Διερεύνηση συμπεριφοράς του συντελεστή μετάδοσης κυματισμού πίσω από ύφαλο κυματοθραύστη με παρουσία ή μη Ποσειδωνίας

Κόγια, Ευφροσύνη Ειρήνη; Kogia, Effrosyni Eirini

dc.contributor.author	Κόγια, Ευφροσύνη Ειρήνη	el
dc.contributor.author	Kogia, Effrosyni Eirini	en
dc.date.accessioned	2019-06-24T08:50:35Z
dc.date.available	2019-06-24T08:50:35Z
dc.date.issued	2019-06-24
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/48884
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.9532
dc.description	Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Επιστήμη και Τεχνολογία Υδατικών Πόρων”	el
dc.rights	Αναφορά Δημιουργού-Όχι Παράγωγα Έργα 3.0 Ελλάδα	*
dc.rights.uri	http://creativecommons.org/licenses/by-nd/3.0/gr/	*
dc.subject	Κυματισμός	el
dc.subject	Ποσειδωνία	el
dc.subject	Ύφαλος	el
dc.subject	Κυματοθραύστης	el
dc.subject	Μετάδοση Ενέργειας	el
dc.subject	Wav	en
dc.subject	Submergede	en
dc.subject	Breakwater	en
dc.subject	Posidonia	en
dc.subject	Transmission	en
dc.title	Διερεύνηση συμπεριφοράς του συντελεστή μετάδοσης κυματισμού πίσω από ύφαλο κυματοθραύστη με παρουσία ή μη Ποσειδωνίας	el
heal.type	masterThesis
heal.classification	Λιμενικά, Ακτομηχανικά	el
heal.language	el
heal.access	free
heal.recordProvider	ntua	el
heal.publicationDate	2019-03-19
heal.abstract	Με την πάροδο των χρόνων η κλιματική αλλαγή επιτάσσει ολοένα και περισσότερο την κατασκευή ήπιων έργων προστασίας των παράκτιων περιοχών με τη λιγότερο δυνατή αρνητική επίδραση στο περιβάλλον. Για το λόγο αυτό τελευταία, η σημασία των ύφαλων διαπερατών κατασκευών ως έργα προστασίας ήπιας μορφής αυξάνεται σταθερά λόγω των περιβαλλοντικών πλεονεκτημάτων τους έναντι των έξαλλων κατασκευών. Μειώνοντας ικανοποιητικά την κυματική ενέργεια στην υπήνεμη πλευρά και μη προκαλώντας οπτική όχληση στο φυσικό τοπίo, πρωταγωνιστούν ως κατασκευαστικές λύσεις στην παράκτια ζώνη ενώ παράλληλα η συμπεριφορά τους διερευνάται συνεχώς πειραματικά ώστε να βελτιωθούν ακόμα περισσότερο. Στην παρούσα μεταπτυχιακή εργασία εξετάζεται το φαινόμενο της κυματικής μετάδοσης πίσω από διατομές ύφαλων διαπερατών κυματοθραυστών στατικά ευσταθών (Υ.Δ.Κ.), με ή χωρίς Ποσειδωνία στη στέψη τους. Η κυματική μετάδοση ποσοτικοποιείται μέσω του συντελεστή κυματικής μετάδοσης Kt, που ορίζεται ως ο λόγος του μεταδιδόμενου ύψους κύματος Ht προς το εισερχόμενο ύψος κύματος Hi. Έχει αποδειχθεί πως ο συντελεστής μετάδοσης Kt εξαρτάται από μία σειρά μεταβλητών που σχετίζονται είτε με τον κυματισμό είτε με την κατασκευή, όπως το Hi, το εισερχόμενο μήκος κύματος Li, το πλάτος στέψης Β, το ελεύθερο βάθος θάλασσας F πάνω από τον Υ.Κ. και το πορώδες n του Υ.Κ. Στη διεθνή βιβλιογραφία υπάρχουν αρκετοί διαθέσιμοι ημι-εμπειρικοί τύποι υπολογισμού του Kt για Υ.Δ.Κ., που λαμβάνουν υπόψη στις εξισώσεις τους μια σειρά αδιάστατων σημαντικών παραμέτρων, με πιο συχνή το σχετικό ελεύθερο βάθος F/Hi. Παρόλα αυτά για την πλειοψηφία των προτεινόμενων αριθμητικών μοντέλων, περιορισμοί στο πεδίο εφαρμογής τους καθώς και η ημι-εμπειρική φύση τους εγείρουν ζητήματα χρήσης τους και αξιοπιστίας στο σχεδιασμό Υ.Κ. Για τις ανάγκες της παρούσας μεταπτυχιακής εργασίας, και συγκεκριμένα για τη διερεύνηση του συντελεστή κυματικής μετάδοσης πίσω από λιθόρριπτους ύφαλους διαπερατούς κυματοθραύστες με παρουσία λειμώνα Ποσειδωνίας στη στέψη τους, αξιοποιήθηκαν οι πειραματικές μετρήσεις που συλλέχθηκαν από τη διεξαγωγή των πειραμάτων των Ziros & Memos (2019), Repousis & Memos (2019), και ειδικότερα αυτές που αντιστοιχούν σε περιπτώσεις μονοχρωματικών κυματισμών. Σε γενικές γραμμές από τα πειράματα παρατηρήθηκε μεγαλύτερη μείωση του Κt στην υπήνεμη πλευρά των Υ.Δ.Κ. που στη στέψη τους είχαν Ποσειδωνία σε σχέση με τις διατομές που δεν είχαν, το οποίο ήταν και αναμενόμενο. Από τα αριθμητικά μοντέλα που εξετάστηκαν, διερευνήθηκαν λεπτομερώς δύο ημι – εμπειρικοί τύποι περαιτέρω, ώστε να αξιολογηθούν ως προς την αξιοπιστία τους σε σχέση με τα πειραματικά αποτελέσματα. Οι δύο αυτοί τύποι είναι των Friebel and Harris (2003) και των Seabrook and Hall (1998). Η σύγκριση των αποτελεσμάτων των αριθμητικών μοντέλων και των μετρήσεων χαρακτηρίζεται απέδειξε μια αξιοπιστία ικανοποιητικού βαθμού των παραπάνω ημι – εμπειρικών σχέσεων. Τέλος πραγματοποιήθηκε μια διερεύνηση πιθανότητας τροποποίησής τους ώστε να λαμβάνουν υπόψιν περιπτώσεις με Ποσειδωνία.	el
