Complex inner shelf environments: Observations and modeling of morphodynamics and scour processes

The inner continental shelf is a complex environmental system marked by sharp variations in bed roughness. Such heterogeneous systems account for 80% of the non-rocky inner shelves worldwide. Interactions among forces (waves, tides, turbulence, and bioturbation) and roughness elements (bed forms, rocks, and anthropogenic objects) exert major controls on sedimentary processes. This study attempts to advance the knowledge and understanding of the morphodynamics of the inner shelf. This study investigates scour and morphodynamic processes at Tairua, New Zealand; Cedar Island, Virginia; Indian Rocks Beach, Florida; and Beaufort Inlet, North Carolina. Using data from the field, the study develops new conceptual models to characterize and quantify the hydrodynamics and morphology of the seabed. The overall dataset includes side-scan sonograms, sub-bottom profiles, grain-size analyses, suspended sediment concentrations and hydrodynamic measurements. Analysis of the morphological data yielded a six-type classification of bottom features previously termed Rippled Scour Depressions (RSDs). The observed stratigraphic signature of RSDs does not agree with the previous interpretation of their formation. Striking spatial and temporal variations in seabed roughness produce significant enhancements of hydraulic roughness and turbulence over different substrates resulting in a self-organized, feed-back system of erosion (scour), deposition, and modified bed forms. The study demonstrates that widely used ripple models inadequately predict bed form geometry and behavior, especially during storms. Improved understanding of scour processes developed in this study leads to a new model of scour and burial of sea-bed objects such as naval mines and archaeological artifacts. When using the model to predict scour and burial, the greatest errors result from the uncertainties in the available forecasts of wave conditions. The model includes vertical variations in sediment characteristics as field observations indicate abrupt changes in substrate substantially alter the scour process. The overall study makes substantial contributions to the general understanding of RSD behavior by tying together detailed field studies with applicable insights from the area of complexity research. A new conceptual model of complex phase-transition is developed, involving critical process factors (hydrodynamics, underlying geology, and depth), which contribute to the observed spatial complexity and temporal variability of different RSD types

Modular Autonomous Biosampler (MAB)- A prototype system for distinct biological size-class sampling and preservation

Presently, there is a community wide deficiency in our ability to collect and preserve multiple size-class biologic samples across a broad spectrum of oceanographic platforms (e.g. AUVs, ROVs, and Ocean Observing System Nodes). This is particularly surprising in comparison to the level of instrumentation that now exists for acquiring physical and geophysical data (e.g. side-scan sonar, current profiles etc.), from these same platforms. We present our effort to develop a low-cost, high sample capacity modular,autonomous biological sampling device (MAB). The unit is designed for filtering and preserving 3 distinct biological size-classes (including bacteria), and is deployable in any aquatic setting from a variety of platform modalities (AUV, ROV, or mooring)

Correction of Bathymetric Survey Artifacts Resulting from Apparent Wave-Induced Vertical Position of an AUV

Recent increases in the capability and reliability of autonomous underwater vehicles (AUVs) have provided the opportunity to conduct bathymetric seafloor surveys in shallow water (\u3c 50 m). Unfortunately, surveys of this water depth may contain artifacts induced by large amplitude wave motion at the surface. The artifacts occur when an onboard pressure sensor determines the depth of the AUV. Waves overhead induce small pressure fluctuations at depth, which modulate the AUV’s pressure sensor output without causing actual vertical movement of the AUV. Since bathymetric measurements are made with respect to the AUV’s depth, these pressure fluctuations, in turn, modulate the measurement of the seafloor. The result is a periodic across-track, vertical offset of the seafloor profile (similar to a heave artifact sometimes common in surface vessel surveys). In this paper we describe our experience with the “Gavia” model AUV (Hafmynd EHF, Iceland) in a recent bathymetric survey during which wave action overhead induced such an artifact with a peak-to-peak amplitude as large as 1 meter. A method for removing the artifact as well as recommendations for modifications to the sonar, INS and AUV to mitigate the effect in the future are provided

A Study to Assess the Effect of Tow Duration and Estimate Dredge Efficiency for the VIMS Sea Scallop Dredge Survey : Final Report

For the sea scallop, Placopecten magellanicus, the concepts of space and time have emerged as the basis of an effective management tool. The strategy of closing or limiting activities in certain areas for specific lengths of time has gained support as a method to conserve and enhance the sea scallop resource. In the last decade, rotational area management has provided a mechanism to protect juvenile scallops from fishing mortality by closing areas based upon scallop abundance and age distribution. Approximately half of the sea scallop industry’s current annual landings come from areas under this rotational harvest strategy. While this represents a management success, it also highlights the extent to which landings are dependent on the success of this strategy. The continued prosperity of scallop spatial management is dependent on both periodic and large incoming year classes, as well as, a mechanism to delineate the scale of a recruitment event and subsequently monitor the growth and abundance of these scallops over time. Current and accurate information related to the abundance and distribution of adult and juvenile scallops is essential for managers to respond to changes in resource subunits

