Managing dynamic flood risks in an urbanizing world

Rebecca E. Morss
Boulder, USA

Despite extensive resources spent on flood mitigation and preparedness, floods continue to have disastrous impacts. Property losses from flooding are growing, and even with current forecast and warning capabilities, floods still cause significant loss of life — on average, 10,000 deaths per year (Jonkman, 2005). Many of the largest flood disasters, with the greatest loss of life, occur in developing countries. However, recent examples (such as Superstorm Sandy, Hurricane Matthew, and the 2016 Louisiana flooding in the U.S.) demonstrate that flooding continues to have devastating impacts in many areas around the world.

Even more troubling, climate change combined with urbanization, land subsidence, and growing population and property at risk are expected to further increase flood impacts, especially in coastal and urban areas (Hallegatte et al., 2013). The growing infrastructure, reliance on technology, and social and economic interconnections associated with urbanization and globalization are also increasing the complexity of flood disasters. These trends threaten lives, property, and human and environmental well-being, as well as longer-term sustainability.

Many approaches to managing flood risks, especially structural mitigation measures, rely on estimates of flood frequencies and associated inundation that treat flood risks as static. Urbanization creates challenges for such approaches by changing flood risks, in multiple ways. The increase in impervious surfaces associated with urbanization decreases water storage capacity and accelerates water runoff, which can increase the likelihood of flooding and its magnitude. Streets, bridges, and other infrastructure alter where flooding occurs and its character, as can the modification of stream channels and water drainage networks that accompanies urban development (Konrad, 2003). As urban, suburban, and peri-urban areas develop and evolve, new structures and infrastructure are built, modified, and then modified again, resulting in continually changing flood risks. Moreover, as people move to and within urbanizing areas, their exposure and sensitivity to flooding and their capacities to prepare, cope, and respond can change dramatically, often in ways that are not apparent until a flood occurs. This means that even without accounting for climate variability and change, flood risk assessment and management in an urbanizing world cannot rely on approaches that are based, for example, on historical records of streamflow or experiential knowledge from historical extreme flood events.

Flood losses occur at the intersection of natural, built, and human systems — all of which are evolving, in different ways in different places around the world. To effectively manage flood risks in this ever-changing world, it is important to broaden the framing of flood risk management, to approach flood risks as dynamic and embedded within larger systems (Merz et al., 2014). As the factors contributing to flood risks evolve and interact, they will continue to bring “surprises” that raise new challenges — especially in places where urbanization is rapidly changing the hydrological system, the built environment, and society.

No matter how effectively societies mitigate and prepare, floods that exceed long-term mitigation efforts can always occur, especially in an urbanizing world with a variable and changing climate. Moreover, long-term flood mitigation activities that focus on alleviating the risks of smaller floods (up to a certain “design” level) can transfer risk to rarer, more extreme floods, by increasing population and property at risk, decreasing people’s experience with flooding, and reducing flood risk awareness and preparedness (Morss et al., 2011; Ludy & Kondolf, 2012). When a flood occurs, advance warning can help people at risk take actions that reduce losses and the costs of post-event response and recovery. Thus, flood forecasting, warning, and risk communication play important roles in effective management of dynamic flood risks, including the inevitable surprises.

Most buildings and infrastructure cannot easily be moved out of harm’s way when a flood threatens. This limits the potential for flood forecasts and warnings to directly reduce large property losses, unless the information can be used to prevent flooding or direct the water away from certain locations. However, many smaller items can be moved, with hours or even a few minutes of warning. Although moving smaller items may not significantly reduce the direct monetary value of losses, it can dramatically reduce the negative human impacts of flooding and facilitate recovery. For example, in interviews that a colleague and I conducted after Hurricane Ike flooded Galveston, TX, some Galveston residents explained that they lost personally valuable items, such as photo albums or important documents, because they did not know they were at risk from flooding as the hurricane approached, or because they moved the items to a higher location but not high enough (Morss & Hayden, 2010). In recent focus groups that several colleagues conducted in areas of New York City flooded by Superstorm Sandy, some lower-income participants discussed how they lost key documents or tools on which their livelihood depended, which greatly complicated their already-difficult post-storm recovery (Morss et al., 2017). In other words, more effective forecast and warning communication as these storms approached could have helped people protect items important for their long-term well-being. As these examples illustrate, approaching flood risks as a dynamic interaction among natural, built, and social systems can help reveal new frames for flood risk management.

