10 research outputs found
High-resolution mapping of mines and ripples at the Martha's Vineyard Coastal Observatory
No full textAuthor 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
No full textAuthor 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
Bonaire and Curaçao
No full textBonaire and Curaçao are oceanic islands surrounded by coral reefs on their leeward sides extending steeply down to mesophotic depths (30–150 m). Easy access from shore, as well as the geopolitical context of the two islands, has made the mesophotic coral ecosystems (MCEs) of Bonaire and Curaçao among the most well studied in the world. MCE research has been conducted in the region since the 1970s, and ongoing research employs state-of-the-art exploration technologies such as manned submersibles, autonomous underwater vehicles, and mixed-gas, rebreather technology. Mesophotic coral communities in Bonaire and Curaçao are typically dominated by agariciids, with Agaricia lamarcki and A. grahamae covering a substantial proportion of the substrate at upper mesophotic depths (30–60 m), while the lower mesophotic (>60 m) consists predominantly of A. grahamae and A. undata. Although much of the habitat is dominated by sediment with only patchy coral growth, in some locations coral assemblages can cover up to 100% of the seafloor down to depths of 70–85 m. Recent biodiversity studies, mostly focusing on fishes, sponges, and corals, documented only limited overlap between shallow and mesophotic reef communities, and that MCE biodiversity is strongly structured by depth. Since MCEs in Bonaire and Curaçao harbor highly specialized communities facing specific threats deriving from their proximity to urbanized land, these ecosystems warrant new management policies and conservation measures. These measures should protect the whole extent of these reefs, which rank among the healthiest in the Caribbean region
Rip current dynamics on an embayed beach
No full textRip currents are fast, narrow currents which traverse the surf-zone in the seaward direction. The most important effect of rip currents is that they can pose a deadly hazard to beach-users. Rip currents and their interaction with waves and underwater morphology are still poorly understood. This is often attributed to a lack of high quality long-term datasets. This shortcoming is due to the difficulty of sampling in the turbulent surf-zone. Past attempts to compare rip current behaviour (e.g. alongshore spacing) to waves have failed to show that they interact.
In this thesis, an improved technique of locating rip channels in video imagery is presented. Previous studies to create computer algorithms to locate rips in video imagery have only looked at one alongshore transect which is averaged in the cross-shore direction, and there have been issues making the algorithms work in complicated cases. The method created in this thesis uses computer algorithms to locate light intensity minima across the entire expanse of the surf-zone in video imagery. This was applied to a dataset from Tairua Beach. The light intensity minima are sorted into distinct rip channels to create a dataset spanning 3.3 years from 1999 until April 2002. Using the high quality rip data output from the algorithms, rip channel morphological reconfiguration events were defined using a measure of change. Wave climate was compared to the timing of these reconfiguration events. It was found that mean wave energy averaged over ten days and wave event duration showed a better relationship to the reconfiguration events than immediate, instantaneous measures of significant wave height.
Rip channel spatial scale (i.e. cross-shore extent) was found to be critical in determining how rip channels behave during high wave events. At Tairua Beach, it was not uncommon for the surf-zone to be wide on one half of the beach and narrow on the other. This 'dual' surf-zone can be attributed to wave shadowing by offshore islands under certain wave directions. Smaller rip channels on the narrow half of the beach changed rapidly whereas larger rips were stable during the same period. This situation shows the importance of both hydrodynamic-control and topographic-control of rips, where rips may respond directly to changes in the wave conditions or be stabilised by the pre-existing morphology respectively. There was also a tendency for rips to form and persist at the headlands of the beach.
A conceptual model was created to demonstrate how rip channels of different spatial scales respond to changes in the wave conditions. Small rips relative to the wave energy are more likely than larger rips to evolve, and vice versa
Bonaire and Curaçao
No full textBonaire and Curaçao are oceanic islands surrounded by coral reefs on their leeward sides extending steeply down to mesophotic depths (30–150 m). Easy access from shore, as well as the geopolitical context of the two islands, has made the mesophotic coral ecosystems (MCEs) of Bonaire and Curaçao among the most well studied in the world. MCE research has been conducted in the region since the 1970s, and ongoing research employs state-of-the-art exploration technologies such as manned submersibles, autonomous underwater vehicles, and mixed-gas, rebreather technology. Mesophotic coral communities in Bonaire and Curaçao are typically dominated by agariciids, with Agaricia lamarcki and A. grahamae covering a substantial proportion of the substrate at upper mesophotic depths (30–60 m), while the lower mesophotic (>60 m) consists predominantly of A. grahamae and A. undata. Although much of the habitat is dominated by sediment with only patchy coral growth, in some locations coral assemblages can cover up to 100% of the seafloor down to depths of 70–85 m. Recent biodiversity studies, mostly focusing on fishes, sponges, and corals, documented only limited overlap between shallow and mesophotic reef communities, and that MCE biodiversity is strongly structured by depth. Since MCEs in Bonaire and Curaçao harbor highly specialized communities facing specific threats deriving from their proximity to urbanized land, these ecosystems warrant new management policies and conservation measures. These measures should protect the whole extent of these reefs, which rank among the healthiest in the Caribbean region
Sistema Joaquina - Morro das Pedras e praias adjacentes da costa leste da Ilha de Santa Catarina: aspectos morfodinâmicos, sedimentológicos e fatores condicionantes
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina. Centro de Filosofia e Ciências Humanas. Programa de Pós-Graduação em GeografiaEntre abril de 2000 e março de 2001 foram monitorados cinco perfis praias ao longo do sistema praial Joaquina - Morro das Pedras objetivando analisar as características morfodinâmicas e sedimentológicas deste sistema. A integração com resultados de pesquisas anteriores permitiu a caracterização morfodinâmica e granulométrica das praias arenosas oceânicas da costa leste da ilha de Santa Catarina. Concluiu-se que as características granulométricas são condicionadas pelas diferentes fontes de sedimentos e pela dinâmica atuante. A granulometria e as modificações sofridas pelas ondas incidentes devido à presença de ilhas e promontórios rochosos condicionam as características morfodinâmicas das praias estudadas
