the same theoretical flow resistance equation can be applied even if a scale effect between the two features is detected

validating the use of 137cs measurements to estimate rates of soil redistribution by wind abstract wind erosion has degraded over one half billion hectares of land worldwide 137 cesium 137 cs has been used as a tracer to study long term rates of soil redistribution by water and to a lesser extent by wind early studies assumed that the decline in 137 cs activity for a potentially eroded soil relative to that for an uneroded soil was linearly proportional to soil loss more recently models have emerged that consider the effects of soil cultivation and the particle surface area dependent partitioning of 137 cs on soils we investigated the partitioning of 137 cs in wind eroded sediments and with soil surface samples sieved into contiguous ranges of particle sizes we also compared the 137 cs activities and stratification of several adjacent soils with known wind erosion and deposition histories finally we tested 137 cs based soil loss models with measured data from sites with documented histories 137 cs activities and mean particle diameters of aeolian samples agreed well with the 137 cs activities and respective mean diameters of the sieved surface soil samples good agreement between model estimations and measured data indicated that 137 cs models developed to estimate soil redistribution by water were also applicable to soil redistribution by wind provided that the models contained an appropriate particle size correction parameter

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

the influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers a reply clifford et al 1992 argue that differences in hydraulic conditions at the moment of sediment entrainment in turkey brook flood by flood are attributable to variations in the form drag that arises from cluster microforms while not denying the possibility of variable cluster concentration we show that the time dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold clifford et al further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event this merits further examination however we point out that clusters are not only observed at low flow and therefore already present and able to contribute significant form drag as the next bedload generating flood waxes but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events the revelation by clifford et al that flow structure in the river quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements perhaps clusters is not surprising there is however some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing nevertheless the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport

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

modeling wave effects on storm surge and coastal inundation abstract we present a parametric study of surface wave effects on storm surge and coastal inundation hurricane wind forcing terms atmosphere pressure gradient terms and radiation open boundary conditions are implemented into an existing quasi 3d nearshore model nearcom tvd which uses a shock capturing tvd scheme and can better simulate the wetting drying process during inundation than a conventional finite difference model systematic numerical experiments are carried out in an idealized continental shelf beach land system to identify the role of waves in modeling storm surge and inundation under different storm characteristics four storm parameters including storm intensity storm size translation speed and incident angle are investigated modeling results reveal that the presence of waves can increase the maximum storm surge heights significantly through wave setup and the contribution of waves varies considerably depending on the storm characteristics in addition to direct wave forcing the wave enhanced bottom stress in the surfzone also promotes higher wave setup and hence higher surge heights waves also have a major influence on the maximum inland inundation distance however we find that waves do not always favor more inundation as expected for storms traveling at very slow translation speeds or nearly parallel to the coast waves surprisingly exert a negative effect on the maximum inundation distance we argue that wave enhanced bottom stress combined with longer forcing duration in such storms counteract the positive effect of wave setup and thus less inundation is predicted in wave current coupled simulations our results highlight the necessity of fully incorporating wave effects in storm surge and inundation modeling

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

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

the effects of kinetic sorting on sediment mobility on steep slopes in poorly mobile static armour sorting is usually considered the result of hiding exposure effects we called this effect static sorting in opposition to very efficient grain to grain mechanisms produced by a mobile mixture called kinetic sorting we hypothesized that kinetic sorting can be an important contributor to the morphodynamics of mountain streams and attempted to demonstrate this with new flume experiments two long runs were produced with natural poorly sorted sediments and with transport stages of the coarse fraction defined by the ratio between the shear stress and the critical shear stress for transport smaller and higher than 1 respectively both runs produced an efficient transfer downstream of the injected material but with a major difference the first run no kinetic sorting produced permanent armour figuring clusters akin to what has already been observed in similar experiments the second run with kinetic sorting also produced bed armouring but this armour was periodically totally destroyed leading to substantial bed erosion this phenomenon was explained by kinetic sorting the effects of which are to produce an efficient downward migration of fine materials and bed surface armouring the consequence is that fine materials are hidden to the flow during aggradation allowing the slope to attain values much steeper than would have been expected at equilibrium for the mixture however whereas the surface armouring tends to stabilize the bed construction of a layer of fine sediments at the subsurface also contributes to making it very unstable these two contradictory effects explain the complex bed behaviours and the existence of very large bedload and slope fluctuations copyright 2014 john wiley sons ltd

