a phase resolving cross shore sediment transport model for beach profile evolution abstract a phase resolving wave transformation module is combined with an intra wave sediment transport module to calculate the on offshore sediment transport rates the wave module is based on the boussinesq equations extended into the surf zone the vertical variation of the mean undertow and the intra wave sediment concentrations are calculated the net sediment transport rates are calculated and the equation for conservation of sediment is solved to predict the beach profile evolution the results of the present paper showed that the undertow contribution to the sediment transport rates is not dominating in all parts of the surf zone even for eroding beaches suggesting that other contributions should not be neglected the present model also showed that for the same offshore wave energy the time series of the oscillatory motion is important and that the effect of wave groups cannot be disregarded

field investigation of bedform morphodynamics under combined flow abstract fieldwork was conducted at the red cliff sandbar located in the upper humber estuary in order to investigate bedform dynamics under different hydrodynamic conditions related to combinations in tidal and wave generated currents a fixed mooring was deployed to obtain current flow and wave properties across the sand bar site at both spring and neap tidal flows and during conditions of high wind generated waves a terrestrial laser scanner was used to scan the sandbar during low water across the various forcing conditions acquiring detailed information of ripple geometries under spring tide and calm wave conditions two dimensional asymmetrical ripples with straight crest lines were generated on the sandbar surveys were also conducted during strong winds prior to high wind waves two dimensional symmetrical and washed out ripples were observed during spring and neap tidal conditions respectively the lengths of these ripples were almost the same as those generated under current only conditions but their heights were relatively smaller after the strong wind periods 2d current induced ripples were replaced by flatbed conditions and 2d symmetrical wash out ripples which indicates wave induced bed shear stress is enhanced by the presence of a tidal current most pertinently our study reveals discrepancies between field observations and existing predictions of bed configurations that are largely based on laboratory investigations this is likely due to a larger grain size distribution in field conditions and due to turbulence dampening by high concentrations of suspended clay particles the study thus highlights a need to extend a range of field investigations that explore the current deficiencies in our abilities to predict bedforms and bedform dynamics in estuarine systems

sediment transport in oscillatory flow over flat beds the motion of sediment in oscillatory flow over a flat bed is investigated both theoretically and experimentally the theory makes use of results for the relationship between stress and strain in steady flow the experiments were performed in two different oscillatory flow water tunnels with two different sediments they were designed to measure the variation of velocity with height in the moving layer of sediment under conditions of intense sediment transport although certainly not perfect the agreement between theory and experiment is reasonable provided the stress ratio is chosen correctly calculated values of the mean sediment transport rate during a half cycle are compared with the predictions of currently available formulas

the hiding exposure effect revisited a method to calculate the mobility of bimodal sediment mixtures abstract predicting seabed mobility is hampered by the limited accuracy of sediment transport models when the bed is composed of mixed sediments the hiding exposure he effect modifies the threshold of motion of individual grain classes in sediment mixtures and its strength is dependent on the grain size distribution however an appropriate method of predicting this effect for bimodal sediment mixtures remains to be developed the prototypical example of a bimodal mixture is that consisting of a well sorted sand and gravel for the fine and coarse fractions respectively through a comprehensive series of laboratory experiments the he effect has been quantified for a full range of sand gravel mixtures from pure sand to pure gravel the choice of which has been underpinned by an integrated study of offshore geophysical and sedimentological data found in coastal and shelf seas in the sand gravel mixtures used in the present study the critical shear stress needed to mobilise the sand and gravel fractions increased by up to 75 and decreased by up to 64 respectively compared to that needed to mobilise well sorted sediment of similar size the he effect was found to be dependent on the percentage of gravel coarse mode present in the bimodal mixture whereby the effect for the mixture is the weighted sum of the he effect for the fine and coarse modes

medium term 2dh coastal area modelling the state of the art in depth averaged mathematical modelling of 3 d coastal morphology is described for the medium term morphodynamic model type in which constituent models of waves currents and sediment transport based on first physical principles are linked together to describe the time evolution of the bed topography various aspects of the combined system of equations are discussed such as its mathematical character its inherent stability and its equilibrium state the results of an intercomparison of different models are shown for two test cases and the potentials and limitations of the model concept are discussed

linkages between climate fire and vegetation in southwest china during the last 18 5ka based on a sedimentary record of black carbon and its isotopic composition abstract fires are sensitive to climate changes and in addition they have a major influence on the global carbon cycle land surface properties and chemical composition of the atmosphere which in turn can affect the climate system projecting the impacts of future climate changes on fire mediated biogeochemical processes requires understanding of how various climatic factors influence regional fire regimes however long term variations in fire activity and their controls are poorly known in this study we report the concentration of black carbon bc and its isotopic composition 13cbc in an upland lake sediment core from southwest china in order to elucidate linkages between changes in fire regime climate and vegetation over the past 18 5 ka the results show that the sedimentary bc content recorded variations in fire activity and exhibits a close negative correlation with the amount of precipitation delivered by the indian summer monsoon ism during heinrich event 1 18 5 15 0 cal ka bp the younger dryas 12 8 11 1 cal ka bp and the mid to late holocene the weakening of the ism led to greatly increased fire activity in southwest china in the last 1 0 ka the bc record may have been affected by human activity in the catchment which may have caused an increased influx of minerogenic material to the lake thereby diluting the sedimentary bc concentration the general trend of changing vegetation compositions inferred from the 13cbc values also tracks the variations of the ism suggesting a change from mixed c3 c4 plant ecosystems during the last glacial maximum to c3 plant dominated forest during the holocene however the abundance of c4 plants may be overestimated during the bolling allerod warm interval and the early holocene due to the fact that fires were probably limited to the savanna ecosystem in the valleys and low lying basins

groin impacts on updrift morphology physical and numerical study abstract groins interfere with coastal dynamics and sediment transport leading to sediments accumulation at the updrift side while at downdrift the erosive process is anticipated due to the lack of sediments to improve numerical modeling capacity to simulate the groins impacts it is necessary to totally understand the shoreline evolution along time and its relationship with the cross shore profiles shape the main goal of this work was to analyze and compare the performance of physical and numerical studies on evaluating the evolution of updrift cross shore profiles geometry and shoreline position after the construction of a groin this study analyzed a coastal stretch updrift of the groin at a prototype and model scales considering the analytical formulation of pelnard considere the numerical model ltc long term configuration and the laboratory tests the laboratory tested scenario was designed with the aim to gather results which could be analyzed and compared with numerical simulations from ltc coelho 2005 allowing its improvement and with the pelnard considere 1956 analytical formulation both at model scale the developed study shows an important difference between ltc and pelnard considere 1956 approaches because the analytical solution for the shoreline in equilibrium does not include the wave refraction effects over the updated bathymetry ltc observed trend of the equilibrium shoreline is not parallel to the initial shoreline and this behavior was confirmed in the laboratory tests it was also observed that the sediment transport capacity has very small impact on the ltc shoreline configuration despite the refraction effects over the updated bathymetries along time the profile shape obtained in laboratory includes bed forms difficult to reproduce in long term numerical modeling evaluation of cross shore profiles because ltc transversal behavior is only based on geometrical considerations and does not represent cross shore sediment transport and its impact on profile geometry

recent advances in modeling swash zone dynamics influence of surf swash interaction on nearshore hydrodynamics and morphodynamics the role of the swash zone in influencing the whole nearshore dynamics is reviewed with a focus on the interaction between surf and swash zone processes local and global hydromorphodynamic phenomena are discussed in detail and a description of the overall swash zone operation is given the effects of swash zone boundary conditions are highlighted together with the importance of surf zone boundary conditions major emphasis is placed on illustrating the interactions of various hydrodynamic modes which in turn control the swash and surf zone morphology finally methods to account for swash zone processes in coastal models with different temporal and spatial resolutions are proposed

