Urban Hazards and Their Mitigation

J.H. Crawford

12 December 2004

Almost all regions of the world face serious hazards. A few risks, such as the volcano that obliterated Pompeii 2000 years ago, are so serious that cities simply must be kept out of the affected area. Other risks can be mitigated, but some level of risk is intrinsic to most sites.

Hazard Identification

One of the first tasks in planning a new urban area is comprehensive hazard identification. New research is sometimes required, but in most cases the hazards have long been known, even if the risk assessment is imperfect. It is essential to gather all available information and then to determine whether the existing studies are sufficiently reliable to guide development plans.


Having suffered from the smoke of a neighbor’s fire in 2002, I am now acutely sensitive to fire risks. I was lucky and lost nothing worse than some time, but this fire might have had a worse outcome. Throughout history, entire cities have burned. However, as fireproof materials were adopted for urban construction, the risk was reduced. (Do remember, though, that a firestorm struck Chelsea, Massachusetts, as recently as 1973.) The risk of fire will never be eliminated, but it can be dramatically reduced and the severity of the consequences greatly mitigated.

In urban areas, structures must be built of non-combustible materials, which is standard practice today. The installation of automatic sprinklers further reduces the chance of a fire getting out of control and probably should be required.

It is, of course, insane to locate incendiary activities in populous areas. A firewroks factory exploded in the Dutch town of Enschede in 2000, with heavy loss of life in the adjacent community. Facilities with such a high disaster index simply must be located where they can do comparatively little damage.

Emergency Access

Considering the Reference Topology, I think that each district ought to be provided with its own small fire company. This would provide very rapid response to a fire, which is critical for life saving efforts. Companies from adjacent districts would be called in to help fight all but the smallest fires.

Access requirements for fire equipment demands analysis. The requirements are affected by the height of buildings, which governs the length of ladders and booms. Street widths and corner radii must be considered when planning fire vehicle access. A small fire vehicles can negotiate narrower streets. In some parts of Venice, ladders have been permanently situated where it would take too long to bring a long ladder.

Fire protection in Venice is provided by boat. Venice’s Fenice opera house burned in 1995, in part because the adjacent canals had been drained for repair, and the fire boats could not reach the scene of what began as a minor fire. One of the tasks of city managers is to ensure that fire brigades can reach the scene of any fire, notwithstanding maintenance work in progress.

Not every street requires fire truck access, but no street that is too narrow to permit such access should be very far removed from wider streets. Standards vary from jurisdiction to jurisdiction, but 75 m (250 ft) is a reasonable point of departure. The provision of force-main fire hydrants at every intersection will minimize the required hose runs and quickly allow water to be put on a fire. If building heights are limited and pressure is high enough, then no pumper is needed, only hose.

The area that must be kept clear of obstructions can be paved in a contrasting color, to define emergency clearways. Corners must be eased to permit larger vehicles to turn. Local authorities will govern in this matter, but their initial requirements should not be blindly accepted. Actual tests with fire trucks have shown that considerably narrower roadways than standard are entirely acceptable. The New Urbanists have struggled with this and usually managed to obtain reasonable regulation.

Fireproof Construction

No urban building should be constructed of flammable materials except for trim and finishing. The entire structure, especially including the floors, should be fireproof. In practice, this means either reinforced concrete or steel members treated with thick coats of fire-resistant material. Flammable roofing material must never be permitted. Conflagrations become nearly impossible when all buildings are substantially fireproof; fire damage will be limited to a few buildings at worst.

Fire Exits

Standard practice in American cities requires fire escapes from upper floors, and these must be independent of the normal stairwell. This has led to the addition of ugly steel fire escapes to many urban buildings in the USA. While these fire escapes were advisable before the advent of sprinklers and smoke alarms, modern fireproof buildings probably do not require the provision of external fire escapes. (Skyscrapers have only internal fire stairs.) European practice has not generally required external escape routes, although a few casualties do arise from this omission. Fire escapes are themselves a regular cause of death from collapse, falls, and intruders. Local ordinances will govern, but it may be possible to negotiate changes. Wildfire A firestorm in which 25 perished swept the hills above Berkeley and Oakland in 1991. The risk should have been anticipated, as a similar although less serious fire had occurred in 1923. California’s long, dry summers make “urban forests” a permanent fire risk. The only protection is either to build dense, fireproof structures the prevent wildfire from intruding into the area or to remove trees and brush from the area. California learned little from the 1991 event, as it was repeated in 2003, near San Diego, albeit with less loss of life.


