Case Studies

In the first phase of the AFPM, a number of case studies on flood management were collected from various regions, based on the experiences of organizations active in flood management. These case studies were essential in formulating the Integrated Flood Management concepts, as they helped to:

  • Identify the extent to which integrated flood management has been carried out;
  • Understand shortcoming in flood management practices worldwide;
  • Extract lessons learned and good practices in flood management;
  • Catalogue the policy changes required to support IFM; and
  • Identify the institutional changes required to achieve IFM.

The case studies are presented here for “historical purposes”: having been compiled almost 20 years ago, they are reflecting national situations that might since have developed. As such, the case studies might be used as baseline or reference material for studies that aim to check the improvements in flood management since the beginning of the century.

The Overview Situation Paper on flood management practices extracts the essence of each case study, emphasizes findings and recommendations with relevance to the aspects of Integrated Flood Management and the potential for practices to be replicated in other locations. Download the Overview Situation Paper  here .

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  • Published: 03 August 2022

The challenge of unprecedented floods and droughts in risk management

  • Heidi Kreibich   ORCID: orcid.org/0000-0001-6274-3625 1 ,
  • Anne F. Van Loon   ORCID: orcid.org/0000-0003-2308-0392 2 ,
  • Kai Schröter   ORCID: orcid.org/0000-0002-3173-7019 1 , 3 ,
  • Philip J. Ward   ORCID: orcid.org/0000-0001-7702-7859 2 ,
  • Maurizio Mazzoleni   ORCID: orcid.org/0000-0002-0913-9370 2 ,
  • Nivedita Sairam   ORCID: orcid.org/0000-0003-4611-9894 1 ,
  • Guta Wakbulcho Abeshu   ORCID: orcid.org/0000-0001-8775-3678 4 ,
  • Svetlana Agafonova   ORCID: orcid.org/0000-0002-6392-1662 5 ,
  • Amir AghaKouchak   ORCID: orcid.org/0000-0003-4689-8357 6 ,
  • Hafzullah Aksoy   ORCID: orcid.org/0000-0001-5807-5660 7 ,
  • Camila Alvarez-Garreton   ORCID: orcid.org/0000-0002-5381-4863 8 , 9 ,
  • Blanca Aznar   ORCID: orcid.org/0000-0001-6952-0790 10 ,
  • Laila Balkhi   ORCID: orcid.org/0000-0001-8224-3556 11 ,
  • Marlies H. Barendrecht   ORCID: orcid.org/0000-0002-3825-0123 2 ,
  • Sylvain Biancamaria   ORCID: orcid.org/0000-0002-6162-0436 12 ,
  • Liduin Bos-Burgering   ORCID: orcid.org/0000-0002-8372-4519 13 ,
  • Chris Bradley   ORCID: orcid.org/0000-0003-4042-867X 14 ,
  • Yus Budiyono   ORCID: orcid.org/0000-0002-6288-6527 15 ,
  • Wouter Buytaert   ORCID: orcid.org/0000-0001-6994-4454 16 ,
  • Lucinda Capewell 14 ,
  • Hayley Carlson 11 ,
  • Yonca Cavus   ORCID: orcid.org/0000-0002-0528-284X 17 , 18 , 19 ,
  • Anaïs Couasnon   ORCID: orcid.org/0000-0001-9372-841X 2 ,
  • Gemma Coxon   ORCID: orcid.org/0000-0002-8837-460X 20 , 21 ,
  • Ioannis Daliakopoulos   ORCID: orcid.org/0000-0001-9333-4963 22 ,
  • Marleen C. de Ruiter   ORCID: orcid.org/0000-0001-5991-8842 2 ,
  • Claire Delus   ORCID: orcid.org/0000-0002-6690-5326 23 ,
  • Mathilde Erfurt   ORCID: orcid.org/0000-0003-1389-451X 19 ,
  • Giuseppe Esposito   ORCID: orcid.org/0000-0001-5638-657X 24 ,
  • Didier François 23 ,
  • Frédéric Frappart   ORCID: orcid.org/0000-0002-4661-8274 25 ,
  • Jim Freer 20 , 21 , 26 ,
  • Natalia Frolova   ORCID: orcid.org/0000-0003-3576-285X 5 ,
  • Animesh K. Gain   ORCID: orcid.org/0000-0003-3814-693X 27 , 28 ,
  • Manolis Grillakis   ORCID: orcid.org/0000-0002-4228-1803 29 ,
  • Jordi Oriol Grima 10 ,
  • Diego A. Guzmán 30 ,
  • Laurie S. Huning   ORCID: orcid.org/0000-0002-0296-4255 6 , 31 ,
  • Monica Ionita   ORCID: orcid.org/0000-0001-8240-4380 32 , 33 , 34 ,
  • Maxim Kharlamov   ORCID: orcid.org/0000-0002-4439-5193 5 , 35 ,
  • Dao Nguyen Khoi   ORCID: orcid.org/0000-0002-1618-1948 36 ,
  • Natalie Kieboom   ORCID: orcid.org/0000-0001-8497-0204 37 ,
  • Maria Kireeva   ORCID: orcid.org/0000-0002-8285-9761 5 ,
  • Aristeidis Koutroulis   ORCID: orcid.org/0000-0002-2999-7575 38 ,
  • Waldo Lavado-Casimiro   ORCID: orcid.org/0000-0002-0051-0743 39 ,
  • Hong-Yi Li   ORCID: orcid.org/0000-0002-9807-3851 4 ,
  • María Carmen LLasat   ORCID: orcid.org/0000-0001-8720-4193 40 , 41 ,
  • David Macdonald   ORCID: orcid.org/0000-0003-3475-636X 42 ,
  • Johanna Mård   ORCID: orcid.org/0000-0002-8789-7628 43 , 44 ,
  • Hannah Mathew-Richards 37 ,
  • Andrew McKenzie   ORCID: orcid.org/0000-0001-8723-4325 42 ,
  • Alfonso Mejia   ORCID: orcid.org/0000-0003-3891-1822 45 ,
  • Eduardo Mario Mendiondo   ORCID: orcid.org/0000-0003-2319-2773 46 ,
  • Marjolein Mens 47 ,
  • Shifteh Mobini   ORCID: orcid.org/0000-0002-3365-7346 48 , 49 ,
  • Guilherme Samprogna Mohor   ORCID: orcid.org/0000-0003-2348-6181 50 ,
  • Viorica Nagavciuc   ORCID: orcid.org/0000-0003-1111-9616 32 , 34 ,
  • Thanh Ngo-Duc   ORCID: orcid.org/0000-0003-1444-7498 51 ,
  • Thi Thao Nguyen Huynh   ORCID: orcid.org/0000-0001-9071-1225 52 ,
  • Pham Thi Thao Nhi   ORCID: orcid.org/0000-0003-4118-8479 36 ,
  • Olga Petrucci   ORCID: orcid.org/0000-0001-6918-1135 24 ,
  • Hong Quan Nguyen 52 , 53 ,
  • Pere Quintana-Seguí   ORCID: orcid.org/0000-0002-7107-9671 54 ,
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  • Elena Ridolfi   ORCID: orcid.org/0000-0002-4714-2511 57 ,
  • Jannik Riegel 58 ,
  • Md Shibly Sadik   ORCID: orcid.org/0000-0001-9205-4791 59 ,
  • Elisa Savelli   ORCID: orcid.org/0000-0002-8948-0316 43 , 44 ,
  • Alexey Sazonov 5 , 35 ,
  • Sanjib Sharma   ORCID: orcid.org/0000-0003-2735-1241 60 ,
  • Johanna Sörensen   ORCID: orcid.org/0000-0002-2312-4917 49 ,
  • Felipe Augusto Arguello Souza   ORCID: orcid.org/0000-0002-2753-9896 46 ,
  • Kerstin Stahl   ORCID: orcid.org/0000-0002-2159-9441 19 ,
  • Max Steinhausen   ORCID: orcid.org/0000-0002-8692-8824 1 ,
  • Michael Stoelzle   ORCID: orcid.org/0000-0003-0021-4351 19 ,
  • Wiwiana Szalińska   ORCID: orcid.org/0000-0001-6828-6963 61 ,
  • Qiuhong Tang 62 ,
  • Fuqiang Tian   ORCID: orcid.org/0000-0001-9414-7019 63 ,
  • Tamara Tokarczyk   ORCID: orcid.org/0000-0001-5862-6338 61 ,
  • Carolina Tovar   ORCID: orcid.org/0000-0002-8256-9174 64 ,
  • Thi Van Thu Tran   ORCID: orcid.org/0000-0003-1187-3520 52 ,
  • Marjolein H. J. Van Huijgevoort   ORCID: orcid.org/0000-0002-9781-6852 65 ,
  • Michelle T. H. van Vliet   ORCID: orcid.org/0000-0002-2597-8422 66 ,
  • Sergiy Vorogushyn   ORCID: orcid.org/0000-0003-4639-7982 1 ,
  • Thorsten Wagener   ORCID: orcid.org/0000-0003-3881-5849 21 , 50 , 67 ,
  • Yueling Wang 62 ,
  • Doris E. Wendt   ORCID: orcid.org/0000-0003-2315-7871 67 ,
  • Elliot Wickham 68 ,
  • Long Yang   ORCID: orcid.org/0000-0002-1872-0175 69 ,
  • Mauricio Zambrano-Bigiarini   ORCID: orcid.org/0000-0002-9536-643X 8 , 9 ,
  • Günter Blöschl   ORCID: orcid.org/0000-0003-2227-8225 70 &
  • Giuliano Di Baldassarre   ORCID: orcid.org/0000-0002-8180-4996 43 , 44 , 71  

Nature volume  608 ,  pages 80–86 ( 2022 ) Cite this article

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  • Natural hazards

Risk management has reduced vulnerability to floods and droughts globally 1 , 2 , yet their impacts are still increasing 3 . An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data 4 , 5 . On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change 3 .

Observed decreasing trends in the vulnerability to floods and droughts, owing to effective risk management, are encouraging 1 . Globally, human and economic vulnerability dropped by approximately 6.5- and 5-fold, respectively, between the periods 1980–1989 and 2007–2016 (ref.  2 ). However, the impacts of floods and droughts are still severe and increasing in many parts of the world 6 . Climate change will probably lead to a further increase in their impacts owing to projected increases in the frequency and severity of floods and droughts 3 . The economic damage of floods is projected to double globally 7 and that of droughts to triple in Europe 8 , for a mean temperature increase of 2 °C.

The purpose of risk management is to reduce the impact of events through modification of the hazard, exposure and/or vulnerability: according to United Nations (UN) terminology 9 , disaster risk management is the application of disaster risk reduction policies and strategies to prevent new disaster risk, reduce existing disaster risk and manage residual risk, contributing to the strengthening of resilience against, and reduction of, disaster losses. Hazard is a process, phenomenon or human activity that may cause loss of life, injury or other health impacts, property damage, social and economic disruption or environmental degradation; exposure is the situation of people, infrastructure, housing, production capacities and other tangible human assets located in hazard-prone areas; and vulnerability is the conditions determined by physical, social, economic and environmental factors or processes 10 , 11 , 12 , 13 that increase the susceptibility of an individual, a community, assets or systems to the impacts of hazards. To be effective, risk management needs to be based on a sound understanding of these controlling risk drivers 14 , 15 . Past studies have identified increasing exposure as a primary driver of increasing impacts 3 , 4 , and vulnerability reduction has been identified as key for reduction of impacts 16 , 17 . However, ascertaining the combined effect of the drivers and the overall effectiveness of risk management has been hampered by a lack of empirical data 4 , 5 .