modeling bottom roughness in the presence of wave generated ripples 1 near bottom wave current and microtopography measurements have been obtained at the leo 15 site off the southern coast of new jersey the measurements are used in conjunction with a recently calibrated continental shelf bottom boundary layer model to refine estimates of bottom roughness for a rippled bed under combined wave and current flows in the presence of nearly two dimensional wave generated ripples application of the grant and madsen 1979 type bottom boundary layer model produces estimates of the bottom roughness felt by the current that are several times greater than established formulations for rippled beds under waves modifying one of the primary length scales in the eddy viscosity profiles returns estimates that are more consistent with the wave roughness results application of the roughness model in conjunction with a refined empirical ripple geometry model produces highly accurate estimates of measured shear velocities and apparent roughnesses during a moderate storm the demonstrated accuracy of this simple combined roughness and ripple geometry formulation reveals its utility when considering applications of coupled bottom boundary layer shelf circulation models

model based interpretation of sediment concentration and vertical flux measurements in a shallow estuarine environment a one dimensional numerical model describing tidally varying vertical mixing and settling was used to interpret sediment concentrations and vertical fluxes observed in the shoals of south san francisco bay by two acoustic doppler velocimeters advs at elevations of 0 36 m and 0 72 m above bed measured sediment concentrations changed by up to 100 g m 3 over the semidiurnal tidal cycle these dynamics were dominated by local resuspension and settling multiple particle class models suggested the existence of a class with fast settling velocities ws of 9 0 10 4 m s 1 in spring and 5 8 10 4 m s 1 in fall and a slowly settling particle fraction ws of 1 10 7 m s 1 in spring and 1 4 10 5 m s 1 in fall modeled concentrations of slowly settling particles at 0 36 m were as high as 20 g m 3 during fall and varied with the spring neap cycle while fine sediment concentrations in spring were constant around 5 g m 3 analysis of in situ water column floc size distributions suggested that floc properties in the lower part of the water column were most likely governed by particle size distribution on the bed and not by coagulation validating our multiple particle size approach a comparison of different sediment bed models with respect to model performance sensitivity and identifiability suggested that the use of a sediment erosion model linear in bottom shear stress b e m b c was the most appropriate choice to describe the field observations when the critical shear stress c and the proportionality factor m were kept constant

geometry prediction for wave generated bedforms published data on wave generated bedform geometry from field and laboratory measurements are re evaluated in terms of relative fluid orbit diameter at the bed and a wave period parameter d 8t2 laboratory bedform data shows a strong dependence on these parameters however for very small values of the period parameter typically field data there is evidence that bedform length depends only on sediment grain size the disappearance of sediment bedforms at high fluid velocities apparently occurs through a decrease in bedform height improved methods for the prediction of bedform geometry under waves are provided in graphical form and computer code

bed load sediment transport on steep longitudinal slopes the results of new experiments on bed load transport on steep longitudinal slopes are presented the experiments were carried out in a large tilting duct with a fine sand median grain diameter of 208 micro m covering the range of slope angles beta of angle of repose for the sand phi sub s of 32 deg 0 56 rad measurements included bed shear stress flow discharge and the sediment pickup rate on slopes the results suggest that existing theories for bed load transport on slopes are not adequate for all bed slopes and the new data are used to formulate a semiempirical relation that satisfactorily predicts the transport rate on horizontal as well as mild and steep slopes the following are two important findings correcting the threshold term for slope in the meyer peter and muller formula adequately describes the transport rate on positive slopes elevation increasing in direction of flow and on small negative slopes while on steeper negative slopes an extra correction term is required although the value of beta phi sub s at which this term becomes necessary is a function of shear stress and the influence of slope vanishes at high shear stresses

formation of fine sediment deposit from a flash flood river in the mediterranean sea we identify the mechanisms controlling fine deposits on the inner shelf in front of the besos river in the northwestern mediterranean sea this river is characterized by a flash flood regime discharging large amounts of water more than 20 times the mean water discharge and sediment in very short periods lasting from hours to few days numerical model output was compared with bottom sediment observations and used to characterize the multiple spatial and temporal scales involved in offshore sediment deposit formation a high resolution 50 m grid size coupled hydrodynamic wave sediment transport model was applied to the initial stages of the sediment dispersal after a storm related flood event after the flood sediment accumulation was predominantly confined to an area near the coastline as a result of preferential deposition during the final stage of the storm subsequent reworking occurred due to wave induced bottom shear stress that resuspended fine materials with seaward flow exporting them toward the midshelf wave characteristics sediment availability and shelf circulation determined the transport after the reworking and the final sediment deposition location one year simulations of the regional area revealed a prevalent southwestward average flow with increased intensity downstream the circulation pattern was consistent with the observed fine deposit depocenter being shifted southward from the river mouth at the southern edge bathymetry controlled the fine deposition by inducing near bottom flow convergence enhancing bottom shear stress according to the short term and long term analyses a seasonal pattern in the fine deposit formation is expected

numerical modelling of the morphological response induced by low crested structures in lido di dante italy this paper analyses the morphological response induced by low crested structures on the adjacent seabed with particular interest in the erosion patterns that frequently develop at gaps and roundheads the mechanisms responsible for erosion processes are examined by means of morphodynamic simulations with the numerical suite mike 21 cams developed by dhi water environment the main purpose of the paper is to verify how and how far a commercial code can predict bed evolution in vicinity of defence structures in a real case in order to get information that may be very useful for structure design and possible maintenance of existing works the code is applied to a long term simulation on a study site that is characterized by a composite intervention and suffers from severe erosion lido di dante italy the simulation covers the period one year and a half between two available multi beam surveys in order to have a detailed real bathymetry as starting point and another one as accurate as the first to compare numerical with surveyed results all the other input data for the model as waves tide wind and sediment characteristics are derived from measurements in the area moreover the code is calibrated using wave and current data acquired during a field campaign the bed evolution derived from simulations shows a good agreement with the survey both in the locations and in the intensity of erosive and depositional areas a sensitivity analysis of results to some selected modelling parameters is performed on a shorter simulation period one month showing that accounting for bed slope in sediment transport modelling has greater effects in bathymetry evolution than the use of a complex sediment bathymetry or the representation of wave diffraction

a surfzone morphological diffusivity estimated from the evolution of excavated holes downslope gravity driven sediment transport smooths steep nearshore bathymetric features such as channels bars troughs cusps mounds pits scarps and bedforms downslope transport appears approximately as a diffusive term in the sediment continuity equation predicting changes in bed level with a morphological diffusivity controlling the rate of seafloor smoothing despite the importance of surfzone sediment transport and morphological evolution the size of the downslope transport term in nearshore models varies widely and theories have not been tested with field measurements here observations of the infill of large excavated holes in an energetic inner surf zone provide the first opportunity to infer the morphological diffusivity in the field the estimated diffusion coefficient is consistent with a theoretical bedload morphological diffusivity that scales with the three halves power of the representative bed shear stress

water masses in the humboldt current system properties distribution and the nitrate deficit as a chemical water mass tracer for equatorial subsurface water off chile abstract three sections are used to analyze the physical and chemical characteristics of the water masses in the eastern south pacific and their distributions oceanographic data were taken from the scorpio may june 1967 piquero may june 1969 and krill june 1974 cruises vertical sections of temperature salinity dissolved oxygen nitrate nitrite phosphate and silicate were used to analyze the water column structure five water masses were identified in the zone through t s diagrams subantarctic water subtropical water equatorial subsurface water antarctic intermediate water and pacific deep water their proportions in the sea water mixture are calculated using the mixing triangle method vertical sections were used to describe the geographical distributions of the water mass cores in the upper 1500 m several characteristic oceanographic features in the study area were analyzed the shallow salinity minimum displacement towards the equator the equatorial subsurface salinity maximum associated with a dissolved oxygen minimum zone and a high nutrient content displacement towards the south and the equatorward intermediate antarctic salinity minimum associated with a dissolved oxygen maximum the nitrate deficit generated in the denitrification area off peru and northern chile is proposed as a conservative chemical tracer for the equatorial subsurface waters off the coast of chile south of 25 s

two dimensional two phase granular sediment transport model with applications to scouring downstream of an apron we present a two dimensional two phase model for non cohesive sediment transport this model solves concentration weighted averaged equations of motion for both fluid and sediment phases the model accounts for the interphase momentum transfer by considering drag forces a collisional theory is used to compute the sediment stresses while a two equation k e fluid turbulence closure is implemented a benchmark sediment transport problem concerning the scouring downstream of an apron is carried out as an example and numerical results agree with existing experimental data

