Peter Traykovski - Academia.edu (original) (raw)

Papers by Peter Traykovski

Research paper thumbnail of Measuring the Spatial Distribution of Ripples Using a REMUS AUV

The long term goal is to develop and apply new technology to make better measurements of the spat... more The long term goal is to develop and apply new technology to make better measurements of the spatial distribution of bedforms during both energetic conditions with ripples that are in equilibrium with forcing and during quiescent conditions where ripples are a product of previous storms.

Research paper thumbnail of Observations and modeling of sand transport in a wave dominated environment

Page 1. Observations and Modeling of Sand Transport in a Wave Dominated Environment by Peter Tray... more Page 1. Observations and Modeling of Sand Transport in a Wave Dominated Environment by Peter Traykovski B.Se. Mechanical Engineering, Duke University, 1988 MS Applied Ocean Sciences and Eng., MIT/ Woods Hole Oceanographic ...

Research paper thumbnail of Horizontal directional spectrum estimation of the Heard Island transmissions

Research paper thumbnail of Coastal Change Processes Project data report for observations near Fire Island, New York, January to April 2012

Research paper thumbnail of Acoustic observations of a fluid mud layer transporting sediment on the northern California shelf

Oceans '99. MTS/IEEE. Riding the Crest into the 21st Century. Conference and Exhibition. Conference Proceedings (IEEE Cat. No.99CH37008), 1999

Page 1. Acoustic Observations of a Fluid Mud Layer Transporting Sediment on the Northern Californ... more Page 1. Acoustic Observations of a Fluid Mud Layer Transporting Sediment on the Northern California Shelf. James D. Irish, Peter Traykovski, and James F. Lynch, Dept. Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution Abstract ...

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

Research paper thumbnail of Motivations for using a pulsed full spectrum Doppler to measure bedload and near-bottom suspended sediment transport

Journal of The Acoustical Society of America, 1998

Acoustic obsewations of bedfomr migration and suspended sediment transport at LEO-15 revaled thaL... more Acoustic obsewations of bedfomr migration and suspended sediment transport at LEO-15 revaled thaLbedload and near bottom suspended load could be the dominant sediment transport mode over wave formed ripples on a sandy bottom. However, suitable observational techniques for measuring bed load on the necessary time and space scales do not exist. Acoustic Doppler basal techniques, which am ideal for this type of measurement since the sediment velocity can be resolved from the station~bed in the frequency domain, are being developed.

Research paper thumbnail of Real-time observations and predictions of scour burial at the Martha's Vineyard Coastal Observatory

Journal of The Acoustical Society of America, 2005

Mine burial by scour was measured in real-time using cylindrical instrumented mines and rotary an... more Mine burial by scour was measured in real-time using cylindrical instrumented mines and rotary and two-axis pencil beam sonar. Two mines and the sonar were connected to a shore-based facility at the Martha's Vineyard Coastal Observatory. Data on mine movement (heading, pitch, and roll), scour pit geometry, percent burial, and environmental processes responsible for scour burial including significant wave height, period, and tidal height were analyzed daily and presented on a mine burial web site. Sonar images show scour pits developing around the mines in response to storm generated significant wave heights greater than 2.0 m. Mines pitch, change heading as they roll into scour pits. A comparison of the observed mine burial and real-time predictions using a modified version of the Whitehouse equations for wave-induced scour model exhibited good agreement suggesting mine burial by scour in similar sandy environments is predictable from bathymetry, sediment type, and measured or predicted surface wave conditions. The Coastal Observatory at Martha's Vineyard provided a unique opportunity for real-time data collection and analysis of both sediment transport events and seafloor-object interactions during storms. [Work supported by ONR.]

