Κατάρτιση χαρτών πλημμύρας στην περιοχή της Ραφήνας

Abstract

Στόχος της παρούσας μεταπτυχιακής εργασίας είναι η ανάπτυξη ενός ολοκληρωμένου μεθοδολογικού πλαισίου για τη δημιουργία χαρτών πλημμυρικής κατάκλυσης, μέσω της χρήσης ειδικού λογισμικού επεξεργασίας της υδρολογικής, υδραυλικής και γεωγραφικής πληροφορίας.Συγκεκριμένα, το λογισμικό που χρησιμοποιήθηκε για την ολοκλήρωσης αυτής της εργασίας είναι τα προγράμματα HEC-HMS 3.5, HEC-RAS 4.0 και Arc-GIS 9.3. Το HEC-HMS χρησιμοποιήθηκε για την υδρολογική προσομοίωση της λεκάνης, το HECRAS για την υδραυλική προσομοίωση ροής σε ανοιχτούς αγωγούς και τελικά το Arc-GIS για την επεξεργασία κάποιων δεδομένων και την οπτικοποίηση των τελικών αποτελεσμάτων υπό μορφή χαρτών.Η περιοχή που εξετάστηκε, είναι η λεκάνη απορροής της Ραφήνας στην Αττική με έκταση 126 km2. Για την περιοχή αυτή, δόθηκαν πρωτογενή δεδομένα όπως το ψηφιακό μοντέλο εδάφους καθώς και οι χάρτες χρήσεων γης και γεωλογίας. Επίσης, έγινε συλλογή βροχομετρικών δεδομένων από εγκατεστημένους σταθμούς στην περιοχή (Hydrologic Observatory of Athens). Έτσι επιλέχθηκε το επεισόδιο βροχόπτωσης της 04/02/2011 και αυτό της 24/02/2011.Με δεδομένα όλα τα παραπάνω πραγματοποιήθηκε η υδρολογική προσομοίωση της λεκάνης, η οποία χωρίστηκε σε υπολεκάνες ανάλογα με τη θέση των σταθμών μέτρησης απορροών. Η υδρολογική προσομοίωση έδωσε πλημμυρογραφήματα σχεδιασμού σε κάθε υπολεκάνη και για τα δύο επεισόδια που επιλέχτηκαν. Ακολούθησε βαθμονόμηση του υδρολογικού μοντέλου και συγκεκριμένα στις θέσεις όπου υπήρχαν παρατηρημένα πλημμυρογραφήματα. Αυτός ήταν και ο σκοπός δημιουργίας υπολεκανών στις θέσεις όπου υπήρχαν σταθμοί μέτρησης απορροών.Στη συνέχεια, πραγματοποιήθηκε η υδραυλική προσομοίωση 12 km του υδρογραφικού δικτύου, σε συνθήκες μόνιμης αλλά και μη μόνιμης ροής. Και στις δύο περιπτώσεις, το εξαγόμενο αποτέλεσμα ήταν ο υπολογισμός της πιεζομετρικής επιφάνειας για κάθε επεισόδιο βροχόπτωσης και στη συνέχεια αντιπαραβολή με το ψηφιακό μοντέλο εδάφους για τον υπολογισμό του βάθους νερού.Έτσι, με τα δεδομένα της υδρολογικής και υδραυλικής προσομοίωσης πραγματοποιήθηκε η χαρτογράφηση των πολυγώνων κατάκλυσης και για τα δύο επεισόδια βροχόπτωσης και η τελική σύγκριση των κατακλυζόμενων επιφανειών.

