Sediment Transport Equivalent Waves for Estimating Annually Averaged Sedimentation and Erosion Trends in Sandy Coastal Areas

In this paper, a simple approach to determine representative offshore wave characteristics for estimating the annually averaged sedimentation and erosion trends in sandy coastal areas is presented. Given the offshore wave climate, the proposed approach breaks down the climate into fixed 22.5-degree bins and based on the sediment transport potential it determines the equivalent wave characteristics for each bin, i.e., a significant wave height, a peak period, a mean wave direction, and a corresponding frequency of occurrence. The approach is validated in idealized cases of uniformly sloping beaches with the presence of a breakwater, for various sediment diameters, sea bottom slopes, and different offshore wave characteristics. The performance of the proposed approach is evaluated against the full climate, returning good results. Furthermore, the proposed approach is applied in a real-life challenge, in the coastal area of Therma in the Island of Samothraki in Greece, where the presence of a fishing shelter has led to sedimentation and erosion problems. The performance of the proposed approach is very satisfactory, given the complexity of the problem. The generic nature of the proposed methodological approach allows it to be applied in numerous sandy coastal regions to estimate the sedimentation and erosion trends, reducing the amount of input parameters and thus requiring significantly less computational efforts

Stochastic simulation of sea waves in shallow waters

During the past decades a need to develop tools describing the wave characteristics in the nearshore keeps rising, from the viewpoint of science, engineering and also from the viewpoint of safety. This need originates from the knowledge that is required for designing port and coastal works. As the waves travel from offshore to the coast and interact with artificial or natural solid boundaries (e.g. sea bottom, islands, headlands, breakwaters, groins etc) a range of physical processes make their appearance, such as shoaling, diffraction, refraction etc, transforming the wave profile. Thus in order to design a coastal work, one should be in place to predict with great accuracy the incoming wave regime in the study area, in order to further calculate the wave loads on the structure, the wave overtopping, the sediment transport etc. in this framework the joint values of the pair “wave-height-wave period” is of paramount importance in many coastal and marine engineering applications.A significant effort has been undertaken so far by various researchers to accurately describe wave statistics in the offshore region as well as in shallower waters and the surf zone. The short-term description of sea states follows generally two distinct methodologies. The first one refers to the description of the wave field in the so-called frequency domain, where spectral components represent simple sine waves. These are derived by making use of a mathematical transform tool (FFT), which decomposes the record time series of the free surface elevation. The spectral representation of surface wind waves has been extensively used in models of wave generation and propagation in deep and shallow waters. Spectral transformation models are routinely used in wave prediction. Due to the nature of the above process, a part of the initial information, concerning the phase difference between the spectral components, is lost. The second method describes the wave field in the time domain, maintaining all of the original information, including phases. Nevertheless this technique involves usually huge computational effort when one need, as usually is the case, to represent satisfactorily a specific wave field in space and time. A third method of representing the wave field that combines advantages of both conventional methods mentioned is the short-term probabilistic description of a sea state. The use of short-term probabilistic simulation of wave characteristics was not introduced, to a large extent, in the research efforts made during the recent past in the field of coastal wave mechanics internationally. Related activity has focused on determining the statistical characteristics of wave height and wave period as independent variables. Thus since the combination of wave height and period by their associated probability is of crucial importance as already said, the determination of joint probability density function (jpdf) of these two variables naturally becomes a key problem in maritime engineering covering both rigid and floating structures.The prime objective of this research work is the derivation of the probabilistic images representing the jpdf of wave heights and periods in intermediate and shallow waters under various sea states and seabed configurations. The specific research objectives are: •To extend the jpdf in deep waters to incorporate the directionality θ of the waves and publish the figures for specific directional angles.•To structure a procedure that describes the decomposition of the deep water probabilistic illustration to time series of surface elevation, in order to serve as input to the wave propagation model.•To search for the optimum numerical model capable of simulating the propagation of the wave trains from deep waters into the surf zone taking into account phenomena such as shoaling, diffraction, refraction, depth-induced breaking, bed dissipation, wave-wave interactions and directional energy spreading.