Meteorlogical Causes Of Flooding Flashcards
What are the main causes of flooding?
Intense rainfall over a short period of time (flash flood)
Prolonged rainfall/ monsoons
Tropical storms
Snowmelt
What does most of the flooding seen in the uk relate to?
Mid-latitude depressions that brings 2 fronts
What 2 types of front does the uk experience?
Showers and rain occur with the passing of a warm front
Heavier rain falls with the cold front
What are isobars?
Lines joining points of equal pressure similar to contours which are shown on weather charts
How is pressure measured?
In Millibars
How many Millibars is an isobar?
4
What does a lower pressure mean on a weather chart?
A greater depression
How can depressions be recognised?
An area of closely spaced isobars often in a roughly circular shape where pressure is lower than the surrounding area
Where is the lowest pressure in a depression?
The middle
What is often associated with depressions?
Strong winds and heavy rain
What way are depressions blown in the northern hemisphere?
Anti-clockwise
How can a cold front be identified?
Bold lines with triangles
What does a cold front indicate?
A change in air mass where warmer air is being replaced by colder air
What are the characteristics of a cold front?
Often bring short spells of heavy rainfall in the form of showers and winds
Accompanied by a decrease in temperature
Veer in wind direction
How can warm front be identified?
Bold lines with semi-circles/ humps
What does a warm front indicate?
Cold air being replaced to a warmer air mass
What are the characteristics of a warm front?
Often bring prolonged sometimes heavy rainfall with strong winds
What happened in July 2007 in the UK?
A major flood event
What were the irregular weather conditions in 2007?
Jet stream followed an abnormal southern direction
What is the role of the jet stream?
Important for control factor for low pressure weather systems in the northern Atlantic
What did a change in the jet stream mean?
Usual anticyclonic/ high pressure weather system from Azores failed
What is the Azores?
Region of Portugal
What happened in the uk due to the shift of the jet stream?
Resulting weather patterns produced exceptional rainfall throughout England and Wales
What were the resultant conditions in the UK during 2007?
Mean temperatures across the UK were generally 1*c warmer
June precipitation was 136mm 190% average form 1961-1990 (wettest June on record)
Rainfall for may to July 2007 highest on record
Why were the soils in 2007 highly saturated?
Early summer rainfall in 2007 meant that soils were already close to saturation and groundwater levels were higher than usual
What are ground water levels like in a normal year in the UK?
Britains usually has lower ground water levels reducing the risk of flooding (acts as extra storage)
When did some intense storms occur during summer 2007?
20th of July
What did the intense storm on the 20th July result in?
A number of localised but severe flash floods
What were some areas that experienced flash floods of 20th July 2007?
Stratford upon Avon
Leamington spa
Tewkesbury
Buckingham
What were the economic effects of the 2007 summer flood?
Cost country £3.2bn
£2bn for homeowners and businesses
What was the average repair cost of a flooded home in 2007?
£23,000- £30,000
What was the problem for homeowners after the 2007 flood?
1/4 of homeowners were not fully covered by insurance
What was the average cost per flooded business after the 2007 flood?
£75,000 - £112,000
How much did the damage to communications and transport cost?
£230m
What was the agricultural loss after the 2007 flood?
Loss of £50m
£1,150 per hectare
What was the cost to the emergency services during/after the 2007 flood?
£27m
What were the social impacts of the 2007 Uk flood?
Education- 400,000 pupil days lost due to school closures
Flooding of water treatment plant in Tewkesbury (worst post WW2 emergency) loss of piped water to 350,000 customers, 138,000 properties for over 2 weeks
What were the demographic impacts of the 2007 UK flood?
13 people were killed
Rural households worse affected- stranded as minor roads not considered as important as major roads in towns and cities
Old and young most affected as they’re least mobile
When did storm Desmond occur?
2015-16
What were the meteorological causes of storm Desmond?
Unusually heavy rain and strong winds due to low pressure from the Atlantic
How much rain did Honister in Cumbria get in 24 hours?
341.4mm
How high did winds get in storm Desmond?
81mph
How many categories are there for depressions?
5 categories
What are the wind speeds for each depression category?
1- 74-95mph
2- 96-110mph
3- 111-129mph
4- 130-156mph
5- 157+mph
How does a tropical depression form?
When low pressure is with thunderstorms that produce circular wind flow with maximum sustained wind flow below 39mph
What is the typical maximum sustained wind speed for tropical depressions?
25-35mph
When was the flood in Bangladesh?
1998
What were some of the causes of flooding in Bangladesh?
Monsoon climate
Spring snowmelt
Deforestation in headwater area
River silt-up
Lying on a floodplain
Diversion for irrigation
Regularly hit by cyclones
How did a monsoon climate cause flooding in Bangladesh?
Brings very heavy rain and snow
Soils are leached and heavy runoff results in soil erosion
How does spring snowmelt cause flooding in Bangladesh?
Results in soil erosion and a rapid increase in river discharge
How does deforestation in headwater area cause flooding in Bangladesh?
Trees are cleared for fuel and grazing land for growing population
Less evapotranspiration
More runoff
Faster soil erosion
How does river silt up cause flooding in Bangladesh?
Raises river bed and reduces capacity of the channel
What % of Bangladesh is built on a floodplain?
80% most of which is only 1m above sea level
How does irrigation cause flooding in Bangladesh?
Removes som of the silt and prevent s the floodplain further downstream from being built up