heal.abstract	Nowadays, the need for protecting the shore from wave action is one of the most frequent yet complex issues scientists, engineers, and managers have to deal with. Considering past experience, conventional installations as emerged shore-parallel breakwaters have been widely applied in defying beach erosion. Despite the overall acceptability of introducing such structures in the coastal zone, it has been observed that they are usually followed by a significant environmental impact, including low level of water renewal, degradation of the aesthetic value of the landscape, occupation of relatively large seabed areas etc. As environmental awareness gradually increases, submerged breakwaters have become a shore protection alternative aiming in confining such side effects most effectively in micro-tidal environments (Mediterranean Sea, Baltic Sea and Caribbean) of up to average exposure. Beyond the basic goal in protecting the coast, it has been deduced that especially permeable rubble mound submerged breakwaters (SPBs) may function similarly to natural reefs as they tend to attract marine life. Lately, as the acceptance of permeable submerged structures due to their environmental merit over emerged breakwaters is steadily increasing, new methods are investigated for further improvement of their technical and ecological efficiency by proposing submerged structures with presence of Posidonia Oceanica over their crest. This study focus on the technical efficiency of rubble-mound submerged permeable breakwaters with presence of Posidonia Oceanica over their crest, and quantitative investigation is performed in terms of wave transmission coefficient. According to the literature, two main mechanisms in reducing notably wave energy are generated over submerged permeable bars: wave breaking and friction. Especially for porous structures the fluid flow through the voids induce significant friction due to turbulent flow within their core (Metallinos et al. 2016). There are indications that positioning Posidonia Oceanica over SPBs increases the reduction of wave energy propagating over them further probably by intensifying the effect of these phenomena. Based on the above assumptions this study was performed considering both experimental data and semi-empirical models found in the literature that are proposed for SPBs. Main goals were primarily to determine the contribution of Posidonia Oceanica on the technical efficiency of SPBs, through a comparative study from experiments covering both cases for similar wave climate and sea conditions, and secondly to examine the possibility of modifying the existing semi-empirical models in order to include the addition of Posidonia Oceanica in their results. Description of the experimental model set-ups Experimental data needed for conducting this study, were gathered by a previous laboratory investigation on rubble mound trapezoidal submerged permeable breakwaters (SPBs) with presence of a Posidonia Oceanica meadow over their crest. The relevant set-ups that were used for this thesis are summarized below.  Laboratory measurements of wave transmission coefficient (Kt for various cases of sea state), induced by regular waves’ action over a series of SPB 2D models, of a scale roughly to 1:10, with steep seaward and leeward slope, with or without Posidonia Oceanica covering their crest, in a canal constructed inside a wave basin (Ziros & Memos 2019, Repousis & Memos 2019). In their studies they calculated wave transmission coefficients that were acquired for investigation within this work, through measurements of free surface elevation obtained by using two twin-wired wave gauges positioned at around 1.0 meter from the windward and leeward of the models respectively. Specifically, the experiments were conducted in the 3-dimensional