Research paper thumbnail of Sediment Delivery to the Seabed on Continental Margins

Continental Margin Sedimentation, 2007

On river-influenced continental margins, terrigenous muds tend to accumulate in the middle of the... more On river-influenced continental margins, terrigenous muds tend to accumulate in the middle of the continental shelf. The common occurrence of mid-shelf mud belts has been attributed to three basic across-margin transport mechanisms. Muds either diffuse to the mid-shelf under the influence of storms, or they are advected there by oceanographic currents, or they arrive at the mid-shelf in dense suspensions that flow across the margin under the influence of gravity. Until recently, observations generally favoured the hypothesis that ocean currents are responsible for advecting dilute suspensions of mud to the mid-shelf. Transport by dense gravity flows was widely rejected, based primarily on the arguments that the bathymetric gradients of continental shelves are too small to sustain gravity flows, and that sediment concentrations cannot grow large enough to cause suspensions to flow down gradient. Observations conducted on the Eel River continental shelf off northern California, however, demonstrate that cross-margin transport by dense suspensions can be an important mechanism for the emplacement of muds on the mid-shelf. Dense suspensions form near the seabed when sediment in the wave boundary layer cannot deposit because of stress exerted on the bottom by waves, and when sediment does not diffuse out of the wave boundary layer because of relatively weak current-induced turbulence. In the future, the importance of these flows on other margins needs to be assessed.

Research paper thumbnail of Simulating Realistic-Looking Sediment Ripple Fields

IEEE Journal of Oceanic Engineering, 2009

Sandy sediment ripples impact sonar performance in coastal waters through Bragg scattering. Obser... more Sandy sediment ripples impact sonar performance in coastal waters through Bragg scattering. Observations from data suggest that sandy ripple elevation relative to the mean seafloor as a function of the horizontal coordinates is not Gaussian distributed; specifically, peak amplitude fading over space associated with a random Gaussian process is largely absent. Such a non-Gaussian nature has implications for modeling acoustic scattering from, and penetration into, sediments. An algorithm is developed to generate ripple fields with a given power spectrum; these fields have non-Gaussian statistics and are visually consistent with data. Higher order statistics of these ripple fields and their implications to sonar detection are discussed.

Research paper thumbnail of High-resolution numerical modeling of wave-supported gravity-driven mudflows

Journal of Geophysical Research, 2009

1] Wave-supported gravity-driven mudflow has been identified as a major offshore fine sediment tr... more 1] 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.

Research paper thumbnail of Mapping epibenthic assemblages and their relations to sedimentary features in shallow-water, high-energy environments

Continental Shelf Research, 2002

Knowledge of spatial relationships among benthic biota and sedimentary features in shallow-water ... more Knowledge of spatial relationships among benthic biota and sedimentary features in shallow-water (<30 m) high-energy environments has been severely limited by sampling technology. We describe and report tests of a SCUBA-diving mapping method specifically for this region. Underwater acoustic location is used to achieve meter-scale resolution over kilometer-scale regions of the sea floor. A triad of acoustic transponders is bottom-mounted at known positions, 300–500 m apart. Transported by underwater personal vehicles, SCUBA-divers map the bed using hand-held acoustic receivers that record ranges to the transponders. The mean error of acoustic fixes was 2.4±1.2 m in a 0.5 km×1.0 km test area. Dense assemblages of epibenthic animals were mapped relative to sediment texture and bedforms off the exposed south coast of Martha's Vineyard Island, Massachusetts, USA. Surveys one month apart within a 0.6 km×0.6 km area (8–12 m depth) revealed 100-m-scale patches of the tube worm Spiophanes bombyx (⩽30,000 m−2) in fine sand and of the sand dollar Echinarachnius parma (⩽55 m−2) in coarse sand. Raised mud patches that, together with fine sand, occurred in two shore-perpendicular belts are likely exposed, ancient marsh deposits. Depth gradients of sand-ripple geometry indicated that ripples in deeper areas were not in equilibrium with wave conditions monitored during surveys; i.e., they were relict ripples. Thus, sand dollars in some areas may have had >1 month to rework surficial sands since their transformation by physical processes. Linear regressions of ripple characteristics against sand dollar or tube worm densities were not significant, although such relationships would be highly dependent on temporal scale. The survey method described here can be used at more frequent intervals to explore such interactions between epibenthic animals and sediment-transport dynamics.

