Groundwater flooding represents a significant part of the overall flood risk in the UK, but is widely missed as it results from subsurface flow processes that are not very well understood and hard to observe. This brief review of last Winter brings a useful illustration of the groundwater hazard. Whilst the worst impacts were avoided this time, it also provides a reminder that climate change is bringing such events to the UK a lot more frequently, so if you are an asset manager or insurer it will be timely to review your risk management strategy.
Paul Ellis, Managing Director
GeoSmart Information provide specialist mapping and forecasting services to quantify and track groundwater risks. GeoSmart’s daily forecasts are helping the Flood Forecasting Centre as well as water companies, insurers and infrastructure operators bring better visibility and early warning to upcoming events. Insights from our work last Winter are highlighted in this post.
Going into Autumn 2019, groundwater levels were widely below average. However, in line with climate change forecasts, the UK saw extreme rainfall and high rates of recharge, bringing aquifer levels rapidly up, to be primed and ready for flooding. Winter 2019 to 2020 saw some localised groundwater flooding causing significant damage, but the impacts were small scale. Conditions however were set for a much wider impact, and as you will see, this was very narrowly avoided.
Groundwater flooding: A national near miss in 2020 and the sign of things to come
Winter 2019 to 2020 saw groundwater flood alerts issued across diverse areas of the UK including North Lincolnshire, Dorset, Wiltshire, Oxfordshire, Hampshire, Kent, Surrey and South London. In an early start to the season, groundwater flooding was experienced around Barton on Humber from 20th November 2019, including residential flooding, road closures and emergency sewage discharge to water courses.
The focus of groundwater flood risk broadened into late January and February with storms Brendon, Ciara and Dennis bringing exceptional rainfall to many parts of the UK, recharging groundwater levels in bedrock aquifers to flood threshold levels in many locations across Southern England. It is only the extreme low rainfall in March and April 2020 which allowed groundwater levels to stabilise, preventing a potential repeat of the regional scale groundwater flood events of the winter 2013 to 2014, or worse still, a national scale groundwater event.
The ‘near miss’ on this occasion could be indicative of future climate change scenarios, with more intense groundwater recharge seasons resulting in flashy responses in groundwater levels according to a recent study by the British Geological Survey (2018*1). Geosmart offer to combine groundwater flood mapping (GW5 data) and forecasting systems to provide a real time simulation of risk as the groundwater season unfolds and for future climate impacts. This will support the aims of the Bank of England PRA (2019)*2 for the insurance sector to ensure that asset managers have better tools to understand the impact of climate change on their decisions.
We present a review of the development of groundwater flood risk over the winter of 2019-20 using the insights gained through our ongoing mapping and daily operation of our groundwater forecasting service in support of the national flood forecast centre and the daily flood guidance statement.
Figure 1.a) Geosmart Groundwater Forecast -hydrographs and flooding threshold levels winter 2019 to 2020
b) Rainfall amount % of 1981 to 2010 average (from MET Office website 2020)
At the peak in February the Environment Agency had in excess of 30 warnings in place for groundwater flooding and the national daily flood guidance statement highlighted groundwater flood risk on a regular basis. Geosmart maintains a database of historic flooding which is updated each year for calibration purposes. We consider that the full economic cost of prolonged groundwater flooding is not yet fully appreciated by the regulatory bodies or the insurance industry, partly due to the difficulty in accurate identification of the actual flood source as groundwater. Analyses of insurance claims from the past winter is likely to yield further insight into the link between the temporal and spatial variation in groundwater flood risk alerts and flood claims data.
The strong groundwater response observed in relation to storm Dennis will have added to the damages incurred from this named event.
