Flood Case Study

Jacqueline Caverly

Flash Flood vs. Lag Response Flood

Flash Flood• The time between the storm

event and the flood is short. • This means that lag time is

short• Usually occurs over dry soil

with little vegetation that water does not penetrate easily

• Excess of water flows quickly, floods then leaves in a short period of time

Lag Response• The length of time between the

storm event and banks full is longer

• The hydrograph slope is less steep

• Lasts longer• Occurs when there is lots of

vegetation to trap water and slow its movement, and in soil that is easily penetrable.

• If pooling occurs before seepage then this is more likely to occur

Case Study: Lag Response Flood

• River Thames, July 2007

• Onset of storm event occurred a month before major flooding

• Tewkesbury, Gloucester, Oxford, Sheffield, Hull and Yorkshire and other towns in southern England flooded

• Lost electricity and freshwater

• 10,000 homes flooded or at risk of flooding

http://www.bargeladycruises.com/filebin/images/maps/EnglandmapLarge.jpg

Location of River Thames (England)

Scene from 2007 flood in Oxford retrieved from http://news.bbc.co.uk/2/hi/uk_news/england/oxfordshire/7192234.stm

Case Study: Flash Flood

• Sudan: Nile River , July 2007• Over 300 people injured• 46,000 homes destroyed• 200,000 people displaced• Ibrahim Adam Yusuf told reporters that it rained

for 5 hours and then he was waist high in water• Thriving agricultural businesses are destroyed• Houses collapsed

Location of Nile River

Homes are flooded and destroyed when the Nile floods

London Case study: Human Impacts

• London escaped flooding in 2007 due to cement dykes built around river– “normal” flooding was hindered by man made dykes

• It was stated that builders did not consider the flood plain when building roads and paving areas which contributed to the flood. – Covering flood large areas with cement increase the volume of

water flowing into streams at one time as infiltration is decreased (less percolation)

– Base flow of water after storm events increases (lag time decreases)

• London’s weight causes it to be sinking on its clay base – increase flood risk as elevation decreases

London is a big city with lots of pavement that intercepts water to reduce infiltration

Nile River Case Study: Human Impacts

• Begins in Burundi- south of the equator. Flows north through Egypt.• Drains into the Mediterranean Sea • 19th century – Dams built across nile to increase water level for

irrigation• Seasonal flooding no longer occurs since the construction of the

Aswan High Dam• At present the dams control flooding

– Less flooding = increased erosion of surrounding area and increased salt content of water and soil in the delta

• Reduce sedimentation and fertility of area– Yearly flooding used to return valuable nutrients to soil– Now large stretches of river above dam become clogged with silt

• Dams caused areas of Egypt and Sudan to flood and become submerged– Clear change in course of original channel/stream flow

Satellite photo of Aswan high dam http://rst.gsfc.nasa.gov/Sect6/Aswan_High_Dam.jpg

Photo of Aswan high dam from Google Earth

Location of Aswan High dam http://www.mead.k12.wa.us/SHILOH/Griffith/MrGsWebPage2/Ancient%20Egypt/ancient_egypt_images/map_aswan_egypt.jpg

China Case study: Human impacts

• 90% of cities built on floodplains– Lots of cement in cities which will decrease infiltration of water and increase

surface runoff• Artificial dams and levees built ex. Along Huang he river and Huai he River

– These rivers now kept in narrow channel, no meandering• Water is used for irrigation

– Amount of water in stream decreases• Infrastructure alters landscape

– Channelization of streams will change• Sewage discharge contaminates water

– Water ecosystems are destroyed– Chemical makeup of stream changes

• Tourism destroys natural habitatsA vicious cycle in China

Here is the location of the Huang He River (China)

Reoccurring Floods Due to Humans

• Chao Phraya basin in Bangkok (Thailand)• Naturally has seasonal flood, but it is intensified

by development.• Urbanization- confines flood plains• Dykes built to reduce flooding, but when flood

does occur the intensity is greater• Roads built over monsoon escape routes• Mining underground for water from aquifers is

causing Bangkok to sink which attracts water draining with gravity

Detailed view of Chao Phraya river basin and its eight sub basins  

Where in the world?

Flood Response: LEDC vs. MEDC

MEDC – London• Methods of warning

– Television teletext– Messaging using home telephone

lines (Floodline)– Mobile telephone text messaging– Mobile telephone voice mail– E-mail, Digital television and radio

broadcast– Real-time internet flood data, including

live – in-home alert systems

• Also there are special considerations for elderly, handicapped, deaf, etc.

• River/ Rain Gauges work electronically • In some instances, floods can be

predicted 36 hours in advance• People buy houses with flood

insurance that in many cases they don’t have to pay for

• LEDC – Sudan (nile River)• People suffer from diseases spread

by floods (Acude Watery Diarrhoea and Mlalaria)

• Livilhoods of farmers destroyed with land and livestock

• Houses collapse (instead of simply flooding in London)

• Extremely vulnerable to natural disasters- they occur often and are a drain on resources

• Received money from aid agencies

Location of Sudan (Africa)

Location of Nile River (Africa)Telephone Flood communication service in London

References• "Building on sand; Britain under water.(The merits of private flood insurance)."  The Economist (US).

384. 8539 (July 28, 2007): 14US. Student Resource Center - Gold. Gale. Port Moody Secondary. 18 Nov. 2008 <http://find.galegroup.com/srcx/infomark.do?&contentSet=IAC-Documents&type=retrieve&tabID=T003&prodId=SRC-1&docId=A166826907&source=gale&srcprod=SRCG&userGroupName=43pmss&version=1.0>. <http://find.galegroup.com/srcx/infomark.do?&contentSet=IAC-Documents&type=retrieve&tabID=T003&prodId=SRC-1&docId=A152639856&source=gale&srcprod=SRCG&userGroupName=43pmss&version=1.0>.

<http://find.galegroup.com/srcx/infomark.do?&contentSet=IAC-Documents&type=retrieve&tabID=T003&prodId=SRC-1&docId=A152639804&source=gale&srcprod=SRCG&userGroupName=43pmss&version=1.0>.

• Codrington, Stephen. (2007) Planet Geography. Stephen Codrington: Hong Kong• Encarta(2003) The Nile River. Retrieved on November 21, 2003 from:

http://www.multimedia-publishing.com/nile_river.htm • "England flood peaks due in days."  UPI NewsTrack. (July 24, 2007): NA. Student Resource Center -

Gold. Gale. Port Moody Secondary. 18 Nov. 2008 <http://find.galegroup.com/srcx/infomark.do?&contentSet=IAC-Documents&type=retrieve&tabID=T004&prodId=SRC-1&docId=A166743335&source=gale&srcprod=SRCG&userGroupName=43pmss&version=1.0>.

• International Federation of Red Cross and Red Crescent Societies (IFRC)- Switzerland (2007) 200,000 displaced, scores killed by floods in Sudan. Retrieved November 21, 2008 from: http://www.alertnet.org/thenews/fromthefield/218536/118546748384.htm

• ThinkQuest (1998)The Nile River. Retrieved on November 21, 2008 from: http://library.thinkquest.org/16645/the_land/nile_river.shtml

• United Nations Educational Scientific and Cultural Organization. Flooding. Retrieved on November 21, 2008 from: http://www.unesco.org/water/wwap/case_studies/chao_phraya/index.shtml

• Zhai, Jinliang & Deng, Wei. (2000) Flooding, floodplain development and management in China. Retrieved on November 21, 2008 from: http://www.biotechnology.uni-koeln.de/inco2-dev/common/contribs/15_zhai.pdf