Here we analyse a new dataset of 45 pairs of flood or drought events that occurred in the same area on average 16 years apart (hereinafter referred to as paired events). The data comprise 26 flood and 19 drought paired events across different socioeconomic and hydroclimatic contexts from all continents (Fig. 1a ). We analyse floods and droughts together, because of the similarity of some of the management methods (for example, warning systems, water reservoir infrastructure), the potential for trade-offs in risk reduction between floods and droughts and therefore value for the management communities to learn from each other 18 . The impact, quantified by direct (fatalities, monetary damage), indirect (for example, disruption of traffic or tourism) and intangible impacts (for example, impact on human health or cultural heritage), is considered to be controlled by three drivers: hazard, exposure and vulnerability 3 . These drivers are quantified using a large range of different indices—for example, the standardized precipitation index, the number of houses in the affected area and risk awareness, respectively (Supplementary Table 1 ). These three drivers are considered to be exacerbated by management shortcomings. Hazard may be exacerbated by problems with water management infrastructure such as levees or reservoirs 19 . Exposure and vulnerability may be worsened by suboptimal implementation of non-structural measures such as risk-aware regional planning 20 or early warning 21 , respectively. We analyse management shortcomings and their effect on the three drivers explicitly, as this is the point at which improvements can start—for example, by the introduction of better strategies and policies. Data availability understandably varies among the paired events, and this can introduce inconsistency and subjectivity. The analyses are therefore based on indicators of change, to account for differences between paired events in respect of measured variables, data quality and uncertainty. These indicators of change represent the differences between the first event (baseline) and the second, categorized as large decreases/increases (−2/+2), small decreases/increases (−1/+1) and no change (0) (Supplementary Table 2 ). To minimize the subjectivity and uncertainty of indicator assignment, a quality assurance protocol is implemented and indicators of change with sub-indicators are used.

figure 1

a , Location of flood and drought paired events ( n = 45). Numbers are paired-event IDs. b , Indicators of change, sorted by impact change. Impact is considered to be controlled by hazard, exposure and vulnerability, which are exacerbated by risk management shortcomings. Maps of the paired events coloured according to drivers and management shortcomings are shown in Extended Data Fig. 1 .

Source data

The majority of paired events show decreases in management shortcomings (71% of paired events; Fig. 1b ), which reflects that societies tend to learn from extreme events 22 . Most cases also show a decrease in vulnerability (80% of paired events) as societies typically reduce their vulnerability after the first event of a pair 21 . The five paired events with a large decrease in impact (dark blue, top left in Fig. 1b ) are associated with decreases or no change of all three drivers.

Drivers of changes in impact

Changes in flood impacts are significantly and positively correlated with changes in hazard ( r  = 0.64, P  ≤ 0.01), exposure ( r  = 0.55, P  ≤ 0.01) and vulnerability ( r  = 0.60, P  ≤ 0.01) (Fig. 2a ), which is in line with risk theory 3 . Although a previous analysis of eight case studies 21 identified vulnerability as a key to reduction of flood impacts, this new, more comprehensive, dataset suggests that changes in hazard, exposure and vulnerability are equally important, given that they correlate equally strongly with changes in flood impact. Changes in drought impacts are significantly correlated with changes in hazard and exposure, but not with changes in vulnerability (Fig. 2c ). This suggests that changes in vulnerability have been less successful in reducing drought impact than flood impact, which is also consistent with those event pairs for which only vulnerability changed (Extended Data Table 1 ). However, quantification of the contribution of individual drivers is difficult with this empirical approach because there are only a limited number of cases in which only one driver changed. There are three cases in which only vulnerability changed between events, two cases in which only hazard changed and no case in which only exposure changed (Extended Data Table 1 ). Additionally, paired events without a change in hazard (0) are analysed in more detail to better understand the role of exposure and vulnerability (Extended Data Fig. 2 ). In all these paired events, a reduction in impact was associated with a reduction in vulnerability, highlighting the importance of vulnerability. In five of these eight cases with a decrease in impact there was also a decrease in exposure, whereas in one case (floods in Jakarta, Indonesia in 2002 and 2007 (ID 18)) there was a large increase in exposure. In the paired event of droughts in California, United States (1987–1992 and 2011–2016, ID 36) an increase in exposure and a reduction in vulnerability increased impact, which points to the more important role of exposure in comparison with vulnerability in this drought case (Extended Data Fig. 2 ).

figure 2

a , c , Correlation matrix of indicators of change for flood ( a ) and drought ( c ) paired events. Colours of squares indicate Spearman’s rank correlation coefficients and their size, the P  value. b , d ,Histograms of indicators of change of flood ( b ) and drought ( d ) stratified by decrease ( n  = 15 and n  = 5 paired events for flood and drought, respectively) and increase ( n  = 5 and n  = 8 paired events, respectively) in impact. The asterisk denotes the success stories of Box 1 ; double asterisks denote pairs for which the second event was much more hazardous than the first (that is, 'unprecedented'). Mgmt shortc, management shortcomings.

Generally the changes in drivers are not significantly correlated with each other, with the exception of hazard and exposure in the case of floods ( r  = 0.55, P  ≤ 0.01) (Fig. 2a ). This finding may be explained by the influence of hazard on the size of the inundation area, and thus on the numbers of people and assets affected, which represent exposure.

The sensitivity analysis suggests that the correlation pattern is robust, as visualized by the colours in Extended Data Fig. 3 . The pattern of P  values is also robust for flood cases, although these become less significant for drought because of the smaller sample size (Extended Data Fig. 3 ).

We split the paired events into groups of decreasing and increasing impact to evaluate their drivers separately (Fig. 2b,d ). Overall, the pattern is similar for floods and droughts. Most flood and drought pairs with decreasing impact show either a decrease in hazard (ten pairs, 50%) or no change (eight pairs, 40%). Exceptions are two flood pairs that are success stories of decreased impact despite an increase in hazard, as detailed in Box 1 . The change in exposure of the pairs with decreased impacts (Fig. 2b,d ) ranges from a large decrease to a large increase, whereas vulnerability always decreased. All cases with a large decrease in vulnerability (−2) are associated with a decrease in impacts. Overall, the pattern suggests that a decrease in impacts is mainly caused by a combination of lower hazard and vulnerability, despite an increase in exposure in 25% of cases.

The role of hazard and vulnerability in impact reduction can be exemplified by the pair of riverine floods in Jakarta, Indonesia (ID 4 in Fig. 1 ). The 2007 event had a flood return period of 50 years, whereas it was 30 years for the 2013 event 23 (that is, the hazard of the second event was smaller). Vulnerability had also decreased as a result of improved preparedness resulting from a flood risk mapping initiative and capacity building programmes implemented after the first flood, to improve citizens' emergency response, as well as by an improvement in official emergency management by establishment of the National Disaster Management Agency in 2008. Additionally, exposure was substantially reduced. Whilst the first flood caused 79 fatalities and direct damage of €1.3 billion, the second event caused 38 fatalities and €0.76 billion of direct damage.

Another example is a pair of Central European droughts (ID 9). During the 2003 event, the minimum 3-month Standardized Precipitation Evapotranspiration Index was −1.62 whereas in 2015 it was −1.18—that is, the hazard of the second event was smaller 24 . The vulnerability was also lower in the second event, because the first event had raised public awareness and triggered an improvement in institutional planning. For instance, the European Commission technical guidance on drought management plans 25 was implemented. Many reservoirs were kept filled until the beginning of summer 2015, which alleviated water shortages for various sectors and, in some cities (for example, Bratislava and Bucharest), water was supplied from tanks 26 . Additionally, water use and abstraction restrictions were implemented for non-priority uses including irrigation 26 . The impact was reduced from €17.1 billion to €2.2 billion, despite an increase in exposure because of the larger drought extent affecting almost all of Europe in 2013.

Most flood and drought pairs with an increase in impact also show a larger hazard (11 cases, 85%; Fig. 2b,d ). For six of these paired events (46%), the second event was much more hazardous than the first (hazard indicator-of-change +2), whereas this was never the case for the pairs with decreasing impact. Of those pairs with an increase in impact, 12 (92%) show an increase in exposure and nine (69%) show a small decrease in vulnerability (vulnerability indicator-of-change −1). Overall, the pattern suggests that the increase in impact is mainly caused by a combination of higher hazard and exposure, which is not compensated by a small decrease in vulnerability.

The role of hazard and exposure in increasing impact is illustrated by a pair of pluvial floods in Corigliano-Rossano City, Calabria, Italy (ID 40). This 2015 event was much more hazardous (+2) than that in 2000, with precipitation return periods of more than 100 and 10–20 years, respectively 27 . Also, the 2000 event occurred during the off-season for tourism in September whereas the exposure was much larger in 2015, because the event occurred in August when many tourists were present. Interruption of the peak holiday season caused severe indirect economic damage. Another example is a pair of droughts (ID 33) affecting North Carolina, United States. Between 2007 and 2009, about 65% of the state was affected by what was classified as an exceptional drought, with a composite drought indicator of the US Drought Monitor of 27 months 28 , whereas between 2000 and 2003 only about 30% of the state was affected by an exceptional drought of 24 months 28 . The crop losses in 2007–2009 were about €535 million, whereas they were €497 million in 2000–2003, even though vulnerability had been reduced due to drought early warning and management by the North Carolina Drought Management Council, established in 2003.

Box 1
 Success stories of decreased impact despite increased hazard

The dataset includes two cases in which a lower impact was achieved despite a larger hazard of the second event, making these interesting success stories (Fig. 3 ). Both cases are flood paired events, but of different types (that is, pluvial and riverine floods (Table 1 )). These cases have in common that institutional changes and improved flood risk management governance were introduced and high investments in integrated management were undertaken, which led to an effective implementation of structural and non-structural measures, such as improved early warning and emergency response to complement structural measures such as levees (Table 1 ).

Effects of changes in management on drivers

The correlations shown in Fig. 2a,c also shed light on how management affects hazard, exposure and vulnerability and thus, indirectly, impact. For flood paired events, changes in management shortcomings are significantly positively correlated with changes in vulnerability ( r  = 0.56, P  ≤ 0.01), and both are significantly positively correlated with changes in impact (Fig. 2a ). For drought, however, these correlations are not significant (Fig. 2c ). Thus, achieving decreases in vulnerability, and consequently in impact, by improving risk management (that is, reducing management shortcomings) seems to be more difficult for droughts than for floods. This difficulty may be related to spillover effects—that is, drought measures designed to reduce impacts in one sector can increase impacts in another. For example, irrigation to alleviate drought in agriculture may increase drought impacts on drinking water supply and ecology 29 .

The paired floods in the Piura region, Peru (ID 13) illustrate how effective management can reduce vulnerability, and consequently impact. At the Piura river, maximum flows of 3,367 and 2,755 m 3  s −1 were recorded during the 1998 and 2017 events, respectively (that is, hazard showed a small decrease (−1)). Around 2000, the national hydrometeorological service started to issue medium-range weather forecasts that allowed preparations months before the 2017 event. In 2011, the National Institute of Civil Defence and the National Centre for the Estimation, Prevention, and Reduction of Disaster Risk were founded which, together with newly established short-range river flow forecasts, allowed more efficient emergency management of the more recent event. Additionally, non-governmental organizations such as Practical Action had implemented disaster risk-reduction activities, including evacuation exercises and awareness campaigns 30 . All of these improvements in management decreased vulnerability. The impact of the second event was smaller, with 366 fatalities in 1998 compared with 159 in 2017, despite an increase in exposure due to urbanization and population increase.

