modelling the effect of time dependent river dune evolution on bed roughness and stage this paper presents an approach to incorporate time dependent dune evolution in the determination of bed roughness coefficients applied in hydraulic models dune roughness is calculated by using the process based dune evolution model of paarlberg et al 2009 and the empirical dune roughness predictor of van rijn 1984 the approach is illustrated by applying it to a river of simple geometry in the 1 d hydraulic model sobek for two different flood wave shapes calculated dune heights clearly show a dependency on rate of change in discharge with time dunes grow to larger heights for a flood wave with a smaller rate of change bed roughness coefficients computed using the new approach can be up to 10 higher than roughness coefficients based on calibration with the largest differences at low flows as a result of this larger bed roughness computed water depths can be up to 15 larger at low flow the new approach helps to reduce uncertainties in bed roughness coefficients of flow models especially for river systems with strong variations in discharge with time

high spatial resolution electron probe microanalysis of tephras and melt inclusions without beam induced chemical modification wavelength dispersive analysis of tephra grains and melt inclusions by epma has been carried out using a focused beam of 3 m diameter without detected loss of sodium or potassium in standard glasses including anhydrous basalts a slightly hydrated rhyolite and a sodium rich intermediate composition the ability to make analyses without chemical modification is strongly dependent upon current density at the analysis site analysis with narrow beams requires extremely low beam currents that are normally associated with energy dispersive analysis experiments indicate that a value of 0 1 na m2 must not be exceeded at least for moderately hydrated samples if sodium loss is to be avoided high resolution analysis without beam induced analytical artefacts enables fully quantitative analysis of very distal and or highly vesicular tephras and very small melt inclusions without the need to use post analysis corrections this development has enabled high quality analyses from crypto tephra layers that were pre

alluvial channel geometry theory and applications the downstream hydraulic geometry of alluvial channels in terms of bank full width average flow depth mean flow velocity and friction slope is examined from a three dimensional stability analysis of noncohesive particles under two dimensional flows four governing equations flow rate resistance to flow secondary flow and particle mobility are solved to analytically define the downstream hydraulic geometry of noncohesive alluvial channels as a function of water discharge sediment size shields number and streamline deviation angle the exponents of hydraulic geometry relationships change with relative submergence four exponent diagrams illustrate the good agreement with several empirical regime equations found in the literature the analytical formulations were tested with a comprehensive data set consisting of 835 field channels and 45 laboratory channels the data set covers a wide range of flow conditions from meandering to braided sand bed and gravel bed rivers with flow depths and channe

poleward heat transport in a barotropic ocean model abstract a barotropic ocean circulation model coupled to a mixed layer temperature equation is used to study the poleward heat transport by the horizontal wind driven ocean currents through scaling analyses two different regimes of the heat transport are found characterized by and 0 respectively where is the ratio between the ocean advection timescale and the restoring timescale of newtonian cooling at the air sea interface in the regime the heat transport is proportional to 1 and to the second power both of the basin east west width and of the magnitude of wind stress curl in the regime 0 the heat transport is proportional to and to the basin width and insensitive to the magnitude of wind stress curl the heat transport is a maximum for intermediate values of and the numerical experiments show that a restoring timescale of 5 months maximizes the heat transport for a barotropic square basin 500 m deep and 400 km wide north atlantic size the corresponding maximum heat tran

sand dune geometry of large rivers during floods the geometry of lower regime bed forms in several large sand bed rivers is investigated during average and flood conditions the van rijn method is revised because it generally underpredicts the dune height of most large rivers around the world during floods in large sand bed rivers upper regime plane bed is not necessarily obtained when t 25 both parameters describing dune height and dune steepness do not decrease as the transport stage parameter t increases in the range 10 t 25 the analysis of bed form data during large floods on the meuse river and the rhine river branches indicates that both the dune height and length generally increase with discharge while dune steepness remains relatively constant a reasonable approximation of the wavelength is 6 5 h where h is the flow depth the dune height varies as a function of the depth h and median grain size d50 estimates can be obtained by 2 5 h 0 7d0 350

lagrangian modeling of the dynamics of river and floodplain flow a new lagrangian dynamic river model is described the model solves the coupled one dimensional hydrostatic flow equations separately in a main river channel and in adjacent floodplains using a two stage predictor corrector scheme the lateral interaction between the main channel and floodplains due to both advective exchange arising from lateral pressure gradients and turbulent exchange due to lateral shear is included the lagrangian moving grid eliminates numerical diffusion and oscillations commonly experienced in eulerian models and can accurately simulate wave propagation and nonlinear steepening until wave breaking the lagrangian moving grid is dynamically adaptive providing variable resolution as the moving fluid parcel s length changes either because the cross sectional flow area or the flow depth changes as the wave moves down a channel of variable cross section the model also allows flows over dry beds and moving boundaries to be handled efficiently the model was successfully validated

dates of holocene icelandic volcanic eruptions from tephra layers in irish peats tephra from historic and prehistoric eruptions of oraefajokull and hekla iceland have been found in irish peats using a series of stratigraphically related high precision radiocarbon measure ments the date of a known age eruption has been satisfactorily estimated based on the same techniques the calendrical date of the hekla 4 eruption has been estimated as 2310 20 bc

tephrostratigraphy of an loch mor inis oirr western ireland implications for holocene tephrochronology in the northeastern atlantic region twelve definable cryptotephra layers younger than c 8600cal bp are reported from lacustrine core material taken from an loch mor inis oirr aran islands western ireland the geochemistry of these shard layers which represent more holocene tephras than previously geochemically characterized from any european site outside the proximal volcanic influence of italy or iceland is presented of these tephras four may correlate with known iceland derived tephra horizons vo 1477 hi lairg a and h5 one appears to have two possible named correlatives hoy lairg b at sluggan bog n ireland another has a plausible correlative ad 860a but two others represent icelandic tephras apparently not previously reported on iceland itself including a tephra that has geochemistry very similar to hl ad 1104 but which dates to c cal ad 840 four tephras that date from the period c ad 40 1400 have a distinct geochemical signature that relates to jan mayen 750km to the northeast of iceland and some 1800km distant from an loch mor the results contribute substantially to the chronology of the sediments from an loch mor and at the international level to the tephrostratigraphy of the northeast atlantic region the well attested icelandic h4 tephra from a hekla eruption of c 4260cal bp was not recorded it is suggested that an loch mor lies south of its recordable distribution in this part of the northeast atlantic region taken together with work at other sites the findings point to a wealth of potential tephra isochrons in holocene deposits of the northeast atlantic seaboard it is clear also that calcareous lake sediments can be as fruitful a source of tephras as peats potential problems relating to dating and geochemical fingerprinting of tephra layers in individual sites are highlighted with reference to the lairg tephras as recorded in scotland and northern ireland and to the hoy tephra that has been described from orkney

