452 research outputs found
Rip currents, morphodynamic simulations
No full textRip currents are narrow and intense, offshore directed flows in the nearshore zone. A rip current is able to transport a considerable amount of water and sediment seaward and may dominate the morphological exchange between surfzone and the offshore. Rip currents are often located nearby lateral boundaries that obstruct the longshore current, but they can appear as well on long straight beaches, Rip currents can arise with a regular spacing along the beach. In case of a barred beach profile the return flow is concentrated in eroded channels across the bar. Rip currents are not a permanent phenomenon. Under a combination of favourable hydrodynamic conditions they may be generated. Within half a day rips can develop more pronounced. The rip channel may migrate along the open beach and vanish after having persisted for some time, up to several months. The objective of this study is to construct a morphological development associated with an initial rip current. It is investigated in what way the simulations can be valdidated by the ARGUS video system. This system generates images of the nearshore zone on an hourly basis. A rip current is a wave induced flow. Transforming waves exert a force on the fluid mass in which they propagate. In order to balance this force, a pressure gradient or a velocity will arise, This leads to variations in the local mean water depth and currents in the nearshore zone. At the same time the generated current influences the waves. A current in the nearshore zone opposing the waves, like a rip current yield an increased wave height and a reduced wave length so that the wave is steepened, even up to breaking. For successful simulations of an evolving rip current, it is required to know plausible mechanisms that are involved in rip current generation. In literature several theories can be found, explaining the generation of rip currents. Most of them assume differences in wave set-up as mechanism that drives the current. This difference in wave set-up can originate from either hydrodynamic interactions or morphological irregularities. The simulations in this study are executed in DELFT2D-MOR, a numerical model developed at Delft Hyrdaulics. It comprehends modules for simulating hydrodynamics and morphodynamics. In order to obtain a realistic and steady state wave-current field, iterations between wave- and current computations are required. The most direct feedback between the hydrodynamic modules along with a short period of the flow simulations give the best results. A barred profile is used as input for the simulations, On a uniform beach, without any longshore perturbation, the simulations show weak, but clearly distinguishable circulation cells. Simulations starting with a small initial rip channel show stronger currents, Increasing the width of the inner trough increases rip velocities. Increasing of the wave height at the offshore boundary results in increase of the rip current velocity. But, beyond a certain value of the applied wave height, the velocity decreases because of rise of the set-up at the rip head. Waves approaching under a relatively large angle ( > 5 dgr to shore normal) fade the effect of the initial rip channel away. Small angles of wave incidence stimulate the development of a rip. On the one hand because the longshore current is much stronger than the current only generated by differences in set-up. On the other hand because, the longshore current drops sediment from the nearshore zone not directly at the offshore end of the rip channel but more downstream. In the initial rip, the breaking of the waves due to the depth at the bar face, coincides with the maximum current velocity. This results in an increase in the dissipation intensity more than a shift of the dissipation peak down stream. So the wave dissipation, as observed on ARGUS images from rips in Noordwijk, is strongly related to the development of sill at the rip head. Morphodynamic simulations show the deposition of a sill at the downstream end of the rip and a slight growth of the rip channel. The morphological development of the rip, resulting from simulations is strongly dependent on the course of the morphodynamic computation. Frequent feedback shows a splitting of the current and sideward discharge. Longer morphological time steps between calling of the hydrodynamic modules show that the rip current is excavating through the sill. Addition of wave mass flux formulations to the model does not modify the flow pattern of the initial rip current significantly. Differences resulting from taking wave mass fluxes into account, arise from other computed shear stresses, because these are based on a different defined velocity. Computations with wave mass flux cause an increase of the sediment transport rates. This effect is not confined to the rip so the whole beach profile strongly changes.Hydraulic EngineeringCivil Engineering and Geoscience
Pictures from an Antarctic expedition: Mikhailov on the Bellingshausen voyage
No full textABSTRACTThe artist Pavel Nikolayevich Mikhailov (1786–1840) took part in the Antarctic expedition of the Imperial Russian Navy commanded by Captain F.F. Bellingshausen from 1819 to 1821. The author was invited to view the collection of Mikhailov's work, from two expeditions, that is held at the Russian Museum, St Petersburg. The Bellingshausen pieces in the collection are described, and their relationships to the smaller collection of Mikhailov's work for the Bellingshausen expedition, held at the State Historical Museum in Moscow, to the lithographs in the Atlas volume of Bellingshausen's published narrative, and to the various versions of Mikhailov's images that have been published in the 20th and 21st centuries, are discussed.</jats:p
