Flooding

The number of floods has increased with the rise in sea levels and the increase in global temperatures. It is critical to explore the causes of floods, so we can discover why they happen, how best to mitigate the risk in potentially affected areas, and limit the impact on the environment and the local population.

Flooding Definition

Main Causes of Flooding

There are physical causes of flooding and human causes of flooding. Let's take a look at the difference!

Meteorological (or physical) causes of flooding

Meteorological causes of floods are physical, but they are to do with the weather.

• Prolonged rainfall: one of the most common causes of floods. Prolonged rainfall saturates the ground and the soil cannot store any more water which leads to increased surface runoff. Rainfall also will enter the river faster compared to when the ground isn't saturated and this means higher discharge levels and floods.

• Storms and flash flooding occur due to intense torrential storms. These flash floods are often associated in the UK with extreme rainfall events in the summer months. The intense amount of rainfall exceeds the capacity of rivers which leads to floods.

• Monsoon rainfall is often heavy rain that occurs in May to September across South and Southeast Asia.

• Snowmelt is when snow melts resulting in the water infiltrating into the soil or ground surface, leading to floods.

Fig. 1 - Prolonged periods of rainfall can cause flooding

Physical factors that increase flood risk

There are also physical factors on earth that increase the risk of flooding such as:

• Permeability of soil and rock in a river drainage basin. When the river drainage basin is impermeable, the precipitation won't infiltrate so the water runs straight into the river, increasing discharge.

• Less vegetation cover of the river drainage basin. If the drainage basin has vegetation cover, the vegetation would intercept precipitation and store it. It can also bind soil, stopping mass wasting which can lead to soil being washed into the river. This could affect the capacity of the river as sediments collect.

• The size and shape of the river drainage basin. This can determine how much precipitation the river can receive and how quickly it will enter the river. The larger the river drainage basin, the larger the catchment area is so there is more water collected meaning increasing discharge.

• The number of tributaries flowing into the river: The more tributaries flowing into the river the more discharge is likely to flow into the river. It is also likely to have flooding at the confluences where the tributaries meet the river.

Human causes of flooding

Human activity can also cause flooding. The way we interact with the earth and a failure to predict the consequences of our actions has caused an increase in flooding.

Urbanisation

Urbanisation leads to an increase of impermeable surfaces such as concrete, tarmac and tiles which can stop infiltration and cause surface runoff of rainfall. Some of the surfaces which have been put down may have had vegetation, and the removal of vegetation can also add to the cause of surface runoff as vegetation can help with infiltration and interception.

Floodplain drainage

Floodplains are seen as flat and fertile land desirable for agriculture yet prone to flooding. In many countries, floodplains are drained to create agriculture possible but in the process destroy wetland habitats. This process also makes the land lower and shrink which leads it to be prone to floods.

Agriculture

Land needed for agriculture has led to deforestation in places such as the Amazon. Agricultural practices increase the amount of soil exposed and the surface runoff which creates soil erosion. The sediment from the erosion is transported to the river which would then collect to reduce the capacity of the river.

Case study – flooding in Cumbria

The northwest of the UK is prone to heavy rain. This is due to the warm westerly winds, with uplands such as the Cumbria fells creating orographic rainfall. Cumbria is seen as the wettest place in England and flooding has happened three times (2005, 2009 and 2015) in a decade. In December 2015, Storm Desmond hit Cumbria. This storm was caused by a deep Atlantic low-pressure system leading to associated fronts stretched across northern Britain, bringing prolonged and heavy rainfall through a mechanism known as a 'warm conveyor belt'.

Fig. 2 - The path of Storm Desmond, 2015

Cause of flooding

Storm Desmond caused severe flooding of the River Eden. The flooding was worsened by the already saturated ground conditions because of wet weather in November. One of the theories for the long periods of wet weather in 2005, 2009 and 2015 is the position of the jet stream. Jet streams are narrow bands of strong wind in the upper levels of the atmosphere. This jet stream was moving north and south, but it remained over the northwest longer than usual, bringing rain-laden depressions from the Atlantic.

Exacerbating flood risk

How we manage and use land can contribute to causing floods. For example, farming has affected the Lake District, in particular the overgrazing of sheep that has created bare slopes instead of forests¹. Previously, trees absorbed and slowly released water, meandering channels slowed the flow, and bogs held water. However land use changes mean that soil is dried out, trees removed, and channels are straightened and dredged. This creates faster runoff, reduced stream lag times, and higher discharge peaks. Rainwater reaches floodplains faster and the risk of flooding is increased.

Mismanagement of rivers

Before 2015 there had been efforts to stop flooding by raising riverbanks and adding pumping stations and diversion channels to help carry excess water from the built-up areas of Cumbria. These hard-engineering schemes are based on flood-return periods which are calculated statistically on past floods to estimate how often a flood of a certain magnitude would occur. However, the flood in 2015 was unexpected and overwhelmed the flood protection schemes.

The long-term impact of floods

After the floods, there were considerable economic disruptions with Carlisle's largest employer having to close the United Biscuits factory for weeks and returning with a smaller workforce². Investors, residents and businesses had to re-evaluate their long term plans reflecting on the social, economic and environmental costs of the floods.

• Social costs – the cost of residents having their homes flooded and needing to live in temporary accommodation. Many local services such as schools, shops, and healthcare were forced to close temporarily. These factors caused stress, anxiety and psychological trauma.

• Economic costs – transport and infrastructure was damaged and many businesses had to close. House prices fell in the areas of flood. Farmers lost livestock as they drowned.

• Environmental costs – the debris blocked the rivers which were contaminated with sewage and pollutants. Habitats and ecosystems were destroyed by the flood. The saturated ground led to landslides and the eroded river banks posed risks of future flood risks.

The Carlisle civic centre amid flood water, Rose and Trev Clough, Wikimedia, CC BY-SA 2.0

Mitigating flood risk

Because of this, the Environment Agency is now focusing on soft-engineering solutions instead of the expensive hard-engineering schemes that cannot deal with the most extreme flooding. This includes:

• restoration of floodplains back to absorbent states so floodwater can be stored.

• restoration of river channels to a natural meandering state.

• reafforestation of upland areas to reduce rapid surface runoff.