large eddy simulation of sediment transport currents over ripples a large eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples the code solves the navier stokes equations and an advection diffusion equation for the sediment and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study the main features of interest in this flow are seen because of the high resolution of the grid especially near the ripple boundary this yields a detailed bottom shear stress distribution properly identified coherent structures and resolved sharp gradients in the sediment concentration globally the sediment is carried up into the flow from locations where the shear stress is high on the upslopes of ripple crests and is advected downstream by the current and upward by the vertical velocity component regions of positive vertical velocity are associated with gortler vortices which represent a significant sediment transport mechanism in this flow

seven tephra isochrones in scotland this paper reports the presence and geochemical characteristics of seven separate tephras in scotland that fell in ad 1510 and about 450 bp 2100 bp 3600 bp 3830 bp 5600 bp and around 6000 bp the results come from 14 peat bogs spread throughout the highlands and islands where 26 tephra deposits have been characterized using over 400 grain discrete electron probe micro analysis firstly tephras are identified and defined by major element characteristics at a reference site secondly tephras are linked between sites on the basis of geochemistry and dated informal names based on british sites are suggested for tephras not yet linked to precise sources in iceland despite some apparent variation in colour all analysed grains in these seven deposits are silicic or intermediate in composition sio2 57 this phenomenon does not appear to be an artifact of preservation for two reasons first small 40 m basic glass shards in peat bogs in northern iceland are found to have undergone minimal alte

statistical characterization of bed roughness due to bed forms a field study in the elbe river at aken germany 1 bed form geometry and dynamics in a straight section of the elbe river in germany is analyzed considering the measured bed surfaces as two dimensional random fields of bed elevations statistically derived roughness parameters are evaluated from high resolution digital elevation models which were available for a range of flow rates from low flows to floods the key results relate to the identification of characteristic scaling regions in the bed surface spectra and to observed relationships between water discharge and both the standard deviation and a factor of the 3 spectral law of bed elevations two dimensional second order structure functions of bed elevations are also analyzed to gain further insight into the spatial structure of sand wave beds in addition the interrelations between flow rate hysteresis and the statistical structure of bed forms as well as effects of channel modification by groynes are highlighted and discussed the reported results demonstrate that statistical parameters of bed forms may be used for characterization and prediction of flow dependent sand bed roughness

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

the adaptation of dunes to changes in river flow abstract the dunes that cover the beds of most alluvial channels change in size and shape over time and in space which in turn affects the flow and sediment transport dynamics of the river however both the precise mechanisms of such adaptation of dunes and the hydraulic variables that control these processes remain inadequately understood this paper provides an overview of the processes involved in the maintenance and adaptation of dunes provides new tools for the analysis of dune dynamics and applies these to a series of bespoke experiments dunes that grow compete for space and dunes that decay need to shed excess sediment therefore dune adaptation necessarily involves the redistribution of sediment over and among dunes the details of sediment redistribution are not captured by mean geometric parameters such as dune height and wavelength therefore new analyses of dune kinematics bed elevation distributions and dune deformation are presented herein that aid the identification and analysis of dune dynamics dune adaptation is often described as a morphological response to changes in water depth at a rate that depends on sediment mobility which itself is a product of flow depth and velocity however depth and velocity are out of phase during the passage of flood waves and they vary spatially across rivers from the thalweg to bar tops and downstream along the river profile in order to improve our understanding of the hydraulic controls on dune morphology and kinematics a series of experiments was performed to investigate the response of dunes in fully mobile sand d50 240 m to changes in flow depth and velocity the experimental results illustrate that water depth and flow velocity have separate effects on the processes that control dune adaptation and that the crests and troughs of dunes do not respond simultaneously to changes in flow trough scour increases with flow velocity but superelevation of the dune crests appear to show only a weak relation with flow depth flattening out of dune crests is related to decreasing depth and increasing flow velocity bedform superimposition a key feature of bedform kinematics was associated with increased flow depth but was also systematically associated with local increases in the crest to crest distance following the dissipation of an upstream dune thus local flow form interactions have a significant effect on the manner in which sediment is redistributed over and among dunes the splitting of dunes decreased in the downstream direction along the length of the flume illustrating that the dunes continue to interact even after dune height has stabilised other processes such as differential migration and dune merging are ubiquitous during all flow conditions these varied responses support the notion that the processes of dune adaptation vary over time and in space analysis of dune deformation through examination of the residuals of cross correlations between successive dune profiles illustrates that local sources and sinks of sediment exist within mobile dune fields these findings highlight that dune adaptation to changes in flow is a dynamic response involving multiple interconnected dunes the redistribution of sediment that is required for dunes to change shape and adapt to new conditions is expected to be an important cause of variability in sediment transport these detailed analyses and findings provide a foundation for further study of dune dynamics in different environments on earth as well as other planetary bodies

spatial variability of tephra and carbon accumulation in a holocene peatland abstract microscopic tephra layers cryptotephras represent important age equivalent stratigraphic markers utilised in many palaeoenvironmental reconstructions when used in conjunction with proximal records of volcanic activity they can also provide information about volcanic ash cloud fallout and frequency however the spatial distributions of tephra layers can be discontinuous even within the same region understanding the deposition and post depositional redistribution of tephra is vital if we are to use cryptotephras as records of ash cloud occurrence and chronostratigraphic markers the discrete nature of tephra layers also allows for detailed study into processes of deposition and reworking which affect many palaeoenvironmental proxy records we undertook a multi core study in order to examine the historical tephrostratigraphy of a raised peatland in northern ireland three tephra layers originating from iceland hekla 1947 hekla 1845 and hekla 1510 are present in 14 of the 15 cores analysed this suggests that in areas not influenced by snowfall or anthropogenic disturbance at the time of tephra delivery the presence or absence of a tephra layer is generally consistent across a peatland of this type however tephra shard counts per unit area vary by an order of magnitude between cores these intra site differences may confound the interpretation of shard counts from single cores as records of regional ash cloud mass density bootstrap resampling analysis suggests that total shard counts from multiple cores are required in order to make a reliable estimate of median shard counts for a site the presence of three historical tephras in 14 cores enables a spatio temporal analysis of the long term apparent rate of carbon accumulation larca in the peatland substantial spatial and temporal variations in larca are identified over the last 450 years this high variability needs to be taken into account when designing studies of peatland carbon accumulation

