Ανατρεπόμενα θυροφράγματα ασφαλείας (fusegates) σε υπερχειλιστές φραγμάτων - Η περίπτωση του ΥΗΕ Λούρου

Abstract

Η τεχνολογία των ανατρεπόμενων θυροφραγμάτων ασφαλείας (fusegates) χρησιμοποιείται για την αύξηση της χωρητικότητας ταμιευτήρων και τη βελτίωση του ελέγχου των πλημμυρών και της παροχετευτικής ικανότητας υπερχειλιστών σε υπάρχοντα ή νέα φράγματα. Τα fusegates είναι ελεύθερα επικαθήμενα θυροφράγματα μίας χρήσης, τα οποία τοποθετούνται κατά μήκος της στέψης του υπερχειλιστή, σχηματίζοντας ένα στεγανό διάφραγμα. Είναι σχεδιασμένα έτσι ώστε να ανατρέπονται ανά ομάδες σε πολύ μεγάλα πλημμυρικά γεγονότα, ενώ σε κανονικές συνθήκες ή σε συνήθεις πλημμύρες το νερό υπερχειλίζει με τα θυροφράγματα να διατηρούν τη θέση τους. Η τεχνολογία των fusegates έχει εφαρμοστεί σε εκατοντάδες έργα ανά τον κόσμο. Στην Ελλάδα σύστημα ανατρεπόμενων θυροφραγμάτων έχει τοποθετηθεί στο Υδροηλεκτρικό Έργο Καστρακίου (ΥΗΕ) της ΔΕΗ (2009) και στο Μικρό Υδροηλεκτρικό Έργο (ΜΥΗΕ) της Δαφνοζωνάρας, στον Αχελώο ποταμό, ιδιοκτησίας ΤΕΡΝΑ ΕΝΕΡΓΕΙΑΚΗ ΑΕ. Υπάρχουν αρκετά φράγματα σε λειτουργία με ελεύθερο υπερχειλιστή στα οποία μπορεί να τοποθετηθεί σύστημα fusegates, εκ των οποίων μερικά είναι ΥΗΕ της ΔΕΗ και τα υπόλοιπα είναι άρδευσης-ύδρευσης και υπάγονται κατα κύριο λόγο στο Υπουργείο Αγροτικής Ανάπτυξης και Τροφίμων (ΥΠ.ΑΓ.Α.Τ.). Για την τοποθέτηση ανατρεπόμενων θυροφραγμάτων απαιτείται έλεγχος ευστάθειας του υπερχειλιστή, του φράγματος και των πρανών του ταμιευτήρα για τα νέα υδροστατικά φορτία, έλεγχος των συνθηκών υδραυλικής λειτουργίας του υπερχειλιστή με/χωρίς fusegates, έλεγχος της συμπεριφοράς του πυρήνα στην περίπτωση γεωφράγματος, μελέτη της επίδρασης σε πληθυσμούς και υποδομές στη γύρω περιοχή και αποτίμηση του ποσοτικού και οικονομικού οφέλους. Στην παρούσα εργασία εξετάζεται η δυνατότητα τοποθέτησης fusegates στο Υδροηλεκτρικό Έργο του Λούρου, που έχει πληρωθεί κατά 80% με φερτά. Η τοποθέτηση ανατρεπόμενων θυροφραγμάτων αναμένεται να αυξήσει τον ωφέλιμο όγκο του ταμιευτήρα κατά 225.000m3 και την παραγωγή ενέργειας κατά 2.150.000kWh/έτος, επαναφέροντας την αποθηκευτική ικανότητα του ταμιευτήρα. Η ευστάθεια φράγματος και υπερχειλιστή δεν αναμένεται να επηρεαστεί από την ανύψωση της Ανώτατης Στάθμης Λειτουργίας, σύμφωνα με τους ελέγχους ευστάθειας. Από οικονομικής πλευράς, η επένδυση κρίνεται συμφέρουσα, όπως προκύπτει από το επιχειρηματικό σχέδιο ενώ η διάρκεια ζωής του φράγματος αναμένεται να παραταθεί κατά 30 χρόνια τουλάχιστον.

