modeling of multilayer cohesive bank erosion with a coupled bank stability and mobile bed model streambank erosion can be an important form of channel change in unstable alluvial environments it should be accounted for in geomorphic studies river restoration dam removal and channel maintenance projects recently one dimensional and two dimensional flows and mobile bed numerical models have become useful tools for predicting morphological responses to stream modifications most however either ignore bank failure mechanisms or implement only simple ad hoc methods in this study a coupled model is developed that incorporates a process based bank stability model within a recently developed two dimensional mobile bed model to predict bank retreat a coupling procedure that emphasizes solution robustness as well as ease of use is developed and described the coupled model is then verified and validated by applying it to multilayer cohesive bank retreat at a bend of goodwin creek mississippi comparisons are made between the predicted and measured data as well as results of a previous modeling study on one hand the study demonstrates that the use of two dimensional mobile bed models leads to promising improvements over that of one dimensional models it therefore encourages the use of multidimensional models in bank erosion predictions on the other hand the study also identifies future research needs in order to improve numerical modeling of complex streams the developed model is shown to be robust and easy to apply it may be used as a practical tool to predict bank erosion caused by fluvial and geotechnical processes

a network scale intermediate complexity model for simulating channel evolution over years to decades abstract excessive river erosion and sedimentation threatens critical infrastructure degrades aquatic habitat and impairs water quality tools for predicting the magnitude of erosion sedimentation and channel evolution processes are needed for effective mitigation and management we present a new numerical model that simulates coupled river bed and bank erosion at the watershed scale the model uses modified versions of bagnold s sediment transport equation to simulate bed erosion and aggradation as well as a simplified bank stability and toe erosion model bstem to simulate bank erosion processes the model is mechanistic and intermediate complexity accounting for the dominant channel evolution processes while limiting data requirements we apply the model to a generic test case of channel network response following a disturbance and the results match physical understanding of channel evolution the model was also tested on two field data sets below parker dam on the lower colorado river and the north fork toutle river nftr which responded dramatically to the 1980 eruption of mount st helens it accurately predicts observed channel incision and bed material coarsening on the colorado river as well as observations for the upstream 18 km of the nftr watershed the model does not include algorithms for extensive lateral migration and avulsions and therefore did not perform well in the lower nftr where the channel migrated across a wide valley bottom rem is parsimonious and useful for simulating network scale channel change in single thread systems responding to disturbance

coevolution of bed surface patchiness and channel morphology 2 numerical experiments in gravel bed rivers bed topography and the bed surface grain size distribution evolve simultaneously but it is not clear how feedbacks between topography and grain sorting affect channel morphology in this the second of a pair of papers examining interactions between bed topography and bed surface sorting in gravel bed rivers we use a two dimensional morphodynamic model to perform numerical experiments designed to explore the coevolution of both free and forced bars and bed surface patches model runs were carried out on a computational grid simulating a 200 m long 2 75 m wide straight rectangular channel with an initially flat bed at a slope of 0 0137 over five numerical experiments we varied a whether an obstruction was present b whether the sediment was a gravel mixture or a single size and c whether the bed surface grain size feeds back on the hydraulic roughness field experiments with channel obstructions developed a train of alternate bars that became stationary and were connected to the obstruction freely migrating alternate bars formed in the experiments without channel obstructions simulations incorporating roughness feedbacks between the bed surface and flow field produced flatter broader and longer bars than simulations using constant roughness or uniform sediment our findings suggest that patches are not simply a by product of bed topography but they interact with the evolving bed and influence morphologic evolution

spatial variability in bank resistance to erosion on a large meandering mixed bedrock alluvial river abstract spatial heterogeneity in the erosion resistance properties of the channel banks and floodplains associated with sediment characteristics vegetation or bedrock can have a substantial influence on the morphodynamics of meandering rivers resulting in highly variable rates of bank erosion and complex patterns of planform evolution although past studies have examined the spatial variability in bank erodibility within small rivers this aspect of the erosion resistance properties for large rivers remains poorly understood furthermore with the exception of recent numerical modeling that incorporates stochastic variability of floodplain erosional resistance most models of meandering river dynamics have assumed uniform erodibility of the bank and floodplain materials the present paper investigates the lateral and vertical heterogeneity in bank material properties and riparian vegetation within two elongate meander loops on a large mixed bedrock alluvial river using several geotechnical field and laboratory methods additionally the bank stability and toe erosion numerical model bstem and repeat terrestrial lidar surveys are used to evaluate the capacity of the bank material properties to modify the rates and mechanisms of bank retreat results show that the textural properties of the bank materials soil cohesion and critical shear stress necessary for sediment entrainment differ substantially between the two bends and are also highly variable within each bend laterally and vertically trees growing along the banks increase the resistance to erosion by contributing to the shear strength of the bank materials and are capable of increasing bank stability along a large river locally outcropping bedrock also influences bank erodibility in both bends the results of this study demonstrate that spatial variability in the erosion resistance properties of the channel banks is an important factor contributing to spatial variability in the rates and mechanisms of bank erosion determined from short term studies of bank retreat and long term analysis of changes in channel planform

the effects of variability in bank material properties on riverbank stability goodwin creek mississippi abstract bank retreat is an important area of research within fluvial geomorphology and is a land management problem of global significance the yazoo river basin in mississippi is one example of a system which is experiencing excessive erosion and bank instability the properties of bank materials are important in controlling the stability of stream banks and past studies have found that these properties are often variable spatially through an investigation of bank material properties on a stretch of goodwin creek in the yazoo basin mississippi this study focuses on i how and why effective bank material properties vary through different scales ii how this variation impacts on the outputs from a bank stability model and iii how best to appropriately represent this variability within a bank stability model the study demonstrates the importance that the variability of effective bank material properties has on bank stability at both the micro scale within a site and at the meso scale between sites in a reach this variability was shown to have important implications for the usage of the bank stability and toe erosion model bstem a deterministic bank stability model that currently uses a single value to describe each bank material property as a result a probabilistic representation of effective bank material strength parameters is recommended as a potential solution for any bank stability model that wishes to account for the important influence of the inherent variability of soil properties

modeling the evolution of incised streams ii streambank erosion incision and ensuing widening of alluvial stream channels is widespread in the midsouth and midwestern united states and represents an important form of channel adjustment streambanks have been found to contribute as much as 80 of the total suspended load the location timing and magnitude of streambank erosion are difficult to predict results from field studies to characterize the resistance of fine grained materials to hydraulic and geotechnical erosion the impact of pore water pressures on failure dimensions and shearing resistance and the role of riparian vegetation on matric suction streambank permeability and shearing resistance are used to enhance the channel evolution model concepts conservational channel evolution and pollutant transport system this paper discusses the conceptualization of the above mentioned physical processes and demonstrates the ability of the derived model to simulate streambank failure processes the model is tested against observed streambank erosion of a bendway on goodwin creek miss between march 1996 and march 2001 where it accurately predicts the rate of retreat of the outside bank of the bendway the observed change in average channel width within the central section of the bendway is 2 96 m over the simulation period whereas a retreat of 3 18 m 7 4 larger is simulated the observed top bank retreat within the central section of the bendway is 3 54 m over the simulation period whereas a retreat of 3 01 m 15 smaller is simulated doi 10 1061 asce0733 94292008134 7905 ce database subject headings bank erosion limit equilibrium mathematical models sensitivity analysis numerical analysis

changes in streambank erodibility and critical shear stress due to subaerial processes along a headwater stream southwestern virginia usa despite more than 40 yr of research attributing temporal changes in streambank erosion rates to subaerial processes little quantitative information is available on the relationships between streambank erodibility kd and critical shear stress c and the environmental conditions and processes that enhance streambank erosion potential the study goal was to evaluate temporal changes in kd and c from soil desiccation and freeze thaw cycling soil erodibility and c were measured monthly in situ using a multiangle submerged jet test device soil moisture temperature and bulk density as well as precipitation air temperature and stream stage were measured continuously to determine changes in soil moisture content and state pairwise mann whitney tests indicted kd was 2 9 and 2 1 times higher p 0 0065 during the winter december march than in the spring fall april may october november and the summer june september respectively regression analysis showed 80 of the variability in kd was explained by freeze thaw cycling alone study results also indicated soil bulk density was highly influenced by winter weather conditions r2 0 86 bulk density was inversely related to both soil water content and freeze thaw cycling results showed that significant changes in the resistance of streambank soils to fluvial erosion can be attributed to subaerial processes water resource professionals should consider the implications of increased soil erodibility during the winter in the development of channel erosion models and stream restoration designs

theoretical regime equations for mobile gravel bed rivers with stable banks a system of rational regime equations is developed for gravel bed rivers with stable banks using the optimality theory ot the optimality theory is based on the premise that equilibrium river geometry is characterised by an optimum configuration defined here as maximum sediment transport efficiency theoretical dimensionless equations are derived for width depth slope width depth ratio and meandering braiding transition independent dimensionless variables comprise discharge sediment concentration and relative bank strength which is defined as the ratio of the critical shear stresses for the bank and bed sediments discharge exponents and general form of the equations agree well with previously developed empirical relations relative bank strength is used to parameterise the influence of riparian vegetation on bank strength and is evaluated by calibrating against observed width depth ratio once calibrated the hydraulic geometry of natural gravel rivers is well described by the theoretical equations including discrimination between meandering and braiding channels the results provide strong support for the assumption that equilibrium or regime river behavior is equivalent to an optimal state and underline the importance of bank strength and sediment load as controls on hydraulic geometry

