Flood Hazard: This case study should address the following aspect of the syllabus

Alexia Jablonski

November 21, 2008

Flood 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)

High vegetation cover = high infiltration

Soil types (desert soil much more prone to flash floods because they absorb less moisture)

Frozen ground = less infiltration

Human constructions such as pavement decreases rate of infiltration

Example: 2007 Sudan floods

First occurred on July 7, 2007

Desert area Caused by heavy rain

According to UN reports:

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 m)

Sudanese affected by 2007 flood

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 Floods Caused by monsoons (torrential rains linked with wind patterns) in India, Nepal, Bhutan, and Bangladesh lasting 20 days

UNICEF: situation "is being described as the worst flooding in living memory"

About 30 million people affected and displaced

Bangladesh: 2/3 of the country left underwater; huge outbreak of flood-related diseases such as dysentery; many areas made isolated

India: more than a thousand people killed; millions of dollars in crops lost

Nepal: 84 people killed

Bhutan: several major roads disrupted

Areas affected by flooding in South Asia

Family marooned by the flood in South Asia

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, 2007

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 Sibu

Example 2: 2007 United Kingdom flood (Wales)

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 Corwen

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

Example: New Orleans (Hurricane Katrina)

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 (1,836deaths)

Flooded New Orleans

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:

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)

Map of Yangtze River in China

Yangtze watershed

People in flooding of the Yangtze

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)

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

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 Ethiopia

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

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

Restores the natural course of tributaries Indirect discharge of rainwater from residential and industrial areas Creates retention and overflow areas within the upper catchment area Development of models and tools for spatial planning 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 system

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 Meuse

Homework Help

https://www.homeworkping.com/

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www.riob.org/ag2000/IRMA.gif