Flooding may result from a large amount of rain or from rapid thawing of snow and in coastal regions may also be caused by a storm surge or a combination of high tide and high river level. The consequences of flooding are often disastrous but there can also be beneficial effects of flooding.
Storm surges
Storm surges are mainly caused by the effect of the wind on the sea, not changes in atmospheric pressure. The effect of wind on the sea surface is known as wind stress. The wind stress on the surface of the sea causes the water level on a coast to rise if the net transport of water is towards land and to fall if it is away from land.
Many coasts are at risk from storm surges and these include the Atlantic side of the United States, the Bay of Bengal area around India, Thailand and Indonesia, the coastal areas of the Netherlands and eastern England coastline. At the head of the Bay of Bengal large storm surges (up to 4m in height) are initiated by hurricane force winds coupled with low pressure tropical cyclone systems. Fortunately for the UK surges large enough to bring about catastrophic flooding of these North Sea coasts are experienced only a few times each century, and improved sea defences should stop all but the most extreme surges. On the Indian sub-continent, especially near the mouths of the River Ganges, death tolls from storm surges caused by tropical cyclones have been huge. In 1970, for example, more than 200,000 died when such a surge struck the area. Here, as in other places where storm surges occur, the sam
Examples of storm surges in the UK and their effects
Flooding of the Thames on 6th -7th January 1928 highlighted the need to find ways of forecasting storm surges. However, the real push to investigate storm-surges was on 31st January and 1st February 1953, when, a surge exceeding 2.7 metres at Southend in Essex and 3.5 metres in parts of Holland killed 307 people died in eastern England and 1,800 in the Netherlands. The storm that caused the 1953 surge was among the worst to hit the UK in the 20th century. Before the storm’s low pressure and storm-force northerly winds raised water levels in the southern North Sea, hurricane-force winds blew down more trees in Scotland than were normally felled in a year; and a car ferry, the Princess Victoria, on passage from Stranraer in Scotland to Larne in Northern Ireland, sank with the loss of 133 lives. Only 41 of the passengers and crew survived. Nowadays, surges are forecast with considerable accuracy and storm-surge barriers are in place in the most vulnerable places in the Low Countries and eastern England, one of them on the Thames a (south of Greenwich).
The effects of pressure on sea level
When pressure falls by one millibar, sea level rises by one centimetre. Thus, a deep depression can cause sea level to rise 60 or 70cm above the level predicted purely on the basis of tidal theory. The pressure-induced rise in sea level caused by a tropical cyclone can be much greater, maybe a metre or more.
Flash flooding
One of the most energetic and destructive of all weather systems are tropical cyclones. The hurricane-force winds can reach 50m/s and torrential rain falls from their towering cumulonimbus clouds.flooding The winds can cause disastrous surges on coasts and the downpours of rain can cause serious flooding. Power and water supplies are disrupted, buildings are damaged, crops are destroyed, people and livestock are drowned, bridges collapse, roads and railways are undermined or blocked by debris, and beaches are scoured. Tropical cyclones nearly always leave behind a trail of destruction and misery.
Flash floods are exceedingly dangerous. When water cannot percolate into the ground, it runs off the surface as it would from impermeable concrete. This is particularly so when the ground is very wet or when baked hard after a hot dry spell. Potholers can be especially at risk, such as on 24 June 1967, when five drowned in Yorkshire. The water which fell in a heavy thunderstorm after a spell of dry weather ran off rapidly into underground streams and caverns at Mossdale. The rise in water levels below ground was too rapid for the potholers to scramble to safety.
Floods resulting from persistently wet weather and thawing of snow case studies
In terms of water flow, the Mississippi is the sixth largest river in the world. Its annual average flow rate is 14,000 cubic metres per second and it discharges into the Gulf of Mexico 580 cubic kilometres of fresh water per year. The greatest flows occur in the period March to May and the least in the period August to October. A large proportion of the United States is drained by this river.
To protect against flooding, which would otherwise occur frequently, the Mississippi River is constrained by levees (embankments) all the way from the State of Missouri to the sea. This barrier, much of it concrete, has isolated the river from a lot of the surrounding countryside, but without it and other means of dealing with excess water in the river, low-lying cities such as New Orleans could not exist.
Tragically, some of the levees around New Orleans were breached on 29 August 2005 when they failed to withstand the battering imposed by the waves and a storm surge generated by Hurricane Katrina. Flood depths reached six metres in places and more than 1,000 people died.