the spatial distribution of holocene cryptotephras in north west europe since 7 ka implications for understanding ash fall events from icelandic eruptions abstract we present distribution maps for all cryptotephras distal volcanic ash layers younger than 7 ka that have been reported from three or more lakes or peatlands in north west europe all but one of the tephras originates from iceland the exception has been attributed to jan mayen we find strong spatial patterning in tephra occurrence at the landscape scale most but not all of the tephra occurrences are significantly spatially clustered which likely reflects atmospheric and weather patterns at the time of the eruptions contrary to expectations based on atmospheric modelling studies tephras appear to be at least as abundant in ireland and northern scotland as in scandinavia rhyolitic and other felsic tephras occur in lakes and peatlands throughout the study region but andesitic and basaltic tephras are largely restricted to lake sites in the faroe islands and ireland explanations of some of these patterns will require further research on the effects of different methodologies for locating and characterizing cryptotephras these new maps will help to guide future investigations in tephrochronology and volcanic hazard analysis

bedform response to flow variability laboratory observations and computational results for the response of bedform fields to rapid variations in discharge are compared and discussed the simple case considered here begins with a relatively low discharge over a flat bed on which bedforms are initiated followed by a short high flow period with double the original discharge during which the morphology of the bedforms adjusts followed in turn by a relatively long period of the original low discharge for the grain size and hydraulic conditions selected the froude number remains subcritical during the experiment and sediment moves predominantly as bedload observations show rapid development of quasi two dimensional bedforms during the initial period of low flow with increasing wavelength and height over the initial low flow period when the flow increases the bedforms rapidly increase in wavelength and height as expected from other empirical results when the flow decreases back to the original discharge the height of the bedforms quickly decreases in response but the wavelength decreases much more slowly computational results using an unsteady two dimensional flow model coupled to a disequilibrium bedload transport model for the same conditions simulate the formation and initial growth of the bedforms fairly accurately and also predict an increase in dimensions during the high flow period however the computational model predicts a much slower rate of wavelength increase and also performs less accurately during the final low flow period where the wavelength remains essentially constant rather than decreasing in addition the numerical results show less variability in bedform wavelength and height than the measured values the bedform shape is also somewhat different based on observations these discrepancies may result from the simplified model for sediment particle step lengths used in the computational approach experiments show that the particle step length varies spatially and temporally over the bedforms during the evolution process assuming a constant value for the step length neglects the role of flow alterations in the bedload sediment transport process which appears to result in predicted bedform wavelength changes smaller than those observed however observations also suggest that three dimensional effects play at least some role in the decrease of bedform wavelength so incorporating better models for particle hop lengths alone may not be sufficient to improve model predictions published in 2011 this article is a us government work and is in the public domain in the usa

transatlantic distribution of the alaskan white river ash volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and or asynchronous behaviors in earth system and archaeological sciences however it is generally assumed that only exceptionally powerful events such as supereruptions 450 km3 of ejecta as dense rock equivalent recurrence interval of 105 yr distribute ash broadly enough to have an impact on human society or allow us to address geologic climatic and cultural questions on an intercontinental scale here we use geochemical age and morphological evidence to show that the alaskan white river ash eastern lobe a d 833 850 correlates to the ad860b ash a d 846 848 found in greenland and northern europe these occurrences represent the distribution of an ash over 7000 km linking marine terrestrial and ice core records our results indicate that tephra from more moderate size eruptions with recurrence intervals of 100 yr can have substantially greater distributions than previously thought with direct implications for volcanic dispersal studies correlation of widely distributed proxy records and volcanic hazard assessment

geochemistry of historical age silicic tephras in iceland the major element chemistry of nine silicic tephras of historical age from iceland is assessed as a key step in the development of the recent tephrochronology of the north atlantic region the tephras include the largest such layers produced by each of the five central volcanoes hekla o raefajokull eyjafjallajokull torfajokull and askja since the ninth century ad h 1104 o1362 e 1821 landnam tephra c 870 a 1875 and four other tephras h 1158 h 1510 h 1947 o1727 the determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra and allows links to be made between icelandic source areas with precise dating evidence and distal deposits elsewhere in the north atlantic region although major element data can be used to discriminate between tephra layers produced by the different central volcanoes on its own it cannot be used to identify all the holocene layers produc

early to middle holocene silicic tephra horizons from the katla volcanic system iceland new results from the faroe islands comparatively few icelandic tephra horizons dated to the early part of the holocene have so far been detected outside iceland here i present several tephra horizons that have been recorded in a holocene peat sequence on the faroe islands geochemical analyses show that at least two dacitic and one rhyolitic tephra layers were erupted from the katla volcanic system on southern iceland between ca 8000 and 5900 cal yr bp the upper two layers can be correlated with the silk tephras described from southern iceland whereas the third dated to ca 8000 cal yr bp has a geochemistry virtually identical to the rhyolitic component of the vedde ash the results suggest that the late weichselian and early holocene eruption history of the katla volcano was probably more complex than inferred from iceland a new early holocene rhyolitic tephra dated to ca 10 500 cal yr bp probably originates in the snaefellsnes volcanic centre in western iceland these new findings may play an important role in developing a holocene tephra framework for northwest europe copyright 2002 john wiley sons ltd

origin of hysteresis in bed form response to unsteady flows 1 field and laboratory studies indicate that changes in riverbed morphology often lag changes in water discharge this lagged response produces hysteresis in the relationship between water discharge and bed form geometry to understand these phenomena we performed flume experiments to observe the response of a sand bed to step increases and decreases in water discharge for an abrupt rise in discharge we observed that bed forms grew rapidly by collision and merger of bed forms migrating with different celerities growth rate slowed as bed forms approached equilibrium with the higher discharge regime after an abrupt discharge drop bed form decay occurred through formation of smaller secondary bed forms in equilibrium with the lower discharge which cannibalized the original relict features we present a simple model framework to quantitatively predict time scales of bed form adjustment to flow changes based on equilibrium bed form heights lengths and celerities at low and high flows for rising discharge the model assumes that all bed form collisions result in irreversible merger due to a dispersion of initial celerities for falling discharge we derive a diffusion model for the decay of relict high stage features our models predict the form and time scale of experimental bed form adjustments additional experiments applying slow and fast triangular flood waves show that bed form hysteresis occurs only when the time scale of flow change is faster than the modeled and measured bed form adjustment time we show that our predicted adjustment time scales can also be used to predict the occurrence of bed form hysteresis in natural floods