Field observations of rip current dynamics
No full textField observations of rip currents have been one of the most difficult measurements to obtain, due to the hazardous nature and the fact the rip current channels tend to migrate. This dissertation examines: 1) innovative methods of obtaining nearshore bathymetry and current profiles within rip current channels, 2) the analysis of long-term video monitoring of rip current morphology at Duck, NC, and 3) the hydrodynamics (steady, sea breeze, infragravity (0.004-.04 Hz), and very low frequency «0.004 Hz)) and morphodynamics of a rip current field experiment (RIPEX) in Sand City, Monterey Bay, CA. In order to acquire proper measurements of nearshore morphology, traditional hydrographic surveying gear was placed in water-tight containers and mounted on a personal watercraft (PWC). The PWC system was tested and evaluated in Myrtle Beach, SC and tilized on the beaches of California, Florida, Oregon, and Washington. An acoustic Doppler current profiler was added to the PWC for the cquisition of vertical velocity profiles within a ip current system, which was evaluated against fixed instruments. hree years of time-averaged video images from an Argus Station at Duck, NC were analyzed to gain insight on rip current channel behavior. Long-term observations suggest that rip current morphology is quasi-stable, and is only modified during extreme storms (Hmo>3) when there is a significant longshore current (V> 1m/s). uring Spring 2001, a rip current field experiment (RIPEX) was carried out in which an extensive array of wave current sensors were deployed on a complex beach, where rip urrents were relatively persistent and plentiful. The easurements obtained during RIPEX llowed for a complete evaluation of rip current behavior through current, pressure, video, and athymetric measurements, currently missing in the existing literature. Particular attention was ocused on rip current pulsations, steady flows, sea breeze conditions, morpho- dynamics, vertical flow structure, and newly observed very low frequency motions
Rip Relating his Story. Rip Van Winkle.
No full textLantern slide of an illustration, circa 1905, from the short story, Rip Van Winkle, by the New York author Washington Irving.https://research.library.kutztown.edu/lanternslideseducation/1642/thumbnail.jp
Beach erosion and rip channels
No full textShort note on the relation between rip channels and beach erosion
Cold war whaling: Bellingshausen and the <i>Slava</i> flotilla
No full textABSTRACTOn 7 December 1945 a captured German whaling factory, Wikinger, was allocated to the Soviet Union under the terms of the Potsdam Agreement between that country, the United States and the United Kingdom. In the first section, this article presents the first detailed account of how Wikinger was seized by the Royal Navy and eventually transferred to Soviet ownership. The second section illustrates the hostile attitudes of western governments towards the Slava whaling flotilla during the cold war, and the degree to which their suspicions were focused on the work of scientists assigned to the flotilla. The next four sections trace the fluctuating perceptions and presentations, during the Tsarist and early Soviet periods, of the Imperial Russian Navy's Antarctic expedition of 1819–1821, the problems in respect of Antarctica which confronted Soviet diplomacy and propaganda in the 1940s, and the new story, about Russians having been the first people to discover Antarctica, which was developed in order to address them. It is then possible, in the seventh section, to explain the political utility of the Slava flotilla in the early 1950s. An eighth section sketches the divergent cultural fortunes of the Bellingshausen expedition and the Slava flotilla after the period under consideration.This article discusses the use of whaling and history in support of Soviet Antarctic policy between the end of World War 2 and the International Geophysical Year (IGY) of 1957–1958. But the Slava whaling flotilla did not just play a part in the historicisation of Soviet Antarctic policy. It was itself a historically constituted object, fraught with meanings on both sides of the cold war. For that reason the opportunity is taken to give a more detailed account of the flotilla's origins than has been available hitherto. The author notes that two contributors to this journal have preceded him in some of these matters (Armstrong 1950, 1971; Gan 2009). He ventures to suggest, however, that the connections between whaling, historiography and public information management in Soviet Antarctic policy have not been fully demonstrated before this.</jats:p
Proposal and verification of rip current detection using AI
No full textThere are from 2,000 to 3,000 rescues including those of unconscious people every year on the beaches of Japan, as shown in Fig. 1. The occurrence of drowning accidents is mainly caused by the rip current (Ishikawa et al., 2014), it accounts for 48 % of drowning accidents, as shown in Fig. 2. Also, in Australia, the United States and the United Kingdom, more than 50 % of rescue accidents are caused by rip currents (Brighton et al., 2013). In order to reduce the rip current accidents, beach users need to recognize rip currents, then they have to avoid them using risk assessment. However, it is the difficulty of risk recognition and judgement under the momentary change in natural phenomenon for beach users. Especially, almost all beach users understand the risk in the case of high wave conditions due to easy visual understanding, whereas they cannot understand rip currents the same way. On the other hand, swimming areas along the shore are very limited, however the number of lifeguards is small at around 1 lifeguard compared to the thousands of beach users. In addition, beach users sometimes enter unpatrolled areas outside the swimming areas. Therefore, we developed a new technology that can automatically detect the rip currents by the Artificial Intelligence (AI), and notify beach users and lifeguards using the Internet of Things (IoT). In this study, we verified the accuracy of the rip current detection by the AI, using a field measurement, an image analysis and a numerical simulation. Also, we examined the log data of 2019 that was actually operated