Dam safety mainly depends on the reliable operation of the spillway in normal or extreme critical conditions. Ungated spillways increase dam safety; however the cost of wasting water and reservoir storage is considerable. Fusegates are used to increase, even maximize, reservoir storage, spillway capacity or both. A fusegate system is installed on the spillway crest followed by the necessary technical works to floaten the surface of the crest. Fusegates are the mechanical equivalent of a fuseplug, held in place by gravity. For a medium range of reservoir levels water flows over the gates, however the gates start to overturn after a predetermined value of water level. The entire system will have tipped until the maximum design reservoir level has been reached. The scope of this postgraduate thesis is an extensive presentation of fusegates technology, an investigation of the necessary parameters to be examined for the installation of a fusegate system on the spillway crest of an existing dam and a project study of such an installation to the Hydroelectric Plant (HEP) of Louros River in the county of Preveza, near the town of Filippiada. FUSEGATE TECHNOLOGY Design of fusegates Fusegates are the mechanical equivalent of a fuseplug, as referred earlier. While a fuseplug fails entirely when operating, in a fusegate system only the number of gates needed to pass a flood are operational. Additionally, tipping levels can be more precisely determined to a fusegate system. A typical fusegate is an L shaped construction and consists of three components: a bucket made of metal or reinforced concrete, a base with a watertight chamber and an intake well connected to the watertight chamber. Lugs cast in the spillway sill at the downstream edge of the gate prevent from sliding and allow the gate to overturn rotating about the downstream edge when operating. Drainage holes in the chamber discharge normal water leakages from the fusegate/sill interface. The height of inlet wells varies depending on the tipping level of the fusegate. Water flows through the inlet well into the watertight chamber of the fusegate base after a certain headwater level, increases the uplift [11] pressure into the chamber until the upstream edge of the fusegate lifts off the spillway sill and causes the overturning of the gate. Upstream seals prevent leakage through the fusegate/sill interface into the chamber in normal conditions. Ballasts placed on the base of fusegates prevent from overturning in critical conditions below the normal tipping level of the gates. Operation conditions A typical fusegate installation consists of multiple gates. During a flood event only the gates needed to discharge the flood are operational. Thus, design flow varies for the several groups of installed fusegates. The well crests are arranged so that adjacent fusegates do not tilt simultaneously. For reservoir elevations up to the fusegate crest there is no discharge. For headwater level higher than the gate crest and up to the inlet well crest, water flows over the gates without affecting the stability of the system. The inlet well crest corresponds to the design flow of the gate. For discharges up to the design flow, the system functions like a labyrinth weir in which each gate represents one cycle of the labyrinth. Normally, the discharge with return period about at least one hundred (100) years is selected as the design flow of a gate. For discharges greater than the design flow (headwater level higher than the inlet well crest), water begins to flow through the well into the watertight chamber in the base of the fusegate. When the inflow overpasses the outflow of the drain holes, water level in the chamber and uplift pressure increase. Due to the increase of the uplift pressure up to a certain value, the upstream edge of the fusegate lifts off the spillway sill and water flows into the chamber directly from the reservoir. This happens when overturning and restoring moments about the downstream edge are equalized. In these conditions the gate finally overturns by rotating about its downstream edge. Each group of fusegates is set to overturn at a progressively higher reservoir level. The last group of gates tips for the maximum design discharge, usually equal to the maximum probable flood of the area. Operational reliability Fusegates are self-operating units needing no mechanical or electrical input. Due to the independent operation of each fusegate, the possibility of the whole system failure is negligible. Fusegates are tested in several operational conditions. Safety against overturning in normal conditions is provided. Operation in extreme critical conditions such as drain hole and inlet well blockage or destroyed upstream seals is foreseen by determining a minimum tipping level and an ultimate stability level for the fusegates. Floating debris, wave, ice and earthquake effects on the operational reliability of the fusegates are not considerable. [12] FUSEGATE SYSTEMS IN EXISTING DAMS Fusegates may be installed on the spillway crest of new or existing dams. In new dams installing fusegates does not require any special study because determination of the Maximum Power Pool of the reservoir takes into consideration the fusegate system. However, installing fusegate system on the spillway crest of an existing dam requires further study of certain parameters because in most of cases the Maximum Operation Level increases. Thus the loads stressing the dam, the spillway and other dam sections in normal and extreme critical conditions increase and need to be studied. The parameters that are influenced by the fusegate installation and need to be examined are: 1. Hydraulic operation conditions of the spillway – discharge coefficient 2. Discharge capacity of fusegate system 3. Spillway stability 4. Dam stability 5. Earth or rockfill dam clay core and slope stability 6. Reservoir banks stability 7. Hydroturbines operation conditions 8. Infrastructures and populations in the area around 9. General and economic benefits FUSEGATE SYSTEM IN LOUROS RIVER DAM Louros River Dam Louros River Dam is located near the town of Filippiada, in the county of Preveza, approximately 420 kilometers away from Athens. It is a solid gravity dam made of reinforced concrete, built in the ages 1951-1954. The dam is arch shaped with crest elevation at +100,50 m, maximum reservoir volume 1.075.000m3, maximum rexervoir surface 0,15 km2 and is exclusively used as Hydroelectric Project of 10MW maximum capacity. The dam has an ungated chute spillway 77m long, located on its main body with crest elevation at +96,00m. The spillway crest elevation corresponds to the Maximum Power Pool of the reservoir. Maximum Flood Level is at +99,75m and corresponds to a design flood of 940m3/s. Extended sediment transport phenomena in the area have [13] reduced reservoir volume and production of hydroelectric power. Due to the advanced age of the dam few designs and info about flood control system are available. Fusegate installation It is proposed to install a fusegate system on the spillway crest of the Louros River Dam in order to increase the annihilated reservoir storage. The proposed fusegate installation consists of 28 labyrinth gates of 1,50m height and 2,75m crest length and will also increase the spillway capacity. The Maximum Power Pool will rise from +96,00m to +97,50m high. All parameters are investigated and it is proved that operational reliability rests unaffected and in some cases increases. Discharge capacity of the spillway is improved, spillway, dam and reservoir banks stability are not affected. In the area around the dam site neither infrastructures nor populations face any problem by the rise of the reservoir elevation. Furthermore the reservoir volume will increase by 225.000m3 and it is expected that Hydroelectric Power production will rise by 2.150.000KWh annually. According to the business plan of the investment there will be a considerable economic benefit. CONCLUSION Taking into consideration international references and the investigation of installing fusegate system on the spillway crest of Louros Dam, it can be safely concluded that fusegate technology is a reliable and economic choice. Fusegates are used to increase reservoir storage, spillway discharge capacity or both and have high operational reliability either in normal or in extreme critical conditions. The gates are self-operating units needing no mechanical or electrical input and the risk of a whole system failure is negligible. Besides, maintenance cost is very low. Installing fusegates on a dam spillway requires a good knowledge of the hydrologic conditions of the area around. The proposed fusegate installation on HEP of Louros River increases reservoir volume, hydroelectric power production and is a profitable choice without affecting the safety of the dam or other sections.