Covering the The human impact (floods) from Hydrology and fluvial geomorphology. Credits are in the document.
Note by chidat, updated more than 1 year ago
Created by chidat almost 9 years ago

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1. What is a flood? A flood is a natural hazard that occurs when a river carries too much water that the banks cannot hold it hence it bursts. The extent to which the river exceeds its limits determines the severity of the flood. 2. What causes flooding? Flooding may be caused by both human and physical factors determine whether a place is more prone flooding than others. Ø Natural Causes · Nature of the drainage basin –Some drainage basins are more likely to flood than others die to the relief, vegetation, soil depth, rock type, drainage density. · Rock Type – Rock that is more permeable allows more infiltration and ground storage to take place hence there are fewer surface run offs. · Soil Depth – The deeper the soil is the more water is absorbed which results in less run off. · Vegetation - In areas where there is more vegetation cover, there are more levels of interception and evapotranspiration which reduces the risk of flooding. · Slope – Gentler slopes means more water is absorbed and less run offs therefore reduced risk. · Drainage Density - Where the drainage density is high, there is a shorter lag time and an increased risk of flooding. · Low lying parts of active floodplains e.g. in Bangladesh, 110 million people are living on the floodplain of the Ganges and Brahmaputra rivers are relatively unprotected as the floods caused during the monsoons usually cover 20-30 % of the flat delta hence in high floods, nearly half of the country can be flooded leading to over 1500 deaths. · Earthquakes – Seismically unstable terrain increases flood risks. · Climatic factors – Excessive levels of precipitation that occur over a long period of time would lead to saturation of the soil. This is because there will be increased amounts of water on the ground surface leading to increased overland flow. For example the heavy autumn and winter rains in the UK are due to the low pressure weather systems (deep depressions that cover a wide area and are long lasting). Intensive precipitation over a short period of time especially when the ground surface was extremely dry for a long period of time would lead to the water passing the rate of infiltration hence the ground cannot soak up the rainfall quickly enough therefore more water will reach the river than it would under normal circumstances. E.g. Monsoon rainfall in India and Pakistan during summer. Melting of snow when the subsoil is still frozen leads to reduced infiltration e.g. in the Alpine and Arctic areas, the melting snow is responsible for widespread flooding. Ø Human Causes of Flooding There are the same factors that affect a storm hydrograph e.g. land use, urbanisation. · Flood-intensifying conditions which alter the drainage basin response to a given storm e.g. urbanisation – Urbanisation (an increase in the proportion of a country’s population living in urban areas) increases the frequency and magnitude of flooding by: i. Increasing the creation of impermeable surfaces e.g. roofs, roads (concrete and tarmac), car parking and pavements leads to less infiltration therefore shorter lag time. ii. Increasing and speeding up drainage density by the construction of artificial conduits e.g. underground sewers. iii. Impending and constricting river channel flow by building alongside or in the river e.g. a bridge supports or riverside facilities which reduce the carrying capacity and slows down discharge. iv. Straightening of channels to increase the velocity which leads to flooding downstream. v. Land use e.g. for deforestation (greater risk of soil erosion leads to less interception), ploughing and overgrazing increases flood run off, reduces interception and decreases channel capacity due to increased deposition in the channel which eventually blocks the channel and increases the risk of flooding. · Climate change – Global warming has increased the frequency of flooding. This is because of increase in the average sea level temperature rise has led to the increased frequency and magnitude of tropical storms especially in the Caribbean. These storms bring heavy rainfall along the coastlines for example Hurricane Katrina in 2005 led to flooding in New Orleans. Higher temperatures will also lead to increased evaporation leading to more precipitation, melting of polar ice caps, rise in sea levels which all increase the risk of floods. 3. What are recurrence intervals? The recurrence interval is the regularity which particular levels and size of floods occur. Once it is calculated, it indicates the number of years which a flood of a particular size is expected. Small floods are expected to occur more regularly than larger floods but are less damaging. Nonetheless, it is a probability based on previous historic evidence and does not mean floods will occur more or less frequently. The recurrence interval can be calculated by the formula: T=n+1m where T is the recurrence interval, n is the number of years on record and m is the ranking of the flood relative to all the other floods on record for a specific river. For example, a flood with a discharge of 200m3s-1 occurred at some point in the river’s past. Out of a data set spanning 199 years5, this flood was the 2nd largest in terms of discharge. Using the formula, this means that a flood of this size is expected to occur once every 100 years (199+12). We’d describe it as a 1 in 100 year flood. All in all, the recurrence interval is not the most reliable as it is based on averages which can have anomalies such as the fact that flood patterns can be unpredictable as they change all the time. 4. How else can floods be predicted? The Environmental Agency constantly monitors rivers and records of floods. Except for the recurrence interval, floods can also be predicted by using the Risk Assessment for Strategic Planning (RASP) which is a method that improves flood warning and prediction. This can be done by weather radar which shows the likely and actual precipitation approaching an area hence it improves the rainfall and snow pack estimates. Automatic rain gauges also help in the prediction because it helps in collecting the meteorological information and maps the channels with the amount of rainfall that has fallen in the catchment area. River gauges record the discharge along a river therefore improving the flood warnings as there will be more complete and timely information of hydrological information that can be shared. 