Offshore Storm Research Articles

Storm-induced inner-continental shelf circulation and sediment transport: Long Bay, South Carolina

Long Bay is a sediment-starved, arcuate embayment located along the US East Coast connecting both South and North Carolina. In this region the rates and pathways of sediment transport are important because they determine the availability of sediments for beach nourishment, seafloor habitat, and navigation. The impact of storms on sediment transport magnitude and direction were investigated during the period October 2003–April 2004 using bottom mounted flow meters, acoustic backscatter sensors and rotary sonars deployed at eight sites offshore of Myrtle Beach, SC, to measure currents, water levels, surface waves, salinity, temperature, suspended sediment concentrations, and bedform morphology. Measurements identify that sediment mobility is caused by waves and wind driven currents from three predominant types of storm patterns that pass through this region: (1) cold fronts, (2) warm fronts and (3) low-pressure storms. The passage of a cold front is accompanied by a rapid change in wind direction from primarily northeastward to southwestward. The passage of a warm front is accompanied by an opposite change in wind direction from mainly southwestward to northeastward. Low-pressure systems passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the offshore storm moves from south to north.During the passage of cold fronts more sediment is transported when winds are northeastward and directed onshore than when the winds are directed offshore, creating a net sediment flux to the north–east. Likewise, even though the warm front has an opposite wind pattern, net sediment flux is typically to the north–east due to the larger fetch when the winds are northeastward and directed onshore. During the passage of low-pressure systems strong winds, waves, and currents to the south are sustained creating a net sediment flux southwestward. During the 3-month deployment a total of 8 cold fronts, 10 warm fronts, and 10 low-pressure systems drove a net sediment flux southwestward. Analysis of a 12-year data record from a local buoy shows an average of 41 cold fronts, 32 warm fronts, and 26 low-pressure systems per year. The culmination of these events would yield a cumulative net inner-continental shelf transport to the south–west, a trend that is further verified by sediment textural analysis and bedform morphology on the inner-continental shelf.

Thresholds for storm impacts on an exposed sandy coastal area in southern Portugal

Storms are one of the most important phenomena responsible for coastal erosion. Their destructive power presents major challenges for coastal management, and knowledge of their characteristics and associated consequences is therefore of paramount importance. In this study, thresholds for storm impacts are defined for a sandy coast in southern Portugal (Faro beach, Ria Formosa) using two different approaches: i) hydrodynamic conditions associated with historical storm impacts (i.e., infrastructural damage); and (ii) computed maximum wave run-up values (RHIGH) compared with beach morphology. Damage thresholds are defined as limits above which the action of a storm exceeds beach front response capability and starts to act directly on human infrastructures. According to method (i), four different thresholds were defined. For individual storms directed from the SW the threshold is a significant wave height of 4.7m with a storm duration of 2days, and for storms directed from the SE is a significant wave height of 6m with a storm duration of at least 2days. Regarding storm groups, for those directed from the SW the threshold is 2 storms lasting at least 2days each with significant wave heights greater than 3.5m, whilst for those directed from the SE the threshold is 3 short storms (lasting 1day each) with significant wave height over 3.9m. The return period for the SW storm group threshold is just 1.7years. For method (ii), beach morphological parameters (DHIGH — highest elevation of the frontal dune; DLOW — elevation of the dune base; and tanβf — foreshore slope) were determined for five chosen cross-shore profiles along the study area, and RHIGH computed for different hydrodynamic conditions. Results were validated through field observations of storm impacts. The lowest thresholds for overwash are along the central part of the study area where a dune crest is absent due to human occupation. The highest thresholds for overwash are on those profiles where a frontal dune is well developed or a foredune ridge is present (western and eastern parts of the study area). This study presents a methodological strategy to compute realistic thresholds for storm impacts along Faro beach based on historical datasets of hydrodynamics and storm impacts information. The approach could be implemented in other coastal areas to provide a comprehensive assessment of the storm impact, requiring only simple information like offshore storm hydrodynamics characteristics, beach morphology and reports of coastal infrastructure damage.