basin in the Laboratory of Harbor Works at National Technical University of Athens, Greece. The models were constructed subsequently between two metal sheets 4.00 m long, placed at a distance of 0.80 m from each other and vertically to a wider canal of 7.00 m width and 22.00 m length so that each SPB section had a width of 0.80 m. Further details for the canal configuration inside the basin can be found in the study by Memos et al. 2018 as the SBP models addressed herein were constructed in the same experimental canal facility. Additional information for SPB cases’ constructional details for similar experiments with or without sea-grass can be found in the study by Ioannis Roupas (2018), who also used the same canal for measuring wave transmission coefficients. Details of the physical models’ set-ups (4 models in total) are mentioned below. Model 1a: Submerged permeable rubble mound breakwater. The height of the model was 0.50 m and the crest width 0.80 m. The structure’s material was natural stones with d50 ≈ 0.12 m and sloping 1:1.5 at both sides. The mean water level was at 0.60 m. Wave conditions were ranging in terms of characteristic wave height between 0.028-0.113 m, and for wave periods between 1,024-1,706 sec. Model 1b: Submerged permeable rubble mound breakwater with placement of a Posidonia Oceanica meadow over its crest. The overall geometry of the structure, sea state and experimental set-up followed Model 1a. Experimental modelling of Posidonia Oceanica was applied by pegging individual rhizomes of a mean 2 cm diameter with leaf clusters reduced to a 0.15 m average length (vertical foliar shoots collected from a natural marine sea-grass bed), on a plastic 2 cm netting mesh (mean sea-grass density of 150 stems per m2) of a total area equal to the crest area of the model which was held with a few stones on top of it. In the under-scaled design of the submerged breakwater-Posidonia set-up following Froude scaling similarity, Ziros & Memos (2019) used real plant parts as one of their main targets was to represent experimentally the plant’s wave interaction and friction induced phenomena by keeping its natural properties such as roughness, rigidity, elasticity, movement pattern to wave-flow, etc. In order for the Posidonia Oceanica bed to cope in terms of length scale with that of the rubbles and sea state reasonably (roughly to a scale of 1:10), the experimental meadow was modelled by managing the grid distance of the rhizomes, their mean diameter and density as described above, but also the mean length and number of the leafs on each stem, aiming in resulting simultaneously to a similar agreeable scale for the overall plant-water contact surface. Model 2a: Submerged permeable rubble mound breakwater The height of the physical model was 0.40 m and the crest width 0.60 m. The structure’s material was natural stones with d50 ≈ 0.12 m and sloping 1:1.5 at both sides. Two mean water levels were examined at 0.50 and 0.60 m. Wave conditions were ranging in terms of characteristic wave height between 0.033-0.15 m, and for wave periods between 1,024-2,048 sec. Model 2b: Submerged permeable breakwater with introduction of a Posidonia Oceanica meadow over its crest. The overall geometry of the structure, sea state and experimental set-up followed Model 2a. Experimental modeling of Posidonia Oceanica was applied likewise model 1b. Analysis of the experimental data and comparison with semi-empirical numerical models The experimental data that were sorted out from Ziros & Memos (2019) and Repousis & Memos (2019) works, were considered satisfactory for a preliminary examination, on both a verification study of semi-empirical models found in the literature for estimating wave transmission coefficient behind SPBs, as well as the contribution of the marine plant meadow on top of the SPBs to their technical efficiency. Last, the information comparing Kt for similar sea states between the two types of SPBs (with or without the sea-grass), was used to examine the possibility of modifying existing