Urban Resilience is defined as the “capability to prepare for, respond to, and recover from significant multi-hazard threats with minimum damage to public safety and health, the economy, and security"[1] of a given urban area. Contemporary academic discussion of urban resilience focuses on three distinct threats; climate change, natural disasters and terrorism.[2][3] This article will initially focus on the challenges and disasters specific to climate change, and future additions on counter-terrorism, other disasters (earthquakes, tsunamis, solar flares, etc.) and sustainable energy strategies are welcome and encouraged.
The urban impacts of climate change vary widely across geographical and developmental scales. This article will define and discussing the challenges of heat waves, droughts and flooding. Resilience-boosting strategies will be introduced and outlined. Resilience is especially important in urban areas, because over the past century there has been a considerable increase in urbanisation and urban sprawl. Half of the world’s population now lives in cities, a figure that is set to rise to 80% by 2050.[4] Mass density of people makes them especially vulnerable both to the impacts of acute disasters and the slow, creeping effects of the changing climate; all making resilience planning critically important.
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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.[5] In August 2003 the UK saw record breaking summer temperatures with average temperatures persistently rising above 32°C. Nearly 3,000 deaths were contributed to the heat wave in the UK during this period, with an increase of 42% in London alone.[6] This heat wave claimed more than 40,000 lives across Europe.[7] 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.[8] The 2010 northern hemisphere summer heat wave was also disastrous, with nearly 5,000 deaths occurring in Moscow.[9] 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.[10] Agricultural losses from heat and drought might not occur directly within the urban area, but it certainly affects the lives of urban dwellers. Crop supply shortages can lead to spikes in food prices, food scarcity, civic unrest and even starvation in extreme cases. In terms of the direct fatalities from these heat waves and droughts, they are statistically concentrated in urban areas,[11] and this is not just in line with increased population densities, but is due to social factors and the urban heat island effect.
Urban heat island (UHI) refers to the presence of an inner-city microclimate in which temperatures are comparatively higher than in the rural surroundings. Recent studies have shown that summer daytime temperatures can reach up to 10°C hotter in a city centre than in rural areas and between 5-6°C warmer at night.[12] The causes of UHI are no mystery, and are mostly based on simple energy balances and geometrics. The materials commonly found in urban areas (concrete and asphalt) absorb and store heat energy much more effectively than the surrounding natural environment. The black colouring 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 sheer size of the buildings also blocks surface heat from naturally radiating back into the cool sky at night.[13] These factors, combined with the heat generated from vehicles, air conditioners and industry ensure that cities create, absorb and hold heat very effectively.
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[14] 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 subsidised apartment buildings and in many cases have little to no contact with the outside world.[15] 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 incidences are increasing each year.[16]
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 green space will both reduce the total quantity of thermally-absorbent artificial material, but the shading effect will reduce the amount of light, heat and electromagnetic radiation that reaches the concrete and asphalt that cannot be replaced by greenery.[17] 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. This both absorbs solar energy for photosynthesis (improving air quality and mitigating global warming), reducing the amount of energy being trapped and held within artificial surfaces, but also casts 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.[18] A popular method of reducing UHI is simply increasing the albedo (light reflectiveness) of urban surfaces that cannot be ‘greened’. This is done by using reflective paints or materials where appropriate, or white and light-coloured options where reflections would be distracting or dangerous. Glazing can also be added to windows to reduce the amount of heat entering buildings.[19] Green roofs are also a resilience-boosting option, and have synergies with flood resilience strategies as well.
There are various strategies to increase the resilience of those most vulnerable to urban heat waves. As established, these vulnerable citizens are primarily socially-isolated seniors. Other vulnerable groups include 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.[20] Local government must quickly communicate with the groups and institutions that work with heat-vulnerable populations. Cooling centres should be opened in libraries, community centres and government buildings. These centres ensure free access to air conditioning and water. In partnership with government and non-government social services, paramedics, police, firefighters, nurses and volunteers; the abovementioned 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 centres.[21]
Heat waves and droughts can reap massive damage on 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 end result can be shortages and price spikes for food and with increasing frequency, shortages of drinking water as observed with increasing severity seasonally in China[22] and throughout most of the developing world.[23] From an agricultural standpoint, farmers can be required to plant more heat and drought-resistant crops. Agricultural practices can also be streamlined to higher levels of hydrological efficiency. Reservoirs should be expanded and new reservoirs and water towers should be constructed in areas facing critical shortages.[24] 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 also plays a role in 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 40-50% of drinking water is lost through leakage.[25] In these types of cases, replacements and repairs are clearly needed.
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 are leading to the increasing occurrence and severity of urban flood events, especially in the developing world.[26][27][28] Storm surges can affect coastal cities and are caused by low pressure weather systems, like cyclones and hurricanes.[29] 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 increasingly vulnerable because of the large quantity of paved and concrete surfaces. These impermeable surfaces cause massive amounts of runoff and can quickly overwhelm the limited infrastructure of storm drains, flood canals and intentional floodplains. Many cities in the developing world simply have no infrastructure to redirect floodwaters whatsoever.[30] Around the world, floods kill thousands of people every year and are responsible for billions of dollars in damages and economic losses.[31] Flooding, much like heat waves and droughts, can also wreak havoc on agricultural areas, quickly destroying large amounts of crops. 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.[32]
The flood flow in urbanised areas constitutes a hazard to the population and infrastructure. Some recent catastrophes included the inundations of Nîmes (France) in 1998 and Vaison-la-Romaine (France) in 1992, the flooding of New Orleans (USA) in 2005, the flooding in Rockhampton, Bundaberg, Brisbane during the 2010-2011 summer in Queensland (Australia). Flood flows in urban environments have been studied relatively recently despite many centuries of flood events.[30] Some researchers mentioned the storage effect in urban areas. Several studies looked into the flow patterns and redistribution in streets during storm events and the implication in terms of flood modelling.[33]
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 account for the hazards caused by the velocity and water depth fluctuations.[30] These considerations ignore further the risks associated with large debris entrained by the flow motion.
Replacing as many non-porous surfaces with green space as possible will create more areas for natural ground (and plant-based) absorption of excess water. Gaining popularity are different types of green roofs. Green roofs vary in their intensity, 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 more structural support.[34] 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. This involves planning or 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, around the new Millennium Stadium.[35] Floodplain clearance is another greening strategy that fundamentally removes structures and pavement built on floodplains and returns them to their natural habitat which is capable of absorbing massive quantities of water that otherwise would have flooded the built urban area.[32]
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[35] 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[36] and the utilisation of innovative porous paving materials on city streets and car parks allow for the absorption and filtration of excess water.[19]
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 raise these structures to appropriate heights[37] or considerations should be made to relocate or rebuild structures on higher ground.
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 right the way up to full military disaster relief plans involving air-based evacuations, search and rescue teams and relocation provisions for 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. For post-flooding repair and reconstruction sufficient emergency funding should be set aside proactively.[38]