Flood resilient construction has become an essential component of the integrated approach to flood risk management, now widely accepted through the concepts of making space for water and living with floods. Resilient construction has been in place for centuries, but only fairly recently has it been recognized as part of this wider strategy to manage flood risk. Buildings and the wider built environment are known to play a key role in flood risk management, and when buildings are constructed on or near to flood plains there is an obvious need to protect these. Engineered flood defense systems date back centuries, with early examples seen in China and Egypt. Levees were first built in the United States some 150 years ago, and were followed by the development of flood control acts and regulations. In 1945, Gilbert Fowler White, the so-called “father of floodplain management,” published his influential thesis which criticized the reliance on engineered flood defenses and began to change these approaches. In Europe, a shortage of farmable land led to the use of land reclamation schemes and the ensuing Land Drainage acts before massive flood events in the mid-20th century led to a shift in thinking towards the engineered defense schemes such as the Thames Barrier and Dutch dyke systems. The early 21st century witnessed the emergence of the “living with water” philosophy, which has resulted in the renewed understanding of flood resilience at a property level.
The scientific study of construction methods and building technologies that are robust to flooding is a fairly recent phenomenon. There are a number of underlying reasons for this, but the change in flood risk philosophy coupled with the experience of flood events and the long process of recovery is helping to drive research and investment in this area. This has led to a more sophisticated understanding of the approaches to avoiding damage at an individual property level, categorized under three strategies, namely avoidance technology, water exclusion technology, and water entry technology. As interest and policy has shifted to water entry approaches, alongside this has been the development of research into flood resilient materials and repair and reinstatement processes, the latter gaining much attention in the recognition that experience will prompt resilient responses and that the point of reinstatement provides a good opportunity to install resilient measures.
State-of-the-art practices now center on avoidance strategies incorporating planning legislation in many regions to prohibit or restrict new development in flood plains. Where development pressures mean that new buildings are permitted, there is now a body of knowledge around the impact of flooding on buildings and flood resilient construction and techniques. However, due to the variety and complexity of architecture and construction styles and varying flood risk exposure, there remain many gaps in our understanding, leading to the use of trial and error and other pragmatic approaches. Some examples of avoidance strategies include the use of earthworks, floating houses, and raised construction.
The concept of property level flood resilience is an emerging concept in the United Kingdom and recognizes that in some cases a hybrid approach might be favored in which the amount of water entering a property is limited, together with the likely damage that is caused. The technology and understanding is moving forward with a greater appreciation of the benefits from combining strategies and property level measures, incorporating water resistant and resilient materials. The process of resilient repair and considerate reinstatement is another emerging feature, recognizing that there will be a need to dry, clean, and repair flood-affected buildings. The importance of effective and timely drying of properties, including the need to use materials that dry rapidly and are easy to decontaminate, has become more apparent and is gaining attention.
Future developments are likely to concentrate on promoting the uptake of flood resilient materials and technologies both in the construction of new and in the retrofit and adaptation of existing properties. Further development of flood resilience technology that enhances the aesthetic appeal of adapted property would support the uptake of measures. Developments that reduce cost or that offer other aesthetic or functional advantages may also reduce the barriers to uptake. A greater understanding of performance standards for resilient materials will help provide confidence in such measures and support uptake, while further research around the breathability of materials and concerns around mold and the need to avoid creating moisture issues inside properties represent some of the key areas.
Resilient construction has been in place for centuries, but only relatively recently has it been used as a systematic component of an integrated flood risk management strategy. Resilient buildings are designed and constructed in such a way to avoid, prevent, or reduce the damage caused when flooding takes place. They can play an important part in flood risk management strategy by reducing damage and, importantly, speeding up the recovery process. This article begins by charting the historical development of the concepts of resilient construction, the use of engineered flood control systems leading to current thinking around living with water, and the acceptance that flooding is unavoidable.
The importance of buildings and the wider built environment within flood risk management is illustrated. An account of the developments in the use of construction technologies and materials follows, including the recognition of the need for more scientific research. The developments of this technology and the understanding of property level measures then follows. This leads to an account of the research and advancements in practice around the repair and reinstatement of flood-damaged buildings.
Looking toward the state of the art, attention is given to the current and future directions around the science of resilient construction, highlighting recent research trends and discoveries. Current developments in the design, construction, and adaptation of flood affected buildings are described. The discussion highlights the development of hybrid approaches to property level resilience combining water exclusion measures with water entry measures. Recent research around water resistant and resilient materials is highlighted, as well as developments in considerate reinstatement practices. This leads to a section on future developments in flood resilient construction before presenting conclusions.
The wider built environment and the buildings and properties that shape this play an integral role in flood risk management. Once structures are constructed on and around flood plains, there is a natural priority to protect these assets, which leads to the development of flood defense schemes or mechanisms to mitigate the damage and disruption that is caused. Flooding can cause a range of damage to urban settlements, including the threat to personal safety when normally dry areas are submerged, leading to the need to escape from buildings. High-velocity floods can sweep people away before emergency services are able to reach them. Damage to buildings and their contents is another major impact, leading to major losses and in some cases severe costs to individuals, businesses, insurers, and government funds. Infrastructure in the form of major transport links, including roads, railways and airports, can also be affected, leading to widespread disruption and interruption to normal business. Further, social impacts, such as the need to close schools, hospitals, and places of worship and also the loss of essential services (electricity, water, and gas supplies), highlight the essential need to protect the wide range of physical assets that make up the built environment.
