What are the reasons for flash flooding and why are flash floods so dangerous in the US?


The most lethal floods in the USA and worldwide are flash floods as they combine speed with and incredible destructive power.

We have seen flash floods in New York and Virginia this month causing millions of dollars damage but it can happen anywhere and at any time..

What causes flash flooding?

Flash flooding can come from torrential downpours or from severely overflowing rivers.

A sudden and very powerful deluge of rain which some would describe as a cloudburst can drop thousands of tonnes of water on an area in just minutes and, basically, the ground it lands on simply can’t cope with that huge amount of water suddenly falling on it.

Flash flooding – which can often happen during thunderstorms - is often caused when ground is either bone hard dry or already saturated with rain so the millions of gallons of water falling on it can’t soak in.

It’s made worse by the fact that every year more green spaces are built over to provide more homes and parking spaces for cars.

Add in the possibility that drains and gullies have not been cleared out and severe flooding can quickly escalate into a disaster.

Although officially defined as floods that can happen within six hours of heavy rain, flash floods usually happen within minutes of a torrential downpour, giving people very little time to react to get flood defences in place.

The National Weather Center in the USA says people need to be always prepared for flooding as flash floods happen so quickly.

It states: “Sometimes floods develop slowly and forecasters can anticipate where a flood will happen. Often times flash floods can occur within minutes and sometimes without any sign of rain. Being prepared can save your life and give you peace of mind.

“If you have access to sandbags or other materials use them to protect your home from flood waters if you have sufficient time to do so. Filling sandbags can take more time than you think.”

This is why many Americans now use FloodSax ‘sandless’ sandbags which are a flexible alternative to traditional sandbags that are, unlike traditional sandbags, space-saving to store and quick and easy to deploy.

They are vacuum-packed and when unwrapped resemble a large pillowcase but when FloodSax are immersed in water they absorb 20 litres which transforms them from being as light as a pillowcase to being more effective than traditional sandbags in around five minutes.

In their dry state they are super-absorbent yet thin with a large surface area so are highly effective at soaking up drips, leaks, spills and floods inside homes and businesses, especially in hard-to-reach places such as beneath pipes, radiators, sinks and boilers.

Almost 3 million have now been sold worldwide.

The best description of flooding comes from the world-famous Met Office in the UK which states: “Flash flooding happens when rain falls so fast that the underlying ground cannot cope or drain it away fast enough. Roads can become like rivers and if there is a lot of water it can flood buildings and carry cars away. So, if the rain is falling too fast for the ground or drains to cope there is a risk of flash flooding.”

The Met Office also reveals how rivers can also cause flash flooding.

It states: “Most rivers flow fairly gently as they slope slowly towards the sea. Therefore, when a river floods it does so quite slowly as it takes time for the rain to percolate through the ground and into the rivers and out to sea − allowing time for some warning. But with flash flooding there is often very little time between the rain falling and flash flooding occurring.

“Flash flooding commonly happens more where rivers are narrow and steep so they flow more quickly. It can also occur away from small rivers in built-up urban areas where hard surfaces such as roads and concrete don’t let the water drain away into the ground. This leads to surface overflow and can often overwhelm local drainage systems leading to flash flooding.”


Why is flash flooding so dangerous?

Flash flooding is dangerous mainly because it happens so quickly no-one has time to prepare flood defences unless they already have them in their homes or businesses and can deploy them immediately.

It means there is very little time to put out any effective warnings.

Flash floods are dangerous because only 6 inches of water is enough to knock a person off their feet. Just 12 inches is enough to move a car and if it goes above 18 inches then that’s powerful enough to sweep a larger vehicle away.

People underestimate the weight and power of water but if you just fill a bucket up and carry it then that’s heavy enough. Flash floods involve millions of gallons of water moving rapidly so the destructive power of a flash flood is immense.

The other danger is that flash floods are filled with debris and all kinds of hidden hazards being swept along beneath the water surface. You just don’t know what is being carried swiftly out of sight in the water such as branches and stones which could cause serious injury.

So, the simple advice is to get up to higher ground away from flash floods as quickly as possible and never try to drive through a flood as it could end up with you losing your vehicle or even worse, your life.

Everyone is responsible for their own flood protection at all times.