a model of channel response in disturbed alluvial channels dredging and straightening of alluvial channels between 1959 and 1978 in west tennessee caused a series of morphologic changes along modified reaches and tributary streams degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed levels by as much as 6 1 m following degradation reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction aggradation downstream of the area of maximum disturbance reached 0 12 m per year with the greatest rates occurring near the stream mouths the adjustment of channel geometry and phases of channel evolution are characterized by six process oriented stages of morphologic development premodified constructed degradation threshold aggradation and restabilization down cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded threshold stage channel widening continues through the aggradation stage as the slough line develops as an initial site of lower bank stability the bank profile develops three dynamic elements 1 vertical face 70 to 90 2 upper bank 25 to 50 and 3 slough line 20 to 25 alternate channel bars form during the restabilization stage and represent incipient meandering of the channel

variability in sediment delivery and storage along river courses in bega catchment nsw australia implications for geomorphic river recovery in many catchments in southeastern australia alluvial stores have been the dominant source of sediments mobilised in the period since european settlement in bega catchment on the south coast of new south wales nsw this has been reflected by dramatic changes to river morphology extensive volumes of material have been released and efficiently flushed to the lowland plain with a sediment delivery ratio of almost 70 however only 16 of these alluvial sediments have been flushed through to the estuary as antecedent controls on valley width have resulted in the lowland plain acting as a large sediment sink the changing nature of sediment source transfer and accumulation zones has varied markedly from subcatchment to subcatchment the volume of material supplied to the lowland plain from differing subcatchments is not related to subcatchment area rather the pattern of river types dictates the spatial variability in storage and transfer over 67 of sediment released has been sourced from just 25 of the catchment from subcatchments characterised by large valley fills cut and fill river style that previously stored extensive volumes of material at the base of the escarpment these parts of bega catchment were especially sensitive to disturbance sediment exhaustion from these parts of the catchment and from river courses elsewhere has major implications for the geomorphic recovery potential of rivers constraining what can be realistically achieved in terms of river rehabilitation

geomorphological variability among microtidal estuaries from the wave dominated south african coast abstract over 300 independent river outlets exist around the microtidal wave dominated south african coast most of these are located in drowned river valley settings and have acquired their present morphology during the holocene marine transgression a variety of geomorphological forms are identified in these estuaries that result from variation in antecedent topography fluvial sediment supply and marine sediment supply five distinctive estuary types are identified on the basis of contemporary morphodynamics these are categorised into three types of normally open estuary that maintain a semi permanent connection with the open sea and two types of normally closed estuary which are separated from the sea for long periods by a continuous supratidal barrier the contemporary morphodynamics and sedimentary environments of each estuary type are discussed open estuaries include barrier inlet systems maintained by fluvial discharge termed river dominated estuaries and tidal discharge termed tide dominated estuaries a third category of open estuary lacks a supratidal barrier due to inadequate marine sediment availability closed estuaries receive marine influence via barrier overwash and occasional breaching but are typically enclosed behind a continuous supratidal barrier two categories of closed estuary are identified perched and non perched perched estuaries develop behind high berms and maintain a water level above high tide level in the open sea they are often dominated by fresh to brackish water and breach and drain periodically non perched estuaries are developed behind low elevation barriers fronted by wide dissipative beach profiles high overwash frequency introduces marine water into such systems breaching of such estuaries does not produce dramatic draining as the water level may only drop according to the stage of the open sea tidal cycle the variety of estuary types suggests several potential pathways may exist in estuarine development and that the progressive infilling model is an over simplification

the chronological sedimentary and environmental context for the archaeological deposits at blombos cave south africa abstract the site of blombos cave bbc is well known for archaeological remains that have advanced our understanding of the development of modern human behaviour during the middle stone age msa occupation of the cave occurred against a backdrop of landscape scale environmental and sedimentary processes that provide the broader context for finer scale interpretations of the site formation history and archaeological patterns detected in the cave deposits aeolian and palaeosol sequences are abundant in the vicinity of bbc and these provide a partial view of the past landscapes available to the inhabitants of the cave an important extension to the palaeo landscape around bbc currently lies submerged on the agulhas bank as sea levels were lower than at present for the entire period of human occupation of bbc in this paper we revisit the optically stimulated luminescence osl chronology for the full sequence of sediment deposition inside bbc increasing the number of dated samples to a total of 40 and revising the period of msa occupation to between 97 7 7 6 and 71 0 5 7 ka uncertainties at 95 4 probability we describe the geological successions at four main areas around bbc estimate the time of sediment deposition using osl and describe and interpret three seismic profiles on the agulhas bank offshore of bbc by correlating these onshore and offshore geological sequences with the sedimentary deposits inside bbc we place the archaeological record within a landscape scale chrono stratigraphic framework to examine how environmental changes may have regulated the presence or absence of humans in the cave and surrounding terrain between about 100 and 70 ka

numerical simulation of downstream fining by selective transport in gravel bed rivers model development and illustration a numerical model has been developed for the routing of gravel sized sediment along a river channel which is free to adjust both its long profile and surface texture hydraulic calculations use a step backwater approach and sediment transport is predicted with the method of parker 1990a which uses a low degree of size selectivity exchange of sediment between the surface and subsurface is described using the modified exner equation of parker and sutherland 1990 the model is applied to an idealized channel based on the highly concave allt dubhaig scotland in which fining by particle wear is minor the rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to 102 years under the present flow regime of the river the evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described it is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective this run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions which are summarized here and will be described in a subsequent paper

modeling the evolution of incised streams i model formulation and validation of flow and streambed evolution components a robust computational model for simulating the long term evolution of incised and restored or rehabilitated stream corridors is presented the physically based model simulates the three main processes that shape incised streams hydraulics sediment transport and streambed and bank adjustments a generalized implicit preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals the solution method introduces several enhancements that increase its robustness specifically to simulate flashy flows transport of cohesive or cohesionless graded bed material is based on a total load concept and suspended and bed load transport modes are accounted for through nonequilibrium effects the model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material the present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds and reports simulations of published experiments on degrading and aggrading channels with graded bed material description and validation of the model s stream bank erosion component and the application of the model to incised stream systems are presented elsewhere

dynamic connectivity in a fluvial network for identifying hotspots of geomorphic change dynamical processes occurring on the hierarchical branching structure of a river network tend to heterogeneously distribute fluxes on the network often concentrating them into clusters i e places of excess flux accumulation here we put forward the hypothesis that places in the network predisposed due to process dynamics and network topology to accumulate excess sediment over a considerable river reach and over a considerable period of time reflect locations where a local imbalance in sediment flux may occur thereby highlighting a susceptibility to potential fluvial geomorphic change we develop a dynamic connectivity framework which uses the river network structure and a simplified lagrangian transport model to trace fluxes through the network and integrate emergent clusters through a cluster persistence index cpi the framework was applied to sand transport in the greater blue earth river network in the minnesota river basin three hotspots of fluvial geomorphic change were defined as locations where high rates of channel migration were observed and places of high cpi coincided with two of these hotspots of possibly sediment driven change the third hotspot was not identified by high cpi but instead is believed to be a hotspot of streamflow driven change based on additional information and the fact that high bed shear stress coincided with this hotspot the proposed network based dynamic connectivity framework has the potential to place dynamical processes occurring at small scales into a network context to understand how reach scale changes cascade into network scale effects useful for informing the large scale consequences of local management actions

seismic stratigraphy of the inner to mid agulhas bank south africa abstract the inner to mid continental shelf of the agulhas bank which forms part of the palaeo agulhas plain is scattered with pleistocene deposits their wide lateral extension is the expression of a flat underlying substrate availability of accommodation space and depositional processes in response to glacio eustatic sea level change we present seismic sub bottom profiles up to 30 m deep sediment cores up to 5 m in length and pleistocene deposits that date back to marine isotope stage mis 7 from the inner to mid shelf between the breede river in the west and plettenberg bay in the east radiocarbon and optically stimulated luminescence dates are integrated with microfossil analysis into a seismic stratigraphic model comprised of twenty quaternary facies units within two depositional sequences bounded by shelf wide unconformities sequence boundary 1 sb1 corresponds to the erosional unconformity with bedrock and sb2 to the mis 2 glacial lowstand incised palaeo river channels are associated with both sequence boundaries and cored deposits also mapped seismically from estuarine lacustrine and fluvial systems are grouped to represent the lower floodplain the most pervasive stratigraphic pattern in these shelf deposits is made up of the depositional sequence remnant of the falling stage systems tract fsst forced regression from mis 5e 2 the other dominant stratigraphic group is the transgressive systems tract tst associated with the post glacial marine transgression the tst makes up an almost equal proportion of deposits in both sequences in the sedimentological record as the fsst despite the shorter temporal span of the tst a wave ravinement surface marks the rise in sea level from the last glacial maximum in a landward direction