latest pleistocene and holocene vegetation and climate history inferred from an alpine lacustrine record northwestern yunnan province southwestern china abstract reconstructing past changes of the summer monsoon in southwestern china is key to understanding the paleoclimatic dynamics of the indian summer monsoon southwest monsoon a major tropical system with impacts beyond the region high resolution pollen and conifer stoma data are presented from a sediment core tiancai lake located just below treeline in the hengduan mountains northwestern yunnan province of china these data record changes in vegetation and climate for the latest pleistocene and holocene c the last 12 230 cal yr bp and by comparing these results with the pollen records from the nearby erhai lake and lugu lake a regional climate history in yunnan province can be inferred during the period of 12 230 11 510 cal yr bp open alpine meadow around tiancai lake indicates a relatively cold and dry climate corresponding to the younger dryas cold event yd between 11 510 and 10 000 cal yr bp the vegetation types around tiancai lake changed into picea and abies forest and alpine rhododendron shrubland reflecting increases in the temperature and humidity from 10 000 to 6100 cal yr bp tsuga forest expanded persistently toward tiancai lake indicating further climate changed towards warmer and wetter conditions between 6100 and 3410 cal yr bp tsuga forest was the dominant vegetation type nearest to tiancai lake denoting warm and humid climatic conditions corresponding to the holocene climatic optimum in the northwestern yunnan province tsuga forest in mountains around lugu lake shrank quickly at 3410 cal yr bp reflecting significant decline of humidity since 2930 cal yr bp tsuga forest around tiancai lake shrank gradually indicating a fluctuating decrease of temperature these climatic changes reflect changes of the southwest monsoon namely warm and humid climate indicating strong southwest monsoon cold and dry climate denoting weak southwest monsoon

wave induced sediment transport and onshore sandbar migration the 25 m onshore migration of a nearshore sandbar observed over a 5 day period near duck nc is simulated with a simplified computationally efficient wave resolving single phase model the modeled sediment transport is assumed to occur close to the seabed and to be in phase with the bottom stress neglected intergranular stresses and fluid granular interactions likely important in concentrated flow are compensated for with an elevated relative to that appropriate for a clear fluid model roughness height that gives the best fit to the observed bar migration model results suggest that when mean current induced transport is small wave induced transport leads to the observed onshore bar migration based on the results from the simplified phase resolving model a wave averaged energetics type model e g only moments of the near bottom velocity field are required with different friction factors for oscillatory and mean flows is developed that also predicts the observed bar migration although the assumptions underlying the models differ the similarity of model results precludes determination of the dominant mechanisms of sediment transport during onshore bar migration

upwelling in the taiwan strait during the summer monsoon detected by satellite and shipboard measurements the taiwan strait is located at the confluence of the east china sea and the south china sea in the west pacific ocean several upwelling zones in the taiwan strait are noted for their high fisheries production these upwelling zones have been studied in the past decade but we have no overall picture on the size and temporal changes of these upwelling zones due to only limited in situ observation over short periods of time the present paper investigates upwelling in the taiwan strait with satellite remote sensing data of noaa derived sea surface temperature sst and seawifs derived chlorophyll a chl a and shipboard measurements during summer 1998 results reveal five upwelling zones 1 coastal upwelling near pingtan island ptu 2 coastal upwelling between meizhou and xiaman mxu 3 big coastal upwelling near dongshan island dsu sometimes extending to offshore 4 small occasional upwelling near the penghu island phu and 5 an intensive upwelling in the taiwan bank tbu tbu was extensively studied over summer 1998 results showed that the tbu looks like a banana in shape in the southern edge of the taiwan bank it intensifies in its southeastern edge the size of tbu was about 2500 3000 km 2 with a mean value of 2796 km 2 the temperature difference between the upwelling and non upwelling areas was about 2 5 3 5 jc for early august with a mean value of 2 3 jc on july to october series of images indicated a short term variation of tbu and showed that high chl a concentrations up to 0 8 2 mg m 3 zones coincided with low sst 25 26 jc zones in terms of location time and shape these upwelling zones change with size and intensity center from time to time field measurements of water temperature salinity and chl a in 1998 are consistent with satellite measurements d 2002 elsevier science inc all rights reserved

short term variability of chlorophyll associated with upwelling events in the taiwan strait during the southwest monsoon of 1998 seawifs chlorophyll andavhrr sst time series as well as in situ measurements in august 1998 were usedto evaluate short term variability of chlorophyll associatedwith upwelling events in the western taiwan strait the extent of eutrophic waters seawifs chlorophyll4 1m g m 3 colder upwelling waters and upwelling index were calculated for the western strait andfor the northern andsouthern portions large extents of eutrophic waters were always accompanied by colder upwelling waters indicative of the tight coupling of chlorophyll with upwelling activities the phytoplankton growth laggedthe upwelling activities by about 2 days the temporal changes of upwelling events were founddifferent in the northern andsouthern taiwan strait a short relaxation of upwelling probably occurredbetween early and mid august in the northern portion one strong upwelling event likely lasted from early through mid august peaking before august 13 in the southern portion it resultedin chlorophyll enhancement but with d ifferent timing of chlorophyll reaching maximum values in these two upwelling zones the duration of one upwelling event in the western taiwan strait in august was estimated to be about 12 days it seems that two distinctive northern and southern upwelling systems in the western taiwan strait were connectedwith andaffectedby the east china sea andthe south china sea respectively

a coupled modeling scheme for longshore sediment transport of wave dominated coasts a case study from the southern baltic sea abstract a modeling scheme based on dynamic coupling of a high resolution 1d cross shore model to a 2dh area model is developed to calculate the total longshore sediment transport lst rate in wave dominated coasts the purpose of this coupling strategy aims at resolving the lst with a high resolution both temporally and spatially inside the surf zone and with a coarser spatial resolution seaward of the surf zone the 2dh area model operates on a fixed pre designed regional grid parent grid and the 1d cross shore model is dynamically coupled to the boundary of the parent grid with a time varying domain starting from the first wave breaking point and ending at the maximum wave set up point the time varying domain is generated in the 1d model by resolving the landward wave propagation from the offshore conditions provided by the 2dh area model at every time step with a high resolution cell size the 1d model resolves the wave propagation processes and resulting lst along the profile the coupled model is applied to study the lst in the pomeranian bight at the southern baltic sea simulation results are compared with three other different hierarchical modeling methods from empirical formulas such as cerc and kamphuis to a 2dh area simulation the comparative study indicates that the dynamically coupled model can be a reliable tool in practical applications especially for the areas where hydrodynamics is controlled by complex bathymetry e g multiple longshore bars or morphologically induced circulation patterns

low nutrient and high chlorophyll a coastal upwelling system a case study in the southern taiwan strait using the field data during four summer cruises of 2005 2006 2008 and 2013 an interesting phenomenon of low nutrient and high chl a lnhc was observed in the coastal upwelling system off dongshan in the southern taiwan strait the results indicated that the upwelling region was dominated by cold 33 9 psu upwelled water and the concentration of nitrate 1 m and phosphate 0 1 m was very low while with high chl a content mean 1 98 3 56 g l 1 maximum 8 3 g l 1 during the stronger summer upwelling cruise of 2005 2006 and 2008 the upwelled water originated from the 50 100 m layer off shanwei with a low concentration of nitrate 7 5 mol l 1 and phosphate 0 5 mol l 1 which was transported to shantou dongshan by the northeastward bottom current during the time that the upwelled water moved the long distance 150 300 km from shanwei to shantou dongshan and was transported through the euphotic zone alongshore the phytoplankton grew rapidly due to the favorable temperature n p ratio and illumination and consumed most of the nutrients in the upwelled water these unique physical chemical and biological processes are the main reasons for the formation of the low nutrients high chlorophyll a in this coastal upwelling system in the southern taiwan strait

