What causes an area to flood?
At its most simple level flooding occurs when an area receives more water than it can drain within a period of time. This can be broken down into inputs, stores and outputs:
The input
The store
The output
Flooding occurs when the input provides more water to the store than the output can remove within a given timeframe. Using this example, flooding occurs when rainfall input provides more water to river channel store than can be removed by the river flow output over a given period of time.
The precise mechanism of flooding differs depending on the type, but the basic principles of inputs, stores and outputs remain the same.
What are the different types of flooding?
Broadly speaking there are three types of flooding:
Surface Water Flooding
Also known as pluvial or flash flooding, are controlled primarily by ground conditions and rainfall intensity. In pluvial floods our input is still rainfall, but our store is now the ground itself. Different soils and ground types allow water to seep in faster or slower, or store less or more water. Our output is a combination of run-off (the movement of water above ground, for example flowing down a slope) and infiltration (the movement of water through the ground)
If the input rainfall is greater than the output run-off and infiltration then the ground store is exceeded, and pluvial flooding occurs.
This type of flood is particularly common where the ground is impermeable (e.g., tarmac), meaning that water cannot infiltrate into the ground, causing it to collect at the surface. Alternatively, intense longer-term rainfall can saturate permeable ground, meaning that no further water can enter it, with this also causing water to collect at the surface.
Riverine flooding
Also know as fluvial flooding, this follows the mechanism described in our first description of how an area floods, with the input being rainfall, the output being river flow, and the store being the river channel. However, input here, and elsewhere, is a little more complex than just rainfall. Movement of water through the soil by infiltration can also be an input, and indeed river flow from upstream is also an input, just as river flow downstream is an output. Other significant inputs are snowmelt and run-off.
Fluvial flooding is also a product of its catchment area. This refers to the physical characteristics of a river valley or collection of valleys which feed a main river. If the catchment is characterized by steep slopes and impermeable ground, we are likely to see a flashy response, while if its flatter with permeable ground then we are more likely to see a gentle response.
So what exactly does flashy and gentle refer to?
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They refer to a storm hydrograph, which essentially shows the relationship between input (rainfall) and output (river discharge or flow).
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A flashy response simply means that the time between the input event, such as a storm, and the output event, such as increased river discharge, is short. A variety of factors can produce flashy catchments, including steepness, ground type, tributary number and catchment size. In general flashy catchments can more often result in flooding as the input rapidly reaches the river over a short time period.
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A gentle response means the opposite to a flashy one. In short, the time between an input event and the output response is longer. As the time is longer this generally means there is less chance of the store being overfilled, and so rivers characterized by gentle response are less likely to flood.
However, while flashy catchments tend to flood more often, they are usually located in upland areas where less property is located. As such flooding here is comparably low cost. Rivers with gentle catchments tend to be in lowland areas such as floodplains, and so when flooding occurs here it is generally more developed, and so has much higher costs.
Additionally, many catchments are interrelated. In practice this means a several flashy catchments may feed into one lowland river which usually exhibits a gentle response. However, if all the feeding flashy catchments experience high input from one storm, this can rapidly provide a large amount of input to the usually gentle catchment, provoking a much flashier response, and with it flooding.
Coastal Flooding
Coastal flooding is related to sea level rise, either short term or long term. Here our concept of inputs, store and outputs suffers, and so we will leave it to one side.
Sea level changes due to a variety of factors:
Tides
Atmospheric Pressure
Isostatic Rebound
Coastal flooding is therefore caused by an amalgamation of these factors, with a healthy does of nearby river and rainfall inputs also sometime contributing in what are known as compound events.
Typically, coastal flooding today occurs when a high tide combines with a storm surge to produce higher than normal sea levels, which can overwhelm costal defences. The difference between the maximum tide and highest sea level is called a skew surge and this essentially describes the ‘extra’ sea level rise produced by storm surges.
Groundwater flooding
Ground water flooding is related to the water table which is the level to which water stored in the ground rises. Here our system of inputs, stores and outputs is once again useful.
Input rainfall infiltrates into the ground where it is stored. As water is stored here the water table rises. Output occurs through further infiltration out of a given storage area, with infiltration acting once again as an output and an input. Flooding occurs when storage is exceeded – in effect when the water table rises to the surface of the ground. In some locations this is a semi-permanent state such as in wetlands.
The speed of this process is much slower than the others described and can take days to occur after a rainfall event. Alternatively, groundwater flooding can occur due to poor management, such as when an area which has had the water table artificially lowered by pumping (say for industry or agriculture) suddenly has pumping cease – following this the water table will return to its natural levels.
How does a changing climate affect the frequency and severity of flooding?
A changing climate has the capacity to alter the frequency and severity of flooding through the alteration of inputs as described in the types of flooding section. The main contributing factors to this are as follows:
Sea level rise
This is a long term and climatic change. This is occurring for several reasons. Perhaps the most obvious and well known is the effect of melting land ice. As glaciers and the like melt the water contained within them eventually finds its way to the sea and so raise sea levels. However, the main driver of sea level rise currently is thermo-dynamic expansion. This is a fancy name for simple heat driven expansion. If water is hotter, it expands. As the Earth warms so to does the ocean, and so it expands.
