OS21E-1204
Numerical Study of Sediment Suspension Over Bedforms in Combined Flows
Sediment suspension in the coastal environment is greatly enhanced by the presence of bedforms under the combined influence of both waves and currents. In order to assess the relative importance of the effects of bedforms, waves, and currents on suspended sediment concentrations, we simulate sediment suspension over sand ripples in a wave-current flow using a three-dimensional Navier-Stokes solver. Hydrodynamics of the wave-current flow is solved in a boundary-fitted curvilinear coordinate system incorporated with Large- Eddy Simulation (LES) to model the turbulence, and transport of suspended sediment is modeled using the Eulerian approach. Using this solver, a laboratory-scale numerical study is conducted in a 1 m x 1 m x 0.25 m (L x W x H) domain with 288 x 288 x 64 grid points. The flow is forced with the sum of oscillating and constant pressure gradients to produce combined wave-current flows in which the mean flow is either aligned with or orthogonal to the wave direction. We adjust the physical parameters based on the experimental set- up of Lacy et al. (2007)* such that hydrodynamics and suspended-sand concentration (SSC) profiles can be obtained under the influence of different wave-current conditions and bedforms. By comparing instantaneous and mean velocity fields from different experimental settings, turbulent features and vortex structures associated with different flow parameters and bedforms are investigated. From the resulting SSC profiles, we assess the relative effects on sediment suspension due to waves, currents and bedforms. The results are validated via comparison to the experimental data of Lacy et al. (2007)*. * J. R. Lacy, D. M. Rubin, H. Ideda, K. Modudai, and D. M. Hanes, ¡§Bed forms created by simulated waves and currents in a large flume¡§, Journal of Geophyiscal Research, 112, C10018, 2007
OS21E-1205
Influence of current on suspended-sand concentration profiles in combined flows
Resuspension of sand in the coastal environment results from the turbulent interaction of waves and currents with the seafloor. Near-bed turbulence is greatly enhanced by the presence of ripples, an effect that is critical to modeling sediment transport. Because ripple dimensions vary with the wave-current environment, it is difficult to distinguish between the effects of bedforms and of waves on hydrodynamic roughness using field data. In this study we investigate the influence of bedforms and current speed on sediment resuspension in experiments in a large flume that used an oscillating plate of sand (median grain diameter 0.27 mm) to produce non-collinear combinations of waves and currents. An instrumented frame mounted on the oscillating plate held two acoustic Doppler velocimeters, two imaging sonars to monitor bedform evolution, and an acoustic backscatter system (ABS) to measure profiles of suspended-sand concentration (SSC) from 2 to 20 cm above the bed. Each experimental run had fixed wave and current velocities and wave-current angle. Runs started from a flat bed, so changes in the time-averaged SSC profiles can be attributed to the development of bedforms during the run. The SSC response to bedform development appears to vary with current speed. For the slowest current (u = 7 cm/s), initial SSC produced by waves with orbital excursion (do) of 60 cm was an order of magnitude less than that produced by waves with 80-cm do. SSC increased by an order of magnitude as bedforms developed. At two greater current speeds (15 and 28 cm/s) the initial SSC is progressively higher, and the increase in SSC due to bedform development is less, than for u = 7 cm/s. We compare the concentration profiles with several vertical mixing models, including a two-layer diffusive model. In the two- layer model, εs is constant near the bed, and is directly related to ripple height. In the outer layer εs = μ z, where μ increases with current strength. We find that once bedforms develop to a height of about 1 cm, the two-layer model compares reasonably well with profiles for all three current speeds, indicating that the differences in evolution of SSC profiles may be accounted for by increased vertical mixing in the current boundary layer at higher current speeds, and by the influence of current on bedform evolution under waves.
OS21E-1206
A Model to Predicting the Evolution of Suspended Sediment Concentration Profiles and Bedforms Before, During, and After Storm Events
Ripples are dominant and vital features on the seabed. Previous studies have focused on equilibrium ripple characteristics. However, relict ripples generally dominate the seabed temporally. These ripples can retain the same dimensions of the active ripples or adjust due to brief periods of sediment mobility. This presentation focuses on the characterization and prediction of the temporal and spatial evolution of continuous and discontinuous 2D and 3D ripples during various degrees of bed mobilization and suspended sediment resuspension. Bedforms on the inner shelf were observed (October 2003 - March 2004) off Long Bay, South Carolina. During this period, waves propagated to the northwest with an average significant wave height of 0.57 m and period of 6.4 s. The site was composed of fine-medium sand (D50=177 microns). The geometry of the bedforms (wavelength, orientation, and plan-view shape) was determined using spectral and visual analysis of sonar images produced by an Imagenex rotating sonar system. Suspended sediment profiles were measured by an Aquatec Aquascat Acoustic Backscatter Sensor. This presentation focuses on the interaction of bedforms, suspended sediment concentration, and wave- current interactions. The bedform parameters were compared with a new 2D time dependent model to predict the evolution of the 2D ripple spectra under changing wave-current forcing and direction. Preliminary indications show the 2D model improves the prediction and evolution of the ripple plan view shape over current 1D models. The 2D model shows the development of ripples from flat bed, appearance of cross ripples and the evolution to a new dominant ripple orientation and wavelength, as well as modification due to slight changes in hydrodynamic forcing. Further investigation characterizes the behavior of suspended sediment profile models against the measured profile above linear transition, irregular, and cross ripples. The evolutionary steps of the bedform and suspended sediment profile are detailed before, during, and after various storm events.