Research paper thumbnail of A laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments

Marine Geology, 1999

. The LISST Laser In Situ Scattering and Transmissometery instrument was designed by Sequoia Scie... more . The LISST Laser In Situ Scattering and Transmissometery instrument was designed by Sequoia Scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment. To understand the LISST's performance with natural sediments from a variety of marine sources, several experiments were performed to compare the LISST's results to traditional sieving, filtering and weighing techniques. The LISST was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1f for sediment sizes of 5 to 250 mm. Unlike a single frequency scattering sensor for sediment concentration, which require sediment size specific calibration constants, the LISST is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 mm. The LISST was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments. q 1999 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Acoustic measurements of the spatial and temporal structure of the near-bottom boundary layer in the 1990-1991 STRESS experiment

Continental Shelf Research, 1997

As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5... more As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5 MHz Acoustic BackScatter System (ABSS) was deployed in 90 m of water to measure vertical profiles of near-bottom suspended sediment concentration. By looking at the vertical profile of concentration from 0 to 50 cm above bottom (cmab) with 1 cm vertical resolution, the ABSS was able to examine the detailed structure of the bottom boundary layer created by combined wave and current stresses. The acoustic profiles clearly showed the wave-current boundary layer, which extends to (order) 10 cmab. The profiles also showed evidence of an “intermediate” boundary layer, also influenced by combined wave and current stresses, just above the wave-current boundary layer. This paper examines the boundary-layer structure by comparing acoustic data obtained by the authors to a 1-D eddy viscosity model formulation. Specifically, these data are compared to a simple extension of the Grant-Glenn-Madsen model formulation. Also of interest is the appearance of apparently 3-D “advective plume” structures in these data. This is an interesting feature in a site which was initially chosen to be a good example of (temporally averaged) 1-D bottom boundary-layer dynamics. Computer modeling and sector-scanning sonar images are presented to justify the plausibility of observing 3-D structure at the STRESS site. © 1997 Elsevier Science Ltd

Research paper thumbnail of Sediment transport and trapping in the Hudson River estuary

Estuaries, 2001

The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term depo... more The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i.e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neapetides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.

Research paper thumbnail of The impact of acoustic oceanographic methods on estuarine dynamics research

ABSTRACT Estuaries present unique challenges for observational oceanographers, due to their inten... more ABSTRACT Estuaries present unique challenges for observational oceanographers, due to their intense spatial gradients and unrelenting temporal variability. The influence of spatial and temporal variation of estuarine structure and flow on the time-averaged regime is the most important research problem in estuarine physical oceanography. Acoustic methods have played an essential role in revealing this spatial and temporal variability, and new advances in acoustic methods are continuing to provide the most important advances in observations of estuarine processes. The measurement of acoustic backscatter has been a mainstay of estuarine physical oceanography, first for providing qualitative images of the density structure, then for quantifying suspended sediment distributions, and most recently for quantifying the intensity of stratified turbulence. Improved resolution of new systems is revealing the internal structure of shear instability and the mechanics of the transition to turbulence. Acoustic Doppler techniques are so routine now as to be taken for granted, but their impact on the field cannot be overstated, and the new advances in pulse-coherent velocity profiling are continuing this revolution in acoustical oceanography. Acoustic propagation in estuaries has not yet received much attention, but its importance to the operation of autonomous vehicles and long-term monitoring should bring this challenging acoustics problem to the forefront.

Research paper thumbnail of A laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments

Marine Geology, 1999

The LISST (Laser In Situ Scattering and Transmissometery) instrument was designed by Sequoia Scie... more The LISST (Laser In Situ Scattering and Transmissometery) instrument was designed by Sequoia Scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment. To understand the LISST's performance with natural sediments from a variety of marine sources, several experiments were performed to compare the LISST's results to traditional sieving, filtering and weighing techniques. The LISST was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1φ for sediment sizes of 5 to 250 μm. Unlike a single frequency scattering sensor for sediment concentration, which require sediment size specific calibration constants, the LISST is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 μm. The LISST was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments.

