Case study for Natural Hazards
- HURRICANES By David
is a hurricane?
"Hurricane" is a regional specific name for a strong tropical cyclone.
The hurricane region is the North Atlantic Ocean, the N E Pacific, east
of the dateline, or the South Pacific east of 160E. "Tropical cyclone"
is the common name for a "non-frontal synoptic scale low-pressure system
over tropical or sub-tropical water with organised convection and definite
cyclonic wind circulation." It is a low pressure system which derives
its primary energy from the evaporation from the sea in the presence of
high winds and lowered surface pressure, and associated condensation in
convective clouds near its centre. Wind speed must reach 33m/s (74 mph)
in order to be classified as a hurricane. Also may be known as a cyclone,
typhoon, or willy-willy.
The formation of hurricanes comes under the general heading of the formation
of tropical cyclones. This is more properly known as "tropical cyclogenesis".
Musk (1988) has identified a list of factors which aid tropical cyclogenesis.
The primary factor is the temperature of the sea, since this is where the
hurricane derives most energy. Favourable environmental conditions for tropical
cyclogenesis; · Warm ocean waters (at least 26.5 deg C, or 80 deg F), throughout
a sufficient depth (about 150ft). Warm water is needed as a fuel for the cyclone.
· A rapidly cooling atmosphere with height such that it is potentially unstable
to moist convection. It is thunderstorm activity which allows heat stored
in the ocean to be liberated for cyclone development. · Relatively moist layers
high up (5km/3 mi) Dry mid troposphere levels are not helpful in allowing
the thunderstorm activity to spread. · A minimum distance of 500km from the
Equator. Tropical cyclogenesis requires a relatively large Coriolis Force
to maintain the low pressure of the wind balance. · A pre existing disturbance,
preferably of weak organisation, but with considerable spin and low level
inflow. · Low values of wind shear (vertical wind direction) between the surface
and the upper troposphere. It is important to note that these conditions are
favourable. They do not NECESSARILY LEAD TO the formation of a cyclone…..
Where do hurricanes occur?
Cyclones only occur in distinct seasons when the sea is warm enough. The Atlantic
hurricane season officially runs from June 1 to Nov. 30, although most tropical
storms and hurricanes typically occur during the August-October peak period.
This season, all of the tropical storms and hurricanes occurred between August
and October They are a relatively frequent occurrence. Between 1968 to 1989;
· the largest number in a year was 65 · the smallest number in a year was
34 · the average was 45 per year. The variations are less in El Nino years,
and more in La Nina years, due to the influence of El Nino/La Nina on the
sea surface temperatures. Size of Hurricanes Hurricanes tend to be large scale
features, which use and release large amounts of energy. There are two ways
of measuring hurricane energy; Total energy released through cloud/rain formation;
Average hurricane produces 1.5cm/day of rain within a 665km radius. This would
require 600 000 000 000 000 Watts of energy - 200 times the world's electrical
generating capacity Total kinetic energy of the wind; By complicated meteorological
calculation, and using some very tricky maths, the average dissipation rate
is about 1 500 000 000 000 Watts; equivalent to half the electrical generating
capacity of the world. It can be seen that the ratio between the energy released
by the hurricane, and the amount required to maintain it (by wind spiral)
is about 400 to 1. The intensity of hurricanes is measured on a logarithmic
scale known as the Saffir Simpson Hurricane Scale. Each category has a defined
set of characteristics, and wind velocity; and each category is 10 times stronger
than the previous one. The categories go from one (smallest) to five (strongest),
and are often referred to by the corresponding Roman numeral.
Impacts of Hurricanes
Hurricanes are one of the most destructive natural hazards, both in terms
of frequency and death toll. They pose a major threat to coastal areas, related
to the storm surge threat. They are a multiple hazard, since loss of life
and property can be a result of one of several factors; · Heavy rain · High
wind velocity · Storm surge, as a result of the above and low central pressure
They also include secondary effects, e.g.. flooding, landslides, and related
damage. Impacts are a function of when and where the hurricane hits. Hurricane
Diane hit in 1955 and caused 184 deaths. Hurricane
Agnes (1972) caused estimated inflation adjusted damages of $6.9 billion.