middle and late pleistocene paleoscape modeling along the southern coast of south africa abstract changing climates environment and sea levels during the middle and late pleistocene must have had significant impacts on early modern humans and their behavior however many important archaeological sites occur along the current coastline of south africa where the gradual slope of the offshore agulhas bank meant that small changes to sea level height potentially caused significant shifts in coastline position the geographic context of these currently coastal sites would have been transformed by sea level shifts from coastal to near coastal to fully terrestrial to understand human adaptations as reflected in the archaeological deposits of these now coastal sites we need to accurately model coastline position through time here we introduce a paleoscape model as a conceptual tool to ground the records for human behavioral evolution within a dynamic model of paleoenvironmental changes using integrated bathymetric datasets gis and a relative sea level curve we estimate the position of the coastline at 1 5 ka increments over the last 420 000 years we compare these model predictions to strontium isotope ratios from speleothems as an independent test and then compare the coastline predictions to evidence for shellfish exploitation through time both tests suggest our model is relatively robust we then widen our paleoscape model to most of the cape region and compare the predictions of this broadened model to evidence from blombos cave

strontium isotope investigation of ungulate movement patterns on the pleistocene paleo agulhas plain of the greater cape floristic region south africa abstract middle stone age sites located within the greater cape floristic region on the south african southern coast have material culture with early evidence for key modern human behaviors such as projectile weaponry large animal hunting and symbolic behavior in order to interpret how and why these changes evolved it is necessary to understand their ecological context as it has direct relevance to foraging behavior during periods of lowered sea level a largely flat and vast expanse of land existed south of the modern coastline but it is now submerged by higher sea levels this exposed area the paleo agulhas plain likely created an ecological context unlike anything in the region today as evidenced by fossil assemblages dominated by migratory ungulates one hypothesis is that the paleo agulhas plain supported a migration ecosystem of large grazers driven by summer rainfall producing palatable forage during summer in the east and winter rainfall producing palatable forage during winter in the west alternatively ungulates may have been moving from the coastal plain in the south to the interior north of the cape fold mountains as observed for elephants in historic times in this study we assess ungulate movement patterns with inter and intra tooth enamel samples for strontium isotopes in fossil fauna from pinnacle point sites pp13b and pp30 to accomplish our goals we created a bioavailable 87sr 86sr isoscape for the region by collecting plants at 171 sampling sites and developing a geospatial model the strontium isotope results indicate that ungulates spent most of their time on the paleo agulhas plain and avoided dissected plain foothill and mountain habitats located more than about 15 km north of the modern coastline the results clearly exclude a north south coastal interior movement or migration pattern and cannot falsify the east west movements hypothesized in the south coast migration ecosystem hypothesis

applicability of thin or thick skinned structural models in a region of multiple inversion episodes southern south africa abstract the deformation of the cape fold belt has been attributed to repeated structural reactivation of a mega detachment from the late proterozoic to the mesozoic 650 65 ma through the integration of onshore cross sections with observations from the offshore mesozoic extensional system this study evaluates the applicability of the mega detachment model regional scale cross sections through the permian triassic cape fold belt reveal that it comprises two main structural domains a northern domain dominated by northward verging and asymmetric folds and a southern domain comprising a series of approximately 8 km wavelength box folds the genesis of these box folds is attributed to motion on underlying high angle 45 reverse faults this variation between north and south in the fold belt is reflected by a similar variation in extensional geometry of the mesozoic normal faults as revealed by subsurface data the normal faults demonstrate a progressive increase in dip from 24 in the north to 60 in the south features commonly attributed to thin and thick skinned tectonic models are observed in both domains therefore it is not appropriate to describe the observed deformation as one of the two end members in addition the structures are inferred to have undergone at least two stages of inversion irrespective of dip this is not predicted by either end member model

a rational sediment transport scaling relation based on dimensionless stream power the concept of stream channel grade according to which a stream channel reach will adjust its gradient s in order to transport the imposed sediment load having magnitude qb and characteristic grain size db with the available discharge q mackin 1948 geological society of america bulletin59 463 512 lane 1955 american society of civil engineers proceedings81 1 17 is one of the most influential ideas in fluvial geomorphology herein we derive a scaling relation that describes how externally imposed changes in either qb or q can be accommodated by changes in the channel configuration described by the energy gradient mean flow depth characteristic grain size and a parameter describing the effect of bed surface structures on grain entrainment one version of this scaling relation is based on the dimensionless bed material transport parameter w presented by parker and klingeman 1982 water resources research18 1409 1423 an equivalent version is based on a new dimensionless transport parameter e using dimensionless unit stream power this version is nearly identical to the relation based on w except that it is independent of flow resistance both versions of the scaling relation are directly comparable to lane s original relation in order to generate this stream power based scaling relation we derived an empirical transport function relation relating e to dimensionless stream power using data from a wide range of stable bed load dominated channels the form of that transport function is based on the understanding that while grain entrainment is related to the forces acting on the bed described by dimensionless shear stress sediment transport rate is related to the transfer of momentum from the fluid to the bed material described by dimensionless stream power copyright 2010 john wiley sons ltd

the last interglacial sea level high stand on the southern cape coastline of south africa abstract the continental margin of southern south africa exhibits an array of emergent marginal marine sediments permitting the reconstruction of long term eustatic sea level changes we report a suite of optical luminescence ages and supplementary amino acid racemization data which provide paleosea level index points for three sites on this coastline deposits in the swartvlei and groot brak estuaries display tidal inlet facies overlain by shoreface or eolian facies contemporary facies relations suggest a probable high stand 6 0 8 5 m above modern sea level amsl at cape agulhas evidence of a past sea level high stand comprises a gravel beach ca 3 8 m amsl and an overlying sandy shoreface facies up to 7 5 m amsl osl ages between 138 7 ka and 118 7 ka confirm a last interglacial age for all marginal marine facies the high stand was followed by a sea level regression that was associated with the accumulation of eolian dunes dating to between 122 7 ka and 113 6 ka these data provide the first rigorous numerical age constraints for last interglacial sea level fluctuations in this region revealing the timing and elevation of the last interglacial high stand to broadly mirror a number of other far field locations

aeolianite and barrier dune construction spanning the last two glacial interglacial cycles from the southern cape coast south africa the southern cape region of south africa has extensive coastal aeolianites and barrier dunes whilst previously reported limited knowledge of their age has precluded an understanding of their relationship with the climatic and sea level fluctuations that have taken place during the late quaternary sedimentological and geomorphological studies combined with an optical dating programme reveal aeolianite development and barrier dune construction spanning at least the last two glacial interglacial cycles aeolianite deposition has occurred on the southern cape coast at ca 67 80 88 90 104 128 160 189 and 200 ka before the present using this and other published data coupled with a better understanding of late quaternary sea level fluctuations and palaeocoastline configurations it is concluded that these depositional phases appear to be controlled by interglacial and subsequent interstadial sea level high stands these marine transgressions and regressions allowed onshore carbonate rich sediment movement and subsequent aeolian reworking to occur at similar points in the landscape on a number of occasions the lack of carbonates in more recent dunes oxygen isotope stages 1 2 and 4 5 is attributed not to leaching but to changes to carbonate production in the sediment source area caused by increased terrigenous material and or changes in the balance between the warm agulhas and nutrient rich benguela ocean currents

geology and archaeology submerged landscapes of the continental shelf an introduction sea level change has influenced human populations globally since prehistoric times even in early phases of cultural development human populations were faced with marine regression and transgression associated with the glacial interglacial climatic cycle amplified by glacio isostatic adjustments in some regions global marine regression during the last glaciation changed the palaeogeography of the continental shelf converting former marine environments to attractive terrestrial habitats for prehistoric human occupation and adding an extensive new increment of land in the case of europe amounting to an additional 40 of the existing land area and on a global scale to some 20 million km of additional territory that is now submerged these areas of the shelf were used as hunting and gathering areas and as pathways of dispersal between regions and between continents until they were resubmerged by the post glacial marine transgression they also most probably witnessed the earliest developments in seafaring marine exploitation and permanent settlements based on modern marine research technologies and the integration of large databases proxy data are now becoming available for the reconstruction of these submerged quaternary landscapes concerted efforts are also now being devoted to the search for prehistoric archaeological sites and artefacts on the seabed often in collaboration with marine scientists this search has been stimulated by the increasing amount of material that has demonstrably survived inundation often with excellent preservation of organic remains by closer collaboration with offshore industry and by the growing realization of the importance of these submerged data for understanding human prehistoric developments during periods of rapidly changing climate and environment moreover these new research trends are not simply being driven by an archaeological need for scientific and technological input from other disciplines but by collaborations involving genuine mutual benefit in which all partners have something to gain archaeological problems often pose new questions about geological change stimulating new techniques of observation new technologies and new investigations which in their turn can offer new data often at higher resolution and with better dates in relation to geological and environmental issues such as sea level change and palaeoclimatic variability on the continental shelf the expansion of early human populations to occupy new territory and new continents is one of the great narratives of human evolution it is currently a theme of wide interest and topicality and has received significant impetus and publicity from the new science of palaeogenetics together with ongoing discoveries of new fossil and archaeological material and new studies of palaeoclimate and palaeoenvironment the field itself is rapidly expanding leading to new discoveries and new controversies grine et al 2009 gamble 2013 smith ahern 2013 all the current indications are that the human species originated in africa with at least two major episodes of dispersal the earliest some time after about 2 myr ago led to the expansion of homo ergaster erectus populations across southern europe and asia extending from britain in the west to china and indonesia in the east and south the second involved our own species homo sapiens anatomically modern humans or amh and took place some time after about 200 ka this resulted in the replacement of earlier hominin populations in europe and asia and expansion further afield into new guinea and australia certainly involving sea crossings over distances of at least 50 km by about 50 ka to the higher latitudes of northern eurasia with entry into the americas by at least 15 ka and into new territory exposed by the melting of the northern hemisphere ice sheets less than 10 kyr ago anthropological and archaeological investigations of these processes are directly connected