parameterization of intrawave ripple averaged sediment pickup above steep ripples near bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models in the present study numerical results from a two dimensional reynolds averaged navier stokes model are used to assess the functional relationship of intrawave ripple averaged sediment pickup above steep ripples the numerical model provides intrawave time histories of ripple averaged near bed velocities and turbulence which are qualitatively interrogated to determine pickup functional relationships several specific sediment pickup formulations are implemented within the numerical model expressions relating pickup to near bed velocity or near bed turbulent kinetic energy via the bed shear stress and expressions relating pickup to near bed shear production of turbulent kinetic energy these are then tested via model data comparisons of near bed suspended sediment concentration the results show that the traditional functions relating sediment pickup to near bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple averaged framework instead relating sediment pickup to near bed turbulence quantities such as turbulent kinetic energy or shear production of turbulent kinetic energy significantly improves the numerical predictions for these conditions this article is protected by copyright all rights reserved

the spatial temporal patterns of asian summer monsoon precipitation in response to holocene insolation change a model data synthesis highlights slice and transient simulations of holocene climate change were performed spatial temporal patterns of holocene asian summer precipitation are investigated a tripole pattern of summer precipitation can be seen over monsoonal asia insolation change is a key factor for holocene asian summer monsoon change internal feedbacks are important to holocene asian summer precipitation changes abstract paleoclimate proxy records of precipitation effective moisture show spatial temporal inhomogeneous over asian monsoon and monsoon marginal regions during the holocene to investigate the spatial differences and diverging temporal evolution over monsoonal asia and monsoon marginal regions we conduct a series of numerical experiments with an atmosphere ocean sea ice coupled climate model the kiel climate model kcm for the period of holocene from 9 5 ka bp to present 0 ka bp the simulations include two time slice equilibrium experiments for early holocene 9 5 ka bp and present day 0 ka bp respectively and one transient simulation ht using a scheme for model acceleration regarding to the earth s orbitally driven insolation forcing for the whole period of holocene from 9 5 to 0 ka bp the simulated summer precipitation in the equilibrium experiments shows a tripole pattern over monsoonal asia as depicted by the first modes of empirical orthogonal function eof1 of h0k and h9k the transient simulation ht exhibits a wave train pattern in the summer precipitation across the asian monsoon region associated with a gradually decreased trend in the strength of asian summer monsoon as a result of the response of asian summer monsoon system to the holocene orbitally forced insolation change both the synthesis of multi proxy records and model experiments confirm the regional dissimilarity of the holocene optimum precipitation effective moisture over the east asian summer monsoon region monsoon marginal region and the westerly dominated areas suggesting the complex response of the regional climate systems to holocene insolation change in association with the internal feedbacks within climate system such as the air sea interactions associated with the el nino southern oscillation enso and shift of the intertropical convergence zone itcz in the evolution of asian summer monsoon during the holocene

morphodynamic modelling of rip channel growth the morphological evolution of a rip channel system due to normally incident waves has been investigated using two different approaches the first approach is a state of the art commercial 2dh coastal area model whereas the second approach is a linear stability analysis the results of the two different approaches have been compared and both models predict rip channel spacings and initial growth rates of similar magnitudes

intensified upwelling over a widened shelf in the northeastern south china sea 1 observational and three dimensional modeling studies reveal that the intensified upwelling in the northeastern south china sea nscs is formed as a result of intensified upslope advection of dense deep waters that cross the middle shelf toward the inner shelf over a distinctly eastward widened shelf the strongest advection occurs over the converging isobaths near the head of the widened shelf as these dense deep waters advance shoreward they are advected downstream by the quickly developed upwelling current over the inner shelf and eventually outcropped at the lee of a coastal cape dynamically the shoreward cross isobath transport over the widened shelf is geostrophically enhanced by a quasi barotropic negative westward along isobath pressure gradient force as a result of the net rate of the momentum influx and by an intensified bottom frictional transport owing to the flow confluence near the head of the widened shelf a negative pressure gradient also exists at the lee of the coastal cape over the inner shelf and locally amplifies shoreward motion induced by the respective widened shelf and the coastal promontory the along isobath variations of cross isobath transport in the water column over the middle and inner shelves interactively characterize intensified upwelling in the nscs

stratification effects by cohesive and noncohesive sediment this study focuses on stratification effects induced by the interaction between suspensions of fine grained cohesive and noncohesive sediment and the turbulent flow in estuarine and coastal environments a concise literature review reveals that this interaction may cause an appreciable modification of the vertical profiles of velocity vertical eddy viscosity diffusivity and reynolds stresses this interaction is further studied with a one dimensional vertical 1dv numerical model which includes the standard k turbulence model with buoyancy destruction terms application of this model to the experimental results by coleman 1981 shows that the measured changes in the velocity profile can be explained entirely by sediment induced buoyancy effects it is argued that capacity for cohesive sediment is cc e a fluid mud layer is formed upon the deposition of the cohesive sediment flocs contrary to the case with noncohesive sediment where the depositing grains immediately form a rigid bed thus a two layer fluid system develops causing significant damping of the vertical mixing processes decreasing the carrying capacity further this positive feed back results in a catastrophic collapse of the turbulence field and the concentration profile a scaling law for this saturation behavior is derived from classical stratified flow theory this law is sustained with a series of numerical experiments with the 1dv model it also predicts the suspended sediment concentrations observed in the yellow river to the right order of magnitude it is concluded that sediment induced buoyancy yields appreciable stratification effects at already moderate suspended sediment concentrations in the case of cohesive sediment these stratification effects can result in a catastrophic collapse of the turbulent flow field and the vertical profile of suspended sediment concentration

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

bottom turbulence on the muddy atchafalaya shelf louisiana usa 1 wave current and sediment observations collected in approximately 5 m depth on the muddy atchafalaya clinoform la usa are used to study the interaction between near bed wave induced turbulent flows and suspended sediment characteristics in a muddy environment low wave bias estimates of near bed reynolds stresses are strongly correlated with flow accelerations and suspended sediment concentration as previously observed on sandy beaches where accelerations have been associated with bed fluidization and sediment transport a detailed numerical analysis of the observations is performed based on a uni dimensional boundary layer model that accounts for the coupling between the fluid and the cohesive sediment phases the numerical simulations suggest that sediment induced stratification effects are of the same order of magnitude as turbulent dissipation and thus play a significant role in the turbulent kinetic energy balance within the tidal boundary layer however inside the wave boundary layer the ratio of stratification to shear induced turbulence production i e gradient richardson number decreases significantly and shear induced turbulence production dominates for these observations the vertical structures of currents and reynolds stresses are relatively insensitive to the exact floc size

wave ripple development on mixed clay sand substrates effects of clay winnowing and armoring based on bed form experiments in a large scale flume we demonstrate that the rate of development of wave ripples on a mixed sand clay bed under regular waves is significantly lower than on a pure sand bed even at clay fractions as low as 4 2 and that this rate of development decreases exponentially from 4 2 to 7 4 clay these experiments also showed that despite the slow growth of the bed forms in the mixed sand clay the equilibrium length and height of the wave ripples were independent of the initial bed clay fraction given that the ripple crests were composed of pure sand at the end of all the experiments that started with well mixed sand clay it is inferred that the clay was removed from the bed during the development of the wave ripples through winnowing into the water column and possibly also by sieving into the subsurface where the final clay fractions were found to be higher than the initial clay fractions these clay removal processes are interpreted to have facilitated the wave ripples to reach equilibrium lengths and heights that are similar to those in pure sand clay carrying pore flow initiated by pressure gradients between the wave ripple troughs and crests might also have contributed to the accumulation of clay in the sediment below the wave ripples the formation of the clay enriched armoring layer in the substrate is likely to further reduce erosion rates and could influence the dispersion of nutrients and pollutants in coastal seas

calculating bottom orbital velocity beneath waves abstract methods are presented for calculating directly from a known wave height period and water depth the orbital velocity produced at the sea bed by surface waves two curves are sufficient to allow the root mean square bottom orbital velocity to be calculated for wave spectra having any one of the jonswap pierson moskowitz bretschneider issc or ittc forms a third curve covers the monochromatic case other curves allow the ratio of the peak periods of bottom orbital velocity and surface elevation to be calculated for the above five spectral forms values are deduced for the height and period of the monochromatic wave which most closely matches the bottom orbital velocity obtained from a full spectral representation optimised over water depths ranging from deep water to breaking