Despite levees, spillways, reservoirs, pumping stations and other constructions, flooding does still occur. The flood-control system proved incapable of containing the flood of 1973, for example, and again proved inadequate in the summer of 1993, when abnormally high rainfall over central parts of the United States caused extensive flooding of the upper and middle Mississippi and lower Missouri rivers. The city of St Louis was particularly badly affected. Discharges of water from the Mississippi into the Gulf of Mexico in the late summer of 1993 were abnormally high. A study using satellite imagery showed that water from the Mississippi spread out far and wide off the States of Louisiana, Texas and Alabama. Indeed, some of the Mississippi water passed through the Florida Strait into the Atlantic.
There was rain, more rain and even more rain in northern California soon after Christmas 1996. From 29 December 1996 to 4 January 1997, depression after depression from the central Pacific brought rain to northern California. As the air was unusually warm, a consequence of the precipitation was that large amounts of snow melted. During the week of the storms, 61 cm of rain was recorded. Significant flooding occurred in northern California and southern Oregon and 43 counties were declared disaster areas. Flooding occurred rapidly because soils became saturated and amounts of snowmelt were large. Flood-control reservoirs could not cope, as their storage capacity was no more than moderate because of near-normal rainfall and run-off prior to the onset of the severe weather. Levee failure occurred on several rivers.
After the United Kingdom’s snowy winter of 1947 came the thaw. In many parts of the British Isles in February and early March 1947, deep drifts of snow, some five metres or more deep, caused villages and hamlets to be cut off for days on end. Then, on 10 March, warm air and rain edged into south-west England and advanced across the British Isles. By 13 March, floods were widespread. Vast areas of Fenland, the Severn Valley and other parts of Britain were submerged. At Selby, Yorkshire, three-quarters of all the houses were under water. A severe south-westerly gale on 16 March drove water ahead and caused dykes in the Fens of eastern England to burst. Warm air and rain are much more effective at thawing large quantities of snow than sunshine. The albedo of old snow (proportion of light reflected) is about 55%. The albedo of fresh snow is about 80%.
On 9 and 10 April 1998, just before Easter, prolonged heavy rain fell over a wide area of Wales and central England. In the Midlands of England, there was serious flooding, with Northampton and Leamington Spa badly affected. In these Easter Floods, as they have come to be known, 4,500 homes were inundated, five people died and the estimated cost of the damage exceeded £350 million. The culprit was a slow-moving depression centred over Brittany. North of it, lying east-west across the Midlands, there were two parallel fronts which were almost stationary, producing prolonged heavy rain that fell on a catchment that was already saturated.
snowflakeIn coastal regions, flooding may be caused not only by a storm surge but also by a combination of high tide and high river level. An example of the latter occurred in South Wales in 1979. On 26 and 27 December of that year, falls of rain exceeded 100mm in many places, especially over the hills. Extensive flooding occurred on the 28th. Cardiff was badly hit when the River Taff in spate met a high tide from the Bristol Channel. Hundreds of homes and offices near the city centre were inundated. Flooding with sea water is bad enough, not least because the salt in the water is left behind when the water evaporates, but the flooding in Cardiff on this occasion was particularly unpleasant, as the water welled up into streets and houses through the sewers.
Health Risks after Flooding
In many parts of the world, there are serious health risks after disastrous flooding. Mosquitoes, flies and other insects may become more abundant than usual, as the filth, debris and stagnant water left by the floods provide suitable breeding conditions. Consequences may include out-breaks of typhoid, dysentery and encephalitis. Rats and mice displaced from their natural habitats may find conditions to their liking in houses, sheds, barns and other buildings.
In some parts of the world, snakes also become a problem after flooding, as they, too, are displaced from their natural homes by the water. Quite often, they appear inside houses. However, snakes can be beneficial, as they help to reduce populations of rodents.
Beneficial Effects of Flooding
Some of the most fertile land in the world lies beside rivers, the Nile valley being the classic example. For thousands of years, the people of Egypt have relied upon the waters of the Nile to overflow their banks every year, carrying with them fertile silt that makes agriculture possible. The flow of this river is nowadays controlled by means of the Aswan High Dam, an operation that can have its advantages and disadvantages. Salinity levels in the Nile Delta have increased, for example, because the outflow of fresh water from the river is much less now than before the High Dam existed. On the other hand, availability of water from the lake behind the dam, Lake Nasser, has allowed water levels downstream to be maintained in drought years, thus benefiting agriculture when in olden times crop failure and famine might have occurred.
The Ganges Delta is another place where flooding brings benefits for agriculture. Here, every June to October, the waters of the Ganges, Brahmaputra and other rivers overflow and inundate the countryside. There is, however, no equivalent of the Aswan High Dam to control the waters of the Ganges Delta. Sometimes, crops are destroyed, hamlets ruined and humans and animals drowned.
