Raising the banks of a river can allow the channel to contain more water. Straightening the channel sends the water through more quickly. Both methods can be used to help prevent flooding, but the channel still has to cope with the same amount of water.
Straightening a river channel upstream can lead to flooding downstream, as the water rushes through the straightened section and then backs up once the meanders begin again.
So the problem doesn’t go away, it is just moved to another area. However, if you can have some control over the location of the flooding, you can at least try to limit the damage caused.
Generally, as you go downstream, river banks and floodplains become more desirable locations for ports and industries, leading to housing and resulting in a built-up area. The floodplain fills up with high value developments, leaving little space for controlled flooding. Hence it is more often the case that flooding is controlled by holding water back upstream.
The Afon Clywedog and the Afon Efyrnwy (River Vyrnwy) are both major tributaries of the River Severn bringing plenty of water down from the wet Welsh mountains. Each river is controlled by a dam so that the flow of water into the River Severn can be regulated, reducing the frequency and size of floods downstream at Shrewsbury and beyond.
But the river is still eventually processing the same amount of water. Is there a way round that? We’ll see next week.