formula for predicting bedload transport rate in oscillatory sheet flows abstract at high bed shear stress sheet flows often occur in coastal waters in which high concentration bedload sediments are transported in a thin layer near the bed this paper firstly constructs a theoretical model partial differential equations pdes for the intense transport of non cohesive bedload sediments by unidirectional currents and then seeks a special solution to the pdes to determine the thickness of the bedload particle water mixture which could serve as the reference height that is often invoked in numerical computation and simulation of suspended sediment transport in turbulent flows moreover a modified formula is presented to determine the reference concentration using a uch approach the present study derives a 1d formula for predicting bedload transport rate in sheet flows driven by asymmetric waves with the help of a novel formula for evaluating wave friction factor the new bedload formula can generically take into account slope angle positive and negative wash load concentration in the driving water flow and other factors that affect bedload transport rate it compares well with measured data in a large scale wave flume dohmen janssen c m hanes d m 2002 sheet flow dynamics under monochromatic non breaking waves journal of geophysical research 107 c10 1301 1321 a large scale oscillatory water tunnel hassan w n ribberink j s 2005 transport processes of uniform and mixed sands in oscillatory sheet flow coastal engineering 52 745 770 and in a swash zone of natural beach masselink g hughes m g 1998 field investigation of sediment transport in the swash zone continental shelf research 18 1179 1199

sediment resuspension in a shallow lake 1 different mathematical formulations for the computation of the entrainment rate of sediment into suspension in lakes can produce widely disparate results under a given set of conditions leading to problematic interpretation in this paper the results of a 4 month field campaign on sediment resuspension in a large shallow hypereutrophic lake in southern california are presented the field measurement program included the observation of currents and waves using a nortek acoustic wave and current awac profiler the observation of water temperatures using a thermistor chain the use of optical backscatter sensors for the measurement of turbidity a surrogate for suspended sediment concentration and the use of meteorological data the paper reports one of the first field experiments in lakes using the awac whose signal strength has not been investigated in detail to date and the correlation of a set of variables coming from different sources during a relatively long period of time the contributions of different forcing mechanisms waves currents and surface seiches to the sediment resuspension in the lake are quantified and the signal strength of the awac is used to address the vertical distribution of sediment in the water column a novel relationship between the awac s backscatter intensity and turbidity is presented turbidity was found to be proportional to wk where w is the wind speed measured at 2 m and k ranged from 4 5 to 6 5 depending on the water depth finally the results of modeling the sediment entrainment into suspension in the salton sea are shown to be well represented by an extension of the garcia and parker formula

near bed turbulence and relict waveformed sand ripples observations from the inner shelf 1 results are presented from a field investigation of near bed turbulence above degrading waveformed sand ripples in 17 m water depth on the inner shelf the heights of the 50 cm wavelength primary ripples were about 5 cm at the start of the observation period and decreased by a factor of 2 within 15 days the principal degradation mechanism involved fish making pits in the seafloor near bed turbulent kinetic energy dissipation rates are estimated both from the energy spectrum and from the vertical structure function within the inertial subrange and ranged from 0 1 10 6 to 3 10 6 w kg the friction velocity u at the bed ranged from 0 3 to 0 5 cm s and the wave friction factor fw from 0 017 to 0 02 the nearbed turbulence intensities and consequently the estimated values of u2 and fw are likely too small by a factor of 2 partly to satisfy the smooth wall constraint and partly to account for the effects of small scale turbulence within the finite volume range cells of the coherent doppler system used to make the turbulence estimates finally the results indicate that the hydraulic roughness of relict ripples is likely a function of both ripple height and steepness and that the relative roughness should also depend on the near bed wave orbital excursion for modeling purposes nielsen s ripple roughness formula is recommended with a reduced proportionality constant to account for the effects of irregular wave forcing and non equilibrium ripple history

