prediction of alluvial channel pattern of perennial rivers abstract purely braided meandering and straight channels can be considered as end members of a continuum of alluvial channel patterns several researchers have succeeded in separating channel patterns in fields defined by flow related parameters however the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry in this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains the method is based on two almost channel pattern independent boundary conditions median grain size of the river bed material and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient this can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition p 1 based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with p 1 5 the function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation with a bankfull or mean annual flood discharge above 10 m3 s and a median bed material grain size between 0 1 and 100 mm

cellular modelling of river catchments and reaches advantages limitations and prospects the last decade has witnessed the development of a series of cellular models that simulate the processes operating within river channels and drive their geomorphic evolution their proliferation can be partly attributed to the relative simplicity of cellular models and their ability to address some of the shortcomings of other numerical models by using relaxed interpretations of the equations determining fluid flow cellular models allow rapid solutions of water depths and velocities these can then be used to drive usually conventional sediment transport relations to determine erosion and deposition and alter the channel form the key advance of using these physically based yet simplified approaches is that they allow us to apply models to a range of spatial scales 1 100 km 2 and time periods 1 100 years that are especially relevant to contemporary management and fluvial studies however these approaches are not without their limitations and technical problems this paper reviews the findings of nearly 10 years of research into modelling fluvial systems with cellular techniques principally focusing on improvements in routing water and how fluvial erosion and deposition including lateral erosion are represented these ideas are illustrated using sample simulations of the river teifi wales a detailed case study is then presented demonstrating how cellular models can explore the interactions between vegetation and the morphological dynamics of the braided waitaki river new zealand finally difficulties associated with model validation and the problems prospects and future issues important to the further development and application of these cellular fluvial models are outlined 2007 elsevier b v all rights reserved

validation of the physical modeling approach for braided rivers 1 laboratory channels are often used to study the complexity of braiding mechanisms for the advantages with respect to field studies nevertheless the extensive use of experimental data raises the question of how representative laboratory braided channels are as compared to real braided rivers this study verifies to what extent laboratory braided patterns reproduce the main features of braided rivers experimental data display isotropic and anisotropic scaling of braided patterns state space plot of total widths anisotropic scaling of islands and statistical distribution of island areas that are similar to those observed in real rivers moreover the data reveals scaling in the perimeter area relation these results support both the reliability of experimental braided channels as physical models of braided rivers and also the possibility of investigating some aspects of braiding in the laboratory that are difficult to address in the field the lack of preferential scales in island characteristics suggests that other phenomena must play a key role in generating island shapes besides classical sediment transport based mechanisms which tend to select well defined length scales

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

hydraulic validation of two dimensional simulations of braided river flow with spatially continuous adcp data 1 gravel bed braided rivers are characterized by shallow branching flow across low relief complex and mobile bed topography these conditions present a major challenge for the application of higher dimensional hydraulic models the predictions of which are nevertheless vital to inform flood risk and ecosystem management this paper demonstrates how high resolution topographic survey and hydraulic monitoring at a density commensurate with model discretization can be used to advance hydrodynamic simulations in braided rivers specifically we detail applications of the shallow water model delft3d to the rees river new zealand at two nested scales a 300 m braid bar unit and a 2 5 km reach in each case terrestrial laser scanning was used to parameterize the topographic boundary condition at hitherto unprecedented resolution and accuracy dense observations of depth and velocity acquired from a mobile acoustic doppler current profiler adcp along with low altitude aerial photography were then used to create a data rich framework for model calibration and testing at a range of discharges calibration focused on the estimation of spatially uniform roughness and horizontal eddy viscosity h through comparison of predictions with distributed hydraulic data results revealed strong sensitivity to h which influenced cross channel velocity and localization of high shear zones the high resolution bed topography partially accounts for form resistance and the recovered roughness was found to scale by 1 2 1 4 d84 grain diameter model performance was good for a range of flows with minimal bias and tight error distributions suggesting that acceptable predictions can be achieved with spatially uniform roughness and h

