Floods

Alexia Jablonski November 21, 2008Flood Hazard Case Study

1. Describe the difference between and provide examples of flash flooding and lag response flooding.

Flash flood Caused by sudden heavy precipitation or dam failures Fast-moving Dangerous because there is little forewarning Caused by low rates of infiltration and interception, which increase surface runoff

(Prof. Stephen A. Nelson from Tulane University)1

o High vegetation cover = high infiltrationo Soil types (desert soil much more prone to flash floods because they

absorb less moisture)o Frozen ground = less infiltration o Human constructions such as pavement decreases rate of infiltration

Example: 2007 Sudan floodso First occurred on July 7, 2007o Desert areao Caused by heavy raino According to UN reports2:

Over 30,000 houses fully destroyed at least 365,000 people directly affected, 64 dead and 335 injured 257 schools destroyed (leaving over 56,000 children without

primary education) At least 12,000 livestock, 16,000 chicken, and 96,000 feddans of

crops lost (1 feddan = 4200 square m²)

Sudanese affected by 2007 flood3

1 http://www.tulane.edu/~sanelson/geol204/riverflooding.htm2 http://www.dawn.com/2007/08/06/rss.htmhttp://www.reliefweb.int/rw/RWB.NSF/db900SID/EVOD-76AJMP?OpenDocument&rc=1&cc=sdn3 http://blog.foreignpolicy.com/files/images/flood.jpg

Lag response flood Has a high lag time: “The time difference between when heavy precipitation

occurs and when peak discharge occurs in the streams draining an area” (Nelson):

Example: 2007 South Asian Monsoon Floods4 o Caused by monsoons (torrential rains linked with wind patterns) in India,

Nepal, Bhutan, and Bangladesh lasting 20 dayso UNICEF: situation "is being described as the worst flooding in living

memory"o About 30 million people affected and displacedo Bangladesh: 2/3 of the country left underwater; huge outbreak of flood-

related diseases such as dysentery; many areas made isolatedo India: more than a thousand people killed; millions of dollars in crops losto Nepal: 84 people killedo Bhutan: several major roads disrupted

Areas affected by flooding in South Asia5

4 http://www.geographyalltheway.com/ib_geography/ib_drainage_basins/ib_south_asian_floods.htm

Family marooned by the flood in South Asia6

2. Use two or more examples of flood events to demonstrate how human activity has affected stream flow, percolation, and siltation.

Definitions:Percolation: The movement of water through the pores in soil or permeable rockSiltation: When a river becomes filled with silt (sedimentary material consisting of very fine particles intermediate in size between sand and clay)

Example 1: Rajang river, Malaysia, 20077

Heavy siltation of the river has been caused by soil erosion, which is due to lack of trees from deforestation upstream

The government has no policies to control logging Silt clogs up the river, making it more likely to flood over its banks The town of Sibu, situated on the lower Rajang delta and on low-level terrain,

faces frequent flash floods

2007 flood in Sibu8

5http://www.geographyalltheway.com/ib_geography/ib_drainage_basins/imagesetc/South_Asia_Flood_July_3_and_August_15.png6 http://newsimg.bbc.co.uk/media/images/44043000/jpg/_44043442_dhaka_ap203long.jpg7 http://malaysiadigest.blogspot.com/2008/04/siltation-in-rajang-river-due-to-heavy.html8 http://i35.photobucket.com/albums/d166/forevermzcm/Flood2small.jpg

Example 2: 2007 United Kingdom flood (Wales)9

Developments like pavement and roads increase make the surface of the ground less permeable, thus reducing percolation rates and increasing runoff and rate of stream flow, which leads to floods

Flash flooding caused by heavy precipitation in June and July 2007 Schools and roads closed Many people evacuated from their houses

Flooding in the town of Corwen10

3. Describe the impact of agriculture and industry on a specific flood event.

Example: New Orleans (Hurricane Katrina)11

New Orleans is already vulnerable because of its low level altitude In New Orleans, as the lower layers of sediment get melted below the crust, new

silt is created and lithifies at the top Levees and canals made to accommodate agriculture have diverted this flow of

sediment, decreasing the amount of lithification and making the land sink (called depressurization)

