Forecasting a change in flood risk management

In March 2020, JBA hosted a Lloyd’s breakfast briefing which examined the recent UK flood events. Together with offering some unique insights and observations, we reviewed the opportunities to use tools like flood forecasting, to explore the value that the additional information can bring to risk managers.

Forecasting flood events – crystal ball?

In terms of forecasting catastrophic events the reinsurance industry has become familiar with looking at forecast hurricane tracks for upcoming events. More recently, we’ve become familiar with forecast loss estimates for those events and, to some degree, the uncertainty associated with them. Even with the advances in numerical weather models however, forecasting loss for flood events remains challenging for several reasons.

The impact of flooding is highly dependent on the intensity and duration of the rainfall combined with the antecedent conditions. Short sharp heavy rainfall over two hours, for example, might cause acute surface water flooding in the headwaters of a catchment. The same volume of rainfall over a two-day duration might cause river flooding further downstream, and the same rainfall spread over five days might cause little or no problem at all. Furthermore, the antecedent conditions or how already saturated a catchment is, can either mitigate or exacerbate the amount of water that can contribute to flooding.

Finally, and critically for portfolio managers, exactly where the water flows and how deep it becomes is dependent on the interplay between localised topography and flood mitigation measures. These factors together with the type of building means flooding can devastate one property while causing little or no damage to an adjacent one.

Forecasting does not offer a crystal ball view into the future, but the tools are sophisticated enough to add real value. JBA has developed a forecasting tool called Flood Foresight to consider all the complicated factors related to forecasting flood and help our clients overcome the challenges. Flood Foresight demonstrates the potential for flooding over the ten-day period leading up to an event and monitors flood throughout the duration of an event using telemetered data in real time. We use this tool along with the JBA rapid post-event response assessment and flood footprints to analyse events for our clients.

But how accurate were our forecasts for the recent UK flood events? We’ve examined our data to find out.

November 2019 Floods: 7–8 November 2019

The November 2019 event followed a wet autumn period. A heavy and persistent band of rain moved down across the country through the first days of November. When it arrived at South Yorkshire it remained stationary for 24 hours between 7-8 November. As a result, 50 to 100mm – the average monthly rainfall – fell in just 24 hours, with Doncaster receiving 81mm in 24 hours and Sheffield 87mm in the same period.

In terms of forecast for this event, Figure 1 is a screenshot from our Flood Foresight system. Here the areas in blue indicate the extent and depth of forecast flooding. This forecast, generated on 6 November for 9 November uses forecast streamflow information from SMHI’s EHYPE model and considers the antecedent conditions, protection offered by flood defences and resultant flood hazard from JBA’s database. The areas forecast as ‘at risk to flooding’ correspond very well to the areas further downstream of the most intense rainfall.

Figure 1: Flood Foresight 6 November 2019 – three-day forecast.

Looking in more detail, one of the most memorable aspects of the Nov ’19 event was the news bulletin that reported people ‘trapped’ in the Meadowhall shopping centre in Sheffield. If we look at Figure 2, the map on the left shows our monitored flood footprint using live telemetered data on 7 November 2019 as compared to the image on the right that shows the actual conditions on the ground. On this occasion the forecasts proved to be on point.

Figure 2: Flooding monitored by Flood Foresight, and observed flooding at Meadowhall, Sheffield, 7 November 2019.

Storm Ciara: 8-9 February 2020

Storm Ciara came through on 8-9 February 2020 with gusts of wind from 70-87mph inland and 97mph recorded on the coast. Record-breaking rainfall was recorded at Honister Pass (177mm in 24hrs) and Calder Valley (100mm in 12-18hrs). The latter is very similar to records set on Boxing Day 2015 during Storm Desmond.

Storm Ciara also brought new peak flows in several upland areas across West and North Yorkshire. The short sharp rainfall impacted these upland areas with a rapid rise in river levels causing local flooding; although further downstream on the Ouse, the Aire and Don, the flows were less extreme.

In terms of our forecasts, from 5 February, five days in advance, we began forecasting flooding in Keswick in the Lake District. The two-day forecast indicated widespread flooding across the north of England.