rotary sidescan images of nearshore bedform evolution during a storm abstract high resolution acoustic images are presented of bedform development on the crest of a nearshore bar during a storm the images were obtained using a 2 25 mhz rotating sidescan sonar the sonar system provides a continuous record of the evolution of the bedform field through time within a 10 m diameter field of view the images document the occurrence of ripples cross ripples and megaripples during the waning stages of the storm transitions among ripple types occurred on time scales of 1 3 h beginning with flat bed and culminating in highly 3 d short crested ripples

the importance of the oxygen minimum zone and sediment geochemistry in the distribution of recent benthic foraminifera in the northwest indian ocean abstract the relationship between the distribution of benthic foraminifera and sediment type and depositional environment in the arabian sea is discussed the benthic foraminiferal fauna were sampled in nineteen recent surface sediment samples and geochemical variables of the sediment of the same samples were measured the water depths for the box core samples varies from 440 to 4040 m a total of 103 species and six species complexes were identified the geochemical properties were found to correspond well to the sediment type and depositional environment and six different sediment depositional environment types could be distinguished analysis of the benthic foraminiferal fauna reveals specific faunal assemblages that are closely related to these sediment depositional environment types

new geophysical results from the south western eurasian basin morris jesup rise gakkel ridge yermak plateau and the fram strait the international multi ship expedition arctic 91 was able to collect a seismic transect between the morris jesup rise and the yermak plateau these conjugate plateau structures in the south western part of the eurasian basin bound one of the slowest mid

depth and current induced effects on wave propagation into the altamaha river estuary georgia abstract a study of sea surface wave propagation and its energy deformation was carried out using field observations and numerical experiments over a region spanning the midshelf of the south atlantic bight sab to the altamaha river estuary ga wave heights on the shelf region correlate with the wind observations and directional observations show that most of the wave energy is incident from the easterly direction comparing midshelf and inner shelf wave heights during a time when there was no wind and hence no wave development led to an estimation of wave energy dissipation due to bottom friction with corresponding wave dissipation factor of 0 07 for the gently sloping continental shelf of the sab after interacting with the shoaling region of the altamaha river the wave energy within the estuary becomes periodic in time showing wave energy during flood to high water phase of the tide and very little wave energy during ebb to low water this periodic modulation inside the estuary is a direct result of enhanced depth and current induced wave breaking that occurs at the ebb shoaling region surrounding the altamaha river mouth at longitude 81 23 w modelling results with stwave showed that depth induced wave breaking is more important during the low water phase of the tide than current induced wave breaking during the ebb phase of the tide during the flood to high water phase of the tide wave energy propagates into the estuary measurements of the significant wave height within the estuary showed a maximum wave height difference of 0 4 m between the slack high water shw and slack low water slw in this shallow environment these wave current interactions lead to an apparent bottom roughness that is increased from typical hydraulic roughness values leading to an enhanced bottom friction coefficient

wind waves and moveable bed bottom friction abstract effects of moveable bed bottom friction for wave observations and wave modeling are investigated using a state of the art bottom friction model this model combines the hydrodynamic friction model of madsen et al with a moveable bed roughness model based on grant and madsen analyzing the present model for idealized swell cases it is shown that swell might result in wave generated sand ripples the large change of roughness corresponding to initial ripple formation results in a preferred wave height for swell related to bathymetric scales as generally occur in shelf seas away from the coast the corresponding wave generated bottom roughness is not defined by the local wave conditions but is related to the overall energy balance of the wave field sediment data thus is imperative for the interpretation of observed decay rates and friction factors for swell for idealized depth limited wind seas near bottom wave motion is expected to generate partially washed out ripples and moderate sheet flo