the presence of holocene cryptotephra in wales and southern england this research was undertaken while e w held a nerc funded doctoral training grant ne k500847 1 this research was also supported by a young research workers award to e w from the quaternary research association

a parameterization of flow separation over subaqueous dunes flow separation plays a key role in the development of dunes and modeling the complicated flow behavior inside the flow separation zone requires much computational effort to make a first step toward modeling dune development at reasonable temporal and spatial scales a parameterization of the shape of the flow separation zone over two dimensional dunes is proposed herein in order to avoid modeling the complex flow inside the flow separation zone flow separation behind dunes with an angle of repose slip face is characterized by a large circulating leeside eddy where a separation streamline forms the upper boundary of the recirculating eddy experimental data of turbulent flow over two dimensional subaqueous bed forms are used to parameterize this separation streamline the bed forms have various heights and height to length ratios and a wide range of flow conditions is analyzed this paper shows that the shape of the flow separation zone can be approximated by a third order polynomial as a function of the distance away from the flow separation point the coefficients of the polynomial can be estimated independent of flow conditions on the basis of bed form shape at the flow separation point and a constant angle of the separation streamline at the flow reattachment point

modeling river dune evolution using a parameterization of flow separation this paper presents an idealized morphodynamic model to predict river dune evolution the flow field is solved in a vertical plane assuming hydrostatic pressure conditions the sediment transport is computed using a meyer peter muller type of equation including gravitational bed slope effects and a critical bed shear stress to avoid the necessity of modeling the complex flow inside the flow separation zone we follow an approach similar to one used earlier to simulate the dynamics of wind blown desert dunes in case of flow separation the separation streamline acts as an artificial bed and sediment avalanches down the leeside distributing evenly on the leeside at the angle of repose model results show that bed slope effects play a crucial role in the determination of the fastest growing wavelength from a linear analysis flow separation is shown to be crucial to take into account if the dune lee exceeds a certain threshold slope if flow separation is not included dune shapes are incorrectly predicted and the dune height saturates at an early stage of bed form evolution yielding an underprediction of dune height and time to equilibrium the local bed slope at the dune crest plays a critical role for obtaining an equilibrium dune height the simulation model is able to predict the main characteristics of dune evolution such as dune asymmetry dune growth and saturation at a certain dune height dune dimensions migration rates and times to equilibrium compare reasonably well to various data sets

a 7000 yr perspective on volcanic ash clouds affecting northern europe the ash cloud resulting from the a d 2010 eruption of eyjafjallajokull in iceland caused severe disruption to air travel across europe but as a geological event it is not unprecedented analysis of peats and lake sediments from northern europe has revealed the presence of microscopic layers of icelandic volcanic ash tephra these sedimentary records together with historical records of holocene ash falls demonstrate that icelandic volcanoes have generated substantial ash clouds that reached northern europe many times here we present the first comprehensive compilation of sedimentary and historical records of ash fall events in northern europe spanning the past 7000 yr ash fall events appear to have been more frequent in the past 1500 yr it is unclear whether this reflects a true increase in eruption frequency or dispersal or is an artifact of the records or the way in which they have been generated in the past 1000 yr volcanic ash clouds reached northern europe with a mean return interval of 56 9 yr the range of return intervals is between 6 and 115 yr probabilistic modeling using the ash records for the last millennium indicates that for any 10 yr period there is a 16 probability of a tephra fallout event in northern europe these values must be considered as conservative estimates due to the nature of tephra capture and preservation in the sedimentary record

retrieving shallow stream bathymetry from uav assisted rgb imagery using a geospatial regression method abstract bathymetric mapping is a prerequisite procedure to conduct assessments of water quality habitat and environmental flow for riverine ecosystems using hydraulic modelling this study evaluates the capability of a geographically weighted regression gwr model which can capture a spatially heterogeneous relationship between inputs and an output to retrieve bathymetry of a shallow stream of which water depth is less than about 1 m from simple rgb imagery a field experiment was performed for measuring water depth and simultaneously for acquiring remotely sensed data with rgb digital numbers dn using a digital camera mounted on an unmanned aerial vehicle uav a 2d shallow water model which was validated by comparison with the field surveyed data was used to simulate the water depth of unmeasured regions band ratios of ln dng dnr was selected as an optimal spectral input of bathymetric inversion models through the principal component analysis pca results showed that global inversion models based on multiple linear regression mlr and artificial neural network ann resulted in large discrepancy between estimation and observation due to the spatially varying response of the pca selected band ratio to water depth over the experimental channel in contrast the gwr model successfully alleviated the biases of the conventional models as r2 increased to 0 85 from 0 60 by accurately modelling the effect of spatial heterogeneity which arose from variable bottom types attributed to submerged vegetation on the remote sensing radiance water depth relationship

tephra analysis in ombrotrophic peatlands a geochemical comparison of acid digestion and density separation techniques ombrotrophic peatlands often lack any substantial local mineral component and therefore present exceptional opportunities for the extraction and identification of tephra horizons following concerns of shard geochemistry alteration during isolation using acid digestion an alternative methodology using heavy liquids has been proposed however harsh basic and acidic digestions are rarely required in organic rich ombrotrophic peatlands consequently this study tested for geochemical differences between tephra shards from a known rhyolitic eruption isolated from the same stratigraphic depth of a peat sequence using density separation and an acid digestion methodology typically applied to ombrotrophic peat samples and analysed using electron probe microanalysis findings suggest that the two samples could not be geochemically distinguished using visual and statistical methods and that in the context of rhyolitic tephras in ombrotrophic peats acid digestion can be used without significant effects on the interpretation of results density separation may still be required in peatlands with high mineral input and further work is required to examine the effect of the acid digestion technique on tephra of non rhyolitic compositions copyright 2015 john wiley sons ltd

the hekla 1947 tephra in the north of ireland regional distribution concentration and geochemistry a cryptotephra layer from the eruption of hekla in 1947 has recently been discovered in irish peatlands this tephra layer represents the most recent deposition of volcanic ash in the uk prior to the eruption of eyjafjallajokull in 2010 here we examine the concentration and geochemistry of the hekla 1947 tephra in 14 peat profiles from across northern ireland electron probe microanalysis of individual tephra shards n 91 reveals that the tephra is of dacitic andesitic geochemistry and is highly similar to the hekla 1510 tephra although spheroidal carbonaceous particle profiles can be used for successful discrimination of the two layers the highest concentrations of hekla 1947 are found in western sites probably reflecting the pathway of the ash fall event due to the prevailing wind direction comparable tephra concentrations from two cores 1 km apart from a single bog and from nearby sites may suggest that tephra shard concentrations in peat profiles reflect ash fallout densities across a specific region rather than site specific factors associated with peatlands this paper firmly establishes hekla 1947 as a useful chronostratigraphic marker for the twentieth century although within a restricted zone copyright 2012 john wiley sons ltd