modelling river history and evolution over the last few decades a suite of numerical models has been developed for studying river history and evolution that is almost as diverse as the subject of river history itself a distinction can be made between landscape evolution models lems alluvial architecture models meander models cellular models and computational fluid dynamics models although these models share some similarities there also are notable differences between them which make them more or less suitable for simulating particular aspects of river history and evolution lems embrace entire drainage basins at the price of detail alluvial architecture models simulate sedimentary facies but oversimplify flow characteristics and computational fluid dynamics models have to assume a fixed channel form while all these models have helped us to predict erosion and depositional processes as well as fluvial landscape evolution some areas of prediction are likely to remain limited and short term owing to the often nonlinear response of fluvial systems nevertheless progress in model algorithms computing and field data capture will lead to greater integration between these approaches and thus the ability to interpret river history more comprehensively

hydroacoustic earthquake detection in the arctic basin with the spinnaker array eighteen arctic basin earthquakes were hydroacoustically detected during 64 days of monitoring with the spinnaker hydrophone array in the lincoln sea arctic ocean epicentral estimates were made for 12 of these events event ranges were estimated via the arrival time difference between the solid earth p wave and the hydroacoustic t wave azimuths were estimated via plane wave beamforming to the array elements in the 5 10 hz band the 12 events fall on the north american eurasian plate boundary at the extreme southern end of the gakkel ridge and along the spitsbergen fracture zone lena trough one of the events m b 5 4 11 may 1996 spitsbergen fracture zone was detected by the global seismic network gsn with an epicenter that differs from our hydroacoustic estimate by 51 km our hydroacoustic detection threshold appears to be m b 3 4 within a 1100 km radius of the spinnaker array confirming that t waves propagate efficiently in the arctic basin and that hydroacoustic techniques hold promise for seismic monitoring of the region

application of a new cellular model for bedload transporting extreme events at steep slopes abstract a cellular model for bedload transport at steep channel gradients and alluvial fans has been developed to simulate lateral distribution of erosion and deposition on alluvial fans of mountain streams the cellular model named fluvised applies a quasi steady state routing of a flood hydrograph over a rectangular grid bedload transport is calculated and the morphological changes are updated after each time step fluvised is designed to complement existing one dimensional bedload transport simulation tools by using their outcome as an input for the simulation of inundated areas the model has been evaluated against tomsed a one dimensional bedload transport model further a back calculation of a laboratory bedload experiment and a real flood event from 2005 at schnannerbach in austria are provided to test the model s suitability for reproducing morphological changes caused by flood events at high channel gradients

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

modeling of hydroecological feedbacks predicts distinct classes of landscape pattern process and restoration potential in shallow aquatic ecosystems article i nfo it is widely recognized that interactions between vegetation and flow cause the emergence of channel patterns that are distinct from the standard schumm classification of river channels although landscape pattern is known to be linked to ecosystem services such as habitat provision pollutant removal and sustaining biodiversity the mechanisms responsible for the development and stability of different landscape patterns in shallow vegetated flows have remained poorly understood fortunately recent advances have made possible large scale models of flow through vegetated environments that can be run over a range of environmental variables and over timescales of millennia we describe a new quasi 3d cellular automata model that couples simulations of shallow water flow bed shear stresses sediment transport and vegetation dynamics in an efficient manner that efficiency allowed us to apply the model widely in order to determine how different hydroecological feedbacks control landscape pattern and process in various types of wetlands and floodplains distinct classes of landscape pattern were uniquely associated with specific types of allogenic and autogenic drivers in wetland flows regular anisotropically patterned wetlands were dominated by allogenic processes i e processes driven by periodic high water levels and flow velocities that redistribute sediment relative to autogenic processes e g vegetation production peat accretion and gravitational erosion these anistropically patterned wetlands are therefore particularly prone to hydrologic disturbance other classes of wetlands that emerged from simulated interactions included maze patterned amorphous and topographically noisy marshes open marsh with islands banded string pool sequences perpendicular to flow parallel deep and narrow channels flanked by marsh and ridge and slough patterned marsh oriented parallel to flow because vegetation both affects and responds to the balance between the transport capacity of the flow and sediment supply these vegetated systems exhibit a feedback that is not dominant in most rivers consequently unlike in most rivers it is not possible to predict the channel pattern of a vegetated landscape based only on discharge characteristics and sediment supply the antecedent vegetation pattern and vegetation dynamics must also be known in general the stability of different wetland pattern types is most strongly related to factors controlling the erosion and deposition of sediment at vegetation patch edges the magnitude of sediment redistribution by flow patch elevation relative to water level and the variability of erosion rates in vegetation patches with low flow resistance as we exemplify in our case study of the everglades ridge and slough landscape feedback between flow and vegetation also causes hysteresis in landscape evolution trajectories that will affect the potential for landscape restoration namely even if the hydrologic conditions that historically produced higher flows are restored degraded portions of the ridge and slough landscape are unlikely to revert to their former patterning as wetlands and floodplains worldwide become increasingly threatened by climate change and urbanization the greater mechanistic understanding of landscape pattern and process that our analysis provides will improve our ability to forecast and manage the behavior of these ecosystems published by elsevier b v