Both were Category One hurricanes. Storm Surges A storm surge is a large dome
of water, 50-100miles wide, which sweeps across the coastline near where the
hurricane makes landfall. It is water pushed towards the shore by the force
of the winds swirling around the storm, and the drop in pressure (260mm sea
level rise for a 30mb fall in pressure) The surge combines with the normal
tide to create a storm tide, and wind waves are then superimposed upon this.
It can be more than 15ft deep. Most of the US Atlantic and Gulf coasts lie
less than 10ft above sea level; therefore at risk. The major danger is from
flooding, particularly when coinciding with natural high tides. This causes
great potential for loss of life. Storm surges have historically been the
cause of 90% of hurricane related deaths. It also causes problems, as large
amounts of salt are deposited inland, e.g. on bayous and estuaries. High wind
velocity Strong winds associated with very low atmospheric pressure can cause
damage. They begin well before the hurricane eye makes landfall. Hurricane
force winds can destroy poorly constructed buildings. They not only damage
structures, but the debris which they carry is dangerous too. External debris
can become deadly missiles in the wind force. Damage to tall objects, esp.
power lines, telephone cables can cause disruption. High rise buildings are
at risk from windows blowing out, particularly at higher levels, since wind
speed increases with height. Winds can stay above hurricane level well inland.
Hurricane Hugo, for example, battered N.Carolina
with gusts up to 100mph, and N.Carolina is nearly 100 miles inland! Heavy
Rainfall Hurricanes and tropical storms typically produce widespread rainfall
of 6-12" often resulting in severe flooding. Inland flooding has been the
primary cause of related fatalities within the last 30 yrs. Rains are generally
heaviest with slower moving storms. (>10mph) The heaviest rains usually occur
to the right of the storm track 6 hours either side of landfall. Storms can
last for days, depending upon what weather systems they interact with. After
Hurricane Agnes (1972) died down, the remnants interacted with another storm
system, and produced floods all the way up to the NE USA, resulting in 122
deaths and $6.9 billion damage. Large amounts of rain can occur more than
100mi inland. When Camille devastated the Gulf coast in 1969, the resulting
storms moved Northeast and combined with a cold front over Virginia to produce
an unexpected 30" of rain, and 109 people died. Occasionally, hurricanes do
not produce the expected rain. In 1966, Hurricane Inez resulted in very little
rain when torrential rains were forecast. As a result, the winds blew salt
spray miles inland, and caused severe damage to vegetation from salt accumulation.
Rainfall figures are variable. They result in events of 100mm/day within 200km
of the eye, to 40mm/day between 200-400km of the eye. The largest rainfall
total was recorded as a result of Cyclone Denise in
1966 in Reunion Island of 1144mm in just 12hrs! Secondary Effects Flooding
can be caused by storm surges or intensive rainfall. In 1974, 800 000 people
died as a result of flooding in Honduras. Landslides can result from total
saturation of porous soils. Waves with increased energy will erode shores.
Other consequences not directly related to the storm; · Fires started by candles
when the electricity fails · Accidents/heart attacks during the clean up phase
· Possible chemical spills from local industry.
CASE STUDY FOR IMPACT Bangladesh
One of the world's poorest nations (US$170 per capita), Bangladesh is frequently
hit by natural hazards. The coastline is funnel shaped, and concentrates storm
surges, and is also vulnerable to warm water cyclones over the Bay of Bengal.
About 5 cyclones p.a. enter the bay outside monsoon season. Money is always
being used for disaster relief, so cannot be spent on population development
projects; in 1988-9, 45% of the country's development budget was being spent
on natural disaster relief. The low lying delta and floodplain are densely
inhabited, which means that large numbers of people are very vulnerable to
storm surges. · November 1970; Tropical cyclone
moved north up the Bay of Bengal. Winds of 250km/hr and an 8m high storm surge
hit the Ganges delta. Over 4m people were affected. 300 000 people died, and
1m were left homeless. 0.5m cattle drowned. Two thirds of the fishing fleet
was lost, and 80% of the rice crop was destroyed. · May
1985; Cyclone hit the coastal islands. 9m high surge penetrates up
to 150km inland. 40 000 people died. Outbreaks of typhoid and cholera. Fresh
water supplies were contaminated. Food shortages - rice crop lost again. Soil
was saturated with salt. Many animals and the much of the coastal fishing
fleet were lost. Management strategies since then have concentrated on warning
and shelters. · April 1991; Intense (738mb) cyclone produces 6-9m storm surge,
early morning. 139 600 killed. 98% of the mud houses destroyed. 500 000 animals
died. · May 1997; Cyclone warning results in
the evacuation of 300 000. Death toll falls to 95. Shows the progress in education
and warning, despite the difficulties.