Research paper thumbnail of The Mechanics of Marine Sediment Gravity Flows

Continental Margin Sedimentation, 2007

Sediment gravity flows, particularly those in the marine environment, are dynamically interesting... more Sediment gravity flows, particularly those in the marine environment, are dynamically interesting because of the non-linear interaction of mixing, sediment entrainment/suspension and water-column stratification. Turbidity currents, which are strongly controlled by mixing at their fronts, are the best understood mode of sediment gravity flows. The type of mixing not only controls flow and deposition near the front, but also changes the dynamics of turbidity currents flowing in self-formed channels. Debris flows, on the other hand, mix little with ambient fluid. In fact, they have been shown to hydroplane, i.e. glide on a thin film of water. Hydroplaning enables marine debris flows to runout much farther than their subaerial equivalents. Some sediment gravity flows require external energy, from sources such as surface waves. When these flows are considered as stratification-limited turbidity currents, models are able to predict observed downslope sediment fluxes. Most marine sediment gravity flows are supercritical and thus controlled by sediment supply to the water column. Therefore, the genesis of the flows is the key to their understanding and prediction. Virtually every subaqueous failure produces a turbidity current, but they engage only a small percentage of the initially failed material. Wave-induced resuspension can produce and sustain sediment gravity flows. Flooding rivers can also do this, but the complex interactions of settling and turbulence need to be better understood and measured to quantify this effect and document its occurrence. Ultimately, only integrative numerical models can connect these related phenomena, and supply realistic predictions of the marine record.

Research paper thumbnail of Including a Near-Bed Turbid Layer in a Three Dimensional Sediment Transport Model With Application to the Eel River Shelf, Northern California

Estuarine and Coastal Modeling (2003), 2004

Including a Near‐Bed Turbid Layer in a Three Dimensional Sediment Transport Model with Applicatio... more Including a Near‐Bed Turbid Layer in a Three Dimensional Sediment Transport Model with Application to the Eel River Shelf, Northern California. [ASCE Conference Proceedings 145, 48 (2003)]. Courtney K. Harris, Peter Traykovski, W. Rockwell Geyer. Abstract. ...

Research paper thumbnail of Geometry, migration, and evolution of wave orbital ripples at LEO15

Journal of Geophysical Research, 1999

Observations of the temporal evolution of the geometric properties and migration of wave-formed r... more Observations of the temporal evolution of the geometric properties and migration of wave-formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO-15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to nonorbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near-bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave-to-current velocities.

Research paper thumbnail of Measuring the Spatial Distribution of Ripples Using a REMUS AUV

The long term goal is to develop and apply new technology to make better measurements of the spat... more The long term goal is to develop and apply new technology to make better measurements of the spatial distribution of bedforms during both energetic conditions with ripples that are in equilibrium with forcing and during quiescent conditions where ripples are a product of previous storms.

Research paper thumbnail of Observations and modeling of sand transport in a wave dominated environment

Page 1. Observations and Modeling of Sand Transport in a Wave Dominated Environment by Peter Tray... more Page 1. Observations and Modeling of Sand Transport in a Wave Dominated Environment by Peter Traykovski B.Se. Mechanical Engineering, Duke University, 1988 MS Applied Ocean Sciences and Eng., MIT/ Woods Hole Oceanographic ...

Research paper thumbnail of Horizontal directional spectrum estimation of the Heard Island transmissions

Research paper thumbnail of Coastal Change Processes Project data report for observations near Fire Island, New York, January to April 2012

Research paper thumbnail of Acoustic observations of a fluid mud layer transporting sediment on the northern California shelf

Oceans '99. MTS/IEEE. Riding the Crest into the 21st Century. Conference and Exhibition. Conference Proceedings (IEEE Cat. No.99CH37008), 1999

Page 1. Acoustic Observations of a Fluid Mud Layer Transporting Sediment on the Northern Californ... more Page 1. Acoustic Observations of a Fluid Mud Layer Transporting Sediment on the Northern California Shelf. James D. Irish, Peter Traykovski, and James F. Lynch, Dept. Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution Abstract ...

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

Research paper thumbnail of Motivations for using a pulsed full spectrum Doppler to measure bedload and near-bottom suspended sediment transport

Journal of The Acoustical Society of America, 1998

Acoustic obsewations of bedfomr migration and suspended sediment transport at LEO-15 revaled thaL... more Acoustic obsewations of bedfomr migration and suspended sediment transport at LEO-15 revaled thaLbedload and near bottom suspended load could be the dominant sediment transport mode over wave formed ripples on a sandy bottom. However, suitable observational techniques for measuring bed load on the necessary time and space scales do not exist. Acoustic Doppler basal techniques, which am ideal for this type of measurement since the sediment velocity can be resolved from the station~bed in the frequency domain, are being developed.

