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Storm Christoph caused widespread surface water flooding

Storm Christoph emphasised growing risk of surface water flooding as main river defences successfully protected thousands of homes

With Storm Christoph bringing heavy and at times record-breaking rain it is no surprise that across the country there was significant instances of surface water and river flooding. Following more than 150 flood alerts and a further 30 flood warnings, river defences were augmented to mitigate the imminent risk of serious flooding from rising water levels. At the River Mersey in Didsbury flood basins were just centimetres from being overwhelmed reflecting the importance of these defences in protecting thousands of homes from flooding[1]. This emulated a wider pattern across much of England and Wales as over 27,500 properties were successfully protected from flooding because of temporary flood defences and the closure of flood barriers[2]. As a result, the insured losses from Storm Christoph appear to be quite modest, with PwC estimating losses at between £80 million and £120 million.

Whilst this close call of severe river flooding is alarming and could have had serious consequences, particularly during the pandemic when many vulnerable residents were reluctant to and, in some cases, unable to leave their homes, the ever-present risk of surface water flooding is brought to the forefront once again.

One of the impacts of the widespread surface water flooding led to several road closures with cars becoming stranded and often forcing further flood water into people’s homes as motorists drive through. This can be seen as a lorry attempts to drive through the B676 under the A46 at Six Hills where there is significant surface water flooding on the roads, causing travel chaos.[3]

Greater Manchester declared a major incident in preparation for Storm Christoph and the emergency response to flooding saw more than 200 Council staff, police officers, Red Cross and Greater Manchester Fire and Rescue officers on the ground supporting local communities[4]. Bury braced for significant river flooding as water levels in the River Irwell were steadily rising, however, surface water flooding was seen to impact highways with drains backing up and properties being affected.

At Previsico, we forecasted the widespread surface water flooding throughout the week. In preparation for the storm we used our visualization platform to identify which areas were forecast to be worst hit. Additionally, as the effects of flooding became evident on social media sites and in news outlets, we gathered ~200 observations of floods, satellite images as well as data from our own sensors to assess what types of flooding were most prominent in these areas and to carry out an accuracy assessment of our forecasts. From these observations it was evident that most floods that took place were from surface water and ordinary watercourses. For example, out of the roads and sites flooded from the Storm Christoph event, 70% of the flooding was due to surface water and ordinary watercourses combined. This is highlighted in the forecast validation showcase above where we were able to forecast for different types of flooding in advance and complementing the alerts provided by the Environment Agencies of all home nations of the UK.

 Following Storm Christoph, the Environment Agency released the number of properties that had flooded from the event[5]. The map below combines the locations of properties that flooded with the validation data we sourced from across the UK to provide an overview of where the reported flooding occurred. The team at Previsico are continuing to gather further evidence of flooding from Storm Christoph from other parts of the UK as evidence continues to emerge.

Over 3 million properties in England are at risk of surface water flooding, even more than those at risk from rivers and the sea (2.7 million).  As the climate becomes more volatile and urbanisation continues, surface water flooding is a risk is rising.  However, most of these flood impacts are avoidable with the right early warning systems. We continue to strive to predicts floods which are missed by traditional approaches to support insurers, businesses and governments to help mitigate the growing devastation caused by flooding.