Redevelopment area Höllgasse
As one of the first projects of the Old Town redevelopment around the town hall square, four historic buildings, including the former city court and the traditional Wilder Mann inn, were combined into a single complex starting in 1983 and converted into a hotel with an attached museum. The planning was based on precise measurements and structural analyses. As with many other historic buildings, the Gothic structure had been severely damaged in the two city fires of 1662 and 1680. The Gothic vaults on the ground floor are partially preserved, while the upper floors were added and remodeled in the Baroque and Neoclassical styles. Decades of neglect had left the buildings in poor condition.
The buildings were extensively damaged due to the soil conditions, frequent flooding, and rising damp in the masonry. Because of the heterogeneous soil – many houses stand partly on the rock of the cathedral hill and partly on alluvial soil – the exterior walls of the buildings leaned by up to 50 cm towards the Danube riverbank. This contributed to structural damage and the destruction of the masonry, resulting in cracks in the facade. The damp walls led to widespread efflorescence on the plaster, the supports for the wooden beam ceilings in the masonry were largely rotten, and the ceilings were also infested with dry rot.
In consultation with the heritage authorities, disruptive additions and later-added partition walls were removed, and the buildings were restored to their original state. The light wells and stairwells were preserved and renovated. Historic plank ceilings were uncovered and restored, and stucco ceilings and doors were reconstructed based on the originals. The facades were restored with the characteristic box windows typical of the Inn-Salzach region, whose flush outer sashes open outwards. (Previously, these exterior windows were replaced with shutters during the summer.)
Remediation measures taking into account the risk of flooding
The foundations were partially underpinned and secured with small-diameter grouted piles. The newly constructed walls and vaults are built using small-scale brickwork, which better accommodates the deformations caused by differential settlement. The entire building complex, with a depth of 40 metres, is braced with threaded rods in the ceiling levels, thus providing internal stability.
Since the ground floor lies below the flood level, its functionality and construction were designed accordingly. Preventive planning and construction measures help to minimize damage: the entrance hall contains only movable furnishings, no permanent fixtures. The large granite slabs – the traditional ground floor flooring in Passau – do not float due to their weight. The slabs are laid in trass cement mortar, which, unlike
Efflorescence does not occur with normal cement mortar; the slight slope towards the Danube-side entrance doors facilitates water drainage. The perimeter insulation is located beneath the reinforced concrete floor slab and therefore cannot float. The new interior walls are constructed of solid bricks or calcium silicate bricks, as these are closed-pore and absorb less water than bricks with cavities. A mineral-based, water-repellent renovation plaster, which is resistant to moisture, was applied both inside and out. Its open-pore structure reduces efflorescence. The paint consists of mineral-based, vapor-permeable paint; emulsion paint would be unsuitable because it forms a film and flakes off. No closed-pore, sealing coatings were used anywhere, as these blister and peel under vapor pressure.
Horizontal damp-proof courses were omitted during the renovation, as this would have been extremely difficult to implement in the existing historic natural stone masonry (underpinning, etc.). The spandrels of the newly constructed vaults are filled with lightweight concrete to close any cavities. The previously common fill materials often became saturated with water and took a long time to dry. Oak doors and windows are finished with a breathable coating. Oak is relatively water-resistant and warps very little. In a newly constructed section of the building, the technical services and heating system are housed on a mezzanine level above the flood line. The ground floor has a separate electrical circuit. The electrical installations are isolated from the rest of the system by an emergency switch, and the power is automatically shut off when a certain water level is reached. Wastewater is pumped above the flood line by a lifting station and flows into the watertight municipal sewer. Rainwater is discharged separately into the Danube; backflow preventers prevent backflow from the storm drain.
All these structural measures have proven their worth over the years. This year's highest Danube level, reached on August 13th, was 10.81 metres (297.27 m above sea level), more than six metres above the normal water level. The water in the hotel's lobby reached a height of 1.60 metres. Nevertheless, the damage was limited: the early flood warning gave the hotel staff ample time to remove furniture and carpets from the lobby. The water rose from below through the gaps in the flooring and also seeped into the lobby through the garage doors. After the water receded a day later, the floor and walls were immediately cleaned to remove the rapidly hardening sludge. The hotel was able to reopen on August 19th. The water left hardly any traces inside; only in a few places did the plaster efflorescence occur.
Other projects in the Höllgasse redevelopment area were renovated according to the same principle of structural flood protection, such as the New Town Hall opposite. Here, too, the ground floor area facing the Danube houses special uses that can be quickly evacuated or are resistant to flooding: The historic arcaded courtyard is used for exhibitions,
The tourist office has no permanent fixtures. The garage in the adjoining building section is designed accordingly: ventilation and electricity are supplied from above. Ancillary rooms are located in the rear of the building, and the partition walls in the restrooms are made of solid, rigid plastic panels.
The adjacent buildings along the Danube riverbank also contain no apartments on the ground floors, but rather uses that leave the riverside ground-floor spaces relatively free from flooding in the event of high water, such as shops or exhibition areas. At the historic inn "Zur blauen Donau" (To the Blue Danube), which is currently being renovated, the restaurant is planned as a two-storey space with a gallery; the ground floor contains only chairs and tables, while the upper floor houses the bar, kitchen, cold storage, and technical equipment. The city of Passau has stipulated that the buildings in the flood zone must provide additional storage space as an alternative for the ground-floor occupants, for example, in the attic.
Living with water means yielding to it when necessary – through adapted use and flexible furnishings that allow for quick removal. Traditional construction methods using materials that can withstand long-term flooding, are not damaged by repeated water exposure, and are easy to clean have proven effective. The aim of the renovation is to reconcile the requirements of historic preservation with functional and building physics considerations and to emphasize the architectural qualities of the buildings in order to enhance the historically developed old town as a diverse residential area.