Elevated groundwater levels led to an increase in the baseflow to rivers and drainage networks, increasing the risk from fluvial and surface water sources. High river levels created a corresponding rise in the shallow water tables within adjacent river gravels, leading to the flooding of buried services, basements, and at the surface, for example during the Flooding of the River Severn at Shrewsbury in February 2020. The evolution of groundwater flood risk over the winter is illustrated in Figure 1 using three borehole hydrographs from different parts of the UK. Variation from the monthly long-term average rainfall, produced by the MET office, is also shown to indicate the spatial and temporal variation in the antecedent conditions that drive groundwater flooding. Geosmart’s national groundwater flood risk mapping highlighted the areas of most concern underlain by major aquifers such as the Chalk, the Cotswold Limestone and Lincolnshire Limestone. These aquifers extend across a wide area of England, from the south coast to the north east. Areas underlain by sandstone tend to have a more muted rise in groundwater level over several years, but can still give rise to flooding, particularly at spring lines along the boundary with less permeable underlying material. Also at risk are sites on shallow perched aquifers and in river valleys where groundwater levels in permeable sand and gravel rise in response to high river and tidal levels, potentially by-passing the above ground flood defences.
Sewer flooding was a significant issue this winter and is often related to groundwater ingress. This reduces the capacity of the treatment works and the sewer to carry foul effluent, potentially resulting in discharges to water courses. According to data obtained by the Guardian (2020*3), water companies in England discharged raw sewage into rivers on more than 200,000 occasions in 2019. The paper also references recent scientific research which has raised concerns that Covid-19 can be carried via sewage discharges, potentially increasing the consequences of sewer pollution events.
In early Autumn groundwater levels in parts of northern England were already elevated and showing an early start to the groundwater recharge season. Figure 1 shows that groundwater levels for Leasingham borehole within the Jurassic limestone were high in October (often when groundwater levels are lowest) and exceeded previous maximum levels in November due to the high preceding rainfall recharge. The Geosmart forecast service issued the first alerts on the 7th October providing the opportunity for flood managers to prepare. Groundwater and sewer flooding was reported in North Lincolnshire on 20th November in New Holland, Barrow upon Haven and Barton upon Humber. North Lincolnshire Council closed sections of B1207 near Broughton due to standing water (Thursday 21st Nov).
The impact of groundwater flooding further south from the Chalk began to develop over Christmas when the railway line between Brighton and Haywards Heath was closed due to flooding of Patcham Tunnel, and the A35 and A354 roads were closed in Dorset due to flooding in areas identified as being at risk of groundwater flooding. Storm Brendan on 13th January 2020 brought heavy rain which continued to drive the seasonal rise in groundwater levels observed in the Chalk at both Woldingham and Kingston Russell boreholes (Figure 1).
February 2020 was the wettest on record. Rainfall from Storm Ciara on 8th February was heaviest in areas less vulnerable to groundwater flooding. Storm Dennis on 15th February led to heavy rainfall over much of the UK including southern England. Groundwater responses to this named storm event appear pronounced, as a result of the already saturated ground. Many of the boreholes included in the Geosmart forecast recorded rapid rises in groundwater levels of approximately five meters over a few days and a distinct response to storm Dennis can be seen as peaks in all three hydrographs in Figure 1. A fast rise in the water table over a short time, would have caused a corresponding increase in discharge of groundwater to the surface drainage systems, potentially increasing the flood extent and duration associated with the main storm event.
The Woldingham borehole in the Chalk (Figure 1) is a good indicator of flows for Caterham Bourne, the Wandle and groundwater related flooding in South London, which was very significant in the winter of 2013/2014. The Caterham Bourne in Croydon began flowing at the end of February and preparations were made for potential flooding by Tandridge District Council on 6th March 2020.
The Environment Agency Groundwater Flood Alert for South East London was in place until the start of April. However, the dry March and April significantly reduced the impact from groundwater flooding which might otherwise have occurred. This highlights the spatial and temporal variations of groundwater flood risk across England and the need for real time monitoring of groundwater levels to give decision makers the key information to manage a response to the costly economic impact of groundwater flooding.
*1 Mansour & Hughes, 2018, British Geological Survey, Summary of results for national scale recharge modelling under conditions of predicted climate change.
*2 Bank of England, May 2019. Prudential Regulation Authority. A framework for assessing financial impacts of physical climate change. A practitioner’s aide for the general insurance sector
*3 The Guardian Newspaper, July 2020, Exclusive: Water firms discharged raw sewage into England’s rivers 200,000 times in 2019.