When the hazard of the second event was larger than that of the first (+1, +2), in 11 out of 18 cases (61%) the impact of the second event was also larger, irrespective of small decreases in vulnerability in eight of these cases (light blue dots/triangles in Fig. 3 ). There are only two paired events in our dataset for which a decrease in impact was achieved despite the second event being more hazardous (highlighted by the green circle in Fig. 3 ). These cases are considered success stories and are further discussed in Box 1 . For the two paired events (ID 21 and 30) for which the only driver that changed was hazard (+1), the impacts did not change (0) (Extended Data Table 1 ). Water retention capacity of 189,881,000 m³ and good irrigation infrastructure with sprinkling machines were apparently sufficient to counteract the slight increase in hazard for the drought paired event in Poland in 2006 and 2015 (ID 21). The improved flood alleviation scheme implemented between the paired flood events (2016 and 2018), protected properties in Birmingham, United Kingdom (ID 30). There are, however, seven cases for which the second event was much more hazardous (+2) than the first (highlighted by the purple ellipse in Fig. 3 )—that is, events of a magnitude that locals had probably not previously experienced. We term these events, subjectively, as unprecedented; almost all had an increased impact despite improvements in management.

figure 3

Categories are: lower hazard and lower impact, ten cases; higher hazard and higher impact, 11 cases; lower hazard and higher impact, one case; higher hazard and lower impact, two cases. Circles and triangles indicate drought and flood paired events, respectively; their colours indicate change in vulnerability. Green circle highlights success stories ( n  = 2) of reduced impact (−1) despite a small increase in hazard (+1). Purple ellipse indicates paired events ( n  = 7) with large increase in hazard (+2)—that is, events that were subjectively unprecedented and probably not previously experienced by local residents.

One unprecedented pluvial flood is the 2014 event in the city of Malmö, Sweden (ID 45). This event was much more hazardous than that experienced a few years before, with precipitation return periods on average of 135 and 24 years, respectively, for 6 h duration 31 . The largest 6 h precipitation measured at one of nine stations during the 2014 event corresponded to a return period of 300 years. The combined sewage system present in the more densely populated areas of the city was overwhelmed, leading to extensive basement flooding in 2014 (ref.  31 ). The direct monetary damage was about €66 million as opposed to €6 million in the first event. An unprecedented drought occurred in the Cape Town metropolitan area of South Africa, in 2015–2018 (ID 44). The drought was much longer (4 years) than that experienced previously in 2003–2004 (2 years). Although the Berg River Dam had been added to the city’s water supply system in 2009, and local authorities had developed various strategies for managing water demands (for example, water restrictions, tariff increases, communication campaign), the second event caused a much higher direct impact of about €180 million 32 because the water reserves were reduced to virtually zero.

Even though it is known that vulnerability reduction plays a key role in reducing risk, our paired-event cases reveal that when the hazard of the second event was higher than the first, a reduction in vulnerability alone was often not sufficient to reduce the impact of the second event to less than that of the first. Our analysis of drivers of impact change reveals the importance of reducing hazard, exposure and vulnerability to achieve an effective impact reduction (Fig. 2 ). Although previous studies have attributed a high priority to vulnerability reduction 17 , 21 , the importance of considering all three drivers identified here may reflect the sometimes limited efficiency of management decisions, resulting in unintended consequences. For example, levee construction aiming at reducing hazards may increase exposure through encouraging settlements in floodplains 33 , 34 . Similarly, construction of reservoirs to abate droughts may enhance exposure through encouraging agricultural development and thus increase water demand 35 , 36 .

Events that are much more hazardous than preceding events (termed unprecedented here) seem to be difficult to manage; in almost all the cases considered they led to increased impact (Fig. 3 ). This finding may be related to two factors. First, large infrastructure such as levees and water reservoirs play an important role in risk management. These structures usually have an upper design limit up to which they are effective but, once a threshold is exceeded, they become ineffective. For example, the unprecedented pluvial flood in 2014 in Malmö, Sweden (ID 45) exceeded the capacity of the sewer system 31 and the unprecedented drought in Cape Town (ID 44) exceeded the storage water capacity 37 . This means that infrastructure is effective in preventing damage during events of a previously experienced magnitude, but often fails for unprecedented events. Non-structural measures, such as risk-aware land-use planning, precautionary measures and early warning, can help mitigate the consequences of water infrastructure failure in such situations 21 , but a residual risk will always remain. Second, risk management is usually implemented after large floods and droughts, whereas proactive strategies are rare. Part of the reason for this behaviour is a cognitive bias associated with the rarity and uniqueness of extremes, and the nature of human risk perception, which makes people attach a large subjective probability to those events they have personally experienced 38 .

On the other hand, two case studies were identified in which impact was reduced despite an increase in hazard (Box 1 ). An analysis of these case studies identifies three success factors: (1) effective governance of risk and emergency management, including transnational collaboration such as in the Danube case; (2) high investments in structural and non-structural measures; and (3) improved early warning and real-time control systems such as in the Barcelona case. We believe there is potential for more universal application of these success factors to counteract the current trend of increasing impacts associated with climate change 3 . These factors may also be effective in the management of unprecedented events, provided they are implemented proactively.

The concept of paired events aims at comparing two events of the same hazard type that occurred in the same area 21 to learn from the differences and similarities. This concept is analogous to paired catchment studies, which compare two neighbouring catchments with different vegetation in terms of their water yield 39 . Our study follows the theoretical risk framework that considers impact as a result of three risk components or drivers 3 : hazard, exposure and vulnerability (Extended Data Fig. 4 ). Hazard reflects the intensity of an event, such as a flooded area or drought deficit—for example, measured by the standardized precipitation index. Exposure reflects the number of people and assets in the area affected by the event. Consequently, the change in exposure between events is influenced by changes in the population density and the assets in the affected area (socioeconomic developments), as well as by changes in the size of the affected area (change of hazard). Vulnerability is a complex concept, with an extensive literature from different disciplines on how to define, measure and quantify it 13 , 40 , 41 , 42 . For instance, Weichselgartner 43 lists more than 20 definitions of vulnerability, and frameworks differ quite substantially—for example, in terms of integration of exposure into vulnerability 11 or separating them 3 . Reviews and attempts to converge on the various vulnerability concepts stress that vulnerability is dynamic and that assessments should be conducted for defined human–environment systems at particular places 12 , 44 , 45 . Every vulnerability analysis requires an approach adapted to its specific objectives and scales 46 . The paired event approach allows detailed context and place-based vulnerability assessments that are presented in the paired event reports, as well as comparisons across paired events based on the indicators-of-change. The selection of sub-indicators for the characterization of vulnerability is undertaken with a particular focus on temporal changes at the same place. All three drivers—hazard, exposure and vulnerability—can be reduced by risk-management measures. Hazard can be reduced by structural measures such as levees or reservoirs 19 , exposure by risk-aware regional planning 20 and vulnerability by non-structural measures, such as early warning 21 .

Our comparative analysis is based on a novel dataset of 45 paired events from around the world, of which 26 event pairs are floods and 19 are droughts. The events occurred between 1947 and 2019, and the average period between the two events of a pair is 16 years. The number of paired events is sufficiently large to cover a broad range of hydroclimatic and socioeconomic settings around the world and allows differentiated, context-specific assessments on the basis of detailed in situ observations. Flood events include riverine, pluvial, groundwater and coastal floods 47 , 48 , 49 , 50 . Drought events include meteorological, soil moisture and hydrological (streamflow, groundwater) droughts 51 . The rationale for analysing floods and droughts together is based on their position at the two extremes of the same hydrological cycle, the similarity of some management strategies (for example, warning systems, water reservoir infrastructure), potential trade-offs in the operation of the same infrastructure 52 and more general interactions between these two risks (for example, water supply to illegal settlements that may spur development and therefore flood risk). There may therefore be value in management communities learning from each other 18 .

The dataset comprises: (1) detailed review-style reports about the events and key processes between the events, such as changes in risk management (open access data; Data Availability statement); (2) a key data table that contains the data (qualitative and quantitative) characterizing the indicators for the paired events, extracted from individual reports (open access data); and (3) an overview table providing indicators-of-change between the first and second events (Supplementary Table 3 ). To minimize the elements of subjectivity and uncertainty in the analysis, we (1) used indicators-of-change as opposed to indicators of absolute values, (2) calculated indicators from a set of sub-indicators (Supplementary Table 1 ) and (3) implemented a quality assurance protocol. Commonly, more than one variable was assessed per sub-indicator (for example, flood discharges at more than one stream gauge, or extreme rainfall at several meteorological stations). A combination or selection of the variables was used based on hydrological reasoning on the most relevant piece of information. Special attention was paid to this step during the quality assurance process, drawing on the in-depth expertise on events of one or more of our co-authors. The assignment of values for the indicators-of-change, including quality assurance, was inspired by the Delphi Method 53 that is built on structured discussion and consensus building among experts. The process was driven by a core group (H.K., A.F.V.L., K. Schröter, P.J.W. and G.D.B.) and was undertaken in the following steps: (1) on the basis of the detailed report, a core group member suggested values for all indicators-of-change for a paired event; (2) a second member of the core group reviewed these suggestions; in case of doubt, both core group members rechecked the paired event report and provided a joint suggestion; (3) all suggestions for the indicators-of-change for all paired events were discussed in the core group to improve consistency across paired events; (4) the suggested values of the indicators-of-change were reviewed by the authors of the paired-event report; and finally (5), the complete table of indicators-of-change (Supplementary Table 3 ) was reviewed by all authors to ensure consistency between paired events. Compound events were given special consideration, and the best possible attempt was made to isolate the direct effects of floods and droughts from those of concurrent phenomena on hazard, exposure and impact, based on expert knowledge of the events of one or more of the co-authors. For instance, in the course of this iterative process it became clear that fatalities during drought events were not caused by a lack of water, but by the concurrent heatwave. It was thus decided to omit the sub-indicator ‘fatalities’ in drought impact characterization. The potential biases introduced by compound events were further reduced by the use of the relative indicators-of-change between similar event types with similar importance of concurrent phenomena.

The indicator-of-change of impact is composed of the following sub-indicators: number of fatalities (for floods only), direct economic impact, indirect impact and intangible impact (Supplementary Table 1 ). Flood hazard is composed of the sub-indicators precipitation/weather severity, severity of flood, antecedent conditions (for pluvial and riverine floods only), as well as the following for coastal floods only: tidal level and storm surge. Drought hazard is composed of the duration and severity of drought. Exposure is composed of the two sub-indicators people/area/assets exposed and exposure hotspots. Vulnerability is composed of the four sub-indicators lack of awareness and precaution, lack of preparedness, imperfect official emergency/crisis management and imperfect coping capacity. Indicators-of-change, including sub-indicators, were designed such that consistently positive correlations with impact changes are expected (Supplementary Table 1 ). For instance, a decrease in 'lack of awareness' leads to a decrease in vulnerability and is thus expected to be positively correlated with a decrease in impacts. Management shortcomings are characterized by problems with water management infrastructure and non-structural risk management shortcomings, which means that non-structural measures were not optimally implemented. These sub-indicators were aggregated into indicators-of-change for impact, hazard, exposure, vulnerability and management shortcomings, to enable a consistent comparison between flood and drought paired events. This set of indicators is intended to be as complementary as possible, but overlaps are hard to avoid because of interactions between physical and socioeconomic processes that control flood and drought risk. Although the management shortcoming indicator is primarily related to the planned functioning of risk management measures, and hazard, exposure and vulnerability primarily reflect the concrete effects of measures during specific events, there is some overlap between the management shortcoming indicator and all three drivers. Supplementary Table 1 provides definitions and examples of description or measurement of sub-indicators for flood and drought paired events.

