Traykovski Peter A.

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Traykovski

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Peter A.

ORCID

0000-0002-8163-6857

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Winter 1993 observations of oceanography and sediment transport at the LEO-15 site

(Woods Hole Oceanographic Institution, 1995-12) Irish, James D. ; Lynch, James F. ; Newhall, Arthur E. ; Witzell, Nick ; Traykovski, Peter A. ; Glenn, Scott M.

The NOAA National Underseas Research Program at Rutgers University is establishing a Long-term Ecosystem Observatory off New Jersey in 15 meters of water. As part of a bottom boundary layer study at this site, WHOI deployed a bottom instrument frame during the winter of 1993-94. The bottom instrument carried a current meter, a vertical array of optical back scattering sensors, temperature, pressure and conductivity sensors and an Acoustical Backscattering Sensor. The deployment was partially successful as the acoustic system failed. The other instrumentation worked well for 3 weeks returning data on winter conditions at the site. The extreme winter waves ended the experiment by tipping the instrument over on its side. The optical instrumentation was calibrated with sediment from the site, and the results from the experiment presented.
Observations and modeling of wave-supported sediment gravity flows on the Po prodelta and comparison to prior observations from the Eel shelf

( 2006-11-14) Traykovski, Peter A. ; Wiberg, Patricia L. ; Geyer, W. Rockwell

A mooring and tripod array was deployed from the fall of 2002 through the spring of 2003 on the Po prodelta to measure sediment transport processes associated with sediment delivered from the Po River. Observations on the prodelta revealed wave-supported gravity flows of high concentration mud suspensions that are dynamically and kinematically similar to those observed on the Eel shelf (Traykovski et al., 2000). Due to the dynamic similarity between the two sites, a simple one-dimensional across-shelf model with the appropriate bottom boundary condition was used to examine fluxes associated with this transport mechanism at both locations. To calculate the sediment concentrations associated with the wave-dominated and wave-current resuspension, a bottom boundary condition using a reference concentration was combined with an “active layer” formulation to limit the amount of sediment in suspension. Whereas the wave-supported gravity flow mechanism dominates the transport on the Eel shelf, on the Po prodelta flux due to this mechanism is equal in magnitude to transport due to wave resuspension and wind-forced mean currents in cross-shore direction. Southward transport due to wave resuspension and wind forced mean currents move an order of magnitude more sediment along-shore than the downslope flux associated wave-supported gravity flows.
Predicting wave-induced ripple equilibrium geometry

(John Wiley & Sons, 2013-06-28) Nelson, Timothy R. ; Voulgaris, George ; Traykovski, Peter A.

A comprehensive database of existing (since 1954) field and laboratory measurements of ripple geometry is compiled and combined with newly collected field data to examine the performance of ripple equilibrium predictors. Reanalysis of this enlarged ripple geometry data set reveals that ripples formed from monochromatic waves scale differently than ripples formed from random waves for many existing ripple predictors. Our analysis indicates that ripple wavelengths from the two data sets collapse into a single scaling when the semiorbital excursion and sediment grain diameter are used as normalizing factors. Ripple steepness remains relatively constant for both regular and irregular wave conditions, and it only slightly increases for shorter ripple wavelengths. These findings allowed for the development of a new equilibrium ripple predictor suitable for application in a wide range of wave and sediment conditions.
Rapid sediment deposition and fine-scale strata formation in the Hudson estuary

(American Geophysical Union, 2004-04-21) Traykovski, Peter A. ; Geyer, W. Rockwell ; Sommerfield, Christopher K.