Engineered flood control dates back centuries for example to China in 400 bce, where steps to protect the agricultural community from the flooding of the Yellow River were undertaken and included the construction of levees, fluvial channels, and natural channels. In the Nile Delta, before the construction of the Aswan Dam, seasonal migration and evacuation were a long-established flood risk management method and were reflected in the seasons of the Egyptian year of Akhet (inundation), Peret (growth), and Shemu (harvest). This approach, while effective, did not protect the built environment. However, these floods brought important nutrients and minerals into the fertile soil, making it rich for farming since ancient times.
In the 20th century in the United States, flooding was the most damaging natural disaster in terms of numbers of lives lost and damage to property. Levees were first built in the United States some 150 years ago. Farmers were attracted to the fertile soils of the flood plains, and were largely responsible for the construction of levees to protect farms and farmlands. Other levees were built to protect cities and towns and following devastating floods. In the early 20th century, the 1917 Flood Control Act was introduced to reduce flood damage along the Mississippi, Ohio, and Sacramento Rivers. Subsequent developments in the Flood Control Acts of 1928 and then 1936 gave greater prominence to flood control as a national priority, giving the US Army Corps of Engineers responsibility to design and construct flood-control projects. These acts also placed a requirement on local communities to undertake maintenance and operation of the levees.
During this era of increased flood control, Gilbert Fowler White, the “father of floodplain management,” wrote his influential book Human Adjustment to Floods, published in 1945 by the University of Chicago (White, 1945). White was critical of US government policy on flood risk management and the overreliance on the development of structural flood defense schemes, claiming that these were actually leading to increased losses when levees and dams were overtopped. The development of areas located in flood plains but protected by these structural systems leads to catastrophic flooding when these systems fail, as was witnessed in Hurricane Katrina’s impact on New Orleans.
In Europe, a lack of farmable land in certain countries (e.g., the Netherlands and parts of the United Kingdom) led to land reclamation schemes, resulting in large swathes of countryside and associated settlements situated below sea level and mechanically drained. In such circumstances, flood risk management is inextricably linked with pumping and drainage, and in the United Kingdom regulated by a series of drainage acts and local drainage bodies. National-scale flood risk management started with the Land Drainage Act 1930 and was further amalgamated by the act of 1961. Massive coastal and riverine flood events in the early to mid-20th century led to a shift in thinking towards engineered defenses and large-scale infrastructure projects, including the Thames Barrier and the extended Dutch dyke system. In recent years, the approach to flood risk management has evolved to a philosophy of living with water (Fleming, 2001), the concept of blue-green cities where flooding is accepted and embraced (Lawson et al., 2014), and the need for renewed understanding of flood resilience at a property level.
The scientific study of construction and building technologies that are robust to the actions of flooding is a relatively newer field than the study of measures to predict and prevent flooding. There are several underlying causes. First, the relative perceived success of flood control measures in the developed world and the framing of property level interventions as “residual risk” with only a small contribution to integrated risk management. Second, the need to accept that floods cannot be prevented and ultimately that floodwater may damage homes despite the massive investment in flood prevention, whereas in developing countries, where flood control has been less prevalent, Hughes (1982) contends that destruction of housing during floods is an expectation and other factors (such as preservation of life) take priority. Third, different vernacular architectures and construction types requiring a much more diverse consideration of materials and methods than that required for large-scale community defenses. As a result, the development of domestic resilient construction technologies has historically been largely a parallel process carried out locally within communities using indigenous knowledge and supported by the construction industry and sometimes by small expert groups. In the low-lying regions of the Netherlands, early houses were built on dwelling mounds called terps (Beeftink, 1975); similar construction was practiced in the United Kingdom (e.g., Glastonbury) (Barrett, 1987), where individual clay mounds were constructed, and in Ireland, where crannogs of stone, earth, and wood were used (O’Sullivan, 2007). Later developments raised individual houses without earthworks, for example traditional stilt housing in Thailand, the Queenslander style in Australia, and raised housing in the United States and Nigeria, as shown in Figure 1. Among other things, raising on stilts allows for free air circulation in hot and humid climates and also assists in flood avoidance. Raising on masonry or concrete yields avoidance and is often more stable in high-velocity flooding.
Often progress has been driven by the reality of experiencing flood events and the process of reconstruction after flooding, as after hurricanes Katrina (Popkin et al., 2006; Eamon et al., 2007; Coulbourne, 2012) and Sandy (John Ingargiola et al., 2015). Some research has been motivated by the need to provide better guidance to support planning restrictions in floodplains, to enable continuation of insurance, and to maintain existing communities, recognizing that they face increased flood risk due to climate change and environmental degradation. As will be demonstrated in the section on State of the Art, the research generated in the late 20th and early 21st century is being shared internationally, and a growing number of studies are emerging that are specifically aimed at understanding the action of flooding on different building types and designing improved technology to reduce future damage to buildings.
The approaches to avoid damage at an individual property level are variously described but basically categorized into three strategies: avoidance by choosing suitable locations or by designing sites or elevating buildings to avoid flooding; water exclusion, also known as dry proofing and resistance where water is prevented from entering the building by barriers and other “resistant” technology; and water entry, also known as wet proofing and resilience, where it is recognized that water will enter a building and the aim is to limit the damage and disruption from flooding.