This increase in sea level means that for the same combination of tidal and storm surge events we are likely to see a greater magnitude of flooding. This has the capacity to overwhelm existing defences. Met Office climate projections indicate sea level rise could reach 1m in areas of the UK. The Environment Agency estimates that under a high emissions scenario this would require an investment of ~3-5 times what is currently expended on maintaining defences.
Increased extreme precipitation
This refers to not just more rainfall, which is projected in winter for the UK, but specifically and increase in extreme events which see a lot of rainfall at once. Essentially this refers to increased storm events bring increased rain. Projections by the Met Office indicate not only that there will be more rainfall, but also indicate that there may be more storm events due to complicated mechanisms in the climate system that we do not need to touch on here. Suffice to say a hotter world allows the atmosphere to store more water, and so in turn results in more rain. The end result is extreme rainfall will increase, and so to will flooding frequency, and likely magnitude as a result. As an example of this a 1 in 100-year event has a 1% chance of occurring presently. By 2080 the chance of this same flood occurring may have increased to 2-4% (making it a 1 in 50 or 1 in 25 year event).
How does flooding affect a property?
Flooding effects a property more or less as you would expect – if the water levels rise high enough then the water will enter the house and cause water damage. This damage can be further compounded if drains or sewage systems overflow.
What is the average cost of flooding?
The annual cost of flooding in the UK totals around £1 billion a year, with the average damages associated with a single property estimated to be around £30 thousand, although this number will vary in any given year based on the severity of flooding.
Where in the UK is most exposed to flooding?
The area’s of the UK most exposed to flooding varies depending on the type of flooding – inland England is not at all likely to suffer from coastal flooding.
More broadly flood risk in is associated with the floodplain, and particularly development on the floodplain. Floodplains tend to be prime targets for development due to their historic access to amenities such as water, and more recently simply for ease of development. They tend to be flat, and so it easier to produce property and infrastructure. The figure below illustrates the percentage of each local authority area contained within the floodplain in England, as well as projected increases in rainfall.
What is the economic effect of climate change on flooding?
Currently 5.2 million properties in the England are exposed to flooding. Under a medium emissions scenario the number of properties exposed is expected to double by the 2080’s. However, the risk from flooding is linked to development, and as such economic and population growth. Under a high growth scenario (with large scale development on the floodplain) the economic risk is projected to increase 20-fold by 2080. In short this suggests a increase in damages from £1 billion a year to £20 billion a year. However, this projection assumes no changes in built defences, and no new regulation on floodplain development. As such it is useful as an illustrative scenario on why we need to take action on flooding under climate change but is not necessarily a realistic scenario.
Beyond property specific considerations flooding also has a huge impact on infrastructure. Currently in England there is:
• 14% of electricity infrastructure sites at flood risk
• 10% of main roads
• 21% of railways
Like the exposure footprint for property, the exposure footprint for infrastructure is likely to increase under all emissions scenarios.
How can conveyancers communicate changing risk of flooding to their client?
Currently 5.2 million properties in the England are exposed to flooding. Under a medium emissions scenario the number of properties exposed is expected to double by the 2080’s. However, the risk from flooding is linked to development, and as such economic and population growth. Under a high growth scenario (with large scale development on the floodplain) the economic risk is projected to increase 20-fold by 2080. In short this suggests a increase in damages from £1 billion a year to £20 billion a year. However, this projection assumes no changes in built defences, and no new regulation on floodplain development. As such it is useful as an illustrative scenario on why we need to take action on flooding under climate change but is not necessarily a realistic scenario.
Beyond property specific considerations flooding also has a huge impact on infrastructure. Currently in England there is:
• 14% of electricity infrastructure sites at flood risk
• 10% of main roads
• 21% of railways
Like the exposure footprint for property, the exposure footprint for infrastructure is likely to increase under all emissions scenarios.
How can properties in an area affected by flooding reduce the risk of damage to the property?
The first step to combat climate-related natural perils must always be mitigation. This is not something you can do specifically to protect your property, but is something you can do to protect the world. Make what lifestyle changes you can to reduce your emissions. On its own these actions may seem pointless, but when we all take such actions, it helps to slow and reduce climate change, which in turn means its effect on increased magnitude and severity of floods will decrease, protecting at risk properties in turn.
The second step to make your property safe is adaption. This is the process of installing physical defences to a property to increase its resilience to flooding. A non-exhaustive list of these measures includes:
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Barriers can be fitted to doors to provide a seal and prevent water entry.
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Resilient floor and wall finishes are finishes which are water resistant and dry out following flooding without undue damage.
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Non-return valves simply prevent the back flow of water back up out of your drains should the drainage system overflow.
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Resistant insulation some more traditional forms of insulation, such as mineral wool, soaks up a huge amount of water and must be replaced at great expense. This issue can be negated by installing water resistant cavity insulation.
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Pump systems can remove water from the property, helping to keep flood levels at a manageable level.
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Raised electrical sockets can prevent wiring having to be replaced following a flood, providing it stays above flood water.