A 9 month time series of tripod-mounted optical and acoustic measurements of sediment concentration and bed elevation was used to examine depositional processes in relationship to hydrodynamic variables in the Hudson River estuary. A series of cores was also taken directly under and adjacent to the acoustic measurements to examine the relation between the depositional processes and the resulting fine-scale stratigraphy. The measurements reveal that deposition occurs as a result of sediment flux convergence behind a salinity front and that the accumulation rates are sufficient to deposit up to 25 cm of new high-porosity sediment in a single ebb-tidal phase. Subsequent dewatering and erosion reduces the thickness of the initial deposit to several centimeters. These depositional events were only observed on spring tides. Ten depositional events during two spring tidal cycles produced a seasonal deposit of 18 cm, consistent with estimates of seasonal deposition from cores. A proxy for near-bed suspended grain size variations was estimated from the combined acoustic and optical measurements, implying that the erosional processes resuspend only the finer-grained sediments, thus leaving behind silt and very fine grained sand beds. The thickness of the deposited homogenous clayey silt beds, and the vertical separation between beds interlaminated with silt and very fine sand, are roughly consistent with the acoustic measurements of changes in bed elevations during deposition and erosion. The variability in individual bed thickness is the result of variations of processes over an individual tidal cycle and is not a product of variations over the spring neap fortnightly timescale.
Mine burial experiments at the Martha’s Vineyard Coastal Observatory

(IEEE, 2007-01) Traykovski, Peter A. ; Richardson, Michael D. ; Mayer, Larry A. ; Irish, James D.

Several experiments to measure postimpact burial of seafloor mines by scour and fill have been conducted near the Woods Hole Oceanographic Institution's Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The sedimentary environment at MVCO consists of a series of rippled scour depressions (RSDs), which are large scale bedforms with alternating areas of coarse and fine sand. This allows simultaneous mine burial experiments in both coarse and fine sand under almost identical hydrodynamic forcing conditions. Two preliminary sets of mine scour burial experiments were conducted during winters 2001?2002 in fine sand and 2002?2003 in coarse sand with a single optically instrumented mine in the field of view of a rotary sidescan sonar. From October 2003 to April of 2004, ten instrumented mines were deployed along with several sonar systems to image mine behavior and to characterize bedform and oceanographic processes. In fine sand, the sonar imagery of the mines revealed that large scour pits form around the mines during energetic wave events. Mines fell into their own scour pits, aligned with the dominant wave crests and became level with the ambient seafloor after several energetic wave events. In quiescent periods, after the energetic wave events, the scour pits episodically infilled with mud. After several scour and infilling events, the scour pits were completely filled and a layer of fine sand covered both the mines and the scour pits, leaving no visible evidence of the mines. In the coarse sand, mines were observed to bury until the exposed height above the ripple crests was approximately the same as the large wave orbital ripple height (wavelengths of 50?125 cm and heights of 10?20 cm). A hypothesis for the physical mechanism responsible for this partial burial in the presence of large bedforms is that the mines bury until they present roughly the same hydrodynamic roughness as the orbital-scale bedforms present in coarse sand.
A method to quantify bedform height and asymmetry from a low-mounted sidescan sonar

(American Meteorological Society, 2018-04-24) Jones, Katie R. ; Traykovski, Peter A.

Rotary sidescan sonars are widely used to image the seabed given their high temporal and spatial resolution. This high resolution is necessary to resolve bedform dynamics and evolution; however, sidescan sonars do not directly measure bathymetry, limiting their utility. When sidescan sonars are mounted close to the seabed, bedforms may create acoustical “shadows” that render previous methods that invert the backscatter intensity to estimate bathymetry and are based on the assumption of a fully insonified seabed ineffective. This is especially true in coastal regions, where bedforms are common features whose large height relative to the water depth may significantly influence the surrounding flow. A method is described that utilizes sonar shadows to estimate bedform height and asymmetry. The method accounts for the periodic structure of bedform fields and the projection of the shadows onto adjacent bedforms. It is validated with bathymetric observations of wave-orbital ripples, with wavelengths ranging from 0.3 to 0.8 m, and tidally reversing megaripples, with wavelengths from 5 to 8 m. In both cases, bathymetric-measuring sonars were deployed in addition to a rotary sidescan sonar to provide a ground truth; however, the bathymetric sonars typically measure different and smaller areas than the rotary sidescan sonar. The shadow-based method and bathymetric-measuring sonar data produce estimates of bedform height that agree by 34.0% ± 27.2% for wave-orbital ripples and 16.6% ± 14.7% for megaripples. Errors for estimates of asymmetry are 1.9% ± 2.1% for wave-orbital ripples and 11.2% ± 9.6% for megaripples.
Observations of wave orbital scale ripples and a nonequilibrium time-dependent model

(American Geophysical Union, 2007-06-30) Traykovski, Peter A.