ballistics and ash plumes discriminated by doppler radar 1 small scale eruptive ash plumes at arenal volcano costa rica were recorded using a ground based doppler radar voldorad the time velocity distribution of the mass load i e doppler radargrams exhibits two contrasted dynamics recorded simultaneously evidenced by distinctive velocities life spans and transit speeds through the radar beam synthetic doppler radargrams computed with a simple ballistic model indicate that the short lived signal is consistent with the instantaneous projection of ballistics blocks accompanying the ash plume emission the mass of centimeter to decimeter sized ballistics is confidently estimated at 0 5 7 tons whereas the ash plume mass is loosely constrained at 5 8 102 tons assuming a particle diameter of 2 mm close to the vent these quantitative estimates of the mass proportion either falling on the slopes of the volcano or ejected into the atmosphere could help in the modeling and monitoring of tephra dispersal

mesozoic exhumation of the southern cape south africa quantified using apatite fission track thermochronology southern africa s topography is distinctive an inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment the origin and evolution of this topography is poorly understood because unlike high plateaus elsewhere its development cannot be easily linked to present day compressional plate boundary processes understanding the development of this regional landscape since the break up of gondwana is a first order step towards resolving regional epeirogenesis we present data that quantifies the timing and extent of exhumation across the southern cape escarpment and coastal plane using apatite fission track analysis afta of 25 outcrop samples and 31 samples from three deep boreholes kw1 67 sa1 66 cr1 68 outcrop fission track aft ages are cretaceous and are significantly younger than the stratigraphic ages of their host rocks indicating that the samples have experienced elevated paleotemperatures mean track lengths vary from 11 86 to 14 23 m the lack of cenozoic apatite ages suggests that major cooling was over by the end cretaceous the results for three boreholes situated seaward south of the escarpment indicate an episode of increased denudation in the mid late cretaceous 100 80 ma an earlier episode of increased denudation 140 120 ma is identified from a borehole north of the escarpment thermal modelling indicates a history involving 2 5 3 5 km of denudation in the mid late cretaceous 100 80 ma at a rate of 175 to 125 m ma the aft data suggest that less than 1 km of overburden has been eroded regionally since the late cretaceous 80 ma at a rate of 10 to 15 m ma but do not discount the possibility of minor in relative amplitude episodes of uplift and river incision through the cenozoic the reasons for rapid denudation in these early and mid cretaceous episodes are less clear but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions e g kimberlites across southern africa and contemporaneous formation of two large mafic igneous provinces 130 and 90 ma flanking its continental margins because cenozoic denudation rates are relatively minimal epeirogenic uplift of southern africa and its distinct topography cannot be primarily related to cenozoic mantle processes consistent with the lack of any significant igneous activity across this region during that time

a middle stone age paleoscape near the pinnacle point caves vleesbaai south africa abstract open air middle stone age msa contexts in southern africa have received relatively little research attention compared to caves rock shelters msa caves rock shelters can provide long sequences of msa behaviors dominated by residential activities in circumscribed contexts but most procurement activities occurred on the landscape in uncircumscribed space we have a limited understanding of these activities at present making studies of open air sites crucial to alleviate this bias the south african coast paleoclimate paleoenvironment paleoecology paleoanthropology sacp4 project expanded its research scope to include msa archaeology from open air contexts we report on a series of msa open air assemblages that are exposed on ancient land surfaces suggestive of intact paleosols at vleesbaai and visbaai south africa importantly these sites occur in close proximity to the long cave rock shelter sequences at pinnacle point this presents the novel potential to study evidence of msa behavior in closed and open settings where their proximity to each other approximates the typical hunter gatherer daily foraging radius documented in ethnography we present a fabric and technological analysis of msa stone tool assemblages from three areas analysis of total station piece plotting of artifact bearing plunge suggests that the lithic assemblages have undergone limited post depositional disturbance the technological analysis and exploratory comparisons between these open air assemblages and msa cave and rock shelter contexts at pinnacle point cave 13b and 9 and cape st blaize cave suggest that the quartzite artifacts from vleesbaai were procured from locally available sources and may have been field processed there before being transported elsewhere perhaps to the caves rock shelters further the analysis suggests a dichotomous pattern of retouched tool discard where quartzite tools are discarded similarly across the landscape in contrast non quartzite tools may have been made primarily at the cave sites and discarded or lost more frequently on the landscape

energy time and channel evolution in catastrophically disturbed fluvial systems abstract two diverse fluvial systems show that with time channels adjust such that the rate of energy dissipation is minimized one fluvial system characterized by high relief and coarse grained sediment was subjected to an explosive volcanic eruption the other system characterized by low relief and fine grained sediment was subjected to dredging and straightening study of the expenditure of kinetic and potential energy components of total mechanical energy provide an energy based rationale of the interdependency between processes and forms during channel evolution spatial and temporal trends of aggradation and degradation are similar although relative amounts of aggradation in the high energy system are greatly enhanced by the deposition of large amounts of eroded bank material from upstream reaches degradation accompanied by widening is the most efficient means of energy dissipation because all components of total mechanical energy decrease with time widening dominates energy dissipation in the coarse grained system to offset increases in hydraulic depth caused by incision in the low energy fine grained system channel adjustment and energy dissipation are dominated by vertical processes because of low relative values of kinetic energy and because eroded bank sediment is transported out of the drainage basin and does not aid in downstream aggradation energy dissipation or channel recovery specific energy is shown to decrease nonlinearly with time during channel evolution and provides a measure of reductions in available energy at the channel bed data from two sites show convergence towards a minimum specific energy with time time dependent reductions in specific energy at a point act in concert with minimization of the rate of energy dissipation over a reach during channel evolution as the fluvial systems adjust to a new equilibrium

morphological evolution of the north fork toutle river following the eruption of mount st helens washington abstract the north fork toutle river nftr has undergone extensive morphological changes following the catastrophic eruption of mount st helens washington in 1980 especially the upper reaches affected by a 2 5 km3 debris avalanche deposit caused by the eruption this paper reports analysis and interpretation of vertical adjustments to the thalweg long profile at some 33 km river reaches redeveloped on the debris avalanche deposit during the 30 year period since the eruption the results confirm that adjustments in the upper part of the study reaches have generally been led by degradation while that in the lower reaches have been led by aggradation with the middle reaches acting as a hinge zone trends of change in the thalweg long profile and bedslope reveal that channel gradients have decreased nonlinearly through time and with distance downstream from the volcano values of stream power have decreased with time commensurately owing to reductions in slope and channel widening while the bed has coarsened so that rates of erosion of the debris avalanche deposit in the upper nftr have slowed to the point that the long profile now perched and slightly steeper is relaxing toward a new equilibrium or graded condition thirty year relaxation paths for thalweg elevation were simulated at seven key cross sections using newly developed comprehensive rate law models based on nonlinear decay in rates of morphological response to perturbation the results indicate that both single and multistep rate law models can simulate the observed records consequently the rate law approach provides an effective method for studying and simulating morphological response of the fluvial system to a major instantaneous disturbance such as a volcanic eruption

process form interactions in unstable sand bed river channels a numerical modeling approach abstract a deterministic numerical model of bed deformation and channel widening which accounts for specific mechanisms of bank erosion and collapse is used to analyze morphological and flow energy parameters in adjusting sand bed channels for a range of simulated fluvial environments the model is based on a set of conservation and process equations solved in conjunction with a set of specific initial and boundary conditions simulated channels with a range of assumed boundary material characteristics were subjected to identical disturbances imposed by reducing the supply of sediment from upstream asymptotic reductions of the rate of energy dissipation energy slope and boundary shear stress were found to be unifying characteristics of channel adjustment in all simulations morphologic responses to an identical disturbance reduction of sediment supply from upstream with identical bed material d 50 1 mm and channel gradient s b 0 001 were diverse the relative magnitude of simulated widening versus simulated bed level change was greatest for sand bank channels and least for clay bank channels stable channel dimensions were attained most rapidly for silt bank channels where simulated adjustments of channel width were similar to simulated vertical adjustments because all components of total mechanical energy flow depth or pressure head bed elevation and channel gradient or datum head and flow velocity or velocity head decrease simultaneously each simulated channel subjected to the baseline disturbance had an initial width to depth ratio f of 13 5 asymptotic values of energy dissipation rate adjusted f values and time taken to reach equilibrium after the disturbance are shown to vary as a function of simulated environmental boundary conditions the importance of channel widening in controlling and reducing the flow depth of a given discharge and average boundary shear stress is highlighted for the silt and sand bank cases channel widening causes a shift from degradation to aggradation during adjustment because of delivery of sand sized sediments from failed bank material to the simulated channel