Of the three approaches to property level measures (avoidance, water exclusion, water acceptance), the avoidance approach is usually preferred. Elevation and landscaping is advocated as a first recourse by most research and guidance (for example Sheaffer, 1960; Hawkesbury-Nepean Floodplain Management Steering Committee, 2007; Bowker et al., 2007). On a given building plot, avoidance can be achieved through landscaping, drainage, and retention features and free-standing structures or barriers to prevent water reaching the building. Much of this might be considered standard construction technology or directly transferable from large-scale water engineering. Avoidance can also be achieved by elevation of the building itself through raising on pillars, extended foundation walls or raised earth structures, or flotation. In the United Kingdom, raising through extended foundation is popular sometimes with garaging underneath. This trend for developments in the floodplain to be elevated has existed for some time but has been accelerated and supported by recent planning guidance (PPG/S25). The advantages of elevation are seen as self-evident if safe access and escape can be ensured, leaving only questions around structural suitability and performance during a flood.
Where wood framed construction is common, for example in the United States and Australia, raising on pillars is more structurally viable. Riverfront/foreshore construction across the globe has often been required to be built on piles for stability on shifting soils and subject to powerful currents. US Army Corps of Engineers (USACE) (1998) examined the performance of flood proofing, including elevation, and learning in the United States continued after Hurricane Katrina (van de Lindt et al., 2007).
An alternative avoidance technique is to create buildings that rise and fall with the water, either permanently floating or designed to float in flood conditions. Arguably avoidance via floating reduces the vulnerability of properties to windstorm damage, as they are not permanently raised and exposed to increased wind loading. Traditionally, houseboats have been a feature of river and coastal living—for example, in the Netherlands and the United States—and based on technology associated with boats. Floating houses are a logical extension from such concepts requiring new research around flotation devices (SGS Economics and Planning Pty Ltd, 2011) and provision of services. However, houses designed to float periodically are a more recent development, requiring studies into stability during and after flood events (English et al., 2017; Mohamad et al., 2012). Much of this underpinning research is based in the Netherlands and the United States.
Water exclusion strategies, also known as resistance and dry flood proofing, are designed to keep water out of a property. Temporary measures are frequently resorted to, and sandbags and homemade flood boards are commonly used by communities to exclude water during an emergency. Sandbags and temporary measures, while they may slow ingress and damage, are neither adequate nor sustainable. In the United States, flood events, in particular the 1927 Mississippi flooding leading to the 1945 Flood Act handing responsibility to USACE, the 1961 Kansas and Missouri flooding, and the formation of the National Flood Insurance Program (NFIP), prompted investment in research to reduce the residual impact of floods on buildings (Perkes, 2011). A pioneering publication is Sheaffer’s (1960) thesis on flood proofing and the ensuing 1967 guidance, and by 1972 the USACE had produced flood-proofing regulations and guidance (United States Army Corps of Engineers, 1972; Federal Insurance Administration, 1976). Canada followed suit in 1978 (Williams, 1978). Figure 2 shows the development of US guidance and regulation up until 2011.
Figure 2. Development of regulation and guidance materials for resilient construction in the United States (after Perkes, 2011).
However, this was based on scant evidence, and the studies that followed by Pace (1978, 1984, and 1988 in Fema, 1993) on waterproofing walls provided improved evidence for the 1993 FEMA technical bulletins. Research on building openings again harks back to Sheaffer (1960), but this has more recently been pursued vigorously in the United Kingdom and Europe as a result of flooding in the late 1990s and 2000s starting with Ogunyoye and Van Heereveld (2002) and Elliot and Leggett (2002), assessments of existing technologies to protect openings and resulting in a proliferation of “resistant technologies” designed to keep water out. Products such as door and window guards, air brick covers, smart air bricks and non-return valves, pumps, cladding systems, plastic skirts, flood-resistant doors, and wall coatings were designed and sold, necessitating the introduction of standards and kite testing to protect property owners and occupiers from investing in substandard technology (PAS1188). Much of the early research on barrier products was conducted in-house and is too numerous to include in this chapter. However, kite mark testing has been carried out in designed facilities in the United Kingdom since 2004 (BSI, 2016). A recent EU-funded project also addressed the performance of flood barriers (Schinke et al., 2013). In the United States, Aglan et al. (2004) tested whole building construction for wood framed domestic buildings, and more recent studies by Perkes (2011) and Uddin et al. (2013) for more contemporary forms of construction. Ingress through masonry walls has also been studied in the United Kingdom by Kelman (2002), Escarameia et al. (2007), and Beddoes and Booth (2015). Work sponsored by CLG in the United Kingdom also examined floor construction technology (Escarameia et al., 2006) and considered the properties of insulation. Water-resistant properties of insulation has also been examined by the Smartest project (Schinke et al., 2013) and Perkes (2011). The consideration of tanking technology has been led in the United Kingdom by the knowledge derived from waterproofing basements, although in general the difference in hydrostatic pressure between normal groundwater and flood conditions has not been studied.
