I don't want to exploit my position as a moderator,
Please take the following as a purely personal view and I will just make the one statement. I have (and continue) to do a lot of work alongside the Environment Agency (going way back to the late 80s, when it was the NRA). This has included quite a lot relating to WFD and catchment management, but also included SUDS, flood defence work and also working with unrelated groups involved with 'top of the catchment' actions.
I have more practical experience on these issues than any engineering topics.
There are powerful counter-arguments on the dredging front, not least that most of the dredging people are lobbying for would protect agricultural land upstream of settlements. Increasing the flows in these areas instead of allowing the low-lying agricultural land (i.e. what is left of the natural flood plain) to flood would just make the situation in towns and villages downstream worse.
Catchment and flood modelling is a fine art and comparable to finite element analysis used in engineering. It is used to plan flood defences and plan dredging and other works, and is remarkably accurate and useful. It is also used to show when 'natural' approaches to waterway management are equally or more effective than the 'traditional' ones which are the root cause of so many of the flooding problems we have now.
I have first hand experience of a site that was meant to be a traditional, engineered, flood control site created by creating a flood bank from dredgings as John suggests, but with the idea it would act as a flood reservoir. It didn't work, not least because some water just shoots in one end and out the other, while most whizzes past in the canalised channel to one side. The original meandering river course would have allowed the whole, uninhabited, area to flood deeper and slowed the flow, protecting several major buinesses downstream.
By-the-bye – the area was classed as being likely to flood every 20-25 years. Increased flood events in the 2000s meant it was flooding once or twice a year – the real problem is climate change, and it won't be solved by dredging in the wrong places.
I'll shut up now.
Neil