•Improvement, development and verification of the numerical wave propagation model with analytical solutions and experimental results.•To structure a second procedure with discrete steps, describing the recompiling of the calculated time series of surface elevation in shallow waters into probabilistic illustrations and comparison with appropriate experimental data.•Finally to produce characteristic figures of jpdf for various storm intensities, and for usual bed slopes in order to provide the reader with useful data ready for implementation.Original results that deserve mentioning have been achieved along the predefined course of basic research of the present dissertation are: Extension of the jpdf to incorporate wave directionality in deep water representations by a simple technique similar to the one that transforms a two dimensional spectrum into three dimensional one. Construction of an integrated algorithm which takes into account the probabilistic representation of the sea state offshore and transforms it into time series of surface elevation. These data combined with the water depths in the study area are given as input to the propagation model. Thus, the frequency of occurrence of every pair of wave height and period along with their associated directionality can be calculated in any desired position. Production of characteristic figures of jpdf for various correlation factors and for usual bed slopes in shallow waters.Μέχρι σήμερα έχει γίνει μια πολύ σημαντική προσπάθεια από πολλούς ερευνητές για την ακριβή περιγραφή των κυματικών στατιστικών στην περιοχή των βαθιών νερών αλλά και των ενδιάμεσων και ρηχών. Η βραχυπρόθεσμη περιγραφή της θαλάσσιας διαταραχής ακολουθεί γενικά δύο διακριτές μεθοδολογίες. Η πρώτη αναφέρεται στην περιγραφή του κυματικού πεδίου στην περιοχή των συχνοτήτων, όπου οι φασματικές συνιστώσες αναπαριστούν απλούς ημιτονοειδείς κυματισμούς. Αυτοί εξάγονται χρησιμοποιώντας μαθηματικά εργαλεία μετασχηματισμού Fourier που αποσυνθέτουν την καταγεγραμμένη χρονοσειρά της ανύψωσης της ελεύθερης επιφάνειας. Λόγω της φύσης της παραπάνω διαδικασίας, ένα μέρος της αρχικής πληροφορίας που αφορά τη διαφορά φάσης μεταξύ των συνιστωσών χάνεται. Η δεύτερη μέθοδος περιγράφει το κυματικό πεδίο στην περιοχή του χρόνου, κρατώντας όλη την πληροφορία ως προς την κυματική φάση, απαιτώντας όμως μεγάλο υπολογιστικό χρόνο για την αναπαράσταση ενός συγκεκριμένου πεδίου στο χώρο και στο χρόνο. Μια τρίτη μέθοδος απεικόνισης του κυματικού πεδίου, που συνδυάζει τα πλεονεκτήματα των δυο προηγούμενων συμβατικών μεθόδων, είναι αυτή που προσπαθεί να κρατήσει όσο το δυνατόν περισσότερη πληροφορία, κυρίως όσον αφορά το ζεύγος του ύψους και της περιόδου του κυματισμού (H, T). Η συγκεκριμένη μεθοδολογία προσπαθεί να συνδέσει κάθε τέτοιο ζεύγος με την πυκνότητα πιθανότητας εμφάνισής του p(H, T). Η μέθοδος αυτή ακολουθείται και αναπτύσσεται στην παρούσα έρευνα. Στόχος της παρούσας διατριβής είναι να μεταφέρει στα ενδιάμεσα και ρηχά νερά την πολύτιμη πληροφορία που αντλείται από τη συνάρτηση της από κοινού πυκνότητας πιθανότητας, όπως έχει ήδη υπολογιστεί σε προηγούμενη έρευνα για ευρέα κυματικά φάσματα στα βαθιά νερά. Προφανώς η κατεύθυνση αυτή ακολουθήθηκε γιατί σε αυτές τις περιοχές των ενδιάμεσων και ρηχών νερών εστιάζεται και το ενδιαφέρον της εφαρμογής, αφού εκεί κατασκευάζεται η πλειονότητα των ακτομηχανικών και λιμενικών έργων και έτσι η γνώση των πιθανοτήτων εμφάνισης των ζευγών ύψους-περιόδου είναι πολύτιμη. . Η βιβλιογραφική επισκόπηση της διεθνούς έρευνας σε αυτό το πεδίο, ανέδειξε ότι υπάρχει μεγάλο κενό στη γνώση της από κοινού πιθανότητας των υψών και περιόδων κυματισμού σε αυτές τις περιοχές. Από αυτή την άποψη, η παρούσα διατριβή είναι πρωτότυπη και η μεθοδολογία που προτείνεται για τον υπολογισμό των πιθανοτικών εικόνων αποτελεί το πρώτο από τα δύο βασικά καινοτόμα σημεία της.Για αυτό το σκοπό, καταστρώνεται ένα σύνθετο μοντέλο που βασίζεται σε μια συνάρτηση πυκνότητας πιθανότητας υψών περιόδων και σε ένα κυματικό μοντέλο τύπου Boussinesq. Η διαδικασία επίλυσης ενός τέτοιου προβλήματος συνίσταται κατ’ αρχήν στην αποσύνθεση μιας δεδομένης εικόνας από κοινού πιθανότητας στα βαθιά νερά σε διαχερίσιμη μορφή δεδομένων, δηλαδή σε μια αντιπροσωπευτική χρονοσειρά ανύψωσης της ελεύθερης επιφάνειας. Τα δεδομένα αυτά τροφοδοτούνται στο μοντέλο Boussinesq, που αναπτύχθηκε στα πλαίσια της διατριβής, για να αποκτηθούν οι αντίστοιχες χρονοσειρές στην παράκτια ζώνη. Το κυματικό μοντέλο που αναπτύχθηκε προσομοιώνει βέλτιστα τη γραμμική και μη γραμμική διασπορά των κυματισμών, τη γραμμική ρήχωση, τα χαρακτηριστικά της γένεσης κυματισμών στο εσωτερικό του αριθμητικού πεδίου και βελτιώνει τη συνάρτηση μεταφοράς ενέργειας μεταξύ των κυματισμών. Συνεπώς, αυτό αποτελεί το δεύτερο καινοτόμο σημείο, καθώς τα υπάρχοντα μοντέλα του ιδίου τύπου υστερούν στην προσομοίωση αυτών των χαρακτηριστικών. Τέλος, η χρονοσειρά που προκύπτει ανασυντίθεται για να δώσει την αντίστοιχη πιθανοτική εικόνα σε οποιοδήποτε βάθος στην περιοχή των ενδιάμεσων ή των ρηχών νερών.Το σύνθετο μοντέλο επαληθεύτηκε με πειραματικές μετρήσεις που διεξήχθησαν στο παρελθόν, αποδεικνύοντας την εγκυρότητα και την ικανότητά του να παράγει ικανοποιητικά αποτελέσματα κοντά στην πραγματικότητα. Τέλος, το σύνθετο μοντέλο χρησιμοποιήθηκε για την παραγωγή αξιόπιστων χαρακτηριστικών αναπαραστάσεων της από κοινού πυκνότητας πιθανότητας μεταξύ ύψους και περιόδου κύματος, για συνήθεις κλίσεις του πυθμένα της θάλασσας δίνοντας αποτελέσματα σε συγκεκριμένα βάθη νερού. Παρέχονται έτσι στον αναγνώστη πληροφορίες που μπορούν να χρησιμοποιηθούν σε εφαρμογές μηχανικού