low angle dunes in the changjiang yangtze estuary flow and sediment dynamics under tidal influence it has long been highlighted that important feedbacks exist between river bed morphology sediment transport and the turbulent flow field and that these feedbacks change in response to forcing mechanisms however our current understanding of bedform dynamics is largely based on studies of steady flow environments and cohesionless bed conditions few investigations have been made under rapidly changing flows here we examine flow and sediment dynamics over low angle dunes in unsteady flows in the changjiang yangtze estuary china topography flow and sediment data were collected over a reach ca 1 8 km long through a semi diurnal tidal cycle in a moderate tide of flood season the results show that 1 roughness length derived from the upper flow changes little with the flow reversing and displays the same value on both the ebb and flood tide moreover the variability of individual bedform features plays an important role in roughness length variation 2 shear stress over the crest of low angle dunes roughly represents the total spatially averaged stress over dunes in this study area which has significant implications for advancing numerical models 3 changes in morphology flow and sediment dynamics over dunes through time reveal how low angle dunes evolve within a tidal cycle 4 the clockwise hysteresis loops between flow dynamics and bedform features height and aspect ratio are also observed the combination of suspended sediment transport and bedload transport on dune transformation and migration attributes to the clockwise hysteresis the specific sediment composition of the riverbed in some extent affects the mechanism of sediment transport related to the exchange between suspended sediment and riverbed but further investigation is needed to figure out the mechanism behind this for extended series of tides such as spring neap tide and tides in flooding and dry season

effect of non migrating bars on dune dynamics in a lowland river as dunes and larger scale bed forms such as bars coexist in rivers the question arises whether dune dynamics are influenced by interaction with the underlying bed topography the present study aims to establish the degree in which dune characteristics in two and three dimensions are influenced by an underlying topography dominated by non migrating bars as a case study a 20km stretch in the waal river in the netherlands is selected which represents a sand bed lowland river at this location longitudinal training dams ltds have recently been constructed to ensure sufficient navigation depth during periods with low water levels and to reduce flood risk by using data covering two year long periods before and after ltd construction the robustness of the results is investigated before ltd construction dune characteristics show large variability both spatially and temporally with dunes being longer lower less steep and having a lower lee side angle when they are located on bar tops the correlation between dune characteristics and the underlying bed topography is disrupted by unsteady conditions for which the dunes are in a state of transition the bar pattern causes tilting of dune crest lines which may result from a transverse gradient in bed load sediment transport as a result of ltd construction the hydraulic and morphological conditions have changed significantly despite this the main conclusions still hold which strengthens the validity of the results

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

case study bed resistance of rhine river during 1998 flood detailed field measurements during the 1998 flood of the rhine river in the netherlands show that both manning in and darcy weisbach friction factor if increase with discharge the changes in bedform roughness height and friction factors are attributed to the increased dune height during floods there is a near peak hysteresis in the dune height measurements at a given discharge dunes are significantly larger after than before the peak discharge the trend is most apparent for the bovenrijn with weaker variations for the waal the methods of engelund and vanoni hwang provide similar estimates of form drag when combined with van rijn s method to estimate grain resistance both methods tend to overpredict the measured bed friction factor after the peak discharge these methods perform best when field bedform measurements are available to estimate form drag the composite effect of primary and secondary dunes should be considered in the analysis of resistance to flow

upper regime plane bed the analysis of a laboratory and field data set for sand bed channels defines the characteristics of upper regime plane bed with sediment transport the occurrence of upper regime plane bed relates to the laminar sublayer thickness and depends on shear velocity u median grain size d50 and grain shear reynolds number r two different boundary conditions are recognized 1 transition to hydraulically smooth when r 11 6 in the first case upper regime plane bed is obtained when u 2 g where g is the gravitational acceleration in the second case upper regime plane bed is observed when d50 2 temperature effects are possible when r approaches 11 6 which explains the observations on the missouri river resistance to flow for plane bed with sediment transport increases with the shields parameter

flow field and morphology mapping using adcp and multibeam techniques survey in the po river an 8 km long and 250 m wide reach of the po river italy was surveyed with two vessels aiming to investigate the channel morphology and flow field characterizing this major italian river and to provide data for the calibration of a numerical model the first vessel was equipped with a multibeam ecosounder mbes whereas an acoustic doppler current profiler adcp was located on the second one fixed vessel adcp measurements were used to preliminarily investigate the turbulence variability affecting the reliability of moving vessel measurements to evaluate the friction velocity by using moving vessel measurements the vertical variance of velocity profiles as an alternative to logarithmic fitting of adcp profiles was estimated an average of 30 m cells of depth averaged velocity friction velocity and surface bottom velocity angle are presented in maps as the most relevant flow field features the flow field and channel morphology at bed form or channel scales showed noticeable correlations 0 9 in

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

numerical simulation of dune flat bed transition and stage discharge relationship with hysteresis effect 1 this work presents recent advances on morphodynamic modeling of bed forms under unsteady discharge this paper includes further development of a morphodynamic model proposed earlier by giri and shimizu 2006a this model reproduces the temporal development of river dunes and accurately replicates the physical properties associated with bed form evolution model results appear to provide accurate predictions of bed form geometry and form drag over bed forms for arbitrary steady flows however accurate predictions of temporal changes of form drag are key to the prediction of stage discharge relation during flood events herein the model capability is extended to replicate the dune flat bed transition and in turn the variation of form drag produced by the temporal growth or decay of bed forms under unsteady flow conditions some numerical experiments are performed to analyze hysteresis of the stage discharge relationship caused by the transition between dune and flat bed regimes during rising and falling stages of varying flows the numerical model successfully simulates dune flat bed transition and the associated hysteresis of the stage discharge relationship this is in good agreement with physical observations but has been treated in the past only using empirical methods a hypothetical relationship for a sediment parameter the mean step length is proposed to a first level of approximation that enables reproduction of the dune flat bed transition the proposed numerical model demonstrates its ability to address an important practical problem associated with bed form evolution and flow resistance in varying flows