This is also variously know as wet proofing, flood resilience, or water acceptance and involves methods and technology designed to limit the damage once water has bypassed the building envelope and entered the occupied space. This is the area least researched, specifically in the flood scenario, much of the knowledge about resilience has emerged from the studies on water exclusion as a side issue, perhaps because water entry has been seen as absolutely the last resort by the risk management and property protection community. In this area, scientific study is bounded and constrained by emotional barriers, and misconceptions and aesthetic and safety considerations can outrank building technology. Historically, this is the area of flood technology most informed by indigenous practice and flood experience. Testimonies tell us that in the past, water was simply accepted and then swept out of buildings (Rogers-Wright, 2013). For example, channels were provided in the floor to facilitate this in the Netherlands. However, with the increased wealth and technology housed in buildings, in building services, and in soft furnishings, the “old-fashioned” methods no longer suffice. Water entry approaches can be subdivided into avoidance, resistant, resilient, and speed of reoccupation, and a recent study identified over one hundred different interventions (Lamond et al., 2016b). There is a large overlap with the research on reinstatement, especially in avoidance and speed of reoccupation approaches. There is also a lively debate in this field around the suitability of retrofitting modern waterproof building materials in existing (sometimes heritage or character) properties (Fidler et al., 2004).
The research specifically on flood resilient materials and methods has usually been a smaller part and has run alongside research on water entry under the catch-all title “flood proofing.” There is a separate branch of related research on building material properties which has been drawn on (sometimes inappropriately) that has also informed the flood-specific studies. Sheaffer is again a major starting point for the work, and FEMA issued guidance on flood resistant materials in 1993 and superseded this in 1999 (FEMA, 1999). Meanwhile, in the United Kingdom the Building Research Establishment also issued guidance (BRE Scottish laboratory, 1996). Subsequently, the ensuing experimental research in the United States, the United Kingdom, and Europe has progressed in parallel with Aglan’s study (Aglan et al., 2004) dovetailing with Escarameia et al.’s (2006) work and some laboratory studies by Wingfield et al. (2005). In Australia, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) invested some effort in various studies by Cole (for example Cole and Bradbury, 1995, as cited in Hawkesbury-Nepean Floodplain Management Steering Committee, 2007). Figure 3 shows the path towards the current British Standards around property level resilience in the United Kingdom, clearly showing the influence of US research.
Figure 3. Development of research, standards, and guidance on water entry (Lamond et al., 2016b).
Research into the recovery and reconstruction of property that has suffered flood damage links to the topic of resilient construction through the simple fact that, particularly in the developed world with increasing restriction on developing new buildings in areas at risk, many construction activities in areas at risk from flooding arise as a result of damage and reconstruction activities. Equally it has been observed that those most likely to prioritize resilience in buildings are those with experience of the loss and damages flood events can bring. Reconstruction, which is the demolition of damaged structures and rebuilding, can often follow design principles for initial construction as described in Jha et al. (2012). However, there may be pressure to maintain cultural heritage that leads to a similar style of buildings being constructed or even direct copies of previous structures.
In a large proportion of flood events, however, the recovery involves refurbishment of existing structures that have been partially damaged and do not need to be demolished. This is particularly the case in the United Kingdom, where structural failure due to flooding is a rare event and the majority of flood damage repair falls under the category repair or reinstatement. Under such circumstances, the property remains substantially intact, and the tendency to replace like with like regardless of the risk of future flooding is strong.
Local practice and “common sense” has informed the damage management industry. Guidance on how to recover from flooding was available as early as 1937 (United States Department of Agriculture, 1945 revised from 1937). However, research in this area related to the building fabric is of more recent origin and largely based in the United Kingdom, and has examined the process and technologies entailed. Key studies in this regard are the 1992 Towyn study (Welsh Consumer Council, 1992) and the 1998 Trading Standards report that followed the 1998 flooding in England (Warwickshire Trading Standards, 1998). The BRE released a guide to repair in 1997 (BRE, 1997). This was followed by a benchmarking study (Nicholas et al., 2001; Nicholas & Proverbs, 2002) of current practice in England and Wales that formed the basis of a Publicly Available Specification (PAS) for flood repair (Netherton, 2006) and a number of associated guidance documents (Proverbs & Soetanto, 2004). CIRIA also released guidance in 2005 (Garvin et al., 2005), and the notions of speeding up drying and resilient reinstatement began to be explored by researchers and industry alike (Association of British Insurers/National Flood Forum, 2006; Lambert, 2006; Escarameia et al., 2007). Further research on the satisfaction of insured households with claims handling and repair (Samwinga & Proverbs, 2003) coincided with further flood incidents where reports of uneven performance by insurers and their contractors demonstrated the difficulties of maintaining standards in time of spate (Association of British Insurers, 2007). The Pitt review (Pitt, 2008) following the 2007 flooding in England and Wales highlighted the delays in returning households to their homes. A proliferation of research at this juncture included Proverbs and Lamond (2008), Soetanto et al. (2008), Woodhead (2008), Association of British Insurers (2009), Kidd et al. (2010), and Taylor et al. (2010). In the heritage arena, advice on non-destructive repair strategies was developed (Fidler et al., 2004; Cassar & Hawkings, 2007).
A separate stream looking at the social and emotional aspects (e.g., Fernández-Bilbao & Twigger-Ross, 2009; Samwinga, 2009; Whittle et al., 2010) concluded that speed of recovery is an important consideration in designing repair strategies. The most recent study that considered the reinstatement process is by Lamond et al. (2017).
Much of the literature in the United States concerns environmental and contamination issues associated with flooded buildings, such as the medical dangers from mold. Curtis et al. (2000) demonstrated that fungus and bacteria were not significantly higher in previously flooded houses. Substantial work was carried out in the aftermath of Katrina where mold was more prevalent (Chew et al., 2006). This research has recently become more widespread—for example, ten Veldhuis et al. (2010), Taylor et al. (2013), and Johanning et al. (2014)—and this has led to recommendations for recovery work.
