[1] Measurements of seafloor ripples under wave-dominated conditions from the LEO15 site and the Martha’s Vineyard coastal observatory were used to develop a time-dependent model for ripple geometry. The measurements consisted of backscatter imagery from rotary side-scan sonars, centimeter resolution bathymetric maps from a two-axis rotary pencil-beam sonar, and forcing hydrodynamics. During moderate energy conditions the ripple wavelength typically scaled with wave orbital diameter. In more energetic conditions the ripples reached a maximum wavelength of 0.8 to 1.2 m and did not continue to increase in wavelength or decrease in height. The observations showed that the relict ripples left after storms typically had wavelengths close to the maximum wavelength. The time-dependent model is based on an equilibrium model that allows the ripples to maintain wavelength proportional to wave orbital diameter until a suspension threshold determined by wave velocity and grain size is reached. The time-dependent model allows the ripple spectra to follow the equilibrium solution with a temporal delay that is based on the ratio of the ripple cross-sectional area to the sediment transport rate. The data was compared to the equilibrium model, a simplified version of the time-dependent model (where the ripples were assumed to follow the equilibrium model only when the bed stress was sufficient to move sediment), and the complete time-dependent model. It was found that only the complete time-dependent model was able to correctly predict the long wavelength relict ripples and that the other approaches underpredicted relict ripple wavelengths.

/pdf/observations-of-wave-orbital-scale-ripples-and-a-1kqp059c2p.pdf

Observations of wave orbital scale ripples and a nonequilibrium time‐dependent model

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 ldquowindowsrdquo to the coarse layer below appear in the fine-grained sands. These ldquowindowrdquo 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 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.

/pdf/high-resolution-mapping-of-mines-and-ripples-at-the-martha-s-psyzv9chhu.pdf

High-Resolution Mapping of Mines and Ripples at the Martha's Vineyard Coastal Observatory

New multibeam echosounder and processing technologies yield sub-meter-scale bathymetric resolution, revealing striking details of bedform morphology that are shaped by complex boundary-layer flow dynamics at a range of spatial and temporal scales. An inertially aided post processed kinematic (IAPPK) technique generates a smoothed best estimate trajectory (SBET) solution to tie the vessel motion-related effects of each sounding directly to the ellipsoid, significantly reducing artifacts commonly found in multibeam data, increasing point density, and sharpening seafloor features. The new technique was applied to a large bedform field in 20–30 m water depths in central San Francisco Bay, California (USA), revealing bedforms that suggest boundary-layer flow deflection by the crests where 12-m-wavelength, 0.2-m-amplitude bedforms are superimposed on 60-m-wavelength, 1-m-amplitude bedforms, with crests that often were strongly oblique (approaching 90°) to the larger features on the lee side, and near-parallel on the stoss side. During one survey in April 2008, superimposed bedform crests were continuous between the crests of the larger features, indicating that flow detachment in the lee of the larger bedforms is not always a dominant process. Assessment of bedform crest peakedness, asymmetry, and small-scale bedform evolution between surveys indicates the impact of different flow regimes on the entire bedform field. This paper presents unique fine-scale imagery of compound and superimposed bedforms, which is used to (1) assess the physical forcing and evolution of a bedform field in San Francisco Bay, and (2) in conjunction with numerical modeling, gain a better fundamental understanding of boundary-layer flow dynamics that result in the observed superimposed bedform orientation.

http://seafloor.otterlabs.org/publications/Barnard_etal_GeoMar2011.pdf

Small-scale sediment transport patterns and bedform morphodynamics: new insights from high-resolution multibeam bathymetry

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

/pdf/mine-burial-experiments-at-the-martha-s-vineyard-coastal-14iy00n2to.pdf

Mine Burial Experiments at the Martha's Vineyard Coastal Observatory

[1] On 30 October 2012, Hurricane Sandy made landfall near Brigantine New Jersey bringing widespread erosion and damage to the coastline. We have obtained a unique set of high-resolution before and after storm measurements of seabed morphology and in situ hydrodynamic conditions (waves and currents) capturing the impact of the storm at an inner continental shelf field site known as the “Redbird reef”. Understanding the signature of this storm event is important for identifying the impacts of such events and for understanding the role that such events have in the transport of sediment and marine debris on the inner continental shelf. As part of an ONR-sponsored program designed to understand and characterize the ripple dynamics and scour processes in an energetic, heterogeneous inner-shelf setting, a series of high-resolution geoacoustic surveys were conducted before and after Hurricane Sandy. Our overall goal is to improve our understanding of bedform dynamics and spatio-temporal length scales and defect densities through the application of a recently developed fingerprint algorithm technique. Utilizing high-resolution swath sonar collected by an AUV and from surface vessel sonars, our study focuses both on bedforms in the vicinity of manmade seabed objects and dynamic natural ripples on the inner shelf in energetic coastal settings with application to critical military operations such as mine countermeasures.

/pdf/a-detailed-seabed-signature-from-hurricane-sandy-revealed-in-14wz6tkyf8.pdf

A detailed seabed signature from Hurricane Sandy revealed in bedforms and scour

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 (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.

/pdf/predicting-seabed-burial-of-cylinders-by-wave-induced-scour-mdcn1s9iz7.pdf

M.D. Richardson

Papers

Predicting Seabed Burial of Cylinders by Wave-Induced Scour: Application to the Sandy Inner Shelf Off Florida and Massachusetts