Research paper thumbnail of Real-time observations and predictions of scour burial at the Martha's Vineyard Coastal Observatory

Journal of The Acoustical Society of America, 2005

Mine burial by scour was measured in real-time using cylindrical instrumented mines and rotary an... more Mine burial by scour was measured in real-time using cylindrical instrumented mines and rotary and two-axis pencil beam sonar. Two mines and the sonar were connected to a shore-based facility at the Martha's Vineyard Coastal Observatory. Data on mine movement (heading, pitch, and roll), scour pit geometry, percent burial, and environmental processes responsible for scour burial including significant wave height, period, and tidal height were analyzed daily and presented on a mine burial web site. Sonar images show scour pits developing around the mines in response to storm generated significant wave heights greater than 2.0 m. Mines pitch, change heading as they roll into scour pits. A comparison of the observed mine burial and real-time predictions using a modified version of the Whitehouse equations for wave-induced scour model exhibited good agreement suggesting mine burial by scour in similar sandy environments is predictable from bathymetry, sediment type, and measured or predicted surface wave conditions. The Coastal Observatory at Martha's Vineyard provided a unique opportunity for real-time data collection and analysis of both sediment transport events and seafloor-object interactions during storms. [Work supported by ONR.]

Research paper thumbnail of Sediment Delivery to the Seabed on Continental Margins

Continental Margin Sedimentation, 2007

On river-influenced continental margins, terrigenous muds tend to accumulate in the middle of the... more On river-influenced continental margins, terrigenous muds tend to accumulate in the middle of the continental shelf. The common occurrence of mid-shelf mud belts has been attributed to three basic across-margin transport mechanisms. Muds either diffuse to the mid-shelf under the influence of storms, or they are advected there by oceanographic currents, or they arrive at the mid-shelf in dense suspensions that flow across the margin under the influence of gravity. Until recently, observations generally favoured the hypothesis that ocean currents are responsible for advecting dilute suspensions of mud to the mid-shelf. Transport by dense gravity flows was widely rejected, based primarily on the arguments that the bathymetric gradients of continental shelves are too small to sustain gravity flows, and that sediment concentrations cannot grow large enough to cause suspensions to flow down gradient. Observations conducted on the Eel River continental shelf off northern California, however, demonstrate that cross-margin transport by dense suspensions can be an important mechanism for the emplacement of muds on the mid-shelf. Dense suspensions form near the seabed when sediment in the wave boundary layer cannot deposit because of stress exerted on the bottom by waves, and when sediment does not diffuse out of the wave boundary layer because of relatively weak current-induced turbulence. In the future, the importance of these flows on other margins needs to be assessed.

Research paper thumbnail of Simulating Realistic-Looking Sediment Ripple Fields

IEEE Journal of Oceanic Engineering, 2009

Sandy sediment ripples impact sonar performance in coastal waters through Bragg scattering. Obser... more Sandy sediment ripples impact sonar performance in coastal waters through Bragg scattering. Observations from data suggest that sandy ripple elevation relative to the mean seafloor as a function of the horizontal coordinates is not Gaussian distributed; specifically, peak amplitude fading over space associated with a random Gaussian process is largely absent. Such a non-Gaussian nature has implications for modeling acoustic scattering from, and penetration into, sediments. An algorithm is developed to generate ripple fields with a given power spectrum; these fields have non-Gaussian statistics and are visually consistent with data. Higher order statistics of these ripple fields and their implications to sonar detection are discussed.

Research paper thumbnail of High-resolution numerical modeling of wave-supported gravity-driven mudflows

Journal of Geophysical Research, 2009

1] Wave-supported gravity-driven mudflow has been identified as a major offshore fine sediment tr... more 1] 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.