The changes are indicated by −2/2 for large decrease or increase, −1/1 for small decrease or increase and 0 for no change. In the case of quantitative comparisons (for example, precipitation intensities and monetary damage), a change of less than around 50% is usually treated as a small change and above approximately 50% as a large change, but always considering the specific measure and paired events. Supplementary Table 2 provides representative examples from flood and drought paired events showing how differences in quantitative variables and qualitative information between the two events of a pair correspond to the values of the sub-indicators, ranging from large decrease (−2) to large increase (+2). We assume that an event is unprecedented in a subjective way—that is, it has probably not been experienced before—if the second event of a pair is much more hazardous than the first (hazard indicator-of-change +2).

Spearman’s rank correlation coefficients are calculated for impact, drivers and management shortcomings, separated for flood and drought paired events. Despite the measures taken to minimize the subjectivity and uncertainty of indicator assignment, there will always be an element of subjectivity. To address this, we carried out a Monte Carlo analysis (1,000 iterations) to test the sensitivity of the results when randomly selecting 80% of flood and drought paired events. For each subsample correlation, coefficients and P  values were calculated to obtain a total of 1,000 correlation and 1,000  P  value matrices. The 25th and 75th quantiles of the correlation coefficients and P  values were calculated separately (Extended Data Fig. 3 ).

Data availability

The dataset containing the individual paired event reports, the key data table and Supplementary Tables 1 – 3 are openly available via GFZ Data Services ( https://doi.org/10.5880/GFZ.4.4.2022.002 ).  Source data are provided with this paper.

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Acknowledgements

The presented work was developed by the Panta Rhei Working Groups 'Changes in flood risk' and 'Drought in the Anthropocene' within the framework of the Panta Rhei Research Initiative of the International Association of Hydrological Sciences. We thank the Barcelona Cicle de l’Aigua S.A., Barcelona City Council, Environment Agency (United Kingdom), Länsförsäkringar Skåne, Steering Centre for Urban Flood Control Programme in HCMC (Vietnam), VA SYD and the West Berkshire Council (United Kingdom) for data. The work was partly undertaken under the framework of the following projects: Alexander von Humboldt Foundation Professorship endowed by the German Federal Ministry of Education and Research (BMBF); British Geological Survey’s Groundwater Resources Topic (core science funding); C3-RiskMed (no. PID2020-113638RB-C22), financed by the Ministry of Science and Innovation of Spain; Centre for Climate and Resilience Research (no. ANID/FONDAP/15110009); CNES, through the TOSCA GRANT SWHYM; DECIDER (BMBF, no. 01LZ1703G); Deltares research programme on water resources; Dutch Research Council VIDI grant (no. 016.161.324); FLOOD (no. BMBF 01LP1903E), as part of the ClimXtreme Research Network. Funding was provided by the Dutch Ministry of Economic Affairs and Climate; Global Water Futures programme of University of Saskatchewan; GlobalHydroPressure (Water JPI); HUMID project (no. CGL2017-85687-R, AEI/FEDER, UE); HydroSocialExtremes (ERC Consolidator Grant no. 771678); MYRIAD-EU (European Union’s Horizon 2020 research and innovation programme under grant agreement no. 101003276); PerfectSTORM (no. ERC-2020-StG 948601); Project EFA210/16 PIRAGUA, co-founded by ERDF through the POCTEFA 2014–2020 programme of the European Union; Research project nos. ANID/FSEQ210001 and ANID/NSFC190018, funded by the National Research and Development Agency of Chile; SECurITY (Marie Skłodowska-Curie grant agreement no. 787419); SPATE (FWF project I 4776-N, DFG research group FOR 2416); the UK Natural Environment Research Council-funded project Land Management in Lowland Catchments for Integrated Flood Risk Reduction (LANDWISE, grant no. NE/R004668/1); UK NERC grant no. NE/S013210/1 (RAHU) (W.B.); Vietnam National Foundation for Science and Technology Development under grant no. 105.06-2019.20.; and Vietnam National University–HCMC under grant no. C2018-48-01. D.M. and A. McKenzie publish with the permission of the Director, British Geological Survey. The views expressed in this paper are those of the authors and not the organizations for which they work.

Open access funding provided by Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ.

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GFZ German Research Centre for Geosciences, Section Hydrology, Potsdam, Germany

Heidi Kreibich, Kai Schröter, Nivedita Sairam, Max Steinhausen & Sergiy Vorogushyn

Institute for Environmental Studies, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands

Anne F. Van Loon, Philip J. Ward, Maurizio Mazzoleni, Marlies H. Barendrecht, Anaïs Couasnon & Marleen C. de Ruiter

Leichtweiss Institute for Hydraulic Engineering and Water Resources, Division of Hydrology and River basin management, Technische Universität Braunschweig, Braunschweig, Germany

Kai Schröter

Department of Civil and Environmental Engineering, University of Houston, Houston, TX, USA

Guta Wakbulcho Abeshu & Hong-Yi Li

Lomonosov Moscow State University, Moscow, Russia

Svetlana Agafonova, Natalia Frolova, Maxim Kharlamov, Maria Kireeva & Alexey Sazonov

University of California, Irvine, CA, USA

Amir AghaKouchak & Laurie S. Huning

Department of Civil Engineering, Istanbul Technical University, Istanbul, Turkey

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Center for Climate and Resilience Research, Santiago, Chile

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Department of Civil Engineering, Universidad de La Frontera, Temuco, Chile

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Global Institute for Water Security, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Laila Balkhi, Hayley Carlson & Saman Razavi

LEGOS, Université de Toulouse, CNES, CNRS, IRD, UPS, Toulouse, France

Sylvain Biancamaria

Department of Groundwater Management, Deltares, Delft, the Netherlands

Liduin Bos-Burgering

School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK

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Agency for the Assessment and Application of Technology, Jakarta, Indonesia

Yus Budiyono

Department of Civil and Environmental Engineering, Imperial College London, London, UK

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Department of Civil Engineering, Beykent University, Istanbul, Turkey

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Graduate School, Istanbul Technical University, Istanbul, Turkey

Faculty of Environment and Natural Resources, University of Freiburg, Freiburg, Germany

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Geographical Sciences, University of Bristol, Bristol, UK

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Cabot Institute, University of Bristol, Bristol, UK

Gemma Coxon, Jim Freer & Thorsten Wagener

Department of Agriculture, Hellenic Mediterranean University, Iraklio, Greece

Ioannis Daliakopoulos

Université de Lorraine, LOTERR, Metz, France

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CNR-IRPI, Research Institute for Geo-Hydrological Protection, Cosenza, Italy

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University of Saskatchewan, Centre for Hydrology, Canmore, Alberta, Canada

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Animesh K. Gain

Department of Economics, Ca’ Foscari University of Venice, Venice, Italy

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Pontificia Bolivariana University, Faculty of Civil Engineering, Bucaramanga, Colombia

Diego A. Guzmán

California State University, Long Beach, CA, USA

Laurie S. Huning

Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Palaeoclimate Dynamics Group, Bremerhaven, Germany

Monica Ionita & Viorica Nagavciuc

Emil Racovita Institute of Speleology, Romanian Academy, Cluj-Napoca, Romania

Monica Ionita

Forest Biometrics Laboratory, Faculty of Forestry, Ștefan cel Mare University, Suceava, Romania

Water Problem Institute Russian Academy of Science, Moscow, Russia

Maxim Kharlamov & Alexey Sazonov

Faculty of Environment, University of Science, Ho Chi Minh City, Vietnam

Dao Nguyen Khoi & Pham Thi Thao Nhi

Environment Agency, Bristol, UK

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School of Chemical and Environmental Engineering, Technical University of Crete, Chania, Greece

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Servicio Nacional de Meteorología e Hidrología del Perú, Lima, Peru

Waldo Lavado-Casimiro

Department of Applied Physics, University of Barcelona, Barcelona, Spain

María Carmen LLasat

Water Research Institute, University of Barcelona, Barcelona, Spain

British Geological Survey, Wallingford, UK

David Macdonald & Andrew McKenzie

Centre of Natural Hazards and Disaster Science, Uppsala, Sweden

Johanna Mård, Elisa Savelli & Giuliano Di Baldassarre

Department of Earth Sciences, Uppsala University, Uppsala, Sweden

Civil and Environmental Engineering, The Pennsylvania State University, State College, PA, USA

Alfonso Mejia

Escola de Engenharia de Sao Carlos, University of São Paulo, São Paulo, Brasil

Eduardo Mario Mendiondo & Felipe Augusto Arguello Souza

Department of Water Resources & Delta Management, Deltares, Delft, the Netherlands

Marjolein Mens

Trelleborg municipality, Trelleborg, Sweden

Shifteh Mobini

Department of Water Resources Engineering, Lund University, Lund, Sweden

Shifteh Mobini & Johanna Sörensen

University of Potsdam, Institute of Environmental Science and Geography, Potsdam, Germany

Guilherme Samprogna Mohor & Thorsten Wagener

University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, Hanoi, Vietnam

Thanh Ngo-Duc

Institute for Environment and Resources, Vietnam National University Ho Chi Minh City, Ho Chi Minh City, Vietnam

Thi Thao Nguyen Huynh, Hong Quan Nguyen & Thi Van Thu Tran

Institute for Circular Economy Development, Vietnam National University Ho Chi Minh City, Ho Chi Minh City, Vietnam

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Observatori de l’Ebre, Ramon Llull University – CSIC, Roquetes, Spain

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School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Saman Razavi

Department of Civil, Geological and Environmental Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Dipartimento di Ingegneria Civile, Edile e Ambientale, Sapienza Università di Roma, Rome, Italy

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University of Applied Sciences, Magdeburg, Germany

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Earth and Environmental Systems Institute, The Pennsylvania State University, State College, PA, USA

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Wiwiana Szalińska & Tamara Tokarczyk

Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China

Qiuhong Tang & Yueling Wang

Department of Hydraulic Engineering, Tsinghua University, Beijing, China

Fuqiang Tian

Royal Botanical Gardens Kew, London, UK

Carolina Tovar

KWR Water Research Institute, Nieuwegein, the Netherlands

Marjolein H. J. Van Huijgevoort

Department of Physical Geography, Utrecht University, Utrecht, the Netherlands

Michelle T. H. van Vliet

Civil Engineering, University of Bristol, Bristol, UK

Thorsten Wagener & Doris E. Wendt

School of Natural Resources, University of Nebraska-Lincoln, Lincoln, NE, USA

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School of Geography and Ocean Science, Nanjing University, Nanjing, China

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Contributions

H.K. initiated the study and led the work. H.K., A.F.V.L., K. Schröter, P.J.W. and G.D.B. coordinated data collection, designed the study and undertook analyses. All co-authors contributed data and provided conclusions and a synthesis of their case study (the authors of each paired event report were responsible for their case study). M. Mazzoleni additionally designed the figures, and he and N.S. contributed to the analyses. H.K., G.D.B., P.J.W., A.F.V.L., K. Schröter and G.D.B. wrote the manuscript with valuable contributions from all co-authors.

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Correspondence to Heidi Kreibich .