This slideshow requires JavaScript.

Flood forecasts and warnings can also reduce loss of life, by helping people move and stay out of harm’s way. Here, urbanization again creates additional challenges, especially for rapid-onset flooding caused by heavy rain. Such flooding evolves rapidly and is often highly spatially variable, due to the complex ways in which rain interacts with the urban hydrology and infrastructure to produce floodwater flow. It is also difficult to forecast, due to these complex interactions combined with the challenges of predicting rain amounts on the spatial and temporal scales needed to anticipate where flooding will occur. In fact, even when an urban flood is already in progress, those with the best information available (such as meteorological and hydrological forecasters and emergency management personnel) may not know where and how severely it is flooding currently, much less in a few minutes or hours (Morss, 2010: Morss et al., 2015).

These characteristics can make it extremely challenging to manage risks proactively rather than reactively when rapid-onset flooding threatens. Knowing who and what is at risk is further complicated by the volume and movement of people in urbanized areas. Moreover, in urban environments, it can be difficult for people to use their own observations during a flood event to assess risks and evaluate the safest course of action (Ruin et al., 2014; Lazrus et al., 2016). To improve flood risk communication and protective decision making for these types of complex urban floods, it is necessary to understand what can be observed and predicted on different spatial and temporal scales, how that intersects with the decisions that people can make to reduce harm, and how to convey that information in ways that enables protective decisions.

For many populations, urbanization is also accompanied by social and technological changes that influence the communication, interpretation, and use of flood risk information. For example, advances in information and communication technology (such as mobile phones, the Internet, and social media) are transforming how people around the world interact with and use information, including information about floods and other types of hazards (Morss et al., 2017). At the same time, even digitally connected people still rely extensively on social networks to communicate, obtain, and process risk information – social networks that often change as people move to and within urbanized areas. In this rapidly evolving modern information environment, improving flood risk communication and protective decision-making will require understanding how different populations interact with, interpret, and respond to different types of risk information from different sources across the physical and digital worlds.

Reducing the growing losses from flooding in an urbanizing world requires multi-faceted, flexible risk management strategies. Although scientific and technological capabilities for assessing and predicting flood risks are advancing rapidly, major gaps remain in translating these advances into reduced harm from floods. Long-term flood risk mitigation efforts, such as flood control and land use planning, must be implemented in ways that approach flood risks as dynamic and interacting, so that they do not inadvertently increase flood losses over the long term. When a flood threatens, effective forecasting, warning, and risk communication are also critical, to enable people at risk and those who help protect them to make decisions that reduce harm. Building sustainable, vibrant cities will require integrating scientific and engineering approaches to flood risk assessment and management with a deep understanding of the social factors that create and help alleviate flood disasters (Godschalk, 2003). By bringing different perspectives together in a dynamic, interactive frame, we can design strategies that empower at-risk populations’ capacities to manage the ever-evolving flood risks that they face in a changing world.


Many of the ideas for this post were developed in collaboration with colleagues in the Weather Risks and Decisions in Society program (, and with colleagues on the Communicating Hazard Information in the Modern Environment project (which is supported by National Science Foundation award 1331490,


Godschalk, D. R., 2003: Urban hazard mitigation: Creating resilient cities. Natural Hazards Review, 4, 136-143.  doi: 10.1061/(ASCE)1527-6988(2003)4:3(136)

Hallegatte, S., C. Green, R. J. Nicholls, and J. Corfee-Morlot, 2013: Future flood losses in major coastal cities. Nature climate change, 3, 802-806.  doi: 10.1038/nclimate1979

Jonkman, S. N., 2005: Global perspectives on loss of human life caused by floods. Natural Hazards, 34, 151-175.  doi: 10.1007/s11069-004-8891-3