OS21E-1207
Observations of intra-wave suspended sediment transport under acceleration-skewed oscillatory flow
In the nearshore, waves transform from skewed shoaling waves to asymmetric breaking waves. Velocity skewness, such as found beneath Stokes second-order waves, generally results in a net onshore sediment transport. Recent numerical studies and limited experimental laboratory data have demonstrated the potential importance of acceleration skewness to onshore sediment transport beneath asymmetric, 'saw- tooth' waves. Here, we investigate new full-scale laboratory tunnel experiments under sheetflow conditions in which regular oscillatory sawtooh flow with varying degrees of acceleration skewness and velocity skewness were generated over a mobile bed. In all experiments the velocity amplitude was 1.2 m/s, the wave period was 7 s, and the bed was composed of well-sorted, fine to medium sand (d50 = 200~μm). In several experiments a counter-current of 0.4 m/s was imposed to imitate undertow. We deployed (1) a 2-MHz Acoustical Doppler Velocimeter Profiler (ADVP) to obtain vertical profiles of (phase-averaged) horizontal and vertical oscillatory and turbulent flow with a temporal and vertical resolution of 50 Hz and 3 mm, respectively, and (2) a triple-frequency Acoustical Backscatter Sensor to obtain vertical profiles of (phase-averaged) suspended sediment concentration with a temporal and vertical resolution of 10 Hz and 5 mm, respectively. Under waves with acceleration skewness only, we observed two concentration peaks, which at the top of the sheetflow layer were approximately in phase with the free-stream velocity. As the concentration peak following the rapid acceleration towards maximum onshore flow was slightly larger than the concentration peak under maximum offshore flow, the depth-integrated and wave-averaged suspended transport was directed onshore. While most sediment stirred during the onshore flow phase had settled back to the bed before flow reversal, some sediment stirred during the offshore flow phase persisted into the onshore flow phase, thus providing an additional means for onshore transport. The larger concentration peak under the maximum onshore flow coincided with the observed smaller thickness of the wave boundary layer under maximum onshore flow and with larger velocity gradients, turbulence intensities, and peak bed shear stress τ, where we determined τ from the ADVP data using the acceleration defect integral method. For instance, for a sawtooth wave with β = amax / (amax + amin) = 0.65, where amax and amin are maximum positive and negative acceleration, respectively, we observed the ratio τmax / |τmin| to be approximately 1.3. With an increase in β and, in particular, with an increase in velocity skewness, we observed τmax / |τmin| to increase, the concentration peak under maximum onshore flow to increasingly dominate over that under maximum offshore flow, and the onshore depth-integrated and wave-averaged suspended sediment transport to increase accordingly. In the experiments with a counter current, the wave-induced depth-integrated and wave-averaged suspended sediment transport opposed the direction of wave propagation because sediment concentrations under maximum offshore flow exceeded those under maximum onshore flow.
OS21E-1208
Sediment Transport as a Function of Position over Dunes
Natural flows over erodible beds are largely controlled by the character of bedforms such as ripples and dunes. Our ability to predict the characteristics of bedforms ultimately depends on developing accurate relations between the near-bed flow and local sediment transport rates. This is often done by applying well- known algorithms such as those developed by Meyer-Peter and Mueller (MPM), Yalin or Einstein to local conditions over a ripple or dune. Because these equations were developed based on the mean transport under steady conditions, they are not accurate when applied to mean local conditions over a dune. Measurements carried out in the re-circulating flume of the Ocean Engineering Laboratory at UCSB clearly show that the relationship between local transport and near-bed flow is more complex than expressions such as that of MPM would suggest. Non-dimensional plots of transport versus bed shear stress (approximated by measuring the streamwise velocity at 12 mm from the bed and assuming a log profile) first of all indicate that there is no apparent critical shear stress. Secondly the relationship between non-dimensional transport and stress is not a single curve, rather a family of curves that depend on the overall mean flow velocity. This results because mean equations like MPM implicitly assume that the turbulent fluctuations in the flow are proportional to the local shear velocity. In flows over dunes where the flow typically separates, especially near reattachment, the turbulent fluctuations are quite large whereas the mean velocity (and stress) are approximately zero. If a simple equation such as that of MPM is applied to a realistic distribution of near-bed velocities rather than the local mean velocity for different discharges, the result is a suite of curves that better match measured transports.