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Extended data figures and tables

Extended data fig. 1 location of flood and drought paired events coloured according to their indicators-of-change..

a , Change in hazard; b , change in exposure; c , change in vulnerability and d , change in management shortcomings.

Extended Data Fig. 2 Parallel plot of paired events with the same hazard of both events.

The hazard change is zero for all shown paired events. The lines show how the different combinations of indicators-of-change result in varying changes in impacts. Small offsets within the grey bars of the indicator-of-change values enable the visualization of all lines.

Extended Data Fig. 3 Results of the sensitivity analyses.

a–d Correlation matrix of indicators-of-change for 25th and 75th quantiles of correlation coefficients and p-values, respectively ( a , c ) and 75th and 25th quantiles of correlation coefficients and p-values, respectively ( b , d ) separate for flood and drought paired events. Quantiles of correlation coefficients and p-values were calculated separately; colours of squares indicate Spearman’s rank correlation coefficients; sizes of squares indicates p-values. Fig. 2a, c is added to the right to ease comparison.

Extended Data Fig. 4 Theoretical framework used in this study (adapted from IPCC 3 ).

This theoretical risk framework considers impact as a result of three risk components or drivers: hazard, exposure and vulnerability, which in turn are modified by management.

Supplementary information

Supplementary tables.

Supplementary Tables 1–3.

Source Data Fig. 1.

Source data fig. 2., source data fig. 3., source data extended data fig. 1., source data extended data fig. 2., source data extended data fig. 3., rights and permissions.

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Kreibich, H., Van Loon, A.F., Schröter, K. et al. The challenge of unprecedented floods and droughts in risk management. Nature 608 , 80–86 (2022). https://doi.org/10.1038/s41586-022-04917-5

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Article contents

Flood resilient construction and adaptation of buildings.

  • David Proverbs David Proverbs Birmingham City University
  •  and  Jessica Lamond Jessica Lamond University of the West of England, Bristol
  • https://doi.org/10.1093/acrefore/9780199389407.013.111
  • Published online: 19 December 2017

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.

  • flood resistance

Introduction

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.

Historical Developments in Flood Risk Management and the Built Environment

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.

Developments in Construction and Building Technologies

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.

flood control case study

Figure 1. (a) Traditional stilt house in on a canal near the Chao Phraya River in Bangkok, Thailand (by Ernie & Katy Newton Lawley from Bowie, MD, USA—Flickr, CC BY 2.0); (b) Flooded Queenslander style architecture in Goondiwindi, Queensland, 1921 (archive of State Library of Queensland) (c) Raised Creole Style building, downtown river corner of Esplanade & Villere Streets, New Orleans (by Infrogmation of New Orleans—Flickr, CC BY 2.0); (d) street of raised houses in Calabar, Nigeria

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.

Avoidance Technology

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 Technology

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.

Water Entry Technology

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 ).

Repair and Reinstatement of Flood-Damaged Property

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 State of the Art

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.

The evidence is underpinned by knowledge of the potential impact of flooding on buildings as outlined in Kelman and Spence ( 2004 ), an understanding of properties and limitations of construction materials, structural engineering principles, and the science of water transport and flood characteristics. Nadal et al. ( 2006 ) summarizes the state of knowledge based on a combination of theoretical and empirical evidence. It is clear that construction elements, furnishings, and occupants all need to be considered from the substructure to provisions. As Kelman and Spence ( 2004 ) observed, the main flood actions on building components are:

Hydrostatic (lateral pressure and capillary rise)

Hydrodynamic (velocity, waves, turbulence)

Erosion (scour under buildings, building fabric)

Buoyancy (lifting the building)

Debris (items in the water colliding with the building)

Nonphysical actions (chemical, nuclear, biological)

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.

Table 1. USACE Flood Proofing Matrix

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 ).

flood control case study

Figure 4. Examples of raised construction in England

Property Level Flood Resilience Technology and Design

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).

Research on Water Resistant and Resilient Materials

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.

flood control case study

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.

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.

Resilient Repair and Considerate Reinstatement

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.

Future Developments in Flood Resilient Construction and Adaptation

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.

Conclusions

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.

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  • v.50(8); 2021 Aug

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How does a nature-based solution for flood control compare to a technical solution? Case study evidence from Belgium

Francis turkelboom.

1 Research team Nature & Society, Institute of Nature and Forest Research (INBO), Havenlaan 88 bus 73, 1000 Brussel, Belgium

Rolinde Demeyer

2 Op de Groei, Rijweg 124, 3020 Herent, Belgium

Liesbet Vranken

3 Agricultural and Resource Economics, Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200 E - Box 2411, 3001 Leuven Heverlee, Belgium

Piet De Becker

4 Research team Environment & Climate, Institute of Nature and Forest Research (INBO)., Havenlaan 88 bus 73, 1000 Brussel, Belgium

Filip Raymaekers

5 Section Demer, Dijle and Maas, Flanders Environment Agency (VMM), VAC Dirk Bouts, Diestsepoort 6 bus 73, 3000 Leuven, Belgium

Lieven De Smet

Associated data.

The strategy of reconnecting rivers with their floodplains currently gains popularity because it not only harnesses natural capacities of floodplains but also increases social co-benefits and biodiversity. In this paper, we present an example of a successfully implemented nature-based solution (NBS) in the Dijle valley in the centre of Belgium. The research objective is to retrospectively assess cost and benefit differences between a technical solution (storm basins) and an alternative NBS, here the restoration of the alluvial floodplain. The method is a comparative social cost–benefit analysis. The case study analysis reveals similar flood security, lower costs, more ecosystem services benefits and higher biodiversity values associated with the NBS option in comparison to the technical alternative. However, the business case for working with NBS depends substantially on the spatial and socio-ecological context. Chances for successful NBS implementation increase in conditions of sufficient space to retain flood water, when flood water is of sufficient quality, and when economic activity and housing in the floodplain is limited.

Supplementary Information

The online version of this article contains supplementary material available at (10.1007/s13280-021-01548-4).

Introduction

For centuries, floodplains have been modified and rivers regulated by the construction of channels and dams to enable agricultural production, to protect settlements against flooding, to enhance navigation or to produce energy (Buijse et al. 2002 ; Posthumus et al. 2010 ). In Europe and North America, up to 90% of floodplains are already ‘cultivated’ and, therefore, functionally extinct (Tockner and Stanford 2002 ). In Germany, a survey of the 79 largest rivers showed that only around 35% of the morphological floodplains still serve for natural flood retention. A further decline between 2010 and 2015 was mainly caused by an increase in settlements and transport infrastructure (Walz et al. 2019 ). While simplification of formerly complex, irregular banks and beds, into straight, uniform (shipping) channels have led to generally more uniform flow conditions, constant water tables and sharply defined embankments, they have given rise to several unintended challenges for society, for instance exacerbating flood risks, diminishing water quality, decreased ecological functioning, biodiversity loss and loss of cultural services related to rivers (such as mental connection to rivers, fishing, water supply, swimming and other recreation activities) (Malmqvist and Rundle 2002 ; Liao 2014 ; Kondolf and Pinto 2016 ; Wantzen 2016 ).

Well-designed, nature-based solutions (NBS, Nesshöver et al. 2017 ; EC 2020 ) are suggested as sustainable ways for addressing water-related risks, as they need less maintenance, are more cost effective, create co-benefits for people, and support high levels of biological diversity today and in the future (Opperman et al. 2009 ; Halbac-Cotoara-Zamfir 2019 ; Albert et al. 2019 ). Floodplain restoration is an example of NBS that can make a significant contribution to a more effective flood risk management, to strengthen multifunctionality of the river landscape and to increase the supply of ecosystem services, although floodplain restoration might not completely eliminate floodings in an era of climate change (Schindler et al. 2014 ; Kiedrzynska et al. 2015 ). However, substantial knowledge gaps regarding NBS in river landscapes still exist, particularly related to planning and implementation practices, effectiveness and monitoring, as well as on governance aspects (Albert et al., 2019 ). Therefore, these authors propose a research and experimentation agenda focussing on the following: (1) effectiveness of NBS, including assessments of the outcomes of both NBS and technical alternatives, (2) co-benefits and costs of NBS using multimetric indicators such as recreation potential, water retention and biodiversity and (3) useful approaches for informed co-design of NBS. In this paper, we present a successful implementation of a NBS in a river landscape in Belgium, where the river was reconnected to its alluvial floodplain in order to ensure flood protection to a nearby city. The objectives of this paper are to assess retrospectively the differences in the costs and benefits between a technical solution for flood control and an alternative NBS, and to describe the process that preceded the decision-making. Based on this experience, implications for policy and floodplain management are formulated.

Case study description

The study area south of leuven.

The catchment of the Dijle river is situated in the central part of Belgium, draining part of a fertile silt plateau (about 100 m above sea level). The river has dug itself some 60–70 m deep in this plateau forming a marked 1-kilometre-wide valley. The study area is situated upstream (south) of the city of Leuven and has a surface of about 800 ha (Fig.  1 ). The Dijle river is strongly meandering (sinuosity ~1.4–1.8) where the meanders spontaneously move each year 1–1.2 m downstream (Vandaele et al. 2002 ).

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Situation map of the study area with technical solution (left) and nature-based solution (right) for flood risk prevention of the city of Leuven (Belgroma 1990 ; flooded floodplain area mapped in situ by INBO during 1990-1994)

Until the 1980’s, the valley was mainly used for agricultural purposes (such as hay making, livestock grazing and poplar tree cultivation) and leisure houses. As the area harbours a high level of biodiversity, including rare species and habitats, the entire valley and surroundings was designated as a “nature area” in 1975 by the Regional Destination Plan. In 1979 the complete valley floor of the Dijle was designated European Bird Directive area, and in 1992 80% of the surface was also designated European Habitat Directive area. The protected habitats are (followed by their Natura 2000 code, European Commission 2013 ): eutrophic lakes with Magnopotamion vegetation (3150), Alopecurion grasslands (6510), Filipendulion tall herb vegetation (6430), mires (7140) and Alion forests/alder carr (91E0). The valley is nowadays covered with grasslands, derelict poplar tree plantations and a limited surface of natural forests, ponds and some drinking water extraction sites. The southern part is called ‘Doode Bemde’ and is a nature reserve managed by a local nature conservation NGO (Vrienden van Heverleebos en Meerdaalwoud), while the northern part is a nature reserve called ‘Vijvers van Oud-Heverlee’, managed by the Agency of Nature and Forestry (ANB). The management of the river itself is under the responsibility of the Flanders Environment Agency (VMM). Residential areas are concentrated along the valley sides, while cropland is situated at the fertile western plateau (Fig.  2 ).

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Birdseye view looking north of the Dijle valley (Doode Bemde), central Belgium. Photo: Yves Adams/Vildaphoto

Flood protection of Leuven: A short historical record

In early medieval times, due to large scale deforestation, the hydrological regime of the river shifted from a fairly constant base flow river into an alluvial (i.e. frequently flooding) river. Heavy rain storms erode soil with associated nutrients, mainly from the western plateaus and slopes, and runoff water transports it towards the valley bottom. Peak discharges brought huge amounts of erosion sediments into the valley. This allowed agricultural practices in the valley, but also increased downstream flood risks. Until 1990 the river water quality used to be very poor with extremely elevated nutrient levels and a ditto load of heavy metals (e.g. Cu, Pb). A decennium of sewer constructions and connecting local sewers to sewage water treatment plants increased the water quality significantly. Similar historical evolutions have occurred all over Western Europe (Huybrechts 1989 ; Notebaert 2009 ).