Extraction of P-wave reflections from microseisms

The last few years there has been a growing number of body-wave observations in noise records. In 1973, Vinnik conjectured that P-waves would even be the dominant wavemode, at epicentral distances of about 40 degrees and onwards from an oceanic source. At arrays far from offshore storms, surface waves induced by nearby storms would not mask the body-wave signal and hence primarily P-waves would be recorded. We measured at such an array in Egypt and indeed found a large proportion of P-waves. At the same time, a new methodology is under development to characterize the lithosphere below an array of receivers, without active sources or local earthquakes. Instead, transmitted waves are used which are caused by distant sources. These sources may either be transient or more stationary. With this new methodology, called seismic interferometry, reflection responses are extracted from the coda of transmissions. Combining the two developments, it is clear that there is a large potential for obtaining reflection responses from low-frequency noise. A potential practical advantage of using noise instead of earthquake responses would be that an array only needs to be deployed for a few days or weeks instead of months, to gather enough illumination. We used a few days of continuous noise, recorded with an array in the Abu Gharadig basin, Egypt. We split up the record in three distinct frequency bands and in many small time windows. Using array techniques and taking advantage of all three-component recordings, we could unravel the dominant wavemodes arriving in each time window and in each frequency band. The recorded wavemodes, and hence the noise sources, varied significantly per frequency band, and – to a lesser extent – per time window. Primarily P-waves were detected on the vertical component for two of the three frequency bands. For these frequency bands, we only selected the time windows with a favorable illumination. By subsequently, applying seismic interferometry, we retrieved P-wave reflection responses and delineated reflectors in the crust, the Moho and possibly the Lehmann discontinuity.

Analysis of Packery Channel Public Access Boat Ramp Shoreline Failure

The shoreline stabilization adjacent to the public access boat ramp in the Packery Channel basin has been damaged in two separate events. For the shoreline damage at the boat ramp bulkhead, toe scour is the likely mechanism for failure. Typical sources of hydrodynamic forcing that can lead to toe erosion include storm currents, locally generated storm waves, and offshore storm waves propagating into the basin through Packery Channel. Quantitative analysis of storm induced wind generated waves and currents eliminated them as possible causes of the damage. However, photographic and movie evidence indicate the presence of low-frequency low-amplitude waves propagated into the basin and impacted the boat ramp. The Coastal System Model (CMS) was used to simulate a range of these low-frequency low-amplitude waves and the results demonstrated that these waves could produce sufficient flows in the vicinity of the boat ramp shoreline to cause the damage. Subsequent modeling was used to develop design criteria for additional shoreline stabilization.

浮体式風力発電の実現可能性に関する総合的評価

This paper describes that an alternative fuel production system provided by offshore wind energy is technically and economically feasible. The following knowledge was acquired as a result of studying element technologies aiming at an initial design of an alternative fuel production system using floating wind power. (1) The sea area where the installation of the wind farm was promising was clarified offshore in Japan. And the energy potential was estimated. The amount of the electric power is three times or more the amount that aims by 2010 (2) The box-girder type floating structure mounted two 5MW wind turbines was proposed as a concept. And it was clarified that the structural safety can be kept in a offshore storm set as a design condition. (3) The conceptual design of a new wind turbine which lowered the center of gravity and is suitable for floating body was carried out, and the effect of inertia force was confirmed. (4) EPR of the system was shown to be about 5 or 6 in the evaluation of the energy balance. (5) It was clarified that the maintenance cost plays an important role in the economic evaluation.

Lack of Impact of Hurricane Michelle on Tidal Flats, Andros Island, Bahamas: Integrated Remote Sensing and Field Observations

ABSTRACT Major storms commonly are cited as important geomorphic and sedimentologic agents, and their significance is perhaps most emphatically noted in interpretations of modern and ancient tidal flats. The purpose of this study was to assess the impact of Hurricane Michelle on the carbonate tidal flats of Andros Island, Bahamas, by coupling remote-sensing data and field observations. Although regional MODIS remote-sensing data reveal that the passage of Hurricane Michelle suspended sediments across the shallow bank, integrating ultra-high-resolution remote-sensing data with field observations demonstrates that the hurricane had almost no impact on the Andros Island tidal flats. This lack of impact is surprising because the storm still had hurricane-force winds as it passed immediately over some of the focal areas. With this hurricane, storm strength and intra-storm interval were probably not limiting factors. Instead, several influences combined to minimize the storm's impact on both sedimentation and erosion, including the low level of water on the bank resulting from ebb tides and, in the northwestern area, from offshore storm winds, and the short duration of the storm resulting from its high forward velocity. Given the apparent prevalence of widespread storm effects on ancient tidal flats, but the spatially limited major geomorphic impacts of storms on many modern systems, we suggest that the geologic evolution and preservation of ancient tidal-flat successions may have been shaped by mega-storms that simply cannot form in the present global climate. Nonetheless, understanding tidal-flat dynamics and changes on human timescales will require continued monitoring and assessment of low-impact storms such as Michelle, as well the more sensational hurricanes that produce widespread death, destruction, and deposition.