In some arid regions (e.g. Namibia and central Australia), floods occur very occasionally. When they do, glorious displays of flowers follow. In these areas, seeds can lie dormant for many years and germinate (grow) very rapidly when the rains come.
The heavy rains brought by tropical cyclones can help revive crops and replenish water supplies. Sometimes, fruit trees have flowered and borne fruit a second time after the passage of a cyclone. Sometimes, floods have flushed away mosquito breeding areas. There is a danger, however, that residual pools of water will, in turn, become mosquito breeding areas.
This is a drought which affects how farmers can use their land. An agricultural drought usually means there is not enough water for the crops to grow as there is a lack of soil moisture. It can also affect livestock such as cows and sheep.
Hydrological droughts are ones which there is a lack of water at the surface of the earth, resulting in less water in streams, lakes and reservoirs and can impact on the use of water for houses and industry
This is usually simply defined as a period of time where there has been less rain recorded. Rainfall amounts can vary by duration (i.e. time the rain fell for) and the intensity of rainfall (how hard it was raining). Meteorological drought is usually recorded in the time there has been little or no rain for e.g. months or years.
A Socioeconomic drought is when physical water shortages affects the lives of people; such as their health and quality of life. It can also affect the supply of food and materials and so affect the economy.
The Climate Change Schools Resources were developed by the Climate Change Schools Project, based at the then Science Learning Centre in Durham and led by Krista McKinzey. A large number of teachers and schools in North East England were involved in their development.
One of these is lightning, though the possibility of its occurrence may be indicated indirectly in a forecast or station report as ‘thundery showers’. In fact, lightning is not such a risk to sailors as it might at first appear. Boats are surrounded by a very good conductor of electricity – water – and unless the boat suffers a direct hit, which is unlikely, the current is dissipated much more quickly than on land.
Mountain tops are often hidden by clouds, which can result in people getting lost. But why does this happen? Clouds are formed by the condensation of water vapour in rising air. Air is very moist (holds lots of water) when it is near the sea or when there is persistent rain. In the UK the cloud base (i.e. the bottom of the clouds) is often below 1000m, so low in fact that much of the time the clouds cover the mountain tops. Cloud cover on the mountains is particularly common in the west of the UK where the moist air blows in across the Atlantic. Clouds can be supercooled i.e. water remains as a liquid even when the temperature is below freezing (0°C)! these droplets of water freeze when they hit solid objects such as fences and even people. The soft ice can build up into a think layer, known as rime which can cause as much difficulty for walkers as lying snow.
The rain and snow which falls over mountains tends to come from nimbostratus clouds, and occasionally cumulonimbus clouds, tend to be heavier and longer lasting than over nearby low lying areas. This is because the air is forced to rise over the mountains, causing the air to cool as it rises condensing the water from a gas to liquid; forming more clouds. However strong winds at the mountain top can blown the heavier rain over the mountain top, so the heaviest rain will not necessarily be at the highest point. Waterproof clothing and footwear on mountains is essential as there can be heavy rain, driving winds and mountain streams can become deep quickly. Deep depressions coming in from across the West coast of the UK can bring heavy rain and strong winds to the UK. Snow combined with wind can be life-threatening on a mountain top.
The higher up the mountain we climb, the colder and windier it usually gets so that the wind chill factor increases. Warm clothing on the top of mountains is definitely needed. On the Munros (the Scottish mountains with tops above 3000m) it can be 10°C cooler at the top of the mountain in comparison to the valley bottom below. In fact air can cool by 6°C in every 1000m and sometimes as much as 10°C. But why does the air get cooler and windier? It is windy high up in the atmosphere as the effect of gravity is reduced and cooler because air temperatures decrease as you get closer to the poles. Therefore gale force winds are stronger and more common at the top of mountains than at sea level. Winds also get faster around mountains as they do around tall buildings in our towns and cities. Over the Himalayas winds of 150km/hr (approximately 42 metres per second) are not uncommon!
Snow combined with poor visibility in cloud also causes problems because shadows disappear. Navigation becomes almost impossible and can lead to blundering into dangerous places ‘ even over the edges of cliffs. This is known as a ‘whiteout’. If there is already much lying snow and a risk of the cloud base descending onto the hills, many climbers often consider it wise to abandon the trip.
Local winds occur on a small spatial scale, their horizontal dimensions typically several tens to a few hundreds of kilometres. They also tend to be short-lived lasting typically several hours to a day. There are many such winds around the world, some of them cold, some warm, some wet, some dry. There are many hazards associated with the winds.
