Frequently Asked Questions

Floods

What is a flood?
A flood is an overflow of water onto normally dry land.  The water may come from a rising river, a ditch, the sea, surcharging drains or sewers or from ponding owing to heavy rainfall.

What is flood hazard?
The potential risk to life and limb and the potential damage to property and infrastructure resulting from flooding. The degree of hazard various according to circumstances across the full range of flooding.

What is a floodplain?
A floodplain is generally a fairly flat area of land bordering a river or the sea on to which water naturally flows from time to time, or would flow on but for the presence of flood defences.

What are the possible sources of flooding in the UK?
These are (1) inland, riverine (fluvial) sources (2) tidal or coastal sources (3) rainfall (pluvial) sources leading to surface water flooding (4) sewers, the surcharging of which may lead to sewer floods (5) groundwater sources (6) dams which in very exceptional cases may lead to dam break floods (7), and combinations of 1-6.

What is meant by ‘flood discharge’?
Flood discharge is the volume of flood water passing a given point in a given period.  It is usually measured in ‘cumecs’ i.e. cubic metres per second.

What is meant by ‘flood stage’?
Flood stage means the same as flood level i.e. the height of flood water at a given point. The height of floodwater above a given point is also called ‘flood depth’.

What is the ‘peak flood’ or ‘peak level’?
This is the maximum flood level occurring during a flood event.

What is ‘flood event’?
This is a term often used by flood engineers and flood risk managers and it simply refers to a discrete or particular flood.

What is a ‘flash flood’?
A flood caused by heavy, intense rainfall over a short period, usually less than six hours although they may occur within minutes of the rainfall.  In the UK flash floods often occur on short, steep gradient rivers such as those in Cornwall or Cumbria (sometimes described as ‘rapid response catchments’), or in urban areas where rainfall permeating into the ground is limited by impermeable surfaces.  A flash flood may also be caused by the collapse of a debris ‘dam’ in a narrow, steep-sided valley, or the breach of a flood defence or dam (the latter being very unusual in the UK). In Scotland ‘torrent’ floods sometimes emanate from upland streams which are ‘flashy’ in nature.

Why do some floods occur quickly while others occur slowly?
Long, large rivers with meandering, low gradient, lower reaches, such as the River Thames are characterised by flood waves which generally take several days to pass from upper reaches through the lower reaches to their estuaries.  The length of such rivers and their comparatively flat long profiles mean that floods travel relatively slowly. Short, smaller rivers with steep gradients are characterised by flash floods in which the flood wave passes rapidly from the headwaters through to the lower reach.  Heavily urbanised river catchments tend to produce ‘flashier’ rivers and streams in which the accumulation and passage of floodwater is rapid.

What is a ‘pluvial’ flood and how does it differ from a ‘fluvial’ flood?
A pluvial flood is one directly caused by rainfall, whereas a ‘fluvial’ flood is a flood caused by the overflow of a river on to its floodplains.  A fluvial flood is usually caused indirectly by rainfall which becomes ‘runoff’ which flows into rivers.  Pluvial floods are very similar to surface water floods.

What is meant by ‘flood extent’?
This is the geographical or spatial area across which a flood extends or covers.

What is meant by a ‘design flood’?
A design flood is a hypothetical flood representing a specific likelihood of occurrence (for example, the 100 year or 1% probability flood). Such a flood may be selected by engineers as the standard of protection which is provided by a flood protection scheme.

Does the level of flood protection provided by the design flood mean that flooding will be prevented?
It means that all floods with a probability of occurrence up to and including the design flood (e.g. the 100 year/1% probability flood) should be prevented, as long as the flood defences are not breached. Floods with a probability of occurrence of more than the design flood will not be prevented, although flooding may be reduced.

Flood risk and flood return periods

What is ‘flood risk’?
Flood risk is the combination of the likelihood of flooding and the consequences of flooding.

What is a flood risk zone?
This is an area of land which is potentially subject to flooding from any source.