OS21E-1209
Sediment Transport Simulations Coupling DEM with RANS Fluid Solver in Multi- dimensions
Multiphase simulations of the sediment-water interface in a wave bottom boundary layer are accomplished by using a Reynolds averaged Navier Stokes (RANS) fluid solver for water motions coupled to the discrete element method (DEM) for modeling the motions of individual sediment grains. Turbulence closure in the ensemble-averaged fluid-phase equations uses balance equations for fluid turbulent kinetic energy and its dissipation rate. Both 1DV and 2DV implementations of the RANS fluid solver have been coupled to the DEM. In both cases, the DEM is fully three-dimensional where sediment particles have spherical shape and point contacts are assumed with normal and tangential forces at the contact point between particle pairs modeled with springs and friction, respectively. Coupling between sediment-water phases varies from simple one-way coupling where fluid drives sediment motions with no feedback from the sediment, up to fully coupled continuity equations and turbulence closure as well as in the fluid momentum equations where Newton's Third Law is strictly enforced at every fluid time step. Fluid-particle interaction forces include drag, added mass, pressure gradient forces, and turbulent suspension implemented through an eddy-particle interaction model based on a random walk. The 1DV DEM-RANS coupled model was used to simulate sheet flow transport conditions under oscillatory flows. The 2DV DEM-RANS coupled model was used to simulate suspension and transport over small-scale sand ripples. For all cases, the DEM used coarse to fine (0.4 mm – 0.2 mm diameter) sized sediments where grain-grain interactions model viscous dissipation through an effective coefficient of restitution as a function of the collisional Stokes number estimated from published laboratory measurements of particle-particle and particle-wall collisions. Initial comparisons were made with laboratory U-tube measurements for bulk transport rates and time-dependent concentration profiles for sheet flow transport conditions.
OS21E-1210
Towards Predicting Sheet Flow Sediment Transport as a Diffusive Process
Numerical investigations are performed of sheet flow transport using a discrete element model (DEM) to simulate the motions of every sediment particle in a small, but physically relevant domain. The transport rates and time series for particle concentration as a function of depth agree well with existing laboratory measurements. Using the detailed output from the DEM simulations we investigate the self-diffusion of particles along and transverse to the direction of instantaneous net transport across the entire sheet flow from the immobile packed bed up through the top of the saltation layer. As one might expect, self-diffusivities are proportional to the local shear rate. As well, the diffusion along the flow direction is much larger than in the transverse direction throughout the bed. Our model is consistent with classic sediment transport formulae that equate the instantaneous sediment transport rate with the instantaneous bed shear stress. However, using the same instantaneous bed shear stress, we have attempted to correlate the sediment transport rate directly to the characteristic advection and diffusion exhibited by the particles in the simulation. Our investigation is aimed at elucidating the role of particle diffusion in sheet flow sediment transport.
OS21E-1211
Sensitivity of a Sediment transport Model for Lake Michigan
A two dimensional (vertical and cross-shore) sediment transport model was used to examine its sensitivity to variations in the input parameters (waves, currents, initial bottom sediment size distribution, settling velocity, and bottom stress required for erosion). The model was applied to several transects in southern Lake Michigan using observations of waves and currents recorded during the spring of 2000. Conditions during this period included several storms that are among the largest observed in the lake. The results show that changing the physical forcing (waves and currents) or the initial bottom sediment size distribution affected the results more than varying the particle properties or the size classes used to describe the size distribution. The results indicate that a relatively simple sediment transport model should produce reasonably accurate simulations of suspended transport in the lake, and that further improvements in specifying the input parameters are more likely to increase the accuracy than including other sediment processes, such as flocculation and bed consolidation.
OS21E-1212
Alongshore Sediment Transport on the South East African Shelf and Associated Upper Slope Depocenters
During R/V Meteor Cruise M75/3 in March/April 2008, multichannel seismic and acoustic measurements have been carried out along the Southeast African Margin. A combined survey on shelf areas offshore the Limpopo and Zambezi Rivers revealed pronounced shore-parallel ridges, indicating strong alongshore transport. The Madagascar and Agulhas current systems represent intense shallow and deepwater currents directed southward along the East African coast, which in turn result in eddy development at indentations of the coastline. Strong alongshore sand transport is the result. Morphological and seismic data collected off the Limpopo river mouth as well as on the Inharrime Terrace, where the sediment cloud is diverted seawards and builds up a drift deposit. Similar features are observed on the Zambezi shelf, where near the shelf edge ridges can be observed, which seem to serve as pathways for slope-parallel sand transport. Leaking sand is deposited on the uppermost slope and increses backscatter strength and small-scale roughness of otherwise fine-grained uppermost slope sediments. A pronounced interaction of deepwater currents and the builtup of sediment drift bodies is observed on the East African and Madagascar margin, having created units of several hundred meters thickness, which in turn indicates long-term current activity. Preliminary results of seismic data processing and interpretation is presented, complemeted by Parasound echosounder from water column and shallow sediments as well as swath bathymetry and backscatter.