the self organization of step pools in mountain streams abstract spontaneous autogenic self organization has been described in numerous geomorphic systems but it has not been investigated in detail with respect to coarse bedforms in general or step pools in particular in this paper we review the spatial organization of step pool systems and present example evidence of step pool development as an autogenic self organization process we then outline the mathematical language for defining spatially divergent self organization and test these ideas using two unique field examples from oregon andrews experimental forest and california baxter creek where step pools developed from planar beds in artificially manipulated channels results show that step pool development is consistent with a spatially divergent self organization phenomenon entropy increases as initially undifferentiated planar channels diverge into steps and pools then declines when a series of steps and pools of consistent size and spacing is established signifying stability in the system the self organization process is accompanied by increasing flow resistance and decreasing slope through increasing the vertical sinuosity of the step pool profile and creation of low or negative gradient pool areas suggesting a minimization of stream power the self adjustment of the step pool bed profile over time represents another manifestation of a general process that results in rhythmic patterns on the surface of earth

relationship between discharge velocity and flow area for rills eroding loose non layered materials a relationship between discharge flow velocity and flow area in rills is established using data from four field and laboratory studies the proposed relationship is shown to predict successfully flow velocities measured in six other studies although slopes range from 0 035 to 0 45 and soil materials range from stony sands over silt loams to vertisols mean flow velocity can be well predicted from discharge alone thus there is no important influence of slope and or soil material characteristics on flow velocities in rills the proposed relationship may be used to improve performance of deterministic flow routing models when applied to rilled catchments furthermore it allows the calculation of unit stream power which has been shown to be related to the transporting capacity of overland flow in terms of slope and discharge

bedload transport resistance in rough open channel flows during bedload movement by saltation streamwise momentum is transferred from the ow to the saltating grains when the grains collide with other grains on the bed or in the ow streamwise momentum is reduced and there is a decrease in streamwise ow velocity and an increase in ow resistance herein termed bedload transport resistance fbt based on experiments in two umes with xed and mobile plane beds and previously published data an equation is developed that may be used to predict fbt for both capacity and non capacity ows the variables in this equation are identi ed by dimensional analysis and the coef cients are determined by non linear regression this equation applies to rough turbulent open channel ows where the relative submergence is between 1 and 20 and the entire sediment load moves by saltation an investigation of the relative magnitudes of fbt and grain resistance fc suggests that where dimensionless shear stress is less than 1 and saltation is the dominant mode of bedload transport fbt fc increases with but never exceeds 1 copyright 2004 john wiley sons ltd

rill hydraulics on a semiarid hillslope southern arizona seventy field experiments were conducted in seven rills located on a semiarid rangeland hillslope underlain by gravelly soils at walnut gulch arizona the rills which are characterized by wide shallow cross sections and gravel covered beds have mean at a station hydraulic geometry exponents of b 0 33 f 0 34 and m 0 33 although the differences between these values and typical values of b 0 30 f 0 40 and m 0 30 for cropland rills are not statistically significant they are thought to be real as cropland rills often have more rectangular cross sections and steeper sides than the rangeland rills under study for rills formed in silty loamy soils govers developed an empirical relation between mean flow velocity and discharge emphasizing the generality of this relation he suggested that it may be used as a simple means of routing runoff through rills he also noted that this relation appeared to be unaffected by either slope or soil materials the present data represent rills underlain by coarser and somewhat more varied gravel rich soils these data do not conform to govers relation and a multiple regression analysis reveals that slope and soil materials either directly or indirectly through bed roughness exert almost as much influence on flow velocity as does discharge three alternative methods are developed for predicting flow velocity in the rills under study all three methods give good results with the largest root mean square deviation being 3 115 cm s 1

