I’m sure everyone has seen the flood warning that have been issued in the UK over the past few days. I have lost count of the amount of times friends have absentmindedly said to me “but we’re supposed to be in a drought, why do we now have floods?” Well as silly as it sounds to have floods during a drought, it’s quite common. Why?
Well, on the one hand, the soil we walk around on is made of lots of tiny particles, and some of these particles are smaller than others. Some parts of the soil can easily be seen (or felt as you’ll know if you’ve ever stood on a sharp stone!), other particles called clays are so small they can only be seen with a very strong microscope, and they behave in a very strange way. When you put a clay mug in a kiln, it sets hard so you can keep water in it, but when clay in the ground gets hot or very dry, it also becomes hard. This means that the rain which falls on the ground cannot get into the soil (or infiltrate) and so is forced to run over the top – if enough water does this then it is a flood.
The second reason is down to something called suction. You will have noticed this phenomena if you have ever tried to build a sandcastle, if the sand is dry, your castle falls down. Too wet? Again the castle falls down. The only way to build a sandcastle is to start with moist sand. The reason for this is that the small amount of water contained within the sand clings to the sand particles and creates something which behaves almost like a vacuum cleaner. It is this suction that holds together the particles in the sand castle.
But what does this have to do with the problems that the UK is facing? Well, more than you’d think! During periods of normal rainfall, the ground has water in it, which means that these vacuum-like interactions work to draw more water down into the ground. During a period of drought, when the top few metres of the topsoil is completely dry, suctions do not exist and water is not pulled into the ground – so when rain falls, it cannot soak into the ground as quickly as it would if the ground was damp.
So as much as we Brits hate the grey, mizzly weather (and so we should, it’s miserable), it does help keep us safe from flooding, so next time you’re stuck indoors on a Saturday, try to feel positive!
Want to introduce topics about flooding to your class? Try our Geography: Floodrisk! activity.
Blog post by Linda Seward of Seward Technical Writing, providers of original science content.