The Different Methods Of Flood Prevention

In the sixth week of the first semester of the current academic year I was informed that I would have to write a technical report on a topic of my choice as well give a presentation on the same topic. After reading around for interesting and relevant topics as well as correspondence with my personal tutor I chose the topic of “Methods of Flood Prevention” for the report. Starting my research I made use of the library and internet to increase my knowledge of the subject as well as find research and discussions by experts in the subject. On the tenth week I gave short presentation on the subject that I presented to my peers this was successful. Furthermore the purpose of this report is to investigate and outline methods of both preventing and dealing with floods, primarily by restricting the places where the water is allowed to flow. This report will span though the causes of flooding, the dangers finally looking at different methods mankind have used to limit the effects of flooding worldwide though a range of solutions, some starting simple and overtime becoming more complex.

An initial examination of flooding

Cause

Flooding is caused by large surges of water over a short period of time this causes existing draining systems, wither natural or manmade to become overwhelmed. This occurs because the ground becomes saturated and is no longer able to absorb more water, in addition other drainage such as rivers, drain pipes and reservoirs may be filled faster than they are able to drain, this causes the excess water to accumulate. The rapid surge of water can come from a variety of different sources as well as a combination of sources. According to J.A.A. Jones heavy rainfall is usually a “prime initiator of flooding worldwide. The key characteristics may include any combination of rainfall intensity, duration, area and location relative to the drainage network.” {{J.A.A Jones 2000}} Other sources include tidal surges, hurricanes, tsunamis, rapid melting of ice or snow. Blockages of drainage systems such as rivers streams and sewers can also cause flooding if water isn’t able to be drained away normally.

Risks

Flooding causes a variety of problems ranging from the short term and more immediately evident risks to long term risks. Immediate risks are often the greatest danger in terms of risk to human life.

Immediate risks pose great danger to human life. According to the world health organization {{World Health Organisation 2010}} floods greatly increase the risk of the spread of communicable diseases. Water-borne diseases, such as typhoid fever, and cholera are likely to spread during flooding due to possible contamination of water supplies especially if the floodwater becomes contaminated by sewerage. Vector -borne diseases are those spread by carriers, floods can lead to a large increase in the habitat of for example mosquitoes which are spreaders of malaria. The sheer mass of water causes risks to anyone trapped in the affected, area, bursting gas pipes, collapsing structures, and drowning.

Long term risks, flooding causes millions of pounds of damage to buildings. After a major flood economic instability is likely to follow, Loss of business, damage to stock and even loss of tourism income. Some countries that have been devastated by floods may suffer the consequences long after the water has subsided, food supplies being destroyed and other economic losses may lead of widespread poverty and famine. Hence the limitation of floods is essential to the well being of any community.

Flood Defences

Over the last few centuries many countries have developed and put in place different measures to control the position of water a limit the effects of heavy freak or seasonal rain, tides as well as tropical storm weather. There are two main branches both closely connected to one another. One is concerned with protecting from floods by preventing or limiting their effects from occurring, the other is concerned with predicting when, how often and how severely floods will strike.

Reservoirs

According to Collins a reservoir can be described as “A natural or artificial lake for storing water for community use” {{ Collins Language 2009}} Reservoirs can be considered a useful form of flood control especially in areas of irregular rain patterns, such as high rainfall in some seasons followed by periods of drought. This way water can be stored during periods of rainfall and then released or used in the community over a longer period of time.

Dams

Dams are often part of a reservoir system; the advantage of dams is they are able to act as more than just a flood defence, they are good sources to be used for hydroelectric power stations. Dams however pose a flood risk caused by dam failure, if improperly built. Dams are also sometimes spanned across rivers or lakes, one example of such damn being the Wilson Damn is dam spanning the Tennessee River. The gates are used to slow or increase the rate of flow through the damn, for the purpose of controlling the water as well as generating large amounts of electricity.

Sea Barriers

Sea barriers are a very effective form of flood protection protecting from sea water. They are useful in places where a costal which are at great risk of flooding from high tides. These can be permanent walls or erecting barriers, examples of such would be the Thames Barrier in London and the Maeslantkering in The Nederlands.

