1,721,049 research outputs found
Estimating extreme water level probabilities: A comparison of the direct methods and recommendations for best practise
No full textOver the past five decades, several approaches for estimating probabilities of extreme still water levels have been developed. Currently, different methods are applied not only on transnational, but also on national scales, resulting in a heterogeneous level of protection. Applying different statistical methods can yield significantly different estimates of return water levels, but even the use of the same technique can produce large discrepancies, because there is subjective parameter choice at several steps in the model setup. In this paper, we compare probabilities of extreme still water levels estimated using the main direct methods (i.e. the block maxima method and the peaks over threshold method) considering a wide range of strategies to create extreme value dataset and a range of different model setups. We primarily use tide gauge records from the German Bight but also consider data from sites around the UK and Australia for comparison. The focus is on testing the influence of the following three main factors, which can affect the estimates of extreme value statistics: (1) detrending the original data sets; (2) building samples of extreme values from the original data sets; and (3) the record lengths of the original data sets. We find that using different detrending techniques biases the results from extreme value statistics. Hence, we recommend using a 1-year moving average of high waters (or hourly records if these are available) to correct the original data sets for seasonal and long-term sea level changes. Our results highlight that the peaks over threshold method yields more reliable and more stable (i.e. using short records leads to the same results as when using long records) estimates of probabilities of extreme still water levels than the block maxima method. In analysing a variety of threshold selection methods we find that using the 99.7th percentile water level leads to the most stable return water level estimates along the German Bight. This is also valid for the international stations considered. Finally, to provide guidance for coastal engineers and operators, we recommend the peaks over threshold method and define an objective approach for setting up the model. If this is applied routinely around a country, it will help overcome the problem of heterogeneous levels of protection resulting from different methods and varying model setups
Form and function of natural and engineered perched beaches
No full textWith predicted sea level rise and possible changes in storminess associated with climate change, it is not known how perched beaches, whether natural or engineered, will respond. Perched beach types range from beaches fronted by wall-like structures in the nearshore that may be connected to the dry beachface, to having underlying and outcropping geological formations. They may also be backed landward by hard structures such as cliffs or infrastructure instead of dunes. In this study, the global distribution of natural perched beaches from existing literature was mapped. 'Hot spots' include the West Indies, Central and South America, Pacific Island Atolls, Indian Ocean islands, the Red Sea and the Mediterranean region. Many of these areas are in the tropics and subtropics and hence are associated with beachrock with is common in areas with coral reefs. Mechanisms of beach control by coastal structures are still poorly quantified. Suggested mechanisms include: limited free-profile fluctuation, decreased sediment availability and increased erosion rates due to less water infiltration and raised beach groundwater tables. Field research in southwest Western Australia indicated that perched beach profiles may recover more slowly from the daily summer sea breeze erosion than exposed profiles. During a storm, perched profiles had less erosion, occurring lower on the beachface than exposed profiles. Perched profiles however did not recover as easily by accretion during low sea level phases of storm activity. Lower rates of recovery for perched profiles could be due to a scour step forming seaward of the rock formations inhibiting cross-shore sediment transport. Results indicate that natural perched beach behaviour has extreme spatial variation. Beach response to changing hydrodynamics is strongly dependent on the configuration and geometry of local coastal structures, whether natural and anthropogenic
Simulation of perched beach accretion using smoothed particle hydrodynamics
No full textYanchep Beach, located 60 km north of Perth in southwest Western Australia, is a perched beach lying on a shallow rock platform. The platform is fronted by a natural sea wall several meters high. Every summer, the perched beach widens significantly. This study evaluates the hydrodynamic conditions in front of the sea wall causing the widening of the beach using a Smooth Particle Hydrodynamic model. The SPH model shows the importance of the reflected wave in the timing of the wave breaking and the presence of a velocity pulse near the seawall. This suggests that the hydrodynamic conditions are capable of significant resuspension and transport of sand on the reef platform
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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