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Flood Protection Measures for The Capital City of Prague - 05 February 2014

Closing the mouth of the Certovka flowing to the Vltava River is clearly the most complicated place of the phase 0002 due to the technical requirements and the demands of conservation authorities. From the beginning of the preparation, one of the main requirements was to carry out such flood protection measures which would not influence the view on the exposed areas of the bank of the Lesser Town.

Based on the evaluation of various institutions and organisations' opinions regarding particular options, in this section, even conservation authorities recommended a route to protecting territory to the fullest extent. Construction preparation was therefore focused on the variant leading flood protection line from the Charles Bridge, following the Vltava to the mouth of the Certovka and, after bridging it, to the wall of Herget's Brickworks.

When considering the closure of the Certovka mouth, to protect the area against flooding from returning high water of the Vltava River, different technical solutions which would not require a bridge construction was assessed by the designer. However, all tightly integrated movable gate or stoppers sliding or tipping out from the bottom, would require a disproportionate intervention to the area during construction. In case of closing the Certovka mouth with the use of mobile constructions, the solution requiring a barrier about eight meters high brought a major problem which was the manipulation with such a construction. The necessary size would eliminate the possibility of manual handling and after the beginning of a flood it would not be possible to carry the device on water and place it. Building a communication which would allow a heavy crane capable of manipulation with such mobile elements come to the Certovka mouth would represent a disproportionate intervention in the whole area.

During the preparation of the suggested project design, bridging the Certovka mouth seemed to be the only acceptable solution. During the processing of the project documentation, this solution was consulted several times with the representatives of conservation authorities. In May 1999 it was agreed that the designer should design a bridge over the Certovka in the profile of the Certovka mouth with the bridge deck about 3.3m. However, in the subsequent zoning approval process the conservation authorities refused any bridging of the Certovka in their assessment and after the appeal of the investor the conservation authorities disagreement was upheld by the Ministry of Culture of the Czech Republic.

Therefore, in the middle of 2000 the designer worked on alternative solutions for the Certovka closure. He considered applying a steel structure, hinged to the bottom of the Certovka, moving both possible ways, a classical mitering gate in both low and high versions or a combination of a subtle bridge with a construction tipping down to the bottom and a mitering gate. The use of a bridge either ejected or lifted from the bottom was considered too.

During co-operation on the architectural designs of the possible solutions the architect suggested a variant of a sliding wall which would not block only the mouth of the Certovka but the wide space ranging with its width of 22 meters to the existing embankment wall. The actual sliding wall in its retracted position would copy the size of the existing embankment wall, where it would be completely hidden in its inactive state. In principle, it would not require any other visible constructions except for two newly designed staircases with stone facing and two subtle columns. During the flood, the sliding wall would be heightened with the mounted mobile barriers to the desired height. This variation was finally judged to be the only acceptable solution and Hydroprojekt incorporated it into adapted documentation for zoning procedure. An appeal from Conservation Institute against the presented solution was finally rejected by the Ministry of Culture in mid-2002, just before the disastrous floods in August.

In the final solution of Hydroprojekt in documentation prepared in May 2003 for the planning permit, the closure of the Certovka was designed in the shape of a sliding wall, which would be located in the area of a newly rebuilt Vltara embankment wall between Charles Bridge and the Certovka. In case of imminent flood threat the closure would ensure protecting the area the the full height of the model flood which is designed according to the flow of the flood in August 2002 including safety margin of 30cm, provided that all the proposed flood protection measures in Prague are already realised and installed. Mobile logs will be set up into blocking position on the top of the closure and will be connected to the continuing mobile wall obstructions of the adjacent areas. The width of the sliding closure is about 23.5cm, its height is 4.9m and the height of the closure's timber extension is 2.97, which in total is overall protecting height of 7.87m. The overall weight of the structure is about 45 tons.

The bottom construction of the closure is formed by reinforced concrete slabs with supports on its sides on the banks of the Certovka. In the reconstructed embankment wall in the direction to the Charles Bridge, space for the lock of the closure and space will be walled off to prevent water from getting inside when in retracted position. In the Certovka mouth profile, there are two intermediate pillars forming the support of the wall. The lower construction includes a substructure for the possibility to create a temporary protection against water from both sides (from the Certovka and from the Vltara) to enable possible repairs and revisions.

The closure is designed as a welded beam construction with the main longitudinal beams and transverse auxiliary beams and with covering metal plate welded on the Certovka side which is fitted with casters and pulleys. The casters carry the entire weight of the closure to the rails and the pulleys provide lateral motion. Profiled rubber seal in the shape of a note will be fixed to the rails welded on the edges of the blocking metal plate in the bottom part and on both sides. Track rails are designed to lighten the seal during the whole time of movement of the closure; only in the final phase due to the fact that the track rails turn aside the rubber seal should press on the sealing frame and the supports in the centre. On the edge of the embankment wall, the track rails are suspended so that the space with the closure in its retracted position can be closed and protected against water to keep the closure dry. In case of manipulation, pieces of rails are placed in the suspended places.

Ejecting timbers are installed in the upper part of the protecting wall which after the relocation of the closure to the protecting position are ejected to the maximum top position and locked. Between those timbers set at a distance of 3 meters, horizontal aluminium EKO - System stop logs are placed. Stainless steel bars are welded in the upper main beam between the timbers, where the horizontal seal of the stop logs fit. The timber barrier is linked to the mobile barrier on the adjacent riverside walls. In the protecting position, the closure sits back against four supports, where the two supports on the sides are formed by vertical sections of the sealing frame fixed in the locks to avoid potential deviation of the body of the closure because of wind or water flow during flooding.

The construction of Part 21 was launched in December 2003, with the construction work in its full range starting in spring 2004. At the end of 2004 the construction part was completed and during 2005 evaluation tests and the trial erection of the mobile protection in the whole section were performed and the construction was presented to the inhabitants of Prague.

Nowadays, the flood protection closure is operated by the state-owned company Povodi Vltavy (The Vltava River). According to the handling and operating rules, the company shall erect the barriers around the Certovka 12 hours before reaching 1st flood level, i.e. flow rate 450 m3/sec with the river surface spot height around 185.60 meters above sea level (normal level is about 184.60 meters above sea level). In case the flow does not reach such values for too long, the barriers are erected at least twice a year to test the readiness. Once every three years the temporary barrier isolating the Certovka profile should be used to carry out maintenance of the closure in a dry environment.

Members of the FloodProBE team were fortunate to visit this flood barrier during one of its maintenance inspections and the following video demonstrates the barrier closing from it's protected position.

Certovka Flood Protection Closure

Prague was a pilot site project for FloodProBE and you can view more information about this pilot site here.