5. How do floods affect places? Boscastle, UK, 2004 Boscastle is a village and fishing port located on the north coast of Cornwall, South West of England and is home to around 2000 people. River Valency flows directly through the towns and meets River Jordan at a confluence in the town which leads to huge amounts of water flowing when discharge increases. The river walls are steep made of impermeable rock and shale. The Boscastle flood of 2004 occurred on Monday 16th August 2004 when 200.2 mm of rainfall (3 normal months of rainfall) was recorded in only four hours. Causes Ø Physical and Human causes · Hurricane Alex and the convectional rainfall triggered the intense heating of the ground which led to heavy rainfall that fell over Bodmin Moor, which is an area made of impermeable shale which increased the surface run off due to a lack of infiltration and interception (sparse vegetation) hence the volume of River Valency and Jordan increased. · The ground was already saturated by previous rainfall which reduced infiltration and increased surface run off therefore most of the water swept downhill between the river walls that resulted in the water reaching 3 m in height that swept through the village. · The surrounding areas of River Valency and Jordan have high relief and steep slopes therefore it encourages surface runoff. · The floodplains around the rivers had been urbanised which reduced infiltration and increased surface runoff. · River Valency had its channel walled off therefore it prevented the channel from adjusting to the increased discharge and limited its efficiency which inevitably caused the flood. Effects Ø Social · Some people suffered from hypothermia and broken bones as people had to seek refuge on higher grounds such as rooftops. 120 people were rescued from rooftops. · Houses became flooded and silt, sewage and debris were deposited inside of them which left many people homeless. Around 1000 people’s homes, businesses and cars were swept away. · There was no water and power supply. The power has to be switched off to protect rescuers and survivors from electrocution. Ø Economic · Potential increase in home insurance due to the extensive damage caused but because floods are so rare hence it was unlikely. Claims came in between £15,000 and £30,000 per property. · Tourism was a main source of income (nearly 90% of the economy was dependent) but after the flood people were less willing to travel to Boscastle because of the potential risk of another flood. This led to sever loss in revenue. · 76 cars were carried through the village by a torrent to the sea because of the low lying nature of the town’s car park. · The “lower bridge” was destroyed by the debris that blocked it. Ø Environmental · Due to the sewage burst, raw sewage was washed out to the sea and in the rivers. · Local wildlife habitats were destroyed. Responses Ø Short Term · A search and rescue team was issued a few hours after the flood which lasted until 2.30 am the next day hence 150 people (120 from the rooftops) were saved by search and rescue operations. · Lifeboats searched the harbour fearing people may have swept out to sea. · North Cornwall Council provided accommodation for 11 tourists who were unable to return home while the night of the disaster. While the night of the disaster, 100 people sought refuge in the Camel ford leisure centres. · A cleanup operation was set up to oversee sewage, road, water and electricity systems. Ø Long term · In 2006, donations summed up to £800,000 which was used in a flood scheme that would incorporate the sewage and drainage system, deepen and widen the riverbed by 6 feet to accommodate more water and to handle an increase in discharge, and, to raise the Boscastle car park. . The Environmental Agency built new flood defences in 2006 at the source and the river base of Boscastle. They also removed the debris and sediments from the source. · Reconstruction was delayed to 2005 as the council was waiting for hydrologists to find the recurrence interval of the flood. But by early 2005 power and water was restored. · The remains of the “lower bridge” that triggered a 3m wave were demolished and replaced with a larger bridge that would be more difficult to block with debris. · Environmental footprint was taken more seriously by the locals as they believed that climate change was exacerbated by human activity which caused the flood therefore all the new construction was eco- friendly e.g. with insulation. 6. How can floods be prevented? a) Loss Sharing – This includes disaster aid and insurance. Disaster aid is the financial and economic resources that are given to an area after a disaster e.g. money, staff, equipment, aid and technical assistance. An example of loss sharing would be insurance which is common in MEDC’S. b) Event Modification – The adjustments include environmental control and hazard resistant design. This kind of physical control of floods depends on two measures: flood abatement and flood diversion. c) Flood abatement – Abatement measures aim to reduce the possibility of flooding by managing land use upstream and decreasing the amount of runoff thereby reducing the peak flow. This includes a variety of methods such as: · Afforestation (requires substantial areas in the catchment to be planted with trees. · Reforestation to increase interception and evapotranspiration and reduce runoff as well as reducing silting of the river channels as the soil will be held together. · Contour ploughing or terracing to reduce to runoff and soil loss, reducing the amount of bare soil to avoid excessive run off by reseeding of sparsely to increase evaporative losses. · Comprehensive protection of vegetation from forest fires, overgrazing and deforestation. · Allowing rivers to meander. · Reduce the amount if sediment and debris washed into the channel. · Construction, restoration and excavation of backwaters, small waters and sediment holding areas to accommodate flood water. d) Flood diversion – This includes the construction of levees, reservoirs and the modification of river channels. · Levees are used to divert and restrict water to low value land on the floodplain. · Channel improvements such as channel enlargement will increase the carrying capacity of the river but the channel becomes clogged with weeds. · Reservoirs store excess rainwater in the upper drainage basin. However this is only appropriate in small drainage networks. Nonetheless, the reservoirs create a water supply area. · Intercepting channels divert parts of the flow away which allows the flow to be used for agricultural purposes. e) Land use planning – This is the mapping of areas that are of a greater risk of flooding and once this is discovered, low value buildings can be developed. However, the poor tend to find it hard to evacuate from this area as it is not always possible to change land uses hence they choose to live on the land. f) Flood proofing (building design) - This is making property less vulnerable to flooding and the damage after. It may be temporary e.g. using sandbags of permanent e.g. buildings with flood proof walls. This will ensure that only low value land uses like car parks occupy ground lives and the potential damage from floods reduced. However temporary flood proofing is only useful for minor flood problems and some people may not be willing or able to afford new advancements to their houses. .

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