What is meant by flood risk zones 1, 2, 3a and 3b?
These are terms used in England and Wales. Flood zone 1 is where the annual probability of flooding is 1 in 1,000 years or less (<0.1%) i.e. where there is a low probability of flooding from rivers or the sea.  Flood zone 2 is where the annual probability of flooding is between 1 in 100 years (<1%) and 1 in 1,000 years (<0.1%) river flooding or between 1 in 200 (<0.5%) and 1 in 1,000 years (<0.1%) for sea flooding i.e. where the probability of flooding is medium. Flood zone 3a is where the annual probability of flooding is 1 in 100 years or greater (>0.1%) of river flooding and 1 in 200 years or greater (>0.5%) for flooding from the sea. Both are regarded has high probability flood zones. Flood zone 3b is the functional floodplain where water has to flow or be stored during a flood. This flood zone is not defined by rigid probabilities of flooding but takes in local circumstances. Land which has a 1 in 30 year probability of flooding (3.3%) or land designed to take the 1 in 100 year flood (0.1%) are starting points in identifying flood zone 3b.

What is meant by a ‘flood return period’?
This is more correctly termed the ‘average flood return period’ although the word ‘average’ is often missed out. The average flood return period has a very similar meaning to the average recurrence interval (see below). It refers to the long term average number of years between a flood of a given magnitude (e.g. a 50 year flood) will ‘return’ or recur.

What is meant by a 100 year flood?
The flood having a 1% or lesser annual probability (or average chance of occurring in a year).  Another way of putting this is that a 100 year flood has 1 chance out of 100 of happening during any one year.  Other floods have different chances of flooding during any one year as follows:
25 year flood – 4 chances in 100
50 year flood – 2 chances in 100
1,000 year flood – 0.1 chances in 100

Can two 100 year floods occur in a 2 consecutive Years?
Yes, although it is unlikely.  100 year floods are very unlikely to occur exactly every hundred years e.g. in 2014, 2114, etc. The chances of them occurring in any single year are exactly the same, and so experiencing a 100 year flood this year makes it no less likely that you will experience a 100 year flood next year, or the year after.

What is a ‘recurrence interval’?
This is more correctly termed the ‘average recurrence interval’ although the word ‘average’ is often missed out. It is the long-term average number of years between the occurrence of a flood as large (or larger than) a selected flood. For example, floods with a volume as great as (or greater than) the 20 year average recurrence interval design flood will occur on average once in every 20 years. Average recurrence interval is a way of expressing the likelihood of occurrence of a flood.

How accurate and reliable are flood return periods or recurrence intervals?
They are estimates, usually the best estimate, and they relate to a particular point, say along a river or on the coast. Generally, the longer the time period (measured in years) that flood levels or flood discharges have been measured at this point, the more accurate and reliable the return period estimate will be as long as there have been no major land use or other changes over that period which alter the relationship between flood level or discharge and frequency.  It is common in the UK for flood records of 50 years’ length or more but many flood records are shorter than this or perhaps only reliable since ‘flood gauges’ have been improved.  Sometimes large floods destroy or overtop the flood gauges designed to record floods and so the larger floods may not be reliably measured.  It is not unusual for return period estimates to be revised upwards or downwards over time as records become lengthier. This means that flood risk management proposals should be tested against the best estimate of return period and also against reasonable low and high estimates (this is called ‘sensitivity testing’ which may also be used to test the effect of varying other variables).

What is a ‘flood frequency-magnitude relationship’?
This is the relationship between the size (which may be measured as discharge or level) of different floods and how often each occurs over a period of time (say 50 years). Flood magnitude is plotted on a graph against flood frequency (measured as flood probability) to generate a frequency-magnitude curve or relationship.  Flood frequency-magnitude relationships are generally different between rivers, estuaries and points along the coast, not least owing to differences in geography there.

Will river dredging reduce flooding?
In some circumstances dredging will reduce flooding, by making a river convey more floodwaters away from urban areas. However the situation is more complicated than that, because dredging a river upstream of an urban area may accelerate the process of flooding and make it worse. Complex calculations are needed to ensure that the adverse effects of flooding do not outweigh its benefits.

Why is river dredging not done sometimes?
Rivers may not be dredged because important wildlife resources would thereby be disturbed, or because such dredging is not hydraulically efficient in terms of reducing flood risk.

Why does it take so long to implement a flood protection project?
Much assessment is needed to ensure that flood risk management measures will be effective and efficient, and much consultation is needed with those likely to be affected. It is also the case that there are many schemes competing for resources, which are finite, and therefore many measures have to take their place in the ‘queue’ rather than see immediate action.