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.
Flood dispersal and deposition by near-bed gravitational sediment flows and oceanographic transport : a numerical modeling study of the Eel River shelf, northern California

(American Geophysical Union, 2005-09-30) Harris, Courtney K. ; Traykovski, Peter A. ; Geyer, W. Rockwell

A large flood of the Eel River, northern California, created a thick sediment deposit between water depths of 50 and 70 m in January 1997. The freshwater plume, however, confined sediment delivery to water depths shallower than 30 m. Mechanisms proposed to explain the apparent cross-shelf transport include dispersal by oceanographic currents, resuspension by energetic waves, and gravitationally forced transport of a thin layer of fluidized mud. Field observations indicate that these processes were all active but cannot determine their relative significance or whether these mechanisms alone explain the location, size, and timing of deposition. Approximately 30% of the sediment delivered by the Eel River is accounted for in the midshelf mud bed and inner shelf, but the fate of the remaining 70% is uncertain. A three-dimensional, hydrodynamic model was used to examine potential mechanisms of sediment transport on the Eel River shelf. The model includes suspended sediment transport and was modified to account for a thin, near-bed layer of fluidized mud. It was used to simulate flood dispersal on the Eel River shelf, to compare the relative importance of transport within the near-bed fluid mud layer to suspended sediment transport, and to evaluate sediment budgets for floods. Settling properties of fine-grained sediment, both within the flood plume and the fluid mud layer, critically impact depositional patterns. To a lesser degree, wind-driven ocean currents influence the volume of sediment that escapes the shelf, and wave magnitude affects the cross-shelf location of flood deposits. Though dilute suspension accounts for a large fraction of total flux, cross-shelf transport by gravitational forcing appears necessary to produce a midshelf mud deposit similar in volume, location, and timing to those seen offshore of the Eel River.
High-resolution numerical modeling of wave-supported gravity-driven mudflows

(American Geophysical Union, 2009-05-14) Hsu, Tian-Jian ; Ozdemir, Celalettin E. ; Traykovski, Peter A.

Wave-supported gravity-driven mudflow has been identified as a major offshore fine sediment transport mechanism of terrestrial sediment into the coastal ocean. This transport process essentially occurs within the wave boundary layer. In this study, wave-supported gravity-driven mudflow is investigated via a wave-phase-resolving high-resolution numerical model for fluid mud transport. The model results are verified with field observation of sediment concentration and near-bed flow velocities at Po prodelta. The characteristics of wave-supported gravity-driven mudflows are diagnosed by varying the bed erodibility, floc properties (fractal dimension), and rheological stresses in the numerical simulations. Model results for moderate concentration suggest that using an appropriately specified fractal dimension, the dynamics of wave-supported gravity-driven mudflow can be predicted without explicitly incorporating rheological stress. However, incorporating rheological stress makes the results less sensitive to prescribed fractal dimension. For high-concentration conditions, it is necessary to incorporate rheological stress in order to match observed intensity of downslope gravity-driven current. Model results are further analyzed to evaluate and calibrate simple parameterizations. Analysis suggests that when neglecting rheological stress, the drag coefficient decreases with increasing wave intensity and seems to follow a power law. However, when rheological stress is incorporated, the resulting drag coefficient is more or less constant (around 0.0013) for different wave intensities. Model results further suggest the bulk Richardson number has a magnitude smaller than 0.25 and is essentially determined by the amount of available soft mud (i.e., the erodibility), suggesting a supply limited condition for unconsolidated mud.
High and variable drag in a sinuous estuary with intermittent stratification

(American Geophysical Union, 2021-09-29) Bo, Tong ; Ralston, David K. ; Kranenburg, Wouter M. ; Geyer, W. Rockwell ; Traykovski, Peter A.

