1,720,966 research outputs found
Wave dissipation patterns as an indicator of rip current hazard
No full textRip currents (rips) are hazardous offshore-directed flows in the surfzone of beaches worldwide. Rips are a major hazard for recreational beach use and are the dominant cause of beach rescues and drownings. It is therefore important to understand what conditions make a rip most hazardous, in order for beach safety practitioners to mitigate the risk. The aim of this thesis was to determine how patterns of wave breaking influence rip channel hazard on beaches. In order to quantify wave breaking, video imagery from three hydrodynamically diverse case studies was used, and validated at two sites against Lagrangian GPS drifter data. This thesis firrst developed pre-processing techniques for video imagery which then subsequently improved the reliability of rip channel detection. Hitherto, the noise inherent in such signals has made automated detection of rip currents problematic. Wave breaking patterns could then be identified with the novel application of synoptic typing methods to the imagery, resulting in a classification scheme for rip currents based on wave breaking. Two dominant types were identified: (1) open channels, whereby the rip channel has free connectivity to the region beyond the surfzone; and (2) closed channels, where wave breaking across the seaward extent of the channel effectively closes this connectivity to the offshore region. Investigation of Lagrangian data for each of the prevailing states shows that under open conditions, drifters were highly likely to be transported beyond the edge of the surfzone by the rip current, with exit rates reaching 100 % at times. Under closed conditions, drifters were more likely to be retained in the surfzone, with typical exit rates between 0 and 35 %. A rip current that exits the surfzone is more hazardous to bathers, and therefore, this thesis subsequently investigated the prevalence of open and closed rip channels in records of rip rescue events. Over two sites for which data were available, upwards of two thirds of major rip rescues occurred when the channel could be classified by this new method as open. Furthermore, the majority of surfer and bodyboarder rescues occurred under open conditions. Despite their over-representation in the rip rescue record, the overall prevalence of open channels over a year is only around 40 %. Normalising the number of rescues in open rips by their occurrence shows open rips to be twice as hazardous as closed channels. This new approach provides a quick and inexpensive means to assess high risk surf conditions at rip beaches worldwide, with the deployment of only a small (often mobile) imaging system
Methods for field measurement and remote sensing of the swash zone
No full textSwash action is the dominant process responsible for the cross-shore exchange of sediment between the subaerial and subaqueous zones, with a significant part of the littoral drift also taking place as a result of swash motions. The swash zone is the area of the beach between the inner surfzone and backbeach that is intermittently submerged and exposed by the processes of wave uprush and backwash. Given the dominant role that swash plays in the morphological evolution of a beach, it is important to understand and quantify the main processes. The extent of swash (horizontally and vertically), current velocities and suspended sediment concentrations are all parameters of interest in the study of swash processes. In situ methods of measurements in this energetic zone were instrumental in developing early understanding of swash processes, however, the field has experienced a shift towards remote sensing methods. This article outlines the emergence of high precision technologies such as video imaging and LIDAR (light detection and ranging) for the study of swash processes. Furthermore, the applicability of these methods to large-scale datasets for quantitative analysis is demonstrated
An analysis of lifejacket wear, environmental factors, and casualty activity on marine accident fatality rates
No full textDrowning and fatalities at sea are a large concern globally. In the UK, many sea rescues are performed by the Royal National Lifeboat Institution, and this study investigates 6 years’ worth of their rescue data to better understand causation of drowning and what makes an incident at sea high risk. A Poisson model is applied to numerous factors recorded as part of each rescue, including environmental conditions (visibility, sea state, etc.), lifejacket wear, and response times for rescue. Increased lifejacket wear is shown to be significantly correlated with lower fatality rates across all spectrum of activities. Survivability among those casualties wearing life jackets was 94%. A seasonal signal is clearly present, with a higher proportion of life at risk incidents occurring during winter months, and a higher than predicted number of fatalities during this time. The analysis identifies high risk groups of beach/sea users, with one of the most at risk being people fishing from shore. Incident survivability is shown to decrease at different rates per activity, as time to rescue increases. This study provides clear evidence that a co-ordinated approach to sea safety is required, and suggests that increased lifejacket wear among coastal and marine users would have a dramatic effect on reducing the number of drowning related deaths each year.</p
Perceptions of rip current myths from the central south coast of England
No full textRip currents (rips) are the global leading cause of fatalities on surf beaches, yet numerous long-standing misconceptions exist. Evidence of the prevalence of these myths is largely anecdotal. This opportunistic, exploratory study presents perceptions on rip current hazards (n = 187), of members of the public attending an open day at the National Oceanography Centre, Southampton. The survey was undertaken as a fun but informative quiz, aimed at families attending the research-facility. It also explored using such events as a conduit to gain valuable knowledge on the understanding of rip currents and other hazards. While most respondents (81%) knew appropriate escape strategies (swim parallel and stay afloat), only 11% identified what makes rips dangerous (panic), with 44% incorrectly saying that rips suck you under. Rip identification is poor, and many are unaware of the meaning of beach safety flags. This study identifies that safety communication messaging needs to focus on debunking rip current myths, to improve understanding of safe swimming areas; and to reduce panic if caught in a rip
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Rip current observations on a low-sloping dissipative beach
No full textRip currents are the main cause of beach rescues and fatalities. Key drivers of rip current hazard are: (1) fast current speeds; and (2) the exit rate of floating material from inside to outside of the surf zone. Exit rates may vary temporally, such as due to Very Low Frequency (VLF) motions, which have a period on the order of 10 minutes. However, there is little field data to determine the driver(s) of exit rate. Therefore, the aim of this research was to determine rip current circulation patterns, and specifically, determine their relationship to surf zone exits, on a high-energy dissipative beach. Three days of field measurements were undertaken at Ngarunui Beach, New Zealand. Three daily surf zone flow patterns were found: (1) alongshore; (2) surf zone eddy with high exit rate; and (3) surf zone eddy with no exits. There were strong infragravity peaks in energy within the surf zone, at 30-45s, although none at VLF (~10 minute) frequencies. Further research is underway to determine what drove the high surf zone exit rate observed at Ngarunui Beach
Synthetic imagery for the automated detection of rip currents
Full text linkRip currents are a major hazard on beaches worldwide. Although it is in-situ measurements of rips can be made in the field, it is generally safer and more cost effective to employ remote sensing methods, such as coastal video imaging systems. However, there is no universal, fully-automated method capable of detecting rips in imagery. In this paper we discuss the benefits of image manipulation, such as filtering, prior to rip detection attempts. Furthermore, we present a new approach to detect rip channels that utilizes synthetic imagery. The creation of a synthetic image involves zonation of the ‘parent’ image into key areas, such as sand bars, channels, shoreline and offshore. Then, pixels in each zone are replaced with the respective dominant color trends observed in the parent image. Using synthetic imagery increased the accuracy of rip detection from 81% to 92%. Synthetics reduce ‘noise’ inherent in surfzone imagery and is another step towards an automated approach for rip current detection
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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