Why is it that we continue to build houses in flood risk areas?
There are many areas in the country where flood risk is very low (for example the areas protected by the Thames Barrier) and it is sensible to build houses there, not least because the effect of overtopping of a structure such as the Thames Barrier would not flood all the area at risk. A balance has to be struck between risk of flooding and the benefits of providing housing for people. This is not an easy balance to strike, but it is quite clear that many areas where flood risk is very low can be developed, not least if they are protected to an even higher standard than hitherto.

What is ‘coastal squeeze’?
Coastal squeeze is the process whereby sea level rises against hard sea defences and the shore is narrowed, whether it be a beach or a saltmarsh.

Why do flood defences breach?
Flood defences can breach if the loading of water on them is greater than the strength of the defence to withstand that loading. This can result if flooding is particularly severe, or if the flood defence is in a poor condition. If both situations occur, then the probability of breaching is enhanced.

What is meant by ‘defence overtopping’?
Flood defences are overtopped when water flows over the top of a flood defence wall, bund or some other barrier.

What is meant by ‘residual flood risk’?
The term of residual flood risk indicates the risk of flooding after a flood defence has been implemented. This recognises that all defences have a finite design standard, and can be overtopped or breached.

What is meant by ‘freeboard’?
Freeboard is the allowance engineers generally use as a contingency against their calculations of flood magnitude or probability being inadequate. Often it takes the form of an extra 50 cm on top of a sea wall or other defence structure, designed to accommodate most extreme floods that have been anticipated or designed for.

Advice on reducing flood risk

Where can I obtain flood risk maps?
Maps showing the extent of flooding in England and Wales can be found on the Environment Agency website, and are currently located here.

What are some of the main myths around flooding?

• floods only occur to other people;
• it has never flooded here, so it never will;
• if flooding is a problem here, someone will have told me;
• we’ve had a 100 year flood now, so the chances of us having another one in my family’s life time is much less;
• you can’t get flooded if you live on the side of a hill;
• you can’t get flooded if you don’t live by a river or the sea;
• if you are protected from flooding by flood defences like flood walls or embankments, you can never be flooded;
• the planning consent process ensures that any house built near a river will be out of the reach of floods.

How might climate change worsen flooding in the UK?
Climate change (e.g. characterised by global temperature rise) is thought likely to lead to warmer, wetter winters in the UK with a subsequent increase in river discharges so that floods become more severe and more frequent.  Higher flood discharges can lead to an expansion of floodplains and increased flood risk making some areas which were formerly free from flooding at risk. Stormier weather and more extreme weather is also expected which, together with sea level rise brought about by global warming of the oceans, is likely to lead to higher storm surges and greater flooding threat along the coast and in tidal estuaries.  More intense rainfall is also expected in the UK increasing the risk of surface water flooding, including in summers. River discharges are expected to increase in their flow by up to c. 20%.  Sea levels are known to be rising by several millimetres per year on parts of the South coast and more than this on parts of the East coast.

If my house is flooded, can I expect the flood level to recede once it has peaked?
Often this expectation is a reasonable one but in some cases a flood peak may be followed by one or more further floods peaks depending upon rainfall patterns and whether or not floods along tributaries contribute to flooding.

If it is flooded, for how long will my house be flooded?
This is difficult to estimate.  However, flooding from the sea is likely to last as long as a tide is high unless seawater becomes trapped behind beaches or flood defences in which case floods may last much longer.  Several storm surges in a row, on high tides, can lead to repeated flooding.  In the UK, river or stream floods often last only a few hours (e.g. 6 – 18 hours), unless, your house is located in a natural basin in which floodwater ponds for days or if flooding is severe and occurs in an area where water levels are naturally high and difficult to drain down (e.g. In low areas reclaimed from the sea).  Surface water flooding is usually short lived  but groundwater flooding can last for weeks.

How might my house be flooded?
Floodwater can enter your house in a variety of ways. It can flow through doorways once water levels exceed the door threshold level but it can also rise through floorboards having already penetrated beneath flooring via ground level air bricks unless they are blocked or shielded. Floodwater can cause the water level in sewers to rise so that the sewer pipe to a ground floor toilet backs up causing the toilet to overflow. Floodwaters may approach from any direction and flood levels may rise very gradually or very suddenly causing a rush of floodwater into your house. If you have a basement or cellar this is likely to be the first part of your house which will be flooded.

