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Multiple Hazards in Cities: Examples from London, Mexico City and Los Angeles

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London has experiences a variety of hazards for a long period of time.

* Royal decree in 1306 banned burning of coal to reduce air pollution

* The Great Fire of London which burned over 13 200 homes

* The little ice age which caused the Thames to freeze over

* The Blitz of WW2 which destroyed many homes and industry

* High tides and storm surges

* Smog, especially in the late 50’s.

London is prone to storm surges due to a combination of reasons;

* Rising sea levels

* Down tilting of the south of England (isostatic uplift)

* Long-term subsidence.

Smog has occurred as a consequence of all of the pollutants pumped

into the air around London. Some of the worst were in December 1952, when;

* Visibility was down to 5m

* Fog had a pH of 1.6

* There were nearly 4000 deaths.

To tackle this problem the following strategies were introduces;

* The 1956 clean air act banned coal fires and provided financial assistance to households to convert to smokeless fuel

* Houses prompted to convert to oil and gas

* Legislation reducing the sulphur content of oil to 1% also helped.

As a result the pollution in London dropped dramatically and continued

Throughout the 80’s.

However the smog’s returned in he 90’s, but rather due to industry it was due to the large number of vehicles in London. The government attempted to reduce he pollution levels in a number of ways;

* Variable speeds on the M25 to reduce traffic jams

* Pedestrianisation of shopping areas

* The reintroduction of trams between Croydon and Wimbledon.

The levels of groundwater in London have been an issue, mainly due to

over extraction since the 1940’s. Some areas of London are now at risk from rising water levels. Those mainly at risk are tube stations and underground structures close to the Thames. To cope with this flooding London underground pumps 14 million litres of water each year.

There is much contaminated land in London, especially in the East Thames region. E.g. Barking Levels has much polluted air and contaminate ground. The marshes have been used as waste dumping grounds and as landfill sites. In places, the land has been contaminated with radon gas, asbestos, fuel, ash and cyanide.

Mexico City

Mexico City experiences many hazards, these include;

* Poor air quality

* Earthquakes

* Shortages of water, as well as infectious diseases, industrial accident and poverty.

Environmental problems are rife in the city as there is a concentration

of sum 3.5 million vehicles, 40 000 factories emitting 12 000 tonnes of gas daily. Smog can reduce visibility to 2km. There are also serious problems due to the illegal dumping of waste.

The Sulphur dioxide levels in the city are very high. The city is surrounded by mountains so experiences frequent temperature inversions. Smog’s can be so severe that schools are closed or a month, they can lead also to the closure of industry as well as many deaths.

The air quality is very poor in the city of a number of reasons;

* Very large population size

* Large number of cars and industry

* Its location in a high altitude basin

* Frequent temperature inversions

* Its urban heat island effect

* Most cars are old and inefficient.

Up to 2 million people suffer diseases caused by air pollution. It’s thought that the health costs related to the ozone are $100 million a year, and PM’s up to $800 million a year. The particular problem is especially due to the dust from the dries lake bed of Texcoco to the north east of the city.

There have been a number of schemes to reduce the air pollution in Mexico City;

* In 1989 the government introduces colour coded permits to reduce traffic flows, which has reduces the no. of cars by up to 400 000 a day and air pollution by 15%

* Drivers face a fine of $600 if they break this restriction

* All taxis over 10 years old have to be replaced

* The amount of lead in petrol has been halved.

The main source of water for the cities 20 million residence is an

aquifer below the city, however this is running dry. The result of this is that Mexico City is sinking t a rate of 50cm a decade.

Dangerous cracks in the clay sediment threaten to contaminate the aquifer which lies just 100m below the surface. The rest of the water is pumped from over 120km away, which requires huge power. The demand for water is very high, and the city looses 30% of its water due to theft from irrigation and pipelines.

The earthquake which hit Mexico City in 1985 caused massive damage along the Pacific Coast, however the most damage was done in the city where 7 000 dies, 40 000 were injured and 30 000 were made homeless. The economic loss was estimated at over $4 billion. The disaster affected some areas more than others. Buildings on ‘susceptible’ soils and alluvial deposits are prone to collapse. E.g. the 1957 earthquake 96% of houses damaged were those built on the lake sediment. When shaken in an earthquake the foundations of structures in these sediments break up , loose strength or become waterlogged.

Los Angeles

Some of the hazards affecting Los Angeles are directly concerned with the physical environment, others are where man has modified the physical environment to his dis-advantage there are also some that occur entirely within the human environment. The main hazards are as follows;

* Earthquakes

* River flooding

* Coastal flooding

* Drought

* Fires

* Mass movement ( mudslides, landslides)

* Smog

* Crime and gang warfare

* Racial violence.

Minor tremors are common in the region, but since 1933 there have

been five earthquakes that have measure at least 5.8 on the Richter scale. The largest of which was the Northridge event that measured 6.8 and killed 57 people, whilst causing $30 billion in damage. L.A is densely populated which accounts for the high loss of life and damage. As it has become very important that both people and buildings are protected, the following schemes have been adopted.