last interglacial fossil elephant trackways dated by osl aar in coastal aeolianites still bay south africa abstract the impressive pleistocene coastal aeolianite exposures in sea cliffs east of still bay on the west southern coast of south africa host a rich archive of fossil mammalian trackways including the african elephant loxodonta africana neither the ichnofossils nor their host sediments have been described in any detail and chronologies remained uncertain this paper presents a new optically stimulated luminescence osl and amino acid racemisation aar chronology the first joint application of osl aar dating in south africa this provides a temporal framework for assessing the palaeoenvironmental significance of dune sedimentation patterns pedogenesis and ichnology the pleistocene aeolianite exposures at still bay represent the recently eroded remnants of a dune cordon mainly built by coalesced parabolic dune systems sedimentary facies are dominated by large scale planar cross stratification formed by foreset progradation in the nose and trailing arms of the dunes and low angle bedding chiefly representing sedimentation in low relief interdune terrain the main dune building wind regime was westerly associated with cyclonic polar frontal systems as at the present time the osl and aar dating demonstrate ages ranging from mis 5e to 5b and termination of pleistocene aeolian sedimentation at 90 ka the pleistocene aeolianite is separated from the overlying holocene dunes dated to 8 ka by a major hiatus recorded by a horizon of intense pedogenesis elephant footprints were seen in profile as casts on the underside of beds and as natural impressions the taphonomic processes controlling the morphology and preservation of these unique ichnofossils were found to be complex the presence of loxodonta africana at still bay represents the southernmost occurrence of this species recorded to date and possibly a closer proximity of woodland during mis 5 this and other observations may indicate a higher moisture regime than at present there is no available evidence that the elephants were predated by contemporary middle stone age people

linking source and sink evaluating the balance between onshore erosion and offshore sediment accumulation since gondwana break up south africa abstract the geomorphic origin and evolution of the tectonically unique interior highland of southern africa the kalahari plateau and its flanking low lying coastal planes remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history here we focus on the southern cape south africa and link onshore denudation based on new apatite fission track thermochronology results to offshore sediment accumulation using abundant well data and a seismic reflection profile we attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern africa the volume of sediment accumulated off south africa s south coast is calculated using 173 wells and a seismic reflection profile a total uncompacted sediment volume of 268 500 km3 accumulated off south africa s south coasts since 136 ma in the outeniqua and southern outeniqua basins accumulation volumes and rates were highest in the early cretaceous 48 800 104 km3 at 8150 km3 ma from 136 to 130 ma and 57 500 104 km3 at 5750 km3 ma from 130 to 120 ma and mid late cretaceous 83 700 104 km3 at 3200 km3 ma from 93 to 67 ma volumes and accumulation rates were lowest for the early mid cretaceous 47 400 104 km3 at 1750 km3 ma from 120 to 93 ma and the cenozoic 31 200 104 km3 at 450 km3 ma from 67 to 0 ma although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion as quantified through apatite fission track thermochronology e g tinker j h de wit m j brown r 2008 mesozoic exhumation of the 439 southern cape south africa quantified using apatite fission track thermochronology tectonophysics doi 10 1016 j tecto 2007 10 009 the timing of increased sediment accumulation closely matches the timing of increased onshore denudation this suggests that the greatest volumes of material were transported from source to sink during two distinct cretaceous episodes and that the processes driving onshore denudation decreased by an order of magnitude during the cenozoic

paleoanthropologically significant south african sea caves dated to 1 1 1 0 million years using a combination of u pb tt osl and palaeomagnetism abstract deposits in sea caves found along the southern coastline of south africa have produced a rich and detailed archaeological record of early modern humans there is however little evidence for coastal cave deposits and human occupation older than mis5e 120 ka based on the correlation of four different chronological methods we present evidence for remnant cave deposits of 1 1 1 0 ma from the quartzite sea cliff of pinnacle point near mossel bay initial uranium thorium ages at isotopic equilibrium indicated an age of 500 ka for two flowstone layers confirmed by uranium lead dating of these flowstones from 1 099 0 012 to 1 047 0 011 ma tt osl thermally transferred optically stimulated luminescence provides an age of 1 02 0 088 ma for the sand grains imbedded in the tufa underlying the flowstone and 0 720 0 066 to 0 665 0 056 for the overlying beach sediments producing an internally consistent age sequence centring on 1 0 1 1 ma the normal palaeomagnetic signal of the younger section of the flowstone is interpreted to represent the jaramillo between 1 07 and 0 99 ma there is a clear hiatus in the middle of this flowstone leading us to interpret the lower normal signal as the punaruu event at 1 115 1 1051 ma together these four techniques point to an age of 1 1 1 0 ma for these cave deposits at pinnacle point far older than anticipated the persistent presence of these 1 1 1 0 ma deposits means that the enigmatic lack of earlier stone age acheulean artefacts in the sea caves along this coastal region can no longer be explained entirely by the age of the caves or through removal of sediments by previous sea level highstands we believe that these and other coastal caves from this region if located high enough above sea level may contain deposits of great antiquity which could provide outstanding records of climate environment sea level change and human occupation back into the early to middle pleistocene

variability of fluvial erodibility parameters for streambanks on a watershed scale abstract typically the erosion rate of cohesive soils is modeled using the excess shear stress equation which includes two soil parameters the erodibility coefficient k d and the critical shear stress c alternatively a mechanistic detachment rate model the wilson model was recently developed to predict the erosion rate of cohesive soils also based on two soil parameters b 0 and b 1 the wilson model is proposed as advantageous in terms of being a more mechanistic fundamentally based erosion equation the objective of this research was to derive the excess shear stress model parameters k d and c from field jet erosion tests jets on numerous streambanks across the illinois river watershed in oklahoma to investigate i erodibility parameter variability or uniformity at a river basin scale ii correlations between the derived parameters and soil texture iii influence of solution technique on the estimated erodibility parameters and iv the applicability of predictive relationships between k d and c the second objective was to demonstrate the applicability of the mechanistic wilson model using field jet data and to investigate correlations between the excess shear stress model parameters k d and c and the wilson model parameters b 0 and b 1 erodibility parameters for streambanks of varying soil texture were measured using a miniature version of the jet device mini jet data from the jets were used to derive the erodibility parameters using a blaisdell and scour depth approach soil samples were acquired at locations of the jets to quantify particle size distribution average particle size d 50 and bulk density no significant relationships existed between k d or c and bulk density d 50 percent clay silt or sand or percent clay silt existing empirical relationships should be used with caution considering the variability between the results observed in this research and previous relationships proposed in the literature strong correlations were observed between b 0 and k d r 2 0 62 to 0 89 and between b 1 and c r 2 0 31 to 0 96 therefore the wilson model parameters closely resemble the empirical excess shear stress parameters

slow rates of rock surface erosion and sediment production across the namib desert and escarpment southern africa slow erosion has characterized the namib desert the namibian escarpment and the adjacent namibian highlands over the pleistocene paired analyses n 66 of in situ produced 10be and 26al in quartz bearing samples of bedrock primarily from inselbergs of sediment from dry river and stream channels and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years bedrock samples n 47 collected in three transects from the coast across the escarpment and into the highlands show no spatial pattern in elevation normalized nuclide abundance despite a difference in mean annual precipitation map between sample sites at the coast map 400 mm yr 1 average model erosion rates inland of the escarpment 3 2 1 5 n 9 are indistinguishable from average rates seaward of the escarpment 3 6 1 9 n 38 indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands sediment samples n 3 from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster 16 m my 1 than either a basin in the highlands 5 m my 1 or a basin in the coastal plain 8 m my 1 data from large rivers n 4 constrain erosion rates averaged over 105 yrs and 104 to 105 km2 between 3 and 9 m my 1 small quartz clasts n 12 collected from four desert surfaces record extraordinarily long variable and in some cases complex exposure histories simple 10be model ages are as high as 1 8 my some minimum total histories considering both 10be and 26al and including both burial and exposure exceed 2 7 my as a group the namibian cosmogenic data do not support the model of significant and on going escarpment retreat the similarity of erosion rates calculated from 10be analysis of fluvial sediments and longer term 107 yr average mass removal rates estimated by others using fission track analysis of rock suggests that namibian erosion rates have reached a steady state and are changing little over time at outcrop scales the concordance of 10be and 26al in most bedrock samples suggests that the model of steady uniform bedrock erosion is valid there is no indication of intermittent burial shedding of thick rock slabs or stripping of previous cover at an intermediate scale a transect of bedrock samples north of gobabeb demonstrates that the northern boundary of the massive namib sand sea has been steady and unshifting similarly low cosmogenically estimated erosion rates across west and central namibia suggest that the landscape is in geomorphic steady state its overall appearance changing only slowly through time

a network based framework for identifying potential synchronizations and amplifications of sediment delivery in river basins long term prediction of environmental response to natural and anthropogenic disturbances in a basin becomes highly uncertain using physically based distributed models particularly when transport time scales range from tens to thousands of years such as for sediment yet such predictions are needed as changes in one part of a basin now might adversely affect other parts of the basin in years to come in this paper we propose a simplified network based predictive framework of sedimentological response in a basin which incorporates network topology channel characteristics and transport process dynamics to perform a nonlinear process based scaling of the river network width function to a time response function we develop the process scaling formulation for transport of mud sand and gravel using simplifying assumptions including neglecting long term storage and apply the methodology to the minnesota river basin we identify a robust bimodal distribution of the sedimentological response for sand of the basin which we attribute to specific source areas and identify a resonant frequency of sediment supply where the disturbance of one area followed by the disturbance of another area after a certain period of time may result in amplification of the effects of sediment inputs which would be otherwise difficult to predict we perform a sensitivity analysis to test the robustness of the proposed formulation to model parameter uncertainty and use observations of suspended sediment at several stations in the basin to diagnose the model the proposed framework has identified an important vulnerability of the minnesota river basin to spatial and temporal structuring of sediment delivery

