rf - flooding case study

8/14/2019 RF - Flooding Case Study

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Geo Case Study

1- Differences between Lag time Flooding and Flash Flooding

In general, Flash Flooding tends to happen more upstream in the river than lag time flooding,

because the river takes less time to overflow its banks since the upstream banks are lower than the

downstream banks. Upstream flooding can be caused by very heavy precipitation mixed with very littlegroundwater infiltration.

Another example of this flash flooding occurred this year in Palestine as well. Soil in Palestine

is generally very resistant to infiltration because of the over all low amount of precipitation inPalestine. This meant that very little rain water was dispersed into the ground, instead most of it pooled

on the surface, doing damage to many homes. Palestine did little ot prepare for such a flooding event,

due to the poor economic conditions and restrictions as well as the political turmoil within the country.

This lack of preparedness became obvious in the floods.

* Flooding in the Gaza strip,showing the surface pooling that occurred.

*-

Conversely, lag time flooding tends to be a downstream occurrence. This can be caused by thewater infiltrating into the ground, and flowing under the ground in through flow or aquifers instead of

pooling on the surface. This causes the water to join the river further downstream rather than travelling

over the surface. This causes the river to flood where the through flow enters the stream channel, which

tends to be further down in the streams course, rather than in its headwaters. Lag time floods tend to beseasonal and associated with a reoccurring weather pattern. An example of this can be seen in the

Ganges river delta, in which the river floods every year, after the monsoon rains. The monsoon rains

cause the waters of the Ganges to swell, but usually they occur farther upstream, and while floods canoccur farther upstream, the much greater risk of flooding occurs downstream where the flood waters

form all of the various rivers meet. This meeting of waters causes the delta to overflow. This can be due

to a number of factors, but the biggest reasons are the permeability of the soil and the size of the riverchannel. The soil is very permeable, and this causes much of the water to go into the oil. As well,

much of the area around the river is covered in vegetation, causing the water to be distributed by stem

flow, and percolate through the earth slower. This causes the water to flood down river, as less of thewater is concentrated in surface pooling. The size of the river channels also causes flooding, since the

rivers have a tendency to be wider and shallower rather than deep and narrow. This means that when

water is put into the river, the water floods more easily, as the river embankments have been eroded


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form years of continuous flooding. Lag time flooding can be as dangerous as flash flooding if there are

not early detection systems, as was seen in 2003 in Bangladesh.

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1- Map of the Ganges river delta. Note the amount of streams present in the lower reach of the river.

The amount of streams shows the wide flood plain, as each stream can gold less water and is more

prone to flooding. 2 Satellite photo of the Ganges delta. The river has many outlets into the ocean,and the area between these show the width of the floodplain.

2) - The Effects of Human Activity on Stream flow and Percolation.

Stream flow and percolation can often be affected by human activity. An excellent example of

this can be seen in the Yangtze river in China. The Yangtze river is currently the site of the Three

Gorges dam. When completed the dam is expected to have a double effect on stream flow, with bothpositive and negative effects for the human beings living there. The positive effects will the be for the

people living downstream on the Yangtze, such as in the Hubei province. The Three Gorges dam is

expected to reduce the amount of seasonal flooding that occurs in these provinces, as well as makingthe land more usable for agriculture, as there will be a control on the water moving through the river,

enabling the farmers to be able to use the land through the entire year, rather than through a single

season. The dam will be able to give a new degree of security to the residents of the lower reaches ofthe Yangtze river.

However, the upper reaches of the Yangtze river are not as lucky. The Three Gorges dam isforcing people to relocate to special relocation settlements created by the Chinese government. As

many as 4 million people could be moving there once the water reaches its expected levels, which

could be as soon as 2009. As well, the increasing amount of water being used of the reservoir would


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make much of the previously arable land unusable for farming due both to the resulting flooding caused

by the dam, and by the amount of soil that will be eroded. This silt will also not be able to be

transported down stream, and will not be able to be included in the soil downriver, in the Hubei

province. This could deprive that soil of essential nutrients upriver.

A interesting example of human interference with percolation can be seen in the Gaza Strip

region of Palestine in 2008. In cities such as Khan Younis in the south, Water was unable to percolatedue to the amount of paving and compacting of the soil that had occurred, but the sewage system and

storm drain system were inadequate to meet the amount of precipitation. Much of it overflowed,

bringing water into peoples houses, and forcing it onto the streets where it was unable to percolate.

Picture of the three gorges dam in China.


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Palestine after the flood. The sewage that ran out of the sewage plant can be seen here, after the floodwater over ran a refugee camp.

Map of Palestine and the affected areas in the Gaza Strip.

3) The Impact of Agriculture and Industry

The effects of Agriculture and industry can be seen in the flooding events of the Yangtze river.

