Impact of Floods on Heritage Structures

 Miloš Drdácký (Photo by Miroslav Kučera, with permission)Throughout human history, floods have played a very important role, mainly due to their adverse impacts on cultural heritage. Floods lead to the loss of historical monuments, devastation of historic sites, changes in the cultural landscape, and also to the disappearance or substantial distortion of intangible heritage. Due to periodic changes in climatic conditions over recent years, we have witnessed increasingly frequent major floods and related events that pose a substantial threat to cultural heritage worldwide. These include the floods in Central Europe in 2002, the New Orleans flood in 2005, numerous floods in South Asia in 2007 and 2009, as well as severe floods in Central Europe again in 2010. Engineering experience acquired during the course of major floods has provided comprehensive knowledge and reliable data about the impact of flooding on historic objects and sites. This experience can serve as a basis for establishing guidelines and recommendations for the effective protection of cultural heritage in emergency situations.

River floods are the most common type of natural disaster in many European and Asian regions. Recent European studies have pointed to a significant increase in the frequency and severity of river floods due to the apparent development of global warming. However, floods, being highly impactful phenomena, are well documented with details including social and economic impacts. Data on the height of water during the flood culmination are usually well documented in situ, in various types of high water line records. Such data show that flooding is a natural occurrence, and has been a frequent loading factor for historic buildings along rivers and historic bridges. For example, a detailed evaluation of the water gauge of the Lahn River in Germany shows that the bridges suffered 60 floods between 1255 and 1984, averaging a flood every $12\text{years}$ . Of these, 22 were major floods. A review of the impact of floods on cultural heritage objects has therefore been chosen as a useful subject for this special issue.

Another reason for choosing this topic is the shortage of systematic information on the damage and failure mechanisms of flooded historic structures and materials. Various reports evaluate the threat to densely urbanized river or coastal regions posed by a rise in sea level. These works typically focus on the impact of floods on human life, housing, and productive infrastructure. However, there is no doubt that the damage will increase significantly due to the loss of cultural and natural heritage assets. The rising sea level and increasing human exploitation of land along rivers have increased the occurrence of floods in areas that have seldom or never experienced them. Quite dangerous for cultural heritage, namely archives and museums, are so-called flash floods, characterized by a rapid rise and fall of floodwaters with peak flows occurring within hours of heavy rain. The occurrence of this hazard, almost forgotten in recent centuries due to rather favorable climatic situations, now seems to be on the rise.

Taking into account this probable increase in threats to cultural heritage sites in Europe, the European Commission (EC) has decided to support a joint research project on Cultural Heritage Protection against Flooding, under the acronym CHEF. The project gathered data on the impact of floods on both moveable and immoveable cultural heritage objects and sites, establishing a sound source of information for further dissemination. This contributed to the decision of the Journal’s editor Ken Carper to prepare a special issue.

Though the main goal of the EC CHEF project was to provide suggestions for preparedness and prevention of damage and loss of cultural heritage, a substantial part of the project was devoted to an analysis of flood impacts. This special issue presents some of the results of the analysis and categorization of damage to materials and structures. The aim is to categorize adaptation strategies and design guidelines for preventive and remedial measures. The outputs include scientific references to specific flood phenomena for professionals, and clearer guidelines and recommendations for nonprofessional readers, mostly owners or managers of architectural or cultural heritage sites.

The sequence of papers follows a scheme, starting with a rather general paper on a methodology to predict the probability of the occurrence of floods, followed by descriptive papers on the effects of floods on materials and structures. Finally, there is a review of some tools for damage assessment, illustrated by a case study of a historic bridge.

The introductory paper (Holický and Sýkora) proposes a general framework for flood risk assessment, with special reference to cultural heritage issues of risk optimization and decision-making about protective measures. It suggests a theoretical model suitable for predicting the flows and the extent of future floods based on knowledge and statistical analysis of historic regional hydrologic data for annual maximum flows of the Vltava River in Prague dating back to 1827. Estimations of extreme flows are provided for various return periods. They can contribute to decisions on protective measures, balancing potential social and economic consequences, and loss of valuable cultural and heritage objects.

The second paper (Drdácký) presents a categorization of flood damage. Cultural heritage objects and sites are subjected to various forces and actions during flood situations. Categorizing the damage to immovable cultural heritage due to flooding facilitates a more general approach to categorizing the threat from natural disasters. The typical examples presented form a basis for subsequent suggestions of preventive or remedial measures for objects characterized as i) flood resistant objects and structures, ii) objects and structures made of materials with high moisture volumetric changes, (e.g. timber structures and elements, combined structures made of materials with different moisture expansions, some soils), iii) structures made of materials whose strength is highly degraded by the impact of moisture, (e.g. dried brick (adobe) masonry, masonry with clay (low lime or cement content) mortars, decayed timber structures and elements, infill subsoil and fine particle subsoil), iv) structures susceptible to partial damage due to flooding (e.g. timber parts prone to uplifting and floating away, large bridges, pavements), and v) structures and elements vulnerable to overall collapse or displacement due to flooding, (e.g. small bridges and walkways, free standing walls, light improperly anchored structures like summer houses).