relict and contemporary influences on the postglacial geomorphology and evolution of a current swept shelf the eastern cape coast south africa abstract few stratigraphic models of continental shelves incorporate the process of geostrophic current sweeping consequently its role in the stratigraphic record is often overlooked we examine the narrow current swept eastern cape shelf of south africa using a combination of geophysical techniques seafloor sampling and video observations and interpret the role of current action on the transgressive stratigraphy of this steep subtropical shelf during the last glacial maximum fluvial valleys incised the acoustic basement rocks during the subsequent transgression two distinct shorelines were formed and preserved at 105 m and 60 m their development and preservation is linked to i high sediment supply from adjacent fluvial sources ii early diagenesis and iii alternating sea level stillstands and periods of rapid sea level rise during melt water pulses 1a and 1b respectively the deeper shoreline formed in a sandy wide coastal plain setting with limited bedrock influence whereas the shallower shoreline comprised alternating rock headlands and embayments like the contemporary coast differences in antecedent topography and geology are responsible for the temporal variability in shoreline type coastal squeeze is exacerbated with increasing bedrock control as the shoreline migrates landward between the two shoreline complexes in the mid shelf the transgressive stratigraphy records initial valley infill by progradation of coast parallel sandy spits these are capped by a stiff lagoonal mud deposited as ongoing sea level rise overspilled the valley interfluves onlapping the adjacent aeolianites the uppermost stratigraphy comprises mounds of rhodoliths which interfinger with a sandy inner to middle shelf highstand wedge after sea level reached its present position ca 7 4 ka yr bp the shelf became subject to reworking by the high energy geostrophic agulhas current this has had the following major effects on the shelf stratigraphy 1 the topographic relief of the cemented palaeo shorelines has been emphasised by removal of the post transgressive cover and 2 the shelf no longer acts as a depocenter instead the seabed consists of rhodoliths gravel streamers bedrock or gravel hash of the wave ravinement surface given the necessary antecedent conditions such as accommodation sediment supply and favourable diagenetic climate prominent shorelines can form and be preserved on the shelf strong current sweeping emphasises these morphological features on subtropical shelves

the evolution of coastal barrier systems a case study of the middle late pleistocene wilderness barriers south africa abstract barrier systems contain lengthy but complex records of long term environmental fluctuations the wilderness embayment south africa contains a system of shore parallel barriers reaching up to 200 m above modern sea level this study reports the results of chronological topographical both on and off shore sedimentological and micromorphological analyses within the wilderness embayment sixty one new luminescence ages from sixteen sites in unconsolidated dunes and three separate barriers are presented which when combined with previously published luminescence ages from the area provide a high resolution chronological framework for the emplacement and evolution of the barrier system the preserved barriers have been constructed within at least the last two glacial interglacial cycles with notable phases between 241 221 ka 159 143 ka 130 120 ka 92 87 ka and post 6 ka multiple phases of barrier construction occurred during sea level highstands with sediment deposition on each individual barrier occurring over at least two interglacials holocene evolution of the system sheds light on earlier events with dune preservation occurring only during early regression from the mid holocene highstand tectonic stability at wilderness allowed glacio eustatically formed shorelines to occupy similar positions on multiple occasions this in conjunction with a relatively humid climate and a well vegetated landscape enabled deflated sediment from beaches to form dunes which stacked upon each other to form an extensive and complex vertical accretionary sequence repeated erosion and recycling of pre existing barriers as well as barrier construction on what is currently the off shore platform during still stands in sea level regressional cycles when sea levels dropped below ca 50 m from the present day has added to the complexity of the preserved terrestrial barrier record the wilderness barrier system contrasts with barriers developed elsewhere in the world where higher rates of crustal uplift have allowed preservation of a more complete and more widely spaced palaeorecord this research also shows the utility of integrating off shore topography as revealed by bathymetry with terrestrial topographic data for the better understanding of the evolution of palaeo coastlines and the preserved dune record found on present day coastal plains local variation in the topography of the continental shelf at wilderness has generated spatial and temporal complexity within the sedimentary records of individual barriers as well as having a significant influence on preservation

characterisation of seafloor substrate using advanced processing of multibeam bathymetry backscatter and sidescan sonar in table bay south africa abstract a method to map seafloor substrates using machine learning based on geophysical data including multibeam bathymetry backscatter and side scan sonar is currently being developed results from a case study in table bay southwestern south africa are presented here showing a method of physical seafloor classification that uses a number of statistical algorithms and software programs in the first step of the process a customised tool was created within arcgis using python scripting language to classify seafloor bathymetry which can be applied to areas beyond south africa the tool created by the authors was based on pioneering work done by the national oceanic and atmospheric administration on a benthic terrain modelling toolbox in step two multibeam bathymetry backscatter and side scan sonar data processed using qimera fledermaus geocoder toolbox and navlog processing software were classified using machine learning techniques including decision trees random forests and k means clustering computer algorithms the results from these algorithms were compared to classify the seafloor substrate distribution our results have allowed a comparison of advantages and disadvantages of each machine learning technique and we found that the k means clustering techniques were the simplest to implement and understand and worked best based on their seafloor segmentation capabilities in table bay with all three data sets multibeam bathymetry backscatter and side scan sonar in future research ground truthing methods for example underwater video and grab samples will be used to validate the interpreted data to create accurate seafloor substrate maps this work provides the initial steps to develop a holistic predictive tool that classifies geophysical data into substrate maps using machine learning techniques the maps can be used to model biological communities and to produce benthic habitat maps for use in marine science and management

downstream variation in stream power abstract stream power has a significant influence on many form and process attributes of the fluvial system using a similar approach to that suggested by d m lawler in 1992 and 1995 a model is developed to explore how stream power might vary in the downstream direction based on the assumption that the longitudinal profile of the river has an exponential form no particular discharge is necessarily specified but near bankfull flows are regarded as significant morphologically and one such discharge is the focus here total stream power per unit channel length qs is predicted to peak at an intermediate location the position of which depends on the ratio b where b and are the downstream rates of change of discharge and slope respectively if b is a relatively conservative quantity then the form of the longitudinal profile and in particular its degree of concavity will have the major influence on the position of maximum stream power specific stream power w is even more sensitive to and is expected to peak closer to the headwaters about half way between source and the location of the stream power maximum although this will depend on how channel width w varies with distance from source the model is tested along three connected streams of different size in the catchment of the river trent taking advantage of a relationship between mean annual flood and link magnitude which enables this near bankfull discharge to be estimated at any point within the network the model is verified in so far as stream power does attain a mid basin maximum but the observed and predicted locations of the maxima do not always correspond largely because of the distinctive variation in discharge and especially slope along individual pathways for example maximum power lies far downstream along the trent because of the large increase in mid basin discharge associated with a series of large closely spaced tributaries the importance of slope is borne out by the near headwater locations for specific power maxima which tend to be further upstream than predicted along all three rivers as a result of the steepness of upper reaches the variety of geomorphic settings produces deviations from expectation which has implications for the movement and storage of material in the fluvial system

erodibility of cohesive streambeds in the loess area of the midwestern usa excess stress parameters critical shear stress c and erodibility coefficient k d for degrading channels in the loess areas of the midwestern usa are presented based on in situ jet testing measurements critical shear stress and k d are used to define the erosion resistance of the streambed the jet testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters streams tested were primarily silt bedded in texture with low densities which is typical of loess soils results indicate that there is a wide variation in the erosion resistance of streambeds spanning six orders of magnitude for c and four orders of magnitude for k d erosion resistance was observed to vary within a streambed from streambed to streambed and from region to region an example of the diversity of materials within a river system is the yalobusha river basin in mississippi the median value of c for the two primary bed materials naheola and porters creek clay formations was 1 31 and 256 pa respectively streambeds composed of the naheola formation are readily eroded over the entire range of shear stresses whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the porters creek clay formation therefore assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams

development of longitudinal profiles of alluvial channels in response to base level lowering degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0 01 cm cm degradation was initiated by lowering base level to a fixed position and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat diffusion equation it is based on the equation of sediment continuity combined with an assumption regarding sediment transport namely that sediment discharge is linearly proportional to the channel slope in accordance with the boundary and initial conditions imposed by the experimental setup and procedure the basic equation is amenable to an analytical solution which defines bed elevation at any distance and time as a function of the amount of base level lowering and a diffusion coefficient additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model the results show that in homogeneous alluvial sediments not subject to armouring the ultimate result of base level lowering by a certain amount is degradation all along the channel by the same amount the main impact of erosion is felt in the early stages after initiation of the process and mainly near the mouth the rate of degradation at any station along the channel reaches a peak and then slowly decreases with time and the peak rate is attenuated with distance from the outlet the model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times

evaluation of the bank stability and toe erosion model bstem for predicting lateral retreat on composite streambanks abstract streambank erosion is known to be a major source of sediment in streams and rivers the bank stability and toe erosion model bstem was developed in order to predict streambank retreat due to both fluvial erosion and geotechnical failure however few if any model evaluations using long term streambank retreat data have been performed the objectives of this research were to 1 monitor long term composite streambank retreat during a hydraulically active period on a rapidly migrating stream 2 evaluate bstem s ability to predict the measured streambank retreat and 3 assess the importance of accurate geotechnical fluvial erosion and near bank pore water pressure properties the barren fork creek in northeastern oklahoma laterally eroded 7 8 to 20 9 m along a 100 m length of stream between april and october 2009 based on regular bank location surveys the most significant lateral retreat occurred in mid to late may and september due to a series of storm events and not necessarily the most extreme events observed during the monitoring period bstem version 5 2 was not originally programmed to run multiple hydrographs iteratively so a subroutine was written that automatically input the temporal sequence of stream stage and to lag the water table in the near bank ground water depending on user settings eight bstem simulations of the barren fork creek streambank were performed using combinations of the following input data with and without a water table lag default bstem geotechnical parameters moderate silt loam versus laboratory measured geotechnical parameters based on direct shear tests on saturated soil samples and default bstem fluvial erosion parameters versus field measured fluvial erosion parameters from submerged jet tests using default bstem input values underestimated the actual erosion that occurred lagging the water table predicted more geotechnical failures resulting in greater streambank retreat using measured fluvial and geotechnical parameters and a water table lag also under predicted retreat approximately 3 3 m but did predict the appropriate timing of streambank collapses the under prediction of retreat was hypothesized to be due to over predicting the critical shear stress of the non cohesive gravel under predicting the erodibility of the non cohesive gravel and or under predicting the imposed shear stress acting on the streambank current research improving our understanding of shear stress distributions streambank pore water pressure dynamics and methods for estimating excess shear stress parameters for noncohesive soils will be critical for improving bstem and other streambank stability models

