Dike Breach Due to Hydraulic Structure Failure Along Fehér-Körös in late 1995

Transcription

1 Dike Breach Due to Hydraulic Structure Failure Along Fehér-Körös in late 1995 L. Nagy, S. Toth Breach Location The Gyula No. I pumping station and gravity sluice was built in The structure is situated in the km section on the left bank of Fehér-Körös in the mouth of the Feed water canal connecting Fehér-Körös with the Élővíz canal. Fig. 1. Location of the site The sluice has two reinforced concrete tubes with rectangular cross sections measuring 1.5 by 1.8 meters. The lock structure facing the water is a Réthy type gate, and as there is no lock on the protected side, the shutter installed in the backpressure pool dike fulfils that function. The pump station is integrated into the flood dike. Pikes were used to eliminate foundation problems. The lifting tube of the station is made of reinforced concrete and is 1.4 m in diameter. The higher flood levels of earlier years resulted in cracks appearing in the wall of the station that faces the water, posing a flood control threat. To retain the capacity of the structure to protect against floods, a reinforced concrete fender wall was built against the wall on the water side. The pump station was designed and constructed in line with the design crest level valid at the time. Later, the dike was raised to match the higher design flood levels, whilst no work was carried out to match the station with the new crest level, hence the height of the station falls short of the height of the dike in its surroundings. To compensate for this deficiency in height, sandbags were used to build an emergency dike on top of the concrete revetment along the crest next to the building when water level was high.

2 2 The bank of the backpressure pool behind the structure formed a semicircular enclosure around the pump station. The height of the bank met the requirements. Shutters were installed in the junction with the Feed water canal. Fig.2. Cross section of Gyula No. I. pump station sluice on the LB, Fehér-Körös, at the tkm section Fig.3. Cross section of the sluice on the feed water canal Events before the Breach: Hydro-meteorological Conditions The earlier dry weather had changed radically by the first day of Christmas in 1995 and more than 100 mm of precipitation, limited initially to a relatively compact area, hit the catchment area of the Körös Rivers in Romania. Rainfall continued for days and clouds carrying loads of rain gathered as if scheduled over the catchment area. Between December 23 and 28 precipitation load totaled at 230 mm in the catchment area of Fekete-Körös and reached unusually high levels at mm elsewhere. The sudden surge in temperature melted the modest blanket of snow covering the highlands in the catchment area of the Körös Rivers.

3 3 After December 27, it was freezing hard for a few days, turning the rain over the catchment area into snow. The second wave of flood, which frequently lags 7-10 days behind the initial high water stage, did not fail to come this time, but it was not significant and caused no problems. The hydrological unit forecasted the following flood peaks: River Gauging Forecasted Design flood level Highest recorded station water level peaking time (a.s.l. Baltic) level (year) Fekete-Körös Ant 920 ± 50 cm Evening, December (94.77 mbf) 1000 (1981) Fekete-Körös Remete 860 ± 50 cm Evening, December (91.78 mbf) 916 (1974) Fehér-Körös Gyula 760 ± 50 cm Evening, December (91.90 mbf) 786 (1974) Kettős-Körös Békés 980 ± 50 cm Night, December (90.58 mbf) 972 (1974) Table1. Forecasted water levels and peaking time Highland precipitation and water level information suggested that, unless action was taken, traveling flood waves would surpass the historical HW recorded on Fehér and Kettős Körös. The severity of the flood control situation forecasted the need to open the emergency flood reservoirs at Mályvád and Mérges to relive the load on the dikes which suffered from extremely high water. Flood control management thought the opening the reservoirs would result in substantially inferior HW levels than projected. Fig. 4. Flood hydrograph Issuing the Flood Warning Alert I. degree was introduced at 16:00 hours along the dikes of the Fekete-Körös to control the wave of flood arriving on December 25. Next, the alert stage changed rapidly and reached 3rd stage along Fekete-Körös and a series of warnings were issued subsequently for the flood control sections along the Rivers Fehér, Kettős, Sebes, and Hármas Körös and Berettyó

