Urban resilience. Urban resilience is defined as the "measurable ability of any urban system, with its inhabitants, to maintain continuity through all shocks and stresses, while positively adapting and transforming towards sustainability". It has conventionally been used to describe the change in structure and function of urban areas. Urban resilience is the ability of a city to survive, adapt, and grow in the face of chronic stresses and acute shocks. It refers to the capacity of a city's systems, institutions, businesses, communities, and individuals to withstand and respond to these stresses and shocks. 

A resilient city is one that assesses, plans and acts to prepare for and respond to hazards, regardless of whether they are natural or human-made, sudden or slow-onset, expected or unexpected. Resilient cities are better positioned to protect and enhance people's lives, secure development gains, and drive positive change.

History
According to urban historian Roger W. Lotchin, World War II had a profound environmental impact on urban areas in the USA. By 1945 Pittsburgh and other cities along the Mississippi River experienced levels of air pollution comparable to the Dust Bowl. The environmental impact of World War II turned urban areas around the world into shock cities. Examples of impacted cities include Hiroshima, Chongqing, Stalingrad, and Dresden. Environmental history first emerged as an academic research topic in the 1970s, focusing initially on rural areas. Pioneers of urban environmental history include Martin Melosi, Christine Rosen, Joel A. Tarr, Peter Brimblecombe, Bill Luckin, and Christopher Hamlin. 

In recent years, urban resilience concerns in the urban planning of cities have become more visible.  Social scientists have taken an increased interest in ecological resilience, because the links between social-ecological systems are being examined. Urban resilience is no longer the preserve of academics, and urban policy groups around the globe are putting forward proposals to enhance the urban resilience of cities. The definition of urban resilience varies, but is no longer limited to the speed at which an urban system recovers after a shock.

Academic research focus
Academic discussion of urban resilience has focused primarily on three threats: climate change, natural disasters, and terrorism. Accordingly, resilience strategies have tended to be conceived of in terms of counter-terrorism, other disasters (earthquakes, wildfires, tsunamis, coastal flooding, solar flares, etc.), and infrastructure adoption of sustainable energy. 

More recently, there has been increased attention to the evolution of urban resilience  and the capability of urban systems to adapt to changing conditions. This branch of resilience theory builds on the notion of cities as highly complex adaptive systems. As a result, academic discussions of urban planning include plans informed by network science, involving less interference in the functioning of cities. Network science provides a way of linking city size to the forms of networks that are likely to enable cities to function. These perspectives can provide further insights into the potential effectiveness of various urban policies. This requires a better understanding of the types of practices and tools that contribute to building urban resilience. Genealogical approaches explore the evolution of these practices over time, including the values and power relations underpinning them.

Investment decisions

Building resilience in cities relies on making investment decisions that prioritize spending on activities that offer alternatives that can perform well in different scenarios. Such decisions need to take into account future risks and uncertainties as risk can never be fully eliminated; emergency and disaster planning is crucial. Improvements in disaster risk management, for example, offer practical opportunities for enhancing resilience.

Since 2007, more than half of the world's human population has lived in cities, and urbanization is calculated to rise to 80% by 2050. The growing urbanization over the past century has been associated with a considerable increase in urban sprawl. Resilience efforts address not only how individuals, communities and businesses cope with multiple shocks and stresses, but also exploit opportunities for transformational development.

One way that national and local governments address disaster risk in urban areas, often supported by international funding agencies, is to consider resettlement. This can be preventative, or occur after a disaster. While resettlement reduces people's exposure to hazards, it can lead to other problems, which can leave people more vulnerable or worse off than they were before. Resettlement needs to be understood as part of long-term sustainable development, not just as a means for disaster risk reduction.

Sustainable Development Goal 11 

In September 2015, world leaders adopted the 17 Sustainable Development Goals (SDGs) as part of the 2030 Agenda for Sustainable Development. The goals, which build on and replace the Millennium Development Goals, officially came into force on 1 January 2016 and are expected to be achieved within the next 15 years. While the SDGs are not legally binding, governments are expected to take ownership and establish national frameworks for their achievement. Countries also have the primary responsibility for follow-up and review of progress based on accessible, timely, and high quality data. National reviews of regional progress will provide information on global progress of the initiative.