the nansen ridge arctic ocean some geophysical observations of the rift valley at slow spreading rate abstract we have examined continous echo soundings across the nansen rift valley and analysed the first gravity and bathymetric data available across the northern flank of the nansen ridge for comparison with other parts of the mid ocean ridge system the observed width of the inner floor 1 7 5 5 km at a total opening rate of 1 5 cm yr supports the idea of progressive narrowing of the inner floor with decreasing spreading rate for slow spreading ridges however the observed distance between the outer walls 18 22 km or rift valley width 30 33 km suggests that these parameters at least to a first approximation tend towards a minimum value with decreasing spreading rate the relation between gravity and bathymetry along the drift track of ice station fram i tentatively suggests that the sea floor topography of this part of the nansen ridge is supported by a plate with an elastic thickness less than what is observed on the mid atlantic ridge

spatial variability in river sediments and its link with river channel geometry 1 this study explores the link between spatial variability within fluvial sedimentary strata and river channel geometry this link is then used to determine the macrodispersion coefficients for solute transport in groundwater flow in river deposits in doing that we combine concepts from sedimentology geostatistics and the stochastic lagrangian theory of subsurface transport it is proposed to analyze the spatial variability of river sediments in terms of transition probabilities the transition probabilities can be determined from the averages and variances of the lengths of stratasets and their volumetric fractions using concepts developed by carle and fogg 1996 and ritzi 2000 strataset length scales are shown to correlate well with the geometry of the river channel and its bed forms and can be determined indirectly from aerial photogrammetry and more directly through surface ground penetrating radar surveys stratal dimensions can also be related using lagrangian transport analysis to macrodispersivity of subsurface transport this allows relationships between the geometry of river channels and subsurface transport parameters of associated fluvial deposits for demonstration the longitudinal macrodispersivity in a compound bar deposit in the gravelly braided sagavanirktok river was investigated

surveyed and modelled one year morphodynamics in the braided lower tana river field measurements and morphodynamic simulations were carried out along a 5 km reach of the sandy braided lower tana river in order to detect temporal and spatial variations in river bed modifications and to determine the relative importance of different magnitude discharges on river bed and braid channel evolution during a time span of one year i e 2008 2009 fulfilling these aims required testing the morphodynamic model s capability to simulate changes in the braided reach we performed the simulations using a 2 d morphodynamic model and different transport equations the survey showed that more deposition than erosion occurred during 2008 2009 continuous bed load transport and bed elevation changes of 1 m and a 70 188 m downstream migration of the thalweg occurred simulation results indicated that during low water periods modifications occurred in both the main channel and in other braid channels thus unlike some gravel bed rivers the sandy lower tana river does not behave like a single thread channel at low discharge however at higher discharge i e exceeding 497 m3 s the river channel resembled a single thread channel when channel banks confined the flow although the spring discharge peaks caused more rapid modifications than slower flows the cumulative volumetric changes of the low water period were greater the importance of low water period flows for channel modifications is emphasized although the 2 d model requires further improvements the results were nevertheless promising for the future use of this approach in braided rivers

