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Following a period of unsettled weather, December 2020 brought two successive rainfall events to the UK causing flooding in some areas over the Christmas period.
Between 19 and 24 December, south Wales and south-west England experienced flooding due to rain falling over already saturated ground. Storm Bella closely followed between 26 and 30 December, bringing high winds and heavy rain which caused disruption across many parts of England and Wales.
Whilst widespread impact to property was limited, the Environment Agency reported 359 properties flooded in England (EA, 2020), with Bedfordshire and Buckinghamshire the worst affected areas. Official figures are yet to be released for Wales, although a number of areas have experienced property flooding.
19 – 24 December
Early December brought higher than average rainfall in the UK with some eastern areas exceeding their typical December rainfall by the middle of the month. A more intense rainfall period followed which was concentrated in a belt across the country from the Severn Estuary in the west to the Wash in the east.
On 18 December, White Barrow in Devon and Llyn-y-Fan in Wales saw a total rainfall of 138mm and 98mm respectively (Sky News, 2020). On 23 December, parts of Wales such as Trevethin and Cogmoors had rainfall of 65.2mm and 57.2mm. Corby in Northamptonshire had nearly 45mm of rain within 13 hours according to the Environment Agency (BBC, 2020a).
In Gloucestershire and Herefordshire, heavy rainfall caused widespread disruption to the road network, while in Oxfordshire localised surface water flooding occurred in Yarnton, Charlbury, Ramsden and Fritwell (BBC, 2020b). People were forced to leave their homes across Cambridgeshire, including in Ramsey, Alconbury, Earith and Broughton, with surface water affecting property in St Neots.
(Above) Total rainfall recorded from 19 to 24 December 2020 for United Kingdom. (Data source: NASA-JAXA Global Precipitation Measurement (GPM). Image: JBA Risk Management).
Surface water flooding also affected homes and business in Wales including in Cardiff, Newport and the Vale of Glamorgan. More than 750 flood calls requesting evacuation and rescue services were made across the south of Wales and Northamptonshire on Wednesday 23 December.
On Christmas Day, 1,300 properties along the River Great Ouse in Bedfordshire were evacuated as water levels rose (iNews, 2020a). A severe flood warning was issued by the Environment Agency indicating that flooding could potentially threaten lives (iNews, 2020b).
JBA’s flood forecasting system Flood Foresight monitored rainfall conditions across the period and indicated that property was at risk in Stroud, Gloucestershire two days in advance. Similarly, Flood Foresight indicated that properties in Clapham, Bedford were at risk three days before being inundated.
JBA Flood Foresight flood footprint forecasted two days in advance for Stroud, Gloucestershire (top) and flood footprint forecasted three days in advance for Clapham, Bedfordshire (bottom). (Flood Foresight data: JBA)
Following these wet conditions and in anticipation of the arrival of Storm Bella, forecast to bring further rain in the coming days, the Environment Agency erected demountable defences along the River Severn at Ironbridge, Bewdley and Upton upon Severn to protect property.
Storm Bella 26 – 30 December
Storm Bella brought strong winds across the UK, recording the highest gust of 2020 at 106mph at the Needles on the Isle of Wight (Met Office UK, 2020a). Similarly, gusts of 83mph were recorded in Aberdaron, north Wales, and approached 80mph in Dorset.
Over 26 and 27 December, rainfall totals of over 30mm were recorded across parts of Kent and Sussex with 30-50mm falling widely across upland parts of south-west England, Wales, and northern England, and over 50mm across much of western Scotland (Met Office UK, 2020a).
More extreme two-day rainfall totals also included 57mm at Capel Curig, Conwy; 65mm at Shap, Cumbria; 80mm at Tyndrum, Perthshire; and 71mm at Achnagart, Highland.
On 26 December 2020, the Environment Agency issued 78 flood warnings and 99 food alerts across England; in Wales, Natural Resources Wales issued two warnings and five alerts, and in Scotland the Scottish Environment Protection Agency issued seven warnings and 13 alerts.