Research paper thumbnail of Mapping epibenthic assemblages and their relations to sedimentary features in shallow-water, high-energy environments

Continental Shelf Research, 2002

Knowledge of spatial relationships among benthic biota and sedimentary features in shallow-water ... more Knowledge of spatial relationships among benthic biota and sedimentary features in shallow-water (<30 m) high-energy environments has been severely limited by sampling technology. We describe and report tests of a SCUBA-diving mapping method specifically for this region. Underwater acoustic location is used to achieve meter-scale resolution over kilometer-scale regions of the sea floor. A triad of acoustic transponders is bottom-mounted at known positions, 300–500 m apart. Transported by underwater personal vehicles, SCUBA-divers map the bed using hand-held acoustic receivers that record ranges to the transponders. The mean error of acoustic fixes was 2.4±1.2 m in a 0.5 km×1.0 km test area. Dense assemblages of epibenthic animals were mapped relative to sediment texture and bedforms off the exposed south coast of Martha's Vineyard Island, Massachusetts, USA. Surveys one month apart within a 0.6 km×0.6 km area (8–12 m depth) revealed 100-m-scale patches of the tube worm Spiophanes bombyx (⩽30,000 m−2) in fine sand and of the sand dollar Echinarachnius parma (⩽55 m−2) in coarse sand. Raised mud patches that, together with fine sand, occurred in two shore-perpendicular belts are likely exposed, ancient marsh deposits. Depth gradients of sand-ripple geometry indicated that ripples in deeper areas were not in equilibrium with wave conditions monitored during surveys; i.e., they were relict ripples. Thus, sand dollars in some areas may have had >1 month to rework surficial sands since their transformation by physical processes. Linear regressions of ripple characteristics against sand dollar or tube worm densities were not significant, although such relationships would be highly dependent on temporal scale. The survey method described here can be used at more frequent intervals to explore such interactions between epibenthic animals and sediment-transport dynamics.

Research paper thumbnail of A laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments

Marine Geology, 1999

. The LISST Laser In Situ Scattering and Transmissometery instrument was designed by Sequoia Scie... more . The LISST Laser In Situ Scattering and Transmissometery instrument was designed by Sequoia Scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment. To understand the LISST's performance with natural sediments from a variety of marine sources, several experiments were performed to compare the LISST's results to traditional sieving, filtering and weighing techniques. The LISST was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1f for sediment sizes of 5 to 250 mm. Unlike a single frequency scattering sensor for sediment concentration, which require sediment size specific calibration constants, the LISST is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 mm. The LISST was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments. q 1999 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Acoustic measurements of the spatial and temporal structure of the near-bottom boundary layer in the 1990-1991 STRESS experiment

Continental Shelf Research, 1997

As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5... more As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5 MHz Acoustic BackScatter System (ABSS) was deployed in 90 m of water to measure vertical profiles of near-bottom suspended sediment concentration. By looking at the vertical profile of concentration from 0 to 50 cm above bottom (cmab) with 1 cm vertical resolution, the ABSS was able to examine the detailed structure of the bottom boundary layer created by combined wave and current stresses. The acoustic profiles clearly showed the wave-current boundary layer, which extends to (order) 10 cmab. The profiles also showed evidence of an “intermediate” boundary layer, also influenced by combined wave and current stresses, just above the wave-current boundary layer. This paper examines the boundary-layer structure by comparing acoustic data obtained by the authors to a 1-D eddy viscosity model formulation. Specifically, these data are compared to a simple extension of the Grant-Glenn-Madsen model formulation. Also of interest is the appearance of apparently 3-D “advective plume” structures in these data. This is an interesting feature in a site which was initially chosen to be a good example of (temporally averaged) 1-D bottom boundary-layer dynamics. Computer modeling and sector-scanning sonar images are presented to justify the plausibility of observing 3-D structure at the STRESS site. © 1997 Elsevier Science Ltd

Research paper thumbnail of Sediment transport and trapping in the Hudson River estuary

Estuaries, 2001

The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term depo... more The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i.e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neapetides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.