In field observations from a sinuous estuary, the drag coefficient C based on the momentum balance was in the range of 5-20 X10-3, much greater than expected from bottom friction alone. C also varied at tidal and seasonal timescales. CD was greater during flood tides than ebbs, most notably during spring tides. The ebb tide CD was negatively correlated with river discharge, while the flood tide CD showed no dependence on discharge. The large values of CD are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of CD, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with CD during flood tides. While CD generally increased with water depth, CD decreased for the highest water levels that corresponded with overbank flow. The decrease in CD may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of CD during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.
A numerical study of onshore ripple migration using a Eulerian two-phase model

(American Geophysical Union, 2020-12-29) Salimi‐Tarazouj, Ali ; Hsu, Tian-Jian ; Traykovski, Peter A. ; Cheng, Zhen ; Chauchat, Julien

A new modeling methodology for ripple dynamics driven by oscillatory flows using a Eulerian two‐phase flow approach is presented in order to bridge the research gap between near‐bed sediment transport via ripple migration and suspended load transport dictated by ripple induced vortices. Reynolds‐averaged Eulerian two‐phase equations for fluid phase and sediment phase are solved in a two‐dimensional vertical domain with a k‐ε closure for flow turbulence and particle stresses closures for short‐lived collision and enduring contact. The model can resolve full profiles of sediment transport without making conventional near‐bed load and suspended load assumptions. The model is validated with an oscillating tunnel experiment of orbital ripple driven by a Stokes second‐order (onshore velocity skewed) oscillatory flow with a good agreement in the flow velocity and sediment concentration. Although the suspended sediment concentration far from the ripple in the dilute region was underpredicted by the present model, the model predicts an onshore ripple migration rate that is in very good agreement with the measured value. Another orbital ripple case driven by symmetric sinusoidal oscillatory flow is also conducted to contrast the effect of velocity skewness. The model is able to capture a net offshore‐directed suspended load transport flux due to the asymmetric primary vortex consistent with laboratory observation. More importantly, the model can resolve the asymmetry of onshore‐directed near‐bed sediment flux associated with more intense boundary layer flow speed‐up during onshore flow cycle and sediment avalanching near the lee ripple flank which force the onshore ripple migration.
Numerical simulations of the flow and sediment transport regimes surrounding a short cylinder

(IEEE, 2007-01) Hatton, Kimberly A. ; Foster, Diane L. ; Traykovski, Peter A. ; Smith, Heather D.

The 3-D flow field and bed stress surrounding a short cylinder in response to combined wave and mean-flow forcing events is examined. Model simulations are performed with a 3-D nonhydrostatic computational fluid dynamics model, FLOW-3D. The model is forced with a range of characteristic tidal and wave velocities as observed in 12–15 m of water at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The 2.4-m-long and 0.5-m diameter cylinder is buried 10% of the diameter on a flat, fixed bed. Regions of incipient motion are identified through local estimates of the Shields parameter exceeding the critical value. Potential areas of sediment deposition are identified with local estimates of the Rouse parameter exceeding ten. The model predictions of sediment response are in general in agreement with field observations of seabed morphology obtained over a one-week period during the 2003–2004 MVCO mine burial experiment. Both observations and simulations show potential transport occurring at the ends of the mine in wave-dominated events. Mean flows greater than 10 cm/s lead to the formation of larger scour pits upstream of the cylinder. Deposition in both cases tends to occur along the sides, near the center of mass of the mine. However, the fixed-bed assumption prohibits the prediction of full perimeter scour as is observed in nature. Predicted scour and burial regimes for a range of wave and mean-flow combinations are established.
Eulerian two-phase model reveals the importance of wave period in ripple evolution and equilibrium geometry

(American Geophysical Union, 2021-06-24) Salimi‐Tarazouj, Ali ; Hsu, Tian-Jian ; Traykovski, Peter A. ; Chauchat, Julien

The evolution of ripple geometries and their equilibrium states due to different wave forcing parameters are investigated by a Reynolds-averaged two-phase model, SedFoam, in a two-dimensional domain. Modeled ripple geometries, for a given uniform grain diameter, show a good agreement with ripple predictors that include the wave period effect explicitly, in addition to the wave orbital excursion length (or wave orbital velocity amplitude). Furthermore, using a series of numerical experiments, the ripple's response to a step-change in the wave forcing is studied. The model is capable of simulating “splitting,” “sliding,” “merging,” and “protruding” as the ripples evolve to a new equilibrium state. The model can also simulate the transition to sheet flow in energetic wave conditions and ripple reformation from a nearly flat bed condition. Simulation results reveal that the equilibrium state is such that the “primary” vortices reach half of the ripple length. Furthermore, an analysis of the suspended load and near-bed load ratio in the equilibrium state indicates that in the orbital ripple regime, the near-bed load is dominant while the suspended load is conducive to the ripple decaying regime (suborbital ripples) and sheet flow condition.
Travel-time perturbations due to internal waves : equivalence of modal and ray solutions

(Acoustical Society of America, 1996-02) Traykovski, Peter A.