UN-Habitat's city resilience profiling tool 
As the UN Agency for Human Settlements, UN-Habitat is working to support local governments and their stakeholders in building urban resilience through the City Resilience Profiling Tool (CRPT). When applied, UN-Habitat's holistic approach to increasing resiliency can improve local government's ability to ensure the well being of citizens, protect development gains, and maintain functionality in the face of hazards. UN-Habitat supports cities to maximize the impact of CRPT implementation. The CRPT follows various stages, including the following: 

Getting started: Local governments and UN-Habitat connect to evaluate the needs, opportunities and context of the city and evaluate the possibility of implementing the tool in their city. They consider the stakeholders that need to be involved in implementation, including civil society organizations, national governments, and the private sector.

Engagement: By signing an agreement with a UN agency, the local government is better able to work with the necessary stakeholders to assess risk and build in resilience across the city.

Diagnosis: The CRPT provides a framework for cities to collect the right data about the city to better evaluate their resilience and identify potential vulnerabilities in their urban system. Diagnosis considers all elements of the urban system, including potential hazards and stakeholders. Effective action requires understanding of the entire urban system. 

Resilience Actions: The main output of the CRPT is a unique Resilience Action Plan (RAP) for each engaged city. The RAP sets out short-, medium- and long-term strategies based on the diagnosis. Actions are prioritized, assigned inter-departmentally, and integrated into existing government policies and plans. The process is iterative; after resilience actions have been implemented, local governments use the tool to monitor impact and identify any necessary next steps.

Taking it further: Resilience actions require the buy-in of all stakeholders and, in many cases, additional funding. However, with the detailed diagnosis resulting from the tool, local governments can leverage the support of national governments, donors, and other international organizations to work towards sustainable urban development.

To date, this approach has been adopted in Barcelona (Spain), Asuncion (Paraguay), Maputo (Mozambique), Port Vila (Vanuatu), Bristol (United Kingdom), Lisbon (Portugal), Yakutsk (Russia), and Dakar (Senegal). The biennial publication, Trends in Urban Resilience, is tracking the most recent efforts to build urban resilience as well as the actors behind these actions and a number of case studies.

Medellin Collaboration for Urban Resilience 
The Medellin Collaboration for Urban Resilience (MCUR) was launched in 2014 at the 7th session of the World Urban Forum in Medellín, Colombia. As a pioneering partnership platform, the MCUR gathers the most prominent actors committed to building resilience globally, including the United Nations Office for Disaster Risk Reduction (UNDRR), The World Bank Group, Global Facility for Disaster Reduction and Recovery, Inter-American Development Bank, Rockefeller Foundation, 100 Resilient Cities, C40, ICLEI and Cities Alliance, and it is chaired by UN-Habitat.

MCUR aims to jointly collaborate on strengthening the resilience of all cities and human settlements around the world by supporting local, regional and national governments through provision of knowledge and research, facilitating access to local-level finance, and raising global awareness on urban resilience through policy advocacy and adaptation diplomacy efforts. Its work is devoted to achieving the main international development agendas set out in the Sustainable Development Goals, the New Urban Agenda, the Paris Agreement on Climate Change and the Sendai Framework for Disaster Risk Reduction.

The MCUR helps local governments and municipal professionals understand the primary utility of the vast array of tools and diagnostics designed to assess, measure, monitor and improve city-level resilience. For example, some tools are intended as rapid assessments to establish a general understanding and baseline of a city's resilience and can be self-deployed, while others are intended as a means to identify and prioritise areas for investment. The Collaboration has produced a guidebook to illustrate how cities are responding to current and future challenges by thinking strategically about design, planning, and management for building resilience. Currently, it is working in a collaborative model in six pilot cities: Accra, Bogotá, Jakarta, Maputo, Mexico City and New York City.

100 Resilient cities and the City Resilience Index (CRI) 
The Rockefeller Foundation, rates 100 cities for resilience. The Rockefeller Foundation states that: "Urban resilience is the capacity of individuals, communities, institutions, businesses, and systems within a city to survive, adapt, and grow no matter what kinds of chronic stresses and acute shocks they experience."

A central program contributing to the achievement of SDG 11 is the Rockefeller Foundation's 100 Resilient Cities. In December 2013, The Rockefeller Foundation launched the 100 Resilient Cities initiative, which is dedicated to promoting urban resilience, defined as "the capacity of individuals, communities, institutions, businesses, and systems within a city to survive, adapt, and grow no matter what kinds of chronic stresses and acute shocks they experience". 

