13,473 research outputs found
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
Rede uitgesproken op 17 september 1956 ter gelegenheid van de erepromotie van Dr. Th. von Karman door de promotor Dr. C.B. Biezeno
No full textRede uitgesproken door prof. C.B. Biezeno ter gelegenheid van de erepromotie aan de TH Delft van dr. Theodore von Karman.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin
A New Technique for Measuring Runup Variation Using Sub-Aerial Video Imagery
No full textVideo monitoring of beaches is becoming the preferred method for observing changes to nearshore morphology. Consequently this work investigates a new technique for predicting the probability of inundation that is based on measuring runup variation using video. Runup is defined as the water-level elevation maxima on the foreshore relative to the still water level and the waterline is defined as the position where the MWL intersects the beach face. Tairua, and Pauanui Beaches, on the north east coast of the North Island of New Zealand, were used as the field site in this study and represent two very different beaches with the same incoming wave and meteorological conditions. Tairua is most frequently in an intermediate beach state, whereas Pauanui is usually flatter in nature.
In order to rectify runup observations, an estimate of the runup elevation was needed (Z). This was estimated by measuring the variation of the waterline over a tidal cycle from time-averaged video images during a storm event and provided beach morphology statistics (i.e. beach slope (α) and beach intercept (b)) used in the rectification process where Z=aX+b. The maximum swash excursions were digitized from time-stacks, and rectified to provide run-up timeseries with duration 20 minutes. Field calibrations revealed a videoed waterline that was seaward of the surveyed waterline. Quantification of this error gave a vertical offset of 0.33m at Tairua and 0.25m at Pauanui.
At Tairua, incident wave energy was dominant in the swash zone, and the runup distributions followed a Rayleigh distribution. At Pauanui, the flatter beach, the runup distributions were approximately bimodal due to the dominance of infragravity energy in the swash signal. The slope of the beach was a major control on the runup elevation; runup at Pauanui was directly affected by the deepwater wave height and the tide, while at Tairua there was no correlation. Overall, the results of the study indicate realistic runup measurements, over a wide range of time scales and, importantly, during storm events. However, comparisons of videoed runup and empirical runup formulae revealed larger deviations as the beach steepness increased. Furthur tests need to be carried out to see if this is a limitation of this technique, used to measure runup. The runup statistics are consistently higher at Tairua and suggests that swash runs up higher on steeper beaches. However, because of the characteristics of flatter beaches (such as high water tables and low drainage efficiencies) the impact of extreme runup elevations on such beaches are more critical in regards to erosion and/ or inundation.
The coastal environment is of great importance to Māori. Damage to the coast and coastal waahi tapu (places of spiritual importance) caused by erosion and inundation, adversely affects the spiritual and cultural well-being of Māori. For this reason, a chapter was dedicated to investigating the practices used by Māori to protect and preserve the coasts in accordance with tikanga Māori (Māori protocols). Mimicking nature was and still is a practice used by Māori to restore the beaches after erosive events, and includes replanting native dune plants and using natural materials on the beaches to stabilize the dunes. Tapu and rahui (the power and influence of the gods) were imposed on communities to prohibit and prevent people from free access to either food resources or to a particular place, in order to protect people and/ or resources. Interpretations of Māori oral histories provide insights into past local hazards and inform about the safety and viability of certain activities within an area. Environmental indicators were used to identify and forecast extreme weather conditions locally. Māori knowledge of past hazards, and the coastal environment as a whole, is a valuable resource and provides a unique source of expertise that can contribute to current coastal hazards management plans in New Zealand and provide insights about the areas that may again be impacted by natural hazards
Integrated Environmental Study for Beach Management: A Methodological Approach
No full textThis paper aims to present a project convened by the University of Genoa and Fondazione Eni Enrico Mattei (FEEM), in collaboration with Local Authorities, concerning the development of tools for beach management in the Riviera del Beigua (Liguria Region, Italy). The aim of the first step of the project is to assess the environmental state of resort beaches examining them interdisciplinary, through a data analysis based on a sound understanding of the components of the physical and the human system. The following step will be the treatment of the data, through the use of various instruments, which use a synthesis analysis, such as the traditional SWOT (Strengths, Weaknesses, Opportunities and Threats) analysis, and the use of a set of environmental and socio-economic indicators. Finally, our ultimate target is to propose guidelines, which will supply an instrument to back up policies concerning beach planning and management.Costal management, Sustainable tourist, Integrated assessment, Indicators
Hydrodynamics and morphodynamics in the swash zone: hydralab III large-scale experiments
No full textThe modelling of swash zone hydrodynamics and sediment transport and the resulting morphodynamics has been an area of very active research over the last decade. However, many details are still to be understood, whose knowledge will be greatly advanced by the collection of high quality data under controlled large-scale laboratory conditions. The advantage of using a large wave flume is that scale effects that affected previous laboratory experiments are minimized.