cross shore migration of lunate megaripples during duck94 1 remote acoustic observations of lunate megaripple migration are presented from two storm events during the duck94 nearshore dynamics experiment for time periods during which longshore currents were weak 20 cm s during these periods significant wave orbital velocities were 50 80 cm s megaripple heights were 0 05 0 5 m megaripple horns were directed shoreward the crest to trough slope of the shoreward advancing ripple face was close to the angle of repose and migration speeds were 10 40 cm h onshore the observations also indicate that the megaripples stalled and may have begun to migrate offshore when the mean offshore flow exceeded 20 cm s during the second storm stress based bed load sediment transport models are moderately successful in predicting the observed dependence of migration velocity on measured fluid velocities separated into zero mean current infragravity wave and sea and swell wave frequency bands wave and mean current friction factors fw and fc are obtained by best fit between the predicted and observed migration velocities for two choices 3 2 and 5 2 for the stress exponent in the bed load transport part of the model net transport is computed using wave velocity amplitudes determined from both run length statistics and wave by wave improved agreement with observations is obtained for the wave by wave net transport predictions and for the wave like treatment of the infragravity band the level of agreement is relatively insensitive to the value of the best fit current and wave friction factor values for 5 2 and 3 2 are fc 4 3 10 3 and 8 0 10 3 and fw 1 7 10 2 and 4 8 10 2 respectively

wave current bedform scales orientation and migration on sable island bank 1 field observations of wave current bedforms on sable island bank show that medium to large bedforms were generally aligned with the wave direction and did not follow the rotating tidal current normalized bedform heights and wavelengths were larger than predictions by nielsen 1992 but agreed well with predictions by khelifa and ouellet 2000 which includes current effects maximum observed bedform wavelengths of 1 9 m were larger than those predicted for bedforms in wave dominated nearshore conditions but this may be expected as the water depths are larger 20 42 m and currents are present measured bedform migration rates had higher vector correlation amplitudes when compared to significant wave velocity than with current velocity or skewness migration rate predictions from three presently available models were not able to predict net migration rate and direction in all cases

mass transfer limitation of nutrient uptake by a wave dominated reef flat community maximum rates of phosphate nitrate and ammonium uptake by a reef flat community were estimated from rates of frictional dissipation that were calculated from the attenuation of waves propagating over the kaneohe bay barrier reef flat time averaged flow speeds across the entire study area ranged from 0 08 to 0 22 m s 21 observed decreases in wave energy fluxes and near bottom orbital velocities gave a mean estimate for the bottom friction coefficient cf of 0 22 6 0 03 friction coefficients were used to estimate nutrient mass transfer coefficients of 5 9md 21 for phosphate and 9 16 m d 21 for ammonium and nitrate wave heights offshore of kaneohe bay were also used to estimate an annual phosphate mass transfer coefficient of 8 6 2md that was determined from measured changes in phosphate concentrations across the reef flat we conclude that rates of phosphate uptake by the kaneohe bay barrier reef flat community are occurring near the limits of mass transfer net primary production is a major pathway by which mass and energy flow into coral reef ecosystems the uptake of dissolved nutrients is often the rate limiting step for net pri mary production in marine ecosystems thus a knowledge of the kinetics of nutrient uptake by coral reef communities is necessary for assessing how nutrient loading rates change in response to natural or anthropogenic increases in nutrient concentrations bilger and atkinson 1992 proposed that nutrient uptake by shallow reef communities was limited by mass transfer the physical transfer of nutrients across con centration boundary layers covering the surfaces of reef au totrophs these authors used mass transfer correlations for rough surfaces taken from the engineering literature to cor relate maximum rates of nutrient uptake with the frictional characteristics of experimental reef communities subse quent studies demonstrated the ability of such mass transfer correlations to predict nutrient uptake kinetics for experi mental reef communities within closed recirculating flumes atkinson and bilger 1992 bilger and atkinson 1995 thomas and atkinson 1997 and a large experimental me socosm atkinson et al 2001 the results of these studies made it clear that shallow water reef communities are ca pable of taking up nutrients from the water column at rates that are limited by mass transfer however it had not yet been demonstrated whether in situ rates of nutrient uptake