Current thinking on flood resilient construction starts from the premise that new construction on the floodplain should be avoided where possible, following the principles of “making space for water.” Examples of planning statements that guide or restrict floodplain development include the Australian Emergency Management Institute’s handbooks (Australian Emergency Management Institute, 2013) and PPS25 in the United Kingdom. In the United States, the USACE/FEMA guidance predominates.
However, where buildings are permitted in the floodplain or where redevelopment, regeneration, or reinstatement activities are carried out in areas at risk from flooding, best practice is represented in guidance documents as highlighted in Figure 3. For the United Kingdom, relevant documents are BS85500, PAS1188, BS1999, and to some extent the CIRA SuDs manual; underlying principles are laid out in PPS25.
Flood resilient construction seeks to minimize the impact of these actions on people and property in the event of a flood using the principles of avoidance, water exclusion, and resilience. The potential actions of flood depend on the likely source, depth, and velocity of flooding within a given area; impacts of high velocity flooding may be dominated by hydrodynamic and debris actions, whereas groundwater flooding may be dominated by hydrostatic and buoyancy actions, and therefore design should always take into account the likely flood attributes.
Local construction traditions also matter, as vernacular and contemporary architecture varies with local climate and available materials. For example, raised housing is traditionally adopted for air circulation in some hot climates and also aids flood avoidance. Therefore it is not practical to propose a generic building design which will suit all flood-prone areas, even where flood patterns are similar. Even within smaller geographical areas, it appears to be accepted by practitioners and academics that there is no simple formula that can determine appropriate adaptation approaches. General principles are offered, for example the USACE flood proofing matrix (Table 1 below), the Australian Hawkesbury-Nepean guide (Hawkesbury-Nepean Floodplain Management Steering Committee, 2007) and the UK guidelines (BS85500). While these are a useful starting point and are based on the available evidence, they somewhat reinforce the preference for water exclusion and categorization that is beginning to be seen as unhelpful (Lamond et al., 2016b). While these matrices imply an either/or approach, the evidence from the field is that many buildings occupants take a more pragmatic and integrated approach. There are also many evidence gaps in the underpinning science that mean practice is often reliant on trial and error techniques.
The latest guidance on raised construction in the United States following learnings from Katrina was issued by the USACE (ASCE, 2015). A critical design factor is the required elevation of structures to limit the chance that flooding will exceed the designed protection. Overelevation causes unnecessary expense and exposure to wind loading, whereas underelevation increases the probability of exceedance. Elevation is usually recommended to above a probabilistic baseline flood (for example 1 in 100 year + climate change adjustment in the United Kingdom, 1 in 100 year in the FEMA guidelines) as represented by flood hazard estimation by government agencies. There is clearly the possibility that these levels will be exceeded and properties may flood, particularly if flooding becomes more intense in the future. UK research under the Technology Strategy Board’s “design for climate” project examined the current and future requirements for flood avoidance (Baca Architects et al., 2013), concluding that uncertainties around future flood risk may render elevated properties more vulnerable than current estimates suggest. The use of the sub-floor space is also a matter for debate. Where this space may be used for garaging or storage, the potential for assets to be destroyed remains. Insurers pay out on loss of motor vehicles due to flooding instead of contents. Furthermore, the items stored may become damaging debris, and structural damage to the raised elevation may ensue.
Flexibly floating houses have the prima facie advantage of rising flexibly above the maximum flood with little increased cost and no increased wind exposure. In practice, however, there will be limitations set by the guidance and tethering mechanisms as well as from attached services.
Concerns around access to raised housing (see examples in Figure 4) during a flood event for emergency services has led to regulation in the United Kingdom that ensures access is provided (Baca Architects et al., 2013).
Moving away from the water exclusion/water entry dichotomy, the concept of property level flood resilience combines the means to limit the amount of water entering a building (where sensible) and approaches that limit damage where water does enter the building envelope, as illustrated in the diagram (Figure 5). This is a concept gaining acceptance in the United Kingdom in recognition that a hybrid approach is often the most pragmatic one. As many UK floods are reasonably shallow, slow in onset, and of relatively short duration, water exclusion is often possible and water entry can be controlled.
The decision about whether to attempt to exclude water from a building is informed by the likely structural consequences in creating increased hydrostatic load due to differences in water levels inside and outside a building. This has been studied In the United Kingdom by Kelman (Kelman, 2002) for masonry structures and in the United States for wooden construction (Aglan et al., 2004). Such research has led to recommended limits to the water exclusion approach, depending on construction type, varying from 0.3 m to 1 m. However, the research does not cover sufficient types of construction, and further testing of construction stability is warranted. If water is to be allowed in for structural stability reasons, then a plan to allow or control flow to ensure rapid equalization of levels may be needed; scant research or guidance exists on this approach.
Other circumstances that may reduce the effectiveness of the water exclusion approach include: groundwater flooding, although it may be possible to create a water resistant flooring system that excludes it, albeit structural considerations may make this undesirable (Bowker et al., 2007); fast onset flooding, which may limit the time for measures to be deployed; high-velocity flooding, where hydrodynamic forces may cause structural issues at lower depths; long-duration flooding, since most walls will allow water through eventually unless steps are taken to treat the wall surface (Beddoes & Booth, 2015); attached property, where an adjoining structure that has a different approach to limiting damage is of different construction or is at a different elevation; historic/character properties, where there may be constraints on the type of measures acceptable for use (Historic England/Pickles et al., 2015); occupant considerations, where both the capacity and preferences are important (JBA, 2012); nonstandard construction; poor-quality/porous brick and poorly maintained structures.