techniques for gis modeling of coastal dunes coastal dunes present a unique problem to coastal scientists because of the dynamic nature of most coastal dune systems coastal dunes can change shape quickly and frequently due to storm generated winds and waves prevailing winds can transport significant amounts of sand throughout the dune system topographic and volumetric changes in a 150 40 m site on the outer banks of north carolina were assessed through a series of monthly field surveys conducted over a 1 year period from may 1997 to may 1998 this paper discusses the geographic information system gis methodology used for data acquisition and analysis and presents one methodology developed to measure 3 d dune morphodynamics using a 2 d and 3 d gis it serves as a guide for other coastal researchers who may have limited surveying or gis experience issues concerning sampling routine data density and grid cell size are discussed the methodology followed results in the production of a grid of interpolated elevation values that can be represented in a variety of ways including as topographic maps digital elevation models dem or two dimensional cross sections of the dune system the grid from the may 1998 survey is subtracted from the may 1997 grid to obtain elevation change information that in turn can be represented graphically the results of the analysis show that volumetric change over the 1 year period was dominated by erosion along the seaward face of the dune the monthly surveys show that this erosion was the result of two northeasters in january and february 1998 the loss of volume is partially compensated for by accumulation to the rear of the foredune ridge primarily in locations where blowouts facilitate aeolian transport of sediment from the beach the implication is that the dune system is eroding rapidly due to storm activity it also suggests that there is a mechanism for offsetting some of the volumetric loss through aeolian transport into the dune system

spatial characterization resolution and volumetric change of coastal dunes using airborne lidar cape hatteras north carolina the technological advancement in topographic mapping known as airborne light detection and ranging lidar allows researchers to gather highly accurate and densely sampled coastal elevation data at a rapid rate the problem is to determine the optimal resolutions at which to represent coastal dunes for volumetric change analysis this study uses digital elevation models dem generated from lidar data and spatial statistics to better understand dune characterization at a series of spatial resolutions the lidar data were collected jointly by the national aeronautics and space administration nasa the national oceanic and atmospheric administration noaa and the u s geological survey usgs dems of two study sites 100 200 m located in cape hatteras national seashore north carolina were generated using a raster based geographic information system gis changes in the dune volume were calculated for a 1 year period of time fall 1996 1997 at grid cell resolutions ranging from 1 1 to 20 20 m directional statistics algorithms were used to calculate local variance and characterize topographic complexity data processing was described in detail in order to provide an introduction to working with lidar data in a gis results from these study sites indicated that a 1 2 m resolution provided the most reliable representation of coastal dunes on cape hatteras and most accurate volumetric change measurements results may vary at other sites and at different spatial extents but the methods developed here can be applied to other locations to determine the optimum resolutions at which to represent and characterize topography using common gis and database software

how do gravel bed rivers braid sedimentary processes and bed forms leading to the onset of braiding were observed in small scale hydraulic models of gravel bed streams the laboratory streams had a variety of combinations of constant discharge and slope but identical bed material particle size distributions from initially straight channels braiding occurred by four different processes deposition and accumulation of a central bar chute cutoff of point bars conversion of single transverse unit bars to mid channel braid bars and dissection of multiple bars in these experiments the chute cutoff mechanism was the most common but the predominant braiding mechanism depends upon sediment mobility excess bed shear stress and the bed form regime at very low excess bed shear stress the central bar process dominates but at higher excess bed shear stress slip face unit bars are more common bed scour at confluences is more pronounced and propogation of alternate convergence scour and divergence deposition is more likely thus chu

topographic changes associated with coastal dune blowouts at island beach state park new jersey topographic changes in two blowouts located in island beach state park new jersey usa were monitored over the winter of 1981 1982 elevation changes were measured with erosion pins and sediment traps placed at comparable locations in each blowout monitored the amount of sand moved by the wind discrete wind events were identified from regional data and morphological data for the intervals with the highest onshore and offshore wind speeds are examined in detail vegetation is the primary influence on the development of the two blowouts blowout a is characterized by eroding sidewalls a stable base and an accreting blowout rim high rates of sediment transport occur through the blowout throat which results in accretion on the vegetated rim this blowout is an active sediment transfer system vegetation causes a large amount of deposition in the throat of blowout b as vegetation was buried over the winter the area of deposition migrated inland sidewall erosion also occurred in blowout b little change was recorded on the blowout rim blowout b is a recovering system where sediment is delivered to the blowout floor from the beach by onshore winds and from the blowout rim by offshore winds where it is stabilized by vegetation the development of foredune blowouts is governed largely by vegetation cover on the dune crest and by sidewall erosion during offshore and onshore winds blowout recovery depends on vegetation growth and sediment deposition in the throat and on the role of the sidewalls as sources of sediment which is deposited elsewhere within the system foredune blowouts are dynamic systems in which positive feedbacks in sediment availability and vegetation growth lead to a cycle of development and closure