Are sandbags any use in stopping flooding of my house?
Sandbags can be used to keep water out if a property for short periods but they are no use in preventing groundwater flooding.  The use of sandbags can be improved by using them in conjunction with plastic sheeting.  For further advice, consult ‘Sandbags and how to use them properly for flood protection’ published by the Environment Agency.

Where do I get sandbags from?
Your local council may have sandbags which you can obtain, possibly at a cost.  Alternatively, you can go to a builder’s merchants to obtain sand and bags which you will then need to fill. Modern non-sand bags can be purchased – consult the National Flood Forum’s Blue Pages for suppliers.

What precautions and preparations should my family take for the possibility of being flooded?
Advice is available from the Environment Agency about these precautions and preparations, and they generally focus on each household developing some sort of ‘Action Plan’ to prepare for the eventuality of flooding. This action plan should prioritise effort in the hours or minutes preceding a flood event, so as to maximise the effectiveness of those efforts.

What precautions should I take after being flooded?
The Food Standards Agency’s advice is that food that has been touched or covered by floodwater should not be eaten because floodwater is often contaminated with sewage.  Also do not eat produce from allotments which have been flooded.  If you run a catering business which is flooded, consult the Environmental Health services before serving any food.

What should I do with flood damaged food?
Put flood damaged food in black plastic bags, double bag them and put them out for refuse collection or take them to a refusal disposal point yourself.  Do not be tempted to salvage flood affected food even in tins as they may be damaged.

Will I be able to get flood insurance?
It should be possible for everyone to get flood insurance, but in high-risk areas it may be necessary to obtain this cover through a specialist broker. Remember that you need to ensure that you have flood insurance cover for both the structure of your property and its contents. See this Defra guidance document for more information (opens as a .pdf).

What is Flood Re?
Flood Re is a joint initiative between the UK Government and insurers to make household flood insurance more affordable. The Flood Re website provides more information on the scheme.

What are the differences between flood alerts and flood warnings?
These are terms used in the UK: different terms are used in other countries. An alert advises about the possibility that flooding could occur, whereas the warning is more specific about the flooding that is likely to occur in the locations specified.

An explanation of the terminology used by the Environment Agency is as follows:

• Severe Flood Warning: Severe flooding. Danger to life;
• Flood Warning: Flooding is expected. Immediate action required;
• Flood Alert: Flooding is possible. Be prepared;
• Warning no longer in force: Flood warnings and flood alerts that have been removed in the last 24 hours.

What are ‘sustainable drainage systems’ (SuDS)?
Sustainable drainage systems are configurations of the urban fabric to hold back flood water or to store it during a flood event. They often take the form of grassed areas, swales, permeable pavements, and other devices to intercept floodwaters and prevent them from accumulating downstream thus causing damage and disruption.

What is the National Flood Forum?
The National Flood Forum is a charitable organisation to help communities at risk from flooding with preparing for and recovering from flood events.

How can I make my property more resistant to flooding?
Making your property more resistant to flooding means keeping out the flood waters that may surround it. This can be done by barriers across doorways, the sealing of air bricks under floor levels, and preventing the ingress of water by other means. The National Flood Forum website provides further guidance on this.

How can I make my property more resilient to flooding?
Making your property more resilient to flooding implies that flood waters will enter the property, but you can do many things to make the damage less and the disruption less problematic. Often this will include raising electrical sockets, making furniture and fittings more resilient when they are covered with floodwaters, and such other measures to make the drying out and cleaning up of your property more effective and rapid.

How can I work out whether it is worth my while investing in flood resistance and resilience measures?
This is a complicated calculation comparing the costs of resistance and resilience measures with the likely flood damage and disruption that you will gain. It can be a completely quantified analysis, or it can be more intuitive. The main aim is to make this comparison, to ensure that you either spend enough on your measures to counter the flood damage that you will suffer, but not spend more than this efficient or worthwhile.

