Traditional flood hazard analyses often rely on univariate probability distributions; however, in many coastal catchments, flooding is the result of complex hydrodynamic interactions between multiple drivers. For example, synoptic meteorological conditions can produce considerable rainfall-runoff, while also generating wind-driven elevated sea-levels. When these drivers interact in space and time, they can exacerbate flood impacts, a phenomenon known as compound flooding. In this paper, we build a Bayesian Network based on Gaussian copulas to generate the equivalent of 500 years of daily stochastic boundary conditions for a coastal watershed in Southeast Texas. In doing so, we overcome many of the limitations of conventional univariate approaches and are able to probabilistically represent compound floods caused by riverine and coastal interactions. We model the resulting water levels using a one-dimensional (1D) steady-state hydraulic model and find that flood stages in the catchment are strongly affected by backwater effects from tributary inflows and downstream water levels. By comparing our results against a bathtub modeling approach, we show that simplifying the multivariate dependence between flood drivers can lead to an underestimation of flood impacts, highlighting that accounting for multivariate dependence is critical for the accurate representation of flood risk in coastal catchments prone to compound events@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

As there is increasing emphasis on transformative education and authentic learning in interdisciplinary research projects, it is meaningful to investigate how to effectively design a multidisciplinary research and education program to ensure beneficial outcomes for participating students. This is especially important for ocean and coastal engineering programs that are likely the most multidisciplinary engineering programs. The NSF PIRE Coastal Flood Risk Reduction Program is an international place and problem-based research education program in which students conduct case studies across the Houston-Galveston metropolitan area in the U.S. and in the Netherlands. There are three to four designated case studies (place-based) annually in each country, covering both surge-based and precipitation-driven flood problems (problem-based). From 2016 to 2018, there were three student research trips to the Netherlands (one each year, after the spring semesters). A total of 42 U.S. students, graduate and undergraduate were selected from four participating U.S. campuses apply for a designated Dutch case study. The three to four case studies change every year. Students from diverse disciplines, including engineering, planning, economics, hydrology, biology, architecture, geography, communications, and computational hydraulics, interested in flood risk reduction can apply. Those accepted into the Program are placed in interdisciplinary research teams composed of 5-6 students: 1-2 PhD, 2-3 Masters, and 2-3 undergraduate students. The teams are guided by project faculty mentors from both U.S. and Dutch partner institutions. A two-week long research trip to the Netherlands provides transformative education and an authentic learning environment through field trips, meetings with Dutch flood experts, lectures, and participation in design workshops. Students are required to present their research work three times while they are in the Netherlands: 5-minute research plan; 10-minute research progress; and 15-minute final presentation. By preparing these presentations, students learn how to collect data, interview stakeholders, lead/participate in brain-storming discussions, and adjust/improve their research products. Students also learn how to interact with people from different disciplines and look at the issues from diverse perspectives. This article describes the design process of the Program, from initial development through implementation. Reflections and lessons learned from the first three years of the Program are shared.@en

Currently there is no internationally accepted framework for assessing the readiness of innovations that reduce disaster risk. To fill this gap, BRIGAID is developing a standard, comprehensive Testing and Implementation Framework (TIF). The TIF is designed to provide innovators with a framework for innovation and guidelines for assessing an innovation’s technical effectiveness, its social acceptance, and its impact on key socio-economic and environmental sectors. The vision is that the TIF will become the standard framework used to assess the effectiveness of climate adaptation innovations and the European quality label for testing.@en