depth averaged two dimensional numerical modeling of unsteady flow and nonuniform sediment transport in open channels a depth averaged two dimensional 2d numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered curvilinear grid the 2d shallow water equations are solved by the isimple c n algorithms with the rhie and chow s momentum interpolation technique the proposed sediment transport model adopts a nonequilibrium approach for nonuniform total load sediment transport the bed load and suspended load are calculated separately or jointly according to sediment transport mode the sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes an empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed load movement direction in channels with steep slopes flow and sediment transport are simulated in a decoupled manner but the sediment module adopts a coupling procedure for the computations of sediment transport bed change and bed material sorting the model has been tested against several experimental and field cases showing good agreement between the simulated results and measured data

asynchronous evolution of the indian and east asian summer monsoon indicated by holocene moisture patterns in monsoonal central asia abstract the numerical meta analysis of 92 proxy records 72 sites of moisture and or temperature change confirms earlier findings that the dominant trends of climatic evolution in monsoonal central asia since the last glacial roughly parallel changes in northern hemisphere summer insolation i e the period following the last glacial maximum was characterized by dry and cold conditions until 15 cal kyr bp followed by a warm wet period coincident with the bolling allerod warm period and terminated by a cold dry reversal during the younger dryas period after an abrupt increase at the start of the holocene warm and wet conditions prevailed until ca 4 cal kyr bp when moisture levels and temperatures started to decrease ordination of moisture records reveals strong spatial heterogeneity in moisture evolution during the last 10 cal kyr the indian summer monsoon ism areas northern india tibetan plateau and southwest china exhibit maximum wet conditions during the early holocene while many records from the area of the east asian summer monsoon indicate relatively dry conditions especially in north central china where the maximum moisture levels occurred during the mid holocene we assign such phenomena to strengthened hadley circulation centered over the tibetan plateau during the early holocene which resulted in subsidence in the east asian monsoonal regions leading to relatively dry conditions our observations of the asynchronous nature of the two asian monsoon subsystems on millennial time scales have also been observed on annual time scales as well as implied through the spatial analysis of vertical air motion patterns after strong ascending airflows over the tibetan plateau area that were calculated from ncep ncar reanalysis data for the last 30 years analogous with the early holocene the enhancement of the ism in a future warming world will result in an increase in the asynchronous nature of the monsoon subsystems this trend is already observed in the meteorological data from the last 15 years

combining and competing effects between precipitation and temperature on holocene fire regime evolution inferred from a sedimentary black carbon record in southwestern china wildfires are sensitive to climate change but their response to changes in temperature and precipitation on long timescales is still disputed in this study we present a 9 4 ka black carbon mass sedimentation rate bcmsr record from lake ximenglongtan xmlt southwestern china to elucidate the holocene fire regime and its linkages to climatic conditions the results indicate that the regional fire activity was low during the early holocene before 7 6 cal ka bp increased notably at 7 6 cal ka bp and continued to increase gradually during the mid to late holocene until 2 2 ka the episodes of higher fire occurrence reflected by higher bcmsr over the last 2 2 ka might be more likely related to the intensified human activities the cool and humid climate during the early holocene limited the spread of fire while warming and drying at 7 6 cal ka bp triggered higher fire occurrence instead of temperature changes in precipitation dominated fire regime variation during the mid to late holocene on millennial timescales we suggest that holocene fire variability has been predominantly controlled by the combined effects of northern hemisphere nh summer and winter insolation that influenced monsoonal precipitation and fire season temperature respectively indian ocean dipole iod events may also have affected fire incidence through influencing monsoon intensity

observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves acoustic measurements of near bed sediment diffusivity profiles are reported the observations were made over two sandy rippled beds classified as medium and fine in terms of sand grain size under slightly asymmetric regular waves for the medium sand the ripples that formed had relatively steep slopes while for the fine sand the slopes were roughly half that of the medium sand in the medium sand case the form of the sediment diffusivity profiles was found to be constant with height above the bed to a height equal approximately to the equivalent roughness of the bed ks while above this the sediment diffusivity increased linearly with height for the case of the fine sand there was no constant region the sediment diffusivity simply increased linearly with height from the bed to understand the difference between the respective diffusivity profiles advantage has been taken of the high temporal spatial resolution available with acoustic systems using intra wave ensemble averaging detailed images have been built up of the variation in concentration with both the phase of the wave and also height above the bed these intra wave observations combined with measurements of the bed forms and concepts of convective and diffusive entrainment have been used to elucidate the mixing mechanisms that underlie the form of the diffusivity profiles observed over the two rippled beds these mechanisms centre on coherent vortex shedding in the case of steeply rippled beds and random turbulent processes above ripples of lower steepness

bed load transport on the shoreface by currents and waves tide driven bed load transport is an important portion of the net annual sediment transport rate in many shoreface and shelf environments however bed load transport under waves cannot be measured in the field and bed load transport by currents without waves is barely measurable even in spring tidal conditions there is consequently a strong lack of field data and validated models the present field site was on the shoreface and inner shelf at 2 to 8 5 km offshore the central dutch coast far outside the surfzone where tidal currents flow parallel to the coast bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13 18 m with fine and medium sands the near bed flow was measured over nearly a year and used for integration to annual transport rates an empirical bed load model was derived which predicts bed load transports that are a factor of n5 smaller than predicted by existing models however they agree with laboratory data of sand and gravel transport in currents near incipient motion the damped transport rates may have been caused by cohesion of sediment or turbulence damping due to mud or biological activity the annual bed load transport rate was calculated using a probability density function pdf derived from the near bed current and orbital velocity data which represented the current and wave climate well when compared to 30 years of data from a nearby wave station the effect of wave stirring was included in the transport calculations the net bed load transport rate is a few m 2 year this is much less than predicted in an earlier model study which is partly due to different bed load models but also due to the difference in velocity pdf the annual transport rate is very sensitive to the probability of the largest current velocities

toward representing wave induced sediment suspension over sand ripples in rans models 1 parameterizations of near bed sediment processes are commonly associated with the poor predictive skill of coastal sediment transport models we implement a two dimensional reynolds averaged navier stokes model to directly assess these parameterizations by reproducing measurements obtained in large scale wave flume experiments a sediment transport model has been coupled to wave hydrodynamics and turbulence and numerical experiments provide temporal and spatial variations of free surface flow velocity sediment concentration and turbulence quantities model data comparisons enable the direct assessment of how key suspension processes are represented and of the inherent variability of the sediment transport model we focus on the different processes occurring above rippled beds versus dynamically flat beds numerical results show that increasing roughness alone is not sufficient to have good predictive capability above steep ripples some parameterization of the vortex entrainment process is necessary and a simple modification which leads to constant sediment diffusivity above steep rippled beds is sufficient to obtain good predictions of wave averaged suspended concentrations model data comparisons for the turbulent kinetic energy are also presented and highlight the need to account for the effect of vortex entrainment on near bed turbulence and transfer of momentum

application of 3d mobile bed hydrodynamic model the prediction of sediment erosion and deposition magnitudes has challenged many investigators one and two dimensional sediment transport models are commonly used for predicting sediment scour and deposition they are adequate for many engineering applications however for some problems only a three dimensional model should be used a three dimensional model is necessary for predicting sediment erosion and deposition whenever significant secondary currents exist such as in river bends crossings distributaries or diversions in diversions for example the sediment load may not split in the same proportions as the flow a three dimensional model can be used to estimate the sediment split this paper presents a description of the three dimensional numerical model ch3d sed and the governing equations for the model the validation of the model and its application to the deep draft navigation project on the lower mississippi river also are presented

resuspension and transport of fine sediments by waves although waves are the primary cause of sediment resuspension in the nearshore zone in existing theoretical models long scale currents induced by the mean wind are often taken to be the only agent for the diffusion and convection of resuspended sediments we present here theoretical examples where waves play a direct role in all aspects of sediment transport details are given for the simple case where only waves are present the wave induced current and diffusivity are shown to be no less important than similar factors in the wind driven current hence in a comprehensive model one should include not only the current forced directly by the mean wind but also the current forced by waves which may or may not be forced by the local wind