OS21E-1213
Physical Factors Controlling Floc Properties in Chesapeake Bay
Extensive data on suspended sediment properties were collected in Chesapeake Bay by two floc camera systems developed recently: (1) a Digital Imaging Particle Settling Tube with In-situ Capture (DIPSTIC), which is simultaneously used for video capturing and bottom withdrawal sampling and (2) an in situ settling tube for remote time series of images characterizing floc size and settling velocity at fixed locations in the field (RIPSCam; Remote In-situ Particle Settling Camera). The DIPSTIC contains a Prosilica GE1380 digital video camera for capturing the images with the pixel size of 10 μm. LED strobes are used to cast a sheet of light on the camera focal plane. The DIPSTIC apparatus also includes a CTD with OBS, an ADV, and a LISST instrument to allow a full suite of measurements. The RIPSCam contains an Olympus SP-500 digital camera with a supermacro lens, resulting in the pixel size of 12 μm. Deployed as bottom mounted, the RIPSCam observes particle properties at approximately 1 m above the bed. Preliminary analysis show that the particle sizes observed in Chesapeake Bay were 40 – 800 μm with settling velocity ranges 0.01 – 6 mm s -1. Floc fractal dimensions of between 1.6 and 2.2 have been observed, where the lower values indicate more loosely packaged flocs. Difference in particle properties between accelerating and decelerating tidal phases were observed in the turbidity maximum where smaller flocs were found during accelerating flow than during decelerating flow even under similar turbulence intensity levels. Additional analyses of interactions between particle properties, turbulence and concentration will be reported.
OS21E-1214
Wave-Turbulence-Sediment Dynamics on the Atchafalaya Shelf, Louisiana, USA
Two synchronized Sontek Hydra ADVs (Acoustic Doppler Velocimeter) were deployed for 2 weeks in early spring 2008 on the muddy Atchafalaya Shelf to observe near-bed wave, turbulence and cohesive sediment transport processes. The ADVs sampled at 10 Hz and were mounted in a vertical array to separate the effects of waves and turbulence and estimate Reynolds stresses. Near bottom velocity profile and suspended sediment concentration were also monitored using a pulse-coherent acoustic Doppler current profiler together with optical and acoustic backscatter sensors. A uni-dimensional boundary layer model for cohesive sediment transport was used to reconstruct the near bed suspended sediment concentration and simulate its interaction with the flow field.
OS21E-1215
A numerical modeling framework for the study of wave-mud interaction
A numerical modeling framework introduced by Hsu et al. (2007, J. Geophys. Res., 112) is extended here for the study of wave-mud interaction. This approach is based on the Fast Equilibrium Eulerian Approximation (Ferry and Balachandar 2001, Int. J. Multiphase Flow, 27) to the two-phase flow equations. As a part of this framework, a well-validated 2DV depth/phase-resolving wave propagation model (COBRAS, Lin & Liu, 1998, J. Fluid. Mech., 359) based on the Reynolds-averaged Navier-Stokes equations has been modified for the study of this problem. The resulting single-phase governing equations and closures reduce to the RANS equations when the sediment concentration approaches zero. Hence, the numerical model is able to simulate continuously and consistently the nonlinear water wave propagation, the fluid-mud generation and transport, the wave-boundary layer processes, turbulence modulation owing to the presence of the fluid-mud, and the rheological effects on attenuating the waves with a single set of balance equations and closures. For monochromatic waves forcing, the time- and depth-dependence of the fluid-mud characteristics were integrated to provide with the input parameters required in Kranenburg's (2008, Delft University of Technology) two-layer wave propagation model. Qualitative agreement between the two models was observed in terms of the dissipation rate. Preliminary results for the case of a wave group propagating over a muddy seabed confirmed recent field observations pointing out the importance of nonlinear wave interaction for explaining the high dissipation rate at the higher and lower frequencies within the incident wave spectrum. Funding of this study is provided by ONR.
OS21E-1216
Dynamic Mud Behavior in Response to Wave Loading: Observations, Predictions and Interpretations of Seawave-Seabed Interaction
Wave interaction with soft marine mud has been a topic of research for many decades with various proposed theories and a moderate number of observations. Models of this interaction have simulated responses assuming viscous, viscoelastic, poroelastic, and plastic beds; however, our ability to monitor the actual rheologic properties of natural mud bottoms under oscillatory strain is extremely limited, especially in terms of field observations under storm wave conditions. Rheological observations made in southern Brazil and in the Atchafalaya Basin during 2005 - 2008 show that marine muds respond dynamically to environmental forcing and suggest a strong disconnect between the assumptions of available models and the reality of the boundary conditions and relevant sediment parameters used to drive them. We find that the mud bottom can assume an almost continuous stratification in terms of density, fluidization thickness, viscosity and consolidation. At times, for specific depths, the fluid mud layer rapidly grew during wave-energy buildup to over 30 cm as the magnitude and duration of wave activity increased. Also, the identification of the top of the consolidated layer is not always precise as this stratum may well exhibit elastic as well as rigid behaviors. Because these dynamic properties and behaviors are difficult to measure and model, we explore the relative importance of fluid and consolidated mud properties that can be observed in the field using theoretical predictions of seawave-seabed interaction. Our simulations of bed fluidization under measured conditions reveal many of the possible complications of working with natural sediments in a field setting.
