There are so many misconceptions on this issue. The capacity of rivers is generally less than 1% of the capacity of the catchment. To prevent floods, you need to slow the flow INTO rivers, and slow the flow OF rivers, above the urban pinch points, to reduce flood peaks. https://twitter.com/SpudSlingsby/status/1194905966078283777

Dredging does not enhance the capacity of the ctachment, but it does accelerate the flow to the towns downstream, hitting them with a wall of water. It is completely counter-productive. See: https://www.anglingtrust.net/core/core_picker/download.asp?id=6971&filetitle=EA+Evidence+Document+on+the+Impact+of+Dredging

The reason that's a weird link, by the way, is that the report was written by the Environment Agency but deleted by the government under Owen Paterson, who had an ideological commitment to dredging, and couldn't stand to see the evidence that it's a really stupid idea.

Some people are arguing that while dredging upstream of towns is clearly a disaster, dredging downstream should speed the flow away from them. But rivers seek equilibrium, and if you dredge downstream they tend to cut back, undermining revetments and flood defences upstream.

Using dredging to relieve floods is like using petrol to douse fires. It is precisely the wrong tool.

Unlike a lot of people on my timeline, @CIWEM knows what it's talking about.
https://www.ciwem.org/policy-reports/floods-and-dredging-a-reality-check