Ginseng is a very popular crop to grow in China, and the government grows extra in order to export.However, a ginseng field cannot be used twice to grow ginseng. To support the increase in demand for

ginseng, the Chinese government began deforesting more and more of the area that surrounded the

Yangtze river. The removal of these trees made the area that surrounded the river much more


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vulnerable to erosion, and when the Yangtze river overflowed its banks in 1998, and then later in 2008,

eroding vast amounts of soil.

The effects of industry can be seen in the Yangtze river, with the construction of the threegorges dam. While the dam will make the river down stream more useable for industry, as well as

supplying a massive amount of electricity for the industry, it will also create a massive flood pain

upriver. While it will mean that some of the industrial plants upriver will be moved, it will also givethem a more steady source of water, and it will make it easier for coal to be moved on the western

reaches of the river, as the water level will be more steady.

The Ganges river has been affected by industry and human demand as well. There are many

Hindus who believe that it if they die or their ashes are placed in the Ganges river, that they will be

carried into paradise in the afterlife. The amount of ashes being placed into the river has given theGanges river an unusually high amount of sediment, and has actually started to slightly alter the course

of the river. Along with this is almost a billion litres of human waste, and a leather industry at the city

of Kanpur that use the water of the Ganges.

Map showing the drainage basin of the Yangtze river. Many of the smaller tributaries have also

been affected by deforestation.

Map of the Yangtze river. The shaded are is the area thatwill become a reservoir for the Yangtze river.


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4) Reoccurring floods due to human use

An example of reoccurring floods due to human use can be seen in the Yangtze river. Due to the

amount of agriculture and industry that is meant to be sustained there, much of the area has beendeforested, which prevents the naturally occurring forest from intercepting the water and distributing it

evenly over the earth, instead, the water begins to percolate immediately, and the ground becomes

super saturated. Water then begins to pool on the surface, and is added quickly to the river, rather thanthe longer time it would take through flow to reach the same location. This increase in water has caused

the Yangtze river to overflow many times. Flooding before the deforestation was not as bad, and serves

to demonstrate the effect that development has had around the Yangtze river.

The Yangtze river has also changed in nature due to the three gorges dam. The three gorges dam

was created a massive floodplain behind the dam, and this floodplain will change the nature of the land.

This land has become very prone to flooding, and the levels could change significantly. In the southernregion though, the floodplain will shrink and the amount of flooding can be contorted significantly by

the presence of the dam. This has sparked regional tension in China, as both sides try to either further

the construction of the dam or halt it.The opposite is being seen in the St Johns river, where 10,000 acres of agricultural land is being

converted into marshes and other wetland habitats. Not only will this have a positive effect by creating

much needed habitat for many species on the verge of extinction, but it will also create a buffer forfloods further down river. The marshes and other wetlands absorb a lot of water in the case of heavy

precipitation, and lessen the flooding effect further downstream.

Map of the St Johns river. 10,00 acres in New Brunswick have been set aside for wetlands use.


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Converted marsh land in New Brunswick.

Deforested area of the Yangtze river. \/

5) Differences in Flooding responses between LEDCs and MEDCs

As is the case for most natural disaster, flooding responses differ drasticaly from MEDCs toLEDCs. In a MEDC such as Canada, early flood detection provides many people with the chance to

either fortify their house or leave. This was seen in the case of the St Johns river flood, where many

people were put on flood alert. This enabled people with the time to try and protect as much of their

property as they could, and enabled many to leave. The government also organized an evacuationschedule for areas very close to the river. The study of the river gave advanced warning to many.

The Chinese government has taken a more physical approach to solving their flood worries. TheChinese government has tried to alter the basin surrounding the Yangtze river, creating several small

reservoir lakes along some of the smaller tributaries. The Military has also been helpful in responding

to flooding events, as can be seen in 2008 when they drained 2/3s of the Dongting lake in order tocreate a pressure gradient to pull more water into the lake. Their efforts have been only partially

successful in eliminating loss of human life due to flooding, as they have also been reluctant to


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evacuate people living near the river.

Sandbagged home near the St John's river/\ Military aiding in the evacuation of dairy cattle \/

Many LEDCs are unable to react to flooding in such a manner. The recent flooding in the GazaStrip highlighted the underdevelopment in the region. The sewage system was unable to take in all of

the water, and overflowed, putting raw sewage into many streets. The water treatment plants were also

threatened by the rain, and some of the more southerly ones overflowed. The Palestinian governmenthas been unable to react to this disaster with any kind of real plan, due to both political instability and

lack of funds. Instead, most of the aid going into the region has come from private aid organization

including the red cross and the red crescent. In Bangladesh, the government is also unable to take muchaction in regards to flood readiness. Every year the Ganges river overflows, and kills people living on

or near the delta. Part of the governments unreadiness is due to lack of funds to study the river, which

has began to show uncharacteristic flood patterns. Without a large group to research this however,

Bangladesh is unable to prepare for floods on the Ganges Delta in the way an MEDC could.


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rf - flooding case study

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