observations and model simulations of wave current interaction on the inner shelf wave directions and mean currents observed for two 1 month long periods in 7 and 2 m water depths along 11 km of the southern shoreline of martha s vineyard ma have strong tidal modulations wave directions are modulated by as much as 70 over a tidal cycle the magnitude of the tidal modulations in the wavefield decreases alongshore to the west consistent with the observed decrease in tidal currents from 2 1 to 0 2 m s along the shoreline a numerical model swan and deflt3d flow simulating waves and currents reproduces the observations accurately model simulations with and without wave current interaction and tidal depth changes demonstrate that the observed tidal modulations of the wavefield primarily are caused by wave current interaction and not by tidal changes to water depths over the nearby complex shoals

understanding coastal morphodynamic patterns from depth averaged sediment concentration this review highlights the important role of the depth averaged sediment concentration dasc to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern beach cusps surf zone transverse and crescentic bars and shoreface connected sand ridges we present a formulation and methodology based on the knowledge of the dasc which equals the sediment load divided by the water depth that has been successfully used to understand the characteristics of these features these sand bodies relevant for coastal engineering and other disciplines are located in different parts of the coastal zone and are characterized by different spatial and temporal scales but the same technique can be used to understand them since the sand bodies occur in the presence of depth averaged currents the sediment transport approximately equals a sediment load times the current moreover it is assumed that waves essentially mobilize the sediment and the current increases this mobilization and advects the sediment in such conditions knowing the spatial distribution of the dasc and the depth averaged currents induced by the forcing waves wind and pressure gradients over the patterns allows inferring the convergence divergence of sediment transport deposition erosion occurs where the current flows from areas of high to low low to high values of dasc the formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features but the physical mechanisms for their migration their finite amplitude behavior and their decay can also be explored

two dimensional numerical modeling of storm deposition on the northern california shelf abstract a two dimensional across shelf sediment transport model has been developed to simulate storm deposition on continental shelves this paper describes the structure of the model and related algorithms in the model the time dependent sediment resuspension transportation and deposition and across shelf transport gradients are simulated as responses to storm waves on the eel shelf northern california the simulations show that the pattern of storm deposition on the sandy inner shelf is different from that on the muddy middle and outer shelf storm bed thickness decreases seaward across the inner shelf sandy zone as far as the 50 m isobath farther seaward storm bed thickness increases as the mud component in the sea floor becomes abundant then decreases again as the weakening of bottom wave motion with greater depth becomes the dominant control storm bed thickness in the mud zone thus has a nearly symmetrical cross shelf pattern with the maximum thickness at a water depth of 70 m the pattern suggests that while the sequence of new beds found in february 1995 on the eel shelf may have had a flood source their geometry reflects storm resuspension and transport the new muds found in these beds may have been reworked transported and redeposited several times by storm waves and other oceanic forces numerical simulations illustrate that the critical bottom shear stress for mud erosion e and the coefficient for sand resuspension 0 are more important than other parameters in determining storm depositional patterns on the eel shelf

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

holocene variations in the asian monsoon inferred from the geochemistry of lake sediments in central tibet we present a record of monsoon variations for the early and middle holocene that is inferred from the geochemistry of sediment cores from ahung co a lake in central tibet the resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal which eliminates errors associated with the lake hard water effect we made down core geochemical measurements of carbonate organic carbon c n and 13c of bulk organic matter 13c and 18o of carbonate and dolomite proxy calibration and modern water balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation we find that lake level and monsoon precipitation have been decreasing at ahung co since the early holocene 7500 cal yr b p superimposed on this trend are rapid declines in monsoon rainfall at 7000 7500 and 4700 cal yr b p and seven century scale wet dry oscillations the cores do not contain sediment from the last 4000 yr surface sediments from the lake accumulated during the 20th century however from this we argue that lake levels have risen again recently following a late holocene dry period

holocene vegetation history precipitation changes and indian summer monsoon evolution documented from sediments of xingyun lake south west china we present a pollen based precipitation reconstruction and multi proxy records from a 485 cm long sequence from a sediment core from xingyun lake yunnan plateau south west china which depicts the evolution of the indian summer monsoon ism during the last 8500 years pollen and other palaeoenvironmental records document several stages of vegetation history and climate change the warmest and wettest climate in the xingyun lake catchment occurred before 5500 cal a bp and subsequently the climate became gradually drier after 2000 cal a bp the regional environmental conditions became unstable and a wet medieval warm period is probably recorded our reconstruction of the ism is similar to that portrayed by holocene speleothem o 18 records from southern china but is distinctly different from the east asian summer monsoon easm evolution which features a mid holocene maximum our results support the hypothesis that the ism and easm evolved asynchronously during the holocene and imply that the chinese speleothem o 18 records from southern china may principally reflect changes in moisture source from the indian monsoon domain and thus record the history of the ism rather than the easm

an evaluation of bed shear stress under turbid flows this paper investigates the effect of various turbidity levels on the drag coefficient and bed shear stress in decelerating flows over a smooth bed turbid flows were simulated in a laboratory annular flume lab carousel using clay suspensions of different known mass concentrations flow velocity measurements were taken in the turbulent boundary layer and the values of bed drag coefficient and shear stress were calculated using the method of flow deceleration results showed that the bed drag coefficient responds significantly to an increase in the suspended sediment concentration up to 20 g l showing a decrease of up to 50 in drag due to turbulence suppression however subsequent increases in mass concentration resulted in an increase in the drag coefficient reaching the clear water value at approximately 200 g l because of the influence of viscosity thus the application of the law of the wall in flows of high concentrations of suspended cohesive sediments up to 200 g l results in overestimation of the bed shear stress

settling speeds of floes in fresh water and seawater the settling speeds of floes of fine grained natural sediments approximately 10 200 m in diameter have been measured in both fresh water and seawater these floes were formed at fluid shears of 100 200 and 400 s 1 and at sediment concentrations of 10 100 and 400 mg l values typical of conditions in the near shore areas of lakes and oceans especially during storm conditions it is demonstrated that the settling speed of a floe is a strong function of fluid shear and sediment concentration as well as of the diameter of the floe but it is a weak function of salinity for the same diameter and salinity floes produced at the lower fluid shears and sediment concentrations have lower settling speeds than do floes produced at higher fluid shears and sediment concentrations if the conditions of fluid shear and sediment concentration under which the floes were produced are unknown or ignored it is shown that the settling speed of a floe is a weak function of diameter and salinity

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

a generalized exner equation for sediment mass balance 1 the advance of morphodynamics research into new areas has led to a proliferation of forms of sediment mass balance equation without a general equation it is often difficult to know what these problem specific versions of sediment mass balance leave out to address this we derive a general form of the standard exner equation for sediment mass balance that includes effects of tectonic uplift and subsidence soil formation and creep compaction and chemical precipitation and dissolution the complete equation 17 allows for independent evolution of two critical interfaces that between bedrock and sediment or soil and that between sediment and flow by eliminating terms from the general equation it is straightforward to derive mass balance equations applicable to a wide range of problems such as short term bed evolution basin evolution bedrock uplift and soil formation and carbonate precipitation and transport dropping terms makes explicit what is not being considered in a given problem and can be done by inspection or by a formal scaling analysis of the terms scaling analysis leads directly to dimensionless numbers that measure the relative importance of terms in the equation for example the relative influence of spatial versus temporal changes in sediment load on bed evolution combining scaling analysis with time averaging shows how the relative importance of terms in the equation can change with timescale for example the term representing bed evolution due to temporal change in sediment load tends to zero as timescale increases

comparison of measurements and models of bed stress bedforms and suspended sediments under combined currents and waves abstract this study presents an analysis of data collected in an embayment off a sandy beach protected by shore parallel breakwaters observations were carried out over a forty day period including spring and neap tidal cycles and low and high wave activity measurements were collected on the waves currents bedforms and suspended sediments and these were used to derive estimates of the bed shear stresses ripple dimensions reference concentrations and suspended sediment profiles the aim of the present study was to use the measurements to assess commonly employed empirical formulations applied to sediment modelling in the coastal marine environment assessments are made of four ripple predictors two formulations of reference concentration and power law and exponential suspended sediment shape functions in some cases predicted parameters are well represented during low wave conditions whilst other parameters are better predicted under high wave events the outcome from the work is an evaluation of the predictive capability of the selected empirical formulae at the sandy location and discussions on the comparison between the observations and predictions