measurements of the evolution of ocean wave spectra due to bottom friction data from a field experiment in which seven wave measurement instruments were placed in a linear transect across the great australian bight are analyzed to determine the spectral decay which can be attributed to bottom friction since other processes such as atmospheric input nonlinear interactions whitecap dissipation refraction and shoaling will also be active the spectral wave model wam is used as an analysis tool explicitly taking these additional processes into account in the analysis the data show marked spectral decay across the shelf analysis reveals that the bottom friction coefficient c is not constant as often assumed in operational wave models the bottom friction coefficient varies approximately inversely with the wave induced bed velocity consistent with the predictions of previous laboratory data and eddy viscosity models it is possible that percolation may also be important in the region but the available data do not allow the relative contributions of bottom friction and percolation to be determined

turbulence based measurements of wave friction factors under irregular waves on a gravel bed very few studies have quantified the wave friction factor fw for coarse sediments at field scale to address this shortcoming high frequency measurements of turbulence obtained within the boundary layer of irregular waves over gravel in the delta flume have been used to calculate values of fw using different evaluation methods in the field scale laboratory experiments reported here three velocimeters were deployed on the seaward side of a 4 m high 5 m wide and 55 m long gravel barrier subject to a jonswap spectrum of waves with significant wave heights ranging from 0 8 m to 1 3 m and peak periods of 3 0 s to 10 s and offshore water depths ranging from 1 75 m to 3 75 m the deployment area was essentially flat with little or no predicted or observed sediment movement under the wave conditions investigated the turbulent kinetic energy method was found to be the most suitable approach for calculating the bed shear stress which can be related to fw wave friction factor values under the conditions tested here fell in the range 0 01 and 0 27 although fw predicted by an existing equation agrees well with the mean measured fw value the application of a new predictor for fw is recommended for improved parameterisation of skin friction over the range of relative roughness values encountered in this study this approach combines the wave reynolds number wave steepness and relative depth to provide a simple expression to assist assessments of coarse sediment transport by waves for uses within a range of practical engineering applications

sediment transport by waves and currents a bed load model based on a generalization of bagnold s 1954 constitutive equations together with the assumption of similarity profiles for velocity and concentration was proposed for sediment transport by waves by sleath 1994 this model is now extended to wave current flows and applied to three different situations a oscillatory flows with superimposed currents b net transport produced by progressive waves over a flat horizontal bed and c longshore transport induced by waves approaching a beach in all three cases the model gives reasonable agreement with experiment in the case of longshore transport the model produces almost the same formula as that of the coastal engineering research center 1977 apart from additional terms for density and grain size

occurrence and orientation of anorbital ripples in near shore sands dimensional and of the anorbital type l0 d50 524 to 535 d0 d50 4300 to 25 000 with 7 6 cm mean wavelength in 150 mm median diameter sand the ripple crests were typically aligned perpendicular to the incident sea and swell direction specifically about 60 of the normals to the crests differ from wave direction by less than 5 and more than 90 by less than 10 during rapid changes in wave direction however ripple orientation adjustment lagged the wave direction by o 1 h ripple reorientation occurred piecewise along the crests with new crest segments sometimes bridging two or more old segments the 1 h reorientation time scale is longer by several orders of magnitude than the adjustment time based on theoretical bed load transport rate and the ripple volume as a possible explanation we suggest that a significant fraction of the transport more than 90 bypasses the ripples during orientation adjustment through large angles

wave friction factors in nearshore sands abstract wave friction factors are estimated from vertical profiles of near bed turbulence and horizontal velocity spanning the wave bottom boundary layer measured values are partitioned by bed state which ranged from irregular ripples to flat bed and are examined as a function of two traditionally selected parameters physical bed roughness and outer flow reynolds number the measurements are from two field experiments in very different nearshore environments a relatively protected unbarred pocket beach and a linear barred beach exposed to the open shelf duck measured wave friction factors are remarkably similar for the two beaches and are highest for low energy rippled beds and lowest for the high energy flat bed conditions the reduction in the friction factor for high energy conditions corresponds to a decrease in the physical roughness of the bed as wave energy increases as a function of relative roughness measured friction factors are generally consistent with previous laboratory results and