Does flooding affect one’s health?
The evidence is that floods can have detrimental physical and mental health effects.  Exposure of skin (e.g. legs, arms etc.) to floodwater can lead to rashes and various skin ailments because floodwater is often contaminated by sewage and other substances such as oil from road surfaces.  Having one’s home or business flooded can feel extraordinarily invasive and often leads to stress and anxiety which can lead to both physical and mental ill-health, including raised blood pressure, disturbed sleep and depression. Flooding can lead to major family disruption which can last for months particularly if you have to move out of your house while drying and repairs are undertaken.  This disruption can itself place individuals and families under great stress and this may be worsened by financial anxieties and uncertainties associated with loss of possessions. Anxieties may also heighten and become intense when it rains or if the level of water in a nearby river rises, even if it does not flood again.

Flood governance

What is an ‘ordinary watercourse’?
An ordinary watercourse is one of two types of statutorily defined watercourses in England and Wales. Ordinary watercourses include every river, stream, ditch, culvert, sewer (other than a public sewer) and passage through which water flows which is not a ‘main river’.

Who has responsibility for managing floods on ordinary watercourses?
Lead Local Authorities or Internal Drainage Boards have ‘permissive’ powers to install flood defences but these are not powers which carry a legal obligation to do so.  Riparian owners also have some rights and responsibilities which relate to certain aspects of flood risk management on rivers and streams. Some ordinary watercourses are defined as ‘critical ordinary watercourses’ for which the Environment Agency has assumed responsibility for certain anti-flooding measures.

What is a ‘main river’?
A main river is one of two types of statutorily defined watercourse in England and Wales. These are usually the larger rivers and streams, although some smaller ones are included as main river. Main rivers are those marked on a special main river map held by central government (Defra).

Who has responsibility for flood risk management on main rivers?
The Environment Agency has ‘permissive’ (but not obligatory) powers to provide flood defences on main rivers.  In Wales this responsibility is held by the Welsh Assembly Government.  Riparian owners also have legal rights and responsibilities regarding certain aspects of flood risk management.

Who is responsible for protection against flooding from the sea?
Local Authorities or the Environment Agency have powers to undertake works to reduce the risk of flooding from the sea; in general, Local Authorities are the major players in this respect.

Flood risk assessment and cost-benefit appraisals

What is a ‘flood risk assessment’?
A flood risk assessment is an evaluation of the magnitude and probability of floods at a given location. It  can also assess the damage caused by floods, and the disruption to local transport and other networks.

How can I assess my flood risk?
You need to look at the probability of your property being flooded, and the consequences of that flooding. This will involve liaising with organisations such as the Environment Agency, to obtain indications of flood probability, and the assessment of potential damage caused by flooding can be obtained from the MCM-online.

What does a ‘benefit’ of flood protection mean?
A benefit of flood protection means the avoidance of future flood damage.

What is the difference between flood protection benefit and flood damage?
The benefits of flood protection are the accumulated potential flood damage in the future that can be avoided by particular flood risk management measures. The terms are similar in many respects, but benefit connotes damage avoided, whereas flood damage may not always be avoided.

Why is benefit-cost analysis used in making decisions about flood risk management?
Benefit cost analysis is used in order to assess how worthwhile is investment in flood risk management measures, gauged in terms of the flood damage avoided in the future (the benefits) and the costs of the required schemes. It is a way of weighing up whether that investment is economically efficient or worthwhile, in order to separate out those schemes which are economically poor value for money and those that are good value for money.

What is the difference between national economic efficiency benefits (of flood protection) and financial/local economic benefits?
This difference arises because many components of flood damage money values are distorted by the fact that repair and replacement involves betterment (a new cooker replaces an old one damaged in flood). And also because such values include taxes such as Value Added Tax (VAT). Converting financial values which include those taxes to economic values which do not (and do not include the element of betterment referred to above) is a process used to convert financial values to economic values. These economic values are the true values to the nation of the costs of flood risk management interventions and the benefits that they generate.

What is meant by ‘annual average damage or AAD’?
Depending on its size or severity, each flood will cause different amounts or levels of damage.  The AAD is the average amount of damage in £ per year that would occur in a designated area from flooding over a very long time period.  In many years there may be no flood damage at all, in some years there will be minor damage (caused by small, relatively frequent floods) and, in a few years, there will be major damage (caused by large, rare floods). Estimating the AAD provides a basis for comparing the economic effectiveness of different flood risk management alternatives (i.e. by comparing their cost in relation to AAD).