The Maeslantkering with its extremely impressive dimensions consists of two 210 meter wide gates that are 22meters high and is hence one of the biggest moving structures on the planet. The purpose of these gates to protect The Nederlands from storm surges. The construction of this barrier took over 6 years to build and was completed in 1997. This is advantageous versus other types of barriers as the fact that it is able to move to allow ships access into The Nederlands. It operates automatically using weather data and closes when there are surges higher than 3 meters over normal are predicted. The automatic computer systems of the Maeslantkering automatically alerts all traffic in advance to halt their course until further notice if the barrier is intended to close. The Nederlands having large portions of its land below sea level such that two-thirds of its population lives there, is at high risk of increasing sea levels and have over the last few hundred years taken many steps to reclaim more and more land from the sea have some of the best flood defences in the Europe. It is hence important that barriers such as the Maeslantkering exist especially in this day and age with the uncertainty concerning rise levels and the continued treat of global climate change. The barrier and other flood defences in The Nederlands are designed to protect the country from floods of estimated frequency of 1 in 10,000 years.

The Thames Barrier in London was completed in 1984 and is designed to protect the city of London from being flooded by large surges of water from the Atlantic Ocean. I consist of 4 main gates as well as several smaller gates; the barriers themselves fill with water and lay submerged when they are not in use, the water in these gates drain out when the barriers are raised. The raising of the barrier is controlled and decided upon using several checks, including predictions not only from its own forecasting systems but also those from the Met Office. Like the Maeslantkering the computer system at the Thames Barrier automatically alerts all traffic coming towards and inside London to cease course and stand by up to 9 hours before it needs to close.

Levees

Levees are slopes of land or walls that are to control water levels next to rivers or the coast. According to {{R. A. Guisepi 2008}} they where first built in the Indus Valley Civilisation over 4.6 thousand years ago. Around the world they are quite popular and very effective as a form of large scale flood control present in rivers both across Europe and in the United States, for example the Mississippi River as while as the Rhine which stretches across seven European counties. Although so widely used and trusted levees are not in fact that reliable when improperly secured especially when it comes to the most extreme weather, most notably during Hurricane Katrina there was wide scale failure in New Orleans. They work by controlling the size and shape of the river, as a result may even change the speed at which the river flows.

Storm drains

Storm drains are systems that are designed to drain rainwater from roofs, paved areas and roads. They generally consist of a series of underground pipes that lead into a reservoir. The areas to be drained are usually tilted at a small angle so that any water that fell on these areas would not remain stagnant, instead would run off into these drainage systems. This process is essential as rainwater is not able to easily penetrate paved areas, this would otherwise cause regular flooding of urbanised areas. The water that runs the tilted pavement runs along little valleys into storm drain entrances. These are usually grids to prevent too objects from falling down, although quite large holes to prevent them becoming blocked. These drains are fitted with traps much like those in a house-hold sink, in this case called a gulley-pot, like traps in sinks they are prevent rising foul gasses to be escape the sewers. As a result of accumulating sediment in these traps require regular maintenance especially during the autumn season where leaves and loose soil are likely to be washed into them, this is easily done using specially designed trucks with their own pumps to simply suck out any blockage, I minor inconvenience to road users however, necessary none the less. Storm drains date back over 4000 years ago according to {{C Michael Hogan 2007}} where evidence of them has been found in Crete. There are several building regulations connected to the storm drains.

Flood plains

Are pieces of land that are usually nearby rivers and are usually very prone to being flooded, this land may because of the proximity to fresh water be very fertile and good for transportation, but control and awareness of developments and usage of these pieces of land are essential for the safety of those living on these areas. Steps such as flood insurance, and evacuation procedures must be put into place to prevent great loss of life or possession.