Research paper thumbnail of The impact of acoustic oceanographic methods on estuarine dynamics research

ABSTRACT Estuaries present unique challenges for observational oceanographers, due to their inten... more ABSTRACT Estuaries present unique challenges for observational oceanographers, due to their intense spatial gradients and unrelenting temporal variability. The influence of spatial and temporal variation of estuarine structure and flow on the time-averaged regime is the most important research problem in estuarine physical oceanography. Acoustic methods have played an essential role in revealing this spatial and temporal variability, and new advances in acoustic methods are continuing to provide the most important advances in observations of estuarine processes. The measurement of acoustic backscatter has been a mainstay of estuarine physical oceanography, first for providing qualitative images of the density structure, then for quantifying suspended sediment distributions, and most recently for quantifying the intensity of stratified turbulence. Improved resolution of new systems is revealing the internal structure of shear instability and the mechanics of the transition to turbulence. Acoustic Doppler techniques are so routine now as to be taken for granted, but their impact on the field cannot be overstated, and the new advances in pulse-coherent velocity profiling are continuing this revolution in acoustical oceanography. Acoustic propagation in estuaries has not yet received much attention, but its importance to the operation of autonomous vehicles and long-term monitoring should bring this challenging acoustics problem to the forefront.

Research paper thumbnail of A laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments

Marine Geology, 1999

The LISST (Laser In Situ Scattering and Transmissometery) instrument was designed by Sequoia Scie... more The LISST (Laser In Situ Scattering and Transmissometery) instrument was designed by Sequoia Scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment. To understand the LISST's performance with natural sediments from a variety of marine sources, several experiments were performed to compare the LISST's results to traditional sieving, filtering and weighing techniques. The LISST was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1φ for sediment sizes of 5 to 250 μm. Unlike a single frequency scattering sensor for sediment concentration, which require sediment size specific calibration constants, the LISST is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 μm. The LISST was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments.

Research paper thumbnail of The Mechanics of Marine Sediment Gravity Flows

Continental Margin Sedimentation, 2007

Sediment gravity flows, particularly those in the marine environment, are dynamically interesting... more Sediment gravity flows, particularly those in the marine environment, are dynamically interesting because of the non-linear interaction of mixing, sediment entrainment/suspension and water-column stratification. Turbidity currents, which are strongly controlled by mixing at their fronts, are the best understood mode of sediment gravity flows. The type of mixing not only controls flow and deposition near the front, but also changes the dynamics of turbidity currents flowing in self-formed channels. Debris flows, on the other hand, mix little with ambient fluid. In fact, they have been shown to hydroplane, i.e. glide on a thin film of water. Hydroplaning enables marine debris flows to runout much farther than their subaerial equivalents. Some sediment gravity flows require external energy, from sources such as surface waves. When these flows are considered as stratification-limited turbidity currents, models are able to predict observed downslope sediment fluxes. Most marine sediment gravity flows are supercritical and thus controlled by sediment supply to the water column. Therefore, the genesis of the flows is the key to their understanding and prediction. Virtually every subaqueous failure produces a turbidity current, but they engage only a small percentage of the initially failed material. Wave-induced resuspension can produce and sustain sediment gravity flows. Flooding rivers can also do this, but the complex interactions of settling and turbulence need to be better understood and measured to quantify this effect and document its occurrence. Ultimately, only integrative numerical models can connect these related phenomena, and supply realistic predictions of the marine record.

Research paper thumbnail of Including a Near-Bed Turbid Layer in a Three Dimensional Sediment Transport Model With Application to the Eel River Shelf, Northern California

Estuarine and Coastal Modeling (2003), 2004

Including a Near‐Bed Turbid Layer in a Three Dimensional Sediment Transport Model with Applicatio... more Including a Near‐Bed Turbid Layer in a Three Dimensional Sediment Transport Model with Application to the Eel River Shelf, Northern California. [ASCE Conference Proceedings 145, 48 (2003)]. Courtney K. Harris, Peter Traykovski, W. Rockwell Geyer. Abstract. ...

Research paper thumbnail of Geometry, migration, and evolution of wave orbital ripples at LEO15

Journal of Geophysical Research, 1999

Observations of the temporal evolution of the geometric properties and migration of wave-formed r... more Observations of the temporal evolution of the geometric properties and migration of wave-formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO-15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to nonorbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near-bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave-to-current velocities.