During August 25-30, 2017, Hurricane Harvey stalled over Texas and caused extreme precipitation, particularly over Houston and the surrounding area on August 26-28. This resulted in extensive flooding with over 80 fatalities and large economic costs. It was an extremely rare event: the return period of the highest observed three-day precipitation amount, 1043.4 mm 3dy-1 at Baytown, is more than 9000 years (97.5% one-sided confidence interval) and return periods exceeded 1000 yr (750 mm 3dy-1) over a large area in the current climate. Observations since 1880 over the region show a clear positive trend in the intensity of extreme precipitation of between 12% and 22%, roughly two times the increase of the moisture holding capacity of the atmosphere expected for 1 °C warming according to the Clausius-Clapeyron (CC) relation. This would indicate that the moisture flux was increased by both the moisture content and stronger winds or updrafts driven by the heat of condensation of the moisture. We also analysed extreme rainfall in the Houston area in three ensembles of 25 km resolution models. The first also shows 2 × CC scaling, the second 1 × CC scaling and the third did not have a realistic representation of extreme rainfall on the Gulf Coast. Extrapolating these results to the 2017 event, we conclude that global warming made the precipitation about 15% (8%-19%) more intense, or equivalently made such an event three (1.5-5) times more likely. This analysis makes clear that extreme rainfall events along the Gulf Coast are on the rise. And while fortifying Houston to fully withstand the impact of an event as extreme as Hurricane Harvey may not be economically feasible, it is critical that information regarding the increasing risk of extreme rainfall events in general should be part of the discussion about future improvements to Houston's flood protection system.@en

The application of risk-based approaches for the design of flood infrastructure has become increasingly common in flood management. This approach, based on risk reduction and reliability, is used to assess the performance of conventional interventions (e.g., flood defences and dams) and to support decisions regarding their implementation. However, for more innovative solutions, performance has often not been quantified by means of these metrics and, therefore, end-users are hesitant to implement them in existing flood risk reduction systems. To overcome the gap between innovators and end-users, we present a framework based on four performance indicators, to ensure the required insights in risk and reliability are provided. The four indicators: effectiveness, durability, reliability and costs, allow end-users to evaluate, select, and implement flood adaptation innovations, and provide innovators with insight into the performance of the technology and the criteria and information necessary for successful market uptake of their innovation. The practical application of the framework is demonstrated for a (hypothetical) case of a hospital complex built in an area that has subsided below the surrounding area, which is subject to tropical rain showers. The following innovations are considered: an early flood warning system, a green roof, and a temporary flood barrier.@en

Floods are the costliest and deadliest weather-related disaster globally. With higher population density in flood prone areas, increased urban development, and projected climate impacts, the frequency and severity of flooding in Europe is predicted to rise. While permanent flood defences have been shown to be more economically effective and reliable in the long-term than temporary flood barriers, they are expensive upfront, socially and politically complex, and time-consuming to build (Lendering et al., 2015). To adapt to climate change and mitigate flooding in the short term, it will be necessary to identify and test temporary flood barriers which can be quickly deployed during a flood event to mitigate risk. Moreover, in some areas, where permanent structures may be phsyically (or socially) infeasible, temporary (or semi-permanent) flood barriers may become a permanent strategy for mitigating floods.@en

In recent years significant emphasis has been placed on quantifying coastal flood hazards in the U.S. using high resolution 2-D hydrodynamic and nearshore wave models. However, these studies are computationally expensive and often neglect to consider the flooding that arises from the combined hazards of precipitation and storm surge in coastal watersheds. This paper describes a method to stochastically simulate a large number of combinations of peak storm surge and cumulative precipitation to determine the hydraulic boundary conditions for a low-lying coastal watershed draining into a semi-enclosed tidal bay. The method is computationally efficient and takes into consideration five tropical cyclone characteristics at landfall: windspeed, angle of approach, landfall location, radius of maximum winds, and forward speed. A precipitation gage network and tidal gage data were used, along with observations from over 300 tropical cyclones in the Gulf of Mexico. A Non-parametric Bayesian Network was built to generate 100,000 synthetic storm events and used as input to an empirical wind set-up model to simulate storm surge within a tidal bay and at the downstream boundary of the watershed. Based on the results, probable combinations of cumulative precipitation and peak storm surge for the watershed during hurricane conditions are determined. These boundary conditions can be easily incorporated into a coastal riverine model to determine flood risk in the watershed. @en