A Permissioned Distributed Ledger for Monitoring Web Content

Online publishing of news and information enables important content to reach a much larger audience than traditional paper publishing, but there is no guarantee of long-term, reliable, and persistent access to the content. Over time, links “decay” because they are not reliably updated when content changes location, or even worse, content is deliberately altered from its original published form or deleted altogether. We present the design of Watchdog, a permissioned distributed ledger that securely and reliably monitors and preserves dynamic web content such as government and news sites. Unlike prior preservation systems, our system design allows nodes to be dispersed across multiple administrative domains, thus eliminating single points of trust and at the same time, is the first to monitor how content changes both over time and across geographical location. Watchdog achieves Byzantine fault-tolerance via a novel interactive consistency algorithm that offers a twist on the traditional definition and use of interactive consistency; the algorithm enables a set of mutually suspicious nodes, with arbitrary-sized, potentially overlapping data collections, to efficiently agree on the exact data collection in possession by each node. We demonstrate how our algorithm enables Watchdog nodes to exchange and agree upon the content each has observed individually, in a fault-tolerant, tamper-proof manner

A social link based private storage cloud

In this paper, we present O-3, a social link based private storage cloud for decentralized collaboration. O-3 allows users to share and collaborate on collections of files (shared folders) with others, in a decentralized manner, without the need for intermediaries (such as public cloud storage servers) to intervene. Thus, users can keep their working relationships (who they work with) and what they work on private from third parties. Using benchmarks and traces from real workloads, we experimentally evaluate O-3 and demonstrate that the system scales linearly when synchronizing increasing numbers of concurrent users, while performing on-par with the ext4 non-version-tracking filesystem

Simulating Nearshore Wave Processes Utilizing an Enhanced Boussinesq-Type Model

The simulation of wave propagation and penetration inside ports and coastal areas is of paramount importance to engineers and scientists desiring to obtain an accurate representation of the wave field. However, this is often a rather daunting task due to the complexity of the processes that need to be resolved, as well as the demanding levels of required computational resources. In the present paper, the enhancements made on an existing sophisticated Boussinesq-type wave model, concerning the accurate generation of irregular multidirectional waves, as well as an empirical methodology to calculate wave overtopping discharges, are presented. The model was extensively validated against 4 experimental test cases, covering a wide range of applications, namely wave propagation over a shoal, wave penetration in ports through a breakwater gap, wave breaking on a plane sloping beach, and wave overtopping behind breakwaters. Good agreement of the model results with all experimental measurements was achieved, rendering the wave model a valuable tool in real-life applications for engineers and scientists desiring to obtain accurate solutions of the wave field in wave basins and complex coastal areas, while keeping computational times at reasonable levels