The professional services firm Arup has helped the Rockefeller Foundation develop the City Resilience Index (CRI) based on extensive stakeholder consultation across a range of cities globally. The CRI is intended as a planning and decision-making tool to help guide urban investments toward results that facilitate sustainable urban growth and the well-being of citizens. The hope is that city officials will utilize the tool to identify areas of improvement, systemic weaknesses and opportunities for mitigating risk. Its generalizable format also allows cities to learn from each other.

The CRI is a holistic articulation of urban resilience premised on the finding that there are 12 universal factors or drivers that contribute to city resilience. The factors vary in importance and are organized into four core dimensions of urban resilience

A total of 100 cities across six continents have signed up for the Rockefeller Center's urban resilience challenge. All 100 cities have developed individual City Resilience Strategies with technical support from a Chief Resilience Officer (CRO). The CRO ideally reports directly to the city's chief executive and helps coordinate all the resilience efforts in a single city.

Medellin in Colombia qualified for the urban resilience challenge in 2013. In 2016, it won the Lee Kuan Yew World City Prize.

Urban governance 
A core factor enabling progress on all dimensions of urban resilience is urban governance. Sustainable, resilient and inclusive cities are often the product of good governance, particularly including effective leadership, inclusive citizen participation, and efficient financing. Public officials also require access to robust data, enabling evidence-based decision making. Open data improves the ability of local governments to share information with citizens, deliver services, and monitor performance. Increased public access to information facilitates more direct citizen involvement in decision-making.

Digital technologies
As part of their resilience strategies, city governments are increasingly relying on digital technology as part of a city's infrastructure and service delivery systems. On the one hand, reliance on digital technologies and electronic service delivery has made cities more vulnerable to phone hacking and cyber-attacks. On the other hand, information technologies have often had a positive impact by supporting innovation and promoting efficiencies in urban infrastructure, thus leading to lower-cost city services. The deployment of new technologies in the initial construction of infrastructure have in some cases even allowed urban economies to leapfrog stages of development. An unintended outcome of the growing digitization of cities is the emergence of a digital divide, which can exacerbate inequality between well-connected affluent neighborhoods and business districts and under-serviced and under-connected low-income neighborhoods. In response, a number of cities have introduced digital inclusion programs to ensure that all citizens have the necessary tools to thrive in an increasingly digitized world.

Climate change 

The urban impacts of climate change vary widely geographically and among levels of development. A recent study  of 616 cities (home to 1.7 billion people, with a combined GDP of US$35 trillion, half of the world's total economic output), found that floods endanger more city residents than any other natural peril, followed by earthquakes and storms. Below is an attempt to define and discuss the challenges of heat waves, droughts and flooding.  Resilience-boosting strategies will be introduced and outlined.

Heat waves and droughts 
Heat waves are becoming increasingly prevalent as the global climate changes.  The 1980 United States heat wave and drought killed 10,000 people.  In 1988 a similar heat wave and drought killed 17,000 American citizens.  In August 2003 Europe saw record breaking summer temperatures with average temperatures persistently rising above 32°C.  In the UK, nearly 3,000 deaths were attributed to the heat wave during this period, with an increase of 42% in London alone,  and the heat wave claimed more than 40,000 lives across Europe.  Research indicates that by 2040 over 50% of summers will be warmer than 2003 and by 2100 those same summer temperatures will be considered cool.  The 2010 northern hemisphere summer heat wave was also disastrous, with nearly 5,000 deaths occurring in Moscow.  In addition to deaths, these heat waves also cause other significant problems.  Extended periods of heat and droughts also cause widespread crop losses, spikes in electricity demand, forest fires, air pollution and reduced biodiversity in vital land and marine ecosystems.  Agricultural losses from heat and drought might not occur directly within the urban area, but certainly affect the lives of urban dwellers. Crop shortages can lead to spikes in food prices, food scarcity, civic unrest and even starvation in extreme cases.  Direct fatalities from heat waves and droughts tend to be concentrated in urban areas,  not just because of increased population density, but because of social factors and the urban heat island effect.