In a recent paper, Lynch et al. used modal and ray based perturbation techniques to compare predicted variances of acoustic travel times due to internal waves to measured variances in the Barents Sea Polar Front experiment [Lynch et al., J. Acoust. Soc. Am. 99, 803–821 (1996)]. One of the interesting results of this work is that the modal and ray travel-time variances are substantially different for rays and modes with the same grazing angle. Specifically, the maximum modal travel-time variance shows a resonant effect in which the variance increases with increasing frequency. Unlike the modal solution, the ray travel-time variance has a geometrically constrained maximum, independent of frequency. In this paper, the linear first-order solutions for the ray and modal variances due to the internal waves are reviewed, and in an Appendix the effects of the linearizing assumptions are examined. The ray and mode solutions are then shown to be consistent by considering a truncated sum of modes that constructively interfere along a geometric ray path. By defining the travel-time perturbation due to a truncated sum of modes, the travel-time variance of the modal sum is derived. With increasing frequency the maximum value of this variance converges to a frequency-independent result with a similar magnitude to the ray maximum variance.
Predicting seabed burial of cylinders by wave-induced scour : application to the sandy inner shelf off Florida and Massachusetts

(IEEE, 2007-01) Trembanis, Arthur C. ; Friedrichs, Carl T. ; Richardson, Michael D. ; Traykovski, Peter A. ; Howd, Peter A. ; Elmore, Paul A. ; Wever, Thomas F.

A simple parameterized model for wave-induced burial of mine-like cylinders as a function of grain-size, time-varying, wave orbital velocity and mine diameter was implemented and assessed against results from inert instrumented mines placed off the Indian Rocks Beach (IRB, FL), and off the Martha’s Vineyard Coastal Observatory (MVCO, Edgartown, MA). The steady flow scour parameters provided by Whitehouse (1998) for self-settling cylinders worked well for predicting burial by depth below the ambient seabed for Ο (0.5 m) diameter mines in fine sand at both sites. By including or excluding scour pit infilling, a range of percent burial by surface area was predicted that was also consistent with observations. Rapid scour pit infilling was often seen at MVCO but never at IRB, suggesting that the environmental presence of fine sediment plays a key role in promoting infilling. Overprediction of mine scour in coarse sand was corrected by assuming a mine within a field of large ripples buries only until it generates no more turbulence than that produced by surrounding bedforms. The feasibility of using a regional wave model to predict mine burial in both hindcast and real-time forecast mode was tested using the National Oceanic and Atmospheric Administration (NOAA, Washington, DC) WaveWatch 3 (WW3) model. Hindcast waves were adequate for useful operational forcing of mine burial predictions, but five-day wave forecasts introduced large errors. This investigation was part of a larger effort to develop simple yet reliable predictions of mine burial suitable for addressing the operational needs of the U.S. Navy.
High-resolution mapping of mines and ripples at the Martha's Vineyard Coastal Observatory

(IEEE, 2007-01) Mayer, Larry A. ; Raymond, Richard ; Glang, Gerd ; Richardson, Michael D. ; Traykovski, Peter A. ; Trembanis, Arthur C.