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

assessing a numerical cellular braided stream model with a physical model a b murray and c paola 1994 nature vol 371 pp 54 57 1997 earth surface processes and landforms vol 22 pp 1001 1025 proposed a cellular model for braided river dynamics as an exploratory device for investigating the conditions necessary for the occurrence of braiding the model reproduces a number of the general morphological and dynamic features of braided rivers in a simplified form here we test the representation of braided channel morphodynamics in the murray paola model against the known characteristics mainly from a sequence of high resolution digital elevation models of a physical model of a braided stream the overall aim is to further the goals of the exploratory modelling approach by first investigating the capabilities and limitations of the existing model and then by proposing modifications and alternative approaches to modelling of the essential features of braiding the model confirms the general inferences of murray and paola 1997 about model performance however the modelled evolution shows little resemblance to the real evolution of the small scale laboratory river although this depends to some extent on the coarseness of the grid used in the model relative to the scale of the topography the model does not reproduce the bar scale topography and dynamics even when the grid scale and amplitude of topography are adapted to be equivalent to the original murray paola results strong dependence of the modelled processes on local bed slopes and the tendency for the model to adopt its own intrinsic scale rather than adapt to the scale of the pre existing topography appear to be the main causes of the differences between numerical model results and the physical model morphology and dynamics the model performance can be improved by modification of the model equations to more closely represent the water surface but as an exploratory approach hierarchical modelling promises greater success in overcoming the identified shortcomings copyright 2005 john wiley sons ltd

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

a precipiton method to calculate river hydrodynamics with applications to flood prediction landscape evolution models and braiding instabilities the precipiton method is a particle based approach that consists of routing elementary water volumes on top of topography with erosive and depositional actions here we present an original way to calculate both river depth and velocity from a method that remains embedded in the precipiton framework the method solves the governing equations for water depth where the water depth is increased by a constant quantity at each precipiton passage and decreased by a value based on a flow resistance equation the precipitons are then routed downstream on top of the resulting water surface the method is applicable even if the precipitons are routed one by one i e independent of each other which makes it simple to implement and computationally fast compared to grid based methods this particle method is not subject to the classic drying wetting issue and allows for a straightforward implementation of sediment transfer functions between the river bed and running water we have applied the method to different cases channel flow flow over topographic barriers and flood prediction on high resolution lidar topography in all cases the method is capable of solving the shallow water equations neglecting inertia when coupled with erosion and sediment transport equations the model is able to reproduce both straight and braided patterns with geometries independent of grid size application of the model in the context of multithread rivers gives new insight into the development of braiding instability

cellular modelling as a tool for interpreting historic braided river evolution abstract results are presented from a new cellular model of braided river dynamics that simulates flow sediment transport morphological change and the effects of braidplain vegetation this model is used to investigate the effect of changes in upstream sediment supply on braided river systems over simulation periods of 200 years modelled changes in channel morphology associated with both aggradation and degradation were seen to be consistent with those reported in the literature in addition simulation results allowed the identification of diagnostic characteristics of aggrading and degrading reaches in the form of relationships between the age extent and relative elevation of fluvial surfaces interpretation of spatial patterns of valley floor surface characteristics in the avoca river new zealand on the basis of these relationships allowed the identification of channel reaches that appear to be experiencing either aggradation or degradation these inferences are shown to be consistent with independent evidence of spatial patterns of sediment supply to the main valley floor derived from aerial photographs and an existing sediment source inventory these results illustrate the potential for using cellular models to develop an improved understanding of natural river behaviour