What is a loss-probability relationship and what is its use?
This is the relationship of flood damage (i.e. flood loss) and flood probability.  The larger the flood, the lower the probability and, usually, the higher is the flood damage.  Small, relatively frequent floods usually generate a low amount of damage whereas large, rare floods usually generate high damage.  This relationship can be graphed producing a loss-probability curve. The loss-probability relationship is used in estimating annual average flood damage (see above) which is an essential step in undertaking a benefit-cost analysis of flood risk management options.

How can I work out how much flood damage will be caused if my property is flooded?
The Professional section of the MCM-Online provides full guidance and flood damage data. You will need to register and purchase a Licence in order to gain access to this information. Please see the Registration page for further details.

How can I work out an estimate of the benefits of flood protection for the village in which I live?
This section of the MCM-Online provides information on estimating the benefits of a flood risk management scheme. The information is given at three levels of complexity, based on your existing knowledge and experience of flood risk management. Please see the home page in order to select the level which best suits your requirements.

What are the main errors likely to be in flood damage estimations?
The main errors will arise from (a) estimates of flood discharge (b) errors in water surface level estimates (c) errors in estimates of the ground floor threshold level or height of a property (threshold level being the height at which floodwater will first enter a property) (d) errors in estimating the value of a property and its contents and (e) errors in estimating the susceptibility of a structure and its contents to flood damage.  There are therefore various errors associated with flood damage estimation and therefore the analyst/estimator usually work outs minimum, maximum and best estimates to fully understand flood damage estimates, the uncertainties surrounding them and their implications.

How accurate are the flood damage data provided on the MCM-Online?
This is a difficult question to answer, but the data has been accumulated over more than 40 years and has remained unchallenged during that period. It is based on the analysis of many flood events in the past, and the experience of loss adjusters and professional surveyors in assessing flood damage to buildings. It should be noted in this respect that the values contained in this website are economic values, being the loss to the nation, rather than the financial values in terms of loss to the individual household.

What is ‘discounting’?
Discounting is an economic technique designed to assess the value of costs or benefits of flood risk management in the future, in order to allow for the fact that alternative investments might be worthwhile. It is generally accepted that sums in the future are worth less than sums acquired or spent now, and discounting is designed to make that adjustment.

What is the ‘test discount rate’?
The test discount rate is the percent used in the discounting process; the higher the test discount rate the less one values future benefits or costs when they are discounted back to present values.

What is flood defence grant-in-aid (FDGiA)?
Flood Defence Grant-in-Aid is the sum that central government is willing to contribute to local and regional flood risk management investment.

What is ‘partnership funding’?
Partnership funding is the arrangement whereby the costs of flood risk management measures are shared between central government and local authorities, businesses, community organisations, residents etc. It is designed to increase the total amount of money available for flood risk management measures, from local community contributions and engagement.

What is meant by ‘Present Value of Damage’
The present value of flood damage in the future is the discounted sum of that damage, brought back to present-day values.

What is a Loss Probability Curve?
The loss probability curve is the relationship between flood probability and the damage caused by those floods. It generally has a concave upwards form, representing the fact that the smaller floods cause less damage than the larger floods. The area under the loss probability curve is the annual average damage at that location.

What is ‘optimism bias’?
Optimism bias is the way of up-rating the costs of flood risk management measures to bring them into line with past experience of cost over-runs. An extra element of cost is added to known costs to allow for future likely cost over-runs. This extra element can be as high as 40%.

What is meant by an ‘Outcome Measure’?
The system of outcome measures was introduced in 2006 to broaden the appraisal process away from just flood damages averted, to include a multi-criteria approach with an emphasis on the numbers of properties with reduced risk, the number of these in economically deprived areas, and the environmental gains from flood risk management schemes.

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Errors
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Reproducing this information and contact details
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Please note: The information contained within these “Frequently Asked Questions” is provided for general information only and may not be completely accurate in every circumstance. Middlesex University, the Flood Hazard Research Centre and the Environment Agency cannot be held responsible for any inappropriate use or misuse of the information contained here.