The process of decision-making regarding flood protection of Leuven is described by Craps et al. ( 2005 ), and complemented with information from interviews with two protagonists.

After the second world war, the urban sprawl of the city of Leuven expanded in the natural floodplain of the Dijle. As a result, flood risk increased sharply. Among the assets at risk are 125 ha of urban area (one third of the buildings in the historic city of Leuven), a university campus, a hospital, major roads and other critical infrastructure (Fig.  3 ). During the 1970’s and 80’s, public water administrations were under rising pressure to come up with a plan to avoid these growing economic and social risks. The interventions should protect Leuven against a one in hundred years’ event. As the flow capacity of the Dijle river within the city centre was presumed to be limited to 21 m 3 /s, the excess volume of a unit hydrograph of a T100 event (1 200 000 m 3 ) had to be stored upstream in the Dijle valley. Discharges more than 25 m 3 /s occur every 2–10 years (Belgroma 1990 , 1996 ). At that time, flood defence designs were based on static calculations with design storms and ignored natural flood conditions. Based on these model results, water managers formulated a plan to install storm basins that can temporarily store the excess volume of river water.

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Extent of the floodable area in the historic city centre of Leuven and the university campus south of the city (after Belgroma 1990 )

During the 1980’s a nature conservation NGO started to create a nature reserve in the valley, and considered these plans as a threat. Government agencies at that time did not recognise these environmentalists as a legitimate party and ignored them. The environmentalists tried to increase public awareness by contacting newspapers, radio and television programs, organizing guided walks and public hearings, and by motivating farmers and recreationists to submit complaints. In this way, they were able to increase the pressure on the decision makers. In 1990 the environmentalists started to incorporate flood prevention aspects in their nature conservation plans, and tried to convince the administrations that a different, nature-oriented flood control approach was an equally valuable solution. In 1993, due to the newly adopted legislation on environmental impact assessment, the administration was required to look for an alternative option that was less damaging for the environment. In the same year, hydrodynamic models were used for the first time in Flanders, which could also assess floods occurring in natural floodplains. The opposing parties gradually reached a consensus due to a number of reasons: the new European natural environment safeguarding directives, fact-based discussions based on new models, the fact that the NGO became recognized as a legitimate discussion partner, and active lobbying of the NGO with the responsible decision makers. After a long decision-making process (lasting about 25 years), a conclusion was reached in 2000, when the technical solution (based on the construction of storm basins) was abandoned in favour of a ‘nature-based solution’ (based on restoration of the alluvial floodplain, plus one emergency storm basin). The implementation of the NBS took 5 years (2000–2005).

Comparison of two flood risk management solutions

In the 1990s, two approaches for flood damage protection were considered in order to guarantee the safety against floods, with a return period of once in 100 years. The most fundamental difference between the technical solution and the NBS is the strategy to store the excess peak discharge water volume.

Technical solution: Storm basins approach

In the technical solution downstream flooding is avoided by storing the excess flood water at peak discharges in storm basins, where the water is retained for a couple of days before being gradually released back into the river. This technical solution would require new measures on top of existing, recurrent interventions (Belgroma 1990 , 1996 ):

Table 1

Technical characteristics of the two alternative floodwater solutions to protect the city of Leuven (Belgroma 1996 ; flooded floodplain area mapped in situ by INBO during 1990-1994)

  • Maintenance works on the river channel would be continued, to avoid turbulent flow and river bank erosion, and consequent financial claims from land owners. Since this smoothening of the river channel reduced the channel roughness, peak discharge water volumes were transported very fast through the river channel, completely surpassing the alluvial floodplain, and thus increasing peak discharges and the consequent downstream flood risks.
  • Regular maintenance of drainage channels would be continued and a siphon would be maintained. These interventions aimed to lower the groundwater level for the predominantly agricultural use of the floodplain, especially in areas with upward groundwater seepage flow.

Nature-based solution (NBS): Restoration of the alluvial floodplain

The main idea of this strategy is to restore a more ‘natural’ flooding regime in the floodplain and to restore the alluvial floodplain ecosystem (Fig.  1 ). This NBS requires four main (non)-interventions:

  • Making use of the storage capacity of the entire natural floodplain: The microtopography of the alluvial floodplain is a crucial feature for this solution. Due to regular floodings since the early medieval times, the river has formed natural elevated banks and lower lying floodplain depressions. When the river channel reaches bank-full (and higher) discharges, the water will overflow the river banks and inundate the floodplain depressions along the entire length of the river simultaneously (± 12 km). Consequently, flood water including the sediment load will spread out over a large part of the natural floodplain.
  • ‘Zero management’ of the river and its banks: This means that the watercourse through the Doode Bemde is no longer cleared from fallen trees, and the river banks are no longer mown. Fallen trees in the river increase the roughness of the watercourse. Consequently, bank-full discharge is reached increasingly sooner (or at lower discharges) and spontaneous floods in the floodplain are induced. Zero management of the river banks of the Dijle started in 1991, but took about a decade before the woody vegetation had the required beneficial impact on increased river bank roughness.
  • Reconnecting the Leigracht to the IJse (i.e. tributary of the Dijle) by removing a siphon (2002) inevitably led to higher groundwater levels, which is favourable for the restoration of groundwater-dependent vegetation types. In addition, this also helped to restore the relation between the river and its surrounding floodplain (La Rivière 2006 ).
  • Minimum infrastructure works (only on a limited number of locations at the fringes of the floodplain). Eventually, it was decided to include the Egenhoven storm basin as an ultimate emergency protection for floods in Leuven in the event of extreme rainfall south of Leuven (completed in 2005).

If the river roughness is high enough and the storage capacity of the natural floodplain depressions is large enough to store the excess stormwater volume, flood damage in the city of Leuven can be avoided. In other words, the flooding of the floodplain should start at a discharge level which is lower than the flow transport capacity of the river through the city centre.

Materials and methods

Comparative social cost–benefit analysis (cscba).

The method we selected to compare between a nature-based and a technical solution is a social cost–benefit analysis (SCBA) (OECD 2018 ). This is a project appraisal approach that enables to inform policy decision-making, by mapping all the costs and benefits for all parties concerned and weighing them against each other. In a classical SCBA, all possible costs and all possible benefits for both solutions are examined. Benefits represent an increase in human welfare or wellbeing, while costs a decrease in human welfare or wellbeing. In this study, we opted for a comparative SCBA where only those costs and benefits are considered for which differences between the two solutions were expected (Demeyer and Turkelboom 2013b ). However, comparison between these two solutions was not always evident. The current situation was considered as a representation for the NBS, for which we could use actual measured data. As the technical solution is a hypothetical approach, we therefore had to use modelled data, data of 20 years ago and expert estimates. To address the uncertainties of this approach, we often calculated low and high estimates. As some assumptions needed to be made, all the results were discussed and validated by an interdisciplinary expert group during 3 meetings. This group included experts from the Flemish Environment Agency (water management), Agency of Nature and Forestry (nature & biodiversity), Department of Environment (policy) and the Institute of Nature and Forest Research (ecohydrology). Our approach entailed 4 sequential steps (Fig.  4 ).

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Different steps of the comparative social cost–benefit analysis (cSCBA) to assess the differences in the costs and benefits between the technical solution and the alternative NBS for flood control

Step 1: Identification of benefit types

In order to identify the local-relevant benefits (ecosystem services (ES) and/or biodiversity), we interviewed 10 key informants who had a helicopter view (during 2013). To ensure a diversity of opinions, we looked for people with opposing views concerning the study area. The interviewees included representatives from a water management organization, drinking water company, nature protection agency, land development organization, service organization for municipalities, local nature NGO, hunters’ association, forest owners’ association, a local farmer and a kayak renting company owner. Respondents were asked to rank pictures of benefits according to their importance for the study area (potential scores: 3 (very important), 2 (medium important), 1 (bit important), 0 (neutral), − 1 (not desired)). The identified benefits were validated by the expert group, who also added some extra ES.

Step 2: Cause-effect analysis

As we wanted to apply a comparative SCBA, it was necessary to withhold only those benefits that respond differently to the two proposed flood management solutions. The tool to justify this selection was a cause-effect flowchart, which illustrates how the two flood management solutions are triggering different responses of ES and biodiversity. This chart was designed based on discussions with the expert group, and it was built in an iterative way.

Step 3: Quantitative/qualitative assessment of differential impacts

The difference in ES responses between both solutions were quantified based on the formulas used in the ‘Nature Value Explorer’ (NVE). The NVE is an online tool for assessing the impact of land use changes on ES in quantitative and monetary terms, based on the best available empirical knowledge in Flanders (Liekens et al. 2013 ). For those impacts for which NVE formulas were lacking, we referred to available empirical data, literature and expert judgement.

Step 4: Monetization of costs and benefits

Monetization is the conversion of the quantified effects into monetary terms. The investment and management costs of the two approaches were extracted from an EIA report (Arcadis 2012 ). To assess the differential impacts on ES, we used the NVE. When the obtained monetary values of cSCBA were prone to uncertainty, a high and low estimate were calculated. Another complication was that cost and benefits appear in different periods over time. To compare all present and future costs and benefits, they were converted to present time value (i.e. 2013). For this purpose, a discount rate of 4% was used (as proposed in the NVE).

As the NVE was not specific enough to determine the recreational value for both solutions, an online survey was drawn to elicit preferences of respondents between the technical solution and the NBS (Supplementary Material S1 ). For this purpose, we used the contingent valuation method (Mitchell and Carson 1989 ; Hanley et al. 2001 ). Respondents were presented with two choice cards with each two flood management approaches: one card contained the NBS and the technical solution, the other the NBS and the 1995 situation (Fig.  5 ). To avoid overload and possible drop-out of respondents, a third card (comparing 1995 situation with the technical solution) was not presented. Five features or attributes were used to describe the approaches: characteristics of river banks (% of banks with and without management, and % concrete dikes), biodiversity (high, medium, low), flooded area (natural flooding area, flooding in a storm basin with concrete dikes, no flooding area), water quality (good, medium, poor), landscape (natural landscape; natural landscape with presence of dikes; agricultural landscape with meadows and poplar trees). In the study we assumed that access to the area would be equal for the three management approaches. Next, respondents were asked about their willingness to pay for their preferred approach using double bounded dichotomous choice questions as the elicitation method (Perman et al. 2011 ). This amount would then have to be paid to a government body (e.g. as a kind of tax) to realize the chosen approach. Twelve starting bids that ranged from 5 euro to 60 euro were used for the first contingent valuation question and were randomized over the sample. In the follow-up question the amount was—depending on the answer on the first question—increased or decreased with 5 euros. At the end, we asked some questions about the socio-demographic background of the respondent. In total 332 people completed the survey, of whom 89% were familiar with the area (Coucke 2013 ).

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Example of a choice card, with two solutions (columns) and five features (rows)

In this section we discuss the differences between the costs and benefits of the technical solution and the NBS.

Identification of benefit types (Step 1)

The interviewed stakeholders identified the following functions as most important for the study area (in declining order): 1) habitat for (typical) animals and plants, 2) protection against floods, 3) clean water, 4) recreation (walking and cycling), and 5) experience of the landscape (i.e. aesthetic value, therapeutic effect, historical landscape). In addition, another 15 ES were positively evaluated. The expert group added two ES that were not considered by local stakeholders: carbon sequestration and air quality improvement. In total 20+ important benefits were identified.