Halifax, Nova Scotia, was substantially destroyed by an exploding munitions ship during WW I. The risks are great in port cities handling bulk cargoes with explosive potential. Wharves handling these cargoes must be situated far from inhabited areas. Fertilizer, cement, flour, and other dusts can explode. The risk from LNG tankers has long been suspected, although no test has ever determined the likely outcome of a collision between an LNG tanker and another big ship. Boston is exposed to this risk, and the port is shut down while an LNG tanker is under way in the harbor. Tank farms are more likely to burn than to explode, but the hazard from smoke downwind requires consideration.


In Bangladesh, flooding is simply a part of life, and floods occur almost every year. Other areas had never known a flood in human times, until a calamity stuck, a situation most likely to arise in arid regions where violent storms occur at long intervals. Watercourses that have been dry in living memory can become torrents within minutes.

Reasonably accurate assessment of the risks is possible, but one common failing is to neglect the “catastrophe index” of flooding events in different areas. In some cases. the 500-year flood only puts the streets under a shallow water; in other places, the 500-year flood may be tens of meters (feet) above normal levels. In the former case, everyone should survive; in the latter case, nearly everyone will perish if the flood arrives without warning. Venice’s regular aqua alta is a major nuisance and appreciable expense, but no great hazard, as the flooding is little more than knee-high.

Risk Estimation

Flood risk is systematically underestimated in the USA due to procedural faults. As a rule of thumb, floods that are predicted to occur once a century are actually likely to occur about once in forty years; the range of error is large, however. River basins with extensive levees (dikes) are most likely to be subject to this error. The US Army Corps of Engineers is aware of the error but has chosen to ignore it. The risk of coastal flooding also appears to be systematically understated.

Coastal Flooding

Storms can flood low-lying coastal areas. Hurricane flooding can be predicted at least a day in advance, and cities are regularly evacuated to prevent a recurrence of the 1906 Galveston tragedy, when an unexpected hurricane flooded the city, the deadliest disaster in US history. Cities in these vulnerable areas must plan not to limit economic damage from moderate events but also to limit loss of life in extreme storms.

Floodplain Management

We cannot afford simply to abandon floodplains. Rather, we need to keep critical uses out of the floodplain to the extent that this is possible and to dedicate flood-prone areas to natural uses whenever alternative sites can be found for habitation. Natural areas should also be arranged to receive flood waters and reduce the height of the flood crest.

Dam & Dike Collapse

Dams upstream of cities must be absolutely proof against any failure, especially if the topography funnels the rushing waters into populated areas. The provision of alarm systems can sometimes give people enough time to flee.

Dike failure can lead to similar calamity, as in the Netherlands during a powerful 1953 North Sea storm. Coastal dikes failed in the south with the loss of 3000 lives. The massive Delta Works was undertaken to prevent this disaster from ever recurring.

Rising Sea Level

It is now clear that global warming will cause sea levels to rise. No matter what is done in the coming decades, we must plan for sea levels to rise as much as a meter (yard) in the coming century. Much of the world’s population lives in coastal areas, and everything possible must be done to prevent the calamity that will result from a large rise in sea level.

Wind Storm

People are often killed by falling trees during thunderstorms, but catastrophic wind storms are normally limited to tornadoes, which can carve a swathe of complete destruction. Little can be done about the destruction, but survival can be nearly assured by providing underground storm shelters, as is routine in the US tornado belt. The area of damage is small and rescue services can usually cope with the scale of injury and destruction.