Mitigating the Hazard.
Very little can be done to stop a hurricane, because it has more power than
all of the electrical capacity of the world. Even if all the nuclear weapons
in the world were detonated simultaneously, they would only power a medium
(Cat 2-3) hurricane for 6-12 hours. Unsuccessful attempts have been made to
'seed' the clouds with silver iodide, in the hope of triggering precipitation.
What can be done is to minimise the exposure of people to the risk. This is
particularly effective in the US. The US has a very effective and co-ordinated
response, at the National Hurricane Center, run with resources and funding
from NOAA, the National Oceanic and Atmospheric Administration. The US has
enough resources to throw at the problem, and enough specialists to help prevent
large loss of life. But the economic losses remain high. There are several
methods of mitigation; Prediction Aims to give an early warning so that people
can prepare, and that loss of life and property are minimised. Monitored by
satellites, land and sea monitoring centres, as well as airborne reconnaissance.
Predictions made on previous experience of tracks and computer modelling.
Inaccurate warnings have human and economic costs; evacuation in the US costs
$50m per 300 miles. Inaccuracy can generate complacency in the target communities.
Time is critical. Accurate predictions can only be issued 12-18 hours before
landfall. Florida, esp the Keys takes an estimated 31 hrs to evacuate.
Management of Adverse effects.
Hurricanes cannot be stopped. Their prediction is still dubious. The most
effective strategy is to manage the effects they cause; · Flash flooding can
be managed by drainage channels, and diversionary spillways. · Coastal areas
can be protected by sea walls, breakwaters and flood barriers. · Galveston,
Texas. After a storm surge which killed 6000 (15000 population), the town
rebuilt, 3.5 metres higher, with a sea wall. It is very expensive, and unlikely
to be repeated elsewhere. · Loss can be minimised by insurance, which is not
compulsory. Mandatory insurance only covers $15,000 worth of damage. · Aid
can be give to LEDCs affected by hurricanes. The international relief effort
following Hurricane Mitch is a good example of this.
Minimising Property Damage
Þ Very little is hurricane proof entirely.
Þ Property damage can be reduced by;
Þ Good, aerodynamic architectural design to minimise the force against it.
Þ Low buildings, to reduce the affected surface area.
Þ External shutters, to prevent glass blowing in from a pressure change.
Þ Raising buildings, in order to protect them from floods.
Þ Effective coastal land planning can limit development in high risk areas,
thereby minimising the risk exposure.
Þ Location may outweigh the risks, e.g. in LEDCs the need for fertile land,
or in MEDCs the need for a beachfront location.
Þ US Growth Management Act (1985) requires limitation of expenditure on development
in high risk areas, and maintaining current evacuation plans and procedures.
· Effective Communication
Þ The entire community needs to be prepared, and to know what to do
Þ Warnings and information need to be rapidly disseminated within the target
Þ People may not evacuate or respond because of their personal beliefs.
Þ The Pan Caribbean Disaster Preparedness and Prevention Project, est. 1980,
concentrated on technical help, and training local people in emergency procedures.
Þ The scheme has had success regarding the loss of life, but economic losses
Hurricanes are large, powerful natural hazards. We cannot control them.
They have multiple impacts on the affected areas, those being determined by
mean sea temperature. Their effects include; · Flooding · Storm surges · Landslides
· Intense rainfall · High winds Their effects can be managed, and this can
take considerable resources, e.g the US, but can be done through education
with equal success at lower cost, e.g. Bangladesh. We can adapt to minimise