This study provides an overview of the impact of the 2021 Summer floods in the Netherlands and the assessment of the effectiveness of various adaptation measures, evacuation strategies, and their impact on society. The floods were characterized by record rainfall in the cross-border region of the Meuse and Rhine basins and resulted in devastating losses in the Netherlands, Germany and Belgium. The study reports on a household survey conducted with 1,509 households in the wake of the 2021 flood event in the southern part of the Netherlands (province of Limburg). Using a descriptive approach, we present household experiences during several stages of the disaster management cycle, reporting on experienced flood hazard and impacts, evacuation, flood damage mitigation measures, the compensation progress, risk perceptions, and stress. Our findings highlight the role of early warnings and flood risk information provision in flood risk management. Risk perceptions influence both adaptation and evacuation behavior, as respondents who were aware of flood risks beforehand took significantly more flood damage mitigation measures compared with those who were not aware. Flood damage mitigation measures, such as building with water-resistant materials and elevating valuables, reduced flood damage by 20% to 50%. Our survey shows that of those who received warnings, the majority actually evacuated. However, residents not aware of any evacuation advice evacuated significantly less. Additionally, the majority (75%) of respondents experienced high or very high stress during and after the flood, which is most likely related to the destructive flood impacts and the slow and uncertain compensation experienced by many respondents. This paper describes the flood event and its consequences to provide insights into Dutch disaster management and what can be learned for potential future disasters in other contexts.@en

In response to the outbreak of SARS-CoV-2, many governments decided in 2020 to impose lockdowns on societies. Although the package of measures that constitute such lockdowns differs between countries, it is a general rule that contact between people, especially in large groups of people, is avoided or prohibited. The main reasoning behind these measures is to prevent healthcare systems from becoming overloaded. As of 2021 vaccines against SARS-CoV-2 are available, but these do not guarantee 100% risk reduction and it will take a while for the world to reach a sufficient immune status. This raises the question of whether and under which conditions events like theater shows, conferences, professional sports events, concerts, and festivals can be organized. The current paper presents a COVID-19 risk quantification method for (large-scale) events. This method can be applied to events to define an alternative package of measures replacing generic social distancing@en

In juli 2021 zijn grote delen van Limburg getroffen door hevige regenval en overstromingen. Ook delen van België en Duitsland overstroomden met zeer veel schade en verlies aan mensenlevens tot gevolg. Dit betrof een extreme en ongeëvenaarde gebeurtenis met enorme impact. Daarom is naar aanleiding van de overstromingen deze verkenning uitgevoerd om een eerste stap te maken om beschikbare informatie over deze gebeurtenis te verzamelen en analyseren. Het onderzoek is uitgevoerd door een breed consortium (TU Delft, Deltares, HKV Lijn in Water, VU Amsterdam, Universiteit Utrecht, KNMI, WUR, Erasmus MC en Universiteit Twente) in opdracht van het Expertise Netwerk Waterveiligheid (ENW). Een overstroming heeft effect op de hele maatschappij. Daarom zijn niet alleen hydrologische en civieltechnische onderwerpen beschouwd, maar ook de maatschappelijke gevolgen van overstromingen, de crisisrespons en de gezondheidseffecten.

Contributors (in alphabetical order):
Nathalie Asselman (Deltares), Hermjan Barneveld (HKV / Wageningen UR), Jules Beersma (KNMI), Eline Boelee (Deltares), Wouter Botzen (VU Amsterdam), Eefke Copper (TU Delft), Dim Coumou (KNMI), Karin de Bruijn (Deltares), Anniek de Jong (Deltares), Jurjen de Jong (Deltares), Hans de Moel (VU Amsterdam), Ferdinand Diermanse (Deltares), Astrid Fischer (Evides) , Gert-Jan Geerling (Deltares), Marie-Louise Geurts (WML), Rob Groenland (KNMI), Mark Hegnauer (Deltares), Bas Jonkman (TU Delft), Nicole Jungermann (KNMI), Frans Klijn (Deltares), Andre Koelewijn (Deltares), Matthijs Kok (HKV / TU Delft), Elco Koks (VU Amsterdam), Bas Kolen (HKV / TU Delft), Marion Koopmans (Erasmus MC), Laurens Leunge (Deltares), Hans Middelkoop (Utrecht University), Roelof Moll (TU Delft), Jaap Mos (Dunea), Sjoukje Philip (KNMI), Gerbert Pleijter (HKV), Joost Pol (HKV / TU Delft), Stephan Rikkert (TU Delft), Guus Rongen (TU Delft), Rinus Scheele (KNMI), Julius Schlumberger (TU Delft), Peter Siegmund (KNMI), Kymo Slager (Deltares), Frederiek Sperna Weiland (Deltares), Bart Strijker (HKV / TU Delft), Henk v.d. Brink (KNMI), Janko van Beek (Erasmus MC), Marion van den Bulk (TU Delft), Bart van den Hurk (Deltares), Tim van Emmerik (Wageningen UR), Kees van Ginkel (VU Amsterdam / Deltares), Mick van Haren (TU Delft), Margreet van Marle (Deltares), Malou van Schaijk (TU Delft), Dennis Wagenaar (Nanyang TU), Davide Wüthrich (TU Delft)@en