OS21E-1217
Computer Simulations of Megaripples in the Nearshore
Megaripples are large bedforms (heights of 20-50 cm and lengths of 1-10 m) that occur frequently in the surf zone of natural beaches. They are dynamically similar to dunes in deserts and rivers. They are important to sediment transport, flow energy dissipation, and hydro- and morpho-dynamics. Unfortunately, their relationship to the flow field is unclear, so predicting their existence and dynamics has been elusive. Here, a self-organization model (similar to models for subaerial bedforms) is used to predict megaripples in the surf zone. Three sediment transport formulae are forced with combined wave and current flows. Random bed irregularities, either imposed or resulting from small variations in transport owing to turbulence, are seeds for bedform development. Feedback between the bumps on the bed and the flow alter the transport such that organized bedforms emerge. Megaripple growth and persistence are dependent on feedback, which supports the idea that megaripples are self-organized. Bedforms form and grow despite the transport formulation employed, only growth rate is different for the different transport formulations. Modeled bedform morphology and dynamics are similar to natural megaripples. The model can be used to extend the field observations and test the hypothesis of Clarke and Werner (2004) that, if conditions remain the same, megaripples will grow indefinitely and do not reach equilibrium. Contrary to traditional bedform models, this model supports the hypothesis.
OS21E-1218
Adjustments to Bedforms and Sediment Transport in Response to Changing Flow
Prediction of an erodible bed's response to changing flow conditions necessitates knowledge of sediment transport over bedforms. Natural bedforms frequently are subject to highly variable flows rendering flume studies with steady flow and equilibrium bedforms somewhat equivocal for accurate prediction of topography under changing conditions. In an attempt to quantify the effect of existing bedforms on changing flow and vice versa, a series of experiments were preformed in a recirculating flume at UCSB's Ocean Engineering Lab. Two equilibrium flows were established, each within ranges that are known to produce dunes for the sediment size present in the flume (0.5mm). Flow A is a low depth, low velocity flow, while flow B is a high depth, high velocity flow. Discharge differs between flow A and flow B by approximately 50 % . Quasi-equilibrium bedforms were allowed to develop at each flow condition by maintaining constant mean velocity and depth, and adjusting the sediment inputs to match the sediment outputs. Once the bedforms reached equilibrium the flow was changed in a variety of ways between these two cases. Detailed observations of the flow and changing bottom boundary were recorded during the quasi equilibrium-flow, the transition period, and the newly established quasi-equilibrium dunes. The bottom boundary was recorded using a multiple transducer array positioned along the centerline of the dunes in a streamwise direction, with the transducers spaced to capture at least one wavelength of the bedform. Application of the Exner equation enables calculation of local transport rates. Flow data collected with an acoustic-Doppler profiler which recorded detailed flow velocity down to 12mm above the dunes yielded estimates of the local boundary shear stress. These two measurements provide a relationship between flow and sediment transport that is more complex than those developed for steady, uniform flows. Understanding this relationship under changing bedform geometry will aid predictive models. Furthermore bedform transition times, erosion and deposition rates, details of transition topography such as height, length, and speed will be discussed for various transitions.
OS21E-1219
Detecting Bedform Migration in Portsmouth Harbor, New Hampshire, USA on Relatively Short Spatial and Temporal Scales
Multibeam echosounder (MBES) systems have enjoyed recent popularity as a tool in bedform-migration
studies due to their ability to produce high-resolution seafloor imagery with complete bottom coverage.
Although shallow-water MBES systems may achieve decimeter-scale data resolution, the use of MBES to
successfully detect and quantify bedform migration on short time-scales (days to weeks) where the migration
distance is relatively small (< 1 m) remains limited by positioning uncertainty. In this study we evaluate
short-term bedform migration and sediment transport in a bedform field at the entrance to Portsmouth
Harbor, New Hampshire, USA. Bedform dynamics over 24-hour and multi-day periods were determined from
high-resolution bathymetry (0.25 m grid resolution) acquired with a Kongsberg EM3002D MBES system.
Position, heading and attitude data were acquired with an Applanix POS/MV system with integrated real-time
kinematic GPS correctors, providing a horizontal positioning uncertainty of < 0.1 m at the GPS receiver.