Pumping up oil causes the pressure underneath the ground to decrease and the land to sink even more (Louisiana produces a third of the nation's oil and a quarter of its natural gas)

The wetland is also being destroyed from levees and oil extraction, which makes the city more exposed to open water

These factors led to the massive destruction caused by Hurricane Katrina (1,836 deaths)

9http://www.conwy.gov.uk/E_MINUTES/e_post2002/e_regulatory/e_planning/e_reports/TAN15_development%20and%20floodrisk.pdfhttp://news.bbc.co.uk/1/hi/wales/6236728.stm10 http://news.bbc.co.uk/2/hi/uk_news/wales/6236728.stm11 http://environmentalchemistry.com/yogi/environmental/200606katrina.html

Flooded New Orleans12

4. Give an example of a location that has recurring flood events due to allegation of the land by human beings.

Yangtze River in China:13

Flooding has become more frequent due to human influence Destruction of vegetation has increased soil erosion (area exposed to erosion

doubled in the past 30 years) This has increased siltation, leading to a decrease in flood storage capacity and

more likeliness of flooding Levees have restricted flood discharge capacity (e.g. Great Jinjiang levee) Sand mining has increased flooding because water seeps into mine shafts and

makes them flood (sand mining along the river has been made illegal on the Yangtze)14

12 http://bbsnews.net/bbsn_photos/topics/hurricane_katrina/uscg_new_orleans_under_water20050829.jpg13 http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V93-448Y47S-5&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=dbceef1e9b8635ff64f07a9e515f724614 english.peopledaily.com.cn/english/200012/13/eng20001213_57716.html

Map of Yangtze River in China15

Yangtze watershed16

15 http://cache.eb.com/eb/image?id=64287&rendTypeId=416 http://content.answers.com/main/content/wp/en-commons/thumb/6/67/650px-Map_of_the_Yangtze_River.gif

People in flooding of the Yangtze17 5. Discuss in regard to specific examples the difference between the manner in which flooding is addressed in both LEDCs and MEDCs

LEDC Little forewarning and flood preparation many casualties and homes destroyed Floods can be beneficial for agriculture by fertilizing crops; this dependence can

be dangerous when the flood bigger than expected (e.g. Ethiopia)18

In densely populated urban areas, there is often not enough room or capital to invest in constructing detention reservoirs or extensive infiltration measures to mitigate the effects of floods19

Flood response (e.g. 2007 South Asian flood): international organizations and foreign countries send supplies (especially food, temporary shelters)

Water-borne diseases are a serious threat from flooding, so international groups supply medical help and clean water

2006 Floods in Ethiopia20

17 http://www.fao.org/docrep/008/ae929e/ae929e01.jpg18 http://www.waterencyclopedia.com/Da-En/Developing-Countries-Issues-in.html19 http://journals.witpress.com/pages/paperinfo.asp?PaperID=275&jID=17&vn=2&in=220 http://www.unsystem.org/SCN/archives/nics10/ethiopia1.jpg

MEDC More money spent on flood prevention and drainage basin management E.g. the EU-funded Interreg Rhine Meuse Activities (IRMA) program21

o For the countries in the catchment area of the Rhine and Meuse (Belgium, France, Germany, Luxembourg and the Netherlands)

o Restores the natural course of tributarieso Indirect discharge of rainwater from residential and industrial areaso Creates retention and overflow areas within the upper catchment areao Development of models and tools for spatial planningo Promotes and implements flood monitoring, flood warning and protection

systems / models Flood response time is short due to better technology and equipment E.g. BC has purchased a $30,000 satellite system22

o It has reduced the time it takes the River Forecast Centre to receive new data by up to six hours

Catchment area of the Rhine and the Meuse23

21 http://www.riob.org/ag2000/IRMA.htm22 http://www.canada.com/victoriatimescolonist/news/story.html?id=f6b36b83-90e1-4b47-aae4-fd5f259b10a623 www.riob.org/ag2000/IRMA.gif