Floods can simply be defined as the physical phenomena, during which an initially dry land area is covered by water. Floods are normally caused by climatic changes, while their evolution depends mainly on geomorphologic factors, such as soil stability, vegetation cover, as well as the geometrical characteristics of the river basin. Aim The aim of this thesis, entitled as «Elaboration of flood inundation maps in Rafina basin», is the development of an integrated methodology, which combines the fields of hydrology, hydraulics, geomatics and cartography for the creation of several maps for observed runoff events. Tools The software, which was used for the accomplishment of this thesis, is HEC-HMS 3.5 and HEC-RAS 4.0 programs, ArcGIS 9.3 and especially Hec-GeoHMS and Hec- GeoRAS toolbars. HEC-HMS: simulates the hydrologic processes of river basins and computes the flood hydrographs. HEC-RAS: simulates the open channel flow (river systems) and computes the energy gradient and piezometric levels along the river. Both of them are compatible with ArcGIS, which can be used for further data processing and visualization for cartographical purposes. Structure of the thesis  Chapter 1 –Introduction  Short analysis of the active EU directive 2007/60/EC for the assessment and management of flood risks  Floods in Europe and Greece xii  Hydrological Models  Aim of thesis  Chapter 2 – Description of thesis’s area  Chapter 3 – Hydrologic simulation of river basin  Data Processing through the use of Hec-GeoHMS  Hydrologic simulation of the 3 runoff episodes through the use of HECHMS  Chapter 4 – Calibration of the hydrologic model  Chapter 5 – Hydraulic Simulation  River mapping through the use of Hec-GeoRAS  Hydraulic open channel flow simulation in steady and unsteady flow conditions, through the use of HEC-RAS  Exportation of the results to Hec-GeoRAS for the digital mapping of water surface and floodplain delineation for each runoff episode  Chapter 6 – Production of flood maps  Water surface maps  Floodplain delineation maps for 2 runoff events Case study The case study of the developed methodology is part of the basin of Rafina in Athens covering an area of 126 km2 and it is divided into 12 sub-basins. For this area the collected raw data, consisted of the digital elevation data, as well as the land use and soil type maps. Concerning the hydrologic data, there were rainfall measures of the HOA (Hydrologic Observatory of Athenes) stations that were installed in this area. The 2 rainfall events, which were collected, are the 04/02/2011 event and 24/02/2011 event. Hydrologic simulation The hydrologic simulation was completed through the use of HEC-HMS combined with Hec-GeoHMS, which is an add-in toolbox in the platform of ArcGIS. The first step of this procedure is the creation of the basin model through the use of Hec- GeoHMS working in the environment of ArcMap. xiii The representation of watershed hydrologic processes was based in the formulation of models of the computation of rainfall losses, direct runoff and channel flow routing. Baseflow components were omitted on purpose, as a result of their minor role in the final target of the thesis. There are many methods to compute the different models. The selection of the methods was determined by the shorage of the reliable data of the area. So, the SCS method was applied for the computation of rainfall losses, lag and muskingun methods for the computation of flow routing and a change of the unit hydrograph of the British hydrological institute was used for the estimation of the direct runoff. The simulation was run for the 2 rainfall events (04/02/2011, 24/02/2011). The output of this procedure is a flood hydrograph in the exit of each sub-basin. Calibration of hydrologic model Calibration is the process, in which the user changes the parameters of the simulation so that the simulation results approach the observed. In this thesis, the calibration was applied to the place where, Rafina station is located, which gives runoff measures. In this case the user changed the CN parameter to bring the simulation’s peak discharge closer to the observed. The best results were accomplished by the reduction of CN near 20% of the initial appreciation for the 04/02/2011 event and 25% for the 24/02/2011 event.

Hydraulic Simulation The hydraulic simulation concerns 12 km of the Rafina river. Especially, it was designed through the use of HEC-RAS combined with Hec-GeoRAS, which is an add-in toolbox in the platform of ArcGIS. The first step of this procedure consists of the computation of the geometric characteristics of the river (stream centerline, bank length, cross sections etc). This was accomplished by the use of Hec-GeoRAS. After this, the river was simulated in steady and unsteady flow conditions with the relevant boundary conditions. In steady flow, the whole procedure computation is based on the solution of the energy balance equation between river cross sections. In this study the flow was mixed (subcritical and supercritical) and the boundary conditions were applied upstream anddownstream of the river. The calculations result in the determination of the piezometric depth, the mean kinetic energy and the energy gradient for every cross section. In unsteady flow, the whole procedure computation is based on the solution of the continuity equation and the momentum equation. In this case, there are several selections to assign the boundary conditions. Definitely, a flow hydrograph was applied for upstream boundary condition and the normal depth for downstream. Comparing the two cases of flow, unsteady flow executes results (piezometric depth, mean kinetic energy and energy gradient for every cross section) for the assigned time interval ( 10 min in this study) , while in steady flow the results concern the peak discharge, while the discharge stays steady. Digital mapping The hydrologic and hydraulic simulation provides all the necessary data for the creation of floodplain maps, which is implemented by the use of Hec-GeoRAS. The software compares the grid values of digital elevation model (topographical elevations) with the corresponding values of piezometric surface (water surface elevations). If the latter is greater than the initial elevation, then the corresponding cell gets the value of the subtraction «water surface elevation – topographical elevations». The output product is a new grid surface, which depicts the water depths in the whole area of the river basin. The following figure, presents the floodplain of the river basin for both events, along with two «zoom in» indicative cross sections. Comparison Comparing the inundated areas between the two rainfall events, someone concludes that there is no important difference. This can be justified by the fact that the slopes of this basin along the river are steep enough to provoke a « blow up» in the inundated areas. Evaluation of the study A recommendation for the continuation of the thesis could be the incorporation of the cost factor for the estimation of the financial damage per land use, as occurred after a flood event. This procedure, results in the production of flood hazard maps, which – along with the floodplain delineation maps – provide a strong operational tool for the elaboration of integrated flood management plans in the direction of prevention or minimization of the adverse flood effects. Another recommendation is to apply all the models that HEC-HMS offers for the computation of losses, direct runoff, baseflow and flow routing. After all these, someone can compare the results and choose the best one.