MBES surveys were conducted on June 8, 14 and 15 in 2007 and July 3 and 9 in 2008. Acoustic current
meters were deployed at two stations within the survey area in 2008 to provide simultaneous observations of
current velocities at a height of 1 m above the bottom. A new approach was developed and used for
detecting and quantifying bedform migration from the bathymetry. Our approach utilizes a ridge-extraction
algorithm to derive a binary map of dune-crest positions from the bathymetric surface, and then calculates
the displacements of small (6.25 m2) subsets of dune crest. Preliminary results indicate that bedform
migrations of ≥ 0.1 m were successfully resolved. Morphology of the bedform field is dominated by
medium and large, two-dimensional, asymmetrical subaqueous dunes (0.4 to 0.8 m height, 8 to 16 m
wavelength). Small, two-dimensional, ebb-oriented subaqueous dunes (0.3 m height, 5 m wavelength) line
the eastern margin of the bedform field, which is adjacent to the channel thalweg. Initial analysis indicates
that bedforms are active on 24-hour and multi-day cycles, with migrations of > 1.2 m observed on multi-day
cycles. The highest bedform-migration rates are observed along the eastern margin where smaller dunes
occur. In 2007 we observed a reciprocal pattern of bedform migration, in which dunes in the western half of
the bedform field migrated in a net flood (northward) direction and dunes in the eastern half migrated in a net
ebb (southward) direction. In 2008, the eastern dune population was still active and southward-migrating,
though the western half of the bedform field appeared to be inactive. The observed pattern of bedform
migration is supported by current-meter data from six tidal cycles (spring tidal conditions) during the 2008
experiment, which reveal a strong cross-channel difference in the flood and ebb currents. The data indicate
ebb-current dominance in the eastern half of the study area and flood-current dominance in the western half
of the study area. Individual bedforms cannot be tracked over the annual period (2007 to 2008) without a
higher survey-repetition rate, suggesting that annual migration distances are comparable with or greater than
the bedform wavelength, and/or that bedform morphology changes significantly over time-scales shorter
than one year.
http://www.ccom.unh.edu
OS21E-1220
Long Wavelength Ripples in the Nearshore
Sediment bedforms are ubiquitous in the nearshore environment, and their characteristics and evolution have a direct effect on the hydrodynamics and the rate of sediment transport. The focus of this study is long wavelength ripples (LWR) observed at two locations in the nearshore at roughly 3m water depth under combined current and wave conditions in Duck, North Carolina. LWR are straight-crested bedforms with wavelengths in the range of 20-200cm, and steepness of about 0.1. They occur in the build up and decay of storms, in a broader range of values of the flow parameters compared to other ripple types. The main goal of the study is to test the maximum gross bedform-normal transport (mGBNT) hypothesis, which states that the orientation of ripples in directionally varying flows is such that the gross sediment transport normal to the ripple crest is maximized. Ripple wavelengths and orientation are measured from rotary fanbeam images and current and wave conditions are obtained from electromagnetic (EM) flowmeters and an offshore pressure gauge array. Preliminary tests in which transport direction is estimated from the combined flow velocity vectors indicate that the mGBNT is not a good predictor of LWR orientation. Results from tests of the mGBNT hypothesis using a sediment transport model will be presented.
OS21E-1221
Morphological Evolution of Sediment Ripples in Coastal Zones
Ripple dynamics, decay and grain sorting in heterogeneous sediments under oscillatory and turbulent background flow conditions were investigated experimentally and theoretically. Experiments were conducted in a water channel with an oscillatory tray filled with bimodal sediment mixture made of two types of glass beads of different diameters, tagged by different colors. The tray was oscillated sinusoidally, thereby generating uniform sinusoidal water motion with respect to the sediment in the tray. Particle image velocimetry and a precision laser displacement sensor were employed to collect data on flow and ripple characteristics. The results with variable oscillatory flow conditions show that a mixture has a stabilizing effect, compared to homogeneous sediment of equivalent size, and the mixture significantly modifies ripple characteristics (length and height) as well as ripple dynamics when compared to homogeneous sediment. Our previous model for the homogeneous sediment (Testik et al., Physics Fluids, 2005) was modified to account for the dynamics of the mixture based on the use of an effective grain size d*, which could explain the observations. It was demonstrated that selective transport leads to sediment segregation, with coarse grains accumulating at the ripple crests and fine grains at the ripple troughs. A physical explanation for the observed grain sorting process was advanced based on the differences in the mobility of two sediments and the presence of coherent structures over established ripples. The degradation/decay of ripples under physical and biological phenomena was also studied. In these experiments, established ripples were subjected either to weak transport flow conditions (physical phenomenon), which was below the threshold for ripple formation, or random turbulence (biological phenomenon), generated by an oscillating grid. In both the cases ripple decay (decrease in height) was documented. To explain the results of observations, a 1-D diffusion equation was used that was modified appropriately for our case. This model was found to fit the experimental data reasonably well and values of coefficient of ripple diffusivity, K0, were estimated and these estimates are in reasonable agreement with the available field measurements. These results are expected to have useful applications ranging from acoustic sensing of seafloor to modeling of shallow water waves and cross-shore sediment transport.