Beach tar concentrations on the Muscat coastline, Gulf of Oman, Indian Ocean, 1993–1995

Beach tar concentrations in the Capital Area near Muscat, Sultanate of Oman, were measured 20 times at nine sites from February 1993 to February 1995. Tar concentrations were highly variable in space and time, with averages for three transects at each station ranging from zero to over 5 kg m −1 of beach front. Tar values for the November 1993 sampling period averaged 10–100 times the concentrations of other periods. These high tar concentrations were measured about 2 weeks after an offshore storm, which may have caused tankers to jettison petroleum. This level of tar pollution is among the highest previously measured in the world, and is a greater amount than was measured anywhere on the Oman coast a decade ago. In contrast to reports that oil and tar pollution have declined elsewhere in the world, the problem appears to have worsened for Oman, if these determinations for the Capital Area are indicative for the entire coastline.

Space-time characteristics of broadband seismic noise: Results of the onshore component of SAMSON.

In Oct. and Nov., 1990, data from an array of 37 three-component seismometers (16 broadband, 6 5 s, 15 1 s) deployed on the coastal plain of northeastern North Carolina and southeastern Virginia were collected as a part of the multidisciplinary SAMSON (Sources of Ambient MicroSeismic Oceanic Noise) experiment. The array extended from a few hundred meters to 170 km from the coastline. Large variations in the broadband noise level are observed as a function of time with the power during the noisiest time periods being more than 100 times higher than during the quiet times. Many of these noise variations are directly related to regional environmental changes (e.g., hurricane Lily and a ‘‘nor-easter’’) and changes in ocean wave heights observed near Duck, NC); however, some seismic noise variation is uncorrelated with local environmental variables, indicating much more distant sources. The correlation with the ocean wave heights is better at higher frequencies (0.2–0.4 Hz) than at lower. Looking more closely, the ‘‘double-frequency’’ (0.15–1 Hz) and ‘‘single frequency’’ (0.05–0.15 Hz) microseisms show interesting patterns. At certain times, the variations of the power in the two spectral bands are independent of each other, but at other times the power increases occur across the entire 0.05–1 Hz frequency band. When broadband increases in the noise power are observed, the higher frequency noise increase always precedes the lower frequency noise increase, and the noise peak in 0.09–0.2 Hz band typically lags the peak in the 0.4–1.0 Hz band by about 7 h. In several instances, ocean wave heights display similar reddening of the spectra with time, indicating an offshore origin for the low-frequency microseismic noise and reflecting the longer time required by the regional offshore storm systems to generate low-frequency waves. The high-frequency (0.2–1 Hz) band is dominated by a number of peaks lasting about 12 h, primarily during daylight hours, and is also correlated with the local sea state. The character of these peaks indicates a regional offshore source in response to increases in wind-speed during daylight hours. Preliminary frequency–wave-number analysis indicates clear directionality for noise associated with the noise peaks that are not correlated with the local sea state. This indicates a dominant source at a large distance from which seismic waves travel essentially as plane waves across the array. The seismic noise field, however, becomes highly heterogeneous when the noise increase is correlated with the local sea state and is presumably of regional origin. In that case, the energy arrives from many azimuths along the coast line.

Geostrophic circulation or shallow marine turbidity currents? The dilemma of paleoflow patterns in strom-influenced prograding shoreline systems