tracer techniques in aeolian research approaches applications and challenges abstract aeolian processes the entrainment transport and deposition of sediments by wind impacts climate biogeochemical cycles food security environmental quality and human health considering the multitude of interactions between aeolian processes and all the major components of the earth system there is a growing interest in the scientific community to quantify the wind related sediment movement process and redistribution rates at different spatial and temporal scales however this quantification is rather challenging due to the complexities of physical mechanisms involved in aeolian processes and the inherent fundamental differences from the rather well studied processes controlling fluvial erosion traditional techniques such as erosion plots and surveying methods for monitoring wind erosion are capable of quantifying sediment movement on small scales but they have a number of limitations in terms of the representativeness of the data obtained spatial and temporal resolution and the patterns over extended areas and the costs involved the demand for alternative methods of soil loss and sediment redistribution assessment in order to complement and enhance the existing methods has directed attention to use tracing approaches for monitoring rates and spatial patterns of sediment redistribution at various scales a comprehensive synthesis of available information from different scientific disciplines on aeolian tracer techniques their applications and limitations are important in understanding the role of aeolian processes and their interactions with the earths systems in changing climate and management scenarios the objective of this paper is to provide a scientific review of the current tracer approaches in aeolian studies including fallout radionuclides rare earth elements sediment finger printing and soil magnetism as well as giving an introduction of the potential tracers that are in development

the limitations of using 137cs for estimating soil redistribution in semi arid environments the 137cs technique for estimating net ca 30 years soil flux has been used successfully in many environments its widespread use is probably because the 137cs technique overcomes many of the problems of monitoring soil erosion and deposition flux over the medium term 5 to 50 years and at the hillslope scale in this respect the technique probably offers the greatest potential for measuring net soil flux in semi arid environments where soil flux monitoring difficulties are compounded by considerable spatial and temporal variability of the controlling factors however there remain uncertainties in the underlying assumptions of the technique and difficulties in satisfying these assumptions especially in semi arid areas several key assumptions of the 137cs technique for measuring net soil flux in semi arid areas were investigated using data from southwest niger samples were obtained along a toposequence typical of the region and at nested grid nodes stratified using geomorphological information for geostatistical estimation at unsampled locations providing complete coverage of the study area the pervasive occurrence of dust made it difficult to identify an undisturbed site a first approximation of the reference 137cs inventory 2066 125 bq m 2 was provided by modelling the 137cs profile at an unvegetated site despite little evidence in the literature on problems of identifying a reference inventory especially those suffering from wind erosion it is likely that similar problems occur in other semi arid areas problems with the preferential transport of 137cs were identified by expressing the 137cs concentration as a proportion of the weight in each grain size fraction however it was partially accounted for in the calibration relationship by reducing by a factor of 10 the 137cs concentration of soil samples from the plateau it is likely that workers in other semi arid regions have similar problems of preferential transport of 137cs and the method discussed here appears to be a valuable tool to indicate the potential for preferential 137cs movement a large disparity was found between the undisturbed model and the structure of a 137cs profile from a small alluvial fan this was due to the removal of 137cs rich soil and its replacement with soil largely devoid of 137cs which probably originated from gully walls two separate models that related 137cs movement to soil redistribution were used to calculate net soil flux additional modelling would be required to account for 137cs dilution in order to measure net soil flux in gullied and badly eroded rangeland areas net soil flux was calculated at sites along the toposequence at the nested grid sites and at unsampled locations across the study using geostatistics the toposequence samples considerably underestimated the net soil loss relative to the nested grid samples and the geostatistical estimates of net soil flux toposequence sampling does not account for the spatial variation in net soil flux unless very well defined geomorphological units that control the soil redistribution processes are evident this form of sampling is of limited value for investigating the variation of 137cs depth distribution at all but the smallest scale largest areas since it cannot easily be used to determine local variation in soil redistribution processes a small improvement in net soil flux accuracy was provided by the geostatistical estimates relative to the nested grid samples of net soil flux this was thought to be due to the efficiency of the nested grid samples suggesting that geomorphological information can be used to limit the number of samples necessary to encompass several scales of net soil flux variation

characteristic dimensions of the step pool bed configuration an experimental study 1 the dominant dimension of the step pool bed form is its spacing which is determined by the locations at which large step forming grains are deposited we conducted laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps direct observation of step development indicates that deposition of step forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development neither mechanism is likely to occur at regular locations on the bed measured step spacing was found to follow a poisson distribution supporting the conclusion that steps do not develop in preferred or regular locations this contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves we observe neither the regular distribution nor the associated depositional mechanism although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing

