Giuliano Di Baldassarre
Fatalities and economic losses caused by floods are dramatically increasing in many regions of the world, and there is serious concern about future flood risk given the potentially negative effects of climatic and socio-economic changes. Over the past decades, numerous socio-economic studies have explored human responses to floods—demographic, policy and institutional changes following the occurrence of extreme events. Meanwhile, many hydrological studies have investigated human influences on floods, such as changes in frequency, magnitude, and spatial distribution of floods caused by urbanization or by implementation of risk reduction measures. Research in socio-hydrology is providing initial insights into the complex dynamics of risk resulting from the interplay (both responses and influences) between floods and people. Empirical research in this field has recently shown that traditional methods for flood risk assessment cannot capture the complex dynamics of risk emerging from mutual interactions and continuous feedback mechanisms between hydrological and social processes. It has also been shown that, while risk reduction strategies built on these traditional methods often work in the short term, they might lead to unintended consequences in the longer term. Besides empirical studies, a number of socio-hydrological models have been recently proposed to conceptualize human/flood interactions, to explain the dynamics emerging from this interplay, and to explore possible future trajectories of flood risk. Understanding the interplay between floods and societies can improve our ability to interpret flood risk changes over time and contribute to developing better policies and measures that will reduce the negative impacts of floods while maintaining the benefits of hydrological variability.
Recent extreme hydrological events (e.g., in the United States in 2005 or 2012, Pakistan in 2010, and Thailand in 2011) revealed increasing flood risks due to climate and societal change. Consequently, the roles of multiple stakeholders in flood risk management have transformed significantly. A central aspect here is the question of sharing responsibilities among global, national, regional, and local stakeholders in organizing flood risk management of all kinds. This new policy agenda of sharing responsibilities strives to delegate responsibilities and costs from the central government to local authorities, and from public administration to private citizens. The main reasons for this decentralization are that local authorities can deal more efficiently with public administration tasks concerned with risks and emergency management. Resulting locally based strategies for risk reduction are expected to tighten the feedback loops between complex environmental dynamics and human decision-making processes. However, there are a series of consequences to this rescaling process in flood risk management, regarding the development of new governance structures and institutions, like resilience teams or flood action groups in the United Kingdom. Additionally, downscaling to local-level tasks without additional resources is particularly challenging. This development has tightened further with fiscal and administrative cuts around the world resulting from the global economic crisis of 2007–2008, which tightening eventually causes budget restrictions for flood risk management. Managing local risks easily exceeds the technical and budgetary capacities of municipal institutions, and individual citizens struggle to carry the full responsibility of flood protection. To manage community engagement in flood risk management, emphasis should be given to the development of multi-level governance structures, so that multiple stakeholders share fairly the power, resources, and responsibility in disaster planning. If we fail to do so, some consequences would be: (1), “hollowing out” the government, including the downscaling of the responsibility towards local stakeholders; and (2), inability of the government to deal with the new tasks due to lack of resources transferred to local authorities.
Prediction of floods at locations where no streamflow data exist is a global issue because most of the countries involved don’t have adequate streamflow records. The United States Geological Survey developed the regional flood frequency (RFF) analysis to predict annual peak flow quantiles, for example, the 100-year flood, in ungauged basins. RFF equations are pure statistical characterizations that use historical streamflow records and the concept of “homogeneous regions.” To supplement the accuracy of flood quantile estimates due to limited record lengths, a physical solution is required. It is further reinforced by the need to predict potential impacts of a changing hydro-climate system on flood frequencies. A nonlinear geophysical theory of floods, or a scaling theory for short, focused on river basins and abandoned the “homogeneous regions” concept in order to incorporate flood producing physical processes. Self-similarity in channel networks plays a foundational role in understanding the observed scaling, or power law relations, between peak flows and drainage areas. Scaling theory of floods offers a unified framework to predict floods in rainfall-runoff (RF-RO) events and in annual peak flow quantiles in ungauged basins.