Cause-effect analysis (Step 2)

However, not all these benefits needed to be assessed for this analysis, as we are only interested in those benefits which respond distinctively to the two flood management approaches. Via the cause-effect analysis, we found that there are three major intermediate controlling factors which respond differently to each management approach (Fig.  6 ):

  • The morphological landscape of the valley floor is mainly affected by the dikes and sluices and the management of the river banks.
  • Flood water characteristics: As in the technical solution the flood water is stored in a limited area, the flood water will be deeper (89, 10, 0 cm for T10 storms for the respective storm basins), the retention time shorter (2, 1, 0 days for T10 storms), and sediments will be mainly concentrated in the storm basins. In the NBS, the excess water volume is spread over a larger surface, resulting in a lower depth of the flood water (average 16 cm), a longer retention time (median: 2.5 days, range: 1–16, measured in Neerijse floodplains 2008–2012), and spread of the sediment over a larger area. This reduces the thickness of the sediment layer to millimetres instead of centimetres compared with the storm basin sedimentation rates (Belgroma 1996 ; De Becker and De Bie 2013 ).
  • Groundwater levels: In the technical solution, the drainage canals remain active and the water is drained faster via the cleared river, resulting in an overall lower groundwater level. The removal of the siphon in the NBS resulted in an increase of the groundwater level (average 9–17 cm higher for resp. high and low groundwater levels) (De Wilde et al. 2001 ; De Becker and De Bie 2013 ).

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Cause-effect flowchart showing the differential impacts of flood control approaches on ES delivery and biodiversity in the Dijle valley (blue blocks: controlling factors; green block: biodiversity; grey blocks: ES)

These controlling factors influence five ES, vegetation and biodiversity in distinct ways (Fig. 6 ).

Impacts of flood management approaches on ES and vegetation (Step 3)

Only those benefits which respond differently for the two solutions are assessed (Table  2 ):

Table 2

Comparative SCBA for floodwater management solutions in the Dijle. Positive values mean that NBS provides more ES (and euros) compared to the technical solution and vice versa

*: 4% discount rate over a time horizon of 30 years

Flood control : Modelling studies using the first generation hydrodynamic computer models showed that the storm basin and nature development approaches are equally capable of protecting Leuven against floods that occur once every 100 years (1 200 000 m 3 , Belgroma 1996 ). As both approaches offer similar protection against floods, this impact was not included in the cSCBA. Recent calculations of high-end climate change scenarios reveal that in the case of an extreme event, additional measures will have to be taken in the future to prevent increasing flood risks in the lower parts of the city (VMM 2014 ).

Water quality improvement via denitrification: Denitrification is the conversion of nitrate (NO3 - ) to nitrogen (N 2 ) by bacteria, which is released into the air. This contributes to improved water quality. An average nitrogen concentration of 5,7 mg N/l in river water and 2,5 mg N/l for ground water were used (De Wilde et al. 2001 ). Separate denitrification estimations were made for terrestrial ecosystems, areas with temporary flooding and running water. The NBS scored better as denitrification efficiency is higher in wet, terrestrial ecosystems (due to higher groundwater level: +20 cm), in flooded areas (due to larger flooded surface) and in running water (due to 10 cm higher water level and hence greater water volume). As a result, an extra 320 tons nitrogen is denitrified over a period of 30 years in the NBS.

Carbon sequestration in soil : The amount of C that is stored depends on land use and hydrology: wetting leads to a greater C stock, while drainage leads to less C storage in soils. The NBS results in an additional 542/554 tons C per year in the soil, due to higher groundwater levels (+20 cm) and a greater area of swamps and reed (+67/78 ha), compared to the technical solution. This amount is possibly an overestimation, as we had to rely on groundwater levels measured in one particular floodplain, where measures were taken to increase the groundwater level.

Vegetation and biodiversity is to a large extent determined by the management of the river banks, the flood water characteristics and the groundwater level (Fig.  6 ). Inside the storm basins of the technical solution, a high water column of standing water with the complete sediment load, would cover the vegetation with inches’ thick sediment cover. Since the Dijle was at that time heavily polluted and richly loaded with nutrients, the resulting vegetation in these storm basins would be of very limited ecological and aesthetic value. Outside the basins, the vegetation would be mainly influenced by the lower water table.

In the NBS, the increased groundwater level is the main source of shifts in vegetation types: Filipendulion tall herb vegetation (Natura2000 code 6430, e.g. meadowsweet) and Alopecurion grasslands (6510, e.g. meadow foxtail) increased in acreage (approx. +73 ha) predominantely at the expense (− 55 ha) of drier grassland types (crested dog’s-tail ( Cynosurion cristati) and tall oat grass ( Arrhenaterion) grasslands) (Demeyer and Turkelboom 2013a ; based on Belgroma 1996 ; De Wilde et al. 2001 ; De Nocker et al. 2006 ; De Becker and De Bie 2013 ). A survey indicated that the changes in the water regime between 1990s and 2019 have also led to an increase in species diversity: typical vegetation types who were present in the Dijle valley before the interventions have remained but shifted to higher grounds (e.g. tall sedge swamps (with Carex acuta & acutiformis) and marsh marigold ( Calthion palustris ) grasslands), while vegetation types of wetter conditions have emerged, with associated invertebrate and vertebrate species (De Becker and De Bie 2013 ; De Becker 2020 ). In addition, as the river channel in the NBS can move more freely, small beaches, eroding banks and steep, vegetation-free banks are formed. The latter are an ideal breeding place for e.g. kingfisher ( Alcedo atthis ). In the river itself, a natural pool-riffle pattern developed, which is favourable for aquatic biodiversity (La Rivière 2006 ). This is in line with other studies showing a greater species diversity in areas where a complete gradient of flood characteristics is present, compared to a river with technical structures and abrupt hydrological conditions (de Nooij et al. 2006 ; Pettifer and Kay 2011 ).

Carbon sequestration in vegetation: C storage in vegetation is mainly dependent on tree growth. For both approaches, there is no difference in forest area, but trees sequester more carbon on drier soils. As the technical solution results in drier soils (groundwater table 20 cm lower), an additional 87 tons of carbon per year are stored compared to the NBS.

Air quality: Vegetation has a positive impact on air quality, as leaves capture particulate matter. Particulate matter is responsible for many of the diseases caused by environmental pollution. The forest areas are equal in both solutions. But as the NBS has more swamps and reed areas (+67/78 ha), there is an additional capture between 279 and 1051 kg particulate matter per year.

Recreation and landscape experience: The majority of the respondents (77%) preferred the NBS, while 23% preferred the technical solution. An impact that was not explicitly assessed was the unique value of the valley in Flanders, as it is one of the few remaining natural alluvial systems in which a freely meandering river is allowed to flood its natural floodplain. This would probably represent an additional benefit for the NBS.

Monetization of costs and benefits (Step 4)

The costs can be divided into two categories: investment costs (for construction of infrastructure and equipment) and maintenance costs (maintenance of infrastructure and nature management). A summary of the differences in costs between both approaches is summarized in Table  3 : the technical solution requires a one-time extra cost between 2.65 and 2.72 million €, while the annual running cost is 10 000 € lower. Overall, the technical solution (investment and maintenance costs) would cost 2.4 to 2.5 million € extra over a 30-year period compared to the NBS.

Table 3

Summary of differences in investment and maintenance costs between the technical and nature-based solutions (Demeyer and Turkelboom 2013a , based on Arcadis 2012 , and estimates of VMM and Piet De Becker)

*For a time horizon of 30 years at a discount rate of 4%

Monetary benefits of ES are calculated with NVE (Table  2 ). Regarding recreation, the respondents who preferred the NBS were willing to pay (WTP) on average 8 €/month/household (p<0.05) to retain the NBS (with a 95% confidence interval from 4.14 to 11.89 €/month/household). Age, income and being unemployed had a significant positive effect on the WTP, while proximity of nature had a significant negative effect on the WTP. As a large part of the survey respondents (44%) were donating money to nature-related organizations and as there was a significant correlation between this characteristic and the degree of WTP, this number is probably an overestimation (Coucke, 2013 ). On the other hand, such a high rating is not exceptional, considering that the nature reserve is surrounded by densely populated areas. Similar values for recreation were also found in previous SCBAs in Flanders (De Nocker et al. 2005 ; De Nocker et al. 2011 ). If we multiply the obtained WTP amount by 12 (to obtain a yearly amount) and the number of households of the municipalities that provide most recreational users for the study area (6 municipalities with 32 500 inhabitants), an average willingness to pay of 3 120 000 €/year was obtained (with a 95% confidence interval from 1 615 000 to 4 637 000 €/year) (Coucke, 2013 ).

If we sum the differences of costs and benefits affected by both solutions, the NBS delivers an additional value between 32 and 100 million € over a period of 30 years, compared to the technical approach (Table  2 ). The used discount rate in a SCBA is always subject of debate (Gowdy et al. 2010 ; Perman et al. 2011 ). A positive discount rate in fact implies that the present value of future costs and benefits is given less weight than the current values. The discount rate can be set lower than 4%, or even a negative rate could be used. A negative discount rate is based on the assumption that nature in future will be more valuable than today. This could be argued based on the fact that there will be an increasing population and a higher demand for ES in the future. When we would have taken a lower discount rate, the additional benefits for the NBS would have been even higher, as compared to the technical solution. For example, if we use a discount rate of 0%, the added value of the NBS would amount between 54 and 147 million € over 30 years.

A NBS for flood control with restoration of the alluvial floodplain can be considered a valid alternative for a technical solution. The major advantages are the potential to provide the same flood protection for less costs, but with additional co-benefits for society and an increase in biodiversity. In addition, the nature-based solution is a “no-regret solution”, as it is able to tackle future challenges, such as climate change. If the authorities would have chosen for the technical solution—fully relying on construction works—any alteration would lead to excessive costs and additional threats to nature values in the Dijle valley. Based on these results, we can conclude that policy-makers made in the year 2000 the right choice to opt for the NBS for the Dijle valley. The ‘Dijle case’ can be considered as an early positive example of integrated basin management, in the spirit of the European Water Framework Directive (2000/60/EC) and the EU Floods directive (2007/60/EC).

On the other hand, it is important to note that the results of a cSCBA for different flood control solutions is highly context-dependent. In the case of the Dijle valley, the floodplain has a protected status, and housing was never very important in the valley due to its waterlogged soils. Consequently, the opportunity costs for housing and agriculture were close to zero. In a valley which is intensively farmed and/or where there are residential areas, the opportunity costs for a NBS would be much higher. When available space is a crucial issue or when opportunity costs are high, a technical solution might be more suitable, although it still needs to be considered that technical solutions will provide less ES and/or biodiversity.

A possible consequence of the advantages of such NBS could be that the ever smaller remaining fragments of wetlands are being considered to provide flood protection services. As these areas are usually under nature conservation legislation, this can entail some risks. Nature conservation and flood damage protection are compatible when some key elements are taken into account: flood water has to be of sufficiently good quality and the amount of excess (flood) water has to be stored on a large as possible surface (De Becker & De Bie 2013 ). In other words, the entire natural floodplain should be used in order to reduce the flood frequency, duration, water depth as well as sediment load per unit surface to a minimum. In this way, the negative impact on biodiversity will be minimized. All those elements were taken into account in the flood damage control discussion for the Dijle. Considering the urbanistic developments in the Dijle river catchment, this approach was the best option to achieve both the Natura 2000 goals and the required flood damage protection. Therefore, this type of NBS fits well in the definition for NBS Type 1: no or minimal intervention in the ecosystem, with the objectives of maintaining or improving the delivery of a range of ES both inside and outside of these preserved ecosystems (Eggermont et al. 2015 ).

