Could a Flash Flood Happen in Your Road?

23 November 2014

A look at the causes of flooding on our streets and what we can all do to prevent it.

Flash flooding

What causes flooding?

Several factors contribute to flooding. Two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play important roles. Most flash flooding in the UK is caused by heavy showers or by slow-moving storms with or without thunder. Floods, on the other hand, can be slow- or fast-rising, but generally develop over a period of hours or days.

Surface water runoff is a step in the water cycle on Earth and provides our drinking water. When precipitation occurs the water can either infiltrate into the ground, evaporate, or become surface water. Water that does not get absorbed into the soil, or rise back into the atmosphere as water vapour, will run off surfaces, collecting in varied locations such as flood plains. Hard surface areas prevent water from infiltrating into the ground especially in urban areas.

Flooding can occur if the amount of precipitation in an area exceeds the evaporation rate and infiltration capacity of the soil. Significant floods can occur as water hits hard surface areas having little, if no, opportunity to soak into the ground. Hard ground surfaces such as sun-baked soil in summer and impermeable clay surfaces will also prevent water from infiltrating. This can cause flash flooding.

Flash flooding in urban areas.

In an urban situation, our streets and roads are designed to promote runoff entering the surface water drains (not sewers) through grates at the side of the road. These entries to the surface water drainage system are important but can be easily blocked in autumn by leaves and other debris. In times of flash flooding it’s even more important that the roadside drains are clear. Fast flowing water can carry significant amounts of solids such as silt, grit and even chippings into the drains. For this reason, roadside drains are equipped with a sump to trap solids. Having a sump does mean that they need cleaning on a regular basis.

Gully cleaning minimises flash flooding.

With an increasing number of cutbacks, local austerity measures are biting deep and we could see a situation where mechanical cleaning wagons are replaced with old fashioned manual labour on an as-needed basis.  In a typical town there may be as many as 30,000 sumps to clean, so there’s very little chance that all the local roadside surface water drains will be serviced. When (not if) these drains are blocked, flash flooding will be inevitable.  And you will only know when the drains are blocked AFTER the flood.

What can you do about flash flooding?

Flash flooding may happen in a street near you. So what can you do about it? It’s simple - report any blocked gully to the relevant authority immediately – it’s in your own interest.

  • Know Your Risk
  • Be Prepared
  • Have a Plan

Free Ebook for communities: How to prepare for flooding and other emergencies.


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