OS21E-1222
A Simple Model for Swash-foreshore Dynamics
Effective coastal management requires an understanding of the behavior of beach morphology, especially under storm conditions. However, this system has proved difficult to model, primarily because of feedbacks in the fluid-morphology system, wherein the fluid motions, sediment transport and evolving foreshore bathymetry all interact. This work aims to characterize the feedback observed in this fluid-morphology system using a simple dynamical model. A system of coupled linear equations is developed where changes in morphology and run-up depend on the exceedence of extreme run-up over the base of the dune and beach slope. Model coefficients are determined by fitting the equations to observations using linear regression. Necessary observations of run-up, beach slope, and elevation of the dune base were collected during a dune erosion experiment at the Oregon State University Wave Research Lab using a stereo pair of Argus video cameras. The experiment was designed to mimic a 1998 Northeaster storm on Assateague Island, MD/VA. Wave conditions were constant during the portion of the experiment to be modeled, so directly forced variability can be neglected. Results will show whether the system tends toward a stable and predictable equilibrium, or whether the system is unpredictable.
OS21E-1223
Sensitivity of a Nearshore Model to Bathymetric Resolution
As nearshore models become more and more complex, sensitivity of the modeled output to the quality and density of the input becomes paramount. This is particularly true of military forecasting models; in these applications expediency is a key metric. In this scenario it is likely that the input to the models could be undersampled and/or of low or unknown quality, and thus have a deleterious effect on the accuracy of the forecast. However, there is no clear standard for sufficient density of input; oversampling of model input can be impractical and unnecessary. In this presentation we describe the effect of reduced input bathymetric resolution on predictions of nearshore waveheights, currents and water levels from a nearshore model. We account for the effect of numerical accuracy by keeping the computational resolution the same during simulations. In all cases we choose a highly resolved bathymetric field with a distinct feature (e.g. a shoal); the resulting predictions form a standard to which ensuing, less-resolved simulations are compared. We also study the sensitivity of the model to the form parameters of each ideal bottom feature in order to give us a better understanding for applying the results of the bottom resolution sensitivity study to different field scenarios. Error statistics are compiled and trends toward higher error with less resolution are noted. Also, the critical parameters that significantly affect model accuracy in each case are noted, and the implications for expedient, accurate forecasting discussed.
OS21E-1224
Video-based Nearshore Depth Inversion using WDM Method
A new remote sensing method for estimating nearshore water depths from video imagery has been developed and applied as part of an ongoing field study at Bethany Beach, Delaware. The new method applies Donelan et al's Wavelet Direction Method (WDM) to compact arrays of pixel intensity time series extracted from video images. The WDM generates a non-stationary time series of the wavenumber and wave direction at different frequencies that can be used to create frequency-wavenumber and directional spectrums. The water depth is estimated at the center of each compact array by fitting the linear dispersion relation to the frequency-wavenumber spectrum. Directional spectral results show good correlation to directional spectral results obtained from a slope array located just offshore of Bethany Beach. Additionally, depth estimations from the WDM are compared to depth measurements taken with a kayak survey system at Bethany Beach. Continuous measurements of the bathymetry at Bethany Beach are needed for inputs to fluid dynamics and sediment transport models to study the morphodynamics in the nearshore zone and can be used to monitor the success of the recent beach replenishment project along the Delaware coast.
OS21E-1225
Modeling Rip Channel and Mega-Cusp Migration With XBeach
The relationship between alongshore rip channel migration and sediment transport is investigated using XBeach, a recently developed 2DH coastal erosion model. XBeach solves the nonlinear shallow water equations and accounts for the effects of breaking waves, wind, turbulent dispersion, and nonlinear bottom friction. It is similar to the more widely used Delft3D but focuses on morphological change to the beach and dune and includes the action of swash on a moving shoreline. Numerics have been simplified to increase model speed and ensure stability in shallow water. XBeach is first validated by recreating a three-year time series of alongshore rip migration patterns measured with video at Fort Ord, near Monterey, CA. The model is initialized with wave spectral data at 15m depth, provided by the Coastal Data Information Program (CDIP). Flow fields and transport patterns are then examined in detail over a single rip channel and mega-cusp to better understand the small scale processes associated with migration, and a range of simulations are conducted to quantify the effects on migration rates of varying wave height, incident angle, or tidal elevation. Results are presented from a four-month period of carefully monitored, accelerated shoreline erosion at the Fort Ord site, which followed the removal of a longstanding riprap barrier that had created a sand dune peninsula extending to the water's edge. Model-predicted erosion rates along the 300m stretch of shoreline are compared with dune retreat measurements for the period.
OS21E-1226
Application of Open Loop H-Adaptation to an Unstructured Grid Tidal Flat Model
The complex topology of tidal flats presents a challenge to coastal ocean models. Recently, several models have been developed employing unstructured grids, which can provide the flexibility in mesh resolution required to resolve the complex bathymetry and coastline. However, the distribution of element size in the initial mesh can be somewhat arbitrary, and is in general the product of the operator tailoring the resolution to the underlying bathymetry and regions of interest. In this work, the flow solution from an idealized tidal flat application is used to drive an open loop h-adaptation of the mesh. The model used for this work is the Finite Volume Coastal Ocean Model (FVCOM), an open source, terrain following model. A background length scale distribution derived from model output is used to generate a new initial mesh for the model run, thus defining an iteration of the procedure. Several metrics for computing the background length scale will be examined. These include direct estimation of spatial discretization error using Richardson's extrapolation from a sequence of meshes as well as heuristics derived from gradients in the primitive variables. Examination of grid independence, computational efficiency, and performance of the scheme for idealized tidal flats with inclusion of morphodynamics will be discussed.
