How can floods be prevented

Flood risk is harder to manage where development, or the right to develop, already exists. Flood risk to existing infrastructure is usually altered through improvements to protection as part of any upgrade. However, for people and property there are basically three ways of managing flood risk to reduce the consequences of flooding: by modifying flood behaviour, property, or community response.

None of these measures is a stand-alone solution for addressing flooding issues. The preferred option is often a combination of flood-, response- and property-modification measures to reduce risk to an acceptable level and to manage this residual risk appropriately.

Flood modification measures aim to reduce flood levels, velocities or flows, or exclude floodwaters from areas under threat for events up to their designed capacity. They are a common and proven means of reducing damage to existing properties under threat from flooding. They tend to be more expensive than response or property modification measures but will often protect a larger number of properties.

Flood mitigation dams can reduce downstream flood levels by temporarily storing and later releasing floodwaters. Most dams are used to supply water to the community, but they can, when purpose built, also provide some flood mitigation for events up to their flood storage capacity. In larger floods, this mitigation capacity can be exceeded and floods pass through with little, if any, reduction.

On the negative side, dams can cause disruption to existing communities, loss of valuable land and negative environmental impacts, and good sites for dams are difficult to locate. Detention basins act like dams but at a much smaller scale and are most suitable for 'green field' developments, where sizing constraints tend not to exist. Levees are generally raised embankments built to eliminate inundation of the areas protected by the levee up to a certain size event.

In larger floods, levees can be overtopped with water flooding into and inundating areas protected in the smaller events. Levees can trap local stormwater, causing damage unless flood gates and pumps are provided.

However, levees, whether temporary or permanent, can increase flood levels in areas not protected by the levee as noted in Question 2. Waterway or floodplain modifications such as widening, deepening, realigning or cleaning rivers and flowpaths can improve the transport of floodwaters downstream and reduce the likelihood of blockage, but can increase velocities and erosion and cause negative environmental impacts.

The benefits of cleaning and clearing are only temporary unless these continue to be maintained. Other structures such as roads, railways and embankments also have an impact on flood risk management because they can alter flood flows and behaviour.

Floodgates can also be used to prevent backflow from river systems into drains. In addition to the zoning and development controls for new and re-developments mentioned above, modifications to existing property are also essential if the growth in future flood damage is to be contained. Land filling involves building up low-lying areas and can improve the flood immunity of structures constructed on that land, but can adversely affect flood behaviour elsewhere and therefore is generally limited to the fringes of the floodplain.

Flood proofing involves the sealing of entrances, windows, vents, etc. Generally it is only suitable for brick commercial buildings with concrete floors and it can prevent ingress for outside water depths up to approximately one metre. Ideally, new developments would use flood resilient designs and materials, as addressed in Question 8. Flood planners 'must prepare for worst'. Farmers urged to do more over floods. Flooding 'devastating' to villagers.

Agency 'worked socks off' for floods. Minister faces anger over flooding. Farmers protest against subsidy cuts. Environment Agency. Chartered Institution of Water and Environmental Management. Image source, Getty Images. Flood barriers. Image source, Environment Agency.

Natural flood management. Sustainable drainage. Dredging rivers. In some cases dredging can even make flooding worse. How much is spent on flood defences? Why are we seeing more floods? Related Topics. Floods Flood defences Severe weather Environment Agency. Published 9 May These can be removed completely when waters recede. Temporary barriers can also be added to permanent flood defences, such as raised embankments, increasing the level of protection.

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