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Flood Factors

Factors affecting River Flooding
Rivers flood for a variety of different reasons, and very few rivers have the same background characteristics. The reasons why rivers flood can be divided into HUMAN and PHYSICAL (or Natural) characteristics.
The physical reasons for flooding

Queensland flooding
River flooding by By Rob and Stephanie Levy from Townsville, Australia (Queensland floods) via Wikimedia Commons

 

  1. Precipitation type, amount and duration are the most obvious reasons for river flooding. Long steady prolonged rainfall will produce rivers which rise slowly but can flood, whilst heavy short showers can cause rivers to rise quickly and burst their banks. Snowfall is another factor to take into account, river levels fall in the UK as precipitation is often stored as snow during cold snaps. However, when temperature warms and that snow melts many days’ worth of precipitation can end up in rivers and cause flooding.
  2. The RELIEF of the land can also have an impact. Steep slopes tend to reduce the amount of infiltration of water into the ground, this water can then flow quickly down to rivers as overland flow. In addition, steep slopes also cause more through flow within the soil. Both can raise river levels. Gentle slopes or flat land allow water to penetrate into the soil and increase lag times.
  3. Vegetation type and coverage plays a big role, with forests intercepting more rainfall than grasses. This interception increases lag time and reduces the risk of a flood. Indeed, deforestation (the removal of trees) can increase soil erosion, reduce interception and increase flood risk. Afforestation, where trees are planted, can have the opposite effect.
  4. GEOLOGY - Soil and rock type can also influence what happens to precipitation when it reaches the ground. Impermeable soils and rocks such as clay or shale do not allow water to infiltrate, this forces water to run off reducing river lag times and increasing flood risk. Permeable rocks allow water to infiltrate into them. If permeable rocks allow water in through cracks, fissures and bedding planes but not through their pores they are said to be pervious (such as limestone). Porous rocks allow water to penetrate into their pores such as sandstone.

Human reasons for river flooding
Humans cause changes in LAND USE which can impact upon river flooding. 
1. Urbanisation can cause flooding because many of the surfaces in towns and cities are Impermeable. The whole urban system is designed to move water from the surface into underground pipes and away from urban areas which have value. This can lead to floods in other regions.
2. Deforestation (the removal of trees) can increase soil erosion, reduce interception and increase flood risk.
3. Increases in population density can also have an impact as it places more people in flood risk areas. It is for this reason that we are building on floodplains and flood risk areas in the UK, this just increases the likelihood of a flood.

Patterns
Historically floods were reasonably unusual events in the UK and mainly occurred during the winter season. This pattern is changing, with major floods becoming more frequent in the UK and many are now occurring in summer. Our case study of a flood in an MEDC, in Morpeth, occurred in early September. If these patterns continue it will cost a lot of money to protect the vulnerable people in flood risk zones.  Winter rainfall is becoming more prolonged and heavier as well. This is shown wonderfully below, using statistics from the Met Office website for 2012;

2012 weather patterns

This resulted in extensive flooding in the UK, even in Newcastle upon Tyne on "Thunder Thursday".  It has been noticed across the UK that flooding is increasing in intensity (how large the floods are), frequency (how often they occur) and are becoming more common in ALL seasons of the year.  This is nicely revealed by the graph from flooding over time in York in Northern England below.
 

York Flooding

 

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