Engineering flooding

Engineering flooding

On Boxing Day massive areas of Yorkshire were flooded. Working on Kirkstall Road, Leeds, and having volunteered with many others to help clean up houses and businesses, I have seen first hand the devastation. Obviously we know that flooding is caused by heavy rainfall, but what is making the situation worse? And what can we do about it?

What might have caused this flood to be so much higher than previous records? 

Climate change is making our weather more unpredictable. Air holds water vapour. If this patch of air then meets a patch of colder air the water vapour condenses, creating rain. With global warming, the air is getting warmer, which means it can hold more water vapour, which means more water falls as rain. Flooding will increasingly occur as climate change affects us globally.

Lastly our own actions can cause flooding in the years to come. A large tree can absorb 100 gallons (450 litres) of water a day, so every time a tree is cut down large quantities of additional water is entering the streams and rivers. When a moor is managed for grouse shooting the ground is drained so that grouse can easily live there. Intensive farming can also mean large run offs into watercourses. Have a look at this article and this article for more information. Lastly, in Greater London, every year the area of Hyde Park, London is concreted over. This means there is no soil and plants to absorb the water, and it runs into the sewer systems instead.

Believe it or not but flood defences can also cause flooding! As a river meanders it has natural eddies, and currents as the water flow interacts with the river banks and river bed. This interaction means the water is slowed down by its surroundings. Flood defences are often built from concrete, so rather than having a soft, bumpy river bank, you get a vertical, flat, solid bank. The water doesn’t get slowed down by the concrete, in fact it speeds up. Flood defences don’t surround every stretch of water, so this style of flood defences simply moves the flooding elsewhere. This could mean onto planned farming flood plains, but can also be into towns and villages further downstream. I live in a village called Woodlesford where Leeds Council kindly built us a small flood defence last year. It protected our village from this winter’s flooding, however the next village downstream, Methley, had the worst flooding on record, with over 50% of the village under water. I expect our new defences had an impact.

So what can we do?

We could continue to build longer, taller, thicker flood defences to keep the water at bay, but knowing how this might cause flooding downstream, surely there’s got to be a better way?

Pickering, North Yorkshire, has a different way of managing the floods. The town was badly hit in the 2007 floods, but rather than build a large, and very expensive, flood defence (costing £3.2million) they looked at catchment measures that would slow down the water movement. Rather than all the water arriving at once, and causing a flood, the water would arrive over a longer time period, keeping the water level low. This article explains the situation in Pickering and this one details the catchment measures used. The measures were completed in 2015 at a cost of £2million. Not only were they cheaper, but planting trees has additional benefits of increased wildlife and CO2 absorbsion too. I should also add that the town didn’t get flooded on Boxing Day!

Obviously these strategies can only be completed in rural towns and villages, but can we mimic this in the city? Turns out we can… Sustainable Drainage Systems (SuDS) provide an urban alternative. SuDS effectively provides large, tanks underground where rainwater can be stored before it enters the city’s drainage system. In normal weather system works as it would in a traditional set-up. In heavy rain though, the water can be stored underground and released in a controlled manner. The city’s water levels are monitored so water is released when capacity allows and stored when it would otherwise cause flooding. Problem solved!

I hope this helps explain what exacerbates and alleviates flooding. Next week I thought I’d continue the theme, but focus on adapting building design in order to protect the property and our possessions when building on flood plains, or river banks.


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