Having experienced the 1989 Loma Prieta earthquake in the San Francisco Bay Area, I am thoroughly impressed with the destructive power of violent earth movements. This quake, measuring only 7.1 on the Richter scale, caused a highway some 100 km (60 miles) from the epicenter to collapse with heavy loss of life. Magnitude 8+ quakes are thirty times more energetic, and the damage in such quakes can be expected to be extreme. The city of Lisbon was destroyed by the 1755 earthquake, thought to have been above magnitude 8, and another quake of the same magnitude is nearly certain, some day.

San Francisco was heavily damaged during the 1906 magnitude 8.6 quake centered quite some distance north of the city. The direct losses during the shake were serious and probably caused most of the deaths, but the quake was followed by fires started when gas mains ruptured, and most of the city burned to the ground in the days following the quake. It would appear that we have learned little from this disaster; the low-pressure gas mains of the day have since been replaced by high-pressure distribution mains that also serve as storage facilities, which is why the enormous “gasometers” of years past have been torn down. This, of course, means that there is a far larger mass of natural gas available to fuel any fire that begins when mains rupture. It seems lunatic to me that this risk has been all but ignored.

Most areas, in fact, run some risk of earthquake damage, even though serious events may be very rare. All that can be done is to obtain the best seismic assessment available and to build well enough to survive the expected shaking. The damage from earthquakes is much higher where the soil is soft and unconsolidated, and extreme caution must be exercised when building on such soils in earthquake country; these sites are much better dedicated to open space uses.

A critical point is that buildings must be designed to protect their occupants, even if the building subsequently requires demolition. Catastrophic failures must be prevented; it will be nearly impossible to entirely avoid irreparable structural damage, but loss of life can be minimized. Volcanic Eruption

The ruins of pompeii are the object lesson of all time: watch where you put your city! Naples is still very much in harm’s way. In 1902, St. Pierre, Martinique, was obliterated by a volcano, but this capital city was subsequently rebuilt far from the volcano; alas, a town is sprining up again in the old location.

Volcanoes have one virtue as a risk: usually do not explode without some warning, permitting populations to be moved out of harm’s way. Volcanoes do their damage in a variety of deadly ways. The risk of a river of hot mud can extend far from the volcano, as can poisonous gas clouds. It may be impossible to avoid the risk entirely, but the areas most likely to suffer deadly effects must be left largely or entirely unbuilt.


Ocean coastlines are subject to tsunamis, which are caused by undersea landslides and earthquakes. Little can be done to reduce the risk except to improve warning systems, which is being done and which can be expected to save many lives. To the extent possible, we should avoid locating new urban areas in low-lying coastal sites subject to tsunami flooding. The Japanese have built tsunami gates to close some harbors to these destructive waves, although they may not be effective against the largest events.

Unstable Soils

Generally, these risks are well understood by engineers, but it is essential to examine the risk and protect against it. The great risk is landslide, and a soils engineer can give a definitive assessment of the risk and necessary protective measures. Subsidence and expansive soils are also troublesom but are rarely fatal. Surveys can identify the problem, so measures can be taken to avoid the risky areas or to prevent damage from occurring.

War & Terrorism

I would hate to plan for war, yet it would be irresponsible to ignore this risk. Thermo-nuclear explosion simply cannot be protected against by any design. Dense urban areas are the worst fighting grounds for invaders and the best defensive positions for retreating armies. Unfortunately, the damage during a siege can be nearly total, as occurred in Stalingrad during WW II. Basements provide some protection for citizens but cannot be constructed in all areas due to high ground water.

The risk of famine can be reduced by providing grain reserves and protecting drinking water supplies. It is prudent to maintain at least a year’s supply of grain in or near the city, and not concentrated at a single location. Cities should grow at least some food within bicycling distance. This provides some protection against famine in the case of a large-scale collapse (from whatever cause) of the infrastructure upon which developed nations rely for their food.