High-resolution multibeam sonar and state-of-the- art data processing and visualization techniques have been used to quantify the evolution of seafloor morphology and the degree of burial of instrumented mines and mine-shapes as part of the U.S. Office of Naval Research (ONR, Arlington, VA) mine burial experiment at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). Four surveys were conducted over two years at the experiment site with a 455-kHz, Reson 8125 dynamically focused multibeam sonar. The region is characterized by shore-perpendicular alternating zones of coarse-grained sand with 5?25-cm-high, wave orbital-scale ripples, and zones of finer grained sands with smaller (2?5-cm-high) anorbital ripples and, on occasion, medium scale 10?20-cm-high, chaotic or hummocky bedforms. The boundaries between the zones appear to respond over periods of days to months to the predominant wave direction and energy. Smoothing and small shifts of the boundaries to the northeast take place during fair-weather wave conditions while erosion (scalloping of the boundary) and shifts to the north-northwest occur during storm conditions. The multibeam sonar was also able to resolve changes in the orientation and height of fields of ripples that were directly related to the differences in the prevailing wave direction and energy. The alignment of the small scale bedforms with the prevailing wave conditions appears to occur rapidly (on the order of hours or days) when the wave conditions exceed the threshold of sediment motion (most of the time for the fine sands) and particularly during moderate storm conditions. During storm events, erosional ?windows? to the coarse layer below appear in the fine-grained sands. These ?window? features are oriented parallel to the prevailing wave direction and reveal orbital-scale ripples that are oriented perpendicular to the prevailing wave direction. The resolution of the multibeam sonar combined with 3-D visualization techniques provided realistic looking images of both both instrumented and noninstrumented mines and mine-like objects (including bomb, Manta, and Rockan shapes) that were dimensionally correct and enabled unambiguous identification of the mine type. In two of the surveys (October and December 2004), the mines in the fine-grained sands scoured into local pits but were still perfectly visible and identifiable with the multibeam sonar. In the April 2004 survey, the mines were not visible and apparently were completely buried. In the coarse-grained sand zone, the mines were extremely difficult to detect after initial scour burial as the mines bury until they present the same hydrodynamic roughness as the orbital-scale bedforms and thus blend into the ambient ripple field. Given the relatively large, 3-D, spatial coverage of the multibeam sonar along with its ability to measure the depth of the seafloor and the depth and dimensions of the mine, it is possible to measure directly, the burial by depth and burial by surface area of the mines. The 3-D nature of the multibeam sonar data also allows the direct determination of the volume of material removed from a scour pit.
Direct measurements of mean Reynolds stress and ripple roughness in the presence of energetic forcing by surface waves

(John Wiley & Sons, 2018-04-10) Scully, Malcolm E. ; Trowbridge, John H. ; Sherwood, Christopher R. ; Jones, Katie R. ; Traykovski, Peter A.

Direct covariance observations of the mean flow Reynolds stress and sonar images of the seafloor collected on a wave‐exposed inner continental shelf demonstrate that the drag exerted by the seabed on the overlying flow is consistent with boundary layer models for wave‐current interaction, provided that the orientation and anisotropy of the bed roughness are appropriately quantified. Large spatial and temporal variations in drag result from nonequilibrium ripple dynamics, ripple anisotropy, and the orientation of the ripples relative to the current. At a location in coarse sand characterized by large two‐dimensional orbital ripples, the observed drag shows a strong dependence on the relative orientation of the mean current to the ripple crests. At a contrasting location in fine sand, where more isotropic sub‐orbital ripples are observed, the sensitivity of the current to the orientation of the ripples is reduced. Further, at the coarse site under conditions when the currents are parallel to the ripple crests and the wave orbital diameter is smaller than the wavelength of the relic orbital ripples, the flow becomes hydraulically smooth. This transition is not observed at the fine site, where the observed wave orbital diameter is always greater than the wavelength of the observed sub‐orbital ripples. Paradoxically, the dominant along‐shelf flows often experience lower drag at the coarse site than at the fine site, despite the larger ripples, highlighting the complex dynamics controlling drag in wave‐exposed environments with heterogeneous roughness.
Mechanisms of surface wave energy dissipation over a high-concentration sediment suspension

(John Wiley & Sons, 2015-03-19) Traykovski, Peter A. ; Trowbridge, John H. ; Kineke, Gail

