Journal of Performance of Constructed Facilities

Fluvial flooding in August 2002 affected a number of structures in the Czech Republic. Considerable damage was observed particularly in the historic city of Prague. Extensive investigations indicated that main observed causes of damage could be classified into geotechnical aspects, inadequate structural properties, and insufficient communication. After the flooding responsible authorities have considered permanent and temporary protective measures to reduce adverse consequences of flooding in the future. Decisions concerning expensive measures should be preferably based on risk optimization, taking into account potential societal and economic consequences and losses of cultural heritage values. General framework of the risk assessment is thus proposed considering specific issues of cultural heritage. Such an assessment needs a theoretical model suitable for predicting flows and extents of future floods. For that reason, the authors statistically analyzed hydrologic data for annual maximum flows of the Vltava River in Prague dating back to 1827. Pearson III and lognormal distributions seem to be suitable models for a considered sample. Estimations of extreme flows, needed for assessment of flooding risk to endangered sites and decisions on protective measures, are provided for different return periods.

This paper presents typical examples of damage to immovable cultural heritage due to flooding. Flooding can damage architectural heritage, historic infrastructure consisting of individual structures, buildings, and objects, as well as objects of art standing alone or firmly attached as an integral part of buildings. All these objects are subjected to various forces and actions during flood situations. These forces can be categorized according to the types of damage that they can cause: horizontal static pressure of raised water; upward hydrostatic pressure; dynamic low velocity streams; dynamic high-velocity streams; dynamic impact of waves; dynamic impact of floating objects; compacting of soils or infill; changes in subsoil conditions; saturation of materials with water; contamination of materials with chemical and biological agents; formation of barriers; ice floes; and postflood effects. These typical actions may occur in combinations and work in synergy. Typical illustrative examples of damage caused by individual actions are presented in the paper, which goes on to present some general lessons applicable for the most vulnerable categories of immovable heritage objects.

Flood events demonstrate devastating effects not only on materials and structures in contact with flowing surface water. Equally important is the behavior of foundations in interaction with subsoil. This contribution gives an overview of different phenomena which arise in subsoil and at the foundation level during the groundwater rise accompanied by several case histories related to cultural heritage. Possible geotechnical measures are discussed as well.

Natural stone is a common material in historic constructions. Flood events may directly affect surfaces of historic stone buildings. Since ashlars and stone sculptures often carry valuable cultural information, a more detailed knowledge about changes in physical properties due to water saturation is crucial for the assessment of their surface stability in case of flooding. Water saturation of stones leads to loss of mechanical strength and to expansion of volume (hydric dilatation). On the basis of data from literature, a rough scheme of vulnerability is suggested for different kinds of building stones. The majority of igneous and metamorphic rock types with dense crystalline structure are not vulnerable to flooding, whereas some types of pyroclastic rocks (tuffs) as well as clay-bearing sandstones are highly vulnerable. Detailed laboratory investigations on Elbe sandstone demonstrate the influence of petrographic features on material behavior due to water saturation. Results of laboratory tests are in good accordance to on-site observations made after the great summer flood in Dresden, Germany in 2002.

Building stones are porous media and they can deteriorate through moisture ingress and secondary damage such as crystallization of soluble salts. Not only is this due to the increasing number of flood events in the past years but also structural damages of houses from activity such as leakage or rising moisture (groundwater) are the main causes. The potential benefit of several nondestructive testing methods to assess water damage in building stone has been studied in a field-scale experiment. Three testing walls made of fired clay brick, sandstone, and spongilite were flooded and their drying behavior monitored using infrared thermography, complex resistivity, ground penetrating radar, and ultrasonics. The results were compared to the average moisture content determined by gravimetric weighing of the specimens. Qualitatively, the results of the different nondestructive testing methods matched well. But in terms of quantitative data, some scatter was observed and the results should be viewed with care. Collecting time-consuming calibration data would help to overcome this problem, but especially when dealing with historic building structures, this is not always possible in practice.

After an investigation on the most recent floods occurred in Italy that damaged the Cultural Heritage masonry buildings, an experimental research started on-site on full-scale masonry models exposed to the environmental agents in Milan. The masonry materials used for the full-scale models were largely investigated in the past and the models were subjected to decay caused by the capillary rise and by the crystallization of sodium sulfate coming from the foundations. These walls can actually simulate the state of naturally contaminated walls before a flood and represent a construction where the main parameters are known. A flood has been simulated by adding water for several days to the walls of the full-scale models previously contaminated by salts, then the walls were left to naturally dry. The objective is to check the effectiveness of nondestructive (ND) techniques in detecting the presence of water and the drying process and also the influence of surface treatments presence. Radar tests, thermography tests, sonic tests, as well as the minor destructive powder drilling tests were applied successfully to evaluate the moisture distribution in the masonry after flooding and during natural drying.

The very long absence of floods with a return period of more than about 100 years in the Vltava river basin dulled the vigilance of the state administration and self-government bodies. Flood safety measures were mostly related to once-in-a-hundred-years floods. When planning the engineering works before major repairs to the medieval Stone Bridge in Písek in the 1990s, an unpleasant but justified question was raised—what would happen to the Stone Bridge if an extremely powerful 500-year return period flood were to occur? The following case study deals with this question and provides a definitive answer to it. Works were carried out on the Stone Bridge, which was just a few years later exposed to the catastrophic flood that occurred in Bohemia in 2002.