late holocene vegetation and palaeoenvironmental history of the dunadd area argyll scotland chronology of events this paper focuses on the chronology and environmental significance of a sediment sequence from an alluvial locality in the vicinity of the historic site of dunadd scotland it outlines the rationale and statistical validity of an age model derived for a sequence of floodplain sediments from which detailed pollen stratigraphical and plant macrofossil records have been derived a series of radiocarbon dates are calibrated using a bayesian modelling approach the results of which can be refined by incorporating two independent age estimates based on tephra layers of known age analysis of the entire floodplain sequence for volcanic glass shards revealed the presence of discrete but geochemically very similar tephra layers within the upper late holocene part of the sequence comparison with published geochemical data obtained from icelandic tephras of historical age indicates strong statistical correlations with the hekla 1947 ad and hekla 1510 ad while the hekla 1510 ad tephra has previously been reported from sites within britain and ireland the dunadd sequence affords the first record of the hekla 1947 ad tephra layer within scotland when the ages and stratigraphic positions of both tephra layers are incorporated into the bayesian age model an overall centennial to decadal precision for the late holocene is achieved with archaeological and environmental transitions discriminated with highest likelihood age uncertainty ranges of 20 50 years at 95 confidence the local environmental record is assessed in the light of this new chronological framework the data support previously reported proposals for two periods of significant climatic deterioration with increased wetness the first during the early medieval period and the second during the late 16th and 17th centuries ad

establishing chronologies for woodland small hollow and mor humus deposits using tephrochronology and radiocarbon dating chronological precision is a pre requisite for all palaeoecological analyses especially in more recent holocene deposits such as small hollow peat and mor humus in woodlands here tephrochronology was employed to improve the chronological control of such deposits to aid subsequent sub fossil insect analyses from three sites in western and southwestern ireland six tephra layers were detected from the three small hollows but only one from the mor humus sites four of the layers were matched to known tephra horizons regularly recorded in ireland while two new tephras of possible azores and grimsvotn origin were recorded the latter tephra may be the first definitive record of laki 1783 4 from ireland and is potentially an important new chronological marker for irish palaeoecological investigations of late holocene deposits results suggest that tephrochronology is a successful additional chronological tool for small hollow palaeoecological studies but is less helpful in mor humus deposits

ripples on stream bed some 30 years ago john f kennedy laid the foundations for the description of the geometry of sand waves on stream beds by the potential flow model since then the analysis has been extensively refined and extended but no rigorous description of the small features called ripples exists the present study attempts a stock taking of the data and the various behavioral concepts with the aim of encouraging researchers to improve our knowledge of the physics of features called ripples various trends are identified from the data it is deduced that ripples act as roughness elements of the boundary the ripple troughs are filled with the energy consuming lee vortices the main flow is over the ripple crests and lee vortices and does not interact with the bed geometry the roughness layer of ripples and vortices is within the constant shear layer and is affected by the flow depth only as far as it affects the velocity distribution and alters the shear stress on the bed

cryptotephras the revolution in correlation and precision dating from its icelandic origins in the study of visible tephra horizons tephrochronology took a remarkable step in the late 1980 s with the discovery of a ca 4300 year old microscopic ash layer in a scottish peat bog since then the search for these cryptotephra deposits in distal areas has gone from strength to strength indeed a recent discovery demonstrates how a few fine grained glass shards from an alaskan eruption have been dispersed more than 7000 km to northern europe instantaneous deposition of geochemically distinct volcanic ash over such large geographical areas gives rise to a powerful correlation tool with considerable potential for addressing a range of scientific questions a prerequisite of this work is the establishment of regional tephrochronological frameworks that include well constrained age estimates and robust geochemical signatures for each deposit with distal sites revealing a complex record of previously unknown volcanic events frameworks are regularly revised and it has become apparent that some closely timed eruptions have similar geochemical signatures the search for unique and robust geochemical fingerprints thus hinges on rigorous analysis by electron microprobe and laser ablation inductively coupled plasma mass spectrometry historical developments and significant breakthroughs are presented to chart the revolution in correlation and precision dating over the last 50 years using tephrochronology and cryptotephrochronology

residual flow bedforms and sediment transport in a tidal channel modelled with variable bed roughness abstract the frictional influence of the seabed on the tidal flow in shelf seas and estuaries is usually modelled via a prescribed spatially temporally invariant drag coefficient in practice the seabed exhibits considerable variability particularly spatially that should in principle be included in simulations local variations in the seabed roughness k s alter the flow strength and hence local sediment transport rates the effect of using a spatially temporally varying k s is assessed here with reference to a tidal channel menai strait n wales in which the variability of the bedforms has been monitored using multi beam surveying the channel not only exhibits strong tidal flow but also a residual induced flow that is used here as diagnostic to assess various bed roughness formulations tested in a telemac model tidal simulations have been carried out with both constant and temporally spatially variable k s and the predicted residual flow is shown to be sensitive to these representations for a mean spring neap sn cycle with variable k s the average residual flow is calculated to be 525 m 3 s 1 consistent with observations this residual flow can be recovered using imposed constant values of k s in the range 0 15 m to 0 3 m the results suggest that the overall effective roughness of the seabed is less than half of the maximum local roughness due to the dunes in mid channel but more than the spatially averaged k s value in the channel as a whole by about 50 simulations carried out with an m 2 alone tide using variable k s produce a somewhat smaller by 7 residual flow of 491 m 3 s 1 the use of an equivalent m 2 tide of amplitude enhanced by 7 3 reconciles these estimates the main contribution to k s is made by dunes which are modelled using van rijn s 2007 formulation subject to an additional history effect the modelled k s is found to equal approximately the observed height of the dunes along mid channel transects rather than half the height as expected this is attributed to the non equilibrium nature of the bedforms in the reversing tidal flow which exhibited shorter wavelength and more symmetrical profiles than dunes in steady flow

