Bellwork

• What factors influence flooding?

• Sort these into physical/human causes

Flooding!!!

Learning Objectives: • Understand the term ‘flood return interval’ • Construct a graph to analyse the flood return frequency and magnitude for Fema Creek, PA

Flood Risk

Risk is affected by: •The frequency of flooding; how often floods can be expected.

•The magnitude of the flood; how severe or large each occurrence will be.

•The number and density of people living in the affected area.

•The amount of flood protection and prediction systems.

•The level of economic development;

this affects an areas ability to afford management strategies and preparedness, and it’s ability to cope in an emergency.

Global Flood Risk

in the next 30 years

Figure 4:

Humanitarian risk hotspots for flooding. The map shows ‘flood risk hotspots’ based on the interaction of extreme and significant flood hazard and high overall human vulnerability. Also shown are areas with significant or extreme flood hazard but lower human vulnerability and areas where climate models predict an increase in extreme precipitation, as an indicator of possible future increases of flood risk. http://www.careclimatechange.org

Flood Risk in the UK

Describe the distribution of flood risk in England

Magnitude-Frequency Flood Risk Analysis

• Floods of a very large size (magnitude) don’t happen very often (they’re not very frequent) • Floods of a small magnitude are more frequent • Large floods usually occur due to unusually heavy or prolonged rainfall There is one exception to this rule! Large floods can still happen in places where there is annual snowmelt or monsoon rains, e.g. Bangladesh Even in areas where this does not happen it is still possible for very large floods to occur more frequently than expected.

• By keeping records over many years hydrologists can predict how often a flood of a certain magnitude may occur • This is called the flood recurrence interval (RI)

Definition…

• Flood return interval…

Magnitude-Frequency Flood Risk Analysis

• • A small flood may have a RI of 2 years A huge flood may have a RI of 100 years • This information can be used to decide what sort of flood protection is needed and where.

Magnitude-Frequency Flood Risk Analysis

TASK

Complete the Flood Frequency worksheet for Fema Creek Extension: Make notes from p. 30 about how floods are managed in the UK and how seasonal discharge can be assessed.

RECURRENCE INTERVALS This can allow planners to zone the flood plain and plan land use. They can also be used to predict the occurrence of low flow.

In order to minimise the damage from floods it is useful to know how often a flood of a certain size might occur. The time between flood

events of the same size is called the recurrence interval.

Work out the discharge for… 1.

1.5 years 2.

3.

5 years 20 years

Post it Plenary

• Suggest how flood recurrence could influence land use planning and building regulations in flood plain areas