Urban heat islands 
Urban heat island (UHI) refers to the presence of an inner-city micro-climate in which temperatures are higher than those in surrounding rural areas.  Recent studies have shown that summer daytime temperatures can be up to 10°C hotter in a city center than in rural areas and between 5–6°C warmer at night.  The causes of UHI are no mystery, and are mostly due to simple energy balances and geometrics.  The building materials commonly present in urban areas (concrete and asphalt) absorb and store heat much more effectively than the surrounding natural environment. The black color of asphalt surfaces (roads, parking lots and highways) is able to absorb significantly more electromagnetic radiation, further encouraging the rapid and effective capture and storage of heat throughout the day.  Geometrics come into play as well, as tall buildings provide large surfaces that both absorb and reflect sunlight and its heat energy onto other absorbent surfaces. These tall buildings also block the wind, which limits convective cooling. The large size of the buildings also blocks surface heat from naturally radiating back into the cool sky at night.  These factors, combined with the heat generated from vehicles, air conditioners, and industry ensure that cities create, absorb and hold heat very effectively.

Social factors for heat vulnerability 
The physical causes of heat waves and droughts and the exacerbation of the UHI effect are only part of the equation in terms of fatalities; social factors play a role as well. Statistically, senior citizens represent the majority of heat (and cold) related deaths within urban areas and this is often due to social isolation. In rural areas, seniors are more likely to live with family or in care homes, whereas in cities they are often concentrated in subsidized apartment buildings and in many cases have little to no contact with the outside world. Like other urban dwellers with little or no income, most urban seniors are unlikely to own an air conditioner. This combination of factors leads to thousands of tragic deaths every season, and the incidence is increasing each year.

Adapting for heat and drought resilience

Greening, reflecting and whitening urban spaces 

Greening urban spaces is among the most frequently mentioned strategies to address heat effects.  The idea is to increase the amount of natural cover within the city.  This cover can be made up of grasses, bushes, trees, vines, water, rock gardens; any natural material. Covering as much surface as possible with greenery will both reduce the total quantity of thermally absorbent artificial material, and by creating shade, will reduce the amount of light and heat that reaches the concrete and asphalt that cannot be replaced by greenery.  

Trees are among the most effective greening tool within urban environments because of their coverage/footprint ratio.  Trees require a very small physical area for planting, but when mature, they provide a much larger coverage area. Trees absorb solar energy for photosynthesis (improving air quality and mitigating global warming), reducing the amount of energy being trapped and held within artificial surfaces, and also cast much-needed shade on the city and its inhabitants.  Shade itself does not lower the ambient air temperature, but it greatly reduces the perceived temperature and comfort of those seeking its refuge. 

An increasingly popular method of preventing the so called urban heat island (UHI) is to increase the albedo (light reflectiveness). This can be done by using reflective paints or materials where appropriate, or white and light colored paints. Glazing can also be added to windows to reduce the amount of heat that buildings or roofs generate and store. 

Green roofs also help reduce the urban heat island effect and improve the resilience to urban flooding. Restoring ponds and lakes and other types of urban open water can also help as shown by Beijing, China's "Dragon-shaped Lake". Depaving urban footpaths and roads has also been found to be effective in urban flood control, and may be a more cost-efficient approach.

Social strategies 
There are various strategies to increase the resilience of those most vulnerable to urban heat waves, primarily socially isolated seniors, but also young children (especially those facing abject poverty or living in informal housing), people with underlying health problems, the infirm or disabled, and the homeless. Accurate and early prediction of heat waves is of fundamental importance, as it gives time for the government to issue extreme heat alerts. Urban areas must prepare and be ready to implement heat-wave emergency response initiatives. Seasonal campaigns aimed to educate the public on the risks associated with heat waves will help prepare the broad community, but in response to impending heat events more direct action is required.  

Local government must quickly communicate with the groups and institutions that work with heat-vulnerable populations. Cooling centers should be opened in libraries, community centers and government buildings. These centers ensure free access to air conditioning and water. In partnership with government and non-government social services, paramedics, police, firefighters, nurses and volunteers; the above-mentioned groups working with vulnerable populations should carry out regular door-to-door visits during these extreme heat scenarios.  These visits should provide risk assessment, advice, bottled water (for areas without potable tap water) and the offer of free transportation to local cooling centers.