OS21E-1227
Temporal Variability of a Multiple Bar System Estimated From Video Imagery
Nearshore sandbars are common morphologic features found in nearshore environments and multiple bar systems are typical of low beach slope and fine sediment size coasts. The aim of the present work is to study the temporal variability of a multiple bar system at Cassino Beach, Brazil. The estimated shoreline and bar positions along with their temporal variability were based on wave dissipation patterns observed over the swash and surf zone, respectively, obtained from video imaging techniques. Based on two years of daily variance images, the innermost two bars were identified in 90 percent of the time, while a third outer bar was also identified 50 percent of the time. Most of the variability registered using the remote sensing technique showed a strong temporal variability at low frequencies (lower than 0.05 cpd) for all bars. Interestingly, it was observed that such low frequency variability decreases from the third bar to the shore.
OS21E-1228
Using a Coupled Ocean - Atmosphere - Wave - Sediment Transport (COAWST) Modeling System to investigate impacts of storms on coastal systems
Coastal storm impacts result from forcing mechanisms on a variety of spatial scales. Numerical modeling approaches typically are limited to a specific range of these scales. Here we increase the range of spatial scales by using a nested (grid refined) and coupled modeling system to investigate the dynamics of coastal storm impacts. We use a newly developed Coupled Ocean – Atmosphere – Wave – Sediment Transport (COAWST) Modeling System to investigate nearshore processes during a major storm event. The modeling system uses the Model Coupling Toolkit to perform concurrent model data exchanges between the ocean model ROMS, the atmospheric model WRF, the wave model SWAN, and the USGS Community Sediment developed routines. We describe the modeling components and the coupling methods. As part of the system the wave and ocean models are run in a coupled mode, on cascading refined grids to resolve nearshore processes driven within a larger coarser scale coastal modeling system. Results are presented that identify the significance of using a coupled system and the importance of utilizing larger-scale forcing to drive nested models.
OS21E-1229
A Comprehensive Study on Coastline Process and Sedimentary Dynamics, Sardinera Beach, Mona Island, P.R.
Sardinera beach in Mona Island, Puerto Rico, has a great recreational and ecological value and is an important research place to gather information on shoreline processes in an area far from the main land and with only scarce man made influences. Beach rock exposures present along the shoreline in Sardinera Beach have increased considerably during the last decade. A new management plan is being developed for Mona Island and the Department of Natural Resources (DNRA) of Puerto Rico wants to better understand the beach sand dynamics on this and other Mona Island beaches. This research includes field and laboratory work that characterize coastal sedimentary processes and helps to better understand the shoreline changes as well as seasonal variations in sand movement and composition. This work also establish the logistics and methodology basis for further studies that will expand to other Mona Island beaches. Benchmarks, GPS coordinates, and landmarks were used to establish ten permanent beach profiles along Sardinera Beach. Beach profiles were (and will be) measured monthly. Sardinera Beach sands are composed mostly of carbonate (CaCO3) components, products of the combination of biological, chemical and diagenetic processes, high grade of micritization, and of lithic limestone fragments. Sand composition differences between Sardinera Beach, the Mona Shelf and adjacent beach, reef crest and reef lagoon systems suggest Sardinera sands are not replenished by the modern marine components produced in these environments. The input of "fresh bioclasts" in this beach seems to be limited by natural (beach rock) and mane made (dock) barriers along the shore and by alteration in the current patterns produced by the man made aperture of the reef. Sardinera's micritized and recrystalized sand deposits seem to have been re-transported between the reefal lagoon and the beach. Sand volume analysis indicates a total sand loss of 1,322 m3 between the months of September to April. Aerial images from the years 1977, 1992 and 2003 show 14 to 27 meters of recession along the coast line.
OS21E-1230
The plight of the beaches of Greece
The coastlines of the Greece are rapidly retreating at a rate that has increased substantially in the past decade. We describe here specific examples of rapid erosion and we speculate as to the causes. In some instances, erosion is advancing at a rate of 1m/year. As in other parts of the Mediterranean, the causes are anthropogenic and include sand mining from the beaches and rivers, poor design of coastal structures that create reflection patterns that focus waves on vulnerable areas, removal of sand dunes to build roads, and coastal construction too close to shoreline. The underlying problem is the complete lack of any semblance of coastal zone management in Greece and antiquated legislation. We conclude that unless urgent salvage measures to protect the beaches and end sand mining and dune removal, several beaches will disappear within the next decade.