dirt road a geomorphological and geochemical record of late holocene human activity in the catchment of lake radacz central pomerania poland abstract the paper offers a discussion of the record of human activity in the relief and sediments of a selected lake catchment in central pomerania in poland one of the basic markers of human activity here is a change in the lake water level the dropping level on average 1 m 100 years made it possible for people to develop successively exposed stretches of land along the shore an analysis of the polish national archeological records and archival maps showed that human development and impact appeared in the lakeshore zone as early as the neolithic in central pomerania the changes were especially intensive in the late holocene in the catchment of the lake under study they included a 100 increase in the length of nearby roads in the archives of its paleoenvironmental data most traces have been left on farming terraces while a poorly studied form of record of human activity is a dirt road running parallel to the shoreline geodetic and geochemical examination has revealed that the road is a stimulant of change its wheeled traffic mixes the accumulated and eroded material and this causes changes in the shoreline section the road cutting which is 2 m wide deepened while material accumulates in the form of 0 5 m wide lateral bars the compaction of the material brings about changes in the structure of the ground or soil its density porosity compactness moisture which affects its level of oxygenation and waterlogging and consequently its iron content to establish this a speciation analysis was carried out using the fe iii fe ii ratio it was determined that the lower the ratio the higher the susceptibility of land to deformation owing to an increase in soil density a drop in its water capacity reduction conditions and the predominance of fe ii the research discussed here is only a pilot study but the authors see extensive opportunities for using speciation analysis in geoarcheology

particle path length distributions in meandering gravel bed streams results from physical models in gravel bed rivers with well de ned pool bar morphology the path length of transported bed particles must be at least during channel forming ows equal to the length scale of the morphology this is the basis for some methods for estimating bed material transport rates however previous data especially from eld tests are often strongly positively skewed with mean much shorter than the pool bar spacing one possible explanation is that positively skewed distributions occur only in channels lacking distinct pool bar topography or only at lower discharges in pool bar channels a series of ume experiments using uorescent tracers was used to measure path length distributions in low sinuosity meandering channels to assess the relation with channel morphology and ow conditions at channel forming ows 55 to 75 per cent of the tracer grains were deposited on the rst point bar downstream of the point of tracer input with 15 per cent passing beyond the rst bar path length distributions are symmetrical with mean equal to the pool bar spacing and can be described with a cauchy distribution in some cases there was a secondary mode close to the point of tracer introduction this bimodal distribution ts a combined gamma cauchy distribution only when discharge was reduced below the channel forming ow were frequency distributions unimodal and positively skewed with no relation to the pool bar spacing thus path length distributions become more symmetrical and mean path length increases to coincide with pool bar spacing as ow approaches channel forming conditions this is a substantial modi cation of existing models of particle transfer in gravel bed rivers copyright 2003 john wiley sons ltd

simulation of braided river flow using a new cellular routing scheme abstract results are presented from a numerical simulation of two dimensional flow patterns in a braided river using a simple cellular routing scheme the results of the routing scheme are compared with field measurements of discharge per unit width obtained within the study reach at low flow and for higher flows with the predictions of a more sophisticated hydraulic model that solves the two dimensional shallow water form of the navier stokes equations an assessment is made of the sensitivity of the routing scheme to variations in the values of its main parameters and appropriate values are determined based on the physical characteristics of the study site and available flow measurements it is shown that despite the simple approach adopted by the cellular routing scheme to simulate processes of water redistribution it is able to replicate accurately both the field data and the results of the more sophisticated hydraulic model these results indicate that the routing scheme outlined here is able to overcome some of the limitations of previous simple cellular automata models and may be suitable for use in modelling bedload transport and channel change in complex fluvial environments as such this research represents a small and ongoing contribution to the field of numerical simulation of braided river processes

crossing the divide representation of channels and processes in reduced complexity river models at reach and landscape scales abstract reduced complexity models have considerable potential as tools for elucidating river behaviour over periods of 10 0 10 4 years and consequently for addressing fundamental questions concerning the scale dependent nature of explanation in geomorphology this paper proposes a simple subdivision of reduced complexity models of river behaviour into two categories that mirror methodological developments in fluvial geomorphology over the past 50 years first high resolution cellular approaches that are implemented within a framework that resolves process form feedbacks at small time and space scales second models that incorporate section averaged representations of channel geometry and processes and that are typically underpinned by regime theory and equilibrium concepts examples of both model types are presented here in the form of a cellular representation of stream braiding and a combined lattice network model of alluvial fan evolution simulations conducted using these models demonstrate how small scale process form interactions determine the emergence of larger scale channel and fan morphology and in so doing regulate system response to external forcing in this sense both models demonstrate that internal feedbacks play a critical role in controlling river responses to environmental change over historic and holocene timescales however both classes of model are characterised by uncertainty in their parameterisation of geomorphic processes such that internal feedbacks and thresholds for channel response to external forcing may vary substantially between competing models methods of refining both approaches are considered and hybrid models based on lattice network structures and mechanistic representations of channel process form interactions are identified as a means of addressing the shortcomings of existing strategies

