Η επίδραση του χρονικού εύρους της διαθέσιμης γεωπληροφορίας στην εκτίμηση του ρυθμού μεταβολής της ακτογραμμής

Abstract

Η μελέτη του ρυθμού μεταβολής μίας ακτογραμμής πραγματοποιείται μέσα από τη συλλογή γεωπληροφορίας που αντιστοιχεί σε θέσεις μίας ακτογραμμής κατά το παρελθόν, και την εφαρμογή μεθόδων που χρησιμοποιούν διαφορετικές διαδικασίες υπολογισμών.

Η συμπεριφορά του εξαγόμενου σφάλματος κάθε μίας από τις μεθόδους υπολογισμού του ρυθμού μεταβολής μίας ακτογραμμής -EPR, AOR, AER, OLS, JK, RLS, WLS, RWLS, WLAD- στην μεταβολή της διαθέσιμης γεωπληροφορίας, εξετάστηκε με μία σειρά εφαρμογών του λογισμικού Shev, κατά τις οποίες ορίστηκαν διαφορετικές αρχικές αβεβαιότητες κλίμακας και διαφορετικοί συνδυασμοί των διαθέσιμων χρονικά ακτογραμμών.

Οι εφαρμογές του λογισμικού πραγματοποιήθηκαν σε δύο περιοχές μελέτης της νήσου Ρόδου, οι οποίες έχουν αντίθετο προσανατολισμό, και συνεπώς, διαφορετικό ανεμολογικό και υδροδυναμικό καθεστώς. Η πρώτη περιοχή ήταν η παραλία Αφάντου, στην οποία χρησιμοποιήθηκαν 6 ακτογραμμές που αναφέρονταν σε ένα εύρος 79 ετών, και η δεύτερη παραλία Φάνες, στην οποία χρησιμοποιήθηκαν 4 ακτογραμμές που αναφέρονταν σε ένα εύρος 81 ετών.

Από τη σύγκριση των αποτελεσμάτων κατά τη μελέτη των δύο περιοχών, προέκυψαν σημαντικά πορίσματα σχετικά με τη συμπεριφορά του τελικού σφάλματος θέσης του ρυθμού μεταβολής της ακτογραμμής στις μεταβολές της διαθέσιμης γεωπληροφορίας.

The aggravation of climate changes, which is caused mainly by the human activity, has brought important consequences to the coastal zones. The temperature modification of the wind and the hydrodynamic condition affect the transition of coastline’s position, even if it is expressed as alluvion or as erosion. Moreover these transitions affect the coastal morphology as well as the human activity in the coastal area.

2.2 Calculation methods of the coastal evolution

There are many methods available for the study of coastline evolution, while each one of them uses different calculation procedure. The variety of those methods provides the ability to the person studying it to choose the one that corresponds better to the demands of the available data sample. The volume of data refers to an amount of coastlines, recorded in a different time period. Therefore, there are methods that require, either large data volume, or wide time range of coastlines, or small initial position error, or all these together. In this project, the applications of simple methods are taking place, such as EPR (End of Point Range), AOR (Average of Rates), AER (Average of Eras Rates), and OLS (Ordinary Least Squares). At the same time, more complex methods are applied: RLS (Reweighted Least Squares), WLS (Weighted Least Squares), RWLS (Reweighted Weighted Least Squares), WLAD (Weighted Least Absolute Deviation) and Binning method. The main characteristic of OLS, RLS, WLS and RWLS methods is that they use the procedure of minimum squares, in order to use the geoinformation available in the best possible way. Furthermore, LAD method considers a Laplace distribution of the initial position errors, while the Binning method, groups the available data while performing constant statistic controls combined with the application of another selected method.

2.3 The study’s application in two areas

The objective of this project was the study of the behavior of the final error of the coastline’s transition rate, as it is calculated with each one of the above methods, and for different combinations of the available coastlines. More specifically, in a primary stage, applications of the methods studied were performed using all the available geoinformation. Next, the application of the same methods was performed, excluding some coastlines. Thus, with the exclusion from the calculation procedure of some time intermediate coastlines, or coastlines that have been determined with a big a priori error, we examined the behavior of the final noise of the transition rate.

This study was performed in two areas of the island of Rhodes, which have opposite orientation, and therefore, different wind and hydrodynamic condition. The first area was Afandou beach, in which 6 coastlines were used that referred to a range of 79 years, and the second was Fanes beach, to which 4 coastlines were used that referred to a range of 81 years.

2.4 Results

From the comparison of results during the study of both areas, some important conclusions were made concerning the behavior of the final error position of the coastline’s transition rate in the transitions of the available geoinformation. More specifically, in methods were minimum squares are used, it was noted that the exclusion of time intermediate coastlines does not affect in a great extent the error of the coastline’s transition rate. On the contrary, the removal from the calculation procedure of older coastlines – which use increased the time range of the study – causes important transitions in the noise of the final transition rate.

Moreover, with the application of those different methods, important conclusions were made concerning the usefulness of each one of those in a specific sample of available data. Thus, in a final stage, the requirements are highlighted under which one method must be selected, in order for the exported results to correspond to reality and to represent, with the best possible way, one actual transition of the coastline’s position.