Despite the limitations we encountered, we suggest that a comparative social cost–benefit analysis, supported by a cause-effect analysis, based on data from models and local knowledge, and validated by an expert group, is a pragmatic approach to make informed decisions. However, a comparison between a NBS and a technical solution is not always a grey vs. green comparison. A NBS may include a number of technical measures (e.g. the NBS for the Dijle valley also included one emergency storm basin). In contrast, technical solutions can also include green elements.

A final interesting observation of this case is that the debate took 25 years, while the implementation only required 5 years. A clear environmental policy framework, availability of appropriate flood risk models, and an active involvement of all stakeholders in the early phase of the debate would have probably reduced the length of the debate period.

As flood damage protection issues are increasingly important all over Europe, NBS are often presented as a valid alternative for technical solutions. To make a comprehensive comparison, it is important to not only focus on the level of flood damage protection and investment and maintenance costs, but also to consider all other impacts on ecosystem services and biodiversity. From a successfully implemented NBS in the Dijle valley in Belgium, we can—despite some uncertainties—confidently state that the NBS provides the required flood security, for fewer costs and with more ecosystem services benefits and biodiversity, compared to the technical solution. The highest additional values are realized via recreation, denitrification, and biodiversity. Recreation comes out as the most valuable ES provided by the NBS (83–91% of the total extra value of the NBS). Only carbon sequestration in vegetation scored better in the technical solution. Reconnecting rivers with their floodplains is, therefore, a valuable policy option when coping with flood risks. However, the business case for working with NBS depend a lot on the spatial and socio-ecological context: the opportunity for a NBS increases when there is sufficient space to retain flood water, when flood water is of sufficient good quality, and when there are only limited economic activities and/or residential areas in the floodplain.

Below is the link to the electronic supplementary material.

Acknowledgements

We are grateful to the expert committee who provided feedback and guidance for the research. Also we are grateful to the 10 respondents who shared their local knowledge during the interviews. A special thanks to Carine Wils (INBO) for the case study maps, Jan Pauwels (VMM) for calculating hydrological data, Laurens Coucke for the choice experiment, and the four reviewers for their very valuable suggestions. The study was requested by the Agency of Nature and Forest (ANB), and was conducted with own INBO resources.

Biographies

is senior researcher of the Team Nature & Society at the Institute of Nature and Forest Research (INBO). His research interests are the application of ecosystem services in planning context, nature-based solutions and participatory approaches.

is a self-employed dietitian and teacher. Her research interests are nature-based solutions, participatory approaches and paediatric nutrition.

is professor in agricultural and resource economics at the University of Leuven (KULeuven), Department of Earth and Environmental Sciences. Her research focusses on assessing and shaping decision-making processes by land managers such as farmers and foresters as well as end-users such as recreationists and consumers to support the development of more sustainable agricultural and food systems and to promote ES and values.

is hydrologist (MSc) and senior researcher in the Team Environment & Climate at the Institute of Nature and Forest Research (INBO). His research interests are ecohydrology and ground- & surface-water management.

is head of section Demer, Dijle and Maas at the Flanders Environment Agency (VMM). His concern is reaching the objectives for local water systems as set out by Flemish and EU legislation like the Water Framework Directive and the Floods Directive.

is researcher of the Team Nature & Society at the Institute of Nature and Forest Research (INBO). His research interests include use of ecosystem service knowledge and tools in policy and land use decisions.

Funding for the open access publication was provided by the FWO project Future Floodplains.

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Contributor Information

Francis Turkelboom, Email: [email protected] .

Rolinde Demeyer, Email: eb.ieorgedpo@edniloR .

Liesbet Vranken, Email: [email protected] .

Piet De Becker, Email: [email protected] .

Filip Raymaekers, Email: [email protected] .

Lieven De Smet, Email: [email protected] .

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flood control case study

Urban River Rehabilitation and Flood Control: Case Study of the Pasig River System

Large-scale river rehabilitation can be a critical strategy for improving flood control and management. This paper presents our analysis of the effects on flooding of the initial phase of the river restoration program for the Pasig River System in urban Metro Manila, Philippines. The Pasig River, a 26-km tidal estuary, was once a center of commerce and political activity in Manila. It is now considered ecologically dead; sediments and large volumes of garbage impede water flow, resulting in frequent flooding in surrounding areas. The national government has initiated a program to restore the river system to its former pristine condition in the 1950s and has completed restoration of the Estero de Paco, a tributary located near the river mouth. Our current study focuses on the effects of this completed work, with particular emphasis on sediment flow and flood control. We developed and implemented a hydraulic model of the Estero in HEC-RAS to examine the extent to which localized flooding has been reduced as a result of the restoration efforts. Collected field data, supplemented by literature data, were used to support model calibration and analysis. Our simulation results show a significant reduction in the extent of flooding in the Estero, however, there is a need to address solid waste disposal and sedimentation from upstream tributaries and surface run-off to control flooding over the long-term. Further analysis is required to examine the long-term effects of in-stream vegetation on channel hydraulics, water quality, and flooding frequencies before similar restoration efforts are replicated in other tributaries.

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Internet Geography

Kerala flood case study

Kerala flood case study.

Kerala is a state on the southwestern Malabar Coast of India. The state has the 13th largest population in India. Kerala, which lies in the tropical region, is mainly subject to the humid tropical wet climate experienced by most of Earth’s rainforests.

A map to show the location of Kerala

A map to show the location of Kerala

Eastern Kerala consists of land infringed upon by the Western Ghats (western mountain range); the region includes high mountains, gorges, and deep-cut valleys. The wildest lands are covered with dense forests, while other areas lie under tea and coffee plantations or other forms of cultivation.

The Indian state of Kerala receives some of India’s highest rainfall during the monsoon season. However, in 2018 the state experienced its highest level of monsoon rainfall in decades. According to the India Meteorological Department (IMD), there was 2346.3 mm of precipitation, instead of the average 1649.55 mm.

Kerala received over two and a half times more rainfall than August’s average. Between August 1 and 19, the state received 758.6 mm of precipitation, compared to the average of 287.6 mm, or 164% more. This was 42% more than during the entire monsoon season.

The unprecedented rainfall was caused by a spell of low pressure over the region. As a result, there was a perfect confluence of the south-west monsoon wind system and the two low-pressure systems formed over the Bay of Bengal and Odisha. The low-pressure regions pull in the moist south-west monsoon winds, increasing their speed, as they then hit the Western Ghats, travel skywards, and form rain-bearing clouds.

Further downpours on already saturated land led to more surface run-off causing landslides and widespread flooding.

Kerala has 41 rivers flowing into the Arabian Sea, and 80 of its dams were opened after being overwhelmed. As a result, water treatment plants were submerged, and motors were damaged.

In some areas, floodwater was between 3-4.5m deep. Floods in the southern Indian state of Kerala have killed more than 410 people since June 2018 in what local officials said was the worst flooding in 100 years. Many of those who died had been crushed under debris caused by landslides. More than 1 million people were left homeless in the 3,200 emergency relief camps set up in the area.

Parts of Kerala’s commercial capital, Cochin, were underwater, snarling up roads and leaving railways across the state impassable. In addition, the state’s airport, which domestic and overseas tourists use, was closed, causing significant disruption.

Local plantations were inundated by water, endangering the local rubber, tea, coffee and spice industries.

Schools in all 14 districts of Kerala were closed, and some districts have banned tourists because of safety concerns.

Maintaining sanitation and preventing disease in relief camps housing more than 800,000 people was a significant challenge. Authorities also had to restore regular clean drinking water and electricity supplies to the state’s 33 million residents.

Officials have estimated more than 83,000km of roads will need to be repaired and that the total recovery cost will be between £2.2bn and $2.7bn.

Indians from different parts of the country used social media to help people stranded in the flood-hit southern state of Kerala. Hundreds took to social media platforms to coordinate search, rescue and food distribution efforts and reach out to people who needed help. Social media was also used to support fundraising for those affected by the flooding. Several Bollywood stars supported this.

Some Indians have opened up their homes for people from Kerala who were stranded in other cities because of the floods.

Thousands of troops were deployed to rescue those caught up in the flooding. Army, navy and air force personnel were deployed to help those stranded in remote and hilly areas. Dozens of helicopters dropped tonnes of food, medicine and water over areas cut off by damaged roads and bridges. Helicopters were also involved in airlifting people marooned by the flooding to safety.

More than 300 boats were involved in rescue attempts. The state government said each boat would get 3,000 rupees (£34) for each day of their work and that authorities would pay for any damage to the vessels.

As the monsoon rains began to ease, efforts increased to get relief supplies to isolated areas along with clean up operations where water levels were falling.

Millions of dollars in donations have poured into Kerala from the rest of India and abroad in recent days. Other state governments have promised more than $50m, while ministers and company chiefs have publicly vowed to give a month’s salary.

Even supreme court judges have donated $360 each, while the British-based Sikh group Khalsa Aid International has set up its own relief camp in Kochi, Kerala’s main city, to provide meals for 3,000 people a day.

International Response

In the wake of the disaster, the UAE, Qatar and the Maldives came forward with offers of financial aid amounting to nearly £82m. The United Arab Emirates promised $100m (£77m) of this aid. This is because of the close relationship between Kerala and the UAE. There are a large number of migrants from Kerala working in the UAE. The amount was more than the $97m promised by India’s central government. However, as it has done since 2004, India declined to accept aid donations. The main reason for this is to protect its image as a newly industrialised country; it does not need to rely on other countries for financial help.

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Effects of runoff generation methods and simulation time steps on flood simulation: a case study in Liulin experimental watershed

  • Original Paper
  • Published: 19 February 2024

Cite this article

  • Jianzhu Li 1 ,
  • Yunfei Peng 1 ,
  • Ting Zhang   ORCID: orcid.org/0000-0002-4567-5306 1 ,
  • Yanfu Kang 2 &
  • Bo Zhang 2  

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Flood simulation in sub-humid regions is one of the difficult issues in hydrology. Liulin experimental watershed, a typical sub-humid region in northern China, was selected for flood simulation. 20 rainfall–runoff events from 1995 to 2021 were selected to calibrate and validate the sub-distributed HEC-HMS model. The applicability of the model to flood simulation in the Liulin experimental watershed was explored. The influences of different runoff generation methods (SCS-CN method and initial constant method) and simulation time steps (1 h and 30 min) on flood simulation were compared. The applicability of the model to different antecedent moisture conditions and different flood characteristics was also analyzed. The results showed that all the schemes of rainfall–runoff models with different runoff generation methods and time steps have satisfactory performance in simulating floods. When the time step is 1 h, the initial constant runoff generation method was more suitable for runoff simulation, however, when the time step is 30 min, the SCS-CN runoff generation method was more robust. As the simulation time step decreased, the model performance was improved, but the improvement amplitude was greater when the SCS-CN method was used. In addition, the model performed better when antecedent moisture was higher, and the flood was single-peak. When the measured peak discharge was lower than 100 m 3 /s, the model could simulate the peak discharge and peak time better, and conversely, the model could simulate the flood volume and flood hydrograph better. This study is valuable for flood forecasting in sub-humid areas.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China [No. 52279022, 52079086].

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Li, J., Peng, Y., Zhang, T. et al. Effects of runoff generation methods and simulation time steps on flood simulation: a case study in Liulin experimental watershed. Nat Hazards (2024). https://doi.org/10.1007/s11069-024-06427-1

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