three dimensional transport speed of wind drifted ash plumes using ground based radar 1 the main utilization of mobile ground based doppler radars is to quantify the dynamics of eruptive activity by aiming directly at the emission source we show that they can also provide information on the initial lateral transport speed of weak ash plumes bent over by crosswind the method is illustrated by measurements made with a transportable volcano doppler radar voldorad at arenal volcano costa rica the near source displacements of the plume are tracked through echo onsets induced by ash entering successive probed volumes in the radar beam a constant transport velocity is commonly reached within a few seconds of the initial ash emission as wind advection and buoyancy take over momentum the plume azimuth and upraise angles are constrained by comparing the amplitude decrease of the radar echoes as a function of distance from the source with results from a simple geometric plume model the three dimensional vector of the ash cloud transport speed is then reconstructed with an accuracy of a few percent this method may have applications for volcano monitoring for determining pyroclast fluxes for the modeling of tephra dispersal and for remote measurements of volcanic gas fluxes

bank and near bank processes in an incised channel abstract gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion the interaction of these forces control streambank mechanics hydraulic forces exerted by flowing water on in situ bank toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower bank profiles seepage forces exerted on in situ bank material by groundwater downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength data from a study site on goodwin creek ms usa clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore water pressures negative pore water pressures matric suction have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear a stable bank can be transformed into an unstable bank during periods of prolonged rainfall through 1 increase in soil bulk unit specific weight 2 decrease or complete loss of matric suction and therefore apparent cohesion 3 generation of positive pore water pressures and therefore reduction or loss of frictional strength 4 entrainment of in situ and failed material at the bank toe and 5 loss of confining pressure during recession of stormflow hydrographs relatively small frequent flows during the winter have the ability to erode failed bank materials maintain oversteepened unstable bank surfaces and promote prolonged periods of bank retreat channel migration and high yields of fine grained sediment confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe which steepens the bank for example more than 2 m of bank retreat occurred during the study period at the research site on goodwin creek northern mississippi the loss of matric suction negative pore pressures due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability apparent cohesion friction angle soil bulk unit weight and moisture content were measured in situ matric suction was measured continuously in situ with a series of five pressure transducer tensiometers a bank failure algorithm which combines the mohr coulomb approach for saturated conditions and the fredlund modification for unsaturated conditions was developed for layered cohesive streambanks the resulting equation has been used successfully to investigate the role of matric suction positive pore water pressures and confining pressure for layered streambanks composed of cohesive materials

archaeology of the continental shelf marine resources submerged landscapes and underwater archaeology we provide a critical review of the evidence for the long term use of marine resources and coastal environments in human evolution and later development we emphasise the importance of the coastal archaeological record in understanding patterns of human settlement and dispersal and draw attention to the large potential biases introduced by the destructive or obscuring effects of pleistocene sea level change we note that lowered sea levels have been the norm for most of the pleistocene and that periods of high sea level have been too short lived to provide other than a fragmentary coastal record and one that is beset with ambiguities and uncertainties we examine the geological processes of coastal uplift and underwater preservation that may help to mitigate these biases coastlines elevated by isostatic and tectonic processes or with very steep offshore drop offs at plate boundaries are important in providing a potential window into coastal landscapes and archaeology formed during periods of lowered sea level however we conclude that the opportunities afforded by these geological factors are too limited to obviate the need for underwater exploration we review the evidence now available from submerged landscapes across the africa eurasia interface from the atlantic coastlines of northwest europe to the southern red sea we show that geomorphological conditions for the preservation of archaeological and palaeoenvironmental data are commonly present that much material has already been discovered and that new techniques technologies and projects are providing the momentum for a rapidly expanding field of investigation the results do not simply add to what we already know from sites on land but are likely to produce qualitatively different evidence for coastal adaptations and landscapes that have no analogue on present day coastlines we note the strong probability that many coastal landscapes exposed at lowered sea level provided relatively fertile and productive refugia for plants land mammals and humans at a time when increased aridity would have reduced or deterred hinterland occupation we conclude that underwater investigation is essential if hypotheses of early human adaptation and dispersal are to be fully tested

pleistocene sea level fluctuations and human evolution on the southern coastal plain of south africa abstract humans evolved in africa but where and how remain unclear here it is proposed that the southern coastal plain scp of south africa may have served as a geographical point of origin through periodic expansion and contraction isolation in response to glacial interglacial changes in sea level and climate during pleistocene interglacial highstands when sea level was above 75 m human populations were isolated for periods of 360 3400 25 yr generations on the scp by the rugged mountains of the cape fold belt climate and vegetation barriers the scp expands five fold as sea level falls from 75 to 120 m during glacial maxima to form a continuous unobstructed coastal plain accessible to the interior an expanded and wet glacial scp may have served as a refuge to humans and large migratory herds and resulted in the mixing of previously isolated groups the expansive glacial scp habitat abruptly contracts by as much as one third in 300 yr during the rapid rise in sea level associated with glacial terminations rapid flooding may have increased population density and competition on the scp to select for humans who expanded their diet to include marine resources or hunted large animals successful adaptations developed on an isolated scp are predicted to widely disperse during glacial terminations when the scp rapidly contracts or during the initial opening of the scp in the transition to glacial maxima the hypothesis that periodic expansion and contraction of the scp as well as the coastal plain of north africa contributed to the stepwise origin of our species over the last 800 thousand years kyr is evaluated by comparing the archeological dna and sea level records these records generally support the hypothesis but more complete and well dated records are required to resolve the extent to which sea level fluctuations influenced the complex history of human evolution

one dimensional numerical modeling of sediment transport and bed deformation in open channels 1 a one dimensional numerical model for simulating unsteady flow and sediment transport in open channels is presented and tested the flow hydrodynamics is represented by the shallow water equations and the bed morphodynamics is represented by the exner equation and an additional equation describing the nonequilibrium sediment transport sediment size distribution is represented by the median grain diameter and the standard deviation instead of the usual modeling with multiple particle size classes various methods for computing bed elevation changes at a cross section due to erosion or deposition of sediment are proposed and tested including an innovative approach that relates the spatial pattern of erosion and deposition rates to boundary shear stress distribution which is calculated by the merged perpendicular method an explicit finite difference scheme is employed for solving the water and sediment governing equations the pertinence of the model is examined for two hypothetical cases the model is then tested on one set of laboratory experiments on bed degradation under steady flow showing excellent model data fit and indicating that incorporating a nonequilibrium sediment transport equation into the model structure is an important element in reproducing the bed degradation process finally the model is applied to simulate the morphological changes taking place in the ha ha river quebec after the failure of the ha ha dyke on july 1996 relevant results can be obtained in terms of changes in longitudinal bed profile and cross sectional geometry as well as water levels although some discrepancies are obtained between the simulated and surveyed cross sectional geometries mainly because bank failure and channel widening are not modeled

spatial heterogeneity of near bed hydraulics above a patch of river gravel 1 the spatial heterogeneity of fully turbulent near bed flows above gravel river beds is examined using a realistic replica of a natural gravel patch in a large flume three dimensional velocity time series were obtained at four heights 0 008 0 1 m above the local bed in each of 99 closely spaced verticals for three flows of increasing intensity the spatial heterogeneity of time averaged velocities and root mean square velocity fluctuations increases under stronger flows and closer to the bed however streamwise velocity becomes spatially homogeneous at a distance from the bed of between 2 4 times the median bed elevation heterogeneity in the direction of the velocity vector is independent of mean flow velocity but in all cases it decreases approximately linearly with distance above the surface skewness of the instantaneous velocity distributions suggests that slowly moving fluid emanating from the near bed region impinges upon higher levels with greater frequency and greater spatial coverage as the average flow velocity increases spatial heterogeneity in turbulent kinetic energy increases with flow velocity and maxima occurr at positions that intercept layers of intense vortex shedding in the lee of obstacle crests the spatial organization of the flow properties is nonrandom and consistent across the three flows simple regression models are developed to provide a basis for investigating the heterogeneity of near bed flow at the patch scale 2 m2 in gravel bed rivers

