Nicked this from another forum...
Obviously dredging works to alleviate local flooding, it just sends the problem downstream to someone else. The problem for the Somerset Levels (which I lived on the edge of for 44 years) is that they are at the downstream end of the line, there is nowhere downstream to send the problem to.
When all the land is less than 6 foot above sea level and the flood banks are up to 12 foot above the land your are fecked once the flood banks overtop. You can't pump the water off the land back into the river channels until it stops raining.
Dredging to drop the bed level will be of no help when the river beds are already at, below or only just above sea level. It would take a huge dredging effort to even double the volume of the river/drain channels but this would make almost no difference because the volume of water flowing down is thousands of times the channel volume.
The problems for the levels are miles upstream in the ever expanding towns and villages which speed up run off. All the agricultural land drainage, maize growing and hedge ripping out doesn't help either. I can't recall the exact figures but I think it is something like every 1 acre of the levels receives the water from 6 acres of hills. The only real hope is to slow the runoff from the upstream areas.