In ancient storm-influenced prograding shoreline sequences, sole marks (mainly tool marks) from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin. Asymmetrical tool marks typically indicate flows directed offshore. Several workers have attributed their formation to storm-generated, shallow marine turbidity currents. This interpretation conflicts with observations from modern shelves, where storm-driven circulation generally is geostrophically balanced, and time-averaged bottom currents approximately parallel bathymetric contours and the local shoreline. The resolution of these apparently conflicting observations may lie in the realization that tool marks (and many other small paleoflow indicators) form almost instantly as the result of instantaneous flow conditions very near the bed. Beneath storm-generated flows in the shallow ocean, instantaneous and time-averaged characteristics of the bottom boundary layer generally exhibit little similarity. Storm-generated tool marks are formed by the movement of large tools within the thin (less than 1 m) inner boundary layer resulting from the superimposition of waves and currents. The orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave-orbital motions, which typically are normal to shore. The magnitude of stress is greatly increased in the offshore direction (and decreased in the onshore direction) by superimposition of a steady current with an offshore component of flow, but the direction of stress is only slightly affected. In ancient storm-influenced sequences, therefore, shore-normal tool marks generally were not formed by turbidity currents; rather, their orientation is best attributed to shoaling waves approaching the coast at a very high angle. Asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component. Tool marks do not reflect the time-averaged bottom-flow direction; in fact, they provide almost no information concerning steady bottom currents. In contrast, high-angle cross-beds (formed in coarser sediment by the migration of dunes and sandwaves), although relatively rare in offshore storm deposits, generally reveal approximately shore-parallel flows in ancient systems. Cross-beds closely reflect the time-averaged flow direction in the outer boundary layer of a geostrophic current, for three reasons: 1) the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time-averaged shear stress; 2) large ripples disrupt the thin inner boundary layer; and 3) long time intervals (relative to wave-induced velocity oscillations) are required to form large ripples.

The Quaternary sediments of Banquereau, Scotian Shelf

The acoustic stratigraphy of Quaternary sediments on Banquereau, Scotian Shelf, interpreted from regional and site-survey seismic lines, is correlated with sedimentary units determined from undisturbed borehole samples. The informal litho-stratigraphic formations of the Quaternary sequence on the Scotian Shelf are subdivided into members and litho-chronozones diagnostic of depositional environments. A late Wisconsinan glacial advance, dated by C 14 analysis of shell material, took place ca. 18,000 to 26,000 yr B.P. It formed a widespread, gravel-rich unit barren of fauna and capped by an erosional surface. The unit occurs within stratified pro-glacial marine sediments known locally as the “Emerald silt formation.” The minimum stand of sea level, 110 m below present sea level, occurred ca. 14,000 yr B.P. and is confirmed by the presence of channel deposits at the top of the Pleistocene section. A marine transgression across Banquereau took place ca. 8,000 yr B.P. and is defined by a widespread erosion surface at the base of the Holocene section. This section, known locally as the “Sable Island sand and gravel formation,” comprises as much as 40 m of reworked sand which is divided into 3 members diagnostic of sand-ridge generation. Eastern Shoal, an offshore storm ridge situated on Banquereau, was formed during Holocene time, created by modern-day, storm-driven and tidal currents. The intensity of these currents is the result of water-mass acceleration over the shallow banks of the Scotian Shelf. Eastern Shoal appears to have moved 30 km across the continental shelf at a rate of ∼3–5 m/yr. The regional setting of this sand body is similar to that postulated for the Shannon and Tocito Sandstones of the Western Interior of the United States and shows that sand transport and large-scale ridge development can take place in a shelf-edge setting on present-day, storm-dominated continental shelves.

Storm- and Tide-Dominated Shorelines in Cretaceous Moosebar-Lower Gates Interval--Outcrop Equivalents of Deep Basin Gas Trap in Western Canada

The Moosebar and Gates Formations crop out in the deformed Foothills belt south of Fort St. John, British Columbia. They permit examination of the depositional environments of sandstones and conglomerates that are gas-bearing in the equivalent Wilrich-Falher interval of the Deep basin of Alberta and British Columbia. The Moosebar-Gates interval consists of several upward-coarsening and marine to nonmarine sequences. Moosebar bioturbated shales pass upward into turbidites and offshore storm deposits. Flow directions indicate a north-dipping paleoslope. A second coarsening-upward sequence at the top of the Moosebar and base of the Gates terminates in a lenticular fluvial conglomerate up to 30 m thick, which is overlain by coals, carbonaceous mudstones, and sandstones. Above the carbonaceous zone, a major lateral facies change is observed. Fluvial conglomerates in the south grade into beach conglomerates northward. The upper part of the beach conglomerate consists of exposed three-dimensional storm berms, at least 500 m long, up to 1.5 m high, with wavelengths of 12 to 24 m. The berms trend east-west. Seaward of the conglomerate beach are herringbone cross-bedded (tidal) sandstones with a marine trace fauna. Graded sharp-based conglomerates also are present, probably washed out to sea by storms. Farther north, at the same stratigraphic level, is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across. The tops of these bars are covered with storm-formed symmetric conglomeratic dunes. Details of the orientation and geometry of these conglomerate bodies, along with their internal structure and pebble fabrics, suggest that four major environments (fluvial, beach, offshore conglomerate bars, and offshore graded storm conglomerates) should be identifiable and predictable in the subsurface.