Theoretical research in the course of time clarified several key ideas: (1) to understand scaling in annual peak flow quantiles in terms of physical processes, it was necessary to consider scaling in individual RF-RO events; (2) a unique partitioning of a drainage basin into hillslopes and channel links is necessary; (3) a continuity equation in terms of link storage and discharge was developed for a link-hillslope pair (to complete the mathematical specification, another equation for a channel link involving storage and discharge can be written that gives the continuity equation in terms of discharge); (4) the self-similarity in channel networks plays a pivotal role in solving the continuity equation, which produces scaling in peak flows as drainage area goes to infinity (scaling is an emergent property that was shown to hold for an idealized case study); (5) a theory of hydraulic-geometry in channel networks is summarized; and (6) highlights of a theory of biological diversity in riparian vegetation along a network are given.
The first observational study in the Goodwin Creek Experimental Watershed, Mississippi, discovered that the scaling slopes and intercepts vary from one RF-RO event to the next. Subsequently, diagnostic studies of this variability showed that it is a reflection of variability in the flood-producing mechanisms. It has led to developing a model that links the scaling in RF-RO events with the annual peak flow quantiles featured here.
Rainfall-runoff models in engineering practice use a variety of techniques to calibrate their parameters using observed streamflow hydrographs. In ungagged basins, streamflow data are not available, and in a changing climate, the reliability of historic data becomes questionable, so calibration of parameters is not a viable option. Recent progress on developing a suitable theoretical framework to test RF-RO model parameterizations without calibration is briefly reviewed.
Contributions to generalizing the scaling theory of floods to medium and large river basins spanning different climates are reviewed. Two studies that have focused on understanding floods at the scale of the entire planet Earth are cited.
Finally, two case studies on the innovative applications of the scaling framework to practical hydrologic engineering problems are highlighted. They include real-time flood forecasting and the effect of spatially distributed small dams in a river network on real-time flood forecasting.
People not only want to be safe from natural hazards; they also want to feel they are safe. Sometimes these two desires pull in different directions, and when they do, this slows the journey to greater physical adaptation and resilience.
All people want to feel safe—especially in their own homes. In fact, although not always a place of actual safety, in many cultures “home” is nonetheless idealized as a place of security and repose. The feeling of having a safe home is one part of what is termed ontological security: freedom from existential doubts and the ability to believe that life will continue in much the same way as it always has, without threat to familiar assumptions about time, space, identity, and well-being. By threatening our homes, floods, earthquakes, and similar events disrupt ontological security: they destroy the possessions that support our sense of who we are; they fracture the social structures that provide us with everyday needs such as friendship, play, and affection; they disrupt the routines that give our lives a sense of predictability; and they challenge the myth of our immortality. Such events, therefore, not only cause physical injury and loss; by damaging ontological security, they also cause emotional distress and jeopardize long-term mental health.
However, ontological security is undermined not only by the occurrence of hazard events but also by their anticipation. This affects people’s willingness to take steps that would reduce hazard vulnerability. Those who are confident that they can eliminate their exposure to a hazard will usually do so. More commonly, however, the available options come with uncertainty and social/psychological risks: often, the available options only reduce vulnerability, and sometimes people doubt the effectiveness of these options or their ability to choose and implement appropriate measures. In these circumstances, the risk to ontological security that is implied by action can have greater influence than the potential benefits. For example, although installing a floodgate might reduce a business’s flood vulnerability, the business owner might feel that its presence would act as an everyday reminder that the business, and the income derived from it, are not secure. Similarly, bolting furniture to the walls of a home might reduce injuries in the next earthquake, but householders might also anticipate that it would remind them that there is a continual threat to their home. Both of these circumstances describe situations in which the anticipation of future feelings can tap into less conscious anxieties about ontological security.