4 4 starting the 27 th to culminate in emergency flood warning conditions due to the severity of the flood. The Government Commissioner issued the extraordinary alert for the dikes along Fekete- Körös starting 18:00 on the 27 th, for Fehér and Kettős Körös rivers starting 08:00 hours on the 29 th, which was called off at noon on the 31 st as the flood eased. Countermeasures The automatically operating, chamber type flood reservoir along the right bank of Fekete- Körös at Tamásda, Romania constructed with a lowered fixed weir, started operating at 00:00 hours on December 28. The 2 million m 3 of water flowed into the reservoir resulted in peaking of the Fekete-Körös at Ant at 908 cm, which was lower than predicted. The emergency reservoir at Mályvád was opened at 04:06 00 on 29. December. Fig.5. The opened Mályvád reservoir. The volume of water stored off was 7.4 million m 3. Fig. 6. The emergency reservoir at Mérges was opened at 01:30 on December 30. The volume of water stored off was 31.8 million m 3. The effective area of the Mérges reservoir extends upstream to the pump station at Hosszúfok. Operated at optimum, the effect on the endangered Fehér-Körösre and the section of Kettős- Körös in Békés County is zero. Furthermore, the reservoir was opened very early this time and the drain-off effect was lost. The relatively large volume of stored water is misleading. Substantial quantities of rising stage water flowed into the reservoir, which only reduced the peak modestly. Fig. 7. The flood wave on Fehér-Körös threatened the dikes at both sides with overtopping.

5 5 Undoubtedly, this attempt at storing water was not such a disaster as at Mályvád but it did not help solve the major problem. As required by flood control procedures, the hydraulic structures dissecting the bank were closed off, including the sluice at Gyula No. I pump station and the lock in the bank of the backpressure pool. An emergency dike had to be raised on the right bank of Fehér-Körös between the river mouth and the connection of the ring dyke at Gyulavár, because the crest was cm-lower than required. People kept working on December 28 and 29 and managed to build 3900 meters of emergency dike along a 6500 m section. The bridge at Gyulavár was surrounded with emergency dikes built of bags filled with earth for protection from the extremely high water on December 29. On December 28, sandbagging managed to block the 80 cm shortfall in height at the concrete covered landing next to the wall of the Gyula No. I pump station. After the pump station was taken off electric supply in the morning of the 29 th the windows opening on the river bed in the front wall were also blocked off. In the morning on the 29 th strengthening leakage was observed midway up the stairs to the concrete covered landing next to the wall of the pump station. By 10 o clock, the leakage had strengthened to a degree that it could disintegrate the material of the dike. The breach started with the components of the concrete stairway tearing up. Short of material and men, no attempt was made to prevent the breach. Fig. 8. The breach closed by Larsen sheetpiles. At on December 29, 1995, the levee connecting to the sidewall of the pump station ruptured and the counter pressure pool had been filled with water to flood level by Restoring the control capacity of the dike started immediately upon the failure. The Control Squad of the Körös Water Directorate used two rows of Larsen sheets anchored together to form a pile wall with 4 meters of crest width to block the breech.

6 6 Fig. 9. The closed breach from the river side a few days later. Fig. 10. The filled counter pressure pool after the failure.

7 7 The bank of the counter pressure pool could withstand the sudden pressure of water and no flood phenomenon developed. That must have been due to the extreme cold. Men from fellow water directorates and the military had built 18 sandbag ribs to support the bank of the counter pressure pool by 14:00 hours on the 30 th. The Cause of the Breach This was a typical case of a failure that was caused by a hydraulic structure integrated improperly into a levee without meeting safety criteria. Deficiencies of the design include: The wing to prevent interface seepage between the building integrated into the levee and the levee body has not been constructed. The development of the levee ignored the harmony between the levee and the integrated building. The level of the concrete surface on the crest was built below the flood level and was made of slabs which could not block water from seeping below the slabs and through borrows. Maintenance errors include: There was no regular use of pesticides under the concrete surface. The direct cause of the breech is impossible to identify exactly in retrospective. Analyses of flood phenomena suggest two possible direct reasons: interface seepage along the wall of the pump station; or borrow(s) below the concrete landing downstream from the pump station. Flood control management did their best to manage the flood in the safest possible manner. The active control interventions along the Fehér-Körös were necessary and were implemented well and in due course. The decision to open the emergency reservoir was fundamentally correct. Abandoning the main line of defense on Fehér-Körös involved a risk in that particular situation (as the control capacity of the bank of the backpressure pool was unknown), but the decision proved to have been right. Evidence from the flood has verified that the existing two emergency reservoirs do not provide effective means for managing the flood problems of Fehér-Körös. After restoring the control capacity of the levee, the reconstruction of Gyula No. I. pumping station started simultaneously with the reconstruction program of the major hydraulic structures in the Tisza valley. To enhance the level of protection along Fehér-Körös, design work on and the construction of the Kisdelta reservoir have started. The theoretical and practical experience regarding the systemic operation of emergency reservoirs is also of appreciable value and could be put to good use in designing and operating future systems.