bedform characteristics during falling flood stage and morphodynamic interpretation of the middle lower changjiang yangtze river channel china article a survey was conducted to examine river bedforms using side scan sonar sub bottom profiler and an acous tic doppler profiler in the middle lower mid lower changjiang yangtze river downstream of wuhan china during august 14 20 2003 when the discharge was 27 000 36 000 m 3 s 1 the results show bedforms of a range of sizes occur in the channel and their heights and lengths follow an exponential correlation below flemming s maximum line not all bedforms were formed under the flow conditions experienced during the survey as shown by comparison to three existing models in the literature small scale bedforms with a length b60 m defined as flat bed mega ripple and small dune in the present study are mostly in equilibrium and are probably formed under the existing flowconditions the large dunes witha lengthofn60 m are not in equilibrium and are believed to be the relicts of previous largedischargesnow undergoing adjustment different bedform types occur in the five riverreaches inthis section of the changjiang wuhan tianjiazhen tianjiazhen anqing anqing maanshan maanshan jiangyin and jiangyin estuary downstream in the narrow and single channel reaches above anqing large small dunes are limited to the channel with higher water surface slope where stream power is strong enough for riverbed sediments to be moved frequently in the downstream reaches be tween anqing and jiangyin characterized by anabranching channels small scale bedforms superimposed on large dunes occur widely due to intensive riverbed sediment transport from frequent diverging converging flow during both floods and low flow periods mega ripples and flat beds prevail in the lowermost channel

computational modeling of dissipation and regeneration of fluvial sand dunes under variable discharges it is observed during flood events that bed forms initially grow in height and make the riverbed rougher but later under high discharge the bed forms grow longer with the opposite effect of making the riverbed smoother after the discharge drops to a lower value new bed forms regenerate on top of the elongated bed forms this mechanism leads to a significant variation in the bed roughness and the water stage and hence determines the behavior of floods and the risk of flood disasters this work presents detailed modeling of bed forms under discharge hydrographs and simulates the conditions under which the bed is flattened out in the upper plane bed regime the flow was simulated by large eddy simulation and the sediments were considered as rigid spheres and modeled in a lagrangian framework the bed morphodynamics were the result of entrainment and deposition of sediment particles we examined several discharge hydrographs in the first case we increased the discharge linearly and then kept it constant after reaching the upper plane bed condition the dunes were generated and grew during the rising stage of discharge when the flow conditions reached the upper plane bed regime high frequency ripples were generated and helped to flatten the bed the results also showed that in contrast with mechanisms in the dune regime the flattening of the bed was associated with a distinct pattern of sediment transport which deposited sediment mainly in the lee side of the dunes and led to flattening of the bed after flattening the sediments were mainly transported in suspension mode as long as flow conditions stayed in the upper plane bed regime the bed remained flat with small high frequency ripples we also examined two other scenarios one with an immediate falling stage of discharge after the rising stage and the other with a period of constant discharge between the rising and falling stages dunes were regenerated during the falling stage of discharge for both cases in the first case the dunes were regenerated before the bed was flattened because of a time lag between the flow and the bed deformation bed flattening was followed by dune regeneration in the second case moreover a significant reduction in the form drag coefficient was observed after the flattening in the second case the reduction in form drag led to a reduction in the flood intensity the model showed its ability to reproduce the key phenomena of the development of subaqueous dunes under conditions of a passing flood wave

mean flow turbulence structure and bed form superimposition across the ripple dune transition 1 this paper describes a laboratory study of the dynamics of flow associated with three different stages of bed form amalgamation across the ripple dune transition measurements of flow velocity were obtained over simplified fixed bed forms designed to simulate conditions at the ripple dune transition using a 2d laser doppler anemometer this yielded information detailing the mean velocity field turbulence statistics local turbulence production local deviations from the mean pressure caused by dynamic effects turbulent kinetic energy and the contributions to the total reynolds stresses from different coherent turbulent events the results broadly confirm previous hypotheses that as bed form amalgamation proceeds across the ripple dune transition a superimposed bedstate can induce a series of critical changes to the flow structure with higher reynolds stresses being produced near and downstream of flow reattachment in particular quadrant 4 events turbulent flow structures with a downstream velocity greater than average and directed towards the bed dominate the vertical turbulent diffusion of longitudinal momentum in near bed regions close to the crest of the next downstream ripple thus providing the potential for increased erosion and sediment transport these experiments using simple fixed beds also provide support for recent measurements that document increased suspended sediment concentrations across the ripple dune transition schindler and robert 2004 2005 and that at the transition the largest bed forms are not necessarily those associated with the most intense shear layer activity schindler and robert 2005 bed form superimposition and amalgamation may thus significantly alter the mean and turbulent flow field as compared to bed forms without superimposition

age extent and climatic significance of the c 3400 bp aniakchak tephra western alaska usa tephra deposits at whitefish lake and cape espenberg on the northernmost seward peninsula of western alaska were derived from the aniakchak caldera on the alaska peninsula more than 1500 km to the south radiocarbon dates on the climactic caldera forming aniakchak eruption and on proximal and distal tephra indicate the aniakchak tephra was deposited about 3435 40 bp 3614 3815 cal bp and forms an isochronous marker horizon across much of western alaska the aniakchak tephra eruption and several other targe eruptions during the seventeenth century bc may have had widespread effects on climate

bed load transport and boundary roughness changes as competing causes of hysteresis in the relationship between river discharge and seismic amplitude recorded near a steep mountain stream hysteresis in the relationship between bedload transport and river stage is a well documented phenomenon with multiple known causes consequently numerous studies have interpreted hysteresis in the relationship between seismic ground motion near rivers and some measure of flow strength i e discharge or stage as the signature of bedload transport here we test this hypothesis in the erlenbach stream swiss prealps using a metric to quantitatively compare hysteresis in seismic data with hysteresis recorded by geophones attached beneath steel plates within the streambed a well calibrated proxy for direct sediment transport measurements we find that while both the geophones and seismometers demonstrate hysteresis the magnitude and direction of hysteresis are not significantly correlated between these data indicating that the seismic signal at this site is primarily reflecting hysteresis in processes other than sediment transport seismic hysteresis also does not correlate significantly with the magnitude of sediment transport recorded by the geophones contrary to previous studies assumptions we suggest that hydrologic sources and changes in water turbulence for instance due to evolving boundary conditions at the bed rather than changes in sediment transport rates may sometimes contribute to or even dominate the hysteresis observed in seismic amplitudes near steep mountain rivers

dunes in the world s big rivers are characterized by low angle lee side slopes and a complex shape dunes form critical agents of bedload transport in all of the world s big rivers and constitute appreciable sources of bed roughness and flow resistance dunes also generate stratification that is the most common depositional feature of ancient riverine sediments however current models of dune dynamics and stratification are conditioned by bedform geometries observed in small rivers and laboratory experiments for these dunes the downstream lee side is often assumed to be simple in shape and sloping at the angle of repose here we show using a unique compilation of high resolution bathymetry from a range of large rivers that dunes are instead characterized predominantly by low angle lee side slopes 10 complex lee side shapes with the steepest portion near the base of the lee side slope and a height that is often only 10 of the local flow depth this radically different shape of river dunes demands that such geometries are incorporated into predictions of flow resistance water levels and flood risk and calls for rethinking of dune scaling relationships when reconstructing palaeoflow depths and a fundamental reappraisal of the character and origin of low angle cross stratification within interpretations of ancient alluvial sediments dunes in the world s big rivers are dominated by lee side slopes with angles of less than 10 according to a bedform analysis of high resolution bathymetric datasets