The manner in which people anticipate impacts on ontological security has several implications for preparedness. For example, it suggests that hazard warnings will be counterproductive if they are not accompanied by suggestions of easy, reliable ways of eliminating risk. It also suggests that adaptation measures should be designed not to enhance awareness of the hazard.
Scott C. Hagen, Davina L. Passeri, Matthew V. Bilskie, Denise E. DeLorme, and David Yoskowitz
The framework presented herein supports a changing paradigm in the approaches used by coastal researchers, engineers, and social scientists to model the impacts of climate change and sea level rise (SLR) in particular along low-gradient coastal landscapes. Use of a System of Systems (SoS) approach to the coastal dynamics of SLR is encouraged to capture the nonlinear feedbacks and dynamic responses of the bio-geo-physical coastal environment to SLR, while assessing the social, economic, and ecologic impacts. The SoS approach divides the coastal environment into smaller subsystems such as morphology, ecology, and hydrodynamics. Integrated models are used to assess the dynamic responses of subsystems to SLR; these models account for complex interactions and feedbacks among individual systems, which provides a more comprehensive evaluation of the future of the coastal system as a whole. Results from the integrated models can be used to inform economic services valuations, in which economic activity is connected back to bio-geo-physical changes in the environment due to SLR by identifying changes in the coastal subsystems, linking them to the understanding of the economic system and assessing the direct and indirect impacts to the economy. These assessments can be translated from scientific data to application through various stakeholder engagement mechanisms, which provide useful feedback for accountability as well as benchmarks and diagnostic insights for future planning. This allows regional and local coastal managers to create more comprehensive policies to reduce the risks associated with future SLR and enhance coastal resilience.
Marian Muste and Ton Hoitink
With a continuous global increase in flood frequency and intensity, there is an immediate need for new science-based solutions for flood mitigation, resilience, and adaptation that can be quickly deployed in any flood-prone area. An integral part of these solutions is the availability of river discharge measurements delivered in real time with high spatiotemporal density and over large-scale areas. Stream stages and the associated discharges are the most perceivable variables of the water cycle and the ones that eventually determine the levels of hazard during floods. Consequently, the availability of discharge records (a.k.a. streamflows) is paramount for flood-risk management because they provide actionable information for organizing the activities before, during, and after floods, and they supply the data for planning and designing floodplain infrastructure. Moreover, the discharge records represent the ground-truth data for developing and continuously improving the accuracy of the hydrologic models used for forecasting streamflows. Acquiring discharge data for streams is critically important not only for flood forecasting and monitoring but also for many other practical uses, such as monitoring water abstractions for supporting decisions in various socioeconomic activities (from agriculture to industry, transportation, and recreation) and for ensuring healthy ecological flows. All these activities require knowledge of past, current, and future flows in rivers and streams.
Given its importance, an ability to measure the flow in channels has preoccupied water users for millennia. Starting with the simplest volumetric methods to estimate flows, the measurement of discharge has evolved through continued innovation to sophisticated methods so that today we can continuously acquire and communicate the data in real time. There is no essential difference between the instruments and methods used to acquire streamflow data during normal conditions versus during floods. The measurements during floods are, however, complex, hazardous, and of limited accuracy compared with those acquired during normal flows. The essential differences in the configuration and operation of the instruments and methods for discharge estimation stem from the type of measurements they acquire—that is, discrete and autonomous measurements (i.e., measurements that can be taken any time any place) and those acquired continuously (i.e., estimates based on indirect methods developed for fixed locations). Regardless of the measurement situation and approach, the main concern of the data providers for flooding (as well as for other areas of water resource management) is the timely delivery of accurate discharge data at flood-prone locations across river basins.
Pedro J. Restrepo
The U.S. National Weather Service (NWS) is the agency responsible for flood forecasting. Operational flow forecasting at the NWS is carried out at the 13 river forecasting centers for main river flows. Flash floods, which occur in small localized areas, are forecast at the 122 weather forecast offices.
