With flooding increasingly in the news, you could be forgiven
for thinking that the nature and extent of the problem in this country is
getting worse. The issue is particularly pertinent for those contemplating a new
property purchase. The recent Planning Practice Guidance from the Department
for Communities and Local Government has certainly raised awareness of the need
to take flooding hazards seriously, thereby significantly reinforcing the need
for good quality data and advice.
So, should we believe the shock headlines? What is the reality, and how does one
establish whether a particular property will face costly and life-threatening
hazards, or whether the perceived risk may be unfounded and won’t impact on the
use or enjoyment of the property at all?
Being well briefed on the degree of risk is key to
making investment decisions, and this is where the quality of information can
make all the difference. One company
that has invested significantly in obtaining in-depth, reliable data is
Landmark Information Group, which uses the statistics as the basis of its
reports on flood mapping, which in turn are used by property developers and planning
consultants to support property investment decisions.
However, we need to be clear on terms. There are many
different flooding mechanisms, but flooding typically results from the
interplay of a number of different variables at any one point. Breaking down
flood risk according to only a single mechanism is unhelpful, and results
largely from a fragmented approach to mapping the problem.
What is a Flood and how does it impact on property?
A flood is described as "where land not normally
covered by water becomes covered by water."
Flooding can happen even if you don't live near a
river, sea or watercourse. During
periods of heavy rain the capacity of the ground to infiltrate rainfall can be
exceeded, and drainage systems can become overwhelmed by the high rate of water
reaching them in a short time. Furthermore if the ground is already saturated
then the runoff will be enhanced and more rapid.
Flooding can devastate lives and cause serious damage
to your property. It is common sense to
avoid purchasing a property in a flood risk area, but with careful
consideration and accurate assessment it might be that the risks are minimal
and acceptable and can be managed to prevent intolerable damage and risk of
injury. In this case, you could still proceed with caution.
First and foremost however, if you are a property
owner in a flood prone area it is essential that you understand whether you are
at risk, so access to good quality data is vital. If you live in an area at risk it is
important to plan in advance for flooding that could occur.
Being prepared can make all the difference and is
your most powerful tool for dealing with flooding. This can include:
- Registering with the Environment Agency’s
flood warning scheme (https://fwd.environment-agency.gov.uk/app/olr/register)
- Prepare a Flood
Evacuation Plan;
- Undertake a property level survey to establish
likely points of water entry;
- Incorporate permanent Flood Resistance measures
designed to keep water out. Some examples include raising water resisting
external doors and windows, raising thresholds, sealant around external doors,
windows, walls and service entry point;
- Incorporate temporary Flood Resistance measures
designed to keep water out. Some examples include the use of sand bags,
de-mountable or free-standing barriers and appliance vent covers;
- Incorporate Flood Resilience measures – designed
to permit water entry but designing the interior of buildings to minimise
damage and speed up the clean-up operation.
Some examples include water compatible internal walls and floors, raised
utilities, and removable fixtures and fittings.
The six types of flood risk that all prospective
purchasers need to be aware of
1)
River
(Fluvial) flooding: Inundation from rivers and watercourses, usually due to
excessive runoff from rainfall, leading to overtopping of river banks. This can
also occur from inundation of areas outside the floodplain due to the influence
of bridges, embankments, and other features that artificially raise water
levels.
River
flooding usually impacts adjacent low-lying properties and can cause widespread
and extensive damage due to the large volume of water and high velocity of
flow. Debris can also cause further
damage and, depending on the setting, flooding can ‘back up’ in the catchment
area and become longer-lasting and more difficult to drain away. Fast-flowing
floodwaters can also be a threat to peoples' and animals' safety and can damage
or even demolish buildings. The
Environment Agency estimates that, when combined with coastal flooding, about
2.4 million properties in England are at risk.
2)
Surface
water (pluvial) flooding: Usually associated with extreme rainfall, but may
also occur when rain falls on land that is already saturated or has a low
permeability. In each case, the rainfall generates overland flow which can lead
to flooding before the runoff is able to enter a drain, sewer or watercourse.
It
may be triggered or made worse in urban areas where the ground consists of hard
surfaces such as concrete or tarmac. It is estimated by the Environment Agency
that beyond those in the fluvial risk areas above, a further 2.6 million
properties are at risk of surface water flooding in England. Extensive flash flooding of this type can
follow periods of high intensity rainfall, and the harm to property is related
to local features in the immediate vicinity rather than overall position in the
catchment. The lack of warning can be a
particular problem. However, due to the flash
nature of this type of flood, it may recede as quickly as it arrived, and the
damage to property may be less serious if the mopping-up and drying-out process
can proceed quickly. Also, it may affect
your garden and the street rather than your home, and features as simple as a
raised kerb may provide property level protection, although it would be wise to
review how reliable these protections are.
3)
Groundwater
flooding: Occurs when the water table rises after prolonged rainfall, only to
emerge above ground level away from a watercourse, or where high river levels
drive water through the river banks. This is most likely to occur in areas
underlain by permeable rock, and can seep directly into properties or
exacerbate other forms of flooding.
This
type of flooding can last for weeks, leading to a higher degree of harm to
buildings because of the impact on the fabric of the building from prolonged
saturation. Groundwater flooding is more
of a seasonal issue because most aquifers need several weeks of high recharge
to fill up before the water table overflows.