geometric scaling of step pools in mountain streams observations and implications understanding the geometric structuring of alluvial step pools is an enduring problem in mountain stream geomorphology many previous studies propose that the key control parameter governing the step pool form is mean bed slope as explored through several examples however we find that slope control formulations are inconsistently demonstrated for broad data sets on this basis we reexamine the bed form architecture of step pools with a new descriptive parameter termed the aspect ratio active stream width divided by step drop height we test with data collected from three different mountain settings for step pools observed over mean bed slopes ranging from 2 to 22 we find that step wavelength and height are well described by when these dimensions are normalized by the step drop height furthermore we observe that varies inversely with mean bed slope context for our findings is provided by recent field and laboratory work which highlights that the cross stream component to the overall flow structure in step pools and scour pools is significant this is important because it expands upon and relates our work to formative models which are based on jet scour and the associated three dimensional flow structure of step pools our work has practical implications for the design of step pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry we observe however that under certain conditions this approach may yield constructed step pools that are unstable as an alternative approach we propose that the geometric relationships developed here can be applied to identify artificial step pool geometries that will be more fully reflective of natural analogs and thus possibly more stable

experiments for testing soil texture effects on flow resistance in mobile bed rills abstract in this paper a recently theoretically deduced rill flow resistance equation based on a power velocity profile was tested experimentally on plots of varying slopes and soil texture in which mobile bed rills are incised measurements of flow velocity water depth cross section area wetted perimeter and bed slope conducted in rill reaches incised on experimental plots having different slope values 9 14 22 24 and 26 and soil texture clay fraction ranging from 42 to 73 and literature data were used to calibrate the flow resistance equation in particular the relationship between the velocity profile parameter the channel slope the flow froude number and texture fractions was firstly calibrated using 147 rill reach data then this relationship was tested using 126 measurements carried out with soils having different texture percentage of clay ranging from 9 6 to 73 and slopes 6 9 26 the measurements allowed to establish that a the darcy weisbach friction factor can be accurately estimated using the proposed theoretical approach and b the data were supportive of the soil texture influence on rill velocity and flow resistance

estimating manning s n for steep slopes abstract hydrological and soil erosion models require calculations of flow velocity for which either the darcy weisbach or the manning equation is generally used a series of field experiments was carried out in a small catchment on the chinese loess plateau to obtain reliable values of manning s n the soils are typically erodible loess soils the experiments were conducted for a range of land uses as well as for different slope angles 6 64 measurements were performed on a 2 5 0 4 m plot on which flow was allowed to find its own path water was evenly applied to the top of the plot and discharge surface velocity flow width and slope were measured the results show that manning s n can just like darcy weisbach f be estimated from reynolds number furthermore for croplands there is an apparent linear increase in manning s n and f with increasing slope angle r2 0 70 as manning s n is usually assumed to be constant this must mean that either velocity increases with slope or hydraulic radius decreases the measurements showed virtually no increase in velocity and a minor increase in hydraulic radius with slope as flow was more concentrated on steeper slopes possible explanations for this lack of increase in velocity include increased roughness water resour res 37 2001 791 decreased effective slope angle because of the development of vertical head cuts and a shift in energy use all three hypotheses only apply to situations involving erosion the trend of manning s n with slope implies that in soil erosion models using manning s equation or darcy weisbach the value of n or f should be a function of the slope for erodible soils for non erodible soil as in woodland no increase in manning s n with slope was observed

the impact of exceptional events on erosion bedload transport and channel stability in a step pool channel sediment transport in the erlenbach a small stream with step pool morphology in the canton of schwyz switzerland has been monitored for more than 20 years during this time three exceptional events events with high sediment yield and long return times that have a large effect on channel morphology have impacted the stream and partly or completely rearranged the existing step pool morphology in the aftermath of the events sediment transport rates at a given discharge and total sediment yield remained elevated for about a year or longer for the last event dated on the 20 june 2007 observations of boulder mobility and step destruction were used to interpret channel stability boulders with median diameters of up to 135 cm and estimated weights of more than 2 5 tons have moved during the 2007 event using hydraulic observations and shear stress calculations boulders up to 65 cm in diameter were predicted to have been fully mobile in peak conditions even if form resistance and increased critical stresses needed for the initiation of motion in steep streams were taken into account for two of the events estimated peak shear stresses at the bed exceeded 1000 pa calculated both from observations of the flow hydraulics and from boulder mobility this suggests that highly energetic flows occur relatively frequently in small steep streams and that large boulders can be transported by fluvial processes in such streams the observations have potential significance for hazard risk mitigation stream engineering and restoration copyright 2009 john wiley sons ltd