Food and water supplies 

Heat waves and droughts can cause massive damage to agricultural areas vital to providing food staples to urban populations.  Reservoirs and aquifers quickly dry up due to increased demand on water for drinking, industrial and agricultural purposes. The result can be food shortages and price spikes, and increasingly, shortages of drinking water as observed with increasing severity seasonally in China and throughout most of the developing world. From an agricultural standpoint, farmers can be encouraged to plant more heat and drought-resistant crops. Agricultural practices can also be modified to higher levels of hydrological efficiency.  Reservoirs should be expanded and new reservoirs and water towers should be constructed in areas facing critical shortages.  Grander schemes of damming and redirecting rivers should also be considered if possible. For saltwater coastal cities, desalination plants provide a possible solution to water shortages. Infrastructure improvements may also enhance resilience, as in many areas aging pipelines result in leakage and possible contamination of drinking water. In Kenya’s major cities, Nairobi and Mombasa, between 40 and 50% of drinking water is lost through leakage.  In such cases, replacements and repairs are clearly needed.

Flooding 

Flooding, either from weather events, rising sea levels or infrastructure failures are a major cause of death, disease and economic losses throughout the world. Climate change and rapidly expanding urban settlements are two factors that increase occurrence and severity of urban flooding, especially in the developing world. Storm surges can affect coastal cities and are caused by low pressure weather systems, like cyclones and hurricanes.  Flash floods and river floods can affect any city within a floodplain or with inadequate drainage infrastructure. These can be caused by large quantities of rain or heavy rapid snow melt. With all forms of flooding, cities are more vulnerable because of the large quantity of paved and concrete surfaces; these impermeable surfaces cause massive amounts of runoff that can quickly overwhelm the limited infrastructure of storm drains, flood canals and intentional floodplains. Many cities in the developing world simply have no infrastructure whatsoever to redirect floodwaters.  Around the world, floods kill thousands of people every year and are responsible for billions of dollars in damages and economic losses.  In cities with poor or absent drainage infrastructure, flooding can also lead to the contamination of drinking water sources (aquifers, wells, inland waterways) with salt water, chemical pollution, and most frequently, viral and bacterial contaminants. Flooding, much like heat waves and droughts, can also wreak havoc on agricultural areas, quickly destroying large amounts of crops.

Flood flow in urban environment 
The flood flow in urbanized areas constitutes a hazard to population and infrastructure. Some recent catastrophes included the inundations of Vaison-la-Romaine (France) in 1992, Nîmes (France) in 1998, New Orleans (USA) in 2005, and the flooding in Rockhampton, Bundaberg, and Brisbane in Queensland (Australia) during the summer of 2010–2011. Flood flows in urban environments have been studied only relatively recently despite many centuries of flood events. Several studies looked into the flow patterns and redistribution in streets during storm events and the implication in terms of flood modelling.

Some research considered the criteria for safe evacuation of individuals in flooded areas. But some recent field measurements during the 2010–2011 Queensland floods showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot fully account for all the hazards due to flooding as they do not take into account risks associated with large debris entrained by the flow.

Adapting for flood resilience

Urban greening 

Replacing as much non-porous surface as possible with greenery will allow the ground and plants to help absorb excess water.  Green roofs are gaining popularity; they vary from very thin layers of soil or rockwool supporting a variety of low or no-maintenance mosses or sedum species to large, deep, intensive roof gardens capable of supporting large plants and trees but requiring regular maintenance and significant structural support.  The deeper the soil, the more rainwater it can absorb and therefore the more potential floodwater it can prevent from reaching the ground. 

One of the best strategies, if possible, is to simply create enough space for the excess water by expanding areas of parkland in or adjacent to the zone where flooding is most likely to occur. Excess water is diverted into these areas when necessary, as in Cardiff, Wales around the new Millennium Stadium and at the main Olympic site in Beijing, China .  

Floodplain clearance is another greening strategy that involves removing structures and pavement built on floodplains and returning the area to its natural habitat which is capable of absorbing massive quantities of water that otherwise would have flooded the built-up urban area.