An analysis was made of the loss of life caused by Hurricane Harvey. Information was collected for 70 fatalities that occurred due to the event and were recovered within the first 2 weeks after landfall. Most fatalities occurred due to drowning (81 %), particularly in and around vehicles. Males (70 %) and people over 50 years old (56 %) were overrepresented in the dataset. More than half of the fatalities occurred in the greater Houston area (n  =  37), where heavy rainfall and dam releases caused unprecedented urban flooding. The majority of fatalities were recovered outside the designated 100- and 500-year flood hazard areas.@en

It is important for decision-makers in emergency response situations to determine the scope, scale, timing, path and resettlement area of an evacuation decision when there is an imminent threat of flooding. In this paper, a method called the "Evacuation Diagram" is described to support risk-based evacuation planning and decision-making. In case of an imminent threat of flooding, we refer to the conditional risk, which is the risk given the forecasted water levels and potential consequences during the next days of the event. Given the threat and potential costs and benefits, evacuation decisions have to mitigate this conditional risk. Since evacuation can be costly, decisionmakers have to make a trade-offbetween costs and benefits. In this research we present a method using a cost-benefit analysis approach, in which we adopt the "Dutch flood risk approach" to define the required strength of levees based on flood risk considerations. The "Evacuation Diagram" method is derived from analytical derivations, based on differential weighing of costs and benefits, which have an impact on a binary choice (or a set of discrete choices) as to whether to instruct an area to evacuate or not. Basically, this is an analysis of behavioural decision-making under risk, investigating how a cost-benefit analysis can yield higher cost effectiveness in risk reduction for human lives lost during possible evacuation incidents. The method is applied to a Dutch case study and the results are compared to the outcomes of the largest evacuation exercise ever held in the Netherlands, called 'Waterproef'. It is concluded that the risk-based evacuation method, as presented in this paper, provides useful insight into collective and authoritative evacuation order decisions.

@en

Sinds het Deltaprogramma zijn risicobenadering en rampenbeheersing definitief ingebed in het waterveiligheidsbeleid. Echter, in een koppeling tussen deze twee is nog veel winst te behalen, betoogt Bas Kolen; vooral voor de systematiek van opschalen, het moment om maatregelen te nemen en manieren om maatregelen beter te kunnen uitleggen. De huidige wijze van voorbereiden blijkt dikwijls risicozoekend, terwijl juist risicoaversie (het voorkomen van slachtoffers) wordt verkondigd en verwacht.@en

An integrated flood risk management (IFRM) strategy consist of a comprehensive set of measures to reduce the risk: protective measures (to reduce the probability of a flood), and land use planning and disaster management (to reduce the consequences of a flood. In the Netherlands this is called a ‘multiple layer safety approach’, other countries refer to ‘multiple lines of defence’. In the development of IFRM strategies one of the main challenges is to define the contribution of disaster management to the reduction of risk, especially when experience with floods is rare and flood awareness is limited.@en

To help decision makers in delta areas to select appropriate adaptation and mitigation strategies it is necessary to have a clear picture of the main drivers of risk and how risk evolves under changing conditions. Acknowledging these problems, the Delta Alliance and the TU Delft took the initiative to develop a new index that helps cities to better understand the dominant drivers of risk, formulate ambitions, and compare and share their results with other delta regions. The approach is based on previous work at TU Delft in collaboration with HKV and Deltares. The goal of this index is not to formulate an “exact” risk number, but aims to serve as a benchmark that supports communicating risk within the political arena and a peer-to-peer learning process with other deltas.@en

The central issue for authorities (as well as the public) is how and when to respond to forecasted extreme water levels on rivers, lakes and along the coast and large-scale flooding is an actual risk. The decision-making process is influenced by contradicting information, overloads and gaps in information, rumours, uncertainties in forecasts, the consequences of a flood and the effectiveness of measures. Emergency measures can be taken to reduce the probability of flooding(e.g. placing sand bags), other measures can be taken to reduce the consequences of a flooding (such as evacuation of inhabitants). For many of these measures, decisions are made days or hours prior to the expected moment of occurrence of the flooding. Using forecasts of water levels, by definition uncertain, and forecasts of the strength of levees, decisions can be made based on the acceptability of the actual flood risk level. The concept of risk can be used to prioritise measures in case of limited time.@en

