**NEW** Checkout my new homepage at www.landscapemodelling.net for details about me and my research on Wildfire and Landscape Modelling **NEW**
Floods are the most frequent and the most lethal of natural hazards. Physically a flood is a high flow of water which overtops either the natural, of the artificial, banks of a river. However such an event cannot be described as a hazard unless it threatens human life and property. As much land is owned, cultivated and used by humans there are not many floods that cannot be perceived as natural hazards.
As with many natural hazards it is the cause of the hazard that most easily determines its classification. The most usual cause of a flood is a period of heavy rain. As rain is caused due to the mechanics of the atmosphere this cause can mean that floods are an atmospheric hazard. Floods caused by storm surges or the melting of snow and ice are also caused due to the atmosphere and come under this classification. However, floods can also be caused by surges on the coast formed by earthquakes or volcanic eruptions on the sea bed and so because of this could be classified as tectonic hazards. The collapse of a dam can also lead to flooding, but if the dam was man made the hazard is not a natural one, while a natural dam collapsing would mean a mass movement hazard. Overall though, all of these causes can be put under one heading as all floods involve varying amounts of water and so all can be classified as Hydrological Hazards.
As I have just mentioned floods can be caused in several ways. These include; periods of heavy rain, the melting of snow and ice, unusually high tides, storm surges and the collapse of a dam. The nature of a flood varies greatly even in developed countries. In most countries river flooding is dominant and this is shown in America where two thirds of all national disasters between 1965 and '85 were due to river flooding. This contrasts with Britain where rivers only represent one third of the total flood risk, as sea flooding represents the major danger.
River floods due to heavy rainfall occur because a too much water gets into a river in such a short space of time that the level of water and discharge of the river increase. This can mean that the water can rise above the levels of the bank and surrounding areas can be flooded. This risk can be increased if factors that allow water into the river faster occur, such as decreased forestation and increased urbanisation. The melting of snow and ice can also contribute to the level of water in the same way. Coastal flooding can be due to unusually high tides and storm surges, caused by strong winds, that rise over sea defences or prevent the discharge of water at a rivers mouth. This type of flooding can also be due to tsunamis caused by underwater earthquakes.
As I have mentioned floods can be semi-predictable but also very random. For example the tsunami created by an earthquake off shore can only be predicted if the earthquake is predicted. As earthquakes are quite a random occurrence these floods will also be random. However, floods caused by an increase in rain fall will be more likely to occur during wet seasons. For this reason floods can be expected annually in a lot of areas, e.g. Bangladesh, during the wet season. The fact that floods are a frequent event is acknowledged by the fact that this very frequency is often used to measure the magnitude of a flood, as floods are given return times. A flood of small magnitude may be given a small return time, say 1 in 10 years, while a large flood may be given a large return time of 1 in 100 years. This return time means that a flood of that magnitude may be expected once every 10 or 100 years time respectively.
However the frequency of floods appears to be increasing and this could be due to either physical or human factors. An annual increase in rainfall or storm frequency may account for increased flood loses in Richmond, Australia but the increasing view is that invasion of the floodplain in the area is the reason for the upward trend.
Floods can occur anywhere where water has the opportunity to flow on the land. The most vulnerable landscapes for flooding are
Low Lying Parts of Active Floodplains and River Estuaries
In a natural state these will experience the most frequent flooding as this is where most water in the system is combined with the low area upon which water will flow when it breaks the banks of a river. In estuarine areas there may be a combined threat from river floods and high tides.
Small Basins Subject to Flash Floods
Flash floods are found mainly in arid areas where there is a combination of steep topography, little vegetation and high intensity, short duration rain storms. Too much water is put into the river in a short space of time and the level of water rises rapidly.
Areas Below Unsafe or Inadequate Dams
Floods resulting from dam failures are an example of a combined natural and technological hazard. As with flash floods there is little time for warning and evacuation.
Low lying Inland Shorelines
These can extend for thousands of kilometres and involve much property, for example the Great Lakes in the USA. Fluctuating lake levels from high river inputs is the main problem.
Alluvial Fans
Alluvial fans are risk prone environments because the drainage channels can meander across steep slope meaning high velocity flows with a large load.
Many urban areas also have flooding problems as the urban area itself causes water to get into the river quickly in different ways :
In all these cases though the flooding is only a hazard because there is human interest in the area. Flooding is a natural occurrence and it is only because human locate themselves in these areas that a hazard is created.
Since man first lived on earth many millions of people have died and billions of pounds worth of damage to property has been done due to flooding worldwide. Flood magnitude is best expressed in terms of peak river flow (discharge) whilst the hazard potential will relate more to the maximum height that the water reaches. The size of any flood hazard is dependant upon two points; the primary causes of floods, such as heavy rainfall, storm surges etc., and any secondary intensifying conditions, such as deforestation. With this in mind we can asses the conditions that might bring the most catastrophic flood. Heavy rainfall and a storm surge at a low lying estuary that is heavily urbanised would probably bring the worst possible flood though there are many other considerations to think of.
Many of these criteria were satisfied in 1988 in Bangladesh when both the Ganges and the Brahmalutra burst their banks affecting 45 million people directly. 30 million people were made homeless, 2 million tons of crop were lost, over 3000 people died and more than 100,000 caught water-borne diseases. This shows what an impact a flood can have when people live in a high risk area. To understand the power that the water has in a flood is difficult. A person standing in water to the waist flowing a 2 mph will have difficulty standing, In floods water can reach 20 times higher. It is also worth remembering that 2.5 million cubic feet of water falling 1000 feet from a mountain has energy equivalent to 3 atomic bombs while the amount of water flowing through the Mississippi in one year is 19 million cubic feet. This does not include floods. Floods have immense power.