a simple model of unsteady sheet flow sediment transport a simple quasi steady one dimensional vertical 1dv model of unsteady sheet flow is presented the central aim is to provide greater realism in the near bed high concentration layer than is possible using models based on the classical referenceconcentration approach this is achieved by tracking erosion and deposition at the bottom of the mobile sediment layer in relation to the amount of sediment present in the sheet flow and suspension layers the model relies on empirical assumptions for the time varying sheet flow layer thickness d and time varying equivalent bed roughness ks the formulations adopted yield realistic instantaneous vertical profiles of velocity and sediment concentration from the stationary bed through the highconcentration sheet flow layer up into the outer suspension layer the suspension layer itself is modelled using a standard k e turbulence closure scheme together with the sediment continuity equation matching conditions are applied at the interface between the sheet flow and suspension layers preliminary results presented here include initial validation comparisons with the data set of horikawa et al horikawa k watanabe a katori s 1982 sediment transport under sheet flow conditions proceedings of the 18th international conference on coastal engineering cape town asce reston va usa 1335 1352 further comparisons with the experiments of dohmen janssen et al j geophys res 106 2001 27103 cover a wide range of wave current conditions and sand grain sizes the results are satisfactory with respect to the measured velocity and concentration profiles apart from cases involving fine sand for which the effects of flow unsteadiness are not accounted for in the formulation in addition the new model provides satisfactory predictions of sand transport rates d 2003 elsevier science b v all rights reserved

sediment dispersal in the northwestern adriatic sea 1 sediment dispersal in the adriatic sea was evaluated using coupled three dimensional circulation and sediment transport models representing conditions from autumn 2002 through spring 2003 the calculations accounted for fluvial sources resuspension by waves and currents and suspended transport sediment fluxes peaked during southwestward bora wind conditions that produced energetic waves and strengthened the western adriatic coastal current transport along the western adriatic continental shelf was nearly always to the south except during brief periods when northward sirocco winds reduced the coastal current much of the modeled fluvial sediment deposition was near river mouths such as the po subaqueous delta nearly all po sediment remained in the northern adriatic material from rivers that drain the apennine mountains traveled farther before deposition than po sediment because it was modeled with a lower settling velocity fluvial sediment delivered to areas with high average bed shear stress was more highly dispersed than material delivered to more quiescent areas modeled depositional patterns were similar to observed patterns that have developed over longer timescales specifically modeled po sediment accumulation was thickest near the river mouth with a very thin deposit extending to the northeast consistent with patterns of modern sediment texture in the northern adriatic sediment resuspended from the bed and delivered by apennine rivers was preferentially deposited on the northern side of the gargano peninsula in the location of thick holocene accumulation deposition here was highest during bora winds when convergences in current velocities and off shelf flux enhanced delivery of material to the midshelf

modeling stratified wave and current bottom boundary layers on the continental shelf the glenn and grant 1987 continental shelf bottom boundary layer model for the flow and suspended sediment concentration profiles in the constant stress layer above a noncohesive movable sediment bed has been updated the reynolds fluxes for sediment mass and fluid momentum are closed using a continuous time invariant linear eddy viscosity modified by a continuous stability parameter to represent the influence of suspended sediment induced stratification throughout the constant stress region glenn and grant 1987 use a less realistic discontinuous eddy viscosity and neglect the stratification correction in the wave boundary layer for typical model parameters the two models produce currents above the wave boundary layer that are in better agreement than the suspended sediment concentrations within the wave boundary layer the differences are much greater for both the current and the sediment concentration this leads to significant differences in the sediment transport throughout the constant stress layer sensitivities of the updated model were examined on the basis of observed wave and current data acquired during storms on the inner continental shelf comparisons between the stratified and neutral versions of the updated model indicate that the stratified version produces a total depth integrated sediment transport that can be 2 orders of magnitude less than and time averaged shear velocities that can be nearly half of that predicted by the neutral version sensitivities to grain size distributions indicate that even a small amount of finer sediment can stratify the storm driven flows sensitivities to closure constants within the range of reported values also produce up to an order of magnitude variation in sediment transport illustrating the need for dedicated field experiments to refine further estimates of these parameters

the effect of sediment stratification on tidal dynamics and sediment transport patterns 1 the western tip of southwest korea is characterized by a tidally dominated turbid coastal environment there are well developed tidal flats along the coast and around islands together with offshore sand ridges several tens of kilometers long in the west the effects of bottom boundary layer bbl sediment stratification on sediment transport and tidal dynamics in this environment were examined using a sediment transport model coupled with a three dimensional tidal hydrodynamic model model experiments using two scenarios with and without the effect of sediment induced stratification showed that bbl sediment stratification influenced the spatial distribution and reduced the magnitude of net sedimentation the presence of a sediment stratified bbl also led to a reduction in suspended sediment fluxes and an increase in the vertical gradient of sediment concentrations in the water column these variations occurred because sediment induced bbl stratification leads to not only a reduction in bottom shear stress but also a decrease in buoyancy production ofturbulent kinetic energy and an associated dampening ofturbulence the significant reductions in turbulence and bottom shear stress result in changes to the vertical current structure of the m 2 tide including alteration of the tidal ellipse configuration and an increase in vertical shears of the tidal current amplitude and phase these reductions also lead to a slight increase in tidal amplitudes due to the decreased tidal energy dissipation the model results indicate that feedback between the sediment transport dynamics and hydrodynamics is an important factor in modeling sediment transport dynamics in tidally dominated turbid coastal environments such environments include the west coast of korea and the yellow sea a suitable approach to simulating the effects of this feedback may be the use of a modified bottom drag coefficient as a stability function together with the inclusion of the effect of sediment stratification on the hydrodynamics

effects of stratification by suspended sediments on turbulent shear flows sediments suspended in turbulent flows of water over plane beds are known to influence the structure of the flows by which they are carried past attempts to model this effect have been based almost exclusively on a theoretical framework in which the dense solid particles stratify the flow and have an influence analogous to that of a downward heat flux in the stably stratified atmospheric surface layer we compare results from a model based on this theoretical framework with laboratory measurements of ensemble averaged velocity and ensemble averaged particle concentration obtained by previous investigators in order to test the applicability of the theoretical approach to dilute suspensions of sand in turbulent flows of water we find that the stratification effect can be observed qualitatively in measured velocity profiles except in one series of experiments in which systematic measurement errors may have obscured the effect estimates of model constants based on measured velocity profiles are overall roughly consistent with expectations based on measurements in thermally stratified flows although there is substantial variability in individual profiles some of this variability is explainable as a consequence of a parameter range in which stratification effects are too weak to be detected accurately other variability is explainable as a consequence of approximations in the model and still other variability is possibly a consequence of weak dependence of model constants on sediment grain size which was not expected from the theoretical development the stratification effect is not observed in individual particle concentration profiles but must instead be observed in experiments in which the flow conditions and particle properties are held fixed while the particle load is increased

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

fine particle retention and deposition in regions of cyclonic tidal current rotation abstract benthic sediments in continental shelf seas control a variety of biogeochemical processes yet their composition especially that of fine sediment remains difficult to predict mechanisms for mud or fine sediment deposition and retention are not fully understood using sediment data and a hydrodynamic model of the northwest european shelf seas a relationship is shown to exist between fine benthic sediment composition and regions of cyclonic tidal current rotation the reduced thickness of cyclonic tidal benthic boundary layers compared with the anticyclonic case promotes deposition of fine sediment and trapping of resuspended material adding the effects of the benthic boundary layer thickness as influenced by ellipticity or not sheds some light on the limitations of approaches only focusing on bed shear stress and sediment pathways to predict the location of mud deposits a tidal boundary layer predictor that includes ellipticity alongside tidal current magnitude and depth was shown to spatially agree with maps of mud deposits

the diversification of paleozoic fire systems and fluctuations in atmospheric oxygen concentration by comparing silurian through end permian 250 million years myr charcoal abundance with contemporaneous macroecological changes in vegetation and climate we aim to demonstrate that long term variations in fire occurrence and fire system diversification are related to fluctuations in late paleozoic atmospheric oxygen concentration charcoal a proxy for fire occurs in the fossil record from the late silurian 420 myr to the present its presence at any interval in the fossil record is already taken to constrain atmospheric oxygen within the range of 13 to 35 the fire window herein we observe that as predicted atmospheric oxygen levels rise from 13 in the late devonian to 30 in the late permian so too fires progressively occur in an increasing diversity of ecosystems sequentially data of note include the occurrence of charcoal in the late silurian early devonian indicating the burning of a diminutive dominantly rhyniophytoid vegetation an apparent paucity of charcoal in the middle to late devonian that coincides with a predicted atmospheric oxygen low and the subsequent diversification of fire systems throughout the remainder of the late paleozoic first fires become widespread during the early mississippian they then become commonplace in mire systems in the middle mississippian in the pennsylvanian they are first recorded in upland settings and finally based on coal petrology become extremely important in many permian mire settings these trends conform well to changes in atmospheric oxygen concentration as predicted by modeling and indicate oxygen levels are a significant control on long term fire occurrence

wildfire history and climatic change in the semi arid loess tableland in the middle reaches of the yellow river of china during the holocene evidence from charcoal records high resolution macroscopic charcoal and sediment analysis was used to reconstruct fire history and environmental changes from three loess paleosol profiles on the semi arid loess tableland landsca