(Above) Total rainfall recorded from 25 to 31 December 2020 for United Kingdom. (Data source: NASA-JAXA Global precipitation Measurement (GPM) Image: JBA Risk Management)
Localised flooding caused transport disruption and affected properties in Oxfordshire, Gloucestershire, Northamptonshire, Cambridgeshire and Bedfordshire. In Oxfordshire, 40 homes were flooded in Witney and Fewcott (BBC, 2020c). Flooding in Bedfordshire was more widespread due to the River Great Ouse overtopping. Between 40 and 60 properties were reported to be inundated in Bedford with further property flooding in Clapham (BBC, 2020d). Similarly, Northamptonshire Fire Service evacuated 1,000 people from 500 caravans in the Billing Aquadrome and Cogenhoe Mill caravan parks as the River Nene inundated the sites up to 1.5m.
JBA’s Flood Foresight indicated the potential for flooding in advance both along the larger watercourse of the River Great Ouse in Bedford and at a more localised scale in Cirencester.
JBA Flood Foresight flood footprint forecasted three days in advance for Bedford (top) and flood footprint forecasted three days in advance for Cirencester, Gloucestershire (bottom). (Flood Foresight data: JBA)
Initial analysis by the Environment Agency indicates new peak flows were recorded on the River Great Ouse at Newport Pagnell in Buckinghamshire, on the River Nene at Northampton, and on the River Avon at Lilbourne, Northamptonshire, and Rugby, Warwickshire.
Comparison of JBA’s 1-in-75-year 5m flood map (top) and ESA Sentinel-1 imagery on 27 December 2020 (bottom) for Clapham, Bedford (Imagery source: Sentinel Hub EO Browser. Map data: JBA Risk Management).
Outlook
Whilst the impacts from December’s wet weather period were relatively localised, the new peak flow records indicate that river levels were extremely high in some areas and flood defences have been tested. Following an initial period of cold dry January weather, forecasters are expecting further rainfall and snow, particularly in the east of the UK. At this stage, however, further flooding is not expected.
JBA Risk Management offers nationwide 5m resolution flood maps for the UK alongside a first of its kind UK Flood Model and UK Climate Change Flood Model.
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References
BBC. 2020a. Flooding in parts of England and Wales after heavy rain. [online] Available at: https://www.bbc.co.uk/newsround/55429223
BBC. 2020b. Flash flooding in Oxfordshire after torrential downpours. [online] Available at: https://www.bbc.com/news/uk-england-oxfordshire-55424827
BBC. 2020c. Christmas flooding in Witney following heavy rainfall. [online] Available at: https://www.bbc.com/news/uk-england-oxfordshire-55452430
BBC. 2020d. Storm Bella: Gusts of more than 100mph recorded in UK. [online] Available at: https://www.bbc.com/news/uk-55454045
Environment Agency 2020. https://twitter.com/johncurtinEA
iNews. 2020a. Five years on from ‘devastating’ Boxing Day floods, Storm Bella brings a fresh deluge and ‘threat to life’. [online] Available at: https://inews.co.uk/news/environment/storm-bella-five-years-on-devastating-boxing-day-floods-808889
iNews. 2020b. Storm Bella flooding: Thousands in Bedfordshire told to evacuate homes for Boxing Day. [online] Available at: https://inews.co.uk/news/uk/storm-bella-thousand-houses-evacuated-bedfordshire-808676
Met Office UK. 2020a. Storm Bella 26 to 27 December 2020. [online] Available at: https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/weather/learn-about/uk-past-events/interesting/2020/2020_11_storm_bella.pdf
Met Office UK. 2020b. A look back at 2020. [online] Available at: https://www.metoffice.gov.uk/about-us/press-office/news/weather-and-climate/2020/2020-round-up
NASA-JAXA Global precipitation Measurement. 2020. [online] Available at: https://gpm.nasa.gov/node/3325
Sky News. 2020. UK weather: Landslides and flooding after half a month’s rain falls in one day. [online] Available at: https://news.sky.com/story/uk-weather-landslides-and-flooding-after-half-a-months-rain-falls-in-one-day-12168053