Field observations from the spring of 2008 on the Louisiana shelf were used to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. After a period of high discharge from the Atchafalaya River, acoustic measurements showed the presence of 20 cm thick mobile fluid-mud layers during and after wave events. While total wave energy dissipation (D) was greatest during the high energy periods, these periods had relatively low normalized attenuation rates (κ = Dissipation/Energy Flux). During declining wave-energy conditions, as the fluid-mud layer settled, the attenuation process became more efficient with high κ and low D. The transition from high D and low κ to high κ and low D was caused by a transition from turbulent to laminar flow in the fluid-mud layer as measured by a Pulse-coherent Doppler profiler. Measurements of the oscillatory boundary layer velocity profile in the fluid-mud layer during laminar flow reveal a very thick wave boundary layer with curvature filling the entire fluid-mud layer, suggesting a kinematic viscosity 2–3 orders of magnitude greater than that of clear water. This high viscosity is also consistent with a high wave-attenuation rates measured by across-shelf energy flux differences. The transition to turbulence was forced by instabilities on the lutocline, with wavelengths consistent with the dispersion relation for this two-layer system. The measurements also provide new insight into the dynamics of wave-supported turbidity flows during the transition from a laminar to turbulent fluid-mud layer.
Tidal flow asymmetry owing to inertia and waves on an unstratified, shallow ebb shoal

(John Wiley & Sons, 2018-09-22) Wargula, Anna E. ; Raubenheimer, Britt ; Elgar, Steve ; Chen, Jia-Lin ; Shi, Fengyan ; Traykovski, Peter A.

Observations of water levels, waves, currents, and bathymetry collected for a month at an unstratified tidal inlet with a shallow (1 to 2 m deep) ebb shoal are used to evaluate the asymmetry in flows and dynamics owing to inertia and waves. Along‐channel currents ranged from −1.5 to 0.6 m/s (positive inland) inside the main (3 to 5 m deep) channel crossing the ebb shoal. Net discharge is negligible, and ebb dominance of the channel flows is owing to inflow and outflow asymmetries near the inlet mouth. Offshore wave heights ranged from 0.5 to 2.5 m. During moderate to large wave events (offshore significant wave heights >1.2 m), wave forcing enhanced onshore mass flux near the shoal edge and inside the inlet, leading to reduced ebb flow dominance. Momentum balances estimated with the water depths, currents, and waves simulated with a quasi 3‐D numerical model reproduce the momentum balances estimated from the observations reasonably well. Both observations and simulations suggest that ebb‐dominant bottom stresses are balanced by the ebb‐dominant pressure gradient and the tidally asymmetric inertia, which is a sink (source) of momentum on flood (ebb). Simulations with and without waves suggest that waves drive local and nonlocal changes in the water levels and flows. Specifically, breaking waves at the offshore edge of the ebb shoal induce setup and partially block the ebb jet (local effects), which leads to a more onshore‐directed mass flux, changes to the advection across the ebb shoal, and increased water levels inside the inlet mouth (nonlocal effects).
Coupled dynamics of interfacial waves and bed forms in fluid muds over erodible seabeds in oscillatory flows

(John Wiley & Sons, 2015-08-16) Trowbridge, John H. ; Traykovski, Peter A.

Recent field investigations of the damping of ocean surface waves over fluid muds have revealed waves on the interface between the thin layer of fluid mud and the overlying much thicker column of clear water, accompanied by bed forms on the erodible seabed beneath the fluid mud. The frequencies and wavelengths of the observed interfacial waves are qualitatively consistent with the linear dispersion relationship for long interfacial waves, but the forcing mechanism is not known. To understand the forcing, a linear model is proposed, based on the layer-averaged hydrostatic equations for the fluid mud, together with the Meyer-Peter-Mueller equation for the sediment transport within the underlying seabed, both subject to oscillatory forcing by the surface waves. If the underlying seabed is nonerodible and flat, the model indicates parametric instability to interfacial waves, but the threshold for instability is not met by the observations. If the underlying seabed is erodible, the model indicates that perturbations to the seabed elevation in the presence of the oscillatory forcing create interfacial waves, which in turn produce stresses within the fluid mud that force a net transport of sediment within the seabed toward the bed form crests, thus causing growth of both bed forms and interfacial waves. The frequencies, wavelengths, and growth rates are in qualitative agreement with the observations. A competition between mixing created by the interfacial waves and gravitational settling might control the thickness, density, and viscosity of the fluid muds during periods of strong forcing.

Traykovski Peter A.

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