stochastic prediction of sediment transport in sand gravel bed rivers classical deterministic bedload transport predictors are applied to sand gravel bed rivers the turbulent bed shear stress is modeled according to a probability distribution to obtain realistic bedload transport rates at incipient motion in extending the predictors to stochastic predictors for nonuniform sediment many parameters that represent near bed turbulence and the particle size distribution must be chosen the parameters that give realistic results are chosen by analyzing the results of a new experimental flume dataset with relatively large water depths choosing other combinations of parameters may give equal total bedload transport rates but at the cost of large errors in fractional transport rates attention is given to the hiding exposure phenomenon and a hindrance effect related to nonuni form sediment validation based on two independent field datasets shows that successful predictions of particle sizes near the threshold for motion are feasible using the stochastic approach while the deterministic approach gives successful predictions well above incipient motion

an outline tephrochronology for the holocene of the north of ireland geochemically typed thin layers of icelandic holocene tephras form a tephrochronology for the north of ireland tephrochronology construction is based on geochemical linkage with dated historic icelandic eruptions and high precision multisample radiocarbon dating comparison with geochemically typed holocene layers at sites throughout scotland and northern england shows that some layers are widespread throughout much of the northern british isles other layers of known historic date have so far been detected only in ireland

geochemistry dispersal volumes and chronology of holocene silicic tephra layers from the katla volcanic system iceland at least 12 silicic tephra layers silk tephras erupted between ca 6600 and ca 1675 yr bp from the katla volcanic system have been identified in southern iceland in addition to providing significant new knowledge on the holocene volcanism of the katla system which typically produces basaltic tephra the silk tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts with one exception the silk tephras have a narrow compositional range with sio2 between 63 and 67 geochemically they are indistinguishable from ocean transported pumice found on beaches in the north atlantic region although they differ significantly from the silicic component of the north atlantic ash zone one naazo volumes of airborne silk tephra range from 0 05 to 0 3 km3 we present new isopach maps of the six largest layers and demonstrate that they originate within the katla caldera the apparently stable magma system conditions that produced the silk tephras may have been established as a consequence of the eruption of the silicic component of naazo ca 10 3 ka and disrupted by another large scale event the tenth century ad eldgja eruption ca 1 ka despite the current long repose silicic activity of this type may occur again in the future presenting hitherto unknown hazards copyright 2001 john wiley sons ltd

a new and less destructive laboratory procedure for the physical separation of distal glass tephra shards from sediments abstract tephrochronology a key tool in the correlation of quaternary sequences relies on the extraction of tephra shards from sediments for visual identification and high precision geochemical comparison a prerequisite for the reliable correlation of tephra layers is that the geochemical composition of glass shards remains unaltered by natural processes e g chemical exchange in the sedimentary environment and or by laboratory analytical procedures however natural glasses particularly when in the form of small shards with a high surface to volume ratio are prone to chemical alteration in both acidic and basic environments current techniques for the extraction of distal tephra from sediments involve the cleaning of samples in precisely such environments and at elevated temperatures the acid phase of the cleaning process risks alteration of the geochemical signature of the shards while the basic phase leads to considerable sample loss through dissolution of the silica network here we illustrate the degree of alteration and loss to which distal tephras may be prone and introduce a less destructive procedure for their extraction this method is based on stepped heavy liquid flotation and which results in samples of sufficient quality for analysis while preserving their geochemical integrity in trials this method out performed chemical extraction procedures in terms of the number of shards recovered and has resulted in the detection of new tephra layers with low shard concentrations the implications of this study are highly significant because i the current database of distal tephra records and their corresponding geochemical signatures may require refinement and ii the record of distal tephras may be incomplete due to sample loss induced by corrosive laboratory procedures it is therefore vital that less corrosive laboratory procedures are developed to make the detection and classification of distal glass tephra more secure

the response of subaqueous dunes to floods in sand and gravel bed reaches of the dutch rhine the branches of the river rhine in the netherlands characterized by a sand gravel bed in the upstream part and a sand bed in the downstream part of the river system show migrating dunes especially during floods in the last 20 years these dunes have been studied extensively high resolution echo sounding measurements of these dunes made with single and multibeam equipment were analysed for three different sections of the rhine river system during several floods this analysis was done to quantify the growth decay and migration rates of the dunes during floods in addition the migrating dunes were used to calculate bedload transport rates with dune tracking the results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain the relations are used to show that the growth and migration rate of dunes and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations

late quaternary icelandic tephras in ireland and great britain detection characterization and usefulness microscopic studies of late quaternary tephras from iceland yield a detailed tephrochronology in deposits of the british isles where the quantities of volcanic glass employed are much less than those used to construct almost any other late quatemary tephrochronology we summarize improved methods of isolating extremely sparse glass shards for light microscope and electron microprobe analysis and provide major element geochemistry for 20 volcanic glass layers detected in ireland including those which comprise an outline historic tephrochronology for ireland the paper highlights possible limitations of chronology construction when attempting to separate dated tephras whose major element geochemistries are almost identical tephras in other parts of the british isles and europe are used with those in ireland to demonstrate further applications of tephrochronology especially for dating deposits where other dating methods e g radiocarbon have proved difficult

response of low angle dunes to variable flow current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments there are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides as a consequence the ability to predict bedform response to variable flow is rudimentary using high resolution multibeam bathymetric data this study explores the dynamics of a dune field developed by tidally modulated fluvially dominated flow in the fraser river estuary british columbia canada the dunes were dominantly low lee angle features characteristic of large deep river channels data were collected over a field ca 1 0 km long and 0 5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet yielding the most comprehensive characterization of low angle dunes ever reported the data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs then increasing as the tide rises and the flow velocities decrease bedform lengths do not appear to respond to the changes in velocity caused by the tides changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity indicating that changes in the bedform geometry lag changes in the flow the data reveal that lee angle slope responds directly to suspended sediment concentration supporting previous speculation that low angle dune morphology is maintained by erosion of the dune stoss and crest at high flow and deposition of that material in the dune trough