wave set up in the storm surge along open coasts during typhoon anita abstract wave set up in storm surges is studied using a numerical model for coasts in tosa bay japan open to the pacific ocean simulation models employing only atmospheric pressures and winds as external forces are unable to properly simulate open coast storm surge heights such as those due to typhoon anita 1970 however the present study shows that a numerical model incorporating wave induced radiation stresses as well as wind stresses and pressure gradients is able to account for the open coast surge heights there is a maximum contribution of 40 by the radiation stresses to the peak sea level rises this study also evaluates the effects of the tides including the tides improves the agreement between the predicted water surface elevations and the observations the difference in predictions between one way coupling from wave to surge models and two way coupling of the surge and wave models is found to be small

the role of sediment specific gravity and availability on cluster evolution the objective of this experimental study was to account for the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics spacing and size to isolate the effects of sediment availability and specific gravity on cluster evolution mono sized spheres were used to simulate the cluster evolutionary cycle overall twelve experimental runs were carried out in the laboratory flume six of these tests were performed by using glass spheres specific gravity sg 2 58 and the other six by employing an equal combination of glass and teflon spheres sg 2 12 of the same diameter to evaluate the role of specific gravity on cluster evolution the three sediment availability conditions that were investigated here simulated isolated gravel elements pool riffle sequences and densely packed gravel bed an advanced image analysis technique was employed to track the evolution of cluster microforms and provide quantitative information about the size and shape of clusters and the number of clusters per unit area the results of this study showed that 1 sediment availability affects the architecture and size of cluster microforms and 2 clusters consisting of mono sized sediments start disintegrating at twice the incipient conditions by performing complementary tests for the isolated gravel elements case it was found that the evolutionary cycle of individual clusters could be described as follows in order of increasing stress no cluster two particle cluster comet triangle rhomboid break up copyright 2003 john wiley sons ltd

alluvial architecture in headwater streams with special emphasis on step pool topography alluvial mountain streams exhibit a range of channel forms pool riffle plane bed step pool and cascades previous work suggested that these forms exist within discrete and progressively steeper slope classes measurements conducted at over 100 sites in west central and central idaho confirm that slope steepens progressively as one moves from pool riffle to plane bed to step pool and finally to cascades median slope for pool riffle topography is 0 0060 for plane beds 0 013 for step pools 0 044 and for cascades 0 068 there is substantial overlap in the slopes associated with these channel forms pool riffle topography was found at slopes between 0 0010 and 0 015 plane beds between 0 0010 and 0 035 step pools between 0 015 and 0 134 and cascades between 0 050 and 0 12 step pools are particularly striking features in headwater streams they are characterized by alternating steep and gentle channel segments the steep segments step risers are transverse accumulations of boulder and cobbles while the gentle segments pools contain finer material step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers this result differs from past studies that have reported channel slope to be the dominant control on step wavelength the presumed geometry and froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step pools are formed by antidunes copyright 2000 john wiley sons ltd

numerical investigation of the morphological dynamics of a step and pool riverbed using dem mps abstractin mountain streams riverbeds with a series of coarse gravel steps are common the sequence flow pattern with a chute and pool is usually formed on series of steps where a transition regi

a coupling module for tides surges and waves a generic module in which tides surges and waves are incorporated has been developed tested and prepared for dissemination within the framework of the mast iii promise pre operational modelling in the seas of europe project two existing pre operational numerical models a wave model and a hydrodynamic model were incorporated into a coupling framework that allows an efficient exchange of information between them minimal adaptation of the models was needed the module has then been implemented and applied to the north sea then a series of experiments were performed to investigate the sensitivity of waves and surges to coupling these experiments show that the sensitivity of waves to coupling increases from deep to shallow water the sensitivity of surges is more uniformly distributed the sensitivity of surges to coupling along the spanish coast was also studied the model results were less sensitive than in the north sea this is explained by the relative importance of the two forcing components the atmospheric pressure and the wind stress in both areas c 2000 elsevier science b v all rights reserved