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness

deterministic coastal morphological and sediment transport modeling a review and discussion 1 modern coastal ocean modeling systems are now capable of numerically simulating a variety of coastal and estuarine problems and can thus provide useful information for managing coastal zones here we review state of the art eulerian implementations of bottom up sediment transport and morphological change in coastal ocean hydrodynamic models in order to investigate the fate of suspended sediment in coastal and estuarine waters as well as the evolution of sea or river beds sediment dynamics need to be represented at a scale relevant to the numerical discretized solution and significant effort is devoted to parameterize sediment processes we discuss boundary layer hydrodynamics and the computation of the bed shear stress we also focus on approaches used to represent near bed processes such as bed load transport and sediment erosion and deposition sediment diffusivities settling velocities and cohesive processes such as flocculation all have an impact on suspended sediment throughout the water column we then describe the implementation of process parameterizations in coastal hydrodynamic models explicitly reviewing five widely used systems the approaches implemented in these coastal models may present distinct strengths and shortcomings with regard to some important issues for coastal zones both numerical and physical while these detailed limitations need to be considered as part of model assessment more general issues also hinder present state of the art models in particular sediment transport is inherently highly empirical which is further compounded by issues arising from turbulence closure schemes we conclude by suggesting some possible directions toward improving sediment dynamics understanding and coastal scale predictive ability

2dh mathematical modelling of morphological evolutions in shallow water abstract a class of compound mathematical models of transient morphological evolutions in shallow water is discussed based on commonly applied horizontally two dimensional formulations of the water and sediment motion in the coastal zone three major aspects of the composition of such models are considered viz the specific adequacy of the constituent models the balance of the total model and the possibility of spurious interactions the lacunae in the knowledge and the tools needed to judge a proposed model at these points are indicated examples of spurious interactions are described in further detail and techniques to trace and avoid them are given these examples concerning morphological evolutions without the influence of waves show that the advective acceleration terms in the flow model play an essential part in the bottom evolution and its interaction with the current and that incorporation of the bottom slope effects on the sediment transport is indispensable in models with a local transport formulation

evidence of a large seasonal coastal upwelling system along the southern shelf of australia 1 we report observational existence of a large seasonal coastal upwelling system that establishes in austral summer december april along australian southern shelves wind driven upwelling events occur simultaneously in three upwelling centres spanning a distance of 800 km during each summer period there are 2 3 major upwelling events each lasting 1 week the simultaneous rapid response of sst to wind forcing in the upwelling centres which display vastly different shelf widths points to the existence of a larger scale process that carries cold water onto the shelf prior to the upwelling season exploration of a major upwelling event in march 1998 shows the evolution of peak surface chlorophyll a concentrations of 4 g l lagging the onset of upwelling by 1 week the associated exponential growth rate can be estimated at 0 4 d 1 another week later we found a distinct sub surface chlorophyll a maximum at a depth of 50 m centred along the upwelling front reasons for the formation of this maximum are not fully understood

observations of wave orbital scale ripples and a nonequilibrium time dependent model 1 measurements of seafloor ripples under wave dominated conditions from the leo15 site and the martha s vineyard coastal observatory were used to develop a time dependent model for ripple geometry the measurements consisted of backscatter imagery from rotary side scan sonars centimeter resolution bathymetric maps from a two axis rotary pencil beam sonar and forcing hydrodynamics during moderate energy conditions the ripple wavelength typically scaled with wave orbital diameter in more energetic conditions the ripples reached a maximum wavelength of 0 8 to 1 2 m and did not continue to increase in wavelength or decrease in height the observations showed that the relict ripples left after storms typically had wavelengths close to the maximum wavelength the time dependent model is based on an equilibrium model that allows the ripples to maintain wavelength proportional to wave orbital diameter until a suspension threshold determined by wave velocity and grain size is reached the time dependent model allows the ripple spectra to follow the equilibrium solution with a temporal delay that is based on the ratio of the ripple cross sectional area to the sediment transport rate the data was compared to the equilibrium model a simplified version of the time dependent model where the ripples were assumed to follow the equilibrium model only when the bed stress was sufficient to move sediment and the complete time dependent model it was found that only the complete time dependent model was able to correctly predict the long wavelength relict ripples and that the other approaches underpredicted relict ripple wavelengths

data analysis of bed concentration of suspended sediment an empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed its derivation is based on the experimental data sets of guy et al in 1966 the values of the near bed concentration were obtained by first isolating the suspended load transport from the observed rates of total load transport by use of a bed load formula the suspended load was then equated to einstein s formulation which allowed the determination of cb the influence of large concentrations of suspended sediment and of the reference level for cb on the calculated values of bed concentration has been analyzed the proposed relation compares satisfactorily to values of near bed concentration obtained from independent data sets the procedure used to derive cb provides a simple method for the calculation of the total transport rate of noncohesive sediment

sediment transport in the surf zone a quasi 3 d numerical model in the preceding paper detailed long crested wave and quasi 3 d surf zone current calculations were presented as the initial steps to a more complete numerical model for coastal processes here these results are used to calculate detailed sediment transport rates in the surf zone and the resulting bathymetry evolution with time the pc based quasi 3 d numerical model results are calibrated with laboratory experiments performed on a simulated infinitely long natural beach tests were carried out using regular and irregular wave fields with different wave conditions and sand sizes

a laboratory examination of floc characteristics with regard to turbulent shearing abstract turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters and their respective aggregate break up this study examines the inter relationships between floc characteristics over increasing turbidity 80 200 mg l 1 and turbulent shear 0 0 6 n m 2 environments by the use of a laboratory flume within which a suspension can be sheared at a controlled rate and with an unintrusive macro lens miniature video camera mounted in a viewing port on the flume channel wall the camera enables the direct simultaneous measurement of both floc size and settling velocity from which accurate estimates of floc effective density and porosity can be made measurements were made 120 s after the induced turbulence has ceased the instrument has an upper viewing turbidity limit of 210 mg l 1 and a lower resolution of 20 m the sediment was collected from the inter tidal mudflats at weir quay on the tamar estuary in devon southwest england the results indicated that increasing turbidity at low shear levels encouraged floc growth but the effect of the increasing turbulent shear 0 35 n m 2 together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process at shears up to 0 35 n m 2 the largest size and settling velocity flocs were produced at high concentrations whereas above 0 35 n m 2 disruption caused smaller flocs at higher concentrations the use of algorithms which were based either on a single floc characteristic i e size or settling velocity or a mean fractal dimension were seen not to accurately approximate the experimental data a multiple regression analysis of the experimental data produced the following formula based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions referred to as max20size mean values settling velocity 0 301 0 00337 rms of the gradient in turbulent velocity fluctuations 0 000606 spm 0 00335 floc size which proved to be the most accurate representation with an r 2 value of 0 95 a similar formula was determined for the average value of the four fastest settling flocs within each sample group max4w s this highlights the importance of modelling algorithms that are developed from data that take into account effective density variations i e simultaneous size and settling velocity measurement