Konrad, C. P., 2003: Effects of urban development on floods. U.S. Geological Survey Fact Sheet 076-03, 4p. Available at:

Lazrus, H., R. E. Morss, J. L. Demuth, A. Bostrom, J. K. Lazo, 2016: “Know what to do if you encounter a flash flood”: Mental models analysis for improving flash flood risk communication and public decision making. Risk Analysis, 36, 411-427.  doi: 10.1111/risa.12480

Ludy, J., and G. M. Kondolf, 2012: Flood risk perception in lands “protected” by 100-year levees. Natural Hazards, 61, 829-842.  doi: 10.1007/s11069-011-0072-6

Merz, B., J. C. J. H. Aerts, K. Arnbjerg-Nielsen, M. Baldi, A. Becker, A. Bichet, G. Blöschl, L. M. Bouwer, A. Brauer, F. Cioffi, J. M. Delgado, M. Gocht, F. Guzzetti, S. Harrigan, K. Hirschboeck, C. Kisby, W. Kron, H.-H. Kwon, U. Lall, R. Merz, K. Nissen, P. Salvatti, T. Swierczynski, U. Ulbrich, A. Viglione, P.J. Ward, M. Weiler, B. Wilhelm, and M. Neid, 2014: Floods and climate: Emerging perspectives for flood risk assessment and management. Natural Hazards and Earth System Sciences, 14, 1921-1942.  doi: 10.5194/nhess-14-1921-2014

Morss, R. E., 2010: Interactions among flood predictions, decisions, and outcomes: A synthesis of three cases. Natural Hazards Review, 11, 83-96. doi: 10.1061/(ASCE)NH.1527-6996.0000011

Morss, R. E., and M. H. Hayden, 2010: Storm surge and “certain death”: Interviews with Texas coastal residents following Hurricane Ike. Weather, Climate, and Society, 2, 174-189.  doi: 10.1175/2010WCAS1041.1

Morss, R. E., O. V. Wilhelmi, G. A. Meehl, and L. Dilling, 2011: Improving societal outcomes of extreme weather in a changing climate: An integrated perspective. Annual Review of Environment and Resources, 36, 1-25.  doi: 10.1146/annurev-environ-060809-100145

Morss, R. E., J. L. Demuth, A. Bostrom, J. K. Lazo, and H. Lazrus, 2015: Flash flood risks and warning decisions in Boulder, Colorado: A mental models study of forecasters, public officials, and media broadcasters. Risk Analysis, 35, 2009-2028.  doi: 10.1111/risa.12403

Morss, R. E., J. L. Demuth, H. Lazrus, L. Palen, K. Anderson, C. M. Barton, C. Davis, C. Snyder, O. Wilhelmi, D. Ahijevych, J. Anderson, M. Bica, K. Fossell, J. Henderson, M. Kogan, K. Stowe, J. Watts, 2017: Hazardous weather prediction and communication in the modern information environment. Bulletin of the American Meteorological Society, accepted pending minor revisions.

Ruin, I., C. Lutoff, B. Boudevillain, J.-D. Creutin, S. Anquetin, M. Bertran Rojo, L. Boissier, L. Bonnifait, M. Borga, L. Colbeau-Justin, L. Creton-Cazanave, G. Delrieu, J. Douvinet, E. Gaume, E. Gruntfest, J.-P. Naulin, O. Payrastre, and O. Vannier, 2014: Social and hydrological responses to extreme precipitations: An interdisciplinary strategy for post-flood investigation. Weather, Climate, and Society, 6, 135–153.  doi: 10.1175/WCAS-D-13-00009.1



Dr. Rebecca E. Morss is a Senior Scientist at the National Center for Atmospheric Research (Boulder, Colorado, USA), where she is also Deputy Director of the Mesoscale and Microscale Meteorology Laboratory.
Web site

Cover Image: Inundated areas in New Orleans following Hurricane Katrina. September 2005.  Credit: National Oceanic and Atmospheric Administration/Department of Commerce / Lieut. Commander Mark Moran, NOAA Corps, NMAO/AOC.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s