Managing flood risk

Flooding occurs naturally but human activity has significantly altered natural drainage processes, often causing greater flood risk.

Urbanisation has reduced the ability of land to absorb rainfall through the introduction of hard, impermeable surfaces. This results in an increase in the volume and rate of surface run-off as less water infiltrates into the ground.

In the past, development has taken place on areas of land at relatively high flood risk, where often building has pressed tightly up to the river bank. These factors make urban areas more prone to river flooding and there is potential for run-off from new development to exacerbate river flooding downstream.

Some natural watercourses – particularly in urban areas – are now confined to narrow channels, often inadequately maintained. In the past culverting of watercourses in built-up areas was quite common, and was seen as a way to release more land for development. Culverting increases flood risk significantly because piped sections restrict flow, pose a serious risk of blockage and restriction and eliminate natural flood routes.

Flooding can be arranged into four broad areas:

• Coastal flooding and erosion
• River (fluvial) flooding
• Surface water (pluvial) flooding
• Groundwater flooding.

There is also potential risk of flooding from failure of infrastructure, including reservoirs and sewer systems.

Tidal levels are a crucial factor especially when combined with other weather conditions, including low pressure and high winds. These conditions can lead to coastal defences being breached or overtopped, resulting in flooding.

During heavy or prolonged rainfall events, rivers can experience large flows which cause them to exceed their capacity. In the natural environment open spaces near the river act as storage areas or ‘flood plains' for out-of-channel flow, alleviating downstream flood risk.

Urbanisation can severely affect this natural process. Faster run-off rates from upstream urban areas can result in an increase in flow in the rivers downstream, and building on the flood plain can significantly increase the risk of flooding both directly to the development concerned and on a wider basis by removing capacity from the flood plain. 

Localised heavy rainfall on impermeable or already saturated surfaces can generate surface water run-off beyond the capacity of the drainage network. Surface water cannot then enter the sewerage system and the drainage network overflows, resulting in overland flow. The majority of flooding experienced in the summer of 2007 was caused by surface water flooding.

In some parts of the UK, water levels in the ground can rise above the surface after prolonged periods of heavy rainfall (not to be confused with the failure of rainwater to infiltrate, especially in clayey soils). This is most likely to occur in areas underlain by permeable rocks, called aquifers. These can be extensive regional aquifers, such as chalk or sandstone, or may be more local sand or river gravels in valley bottoms underlain by less permeable rocks.

Groundwater flooding can be prolonged, lasting up to months in some cases. Groundwater flooding by its nature can also be very difficult to address as substantial works may be necessary to conduct the flows away to a natural outfall by gravity. However, it is worth noting that many aquifers are valuable resources for water abstraction and as such are protected/works restricted.

Climate change poses a major challenge in our management of flood risk. Increases in global temperatures and changing weather patterns indicate that climate change will cause more extreme weather events.

The impacts of climate change are likely to:

• increase the severity and frequency of storm events
• increase the frequency of intensive rainfall events that may cause significant run-off from urban and agricultural areas, exceeding the capacity of artificial drainage systems and natural watercourses reaching their capacity
• increase the saturation of groundwater from large rainfall events that may cause large overland flows and watercourses to overflow
• contribute to rising sea levels that may increase the risk of coastal flooding.