Excluding water requires the consideration of multiple entry points: windows and doors, floor voids (particularly suspended floors), cracks or gaps in walls, air vents or air bricks (designed for ventilation), service ducts and pipes, toilets and drains, or seepage through floors (particularly earth or stone floors where there is no damp-proof membrane). In addition, the quality of building components is critical, as failure of any one element can compromise the whole design.
Figure 5. Graphic illustrating combined resistance and resilience measures (Dhonau & Rose, 2016).
Aperture technology has evolved from simple wooden boards held up by sandbags to an industry creating innovative, ready-made door guards, smart air bricks, non-return valves, etc. These products have been subjected to laboratory testing, particularly in the United Kingdom as a result of the establishment of kite mark standards defining the acceptable leakage rates of barriers (BSI, 2016).
In short-duration flooding, blocking apertures may be sufficient, but in long-duration flooding water will potentially permeate through the building fabric itself. This has led to the increased use of “tanking” technology to increase the water tightness of walls, led in the United Kingdom by the knowledge derived from waterproofing basements. Membranes and assemblages to improve water-tightness of walls have also been tested in the United Kingdom by Escarameia and Tagg (Escarameia et al., 2006) and in the United States, showing that combinations involving sprayed and sheet-applied water-resistant membranes, insulated concrete formwork, and metal structural insulated panels were suitable to exclude water up to 1 m (Perkes, 2011). Work has also been carried out by CSIRO in Australia and by Branz in New Zealand. Further research in the United Kingdom on Silane-based products show that coating walls and regrouting with admixtured grout can reduce ingress to levels that can be controlled and expelled by pumps (Beddoes & Booth, 2015).
Once water enters the building, a wide range of building elements, fixtures, and fittings become vulnerable to damage. Approaches to limit damage (as illustrated in Figure 6) within a building mirror building-level approaches, avoidance, water-resistant materials, water-resilient materials, and speedy recovery (Lamond et al., 2016a). The efficacy of avoidance measures is self-evident, subject to the height to which building elements, fixtures, and contents may be raised. Items may be permanently raised above the height of expected flooding—for example, electrical sockets, wall-mounted cabinets, meters, control panels and boilers, etc. Dropping electrical services from above and isolating circuits likely to be affected from the rest of the wiring are in line with current electrical practice, and modern cabling and piping within walls and floors are usually well protected (Lamond et al., 2016a).
Alternatively, items such as carpets and reasonably lightweight furniture may be moved in anticipation of an impending flood, if a suitably high storage space is available or one that can be raised temporarily on trestles. In these circumstances, construction should allow for ease of removal (e.g., easy-remove hinges for doors and cabinet doors) and also allow easy access to upper levels for removal (avoiding steep, narrow, and winding staircases).
Advice on the properties of materials in relation to flooding is provided in some guidance; for example, the Hawkesbury-Nepean guidance (Hawkesbury-Nepean Floodplain Management Steering Committee, 2007) contains tables of material absorbency and of suitability of materials for 96-hour immersion. This information is based on research carried out in the 1990s by Cole for CSIRO. This information is also provided by UK publications (Bowker et al., 2007) based on work carried out for CLG in 2003–2005.
Research on materials subject to hydrostatic pressure, which might be experienced during deep flooding, demonstrates that the porosity of construction materials can affect both ingress and drying properties. Properties can be constructed of materials such as engineered bricks in an effort to limit water ingress into and through walls (Escarameia et al., 2006). Different types of plaster and plasterboard have also been studied. The instability of gypsum-based plasters is well documented, as they absorb large quantities of water and are vulnerable to deterioration and salt transport (Environment Agency & CIRIA, 2001; Bowker, 2002; Drdácký, 2010). Therefore, lime-based or cement-based alternatives are often recommended. However, gypsum is quick to dry out and may be suitable in circumstances where short-duration floods are expected. Solid plaster directly applied to walls or on battens on top of masonry represents an example of a traditional construction method that is increasingly being replaced by alternatives such as “dry lining” with pre-prepared boards made of gypsum and a light layer of skimming plaster. Standard gypsum boards suffer from similar issues to gypsum plaster (Lambert, 2006; Escarameia et al., 2007). However, an increasing range of moisture- and water-resistant boards are available, and some have been tested for performance under flood conditions. Aglan et al. (2004, 2014) found that water-resistant boards (Fiberock) were suitable for floods of up to three days’ duration. “Splash proof” board (Fermacell) was found to resist water penetration by Escarameia et al. (2006), although it was distorted due to hydrostatic pressure. Cement-based boards and fully waterproof boards (for example, made of magnsium oxide) have been recommended by professionals but no independent testing evidence is yet available (Lamond et al., 2016a).
The role of insulation materials in property level resilience is complicated, because it is often inaccessible, being situated within the cavity, under floor structure, or behind other finishes. Therefore it is important for insulation to retain integrity when flooded and not slump within a cavity, dry quickly and retain thermal performance, and not impede drying of adjacent materials. Experimental evidence and experience suggest that fiberglass, mineral fiber (aka mineral wool/rock wool/stone wool), and blown-in mica can slump and degrade during wetting (Escarameia et al., 2006). Although recent tests on mineral batt insulation shows that it can dry out without degradation when sufficiently supported and drained (Sanders, 2014), it is slow to dry out, particularly within a cavity. Closed-cell insulation is more rigid and is therefore often recommended, but there are very few tests that demonstrate the post-flood thermal performance. Waterproof insulation materials have been tested (Technitherm), and as they can be demonstrated to resist penetration by floodwater, their thermal integrity is retained (Gabalda et al., 2012; CORDIS, 2015). Considerations of insulation and drying are covered in the section on repair and reinstatement.
