Causes of river flooding: rainfall, relief, vegetation, geology

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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.

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.

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.

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.]

Sometimes snowmelt can cause flooding. When air temperatures stay below freezing in the spring, a large amount of snowpack can remain intact well into the late spring. Once the snowpack has accumulated enough energy, melting can occur rapidly. This is known as freshet flooding. Generally, snow melts rapidly in the prairies, so freshet flooding is minor and occurs on small streams. In the mountains, snowmelt is very gradual; it takes several months for the snow to melt. Due to the slow rate of melting, mountain snowmelt usually is not a primary cause of flooding

1. A warmer atmosphere holds more moisture

According to the Intergovernmental Panel on Climate Change ( IPCC), the atmosphere is about 0.75 degrees warmer than it was at the start of the century, which means it can hold 5-6 per cent more moisture.

That doesn’t automatically mean more heavy rainfall for the UK because complex weather patterns govern the amount, timing and distribution of rainfall. But it does mean that with more water in the atmosphere, the volume of rainfall may increasewhen it does pour.

This isn’t the end of it though, because climate change may affect atmospheric circulations like the Jet Stream, which control the movement of weather systems over the UK. Research on this is still fairly new, however, and the link remains less than certain for now. But it’s another reason why the climate change link to flooding is complicated.

2. Evidence of heavier rainfall in the past is limited, but growing

Extreme events are rare, and detecting trends outside natural variability requires decades of continuous observations. So even though there’s a theoretical link between climate change and rainfall, it’s hard to find clear evidence yet because the record of measurements is short and doesn’t cover all parts of the world.

Globally, that makes it hard to distinguish any trend in the intensity or frequency of flooding due to climate change, as the IPCC concluded in its Special Report on Extreme Events. But in Europe, where more data on rainfall exists, there’s some evidence for a trend towards heavier rainfall.

A recent study finds greenhouse gas emissions contributed to observations of more intense precipitation over two thirds of the northern hemisphere between 1950-2000. Finding trends like this is difficult with a small data set on rainfall, but should become clearer as more extreme events occur over time.

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.