Flood-water control 

Levees and other flood barriers are indispensable for cities on floodplains or along rivers and coasts. In areas with lower financial and engineering capital, there are cheaper and simpler options for flood barriers.  UK engineers are currently conducting field tests of a new technology called the SELOC (Self-Erecting Low-Cost Barrier).  The barrier itself lies flat on the ground, and as the water rises, the SELOC floats up, with its top edge rising with the water level. A restraint holds the barrier in the vertical position. This simple, inexpensive flood barrier has great potential for increasing urban resilience to flood events and shows significant promise for developing nations with its low cost and simple, fool-proof design. The creation or expansion of flood canals and/or drainage basins can help direct excess water away from critical areas and the utilization of innovative porous paving materials on city streets and car parks allow for the absorption and filtration of excess water.

During the January 2011 flood of the Brisbane River (Australia), some unique field measurements about the peak of the flood showed very substantial sediment fluxes in the Brisbane River flood plain, consistent with the murky appearance of floodwaters.

Structural resilience 
In most developed nations, all new developments are assessed for flood risks. The aim is to ensure flood risk is taken into account in all stages of the planning process to avoid inappropriate development in areas of high risk. When development is required in areas of high risk, structures should be built to flood-resistant standards and living or working areas should be raised well above the worst-case scenario flood levels. For existing structures in high-risk areas, funding should be allocated to remediation, for example raising electrical wiring/sockets so any water that enters the home can not reach the electrics. Other solutions are to raise structures to appropriate heights or make them floating; as a last resort, considerations should be made to relocate or rebuild structures on higher ground. A house in Mexico Beach, Florida which survived Hurricane Michael is an example of a house built to survive tidal surge.

The pre-Incan Uru people of Lake Titicaca in Peru have lived on floating islands made of reeds for hundreds of years. The practice began as an innovative form of protection from competition for land by various groups, and it continues to support the Uru homeland. The manual technique is used to build homes resting on hand-made islands all from simple reeds from the totora plant. Similarly, in the southern wetlands of Iraq, the Marsh Arabs (Arab al-Ahwār) have lived for centuries on floating islands and in arched buildings all constructed exclusively from local qasab reeds. Without any nails, wood, or glass, buildings are assembled by hand as quickly as within a day; such homes can also be disassembled in a day, transported, and reassembled.

Emergency response 
As with all disasters, flooding requires a specific set of disaster response plans. Various levels of contingency planning should be established, from basic medical and selective evacuation provisions involving local emergency responders all the way up to full military disaster relief plans involving air-based evacuations, search and rescue teams, and provisions for relocation of entire urban populations. Clear lines of responsibility and chains of command must be laid out, and tiered priority response levels should be established to address the immediate needs of the most vulnerable citizens first. Sufficient emergency funding should be set aside for post-flooding repair and reconstruction.

World education and research relating to urban resilience
The United States

Urban resilience as an educational topic in the USA has experienced an unprecedented level of growth due in large part to a series of natural disasters including the 2004 Indian Ocean earthquake and tsunami, 2005 Hurricane Katrina, the 2011 Tohoku earthquake and tsunami, and Hurricane Sandy in 2012. Two of the more well-recognized programs are Harvard Graduate School of Design's Master's program in Risk and Resilience, and Tulane University's Disaster Resilience Leadership Academy. There are also several workshops available related to the U.S. Federal Emergency Management Agency and the Department of Homeland Security.

China

China's resilient cities research started relatively late, involving theories, scholars, and disciplines mostly from the United States. However, with the establishment of China's Ministry of Emergency Management and the country's deepening awareness of and emphasis on earthquake prevention and mitigation, related research and institutions have developed rapidly. A number of universities, including Zhejiang University's Ren Center for Resilience, have made significant contributions to the promotion and application of resilient cities concepts in China.

Challenges with further mainstreaming of urban resilience approaches

There are at least three key challenges to further mainstreaming innovative approaches to urban resilience. First, urban development systems have tended to see urban resilience schemes as public projects entailing a significant burden on the state to finance, plan and manage them. This is a classic problem of externalities, in which private developers are too often not required to bear the costs of remediating the consequences of their activities. Second, urban planning regulations typically do not require urban resilience measures in the same way they require fire detection and suppression or road access. Third, too many professionals in urban design, engineering and the environmental sciences lack awareness of innovative approaches to resilience and so cannot practice them.

See also 
Co-benefits of climate change mitigation
Energy security

 New Urbanism
 Sustainable urbanism
 Urban vitality

References 

Urban planning
Disaster management