identifying the effects of parameter uncertainty on the reliability of riverbank stability modelling bank retreat is a key process in fluvial dynamics affecting a wide range of physical ecological and socioeconomic issues in the fluvial environment to predict the undesirable effects of bank retreat and to inform effective measures to prevent it a wide range of bank stability models have been presented in the literature these models typically express bank stability by defining a factor of safety as the ratio of driving and resisting forces acting on the incipient failure block these forces are affected by a range of controlling factors that include such aspects as the bank profile bank height and angle the geotechnical properties of the bank materials as well as the hydrological status of the riverbanks in this paper we evaluate the extent to which uncertainties in the parameterization of these controlling factors feed through to influence the reliability of the resulting bank stability estimate this is achieved by employing a simple model of riverbank stability with respect to planar failure which is the most common type of bank stability model in a series of sensitivity tests and monte carlo analyses to identify for each model parameter the range of values that induce significant changes in the simulated factor of safety these identified parameter value ranges are compared to empirically derived parameter uncertainties to determine whether they are likely to confound the reliability of the resulting bank stability calculations our results show that parameter uncertainties are typically high enough that the likelihood of generating unreliable predictions is typically very high 80 for predictions requiring a precision of 15 because parameter uncertainties are derived primarily from the natural variability of the parameters rather than measurement errors much more careful attention should be paid to field sampling strategies such that the parameter uncertainties and consequent prediction unreliabilities can be quantified more robustly

numerical simulation of widening and bed deformation of straight sand bed rivers ii model evaluation in this paper the numerical model presented in the companion paper is tested and applied assessment of model accuracy was based on two approaches first predictions of evolution of a 13 5 km reach of the south fork of the forked deer river in west tennessee were compared to observations over a 24 yr period results suggest that although the model was able to qualitatively predict trends of widening and deepening quantitative predictions were not reliable simulated widths and depths were within 15 of the corresponding observed values but observed change in these parameters at the study sites were also close to these values simulated rates of depth adjustment were within 15 of observed rates but observed rates of channel widening at the study sites were approximately three times those simulated by the model in the second approach the model was used to generate relationships between stable channel width and bank full discharge the model was able to successfully replicate the form of empirically

holocene geoarchaeology of the sixteen mile beach barrier dunes in the western cape south africa holocene evolution and human occupation of the sixteen mile beach barrier dunes on the southwest coast of south africa between yzerfontein and saldanha bay are inferred from the radiocarbon ages of calcareous dune sand limpet shell patella spp manuports and gull dropped white mussel shells donax serra a series of coast parallel dunes have prograded seaward in response to an overall marine regression since the mid holocene with dated shell from relict foredunes indicating periods of shoreline progradation that correspond to drops in sea level at around 5900 4500 and 2400 calibrated years before the present cal yr b p however the active foredune extensively covered by a layer of gull dropped shell has migrated 500 m inland by the recycling of eroded dune sand in response to an approximate 1 m sea level rise over the last 700 yr manuported limpet shells from relict blowouts on landward vegetated dunes indicate human occupation of coastal dune sites at 6200 and 6000 cal yr b p and help to fill the mid holocene gap in the regional archaeological record coastal midden shells associated with small hearth sites exposed in blowouts on the active foredune are contemporaneous 1600 500 cal yr b p with large midden sites on the western margin of langebaan lagoon and suggest an increase in marine resource utilisation associated with the arrival of pastoralism in the western cape

disturbance stream incision and channel evolution the roles of excess transport capacity and boundary materials in controlling channel response abstract channel incision is part of denudation drainage network development and landscape evolution rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non incised surfaces large anthropogenic disturbances similar to large or catastrophic natural events greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power field examples of channel responses to antrhopogenic and natural disturbances are presented for fluvial systems in the mid continent and pacific northwest usa and central italy responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems similar disturbances are shown to result in varying relative magnitudes of vertical and lateral widening processes and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks this apparent contradiction is explained through an analysis of temporal adjustments to flow energy shear stress and stream power with time numerical simulations of sand bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half show identical adjustments in flow energy and the rate of energy dissipation the processes that dominate adjustment and the ultimate stable geometries however are vastly different depending on the cohesion of the channel banks and the supply of hydraulically controlled sediment sand provided by bank erosion the non linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems the sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape with erosion from the channel banks generally the dominant source disturbances that effect available force stream power or flow energy or change erosional resistance such that an excess of flow energy occurs can result in incision channel incision therefore can be considered a quintessential feature of dis equilibrated fluvial systems

effectiveness of grade control structures in reducing erosion along incised river channels the case of hotophia creek mississippi abstract herein we undertake a geomorphological analysis in which spatial and temporal trends of bed and bank erosion along an 18 km length of hotophia creek mississippi are estimated for the period between 1961 and 2050 the evaluation was undertaken for two scenarios of channel response to channelization during 1961 1963 one scenario represents the actual response of the channel and includes the effects of installing a series of grade control structures gcs between 1980 and 1996 while the other represents a hypothetical scenario in which the channel is left to adjust naturally this allows the effectiveness of gcs in reducing in channel erosion to be assessed the analysis relies on the availability of channel survey data to develop empirical bed and bank response models for each adjustment scenario supplemented by bank stability modelling to predict future rates of bank erosion results indicate that channel erosion rates decline nonlinearly with respect to time since 1961 for both adjustment scenarios however by the year 2050 the with gcs adjustment scenario results in the cumulative removal of some 663 000 9 extra tonnes of sediment relative to the without gcs scenario most 63 of this excess is derived from enhanced bed erosion during 1976 1985 and 1985 1992 with the remainder derived from increased bank erosion during 1985 1992 detailed analysis of the patterns of erosion and deposition and their association with the gcs provides evidence to support the view that gcs installed along hotophia creek have for the most part been ineffective in reducing channel erosion rates this is because the gcs were installed too late to prevent bed degradation caused by the 1961 1963 channelization migrating upstream in addition some structures have disrupted the downstream transmission of bed material from eroded reaches upstream exacerbating bed degradation and bank erosion in incised reaches downstream

doppler radar sounding of volcanic eruption dynamics at mount etna besides their common use in atmospheric studies doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field we present the observations made with a mid power uhf doppler radar voldorad during a 7 h strombolian eruption at the se crater of mount etna on 11 12 october 1998 main characteristics of radar echoes are retrieved from analysis of doppler spectra recorded in the two range gates on either side of the jet axis from the geometry of the sounding the contribution of uprising and falling ejecta to each doppler spectrum can be discriminated the temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before during and after its paroxysm moreover during the sharp decrease of eruptive activity following the paroxysm detailed analysis of video from camera recording radar and seismic measurements reveals that radar and video signals start to decrease simultaneously approximately 2 5 min after the tremor decline this delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the se crater detailed analysis of eruptive processes has been also made with voldorad operating in a high sampling rate mode signature of individual outburst is clearly identified on the half part of doppler spectra corresponding to rising ejecta temporal variations of the backscattered power exhibit quasi periodic undulations whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time periodicity of power variations between 3 8 and 5 5 s is in agreement with the occurrence of explosions visually observed at the se vent maximum vertical velocities of over 160 m s 1 were measured during the paraoxysmal stage and the renewed activity finally by using a simplified model simulating the radar echoes characteristics we show that when voldorad is operating in high sampling rate mode the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam

interplay between spatially explicit sediment sourcing hierarchical river network structure and in channel bed material sediment transport and storage dynamics understanding how sediment moves along source to sink pathways through watersheds from hillslopes to channels and in and out of floodplains is a fundamental problem in geomorphology we contribute to advancing this understanding by modeling the transport and in channel storage dynamics of bed material sediment on a river network over a 600 year time period specifically we present spatiotemporal changes in bed sediment thickness along an entire river network to elucidate how river networks organize and process sediment supply we apply our model to sand transport in the agricultural greater blue earth river basin in minnesota by casting the arrival of sediment to links of the network as a poisson process we derive analytically under supply limited conditions the time averaged probability distribution function of bed sediment thickness for each link of the river network for any spatial distribution of inputs under transport limited conditions the analytical assumptions of the poisson arrival process are violated due to in channel storage dynamics where we find large fluctuations and periodicity in the time series of bed sediment thickness the time series of bed sediment thickness is the result of dynamics on a network in propagating altering and amalgamating sediment inputs in sometimes unexpected ways one key insight gleaned from the model is that there can be a small fraction of reaches with relatively low transport capacity within a nonequilibrium river network acting as bottlenecks that control sediment to downstream reaches whereby fluctuations in bed elevation can dissociate from signals in sediment supply

sedimentation from strong volcanic plumes 1 we develop descriptions of the key processes influencing tephra dispersal from strong volcanic plumes these are characterized by the development of a subvertical eruption column in the atmosphere that forms a spreading current at a level of neutral buoyancy we describe the propagation of the spreading current due to both gravity and wind advection using scaling arguments and a simplified geometry to model particle sedimentation in a windy as well as a wind free environment new parameterizations are used to describe the wind field below the spreading current particle aggregation and particle density variations we conducted a broad study to investigate the effects of these processes and made comparisons with field observations the greatest variations resulted from wind advection below the spreading current which shifts the plume corner mass accumulation and the position of transitions in fallout regimes downwind particle aggregation strongly depends on the initial grain size distribution and significantly affects deposit thinning depending on the aggregate size and density and on the relative amount of aggregating particles small aggregates can reproduce secondary maxima of mass accumulation when sedimenting in a wind field variation of particle density also affects the resulting thinning trend the model provides acceptable reproduction of observations of the propagation of the spreading current tested with the plinian phase of the 18 may 1980 eruption of mount st helens and of observed thinning of tephra fallout deposits tested with deposits from mount st helens 1980 askja d 1875 quizapu 1932 minoan eruption 3 6 ka b p novarupta a 1912 and hudson 1991