Water Cycle Pack G Flashcards
What is a flood?
When a sufficient quantity of discharge has caused a body of water to overflow its channel and submerge the surrounding land
How does prolonged rainfall cause excessive runoff?
- Usually in autumn or winter but jet stream can make it happen at other times
- The ground becomes saturated
- Speeds up overland flow and shortens lag time
- There is a rapid rise in water level, which exceeds bankfull discharge
- Prolonged rainfall in the UK is associated with low pressure systems or depressions
What is an example of prolonged precipitation?
- Storm Ciara and Dennis in the UK 2020
- 237% more rainfall than average for the month in February (total of 209mm) and it was very mild
- Rainfall was 2x the norm, 3x in Lancashire and West Yorkshire, 4x locally
- Lasted for 9 days
- 100mm of rainfall in the western UK and 150-250mm in higher land like Dartmoor and Wales
How do intense storms cause excess runoff?
- Intensity of the rain exceeds capacity of the river to cope with the amount of water
What is an example of an intense storm?
- Boscastle, Cornwall
- 2 sea breezes converged over Boscastle
- Air was forced up 13,000m where it cooled and condensed
- 6 hours of torrential rain as storm didn’t move on
- 130mm of rain
- Steep sided valley meant water travelled straight over land
- 2 million tonnes of water came down the river
- 1 in 400 year event
How does monsoon rain cause excess runoff?
- Wet, seasonal prevailing wind wind found over S and SE Asia
- Intense seasonal monsoon rain between July and August
- 70% of the usual average rain may fall in 2 months
What in an example of monsoon rain?
- Low-lying plains of India and Bangladesh frequently flood
- Dehli saw over 153mm rain on 9 July 2023
- This was the highest precipitation in a single day in July in over 40 years
How does snowmelt cause excess runoff?
- Snow melt which cannot infiltrate the soil or ground leads to flooding
- Snow and ice are responsible for many flood events in high latitudes or mountainous areas
- Melting snow usually happens in late spring when there is a quick transition from winter to spring
- Flood water can be held in ice dams
What are examples of snowmelt?
- GLOFs occur in the Himalayas
- Yosemite, California
- 2x to 3x more snow than usual due to higher temperatures
- Merced River had high discharge causing excess runoff downstream
How does land use change (urbanisation) cause excess runoff?
- Creation of impermeable surfaces (e.g. roofs, pavements, roads and car parking)
- Speeds up drainage of water into rivers from sewers and drains
- Bridge supports may impede river flow
What are examples of urbanisation?
- Houston, Texas
- Mumbai, India
- Population density, impermeable surfaces, deforestation of mangroves, coastal and artificial land, not enough drains, plastic and silt clogging drains, sea levels rising causing backups of water, siltation of rivers
How does land use change (deforestation) cause excess runoff?
- Less interception
- Less absorption
- More surface runoff
What is an example of deforestation?
- Freetown, Sierra Leone
- Deforestation happened as a result of urban sprawl, illegal logging and quarrying
- Freetown the tree town was the solution
- Aim is plant 1 million trees in which locals are paid for planting trees and the most trees are planted in areas of highest risk
How do cyclones/typhoons cause excess runoff?
- System of winds rotating counterclockwise in the N hemisphere around a low pressure centre
- Swirling air rises and cools
- Creates clouds and precipitation
What is an example of a cyclone?
- Cyclone Nargis in Myanmar
- Low wind shear and low pressure area of the Bay of Bengal
- 600mm of rain and a large storm surge
How can El Nino and La Nina cause excess runoff?
El Nino:
- Reversal of normal patterns of wind over the tropical Pacific Ocean and the associated changes in oceanic currents
La Nina:
- Intensification of the normal patterns of wind over the tropical Pacific Ocean and associated changes in oceanic currents
What an example of El Nino and La Nina?
La Nina:
- 2008 event had significant effects on hurricanes and cycles
- The Atlantic hurricane season was one of the five most active since 1944
- 2010 event brought record high ocean temperatures in the NE of the Indian Ocean
- There were exceptionally heavy monsoon rains in Pakistan with 2000 deaths and 20 million people affected by damage to property/infrastructure
- £1.5billion worth of property damage due to flooding in Queensland, Australia
- Historic blizzards in eastern USA where 30-80cm of snow fell in just over 24h from N Florida to S Canada
What is an example of river basin mismanagement?
- Somerset Levels
- Land was drained in the past for farming
- It shrank and lowered so rivers ended up higher than the land
- Pumps now pump water up into the rivers but pumps cannot be turned on in extreme weather
- There is excess water on the land
- Grasslands have been replaced with cereal crops
- After harvests fields are left bare, so there is higher/earlier peak discharge
- Silt enters the rivers as rain makes soil erode
- Solutions by the Flood Action Group include mud banks along roads, dredging, embankments, retention pons, trees, silt traps and planting different crops on the same fields
What are the multiple causes of Bangladesh having excess runoff?
- Tropical climate
- Monsoons
- Flat, low-lying land
- Crossed by major rivers which creates a high drainage density
- Densely population (4x that of the UK) so there are more impermeable surfaces
- Rising sea levels (30cm to 50cm by 2050)
- Meltwater from the Himlayas
- More intense rainfall
- Climate changes increasing temperatures
- Cyclones
