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10. Building on the flood plain

Flooding on the Thames is a natural process. When the river overflows its banks and floods, the water must have somewhere to go. The water spreads out over the flood plain. People have built upon the Thames floodplain, so the water has nowhere to go. The result is that when the river floods, property and homes are destroyed. We must find a balance that allows the river to flood naturally, but doesn’t damage people’s property.

Flooding is a natural process. When a river gets full with rain water, it breaks its banks and the water spreads out over the flood plain. The water then soaks into the ground or returns to the river as levels drop. The rich river silt (mud) that is left behind makes the earth good for agriculture. The wide flat Thames Valley has a large fertile flood plain, which is why it has been an important area for farming for hundreds of years.

Problems arise when people build on the flood plain. Flat river valleys have always been popular places to build houses, so for hundreds of years people have been battling to stop the river flooding out over the valley floor.

London is protected from the Thames flooding by the Thames Barrier and by an unbroken line of flood defence that stretches alongside the river from Teddington down to the Barrier and beyond. This defence consists of flood walls, humps in roads and pavements or even defences built into houses themselves.

Building on the flood plain and erecting flood protection can make the risk of flooding worse. For example, in London:

* covering the flood plain with tarmac and buildings means that the water cannot drain away slowly and naturally. Instead the water escapes down drains in the road and gets back into the river very quickly. This makes the river levels rise and speeds up the flow of water * building out into the river (encroachment) has made the Thames narrower, making the river deeper and faster and more liable to flood * building high flood defences to contain the Thames has made the level of the river higher.

Impacts

* building on the flood plain reduces the amount and variety of habitats available to other creatures * rain falling on London washes oil and dirt down the drains into the river, polluting the water * when the drainage system gets very full, rain water and sewage mix. Raw sewage then gets swept into the river, reducing oxygen levels * speeding up the river flow makes it a more hostile environment for river creatures, who can get swept away by the current * high flood walls make it difficult for people to see the river

What we can do?

* do not build in areas that are liable to flood * stop building on sites that are not already covered with buildings or concrete, for example, open land, school playing fields * where possible, remove high flood walls and build flood defences further back from the river, allowing the river to flood more naturally * build roads, car parks etc of materials that allow water to seep into the ground and not rush down the drains * keep roads as oil free as possible
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