Friday, 21 March 2014

Understanding Groundwater and its role in flooding


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).



No comments:

Post a Comment