Temporary structures

Temporary flood barriers are available from a large range of sources online. There are businesses which specialise in fast erecting cheap but incredibly effective flood barriers which can provide protection on a small scale for specific buildings. These types of barriers can consist of a number of things, for example inflatable barriers or strong polymer barriers that lock into alleyways or doors. Packing bags of sand may also be done as a way of protecting areas that are sensitive to flood waters such as power stations. An example of one such company which I discussed in my talk was Revetment Limited claiming to be able to erect barriers “20 meters can be erected in less than 15 minutes” {{Revetment Limited 2003}}

Environment Agency

In the United Kingdom the Environment Agency issues flood maps which consists of area based risks assessments for the whole of England and Whales; anyone can request copies of theses maps or simply view them online. They outline which parts of the country are at risk of flooding, to what degree, and from what source. If for example I was to look at a flood map and zoom onto the area that is Loughborough I could see that there is very low risk of flooding and only river flooding, at the canals especially near the leisure centre. In addition whenever any landscaping or building work is to be done there is a requirement that a risk assessment of the area is to be carried out; not only on the area being developed but on the affect that this development would have on the surrounding area and wither it would increase the flood risk of this area. The development would have to not interfere with the drainage of the area surrounding, all this is outlines within “Planning Policy Statement 25: Development and Flood Risk” {{The Environment Agency 2009}}

Meteorological

There are scientists out there who devote their entire careers to understanding and predicting the way that the weather will sway. They make use of measurements of air pressure, air temperature, weather balloons and in more recent year’s weather satellites. These organisations are essential and one of the frontier lines as far as flood defence goes. They provide data for the engineers to build upon, they also provide the data for certain emergency system to run for example, evacuation of high risk areas the raising for flood barriers such as the Thames and the Maeslantkering. One of the most essential qualities of such an organisation, for example the UK Met Office is making predictions clear and ahead of time then communicating with the necessary authorities and making sure the what needs to be done is carrier out.

Frequency Based Risk Assessment

“Frequency analysis can be based either on annual maximum (AM) data or peak-over-threshold (POT) data. The AM series comprises the largest rainfall observed in each year whereas POT data consists of the time and magnitude of rainfalls exceeding a threshold.” {{Duncan. F 1999}} Frequency based risk assessment is process used by meteorologists to provide estimations for governments, engineers, and insurance companies alike. Currently frequency based estimations are being used in the Netherlands to set the heights of the river dikes. Hoekstra explains so in an article for the journal Natural Hazards “In the Netherlands the current dike design policy is to design flood defence structures corresponding to an agreed flooding probability with an extra safety board of at least 0.5 m.” {{Hoekstra, Arjen Y. 2008}} He later goes on to explain that “A problem with this strategy is that it builds on assumptions with regard to the intrinsically uncertain probably distributions for the peak discharges”. {{Hoekstra, Arjen Y. 2008}}. As explained by the above quote these analyses and risk assessments are rather vital as it provides engineers and designers for a template to design other flood defences to. Without this data we would simply be building walls and would have no idea how high they would need to be and how likely it would be to be a sufficient or effective flood defence. It also sometimes hints on patterns of likely hood on when a flood is likely to occur. In years to come however these estimations become more and more unreliable as sea levels increase and climate patterns that have been observed for hundreds of years slowly change.

Conclusions

Floods are an unavoidable natural process caused by large surges of water in short periods of time, leaving the drainage system that is in place, wither man made or natural unable to cope regardless if it simply controls where the water is kept or even controls how to dissipate the water. Therefore we have to do everything in our powers to limit the damage caused by these floods as to ensure the livelihood of the people of the community. Wither we believe that the oncoming climate change is due to human negligence or if it simply part of longer cycle of changes we must consider the steps to prepare for the worse. Even if we do build defences and there turns out to be no massive rise in sea levels it may still be useful to prepare because when the proof arrives it may already be too late. Furthermore such a large building project could be beneficial to many communities providing not only security but employment and economic stability. In this report outlined the different measures that can be taken both as emergencies and on the long term, including but not limited the storing of water in reservoirs, the use of sea barriers, levees, storm drains, temporary emergency structures, and proper planning and predictions.

In the light this, I would suggest that other countries follow the example of the Nederlands and attempt to provide its most sensitive areas with flood defence that is capable of protecting from one in 10,000 year storms, as it is a lifetime investment that will provide long term piece of mind regardless of wither it will be used against freak floods or used to protect people on the advent of global sea levels continuing to rise.