Dive into this article as we uncover the impact climate change has on flooding, addressing the urgent need to think differently when assessing this subterranean hazard and the innovative approaches being taken to protect our cities and lives from the unseen dangers that lie beneath the surface.
Understanding Groundwater Flooding
Flooding will generally bring to mind images of rivers overtopping their banks; large waves and high tides exceeding coastal defences; reservoirs breaching their impoundment; and heavy rainfall overwhelming drainage infrastructure. These occurrences can be rapid, dramatic and will often make headline news. The processes behind such flood events can be observed, measured and predicted. These sources have also been studied and mapped extensively, with ever-increasing levels of detail, allowing emergency planners, local authorities, and developers to appraise risk and make appropriate decisions. However, one source of flood risk lies hidden beneath our feet and, as a result, is much less understood – groundwater flooding.
Causes and Characteristics of Groundwater Flooding
Groundwater flooding occurs when the water table rises above ground elevations or rises to depths containing basement-level development. It is most likely to happen in low-lying areas underlain by permeable geology. This is most common on regional scale chalk aquifers, but there may also be a risk on sandstone and limestone aquifers or on thick deposits of sands and gravels such as that in a river valley. Rising groundwater can be caused by long-duration rainfall events that recharge aquifers beyond their capacity, or it may be caused by high river levels or tides, driving water through the ground due to a difference in hydraulic pressure. This means flooding can bypass traditional flood defences and rise directly through floors or underground rooms. Also, groundwater may not appear where you would expect it to, such as valley bottoms; it can also emerge on hillsides. Groundwater flooding can also be much slower to occur than surface flooding. It will usually happen days or weeks after heavy or prolonged rainfall and may last weeks or even months.
Impacts and Challenges of High Groundwater Levels
The potential effect of high groundwater levels is not limited to the observable above-ground or basement-level flooding. Shallow groundwater can impede the drainage of septic tanks and soakaways, undermine/ destabilise footings and foundations, and damage below-ground infrastructure due to water ingress and buoyancy. High groundwater can raise river levels, surcharging drainage outfalls and reducing a river’s capacity for a fluvial flood event. It can also waterlog land leading to a larger than expected flooding response from storm events.
The Lead Local Flood Authorities are responsible for managing groundwater flooding risk. However, aquifers can be of a regional scale, spanning many authorities, hindering a coordinated appraisal and management of risk. An Environment Agency study found limited evidence of groundwater flood risk being considered in spatial planning. The Environment Agency, which has a strategic overview of all sources of flooding, offers a groundwater alert and warning service, but only in areas that have historically experienced groundwater flooding. Historical incidents of groundwater flood events may have been masked by, or perceived to be caused by, surface water or fluvial events, possibly hiding the true extent of the groundwater issue.
Evaluating National Groundwater Flooding Susceptibility Maps
The British Geological Survey (BGS) has prepared national “susceptibility of groundwater flooding” maps that attempt to identify areas with potential for groundwater flooding. However, this data is based on broadscale geological and hydrogeological data that may not reflect local topographical and geological intricacies or drainage infrastructure.
Limitations of Existing Data and Modeling Approaches
Several private consultancies have tried to improve upon the BGS data by considering generalised topography and the development of in-house models. But the proprietary models are not published to allow for scrutiny, and they still relate back to the same national scale datasets1. Importantly, none of these data accurately quantify the probability of flooding, the rate and duration of flooding, nor the consequence of the flooding – all factors required to identify and manage risk robustly. And these are just the limitations when assessing the present-day flood risk; things get more complicated when future climate change is considered.
The Influence of Climate Change on Groundwater Flooding
Climate change in the UK is predicted to lead to milder, wetter winters and hotter, drier summers. Rising temperatures and variation in rainfall patterns will change how aquifers recharge. The recharge season could become shorter, and as aquifers are recharged more effectively by prolonged rainfall, a long-term decline in groundwater level may be expected. However, the winter recharge period may be much wetter, leading to larger and more rapid fluctuations in groundwater levels and, consequently, a potential increase in the severity of groundwater flooding events2.
Complexities of Predicting Future Changes in Aquifer Levels
Predicting the future change in aquifer water levels is also complicated by groundwater rebound, where artificially depressed groundwater levels are returning to natural levels due to a decrease in abstraction for industrial activities in major urban centres such as London, Birmingham, Nottingham, and Liverpool3. Conversely, it could be expected that greater abstractions could be made in the future due to increased demand for clean and reliable water supplies.
Interplay Between Climate Change and High River Flows
Climate change is also predicted to bring with it an increase in the probability and magnitude of high river flows4. Where the underlying aquafer is very conductive, higher river levels brought about by climate change will directly impact local groundwater levels. This may extend the influence of a fluvial flood event to affect below-ground infrastructure and development beyond the floodplain. It may lead to traditional flood defences being bypassed entirely, as water levels are driven through the ground by the hydraulic head.
Coastal Impacts: Rising Sea Levels and Groundwater
Sea levels around the UK coast are predicted to rise by between 0.4m and 1m by 2100. The increase will generate a corresponding rise in groundwater levels in coastal aquifers5. The potential effects of this could extend well beyond the coastal floodplain due to the low-lying coastal regions in which some of the largest UK cities are located. In addition to rising groundwater levels, coastal aquifers may be subject to saline ingress affecting the quality of this important water resource.
Uncertainties and Challenges in Assessing Groundwater Flooding Risk
Nationally, much focus is given to the potential direct impacts of future sea level rise and the increased risk from river and surface water flooding due to climate change. However, the uncertainties in the current understanding of groundwater processes are compounded by uncertainties in the future effects of climate change and the future demand on water resources, making assessing the probability, consequence, and risk of groundwater flooding difficult.
BWB’s Expertise
BWB has recognised this gap in industry understanding, which places doubt on the adequacy of current flood predictions, thereby putting doubt on the long-term risk to communities, properties and assets. This is why BWB have combined the expertise of our flood risk and ground investigation teams to ensure that our assessments are holistic and that the potential interactions between groundwater and surface water are taken into account when considering the effects of Climate Change and the Resilience and Adaptation of our designs.
For more information, please contact Iqbal Rassool.
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- Environment Agency, 2021. Rapid evidence assessment and overview of groundwater flood risk management in England. FRS19217.
- Mansour & Hughes, 2018, British Geological Survey, Summary of results for national scale recharge modelling under conditions of predicted climate change.
- Dr Michael jones, UK groundwater Forum. Undated. Rising Groundwater in Central London, http://www.groundwateruk.org/Rising_Groundwater_in_Central_London.aspx
- Environment Agency, May 2022, Climate Change Allowances. (https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances).
- 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)
- Rotzoll, K., Fletcher, C. Assessment of groundwater inundation as a consequence of sea-level rise. Nature Clim Change 3, 477–481 (2013). https://doi.org/10.1038/nclimate1725