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

southward shift of westerlies intensifies the east asian early summer rainband following el nino author s kong w chiang jch abstract 2020 american geophysical union all rights reserved an intensification of east asian rainfall usually occurs in summers following el nino we propose that this teleconnection is mediated via the westerlies impinging on the tibetan plateau through el nino s control on tropical tropospheric temperature this is distinct from previous studies that attribute the el nino s influence to changes in the western pacific subtropical anticyclone the warming in the eastern equatorial pacific leads to uniform warming of the entire tropical troposphere which sharpens the temperature gradient between the tropics and the subtropics and shifts the westerlies southward the westerlies impinge over the tibetan plateau for longer and through interaction with the topography induce intense and persistent extratropical northerlies downstream of the plateau that in turn intensifies the east asian rainband the rainband has previously been shown to intensify in a similar manner in a warming climate suggesting that the el nino response provides an analog for future changes

acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface seawards of the breaking zone during a mild storm event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break off bed shear stress is exceeded no direct measurements of bedforms were available and the combined wave current interaction model of grant and madsen j geophys res vols 84 1979 and 87 1982 with extensions to movable bed roughness ripple dimensions from carstens et al s 1969 laboratory data u s army corps eng cerc tech mem 28 and to sediment induced self stratification glenn and grant j geophys res vol 92 1983 was used to predict the current and suspended sand concentration profile matching the measured and modeled concentrations at 2 cm above the bed the values of the resuspension coefficient 0 were found to decrease as excess skin friction increased in a manner similar to that suggested by drake and cacchione cont shelf res vol 9 1989 but were an order of magnitude larger using the much smaller ripple dimensions of nielsen j geophys res vol 86 1981 changed the model results very little significant differences were found between the time averaged suspended sand profiles and those predicted by the model sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents the current being the sum of steady and wave induced components the wave induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5 10 cm above the bed and offshore transport in the few centimetres closest to the bed little net transport occurred above 15 cm these fluxes show a weak dependence on the wave energy becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased again consistent with the influence of the ripple steepness

coherent doppler profiler measurements of near bed suspended sediment fluxes and the influence of bed forms 1 this paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3 4 m water depth the measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0 7 cm vertical resolution normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states irregular ripples cross ripples linear transition ripples and flat bed indicate a small but significant peak at incident wave frequencies but are otherwise rather flat with weak redness estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples the contribution from the high frequency turbulent component is small wave phase averages for low energy bed states exhibit near bed peaks in the suspended sediment flux following wave phase reversal wave phase averages for the high energy cases do not exhibit a diffusive signature observed vertical profiles of turbulence intensity for different bed states reveal that the near bed turbulence levels are relatively independent of bed state friction velocity predictions from presently available models including a bed stress model and a sediment eddy diffusion model are compared to measured values of near bed turbulence intensity reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors from tolman 1994 are used

a hybrid eulerian lagrangian model for spectral wave evolution with application to bottom friction on the continental shelf abstract a hybrid eulerian lagrangian wave model is presented that solves the spectral energy balance equation for surface gravity waves in varying depth the energy of each spectral component is advected along lagrangian ray trajectories the source terms in the energy balance equation e g interactions between wave components and nonconservative processes are computed on a fixed eulerian grid and interpolated onto the ray trajectories the source terms are integrated in time along the rays this integration is performed in parallel over the entire model domain the main advantage of this new model named crest coupled rays with eulerian source terms is that refraction of waves by subgrid scale depth variations is evaluated accurately using precomputed rays and thus the model can be applied with relatively coarse source term grids to large coastal areas hindcasts of swell evolution across the north carolina continental shelf are presented for a source term restricted to energy dissipation in th

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