This wind is caused by thermal (heat) processes. Anabatic (upslope) winds occur over slopes which are heated by the sun. Air which is in contact with slopes that are warmed expands upward and cool and sinks over neighbouring valleys (see diagram). Anabatic winds are usually slow, at only 1-2m/s and are rarely important expect near coasts where they can increase the strength of sea breezes.
Katabatic (downslope) winds occur over slopes which are cooled. Katabatic winds occur where air in contact with sloping ground is colder than air at the same level away from the hillside over the valley (see diagram). Katabatic winds are nocturnal phenomena in most parts of the world (i.e. they tend to happen at night) as there is surface cooling, especially when there is little cloud and due to lack of heating by the sun.
The bora is a strong, cold and gusty north-easterly wind which descends to the Adriatic Sea from the Dinaric Alps, the mountains behind the Dalmatian coast (the coast of Croatia). It is a winter phenomenon that develops when a slow-moving depression is centred over the Plain of Hungary and western Balkans so that winds are blowing from the east towards the Dinaric Alps. These mountains form a barrier which trap the cold air to the east of them whilst the Adriatic coast remains comparatively mild. Gradually, though, the depth of the cold air increases until the air flows over passes and through valleys to reach the Adriatic Sea.
The Föhn is a warm, dry, gusty wind which occurs over the lower slopes on the lee side (the side which is not directly exposed to wind and weather) of a mountain barrier. It is a result of forcing stable air over a mountain barrier. The onset of a Föhn is generally sudden. For example, the temperature may rise more than 10°C in five minutes and the wind strength increase from almost calm to gale force just as quickly. Föhn winds occur quite often in the Alps (where the name Föhn originated) and in the Rockies (where the name chinook is used). They also occur in the Moray Firth and over eastern parts of New Zealand’s South Island. In addition, they occur over eastern Sri Lanka during the south-west monsoon.
The danger of a Föhn where there are steep snow-covered slopes is that avalanches may result from the sudden warming and blustery conditions. In Föhn conditions, relative humidity may fall to less than 30%, causing vegetation and wooden buildings to dry out. This is a long-standing problem in Switzerland, where so many fire disasters have occurred during Föhn conditions that fire-watching is obligatory when a Föhn is blowing.
The mistral is also a strong and often violent wind. It blows from the north or north-west down the Rhône Valley of southern France and across the Rhône Delta to the Golfe du Lion and sometimes beyond. Though strongest and most frequent in winter, it may blow at any time of year and develops when stable air is forced through the Rhône Valley. It occurs when a depression is centred over north-west Italy and the Ligurian Sea and a ridge of high pressure extends north-eastward across the Bay of Biscay.
Thunder is the loud noise which follows a flash of lightning. Lightning can be seen before thunder is heard as light travels faster than sound. The speed of sound in air is just over 300m/s. This means that if you count the seconds between seeing the lightning and hearing the thunder, and divide by three, you can work out how many kilometres away the storm is (for example, if you start counting when you see the lightning and get to 9, then the storm is about 3km away). The noise of thunder is caused by the rapid expansion of heating the air. You can normally hear thunder up to 6 miles (10km) away from the lightning flash. The sound can last quite a few seconds!
Lightning can be seen virtually instantaneously as light travels very fast (about 300,000,000 m/s!). Lightning can be seen up to 50 miles away! lightning. Lightning is produced by discharges of electricity from cloud to cloud or from cloud to ground. A large positive charge builds up in the upper part of a thunder cloud and a negative charge builds up near the base of the cloud. When the potential difference between the charged areas becomes large enough, electrical energy is discharged and a flash of lightning occurs. Huge quantities of electricity are discharged in lightning flashes and temperatures of over 30,000°C or more can be reached!
In a thunderstorm you should not stand under a tree! Lightning tends to strike the highest point around and everything near this can be a target for the lightning too. Very few people survive being hit by lightning. To increase your safety in a thunderstorm you should avoid high ground, water, open spaces such as parks and golf courses, staying in a tent or shed, being within 30m of wire fences or using your umbrella. You should make yourself as small as possible – curling up in a ball is good. It is however safe to stay in the car…do you know why?! It is because the car acts as what is known as a Faraday cage, protecting you from the electric field generated by the storm.
For thunderstorms to occur, cumulonimbus clouds are required. These are heavy, dense, towering clouds with tops shaped like anvils or vast plumes, where the speed of air rising through the cloud can reach 20m/s. Pilots tend to fly around these clouds if they can. They can fly around them as often they are only 10-12km in width. In cumulonimbus clouds weather such as heavy rain, lightning, hail, turbulence and strong winds can occur.