Real-time flood forecasting is a complex process that requires the acquisition and quality control of remotely sensed and ground-based observations, weather and climate forecasts, and operation of reservoirs, water diversions, and returns. Currently used remote-sense observations for operational hydrologic forecasts include satellite observations of precipitation, temperature, snow cover, radar observations of precipitation, and airborne observations of snow water equivalent. Ground-based observations include point precipitation, temperature, snow water equivalent, soil moisture and temperature, river stages, and discharge. Observations are collected by a number of federal, state, municipal, tribal and private entities, and transmitted to the NWS on a daily basis.
Once the observations have been checked for quality, a hydrologic forecaster uses the Community Hydrologic Prediction System (CHPS), which takes care of managing the sequence of models and their corresponding data needs along river reaches. Current operational forecasting requires an interaction between the forecaster and the models, in order to adjust differences between the model predictions and the observations, thus improving the forecasts. The final step in the forecast process is the publication of forecasts.
Philip Bubeck, Antje Otto, and Juergen Weichselgartner
Floods remain the most devastating natural hazard globally, despite substantial investments in flood prevention and management in recent decades. Fluvial floods, such as the ones in Pakistan in 2010 and Thailand in 2011, can affect entire countries and cause severe economic and human losses. Also, coastal floods can inflict substantial harm owing to their destructive forces in terms of wave and tidal energy. A flood type that received growing attention in recent years is flooding from pluvial events (heavy rainfall). Even though these are locally confined, their sudden onset and unpredictability pose a danger to areas that are generally not at risk from flooding. In the future, it is projected that flood risk will increase in many regions both because of the effects of global warming on the hydrological cycle and the continuing concentration of people and economic assets in risk-prone areas.
Floods have a large variety of societal impacts that span across space and time. While some of these impacts are obvious and have been well researched, others are more subtle and less is known about their complex processes and long-term effects. The most immediate and apparent impact of floods is direct damage caused by physical contact between floodwaters and economic assets, cultural heritage, or human beings, with the result for humans being injuries and deaths. Direct flood damage can amount to billions of US dollars for single events, such as the floods in the Danube and Elbe catchment in Central Europe in 2002 and 2013. More indirect economic implications are the losses that occur outside of the flood event in space and time, such as losses due to business disruption. The flood in Thailand in 2011, for instance, resulted in a lack of auto parts supplies and consequently the shutdown of car manufacturing within and outside the flood zone.
Floods also have long-term indirect impacts on flood-affected people and communities. Experiencing property damage and losing important personal belongings can have a negative psychological effect on flood victims. Much less is known about this type of flood impact: how long do these impacts last? What makes some people or communities recover faster than others from financial losses and emotional stress? Moreover, flood impacts are not equally distributed across different groups of society. Often, poor, elderly, and marginalized societal groups are particularly vulnerable to the effects of flooding inasmuch as these groups generally have little social, human, and financial coping capacities. In many countries, women regularly bear a disproportionately high burden because of their societal status.
Finally, severe floods often provide so-called windows of opportunities, enabling rapid policy change, resulting in new flood risk management policies. Such newly adopted policy arrangements can lead to societal conflicts over issues of interests, equity, and fairness. For instance, flood events often trigger large-scale investment in flood defense infrastructure, which are associated with high construction costs. Although these costs are usually borne by the taxpayer, often only a small proportion of society shares in their benefits. In addition, societal conflict can arise concerning where to build structural measures; what impacts these measures have on the ground regarding economic development potentials, different kinds of uses, and nature protection; and which effects are expected downstream. In such controversies, issues of participation and decision making are central and often highly contested.
While floods are usually associated with negative societal impacts in industrialized countries, they also have beneficial impacts on nature and society. In many parts of the world, the livelihood of millions of people depends on the recurring occurrence of flooding. For instance, farming communities in or near floodplains rely upon regular floodwaters that carry nutrients and sediments, enriching the soil and making it fertile for cultivation.