In this work new large-scale laboratory data from two sets of experiments are presented. Physical model tests were performed in the large-scale wave flumes at the Grosser Wellen Kanal (GWK) in Hannover and at the Catalonia University of Technology (UPC) in Barcelona, within the Hydralab III program.
The tests carried out at the GWK aimed at improving the knowledge of the hydrodynamic and morphodynamic behaviour of a beach containing a buried drainage system. Experiments were undertaken using a set of multiple drains, up to three working simultaneously, located within the beach and at variable distances from the shoreline. The experimental program was organized in series of tests with variable wave energy. While a positive effect was observed under low energy conditions, for medium and high energy conditions the benefit of having the drains operative was not always clear. In any case, it was evident that any positive effect of the drains on the beachface was confined by the position of the cone of depression in the aquifer’s surface.
The tests carried out in the large wave flume at UPC had the intent to investigate swash zone under storm conditions. The main aim was to compare beach profile response for monochromatic waves, monochromatic waves plus free long waves, bichromatic waves and random waves. Both erosive and accretive conditions were considered. The experiments suggest that the inclusion of long wave and wave group sediment transport is important for improved nearshore morphological modelling of cross-shore beach profile evolution, and provide a very comprehensive and controlled series of tests for evaluating numerical models. It is suggested that the large change in the beach response between monochromatic conditions and wave group conditions is a result of the increased significant and maximum wave heights in the wave groups, as much as the presence of the forced and free long waves induced by the groupiness. The equilibrium state model concept can provide a heuristic explanation of the influence of the wave groups on the bulk beach profile response if their effective relative fall velocity is larger than that of monochromatic waves with the same incident energy flux
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
Prof. Th. W. Adorno and the author Hans Erich Nossack.
No full textProf. Th. W. Adorno and the author Hans Erich Nossack at a reception of Insel Verlag, Buchmesse Frankfurt 1966LB
Author, publisher and bookseller : a tripartite synergy in Nigerian book industry
No full textThis work is about the roles of Author, Publisher and Bookseller in Book development in
Nigeria. The paper started by delving into the history of Book Publishing in Nigeria after
which it proceeded by defining who an author, a publisher, and a bookseller is and
expatiated on the indispensable roles of these key actors in Nigerian Book Industry and in
the emerging Information Society. Furthermore, the various constraints to book
development were identified while the paper advised on how the Book Industry can be
further promoted in Nigeria. However, the paper concluded and made recommendations
on how the Book sector can help in enhancing scholarship in the country
Estimating Peak Demand for Beach Parking Spaces
No full textThe United States Army Corps of Engineers planning guidance stipulates that in order for local beach communities to qualify for Federal cost share funds for Hurricane and Storm Damage Reduction beach renourishment projects, the community must provide public beach access and parking to satisfy peak demand. This study presents a method for estimating peak demand for beach parking spaces in the presence of parking constraints. A Tobit regression model is developed to estimate the number of parking spaces that would be necessary to meet unconstrained demand on a given percentage of peak demand days. For example, the model can be used to estimate the number of parking spaces that would be adequate to meet peak demand on 90% of peak parking days. The Tobit model provides a promising framework for estimating peak parking demand under constrained parking conditions, a situation that characterizes most beach communities.
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