A Personal Distributed Real-time Collaborative System

In this paper, we present O-3 REAL, a privacy-preserving distributed middleware for real-time collaborative editing of documents. O-3 REAL introduces a novel approach for building peer-to-peer real-time collaborative applications, using a reliable broadcast channel mechanism for network communication, but at the same time provides for persistent storage management of collaborative documents using the filesystem interface of a POSIX compliant filesystem. This approach enables real-time, completely decentralized collaboration among users, without the need for a third party to intervene, and significantly simplifies the creation of peer-to-peer collaborative applications. We demonstrate that O-3 REAL scales well for real-time collaboration use-cases. For example, with 33 users simultaneously collaborating on a document in real time over a WAN with a 50 ms link delay, the average perceived latency is approximately 54 ms, which is very close to the optimal baseline. In comparison, Etherpad exhibits nearly twice the perceived latency

A Wave Input-Reduction Method Incorporating Initiation of Sediment Motion

The long-term prediction of morphological bed evolution has been of interest to engineers and scientists for many decades. Usually, process-based models are employed to simulate bed-level changes in the scale of years to decades. To compensate for the major computational effort required by these models, various acceleration techniques have been developed, namely input-reduction, model-reduction and behaviour-oriented modelling. The present paper presents a new input-reduction method to obtain representative wave conditions based on the Shields criterion of incipient motion and subsequent calculation of the sediment pick-up rate. Elimination of waves unable to initiate sediment movement leads to additional reduction of model run-times. The proposed method was implemented in the sandy coastline adjusted to the port of Rethymno, Greece, and validated against two datasets consisting of 7 and 20 and 365 days, respectively, using the model MIKE21 Coupled Model FM. The method was compared with a well-established method of wave schematization and evaluation of the model’s skill deemed the simulations based on the pick-up rate schematization method as “excellent”. Additionally, a model run-time reduction of about 50% was observed, rendering this input-reduction method a valuable tool for the medium to long-term modelling of bed evolution

Sediment Transport Equivalent Waves for Estimating Annually Averaged Sedimentation and Erosion Trends in Sandy Coastal Areas

In this paper, a simple approach to determine representative offshore wave characteristics for estimating the annually averaged sedimentation and erosion trends in sandy coastal areas is presented. Given the offshore wave climate, the proposed approach breaks down the climate into fixed 22.5-degree bins and based on the sediment transport potential it determines the equivalent wave characteristics for each bin, i.e., a significant wave height, a peak period, a mean wave direction, and a corresponding frequency of occurrence. The approach is validated in idealized cases of uniformly sloping beaches with the presence of a breakwater, for various sediment diameters, sea bottom slopes, and different offshore wave characteristics. The performance of the proposed approach is evaluated against the full climate, returning good results. Furthermore, the proposed approach is applied in a real-life challenge, in the coastal area of Therma in the Island of Samothraki in Greece, where the presence of a fishing shelter has led to sedimentation and erosion problems. The performance of the proposed approach is very satisfactory, given the complexity of the problem. The generic nature of the proposed methodological approach allows it to be applied in numerous sandy coastal regions to estimate the sedimentation and erosion trends, reducing the amount of input parameters and thus requiring significantly less computational efforts

Accelerating Predictions of Morphological Bed Evolution by Combining Numerical Modelling and Artificial Neural Networks

Process-based models have been employed extensively in the last decades for the prediction of coastal bed evolution in the medium term (1–5 years), under the combined action of waves and currents, due to their ability to resolve the dominant coastal processes. Despite their widespread application, they are associated with high demand for computational resources, rendering the annual prediction of the coastal bed evolution a tedious task. To combat this, wave input reduction methods are generally employed to reduce the sheer amount of sea-states to be simulated to assess the bed level changes. The purpose of this research is to further expand on the concept of input reduction methods by presenting a methodology combining numerical modelling and an Artificial Neural Network (ANN). The trained ANN is tasked with eliminating wave records unable to initiate sediment motion and hence further reduce the required computational times. The methodology was implemented in both an idealized and a real-field case study to examine the sensitivity, and produced very satisfactory predictions of the rates of bed level change, with respect to a benchmark simulation containing a very detailed wave climate. The obtained results have strong implications for further accelerating the demanding morphological simulations while enhancing the reliability and accuracy of model predictions