* Mapping the geological structure, ( mapping of all faults and predicting in the future)

* Land-use planning, the placing of schools and hospitals in low risk areas

* Hazard-resistant structures, counterbalancing and X-bracing etc.

* Fire prevention, ‘smart metres’ cut off gas in the event

* Emergency service readiness, equipment is distribute throughout the city.

* Education and planning, there are classes in schools and on TV to educate people in what to do in the event of an earthquake.

River flooding during an El Nino event is common. Rainfall of 100mm/

day is common. The 1914 flood caused $10 million in damage and brought a public outcry for action to address the recurrent flooding problems. The following schemes were adopted;

* Channelising, which involved both straightening and paving

* Building small dams on the mountain tributaries

* Larger dams with reservoirs

* Debris dams to prevent a very high bed load

* Land set aside for controlled flooding

* Spreading grounds for surplus water to be dissipated over.

The heavy winter rains of southern California have brought some coastal flooding. Sea walls and injections have been put in place to minimise he damage.

The rivers tend to dry up in the summer months, an as there is a population of over 20 million drought is a serious problem. Many reservoirs have been built to hold water hat has been pumped in from the Colorado and the wetter northern parts of California. The other solution is to plant desalinate plants along the coast, though this has high energy problems.

Much of L.A’s basin is covered with chaparral vegetation, so in the dry summers any spark can cause a massive bushfire. When there is a fire there is much vegetation that is at risk. In 1993 a firestorm swept through Malibu destroying over1000 homes. The strategies adopted are increasing the amount of equipment to fight the fires, planning, education, clearance zones and a voluntary watch service.

Mass movement is a major problem, as it is destroying the coastline. To try and prevent this from happening there are regulations to stop building on the cliffs, and at the sites at greatest risk. Also deflection barriers are built and local people are educated about the risk.

Smog is a large problem for a number of reasons;

* The bowl like setting

* Sea breezes

* Frequent temperature inversions

* Abundant sunshine

* The amount vehicles

In 1989 an Air Quality management plan for southern California was

Approved, its aim to reduce the amount of pollutants in the air. In 1990 the clean air act was introduced trying to encourage public transport and installing catalytic converters. A rail service was devised in the 80’s to try and reduce the number of cars on the road. Also people were encouraged to use electric and LPG cars.

Los Angeles has a very high crime rate, in 2001 there were 190 000 crimes recorded, of which 600 were murders. There are neighbourhood watch schemes and classes on personal safety. There are many different racial gangs in the city which are responsible for much of the crime. To try and stop the gangs, special police units have been set up.

The Philippines

The Philippines consists of over 7000 islands, of which only about 2000 are inhabited. The Philippines contains 37 volcanoes, 18 of which are active. These include Mt. Maon, Mt. Pinatubo, and Taal volcano. They have formed here due to the Philippines lying on a plate boundary, (ring of fire).

One of the largest earthquakes this centaury took place in July 1990. The worst affected areas were Luzon, Balgo and Dagupan. The earthquake measured 7.7 on the Richter scale and was responsible for the deaths of over 1600 people and the destruction of 20 000 buildings. Volcanoes are a major hazard in the region;

* In June 1991 Mt. Unzen erupted and the surrounding towns had to be


* In May 1991 M. Pinatubo erupted scattering ash up to 100km from the

base of the volcano and killing over 1600, and injuring a further 3000. Clouds of hot gas, pumice and ash swept down the sides of the volcano at speeds of


The mud storms and mudslides covered over 50 000 ha of cropland, destroying all crops and over 10 000 homes. Suppliers of electricity were cut off for weeks, water became contaminated and roads and telecommunication links were destroyed. An epidemic of respiratory and gastric diseases broke out in the temporary housing. The government estimated that 60 000 lost heir jobs.

Over 200 000 people were evacuated as a result of the early warning systems. Management of the 1991 eruptions have been effective;

* State of the art volcano detecting equipment

* Hazard zonation mps

* An alert warning system

* Disaster response machinery

Typhoons and cyclones are also a hazard to the Philippines. E.g. in 1989

in a single week a typhoon caused 167 deaths, and 50 000 were made homeless. Landslides are also common due to illegal logging on the hillsides.

There have been many seismometers placed around the islands to try and detect when volcanoes are going to erupt, but there is no way of stopping them.

* Satellites offer the prospect of global coverage from space

* Hazard zonation maps

* The monitoring of active volcanoes all help in managing the crisis when it has happened.

Flooding is common in the Philippines due to tsunamis and typhoons.

Lowland floodplains can’t cope with the huge amounts of water. The result of flooding e.g. in July 1972, 600 were left dead, 370 647 were left homeless and 200 000 ha of crop land was destroyed. This resulted in �20 million of damage to roads, bridges and infrastructure.

Drought is also a problem as there are often several months with very little

rainfall (up to 40% less). The last significant drought was in December 1986 when most of the country was affected. In 1989 a drought weakened the economy with a loss of agricultural production, just before the 1990 earthquake and the 1991 eruption.

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