flume experimental evaluation of the effect of rill flow path tortuosity on rill roughness based on the manning strickler equation abstract numerous soil erosion models compute concentrated flow hydraulics based on the manning strickler equation v kst r2 3 i1 2 even though the range of the application on rill flow is unclear unconfined rill morphologies generate local friction effects and consequently spatially variable rill roughness which is in conflict with the assumptions of sectional uniform channel flow and constant channel roughness of the manning strickler equation the objective of this study is to evaluate the effect of rill morphology on roughness and hence to assess the manning strickler roughness coefficient kst by rill morphological data a laboratory experiment was set up to analyse rill hydraulics and roughness of i free developed rill fdr flows and ii straight constrained rill scr flows in the flume the flume experiment generated manning strickler roughness coefficients kst between 22 m1 3 s 1 and 44 m1 3 s 1 reflecting a potential area of the roughness parameter uncertainty it was found that fdr experiments generated significantly lower kst values compared to scr experiments because skin and local friction effects in the fdr experiments were more efficient reducing flow velocity probably due to higher energy dissipation rill flow path tortuosity tort was used to describe the rill morphology of the experiments and correlation statistics between tort and kst identified considerable explanatory capacity of rill flow path tortuosity on rill roughness the flume study demonstrated that a regression model between tort and kst can be used to assess local friction effects of unconfined rill morphologies and hence to reduce the area of uncertainty of the manning strickler roughness parameter

morphological effects of local scouring in step pool streams step pool morphology characterizes many high gradient streams in a variety of natural settings but formative processes and evolutionary dynamics are still poorly understood in this paper natural step pool geometry is compared with steep alluvial channels where grade control structures such as check dams and bed sills make the stream profile resemble a natural stepped stream along these channels local scouring due to falling jets forms plunge pools under each structure analogous to natural steps determining the formation of pools in order to test the hypothesis that natural pools are analogous to pools formed below grade control works with respect to their dimensions shape and formative dynamics 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern italian alps pools below grade control works featured a transitional zone between the scour hole and the downstream sloping bed marked by a depositional berm when geometric parameters such as maximum pool depth length and step berm distances are normalized to the jet virtual energy no statistically significant differences were detected between natural and artificial systems these results lend support to an upstream forced cascade model for step pool formation where the energy of falling jets controls the geometry of the pools and is therefore regarded as the most important scaling independent variable copyright 2005 john wiley sons ltd

modeling the topography of shallow braided rivers using structure from motion photogrammetry abstract recent advances in computer vision and image analysis have led to the development of a novel fully automated photogrammetric method to generate dense 3d point cloud data this approach termed structure from motion or sfm requires only limited ground control and is ideally suited to imagery obtained from low cost non metric cameras acquired either at close range or using aerial platforms terrain models generated using sfm have begun to emerge recently and with a growing spectrum of software now available there is an urgent need to provide a robust quality assessment of the data products generated using standard field and computational workflows to address this demand we present a detailed error analysis of sub meter resolution terrain models of two contiguous reaches 1 6 and 1 7 km long of the braided ahuriri river new zealand generated using sfm a six stage methodology is described involving i hand held image acquisition from an aerial platform ii 3d point cloud extraction modeling using agisoft photoscan iii georeferencing on a redundant network of gps surveyed ground control points iv point cloud filtering to reduce computational demand as well as reduce vegetation noise v optical bathymetric modeling of inundated areas and vi data fusion and surface modeling to generate sub meter raster terrain models bootstrapped geo registration as well as extensive distributed gps and sonar based bathymetric check data were used to quantify the quality of the models generated after each processing step the results obtained provide the first quantified analysis of sfm applied to model the complex terrain of a braided river results indicate that geo registration errors of 0 04 m planar and 0 10 m elevation and vertical surface errors of 0 10 m in non vegetation areas can be achieved from a dataset of photographs taken at 600 m and 800 m above the ground level these encouraging results suggest that this low cost logistically simple method can deliver high quality terrain datasets competitive with those obtained with significantly more expensive laser scanning and suitable for geomorphic change detection and hydrodynamic modeling