Analysing Storm Desmond using Flood Foresight

Location: Northern England, Southern Scotland, Wales and Northern Ireland

Storm Desmond was a significant storm event that brought severe gales with gusts up to 81mph and record-breaking rainfall to southern Scotland, the north of England, Wales and Ireland from Friday 4 – Sunday 6 December 2015. The worst impacts were felt in Cumbria, England, where the Honister Pass recorded 341.4mm of rainfall in the 24-hours up to 18:00hrs GMT on 5 December, a record rainfall for the UK. The period between 09:00hrs on 4 December to 09:00hrs on 6 December also set a new 48-hour rainfall record with 405mm rainfall recorded at Thirlmere in Cumbria in just 38 hours.

Challenge

To retrospectively apply Flood Foresight using Storm Desmond data to develop future response actions.

The record-breaking rainfall associated with Storm Desmond caused severe disruption, flooding 5,200 homes across Lancashire and Cumbria (Met Office, 2016). Several major roads across the north of England and Scotland were flooded. There was major disruption to rail services in the north of England and a landslide closed a section of the West Coast Mainline rail tracks between Preston and Carlisle. 43,000 homes across north-east England were left without power and on 5 December 61,000 homes in Lancaster lost power when the electrical substation was flooded (Met Office, 2016).

Solution

Flood Forecast inundation and depth forecast on the 3 December for the 5 December 2015.

Real-time and forecast flood maps, before, during and after flood events.

There are three modules within Flood Foresight; Rainfall Screening, Flood Forecasting and Flood Monitoring. Flood Forecasting provides combined flood extent and depth maps for fluvial flooding up to 10 days in advance for the whole of the UK and Ireland. The maps are driven by streamflow forecasting models and are updated daily. The worst affected areas were picked up three days in advance. Locations such as Appleby, Carlisle, Cockermouth, Kendal and Keswick showed a clear and persistent flooding signal, and the forecasted timing of the event corresponded well with the peak flow measured by river gauges. However, difficulty forecasting the extreme amount of rainfall in combination with high relief topography in and around the Lake District meant that the impacts on the ground were challenging to forecast accurately.

Rainfall screening Maximum Return Period (RP) forecast on the 3 December for the 5 December 2015.

Rainfall Screening a forecast rainfall amount and associated return period for up to six days in advance.

Flood Monitoring inundation and depth footprint on 5 December 2015 18:00 UTC. The footprint shown is based on river gauges from England and Wales.

Values are updated daily and can be visualised either on a grid view or average by river catchment. The possibility of an event in northwest England and west Wales was identified in the five-day forecast. As you can see in the image, the forecasts on 2 December for the 5 December (three-day lead time) showed affected catchments in northwest England, southern Scotland and Ireland – mostly correctly identified. However, with uncertainty and changes in predicted rainfall, the affected catchments in central Scotland were not persistent on the forecasts. The same day forecast accumulations for 5 December highlights an exceptional event for central Scotland and a widespread major event for much of southern Scotland, northwest England and a large portion of Ireland. The affected catchments were correctly identified, although the magnitudes of the forecast rainfall return periods were larger than observed.

Results from this module rely on the accuracy of weather forecasts which cannot be guaranteed, but the use of the rainfall return period alongside forecast rainfall amounts can be used to successfully identify areas likely to experience flooding. Rainfall Screening Maximum Return Period (RP) forecast on the 3 December for the 5 December 2015 (two- days lead-time).

Flood Monitoring observed streamflow data from a network of river gauges, updated every three hours, to produce real-time flood extent and depth maps.

Flood Foresight showed that Cumbria was the most affected region in terms of area flooded, followed by some areas in Lancashire and North Yorkshire. Flooding started to show on the 5 December 03:00 UTC footprint, but still with most of the flooding levels below the level of standard of protection of the defences. On the 06:00 UTC footprint, more flooding is displayed where many of the defences exceeded their standard protection level at many locations. The peak flood extent was maintained during the evening of 5 and 6 December, with flooding beginning to recede in the early hours of 7 December for most areas. The image shows flood extents and depths as Storm Desmond made its way across the UK and Ireland.

Benefit

Flood Foresight provided a comprehensive overview of the Storm Desmond event through all three modules.

  • An operational rainfall and flood forecasting and monitoring system, currently operational for UK and Ireland and with pilot demos set up for India, Bangladesh and Southeast Europe
  • Provides consistent methodologies and data for (trans)national flood inundation and impact analysis
  • Provides a framework for flood forecasting to make use of the best available data: flood maps, gauge data, forecast data, EO data
  • Opportunities for new data providers and commercial evaluations

 

Want to know more?

Email John Bevington for more information about Flood Foresight, JBA’s flood forecasting and real-time flood monitoring system visit our Flood Foresight webpage.



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