numerical simulation of bar and island morphodynamics in anabranching megarivers 1 bar and island morphodynamics in the world s largest anabranching rivers are investigated using a new numerical model of hydrodynamics sediment transport bank erosion and floodplain development operating over periods of several hundred years simulated channel morphology is compared to that of natural rivers and shown to be realistic both in terms of the statistical characteristics of channel width depth and bar shape distributions and mechanisms of unit bar compound bar and island evolution results demonstrate that bar and island stability may be sensitive to hydrologic regime because greater variability in flood magnitude encourages the formation of emergent bars that can be stabilized by vegetation colonization simulations illustrate a range of mechanisms of unit bar generation that are linked to local bed or bank instabilities this link may explain the reduced frequency of unit bars observed in some large anabranching rivers that are characterized by stable vegetated islands and slow rates of channel change model results suggest that the degree to which sand sized bed material is carried in suspension likely represents an important control on bar morphodynamics and channel network evolution because of its influence on sand transport direction consequently differences in the partitioning of the total sand load between bed load and suspension may provide a partial explanation for contrasting styles of anabranching in the world s largest sand bed rivers these results highlight a need for spatially distributed flow and sediment transport data sets from large rivers in order to support improved parameterizations of sand transport mechanics in morphodynamic models

self organization of sorted patterned ground striking circular labyrinthine polygonal and striped patterns of stones and soil self organize in many polar and high alpine environments these forms emerge because freeze thaw cycles drive an interplay between two feedback mechanisms first formation of ice lenses in freezing soil sorts stones and soil by displacing soil toward soil rich domains and stones toward stone rich domains second stones are transported along the axis of elongate stone domains which are squeezed and confined as freezing soil domains expand in a numerical model implementing these feedbacks circles labyrinths and islands form when sorting dominates polygonal networks form when stone domain squeezing and confinement dominate and stripes form as hillslope gradient is increased

a cellular model of braided rivers a broad sheet of water flowing over non cohesive sediment typically breaks up into a network of interconnected channels called a braided stream fig 1 the dynamics of such networks are complex channels migrate laterally split rejoin and develop bars with the flow shifting unpredictably from one part of the network to another many processes are known to operate in a braided river1 3 but it is unclear which of these are essential to explain the observed dynamics we describe here a simple deterministic numerical model of water flow over a cohesionless bed that captures the main spatial and temporal features of real braided rivers the patterns arise from local scour and deposition caused by a nonlinear dependence of bedload sediment flux on water discharge although the morphology of the resulting network depends in detail on the sediment transport rule used in the model our results suggest that the only factors essential for braiding are bedload sediment transport and laterally unconstrained free surface flow

erosion dynamics modelling in a coupled catchment fan system with constant external forcing article i nfo recent alluvial fan models have suggested that deep alluvial fan entrenchment could occur without any change in sediment and water influx moreover other studies have shown that the evolution of a fan could strongly depend on feedback between the fan and the mountain catchment we evaluate if natural entrenchment still occurs in a coupled catchment fan system and we evaluate its possible impact on the evolution of mountain erosion we use a landscape evolution model where the mountain corresponds to an uplifting block and where fans form over an initial horizontal surface our experiments confirm that deep entrenchment at the fan apex can occur without a change in climate or in uplift rate under two conditions 1 the transport threshold critical shear stress is significant and 2 the downstream boundary condition corresponds to an open boundary with fixed elevation which stops the progradation of alluvial fans a stable entrenchment occurs when sediments reach this limit the entrenchment can be explained by a strong nonlinear dependence of the transport law on the slope when the shear stress is close to critical a fan slope increase occurs when sediments reach the model limit and this drives entrenchment fan entrenchment drives strong erosion in the mountain with intensity and a response time similar to those observed for the initial mountain uplift these results indicate that determining how much of natural erosion is from autogenic mechanisms is essential in order to link landscape entrenchment with past external changes 2010 elsevier b v all rights reserved