Lower Silurian distal shelf storm-induced turbidites in the Welsh Borders: sediments, tool marks and trace fossils

The Upper Llandovery Hughley Shales of Shropshire are interpreted as storm-induced distal shelf turbidites. They comprise a sequence of calcareous sandstones interbedded with purple mudstones. The trace fossils, body fossils, and palaeogeography indicate a deep shelf position for their deposition. The thin sandstone units are interpreted as having been deposited from density current bedload and suspension. Storm surge ebb traction currents, arising from intermittent major offshore storms, are considered to have initiated the density currents. A deepened storm wave base allowed transportation of fine sand in suspension out onto the distal shelf. Internally, the resultant sands are differentiated from distal (fan) turbidites by multi-directional bedload currents, which are shown most clearly by spectacular tool marks on sandstone bases. The tools are clearly identifiable in many cases as indigenous benthonic organisms such as horn corals, coral colonies, crinoids, brachiopods and trilobites. The sandstone beds were later colonized by burrowers and other track-producing organisms. The trace fossils include Diplocraterion (or Corophioides ), Palaeophycus , Skolithos, Chondrites , Scolicia , Rusophycus , Cruziana and Diplichnires (Cruziana facies assemblage). A new ichno-species, Walcottia devilsdingli is established for small burrows with regular lateral appendage marks at certain points.

EARLY PERMIAN DEPOSITIONAL ENVIRONMENTS, SOUTHERN SYDNEY BASIN

Seven major lithofacies are present in the Lower Permian sediments of the Jervis Bay area (southern Sydney Basin). These facies, which are areally extensive, are members of the Pebbly Beach Formation (facies A), the overlying Snapper Point Formation (facies B-F) and the Wandrawandian Siltstone (facies G).Facies A to D represent a regressive shoreface succession which passes upwards into lagoonal and marsh sediments. They were deposited during the Snapper Point regression. Currents were predominantly long-shore with rip and onshore and offshore storm current components.Facies E to F constitute a regressive (i.e. shoaling upwards) spit profile which Accreted by longshore progradation during the early stages of the Wandrawandian transgression. They form the upper part of the Snapper Point Formation.Facies G comprises sandy muds of the Wandrawandian Siltstone which were deposited in a marine embayment at temperatures below 0°C as evidenced by glendonites (pseudomorphs after thenardite). The muds represent the culmination of the Wandrawandian transgression which followed upon the disruption of sediment supply to the spits and their consequent inundation.This depositional history resolves earlier conflicting interpretations of this part of the southern Sydney Basin sequence. It provides a better understanding of the origin of the Snapper Point Formation, which has been a common target for onshore petroleum exploration drilling.

HURRICANE TIDE PREDICTION FOR NEW YORK BAY

This paper is concerned with the solution of the problem of correlating, on a two-dimensional basis, the meteorological parameters of several off-shore storms with the known surge induced by them in New York Bay and with the application of the results to the prediction of likely effects in New York Bay from a design hurricane of given strength traversing a given path at a given speed. A purely theoretical approach would have been beyond practical possibility within the time available for this study; the method adopted therefore is empirical but with some degree of theoretical guidance. A recursion formula is evolved, using the method of finite differences for time increments of 1/3 hour, which relates tide elevation at the bay-mouth with two values of the elevation at 1/3 and 2/3 hour earlier and with values of wind-stress and pressure-gradient driving-force components (directed towards New York Bay from several remote two-dimensionally spaced offshore-stations on the continental shelf) at times earlier by the periods taken for free long gravity waves to travel from the stations to the bay-mouth. The formula includes a comulative forcing function term which allows for the geostrophic influence of the earth's rotation and also for an "edge-wave" effect northward along the eastern seaboard. Moreover it takes into account the observed tendencies of hurricane storm tides in New York Bay to develop resurgences at periods of 7 hours with decay rates of 50% amplitude decrease per cycle. The coefficients of the "forcing functions", determined by correlation, tend to represent the storm size and speed and also the dynamic augmentation of the forced wave. Predicted maximum storm tide heights are in fair agreement with crude empirical estimates based on central pressures within the hurricanes. Predictions, however, provide complete time-sequences of water level for periods up to 24 hours inclusive of the first resurgence after the main surge.