On 3 December 2015, Storm Desmond formed, bringing record-breaking rainfall and flooding to the UK. The worst impacts were felt in Cumbria, although 19,000 homes were flooded across northern England as a whole and insured losses were estimated at £604m at the time of writing (PERILS, 2016).
With increased flooding expected across most of the country under future climate scenarios, it’s more important than ever to consider how we, as an industry, will respond to the next flood event of this scale in the UK.
One of the key challenges during and in the immediate aftermath of an event like Desmond is to identify the areas impacted and understand the extent of the damage. Despite recent advances in earth observation, there are still limitations in satellite coverage due to their revisit time and cloud cover can obscure the view for some satellite sensors (unfortunately a common occurrence when it has been raining). This means that data is intermittently available and can provide an inconsistent and incomplete view of flooding.
An alternative approach is to use forecast rainfall and river flows, in combination with real-time observations, to understand the potential for flooding and to model likely flood extents and depth before, during and after an event. This is the approach used by Flood Foresight® and, in this blog, we will take a look at some of the insights Flood Foresight would have provided for Storm Desmond.
Rainfall Screening
There are three modules within Flood Foresight, the first of which is the Rainfall Screening Module. This module provides a forecast rainfall amount and associated return period for up to 6 days in advance. Values are updated daily and can be visualised either on a grid view or average by river catchment.
So, what did this module show in the days leading up to Storm Desmond?
- The possibility of an event in northwest England and west Wales was identified in the 5-day forecast.
- As seen in figure 1, in forecasts from 2 December (3-day lead time) for 5 December, affected catchments in northwest England, southern Scotland and Ireland are 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 highlight 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 rainfall return period alongside forecast rainfall amount can be used to successfully identify areas likely to experience flooding.
Figure 1: Rainfall Screening forecast on 2 December for 5 December 2015 (3-day lead time) showing the forecast return period for each identified catchment.
Flood Forecasting
The Flood Forecasting Module provides flood extent and depth maps for river (fluvial) flooding up to 10 days in advance. The maps are driven by streamflow forecasting models and are updated daily.
Did this module correctly identify flooding?
- The worst affected areas were picked up three days in advance. Locations such as Appleby, Carlisle, Cockermouth, Kendal and Keswick showed a clear 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.
Figure 2: Flood Forecast flood extent and depth forecast for Kendal on 3 December for 5 December 2015 (2-day lead time).
Flood Monitoring
The Flood Monitoring Module uses observed streamflow data from a network of river gauges, updated every three hours, to produce real-time flood extent and depth maps.
How did this module identify flooding?
- 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 began to show on the 5 December 03:00 UTC footprint but most of the flooding levels were below the standard protection of the defences. On the 06:00 UTC footprint, more flooding is displayed with defences in many locations exceeding their standard protection level.
- 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. Figure 3 shows flood extents and depths on the evening of 5 December in Kendal.
Figure 3: Flood Monitoring extent and depth footprint for Kendal on 5 December 2015 18:00 UTC.
Overall, although rainfall amounts were challenging to forecast accurately due to uncertainty in weather forecasts and the record amounts of orographic rainfall to upland areas, Flood Foresight gave a comprehensive overview of Storm Desmond throughout all three modules. This type of early view into expected flooding has a range of benefits for insurance companies – from outreach to policy holders, giving them the opportunity to implement flood mitigation measures, to prioritisation of claims resources – all enabling better customer service and a minimisation of the costs.
As well as visualising hazard data through a web application, Flood Foresight data can be downloaded or integrated directly into risk management platforms to evaluate alongside exposure data and claims, estimating losses throughout the lifecycle of an event.
Producing flood footprints in the aftermath of an event has traditionally been a time-consuming process, dependent on manual steps and often limited observational data. Rainfall screening, flood forecasting using sophisticated streamflow modelling and near real-time monitoring provide a valuable new set of tools to understanding flood events as they evolve. And, importantly, access to these early insights into flooding and timely availability of reliable flood footprints can help insurers and reinsurers respond more effectively to the next large-scale event.
For more information on Flood Foresight® , get in touch or read our Flood Foresight Executive Briefing.