There can be many disastrous consequences of a flood in a heavily populated area. Bangladesh is a low-lying country that is very flood prone because of its situation. Over 40% of the country is no more than 1 metre above sea level and many major floods often follow the cyclones which hit the coast or when the Ganges or Brahmaputra rivers burst their banks due to heavy rain. Bangladesh has experienced five major floods in recent years, 200,000 were killed in 1970 by a tidal surge and I have just talked about another bad flood. Bangladesh also suffered flooding during 1998 as two months of rain inundated two-thirds of the country. Officials have said the death toll from more than two months of flooding passed 1,000 and could keep rising if a looming diarrhea epidemic is not contained. Diarrhea, caused by polluted water or rotten food, accounted for 227 of the 1,040 deaths caused by the disaster, the worst flooding in Bangladesh's history, officials said. Other deaths were caused by drowning, snakebites, collapsing houses and mudslides, disaster management officials said. The floods have destroyed crops worth $300 million and damaged close to 10,000 km (6,200 miles) of roads.
Water Resources Minister Abdur Razzak said in a newspaper interview that the disaster had mainly been caused by heavy rainfall in the upper reaches of the country's major rivers, including the Ganges.He added silting up of rivers and blockage of drainage systems had also been partly responsible for the country's most prolonged natural disaster.
This is a good example of coastal flooding allied to river flooding in an ELDC.
Another area where flooding is common is on the Mississippi river in the USA (shown on right). This river is one of the largest in the world draining 1.25 million square miles - 40% of the USA - and is 2348 miles long with an average discharge of 17,000 cubic metres a second. It is one of the most extensively managed river basins with a system of dams, levees, walls and channelisation trying to protect people living on its floodplain. However in the summer of 1993 these were rendered useless as a flood killed 50 people and caused $10 billion of damage. This flood occurred because of natural and human factors including:
An example of a flood caused because of a dam burst was at the South Fork Dam in 1889. A torrential storm caused the dam to break and let a wave of 45m high crash through two towns killing 2210 people and destroying both dams. It is very unusual for a dam to break however as, by the laws of physics, the water pushing on it holds it up. Floods due to dams are more commonly due to waves in the reservoir flowing over the top of the dam.
Tsunamis are also a viable cause of flooding and one in 1946, killing 159 in a surge that was so powerful it bent parking meters double.
The obvious human reaction to any flood will be to move away from the affected area initially and to move back when the water has subsided. It is important that shelter, warmth and clean drinking water are provided or found as many people can catch water-borne or other diseases. For many of the flood prone ELDC's international aid for disaster relief is important if not vital to prevent this happening. In many EMDC's there are often calls for government sponsored relief but these can be resisted on the grounds that possessions should have been insured. Many insurance policies are dependant upon the distance of the policy holder's possessions from the river, coast or their position within the risk area.
To prevent such hazards or to reduce their scale attempts at control are taken in many flood prone areas. This control can be in the form of flood abatement or flood diversion.
Flood abatement is targeted at the drainage basin itself and includes the use of afforestation, removing urban areas from the flood plain and the reshaping of the land, in the form of terraced slopes to reduce the speed of run off. Flood diversion means the controlling or redirection of water away from specific areas at risk and include:
Techniques that use levees, channelisation and large amounts of such mechanical engineering are known as hard engineering techniques. Those that try to restore the natural environment and reduce the amount of water flowing into the actual river are known as soft engineering techniques.
Other techniques used to protect housing can involve blocking up certain entrances with seals and sand bags to keep flood waters from getting into homes. Flood-proofing, as it is known, is also being increasingly used to zone areas of the flood plain and ensure that housing is as far away from the river as possible.
Forecasting and warning systems are rapidly developing as technology increases. However it is estimated that of all the countries in the world around 15 have no warning system at all and 40 have an inadequate system. Flood hydrology has improved so much that storm rainfall and runoff conditions can be modelled to high levels of accuracy. Basin stations can monitor runoff while weather satellites can monitor the movement of storms and so warnings can be formulated in time to give adequate warnings. Large amounts of money have been spent on these systems in EMDC's and now in heavily urbanised areas warnings have been reduced to 30 minutes. In ELDC's there is little money spared for these systems to be developed and warnings of even major floods can be very small. In Bangladesh most floods start in the upper regions of the basin which is in India and communication between the countries is limited.
Since man has been making settlement he has been drawn to the benefits of living near to the river. These include a close supply of water, good soils for cultivation and flat land for building. Since then people have been living on the flood plains with a very laid back attitude towards any flooding. Recently as more and more people have moved to flood prone areas of EMDC's people have demanded flood protection. With this in place more people have moved to the flat lands so contributing to the factors that can increase the size and danger of flooding while becoming more and more complacent that they are living in a safe area. However, as was shown in the 1993 Mississippi flood no matter how much engineering is done the river is still a powerful force that can defeat the attempts of man to suppress it.
In ELDC's perceptions are much more realistic with people appreciating how much damage and destruction they can cause as they often feel the brunt of even small floods as there is little or no protection from any flooding.
Flooding is a natural phenomena that will always occur on a regular basis and it is part of the cycle of a river. No matter what protection techniques are used floods will always cause damage and, with the increase of urban areas on the flood plain, more people are within the flood prone areas of the world. Once again if people did not settle so close to the hazard they would not be in so much potential danger. It is also important that with all the techniques that engineers have developed that people do not feel complacent and the river has been tamed and they are safe as this is not the case.
USGS Resource, Floods and Flooplains
The Floodplain Management Web Site
Louisiana Floods
Federal Emergency Management Agency
(US) National Weather Service, Flood Information. (Examples of flood warnings available on the net.)
The Sacramento Bee
The 1997 Flood in Grand Forks North Dakota
Mississippi Flood Pictures