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, sewers, 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 drainage and spring flows. Groundwater flooding or a shallow water table prevents rainfall infiltration and increases the risk of surface water flooding. High groundwater discharge reduces the capacity in the drainage system. This means that many river and surface floods are driven by groundwater conditions. The Environment Agency*1 suggest ‘bigger, more frequent floods are expected over the 21st century, particularly during winter’ which is when groundwater levels are high, increasing the overall flood risk.
Geosmart’s national scale mapping of groundwater flood risk incorporates several mechanisms that are affected by climate change. Climate models cover a range of scenarios and include significant uncertainties which make precise predictions difficult. However, we have reviewed how the groundwater flood risk currently identified may change in the future. A site specific assessment can then be undertaken to determine the groundwater flood mechanism that is applicable to the site and the climate change impacts on this.
The UK Met Office and UK Climate Impacts Programme have developed predictions for the climate of the UK over the coming century, in 2009 (UKCP09) and most recently in 2018 (UKCP18*2). Predictions for the UK over the next century suggest milder, wetter winters and hot, drier summers, with an increase in the frequency and intensity of extremes, both flood and drought. It is highly uncertain how climate change will affect the possibility of extreme events such as coastal surges or heavy rainfall happening at the same time. However, as the chances of individual events increase, so does the probability of them coinciding. The consequences can be severe when these events occur together*1. Given the prolonged duration of groundwater flooding (weeks to months), groundwater is likely to make a significant contribution to these combined events.
The Future Flows and Groundwater Levels project *3 (FFGWL) produced a national ensemble of daily river flow and monthly groundwater levels using Future Flows Climate (FFC). FFC contains an ensemble of 11 plausible time series from 1950 to 2098, capturing natural climate variability and some climate change uncertainty. Predictions included 282 river sites and 24 boreholes within chalk, limestone and sandstone aquifers.
The British Geological Survey (BGS) have used climate change predictions (UKCP09) to model potential changes in groundwater recharge across the UK as part of the Future Flows and Groundwater Levels project *3. The uncertainties relating to the modelling were considered acceptable, with the climate signal likely to remain the dominant factor of change in groundwater levels in most catchments but also changing land use was a contributory factor. The overall pattern of changes is very complex spatially and no clear spatial pattern of changes is shared by all scenarios.
Rising temperatures and variation in rainfall patterns will change the recharge to groundwater in UK aquifers. The groundwater recharge season, which typically runs from September to April, could become shorter (3 to 4 months), but more recharge may occur within the shorter period, leading to flashy responses in groundwater level. The potential for higher peaks in groundwater level and more flooding, driven by rainfall recharge, increases under many of the climate change scenarios, but results are not uniform across the UK. Results show a wide range of potential outcomes with the possibility for mean monthly groundwater levels to move both up or down by several meters or more. Current rainfall patterns vary significantly across the UK on seasonal and regional scales and will continue to vary in the future. A range of scenarios are possible and there is uncertainty in the predictions. By 2070, under a high emission scenario, changes in rainfall could range from -47% to +2% in summer, and -1% to +35% in winter. Additional assessment is underway by the BGS using the more recent climate change predictions (UKCP18).
A rise in peak river levels in response to extreme weather events is predicted. When rivers are in hydraulic continuity with adjacent aquifer systems, such as river gravels, the rise in river level causes a response and increase in adjacent groundwater levels. The Environment Agency*4 have set out climate change allowances for changes in peak flow ranging from +25% to +105% in extreme cases that suggests very significant increases in some areas, particularly in South East England. Therefore, it is likely that there will be a corresponding increase in the frequency and severity of the groundwater flood events driven by the peak river levels. Average winter river levels are also likely to rise under a number of the scenarios considered*5.
Despite on-going efforts to build higher river flood defences, groundwater may by-pass these, causing flooding even if the river remains behind the flood barriers. Changes to groundwater flood risk for sites located on permeable material adjacent to river flood plains would be subject to the predicted climate change increases in peak river level for the local catchment and a risk assessment is recommended on this basis.
Percentage change in winter flow (DJF) for the 2050s – CERF hydrological (Extract – Figure 45 From *5 Prudhomme et al.)
Sea level rises of between 0.4m and 1m are predicted by 2100, leading to a rise in average groundwater levels in the adjacent coastal aquifer systems, and potential increases in water levels in the associated drainage systems*1. There is potential that these rises may occur more quickly, even as soon as 2050. Changes to groundwater flood risk for sites located on permeable material adjacent to the coast and tidal rivers would be subject to these predicted climate change increases in average sea level. The recent UKCP18 predictions for sea level rise are higher than the previous modelling last undertaken in 2009 (UKCP09). The Earth is already locked in to sea level rise for decades and centuries ahead. If action is not taken to reduce emissions, UK sea levels could rise by up to 4 metres by 2300.
The ‘backing up’ of groundwater levels from both coast and tidal estuary locations may extend a significant distance inland and affect infrastructure previously constructed above average groundwater levels. The impact of coastal surges will be in addition to the rise in average sea levels. Many of the UKs coastal areas are relatively low lying and therefore susceptible to small changes in groundwater levels. Several UK towns and cities have infrastructure that copes with current groundwater levels but which have never experienced the increases that are now expected, and this can lead to new and unexpected impacts rather than purely incremental changes to previous risks.
The Environment Agency *1 have identified that careful water level management ensures that farmland, properties and infrastructure are protected from water logging and helps to ensure resilience to flooding. As sea levels rise some catchments that can currently be drained to the sea by gravity will need to be pumped with increasing frequency. Catchments that already require the assistance of pumped drainage will require additional pumping capacity. Historic assets such as landfill sites also need to be protected against changing groundwater levels as a result of climate change. For many historic landfills there are few records and much uncertainty about the scale of the legacy problems.
Groundwater flooding in response to sea level change and climate change impacts on recharge has not been systematically assessed hitherto, and is the subject of ongoing work at GeoSmart.
Contact us for further information or see the additional information on our web site in relation to groundwater flood risk mapping, groundwater forecasting and our expert advice on groundwater risk assessment.
*1 Environment Agency, November 2018, Climate Change Impacts. (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/758983/Climate_change_impacts_and_adaptation.pdf).
*2 Met Office, 2018, UKCP18, UK Climate Projections. (https://www.metoffice.gov.uk/research/approach/collaboration/ukcp/index).
*3 Mansour & Hughes, 2018, British Geological Survey, Summary of results for national scale recharge modelling under conditions of predicted climate change. ( http://nora.nerc.ac.uk/id/eprint/521605/ )
*4 Environment Agency, February 2019, Climate Change Allowances. (https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances).
*5 Prudhomme et al., October 2012, Future flows and Groundwater Levels – SC090016, Final Technical Report. (https://webarchive.nationalarchives.gov.uk/20130301204712/http://www.ceh.ac.uk//sci_programmes/Water/Future%20Flows/documents/FutureflowsandGroundwaterLevels_PN9_FinalReport_FinalOct2012.pdf).
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