semi-empirical models of certain reliability for the SPBs addressed herein; i.e. if they could incorporate adequately the contribution of an added Posidonia Oceanica bed over a SPB crest in their numerical calculations and results of Kt. At this point it should be noted that semi-empirical models for the case of SPBs with Posidonia Oceanica have not been presented yet. Due to lack of adequate experimental data that can be found in the literature, the proposal of a relevant new reliable numerical model covering a series for SPBs with seagrass over their crest, was not possible within this work. For this reason the possibility for proposing such a module was approached by investigating the behavior of the existing semi-empirical models for SPBs in order to use them as a base, by making the assumption of a similar behavior for SPBs of the same layout with or without a bed of Posidonia Oceanica covering their crest. Specifically for the proposed model that was found to be the most reliable according to the experimental data for SPBs within this work, targeted calibrations were attempted to its structure in order to examine the possibility of producing a reliable modification that could capture the presence of Posidonia Oceanica. Conclusions Considering the experimental data the conclusions can be summoned below:  The wave transmission coefficients (Kt) that were derived from the experimental data for the models 1b and 2b (sections with presence of Posidonia Oceanica ) were smaller than those for the relevant SPBs models 1a and 2a for the same wave conditions and sea state.  Specifically for Section 1 (b=0.8m, h=0.6m, h’=0.5m) the reduction of Kt for the layout with the Posidonia Oceanica reached up to 30%.  For Section 2 (b=0.6m, h=0.6m & h=0.5m, h’=0.4m) it was observed that for increased freeboard (h=0.6m), the wave transmission coefficients (Kt) did not present the same percentage of reduction as for the smaller one freeboard ( h=0.5m).  Between the model 1b and 2b with water depth of 0.5 m, better technical efficiency had the model 1b. The main reason is because of its largest crest width and overall volume.  Last for all physical models was observed a tendency for significant wave energy reduction for certain wave period and of certain wave height. Considering the comparison of the experimental data and with semi-empirical numerical models the main conclusions are mentioned below:  The majority of the semi empirical models that were investigated in this work could not reproduce sufficiently the experimental wave transmission coefficients (Kt).  Those that presented the best behavior and degree of a certain reliability were the Friebel and Harris, and Seabrook and Hall numerical models. However they could not cover adequately all the wave scenarios.  Specifically for the Friebel and Harris numerical model, the cases that could not be reproduced with accuracy were those for waves of significant wave height for some wave periods without a clear tendency of their influence on these certain miscalculation. The same was observed also for Seabrook and Hall numerical model.  In general, the Friebel and Harris numerical model was the most reliable.  Last, the investigation of the possibility of modifying reasonably Friebel and Harris model and proposing a new equation integrating Posidonia Oceanica effect on Kt led to the conclusion that this could not be achieved due to the structure of the initial equation. This is probably because of the underestimation of the phenomena that are connected to the porosity and the roughness of the structure with wave energy reduction that led Friebel and Harris to neglect extreme values of Kt by proposing best fit curves.	en
heal.advisorName	Μέμος, Κωσταντίνος	el
heal.committeeMemberName	Μέμος, Κωνσταντίνος	el
heal.committeeMemberName	Τσουκαλά, Βασιλική	el
heal.committeeMemberName	Νάνου, Αικατερίνη	el
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Πολιτικών Μηχανικών	el
heal.academicPublisherID	ntua
heal.numberOfPages	99 σ.	el
heal.fullTextAvailability	true