Figure 6. Examples of resilient materials in situ: a) Marine Ply Kitchen has survived a flood, tiled floor; b) hydraulic lime plaster with salt resistant additive over a wire mesh to provide air gap; c) Concrete floor with removable carpet tiles, sump and pump to control flow; d) Tiled floor and well-seasoned, varnished, and painted hardwood stairs and skirting has survived several floods.
Photos with kind permission of the homeowners (all rights reserved).
Timber is another commonly used building material that under some circumstances can be regarded as highly resilient. Solid, dense, and well-seasoned wood building elements, fittings, and furniture can survive inundation (Lambert, 2006; O’Leary, 2014; Lamond et al., 2016b). But more modern, lighter-density, and fast-treated wood is less resilient; such wood can be made more resilient by surface treatment with varnish and paints on all surfaces and renewed as necessary.
Composite wood products, for example paneling and veneers and MDF/particleboard, are not regarded as resilient, with the exception of highest-grade marine ply (e.g., compliant with BS1088). The type of timber framing used in modern UK buildings requires specialist treatment, and panels will usually need to be removed for restoration after a flood.
Table 2 shows an example of guidance helpful in selecting suitable materials for long-duration flooding (over 96 hours’ immersion). This demonstrates how the research can be made highly relevant in assisting competent building professionals in selecting materials and assemblages. However, it needs to be considered in a whole-building context and also in the light of occupant capacity and preference, availability, and cost of materials and skilled workers and the reinstatement protocols that may be followed in the event of a flood.
Table 2. Example of guidance for selecting materials suitable for 96-hour immersion (adapted from Hawkesbury-Nepean Floodplain Management Steering Committee, 2007).
SUITABLE: these materials or products are relatively unaffected by submersion and flood exposure and are the best available for the particular application.
MILD EFFECTS: these materials or products suffer only mild effects from flooding and are the next best choice if the most suitable materials or products are too expensive or unavailable.
MARKED EFFECTS: these materials or products are more liable to damage under flood than the above category.
SEVERE EFFECTS: these materials or products are seriously affected by floodwaters and have to be replaced if inundated.
As an alternative or as a complement to designing a property to be resilient, it has to be recognized that when a flood event occurs there will be a need to dry, clean, and perhaps repair the affected buildings as quickly and sympathetically as possible. The trauma faced by flood-affected occupants is well documented (Whittle & Medd, 2011), and the desire to return quickly after a flood is widespread (Soetanto et al., 2008). Faster recovery can even limit psycho-social symptoms from flooding (Lamond et al., 2015). Considerate reinstatement as advocated by Woodhead (2011) and the sensitive and professional handling of the recovery process are represented in guidance such as PAS64.
Fast and effective drying of flooded buildings is therefore a key criterion in recovery, and the avoidance of trapped water, slow-drying material, or water vapor between building layers and behind finishes is desirable. The potential exists for secondary damage to occur if drying is delayed or badly controlled, and therefore the choice of resilience approaches should be contextualized within a recovery/reinstatement plan.
Resilient materials that are slow to dry out—for example, lime plasters (Office of the Deputy Prime Minister, 2003)—can slow recovery, even though they can be retained. Solid plaster of any kind that remains in situ has the potential to slow the drying of the underlying masonry (Office of the Deputy Prime Minister, 2003), so to avoid delay the option of removing the plaster, an air-gap method such as plastering over a metal mesh, can be considered (Sheaffer, 1960).
Another consideration in the retention of resilient materials is the need to decontaminate them. There is very little evidence available on the scale of the contamination issue in a post-flood situation. However, professionals generally should provide drying and decontamination certificates (PAS64), and biocidal cleaning agents are widely available for occupants to use if professionals are not required for other purposes. Heat-assisted and speed-drying techniques can accelerate the reinstatement of property, and there are a wide variety of specialized tools to aid cleaning and drying and to access voids where water may be trapped. In planning a resilient property scheme, it may be important to select materials that will not be damaged by the cleaning and drying processes. However, there is very little research into the impacts of cleaning and drying that can guide building occupants in these choices.
It is clear from the above that the materials and technology to create and retrofit properties that are more resilient to flooding already exist. However, the adoption of such measures is limited by a number of factors, underlying which are important limitations that indicate the need for future developments in resilient technologies and construction (Proverbs & Lamond, 2008).
Recommended adaptations are often rejected on aesthetic or familiarization grounds because they make properties look different (Harries, 2008; Thurston et al., 2008) or are designed to be functional without adequate consideration of good design. In the United Kingdom, recently developed flood doors are designed to look more conventional and potentially enhance the appearance of homes. Further development of flood resilience technology that enhances the aesthetic appeal of adapted property would support uptake of measures.
Cost of adaptation is also a consideration (Thurston et al., 2008). Future developments that reduce cost or that offer other aesthetic or functional advantages may also reduce the barriers to uptake (Lamond et al., 2017). For example, better understanding of the link between resilient insulation and thermal tightness might lead to the development of multi-purpose flood resilience products or protocols.
Performance standards for resistance products exist, at least in the United Kingdom, but performance standards for resilient materials and designed schemes are not available. Lack of confidence in the performance of measures is a barrier to uptake, and therefore future developments should aim to establish standards or performance indicators to enhance belief that measures will limit damage and reduce disruption.