Parvin Sultana and Paul Thompson
Floodplains are ecologically diverse and important sources of livelihood for rural people. Bangladesh is one of the most floodplain-dominated countries and supports the highest density of rural population in the world. The experience of Bangladesh in floodplain management efforts provides evidence, lessons, and insights on a range of debates and advances in the management of floodplain natural resources, the challenges of climate change, and the role of local communities in sustaining these resources and thereby their livelihoods. Although floodplain areas are primarily used for agriculture, the significance and value of wild common natural resources—mainly fish and aquatic plants—as sources of income and nutrition for floodplain inhabitants has been underrecognized in the past, particularly with respect to poorer households. For example, capture fisheries—a common resource—have been adversely impacted by the building of embankments and sluice gates and by the conversion of floodplains into aquaculture farms, which also exclude poor subsistence users from wetland resources. More generally, an overreliance on engineering “solutions” to flooding that focused on enabling more secure rice cultivation was criticized, particularly in the early 1990s during the Flood Action Plan, for being top down and for ignoring some of the most vulnerable people who live on islands in the braided main rivers. Coastal embankments have also been found to have longer term environmental impacts that undermine their performance because they constrain rivers, which silt up outside these polders, contributing, along with land shrinkage, to drainage congestion. Locals responded in an innovative way by breaking embankments to allow flood water and silt deposition in to regain relative land levels.
Since the early 1990s Bangladesh has adopted a more participatory approach to floodplain management, piloting and then expanding new approaches; these have provided lessons that can be more general applied within Asia and beyond. Participatory planning for water and natural resource management has also been adopted at the local level. Good practices have been developed to ensure that disadvantaged, poor stakeholders can articulate their views and find consensus with other local stakeholders. The management of smaller water-control projects (up to 1,000 ha) has been taken on by community organizations, and in larger water-control projects, there is collaborative management (also called “co-management”) among a hierarchy of groups and associations and the appropriate government agency. In fishery and wetland management, many areas have been managed by community organizations to sustainably restore common resources, although their rights to do this were lost in some cases. Associated with community management are successful experiments in adopting a more system-based approach, called “integrated floodplain management,” which balances the needs of agriculture and common natural resources, for example, by adopting crops with lower water demands that are resilient to less predictable rainfall and drier winters, and enable communities to preserve surface water for wild aquatic resources. Bangladesh also has had success in demonstrating the benefits of systematic learning among networks of community organizations, which enhances innovation and adaptation to the ever-changing environmental challenges in floodplains.
Russ S. Schumacher
Heavy precipitation, which in many contexts is welcomed because it provides the water necessary for agriculture and human use, in other situations is responsible for deadly and destructive flash flooding. Over the 30-year period from 1986 to 2015, floods were responsible for more fatalities in the United States than any other convective weather hazard (www.nws.noaa.gov/om/hazstats.shtml), and similar findings are true in other regions of the world. Although scientific understanding of the processes responsible for heavy rainfall continues to advance, there are still many challenges associated with predicting where, when, and how much precipitation will occur. Common ingredients are required for heavy rainfall to occur, but there are vastly different ways in which the atmosphere brings the ingredients together in different parts of the world. Heavy precipitation often occurs on very small spatial scales in association with deep convection (thunderstorms), factors that limit the ability of numerical models to represent or predict the location and intensity of rainfall. Furthermore, because flash floods are dependent not only on precipitation but also on the characteristics of the underlying land surface, there are fundamental difficulties in accurately representing these coupled processes. Areas of active current research on heavy rainfall and flash flooding include investigating the storm-scale atmospheric processes that promote extreme precipitation, analyzing the reasons that some rainfall predictions are very accurate while others fail, improving the understanding and prediction of the flooding response to heavy precipitation, and determining how heavy rainfall and floods have changed and may continue to change in a changing climate.