Figure 3: Flood Foresight 9 February 2020 – one day forecast indicating areas likely to be affected across the north of England.

One day in advance, Flood Foresight picked up issues in the Calder Valley but it’s fair to say the rain had come earlier than expected and we were already starting to see the impact of flooding in the area by 9 February.

Figure 4: Flood Foresight forecast for the Calder Valley from 9 February for 10 February.

So what happened with the forecasting here? This area is a very small upland catchment. If the rain falls on the other side of the hill, only a couple of kilometres away in the adjacent valley, there’s no problem. However, two kilometres in the other direction and we experience the significant flooding that we witnessed during this event. At this sort of scale, you start to see some limitations in the forecast.

At locations further downstream where there is a larger catchment, such as Appleby-in-Westmorland in Cumbria, the area is less affected by short sharp rainfall events and is therefore easier to forecast. Flood Foresight captured flooding here two days prior to the event (Figure 5).

Figure 5: Appleby-in-Westmorland, 9 February 2020.

Storm Dennis: 15-17 February 2020

Storm Dennis followed one week after Storm Ciara and brought strong winds and sustained heavy rainfall. This rainfall fell across ground that was already saturated with accumulated rainfall from Storm Ciara. Rainfall levels were recorded to be between 180–205% of average in Herefordshire, 130-180% of average across South Wales and 140-210% of average across the Calder Valley.

The five-day forecast shows the potential for significant flooding across much of central England with the most intensely affected areas forecast to be south Wales, the Severn and Avon catchments and the Trent catchment.

Figure 6: Flood Foresight forecast Storm Dennis, five days in advance.

Looking in more detail across the Welsh Valleys, there is a strong forecast for this area with all the major areas affected forecast at least one day in advance.

Figure 7: Flood Foresight forecast for the Welsh Valleys, one day in advance.

In Figures 8 and 9, we show monitored flood footprints on the left and snapshots from the ground on the right. Figure 8 captures the scene in Pontypridd in Wales and Figure 9 shows the scene just over the border into England in Worcester.

Figure 8: Pontypridd, 16 February 2020.

Figure 9: Worcester, 17 February 2020.

Overall, our forecasts for Storm Dennis captured the areas worst hit by flooding. One aspect that is difficult for us to capture is the deployment of temporary flood defences. Given the clear signal ahead of Storm Dennis, the Environment Agency was able to deploy temporary barriers (up to 6 km) and engage all gates, barriers and demountable defences. Our flood footprints in these areas overestimate the true extent of flooding where the water is being held back by flood defences. Interestingly, due to the sustained high flows during this event, temporary defences in several locations were overwhelmed with resultant damage to nearby property. 

Storm Jorge: 17 to end of February 2020

Finally, hot on the heels of Storm Dennis, Storm Jorge brought further rainfall towards the end of February. Whilst Jorge did not bring the quantity or intensity of rainfall we’d seen over previous weeks, once again the rain fell on already saturated ground and was enough to keep river levels high. Figure 11 shows the forecast flooding across the downstream section of the River Aire which resulted in properties being affected in Snaith and East Cowick.

Figure 11: Flooding at the confluence of the Rivers Ouse, Aire and Don, 25 February 2020.

As we can see from these recent events, flooding remains an extremely complex peril, but the capability of the models to forecast realistic outcomes is improving.

Forecast footprints now present a reliable warning when big events are approaching. Furthermore, with the seamless integration of our new Global Flood Model and forecasting tool, the ability to calculate potential loss to your portfolio in advance is also a reality. Monitoring of ongoing events can also be particularly helpful in triaging claims and directing response teams to the worst-hit areas during the event. Time-stamped footprints can also be produced post-event to help determine best reinsurance recovery.

Crucially, flood forecasting may start to allow the mitigation of flood losses by giving extra time to set up emergency contingency plans, have barriers put in place and make arrangements for contractors to be on standby for remedial work. Using this information in a timely fashion is a valuable tool for mitigating flood losses in the future.
This blog is a summary of a presentation given by Ian Millinship at Lloyd’s of London on 3 March 2020. If you want to learn more or talk to somebody about JBA’s Flood Foresight and Event Response service please get in touch.