Breathability is also a major consideration that limits the specification of measures by professionals concerned not to create moisture issues inside properties. Further developments in technology may need to build on the vapor-permeable coatings already existing (Beddoes & Booth, 2015). Mold inhibition through biocides or assemblages that can be easily dismantled for cleaning and drying are alternative routes to circumvent moisture trap problems.
As kitchens are typically the costliest area damaged in domestic flood incidents, there is further scope to develop the science and resilience of white goods and appliances. The design of kitchens that can easily be adapted or protected is useful. Practical steps using off-the-shelf products can make real improvements to the resilience of kitchens. Again, recent research involving flood-affected communities has highlighted the importance of aesthetic considerations, as people prefer to keep to norms in design and appearance.
Lastly, much of the discussion here and in the literature generally relates to traditional construction types typical for residential and small business premises. A greater focus on modern construction types and commercial premises will be needed in order to meet adaptation challenges in the 2020s and beyond.
Increasingly, flood resilient construction has become an important component of an integrated approach to flood risk management. This largely underresearched area has become more important in recent years due to development pressures and planning regulations and a general acceptance of the need to live with flooding. The design and construction of new buildings as well as the adaptation or retrofit of existing buildings to make them resilient to flooding can play an important part in mitigating the damage caused by flooding and in speeding up the recovery process. The concepts and principles of flood resilient construction date back centuries, but the scientific study of construction and building technologies in this context is a much more recent development, prompted by a growing realization that flooding cannot be prevented, the advancements in building technologies and materials, and the development of property level resilience and resistance measures.
Flood resilient construction strategies are categorized into three types as avoidance, water exclusion, and water entry, with the avoidance approach being the most commonly adopted, mainly through elevation and landscaping systems. Other approaches include buildings that are designed to permanently float or to float in flood conditions. Water exclusion involves steps to keep water out of a property, and recent interest in this has led to the development of many new products designed to be installed at the individual property level. Water entry technology or flood resilience approaches which assume water will enter the occupied space is an underresearched area, but one that is gaining interest, especially in the United Kingdom, due to the nature of flooding and the vernacular characteristics of buildings which lend themselves to this approach.
Advancements in the domain of flood repair and reinstatement have been witnessed especially in the United Kingdom, where much research followed the critical Pitt Report. This has led to the introduction of more guidance and the development of standards which have improved our understanding and raised awareness. Increased recognition of the importance of the repair and reinstatement process has given rise to the need to dry and restore buildings as quickly as possible. The need to avoid materials that can take a long time to dry out and the avoidance of water traps and the need to decontaminate materials have also been highlighted.
The state of the art in flood resilient construction stems from the principle that construction on the flood plain should be avoided wherever possible, in line with “making space for water.” However, despite planning restrictions and guidance, other developmental pressures, and the desire to be close to waterways, result in many buildings still being constructed within floodplains. Flood resilient construction is therefore needed to minimize the impacts of the main flood actions on buildings, including hydrostatic pressures and damages caused by debris and erosion. The emergence of a hybrid approach to flood resilience which restricts the amount of water entering a building while limiting the damage caused by water that does enter is gaining recognition, especially in Europe. This hybrid approach involves a combination of water exclusion measures together with some resilience measures to address the residual risk. Developments in the technologies and products designed to keep water out of buildings have advanced significantly, and standards are now in place to provide some assurance of the efficacy of these to the extent that they are now becoming more commonplace. There has been much research around the properties of materials under the effects of water, and this has led to a better understanding of the need to consider material specification as part of the overall strategy. This includes materials such as plaster, dry lining, water tanking, insulation, and timber, with guidance now available to help select materials.
Future developments in the field of flood resilient construction and adaptation have been highlighted; they include the need to develop a better understanding of the preferences of property owners and the need to develop more affordable solutions. The importance of resilient construction is likely to continue to increase with the demands for new housing, increased likelihood of flooding, and continuing urbanization. There is much scope for further research to improve the science around materials and new technologies as well as some of the less technical themes linked to behaviors and preferences of property owners. A more scientific understanding to the measurement of resilience at the property level would help to gauge improvements and understanding around residual risk and the likely costs and disruption to be expected.
BSI. (2016). Flood protection: Certification and testing services for flood protection products [Web log post].
CORDIS. 2015. SMARTeST Report Summary (Project reference: 244102) [Online]. Available online.
Dhonau, M. & Rose, C. B. (2016). Homeowners' Guide to Flood Resilience. [Online] Know Your Flood Risk Campaign. Available online and http://knowyourfloodrisk.co.uk/.
Hawkesbury-Nepean Floodplain Management Steering Committee. (2007). Reducing vulnerability of buildings to flood damage: Guidance on building in flood prone areas. Parramatta, Australia: Hawkesbury-Nepean Floodplain Management Steering Committee.
Ingargiola, J., Sheldon, A., & Ghorbi, L. (2015). Building code changes resulting from FEMA’s hurricane Sandy mitigation assessment team report. Seventh Congress on Forensic Engineering. Miami, Florida, November 15–18.
O’Leary, P. 2014. Timber suits flood situations. TRADA Timber Industry Yearbook 2014 [Online]. Available online.
SGS Economics and Planning Pty Ltd. (2011). Coastal hazards—adaptations: Part 1. Melbourne, Australia: Local Government Association of Tasmania.