Some of the problems mentioned above are discussed in the third paper (Herle, Herbstová, Kupka and Kolymbas), which deals with the behavior of foundations in interaction with the subsoil. Adverse damage to foundations when the groundwater rises frequently can cause severe damage to cultural heritage objects, initiating partial or total collapse. In fact, geotechnical problems are the most important and difficult issues related to the impact of floods on historic structures. This paper includes suggestions for geotechnical measures that can protect historic structures from disastrous effects.

After subsoil instability, we move on to significant changes in the mechanical characteristics of some materials, namely brick and natural stone that endanger architectural heritage and sculptural monuments. Loss of strength due to water saturation substantially decreases the load-carrying capacity of stone masonry or stone elements. This is further worsened by the stress gradients that may arise as a consequence of uneven volumetric changes due to hydric dilation. A detailed study of the vulnerability of stone during and after a flood, based on laboratory investigations, is presented in the fourth paper (Siedel). The results are then compared to site observations during the catastrophic flood in Central Europe in 2002.

Building stones, mortars, and their masonry composites are porous media that frequently deteriorate due to wetting or water saturation. Secondary damage may also occur, such as crystallization of the mobilized soluble salts present in the composite or transported by contaminated flood water. A reliable assessment of the effects of elevated moisture on masonry materials and structures requires appropriate and effective nondestructive testing methods (NDTs). The fifth paper (Válek, Kruschwitz, Wöstmann, Kind, Valach, Köpp and Lesák) illustrates the capacity of selected techniques, viz., thermography, complex resistivity, radar and ultrasonic wave velocity measurements, applied in monitoring the drying of three test walls made of fired clay brick, sandstone, and spongilite after flooding modeled by deep immersion in water. The example helps users select a suitable method and provides information about possible limitations of each reported technique.

Nondestructive techniques for detecting the presence of water and the drying process in masonry also play an important role in the next paper (Binda, Cardani and Zanzi). It deals with the application of radar, thermography, sonic tests, and powder drilling to assess the moisture distribution in masonry after flooding and during natural drying. The tested situations include a study of the influence of surface treatment layers. The test walls simulated the state of naturally contaminated walls before a flood, and represented a structure where the main material and geometrical parameters are known. Flooding was simulated by supplying water over a period of several days to full-scale test walls that had been contaminated by salts. The walls were monitored when left to dry naturally. The reader may compare various applications of NDTs for a better understanding of drying processes and salt efflorescence processes.

The final paper (Barták and Slížková) presents a case study of one of the oldest medieval bridges in the world. The bridge in Písek survived the extreme Central European flood in August 2002 without any signs of impairment to its stability, unlike many modern bridge structures. This was probably due to additional anchoring of its pillars carried out in the course of the bridge’s renovation between 1996 and 1998. Among the lessons to be taken from this event, it should be noted that the extreme flood in 2002 finally put an end to the prevalent assumption in a number of engineering works that the $Q_{\mathit{100}}$ ( $100\text{-year}$ return period flood) is the decisive safety factor for proposed anti-flood measures. It was unequivocally confirmed by this flood that there is a considerable margin between the $Q_{\mathit{100}}$ flow rate and the theoretically largest outflow $Q_{\mathit{MAX}}$ , generated by the maximum possible flow rate in a given river basin. An extraordinarily large flood at an unknown time is a real danger, and not a figment of imagination. The evolution of opinion on the reliability of anti-flood protection of large cities or historical parts of cities leads to the conclusion that structures should be protected not against a specific n-year flow, but against the maximum possible flood. Protection of human lives has the highest priority and it is necessary to differentiate the types of assets that need to be protected. Political, rather than professional, considerations and points of view are usually decisive for projects requiring considerable investments.

Although this special issue is not oriented toward preventive and temporary measures against flood damage to historical objects and sites, it does provide some useful and practical comments on this issue. The measures are typically sorted into two categories: structural and nonstructural, rather than organizational or operational. Structural measures are sometimes difficult to implement in the case of cultural heritage protection, because they are mostly visible and disturbing, and are often not cost-effective. This subject needs further research and comparisons should be made with best practice nonstructural measures. As far as nonstructural measures are concerned, the application of standards to protect cultural heritage from floods may lead to the originality, authenticity and aesthetic qualities and values of historic monuments being compromised.

Best practice is usually difficult to generalize in a sufficiently informative way. However, here are the summarized four pillars to help mitigate any adverse effects of natural disasters on cultural heritage: i) regular inspection and careful maintenance of the historical stock & improved land use planning and management, ii) awareness creation and regular coordinated training, iii) international cooperation and availability of funding, and iv) legislative support.