UPPER SIXTH
Case study for Natural Hazards - HURRICANES By David Preece
What 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.
Origin of Hurricanes
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
Very little is hurricane proof entirely.
Property damage can be reduced by;
Good, aerodynamic architectural design to minimise the force against it.
Low buildings, to reduce the affected surface area.
External shutters, to prevent glass blowing in from a pressure change.
Raising buildings, in order to protect them from floods.
Effective coastal land planning can limit development in high risk areas, thereby minimising the risk exposure.
Location may outweigh the risks, e.g. in LEDCs the need for fertile land, or in MEDCs the need for a beachfront location.
US Growth Management Act (1985) requires limitation of expenditure on development in high risk areas, and maintaining current evacuation plans and procedures. Effective Communication
The entire community needs to be prepared, and to know what to do
Warnings and information need to be rapidly disseminated within the target community.
People may not evacuate or respond because of their personal beliefs.
The Pan Caribbean Disaster Preparedness and Prevention Project, est. 1980, concentrated on technical help, and training local people in emergency procedures.
The scheme has had success regarding the loss of life, but economic losses remain large.
Conclusions.
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 their effects.