# Meteorological Causes of Flooding

## Natural Causes of Flooding

__Meteorological __causes of flooding, including:

• intense storms leading to flash flooding,
• unusually heavy or prolonged rainfall,
• extreme monsoonal rainfall
• snowmelt.

Explain the meteorological causes of flooding.
Your answer should include: Snowmelt / drainage basin / natural disasters / vegetation / lithology.

## Variations in Rainfall

In the UK flooding is:

prolonged and heavy rain

associated with low-pressure systems

occurring in the progressive cycle (during autumn and early winter)

In other areas e.g. south east Asia:

• flooding is associated weather systems (with monsoonal rainfall during July to September), glacial outburst flooding (Jokulhaup),
• storm surges
• fluvial convergence.

The apparent position of the Sun with reference to the Earth oscillates from the

Tropic of Cancer to the Tropic of Capricorn.

• low pressure region created by solar heating also changes latitude.
• The northeast and southeast trade winds converge in this low pressure zone - Intertropical Convergence Zone or ITCZ
• This low pressure regions sees continued rise of moist wind from the sea surface to the upper layers of the atmosphere, where the cooling means the air can no longer hold so much moisture resulting in precipitation.

## Case Studies on Human Actions

Why are human actions making floods worse, especially in LICs?

## Flood Frequency

We can calculate the ‘recurrence interval’ of flood events using this calculation:

Recurrence Interval = (N + 1) / Rank

n= number of years of observation

Rank = rank order (large flood events are ranked each year according to their magnitude in runoff volume, not damage/impacts)

This tells us the number of years within which a flood of this size can be expected e.g. a 1 in 10 year or a 1 in 500 year flood event.

This calculation uses historical data and therefore doesn’t take climate change into account.

The floods of highest magnitude will have the longest return period e.g. 1 in 500 year events.

This doesn’t mean this exact flood will happen every 500 years.

Each year has a 1 in 500 chance of experiencing a flood of this magnitude.

## Case Study of Impact of Flooding in the UK

During April-July 2012 severe flooding occurred across the UK.

We will study the impacts on the north east of England.

Causes:

The yo-yo pattern is the result of two weather systems

1. A warm front from the Atlantic and a colder one drawn from north of Scotland meeting
2. Dumping persistent rain.

Although some areas have experienced gusts of up to 70mph, the reluctance of the dismal weather to move off north and east is also due to a weaker jetstream than usual.

The UK’s exceptionally wet summer in 2012 __and extreme rainfall events in China, Japan, Australia and New Zealand were all found to be __influenced heavily by natural variations of weather and climate.

However, rainfall totals can be affected by increases in the amount of moisture held in the atmosphere directly related to the warming we have seen due to greenhouse gas emissions.

Socio Economic Impacts:

• Over 1000 properties flooded overnight in the NW and Yorkshire

• Residents rescued from their homes by dinghy in Greater Manchester

• Northern Northern Rail services services disrupted disrupted and West Coast mainline services between Glasgow, Edinburgh and Carlisle suspended

• 9 people rescued from cars in the Yorkshire Dales

• 20 people rescued by boat in West Yorkshire

• Boats ferried residents to safety from St Helen’s in Lancashire and Morpeth in Northumberland
• Transport was devastated as parts of the M6 and the East Coast main line became temporary rivers
• Police and local authorities urged people to leave work early in much of the north-east, where many schools in Stockton-on-Tees also closed at lunchtime.
• Drivers were evacuated from cars on the Great North Road in Gateshead when the carriageway became awash.

Environmental Impacts:

• Soil erosion