Irrespective
of whether water shows at the surface, rising groundwater levels are posing an
increased threat to buildings with basements. Such flooding may occur
separately or in conjunction with flooding from other sources, such as surface
water flooding. ESI estimates that up to one third of flooding problems in the
UK are due to groundwater, which typically causes more property damage than
other forms of flooding, making this one of the major flood risks when it comes
to property. However, the Environment
Agency are yet to include this data on their maps, so we cannot yet put this
risk into the same context as their mapped 5M properties in England within
areas at risk of fluvial and pluvial flooding.
Thankfully,
the slower flow rates associated with groundwater mean that risk to life is
significantly lower. Nevertheless, information
on groundwater is clearly vital to the overall understanding of flood risk and,
with the Environment Agency reporting that last winter 24% of flooding was
caused by groundwater, prospective purchasers need to review the data
carefully.
4)
Coastal
flooding: Caused by high tides and/or inclement weather breaching sea
defences and inundating the surrounding area.
It can also cause backing up in rivers and groundwater systems that will
then flood upstream. Coastal flooding
may affect not only a property on the coast itself, but also property in tidal
rivers some distance inland, due to floodwater being forced up the tidal
reaches of rivers and estuaries by raised sea levels and gales.
Coastal flooding is often associated with
high energy storm events, posing a very real risk to life. Those considering purchasing a coastal
property, particularly where the coastline is very exposed to storms, should
review mapped flood zones carefully and consider the wider disruption that
could arise, as well as whether flood water could reach within their own
property boundaries.
5)
Sewer
flooding: Sewers are generally not sealed and during periods of extreme
rainfall can overflow into the surrounding subsurface, ground surface, or into
properties via drains, toilets and basements, causing considerable and costly
damage, along with many unpleasant and harmful consequences.
Groundwater can be the decisive factor,
and typically the cause is too much water entering the sewers from storm run off, or where rising groundwater may infiltrate the sewer network. Many of the water companies have access to
the ESI national groundwater flood risk data and this is helping them to
implement sewer infiltration reduction plans that should help to reduce this
problem over the next five years, but sewer flooding currently remains a very
significant problem in some areas.
6)
Reservoir
flooding: The UK has approximately 5,000 reservoirs, and flooding can occur
if there is a catastrophic failure of a reservoir wall or embankment.
Fortunately dam failures in the UK are rare, but have occurred to considerable
catastrophic effect in the past. While
the chances of reservoir failure are very small, the consequences could be
fatal and affect large areas many miles from the dam itself.
Is flood risk increasing?
There are several aspects to this:
1)
Some risks such as groundwater flooding have
been largely ignored, but new data has demonstrated the full nature and extent
of the problem, in turn leading to increased recognition of a risk that has
always been present. This is good news
because the data enables risk management action to be taken and problem
locations can either be avoided or mitigation measures incorporated.
2)
Climate change is leading to rising sea levels,
and these in turn increase coastal flooding risk. There is a very real increase in risk over
time, but the fact that it is largely predictable means that the problems can
be managed if attention is paid now.
3)
Climate change is leading to different rainfall
patterns in the UK, with long-term averages over a 30-year period showing an
increase in annual rainfall of about 5% from 1961-1990 to 1981-2010. Preliminary research from the Met Office
suggests we may have also seen a change in the nature of the rain we get, with
'extreme' daily rainfall becoming more frequent.
Increased rainfall can lead to an increased
likelihood of flooding, and there is a very real threat that the risk will increase
over time. However, it is highly uncertain in its impact due to limitations in
climate prediction models. The latest
work from the Climate Change Committee, for example, suggests that the risk of
flash flooding may increase in frequency due to more high intensity rainfall,
but severity may remain at similar levels.
4)
In the case of groundwater there may be a real
increase in the number of areas that are at risk due to the impact of seasonal
recharge of the aquifer and the increase in wet winters. Clearly more work is needed, but in the
meantime a precautionary approach is sensible, and a ‘factor of safety’ is
recommended.
5)
Catchments are changing. Over many decades, urban development the
increased amount of ‘impermeable surfaces’ and led to more runoff in catchments
where the natural catchment response would be more attenuated and less
flood-prone. The recent move towards sustainable urban drainage systems (SuDS) such
as attenuation ponds and soakaways is intended to return catchments partly back
to their natural state to reduce flood risk, but the regulations have been
delayed by Defra so are not yet fully in force.
6)
Traditionally flood defence designs used
over-sized flood channels engineered to remove water as fast as possible, but
these did not sufficiently recognise the impact further downstream this may
cause.
In
summary, we have seen some increase in certain risks, but the biggest change in
recent years is actually the recent increased access to better data, and in
general this is helping us to more accurately identify risks that have always
been present. A flood risk map such as
that provided by Landmark Information Group can form part of a structured risk
assessment process enabling those advising on property purchases to decide
whether the next steps indicated by the initial risk screening report towards
site-specific risk assessment, or risk mitigation measures, or indeed in
extreme cases, withdrawal from the purchase process, are warranted.
What to do next if your proposed purchase falls
within any mapped flood zone?
If the property has been identified as located in an
area at risk, it is important to appreciate that this does not automatically
mean that the property will flood.
The risk is based on the probability that a flood event
will occur based on the environmental setting of the property and the climatic
conditions it is exposed to. If your
property is classified as being “at risk”, it simply means that your property
is located in an area that is considered to be hazardous in terms of flood risk,
but the risk isn’t actually present unless the correct environmental and
climatic conditions occur and the local features immediately surrounding the
property do not prevent it impacting the property.
There are a number of features including property
level protection that may prevent a real risk arising, so the best response
will be to discuss the matter with your legal advisor and, if appropriate,
consider the next level of detail in a desktop mapping report to identify the
true extent of the problem.
Author: Mark Fermor, Managing Director, ESI Limited
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