photogrammetric monitoring of small streams under a riparian forest canopy the recent advent of digital photogrammetry has enabled the modeling and monitoring of river beds at relatively high spatial resolution 0 01 to 1 m through the extraction of digital elevation models dems the traditional approach to image capture has been to mount a metric camera to an aircraft although non metric cameras have been mounted to a variety of novel aerial platforms to acquire river based imagery e g helicopters radio controlled motorized vehicles tethered blimps and balloons however most of these techniques are designed to acquire imagery at flying heights above the riparian tree canopy in relatively narrow channels e g 20 m bankfull width streamside trees can obscure the channel and limit continuous photogrammetric data acquisition of both the channel bed and banks while still providing useful information regarding the riparian canopy and even spot elevations of the channel this paper presents a technique for the capture and analysis of close range photogrammetric data acquired from a vertically mounted non metric camera suspended 10 m above the channel bed by a unipod the camera is positioned under the riparian forest canopy so that the channel bed can be imaged without obstruction the system is portable and permits relatively rapid image acquisition over rough terrain and in dense forest the platform was used to generate dems with a nominal ground resolution of 0 03 m dems generated from this platform required post possessing to either adjust or eliminate erroneous cells introduced by the extraction process overhanging branches and by the effects of refraction at the air water interface for submerged portions of the channel bed the vertical precision in the post processed surface generally ranged from 0 01 to 0 1 m depending on the quality of triangulation and the characteristics of the surface being imaged copyright 2010 john wiley sons ltd

spinifex basalts with komatiite tholeiite trend from the nansen gakkel ridge arctic ocean abstract during cruise ark iv 3 with rv polarstern 1987 volcanic rocks were recovered from the nansen gakkel ridge ngr a slow spreading half rate approximately 0 5 cm ridge with an axial depth of more than 5000 m the ngr is one of the slowest and deepest mid ocean ridges so far known and calculations based on the distance of sampling location from the axial valley yielded ages of approximately 600 ka for the rocks investigated here according to petrographic and geochemical results i e spinifex textures mg 70 and mgo 9 wt the volcanics are termed komatiitic basalts dark spherical droplets of basanitic composition within the komatiitic basalts are believed to be relicts of an incomplete magma mixing whose basanitic end member could well account for the enriched character of the ngr basalts in terms of rare earth elements ti and incompatible trace elements based on nd isotope as well as high sm nd ratios mantle metasomatism i e veined mantle model could be responsible for the enrichment of incompatible trace elements in the source region of komatiitic basalts of the ngr

beach dune morphological relationships and erosion accretion an investigation at five sites in england and wales using lidar data abstract the critical relationships between beach and nearshore parameters and the erosion accretion status of frontal dunes have been investigated using lidar data complemented by data obtained from conventional topographic and bathymetric surveys at five dune systems in england and wales morphometric parameters calculated from the lidar data demonstrate the presence of a general relationship between beach morphology dune morphology and erosion accretion eroding dunes were found to be associated with narrower steeper beaches whilst accreting dunes were associated with wider low angle beaches however the critical beach width mean slope angle separating eroding dunes from stable or accreting dunes was found to vary between sites at any given site changes in beach width slope over time can be used to provide an early warning of changes in frontal dune erosion accretion behaviour

sedimentary and crustal velocities in the norwegian greenland sea refraction and wide angle reflection velocities from the oceanic crust and the overlying sediments in the norwegian greenland sea have been analyzed there is a relatively rapid increase in velocity with depth in the uppermost sediments whereas a low constant gradient is representative of the main sediment sequence two regions the northeastern greenland sea and the oldest parts of the basins have particularly high velocity gradients 0 71 and 0 85 s 1 respectively the high gradient in the northeastern greenland sea is associated with the shear rifted margin off svalbard relatively high sea floor sediment sound velocities are typical of the norwegian greenland sea the crustal velocities fit the houtz ewing model which divides layer 2 into layers 2a 2b and 2c average velocities in the norwegian greenland sea are 3 88 2a 5 18 2b 6 05 2c 6 66 3 and 7 80 4 km s layer 2a is age dependent and has not been observed on crust older than 36 m y the model is not representative of the uppermost crust near the active spreading axis where one has to introduce additional layers or assume continuous velocity changes with depth