Floods

Mississippi River during the Great Flood of 1993

1. Riverine

Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction.

Fast kinds: include flash floods resulting from convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier.

Types of Floods

2. Estuarine

Commonly caused by a combination of sea tidal surges caused by storm-force winds. A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

3. Coastal

Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

4. Catastrophic

Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption)

Types of Floods

Katrina flooding

Dec 26 2004 Tsunami

Picture of Banda Aceh before and after 2004 Tsunami

Table 24.1: Flood probabilities for any single year

“X”-year flood Probability of occurrence in a year

5 20%

10 10%

25 4%

100 1%

500 0.2%

151 flash floods over 5-year period in 1970s

Primary effects

•  Physical damage - Can range anywhere from bridges, cars, buildings, sewer systems, roadways, canals and any other type of structure.

•  Casualties - People and livestock die due to drowning. It can also lead to epidemics and waterborne diseases.

Effects of floods

Secondary effects

• Water supplies - Contamination of water. Clean drinking water becomes scarce.

• Diseases - Unhygienic conditions. Spread of water-borne diseases.

• Crops and food supplies - Shortage of food crops can be caused due to loss of entire harvest. However, lowlands near rivers depend upon river silt deposited by floods in order to add nutrients to the local soil.

• Trees - Non-tolerant species can die from suffocation.

Effects of floods

Tertiary/long-term effects

• Economic - Economic hardship, due to: temporary decline in tourism, rebuilding costs, food shortage leading to price increase etc.

Effects of floods

Figure 24A

Figure 24B

Levee break during Great Flood of 1993

Region of the U.S. affected by flooding form tropical cyclones

Figure 24.3

Hurricane Georges

Hurricane Fran

Figure 24D

Flooding from frontal squall lines

The Great Flood of 1993

Mesoscale Convective Systems

Mesoscale Convective Systems

Great Flood of 1993

Figure 24F

850 mb

Mean flow for 5 June -19 July, 1993

Departure from 1979 – 1988 average

Figure 24H

250 mb

Mean flow for 5 June -19 July, 1993

Departure from 1979 – 1988 average

Flooding from frontal overrunning

Flooding compounded by

snowmelt

Red Lake River, North Dakota, April 1997

Figure 24.9

Figure 24.10

Figure 24.11

Figure 24.12

Figure 24.13

Figure 24.14

Figure 24.15