Flat floodplains are easy to build on, but it is not so easy to keep the river from flooding over them.
If a floodplain is simply used for grazing, then it is no real problem if it floods, but as soon as a housing estate is sitting there, there is a lot more at stake.
So how to keep the river in its channel.
We’ve already seen that the river tends to dump material on its banks and these levees make the banks higher. The natural levees can be added to, to further increase their size. If the banks are higher then more water can fit into the channel.
Another option is to make the channel more efficient at carrying the water away. Rivers cross their floodplains in wide sweeping meanders. If the channel is straightened the water can flow through much more quickly and is less likely to build up and overflow.
Both of these methods are relatively cheap. It just needs some heavy machinery to move some soil and rocks around and maybe some concrete to line the new channel. However, the channel still has to cope with whatever comes along and a long spell of heavy rain could still provide too much water.
To be more confident of preventing a flood you need to be able to hold back some of the water, or even divert it, which is a bit more expensive. We’ll look at these methods next week.