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Flash Flooding

In the past year there have been a number of occasions where towns and villages have been affected by flash flooding due to very localised and intense rainfall. This unpredictable nature of flash flooding events can lead to them having damaging consequences.

Flash floods can happen very quickly, meaning that weather monitoring bodies such as the Met Office and Environment Agency may have very little or no prior warning. As a result, places that are in a flood risk area may not receive flood alerts or warnings before the flood happens and residents will have less time to prepare for it. If the place affected is not in a flood risk area, flooding conditions would not be monitored by the Environment Agency and residents in these areas would not receive flood alerts or warnings either.

It is important to remember that although you are not in a flood risk area it doesn’t mean that you won’t be affected by flooding from other sources, which can also be difficult to predict. Flash flooding is often associated with surface water flooding as rainfall can overwhelm drainage systems, and as this is not always related to rivers it is not directly monitored by Environment Agency.

Other weather conditions and potential warnings can still be monitored by individuals, for example by visiting the Met Office website or checking the level of local river monitoring stations. Again a problem is presented here as not all areas have river level monitoring stations, and so high abnormalities in river levels could go unnoticed meaning even less chance of residents having warning of a flash flood event. This can be a particular issue for small watercourses such as becks where high levels of localised rainfall can mean they are quickly overwhelmed.

 

Here are some examples of this type of flooding that have happened across England:

Millom, Cumbria

The flooding event that happened in Millom in September 2017 was a shock to all residents as the town had never experienced a flood event before. Heavy and localised rainfall hit the town of Millom in south west Cumbria on 30thSeptember causing surface water flooding that affected over 1,000 residents in 317 properties. This town was isolated by the incident due to having only one road in and one road out. However, the community came together to address the issue and are now more prepared and working towards a community resilience plan.

 

Coverack, Cornwall

In July 2017, the coastal village of Coverack in Cornwall was subject to localised thunderstorms followed by sudden, heavy rainfall, causing flash flooding that affected round 50 properties. The intense weather had such an effect due to rapid rainwater runoff into the village from higher ground, causing drains and becks to be inundated with water and resulting in surface water flooding. Coverack being situated in a small river basin adds to the likelihood of localised and intense rainfall resulting in flash flooding, as the area has a smaller capacity to hold the water.

 

Galgate, Lancashire

The village of Galgate in Lancaster was badly affected by torrential rain that swept through the North West in November 2017, causing the River Conder to breach it banks and flood around 100 homes. The rain level was the highest recorded in more than 50 years at the Hazelrigg, Lancaster University weather monitoring station, with 73.6mm falling in 24 hours. Despite neighbouring Lancaster being hit badly by flooding during Storm Desmond in 2015, for most residents the November 2017 flooding event was the first in living memory.

 

POYNTON, CHESHIRE

On the 31st July 2019, heavy and intense rainfall hit the area, overwhelming drains which caused Poynton Brook to overtop. Homes, businesses and infrastructure were flooded and damaged by the floodwater. A major incident was declared and many homes were evacuated as nearly a month’s worth of rain (~2 inches) fell in just 24 hours. Poynton Fire Station responded to more than 20 incidents over a four hour stretch including rescuing at least 11 people from flood water. Roads were closed, including the new £290million A555 dual carriageway, where two cars were abandoned and 800,000 litres of water had to be pumped away.

 

North Yorkshire

In July 2019, North Yorkshire was badly affected by flash flooding with a month’s worth of rain falling in just 4 hours. Damage to homes and businesses was widespread, with reports of pets and livestock drowning as a result. The floodwater also impacted local infrastructure, leading to the collapse of Grinton Moor Bridge and the road becoming completely impassable.

 

As you can see from the examples given above, flash flooding is not seasonal and can occur at any time of year. A common misconception is that flooding only occurs in winter, this is not the case and it can even happen in the drier, warmer conditions of summer. During summer, the amount of evaporation from the ground is larger than the amount of rainfall, this can lead to a water deficit in the ground. As a result, the ground can become harder, more compacted and drier and it becomes more difficult for water to infiltrate and percolate through the soil, leading to increased runoff and a greater chance of flooding. Certain locations can also have an increased chance of flash flooding occurring due to the size and shape of the river basin, larger river basins can collect more rainwater and a rounded basin means water can percolate and flow at the same time, increasing the amount of water reaching the river channel and increasing discharge.

One well known example of a flash flood event occurring in summer is that of Boscastle in the August of 2004, when 58 properties flooded after an unusually high amount of rainfall fell over eight hours. A small, steep river flows through the village and heavy, thundery showers occurred at the head of the river. The river and hillside was unable to contain the excess water and water gushed down the valley and into the village having devastating consequences.

These three events from 2017 are clear examples of why it is important for all properties to consider having a flood plan or an emergency plan in place. They show that such a localised weather event can have a large effect and it is not just people in flood risk areas that could be affected.

 

 

Sources: JBA Risk Management, Met Office, .gov.uk

 

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