On August 25, 2017, Hurricane Harvey made landfall near Rockport, Texas as a Category 4 hurricane with maximum sustained winds of approximately 200 km/hour. Harvey caused severe damages in coastal Texas due to extreme winds and storm surge, but will go down in history for record-setting rainfall totals and flood-related damages. Across large portions of southeast Texas, rainfall totals during the six-day period between August 25 and 31, 2017 were amongst the highest ever recorded, causing flooding at an unprecedented scale. More than 100,000 residential properties are estimated to have been affected in southeast Texas. It is likely that Harvey will rank among the costliest storms in U.S. history. In the wake of Hurricane Harvey, Delft University of Technology has initiated a Harvey Research Team to undertake a coordinated multidisciplinary investigation of the events with a focus on the greater Houston area. This ‘fact-finding’ research is based on information available from public sources during and in the first weeks after the event. Results are therefore preliminary, but aim to provide insight into lessons that can be learned for both Texas and the Netherlands. As part of the investigations, a hackathon with more than 80 participants was organized to collect and analyze available public information. Houston was especially hard hit by flooding. During the event, all 22 watersheds in the greater Houston area experienced flooding. Many of Houston’s creeks and bayous exceeded their channel capacities, reaching water levels never before recorded. Across large portions of Harris County, rainfall totals exceeded the 1000-year return period. In addition, the water from the two reservoirs protecting downtown Houston (Addicks and Barker) were opened on August 28 to prevent catastrophic damages to the dams and further flooding in upstream communities. The releases exacerbated flooding in the areas downstream of the dams and an estimated 4,000 homes in neighborhoods downstream of the dams were impacted by flooding. The consequences of the event in the greater Houston area have been characterized in terms of economic damages, loss of life and impacts on critical infrastructure, airports and industry. In total, more than 100,000 homes were affected more than 70 fatalities were reported in the greater Houston area. The event highlighted the vulnerability of industrial facilities, as several cascading impacts (releases of toxic materials and explosions) were reported. Emergency response has been assessed. No large-scale mandatory evacuation was ordered before or during Harvey. However, it appeared that several local evacuations were ordered for areas with specific risks and circumstances. During the event, many people were trapped by rising waters necessitating a major rescue operation. In total, more than 10,000 rescues were made by professional and volunteer rescuers. Social media played an important role during the event and recovery, as an additional source of information, to inform emergency managers and as a means to organize community response e.g. for clean-up. Also, messages were conveyed through social media, e.g. a report of a levee breach that appeared to be incorrect afterwards. Major flooding is a problem that has multiple causes from both physical and social origin. Based on the investigations, recommendations for future research and lessons for flood management have been formulated. A better understanding of the issues studied in this report is expected to contribute to a knowledge basis for further in-depth investigations and future directions for flood risk reduction. Data collection and Report production funded by DIMI and DSys Special Case 'Houston Galveston Bay Region, Texas, USA' Project 'Harvey hackathon' and follow-up research @en

Design-criteria for flood-defences have a legal character in the Netherlands. Formerly, these flood protection standards were based on design-waterlevels, where a flood defence was implicitly supposed not to fail below this waterlevel. Since knowledge regarding dike-failure improved, a new risk approach has been developed based on a social cost-benefit analysis which addresses a monetary value to a (prevented) fatality. Also, loss of life is part of the risk analysis. The main improvement of the new approach is the explicit inclusion of failure probabilities in flood-risk calculations. A recent floodrisk study showed the risk of flooding for some regions in the Netherlands is larger than previously thought [Rijkswaterstaat 2015].@en

This paper presents an overview and review of methods developed for loss of life estimation in flood risk assessment. These methods range from empirical to simulation based approaches that are used to support flood risk analyses and emergency management. Similarities and differences between the modelling approaches, input and output types and applications are discussed. Challenges to the field are summarized, including empirical data collection for validation and benchmarking and comparison studies.@en

Both in the USA and in the Netherlands, extensive studies on approaches for identifying flood risks (the combination of levee failure probabilities and consequences) are ongoing. A related topic concerns evacuation and emergency management (EEM). The magnitude of life loss is directly influenced by the effectiveness of EEM. Different models can be used to define the effectiveness of evacuation. The main objective of this study is to compare Dutch and American methods for evacuation as part of flood risk management strategies for a number of case studies in the US. A second, additional objective is to explore how approaches for EEM that have been recently developed in the Netherlands, can be applied in the United States. This research shows the benefits and limitations for the application of different types of models. This research contributes to the improvement of methods for emergency management, in the Netherlands, the US and other countries.@en