What is groundwater and why is it a
problem?
Rainfall runs
off into rivers and the sea, evaporates, and percolates into permeable ground
to recharge aquifers. These processes
are collectively known as the water cycle, and the part that flows underground
is called groundwater. It normally re-emerges
at springs and into river beds and can keep the rivers flowing for months
without rainfall. It is clearly a slower
moving part of the system, and flow through rocks is often a hundred times
slower than surface flows. It is better
to think of flow through a permeable sponge rather than the popular conception
of underground rivers or lakes, although in extreme cases such as karstic
limestone groundwater may have dissolved channels to become fast flowing
underground rivers, and the Chalk in England tends to have fast flow in
fissures, which explains why Chalk areas suffer most of the UK’s highest
groundwater flood risk.
After a large
rainfall event we see surface water flood where it exceeds local drainage
capacity and may pond for a short time, and also high river levels leading to
flood plain inundation, but these will recede in a few hours or days. Because groundwater moves so slowly the
rainfall tends to accumulate over the winter months leading to seasonal peak
levels in the spring and lows in the autumn (rainfall is actually distributed
quite evenly through the year on average in Britain, and it is the low
evaporation in the colder winter months that leaves more water for run off and
recharge). This means that groundwater
flooding also generally peaks in the spring.
Under average
conditions the zones where groundwater discharges to the surface or river
channels are well defined, but after high rainfall the flows will emerge in
different places to cause groundwater flooding.
Just as flows are slower than rivers, once flooding occurs it will
recede a lot more slowly too, leading to flooding for far longer (3 months is
not unusual). It is harder to observe
the processes that lead to groundwater flooding than those that happen on the
surface, so uncertainties are significantly greater. However, a reasonable understanding can be
achieved once the available evidence is put into a coherent framework for how
the system works (known as the conceptual
model).
So, groundwater
flooding occurs when sub-surface water emerges from the ground at the surface
or into Made Ground and structures. This
may be as a result of persistent rainfall that recharges aquifers until they
are full; or may be as a result of high river levels, or tides, driving water
through near-surface deposits. Groundwater flooding is characterised by:
- Water
flows to the surface or into basements, services ducts and other
subsurface infrastructure rising up through floors or directly from the
ground. This may be seen as diffuse seepage from the ground, as emergence
of new springs or as an increase in spring flows
- Flooding
may last a long time compared to surface water flooding, from weeks to
months. Hence the amount of damage that is caused to property may be
substantially higher. Likewise closures of access routes, roads, railways
etc may be prolonged
- Flooding
may occur with a delay following periods of high rainfall rather than
immediately during storms
- Emergent
groundwater tends to be clear and relatively clean compared to muddy fluvial
flood waters, but contamination by sewers and brownfield sites poses
additional hazards
- Groundwater
flooding or a shallow water table prevents rainfall infiltration and
increases the risk of surface water flooding. This means that many surface floods are
actually driven by groundwater conditions.
But consideration of surface water in isolation and lack of
evidence for groundwater conditions leads to incorrect analysis of overall
causes.
It is also
important to understand that surface water, rivers, groundwater and other
factors all interact in permeable catchments, contributing to overall flood
risk.
Whilst
groundwater flooding is generally less hazardous to human health than surface
flooding, it is more hazardous to property producing 2 to 4 times the damage to
building fabric and greater disruption to economic activity due to the longer
duration of flood events. Also, the impact may be less about surface water
depths or velocities and more about the extended saturation of the shallow
subsurface with the following consequences:
1. Damage to basements and other
structures below ground
2. Damage to infrastructure such as
buried services and ducts
3. Sewer flooding
4. Water damage to property, cultural
heritage, crops or sensitive habitats due to saturated conditions
5. Leaching of contamination from
brownfield sites and other sources of contamination
6. Increased likelihood, intensity and
duration of surface water flooding due to saturated ground conditions
7. Increased cost of construction
projects, which will need to incorporate preventive groundwater control
measures to prevent what would otherwise cause harm.
This winter has
seen the highest rainfall on record, leading to the highest groundwater levels
and therefore record groundwater flooding and groundwater driven surface water
flooding. The Environment Agency has
reported that 24% of flooding incidents are categorised as groundwater
flooding. Given that flooding will often
appear as a surface event it is no surprise that groundwater flooding is
under-reported, so the full extent of the problem will be bigger than this. Furthermore, it is helpful to understand that
groundwater will interact with the other parts of the water cycle described
above so that flooding events will often have some groundwater component, and
even where flooding may appear to be a simple result of the overtopping of a
river bank, the consequences will be more severe in those parts of the flood plain
that are permeable because of the longer time it will take for the flooding
problems to recede. For example, with
the River Thames flooding events of February 2014 it is those areas such as the
Chalk winterbournes, Oxford and Wraysbury where the consequences will tend to
be excacerbated by long duration as the groundwater system slowly recedes. Because risk is a combination of likelihood
and consequences we can see that overall risk associated with the permeable
parts of catchments will have higher risks, even if these are associated with
river water driven events rather than pure groundwater emergence.
Current
groundwater problems will continue for several months, but if we see a return
to average rainfall from now on, the more extreme events will recede to more
normal levels in a few weeks. However,
now that the system is ‘primed’, further wet periods will lead to more rapid
groundwater flooding. On the plus side,
this year is giving us excellent data to further improve the models and future
predictive capability! With careful
consideration of groundwater flooding risks it will be possible to plan better
land development to avoid problems in the future.
Landmark in
partnership with ESI will be incorporating groundwater flood risk data into a
wide range of its risk assessment reports including Homecheck
Flood, Envirosearch Residential, SiteSolutions Residential, Sitecheck Combined,
SiteSolutions Commerical and SiteSolutions Combined.
ESI is building
an extensive national database to help improve knowledge of groundwater
flooding and we are keen to hear from you if you have any incident to report or
information on where our map can be improved.
We are also pleased to help if you are facing groundwater flooding
issues, and give a free initial consultation, so please do not hesitate to get
in touch on markfermor@esinternational.com
Glossary of Terms
Groundwater is water that
normally resides in the subsurface; it fills the pore spaces within rocks
Groundwater flooding is
defined as the emergence of groundwater at the surface or into Made Ground or
infrastructure away from perennial surface water bodies
An aquifer is a body of rock
containing water that is sufficiently permeable to allow significant amounts of
groundwater flow. Principal aquifers in
the UK are limestones (including Chalk) and sandstones
The water table is an
irregular surface that generally resembles a gentler version of the overlying
ground surface. It rises when recharge adds more water to the ground, and falls
when drought reduces the recharge
Recharge is rainfall that has
not been evaporated, used by plants or run off to streams; and has got through
the soil zone into an aquifer
Permeability is a measure of
how easily water can flow through a rock: aquifers are highly permeable, clays
are not
Risk is defined (consistent
with the Flood and Water Management Act 2010) as ‘a risk in respect of an
occurrence assessed and expressed (as for insurance and scientific purposes) as
a combination of the probability of the occurrence with its potential
consequences. In each case the potential harmful consequences to be considered
in assessing risk include, in particular, consequences for
(a)human health
(b)the social and economic welfare of individuals and communities
(c)infrastructure
(d)the environment (including cultural heritage).
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