Adverse consequences of floods change in time and are influenced by both natural and socio-economic trends and interactions. In Europe, previous studies of historical flood losses corrected for demographic and economic growth (‘normalized’) have been limited in temporal and spatial extent, leading to an incomplete representation of trends in losses over time. Here we utilize a gridded reconstruction of flood exposure in 37 European countries and a new database of damaging floods since 1870. Our results indicate that, after correcting for changes in flood exposure, there has been an increase in annually inundated area and number of persons affected since 1870, contrasted by a substantial decrease in flood fatalities. For more recent decades we also found a considerable decline in financial losses per year. We estimate, however, that there is large underreporting of smaller floods beyond most recent years, and show that underreporting has a substantial impact on observed trends.@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

In this study, we compare the ability of two riverine flood control approaches: channelization and stream preservation/setbacks, to alleviate the adverse impacts of rapid urbanization. To study the effects of structural intervention and urban development on the evolution of the floodplain, we have chosen two neighboring urban watersheds in Houston, TX: Brays Bayou and Buffalo Bayou. While the two watersheds are similar in size, topography, and development level, they have contrasting riverine flood management approaches. Brays Bayou is channelized, whereas Buffalo Bayou remains mostly unchannelized. We use the distributed hydrologic model, Vflo®, and the hydraulic model, HEC-RAS, to analyze channel hydraulics and floodplain extent in the two watersheds under the 10- and 100-year rainfall scenarios at three points in time: 1970s (early development), 2011 (current development), and 2040 (future development). We find that, while floodplain extent in both watersheds increases over time, the relative change in floodplain extent for Brays Bayou (channelized) is substantially larger than that for Buffalo Bayou (unchannelized). The results in this study contribute to a better understanding of the long-term performance of two flood mitigation approaches (channelization and setbacks) on riverine flood risk and provide insight into best management practices for cities experiencing rapid urban growth.@en

Rescue requests during large-scale urban flood disasters can be difficult to validate and prioritise. High-resolution aerial imagery is often unavailable or lacks the necessary geographic extent, making it difficult to obtain real-time information about where flooding is occurring. In this paper, we present a novel approach to map the extent of urban flooding in Harris County, Texas during Hurricane Harvey (August 25–30, 2017) and identify where people were most likely to need immediate emergency assistance. Using Maximum Entropy, we predict the probability of flooding based on several spatially-distributed physical and socio-economic characteristics coupled with crowdsourced data. We compare the results against two alternative flood datasets available after Hurricane Harvey (i.e., Copernicus satellite imagery and riverine flood depths estimated by FEMA), and we validate the performance of the model using a 15% subset of the rescue requests, Houston 311 flood calls, and inundated roadways. We find that the model predicts a much larger area of flooding than was shown by either Copernicus or FEMA when compared against the locations of rescue requests, and that it performs well using both a subset of rescue requests (AUC 0.917) and 311 calls (AUC 0.929) but is less sensitive to inundated roads (AUC 0.721).@en

This report focuses on the methodological development of the testing and implementation framework (TIF) for increasing the socio-technical readiness of climate adaptation innovations and assessing their impact on different socio-economic and environmental sectors. It is designed to be read by innovators and used as a supporting document for the application of different toolboxes made available through BRIGAID. In this report, Chapter 2 provides an overview of the different components of the TIF, including an overview of the planned testing phases. Definitions for the Performance Indicators (PI) are provided in Chapter 3, which also includes a description of how the test results which are to be integrated into the Climate Innovation Window (CIW) (in WP7). Elaborated guidelines for testing are provided in Chapters 4-6. Specifically, guidelines for assessing the technical effectiveness of innovations are provided in Chapter 4; guidelines for assessing the impact of an innovation on the environment and socio-economic sectors that will feel direct consequences of climate change are provided in Chapter 5; guidelines for assessing the societal acceptance of innovations in Chapter 6.@en

After five years of exchange programmes across the Atlantic, Texas research veterans Nikki Brand (spatial planning), Antonia Sebastian (urban hydrology) and Baukje Kothuis (anthropology) assess the accomplishments of the Dutch Delta Approach in the Houston-Galveston Bay region. Looking at the academic discourse, the public debate and the design of a flood defense along Galveston island, they conclude that the approach has worked.@en