Designing Improved Sediment Transport Visualizations

Monitoring, or more commonly, modeling of sediment transport in the coastal environment is a critical task with relevance to coastline stability, beach erosion, tracking environmental contaminants, and safety of navigation. Increased intensity and regularity of storms such as Superstorm Sandy heighten the importance of our understanding of sediment transport processes. A weakness of current modeling capabilities is the ability to easily visualize the result in an intuitive manner. Many of the available visualization software packages display only a single variable at once, usually as a two-dimensional, plan-view cross-section. With such limited display capabilities, sophisticated 3D models are undermined in both the interpretation of results and dissemination of information to the public. Here we explore a subset of existing modeling capabilities (specifically, modeling scour around man-made structures) and visualization solutions, examine their shortcomings and present a design for a 4D visualization for sediment transport studies that is based on perceptually-focused data visualization research and recent and ongoing developments in multivariate displays. Vector and scalar fields are co-displayed, yet kept independently identifiable utilizing human perception\u27s separation of color, texture, and motion. Bathymetry, sediment grain-size distribution, and forcing hydrodynamics are a subset of the variables investigated for simultaneous representation. Direct interaction with field data is tested to support rapid validation of sediment transport model results. Our goal is a tight integration of both simulated data and real world observations to support analysis and simulation of the impact of major sediment transport events such as hurricanes. We unite modeled results and field observations within a geodatabase designed as an application schema of the Arc Marine Data Model. Our real-world focus is on the Redbird Artificial Reef Site, roughly 18 nautical miles offshor- Delaware Bay, Delaware, where repeated surveys have identified active scour and bedform migration in 27 m water depth amongst the more than 900 deliberately sunken subway cars and vessels. Coincidently collected high-resolution multibeam bathymetry, backscatter, and side-scan sonar data from surface and autonomous underwater vehicle (AUV) systems along with complementary sub-bottom, grab sample, bottom imagery, and wave and current (via ADCP) datasets provide the basis for analysis. This site is particularly attractive due to overlap with the Delaware Bay Operational Forecast System (DBOFS), a model that provides historical and forecast oceanographic data that can be tested in hindcast against significant changes observed at the site during Superstorm Sandy and in predicting future changes through small-scale modeling around the individual reef objects

Local-scale post-event assessments with GPS and UAV-based quick-response surveys:A pilot case from the Emilia-Romagna (Italy) coast

Coastal communities and assets are exposed to flooding and erosion hazards due to extreme storm events, which may increase in intensity due to climatological factors in the incoming future. Coastal managers are tasked with developing risk-management plans mitigating risk during all phases of the disaster cycle. This necessitates rapid, time-efficient post-event beach surveys that collect physical data in the immediate aftermath of an event. Additionally, the inclusion of local stakeholders in the assessment process via personal interviews captures the social dimension of the impact of the event. In this study, a local protocol for post-event assessment, the quick-response protocol, was tested on a pilot site on the Emilia-Romagna (Italy) coast in the aftermath of an extreme meteorological event that occurred in February 2015. Physical data were collected using both real-time kinematic Geographical Positions Systems and unmanned aerial vehicle platforms. Local stakeholders were interviewed by collecting qualitative information on their experiences before, during, and after the event. Data comparisons between local and regional surveys of this event highlighted higher data resolution and accuracy at the local level, enabling improved risk assessment for future events of this magnitude. The local survey methodology, although improvable from different technical aspects, can be readily integrated into regional surveys for improved data resolution and accuracy of storm impact assessments on the regional scale to better inform coastal risk managers during mitigation planning

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

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 situhydrodynamic 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

High-resolution mapping of mines and ripples at the Martha's Vineyard Coastal Observatory

Author Posting. © IEEE, 2007. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 32 (2007): 133-149, doi:10.1109/JOE.2007.890953.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.The work of L. A. Mayer, R. Raymond, G. Glang, P. Traykovski, and A. C. Trembanis was supported by the U.S. Office of Naval Research (ONR) under the Grants N00014-01-1-0847, N00014-01-10564, and N00014-03-1-0298. The work of M. D. Richardson was supported by the U.S. Office of Naval Research (NRL) under the Core funding. The work of L. A. Mayer, R. Raymond, and G. Gland was also supported by the National Oceanic and Atmospheric Administration (NOAA) under the Grant NA17OG2285

Predicting seabed burial of cylinders by wave-induced scour : application to the sandy inner shelf off Florida and Massachusetts

Author Posting. © IEEE, 2007. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 32 (2007): 167-183, doi:10.1109/JOE.2007.890958.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.This work was supported by the grants from the U.S. Office of Naval Research Marine Geosciences Program. The work of A. C. Trembanis was supported by the USGS/WHOI Postdoctoral Fellowship