In a world that seems to become more dangerous with each passing day, security considerations cannot be neglected. It is instructive to consider the 11 September 2001 attacks in this regard. The Twin Towers were each struck by one aircraft, causing the collapse of both towers with very heavy loss of life. The Pentagon, a building of comparable size, was also struck by a single aircraft, which resulted in heavy damage to a small section of the building, with much less loss of life.

The carfree city is nearly immune to car bombs; all that can be destroyed with a bomb in a car is part of a parking garage. In fact, security concerns during the worst of the crisis in Northern Ireland led to the virtual closure of parts of London to car traffic, as this was seen as the only effective means to foil car bombers.


As the 2003 outbreak of Severe Acute Respiratory Disease (SARS) showed, urbanization can increase the risk of epidemics. In the long term, however, cities almost certainly reduce the risk from epidemics by making possible the intense medical and biological research that has done so much to protect us against infectious disease. We may some day have reason to regret this concentration of population, at least in individual cities, but over the long term it seems likely that we will continue to reduce risk by living in cities and pursuing the knowledge needed to protect us from epidemic diseases, a task that requires the riches that flow from urban areas.

Some practical measures can be taken to prevent epidemics. It goes without saying that clean drinking water and sanitary disposal of sewage must continue despite other calamities that may occur. The design of these systems must take this into account, which may be one reason to keep these critical systems in public hands, rather than trusting private interests to take the necessary, expensive steps to protect populations against a risk that may seem vanishingly small. The use of dry composting toilets is one possible strategy to prevent infrastructure collapse from exposing a city’s population to waterborne infections.

It is true that some infectious diseases can spread faster in densely populated areas than in sparsely populated areas. It is worth noting, however, that diseases spread by the bite of infected insects can be more or less eradicated in cities if standing water is properly controlled, which is not practical in rural areas.

Air filtration and the provision of fresh air may also be helpful in preventing epidemics, but we must never forget that ventilation systems can spread disease, as in the first known outbreak of legionella in Philadelphia. In this respect, large systems may present larger risks than small systems. Natural ventilation is preferable, as it can be relied upon even if power is interrupted.


Extreme air pollution events have long claimed many lives. In a real crisis, the combustion of any fuel must be suspended. In 1997, monsoon rains failed in Southeast Asia and unprecedented wildfires occurred. The region was blanketed by extremely polluted air, with many deaths. Little can be done to protect against this latter risk except not to disturb climate systems that have historically provided protection.

Siting of Emergency Facilities

Countless bitter lessons have shown the importance of correctly siting hospitals and other vital facilities. Too many times, hospitals are damaged or destroyed in a disaster, just at the moment when their services are most needed. Hospitals and fire stations must be sited where they are as immune from disaster as possible, while still remaining readily accessible to all areas of the city. The buildings themselves must be proof against the most catastrophic events that can be anticipated. Attention must also be given to the possibility that a major earthquake will cause widespread collapse of buildings with consequent blockages of routes leading to the hospital. Multiple routes help to mitigate this risk. No hospital should be sited in a flood zone if an alternative exists. Back-up power supplies are essential.

Hospitals should be in quiet locations, preferably surrounded by green areas. With respect to the Reference Topology, I have always thought that six hospitals should be built, one in the center of each lobe, where they can be entirely surrounded by green space and at the same time be within quick reach of any part of the city. This also would provide multiple routes to the hospital. Most of the time, these routes can be used for bicycle traffic crossing the lobe from one district to another; clearly, of course, ambulances using these roads must be given priority.

Extremely Rare Events

Large meteor impacts, massive tsunamis, and super-volcano explosions have never occurred in recorded history, but these events are nearly certain to occur sooner or later. There is not much to be done to protect against them, and they should probably be ignored at the local level. Technology may some day provide some level of protection.


The rapid evacuation of a city in the face of some calamity must always be considered. It should be possible for the entire population to quickly relocate into the surrounding countryside. In the case of the Reference Topology, it would be possible for the entire population to cycle into the surrounding countryside. Every family should have a tent and sleeping bags, which can easily be transported by bike. Safe greenbelts near the city can provide a place of refuge. Public transport systems may be able to assist, and consideration should be given to this use when designing these systems.

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