Rip currents: A laboratory study of a rip current in the presence of a submerged reef
No full textDetailed laboratory measurements of the wave and current field induced by bi-chromatic waves incident on an artificial surfing reef are presented. Strong wave focussing occurs on the surfing reef. Both PTV and in-situ measurements are reported that show the presence of a strong rip current flowing oblique off-shore. Observed water level set-up and set-down indicate the locations of feeder currents, driving the rip current. Wave-current interaction between the incoming bi-chromatic waves and the outgoing rip-current are apparent in the measurements. The stochastic character of meandering of the rip current is shown by the variance of the low pass filtered time-averaged local velocities. The vertical velocity distribution could not be determined reliably due to limitations in the test set-up and measurement devices. The dispersion relationship is utilized to connect the PTV surface measurement with the underlying velocity field obtained by in-situ measurements. The total dataset gives a detailed synoptic view of the wave-current field that can be used for model verification and validation.Civil Engineering and Geoscience
Rip at the Village Inn. Irving\u27s Rip van Winkle.
No full textLantern slide of an illustration, circa 1905, from the short story, Rip Van Winkle, by the New York author Washington Irving.https://research.library.kutztown.edu/lanternslideseducation/1641/thumbnail.jp
Rip Current Characteristics at the Dutch Coast: Egmond aan Zee
No full textRip currents are narrow, seaward directed flows in the surf zone that can pose a serious threat to swimmers. This issue has received attention particularly on swell dominated coasts (such as the US, Australia, France and UK) where numerous field experiments have been undertaken. However, the threat of rip currents is less recognised on wind-sea dominated coasts such as the North Sea, even though a consistent number of swimmers drift offshore (in rip currents) and require rescue by surf lifeguards each year (for example at Egmond aan Zee, The Netherlands). In August 2011, a five day field experiment was conducted at Egmond aan Zee. Lagrangian velocities in the surf zone were measured with drifter instruments and human drifters that were tracked via GPS. An extensive dataset of measurements was collected from which parameters that govern the strength of rip currents and affect their mean flow properties were identified. Three flow patterns were observed in the experiment: (1) a locally governed circulation cell, (2) a pattern in which the drifter initially floats offshore and then is advected by a strong long-shore current and (3) a meandering longshore current. A variety of rip current velocities were measured with the strongest being approximately 0.6 m/s. A statistically significant correlation between the ratio of offshore wave height over water depth on the bar and rip current speeds was established from the data. A 2-dimensional hydrostatic XBeach model was validated against laboratory rip current experiments and field data from Egmond aan Zee. A sensitivity analysis was performed to test a range of parameters that were categorised into two groups of different time scales: hourly and daily altering hydrodynamic parameters, and daily to weekly (sometimes longer) varying geometric parameters. Various hydrodynamic scenarios of wave height, wave period, wave angle and tidal water level were tested to evaluate the influence on rip current initiation and mean flow properties. Additionally, the importance of wave and tidal driven longshore currents was investigated. The key geometric parameters tested were channel width and depth. A reduction of the hydrodynamic parameters along with simplification of the model bathymetry allowed for identification of the governing rip current parameters. The sensitivity analysis demonstrated that rip currents strengthen with increasing wave height, decreasing water depth over the updrift bar and increasing channel depth. The influence of the wave period was of secondary importance and the wave angle did not affect the offshore rip current velocity for site specific channel dimensions. The wave angle was observed to only have an impact for relatively narrow channels (relative to the forcing). This study revealed similarities and differences of rip currents at the Dutch coast and rip currents at previous field sites. The driving parameters of rip currents were identical; however, the flow patterns differed. While in previous field experiments drifters were predominantly retained within the surf zone, most drifters at Egmond aan Zee were ejected from the surf zone and did not return shoreward. Offshore of the channel the drifter behaviour was governed by the tidal current that advected the drifters alongshore. In case of weak tidal currents (slack water) the rip currents extended far offshore.Coastal EngineeringHydraulic EngineeringCivil Engineering and Geoscience
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