Sorry, as you can imagine its a bit of a soapbox issue of mine at present. But not really helpful for people that are struggling.
On a point about river dredging, the following is a post from a FB page I have an interest in, they were talking about the usefulness of dredging and a chap posted something I thought was worth reading. He is talking about the River Severn into Worcestershire, but it would be relevant across the country I think:
"The weirs were placed into the river, as Richard said to set the depth of water. At the time it was because the river was a major trading route. But silt still gets deposited, so the presence of weirs is neither here nor there. The only exception to that I am aware of is Maisemore weir, which serves to break up the Severn Bore, enabling safe navigation upstream of Gloucester.
Dredging is extremely expensive, and creates a need for further dredging. In essence it's a case of fighting a battle that can never be won. You dredge, you speed up the flow. You speed up the flow, you enable the water to cause further erosion...which means more silt held in suspension and deposited wherever the flow slows enough.
The relationship between speed of the water and its ability to hold sediments in suspension needs to be taken into account. It is something of a double edged sword. If you double the speed of the water you increase its ability to hold particles in suspension by an order of magnitude, so speeding up the flow is a bad idea.
But conversely, slowing down the flow has the same effect... halving the speed means reducing the capacity to hold particles in suspension by an order of magnitude, so silt will be deposited in large quantities.
This is, in essence an historic time bomb we have made for ourselves by "managing"our rivers. Most people have no idea what an entirely natural river looks like, because I'm not aware that we have any left here. They tend (in middle and lower reaches anyway) not to have banks as we know them. They have wetland margins, gradually shallowing and becoming dry as you move further from the main channel. Wetland was drained over most of Britain for various reasons.."improving" land, gaining better access to fisheries and so on. But wetland is just about the best flood management regime you can have.
We have now reached a point where we seem to seriously believe that we can entirely dictate terms to nature. So large portions of the Severn are bounded by revetted banks... well pretty much all of it actually.. and we seem to think that by rights the river should bloody well do as it's told and stay in its bed. But Sabrina has other ideas. Natural rivers when viewed over time don't have such neatly defined courses. Big flooding events will always happen and whilst there is nothing we can do to stop the rain from falling, or the tides from surging, we can stop deluding ourselves. We're still building on flood plains... how dumb is that?
It's worth bearing in mind that flood defence procedures CAUSED the disaster called Katrina. Not the hurricane..that was going to happen anyway. But what happened to New Orleans. Many of the suburbs that flooded were at levels BELOW that of the riverbed, and that was caused by flood defences. In that river, defences were built higher, and given the scale of the task of dredging they couldn't keep up with it. And this sort of flood defence only works if groundwater cannot penetrate and as long as any physical obstruction to the water is 100% secure. One small leak and experience shows that 100% of the defence is pointless.
The best flood defence is the regeneration of wetland and planting appropriate native trees upstream. Such measures have been shown to be highly effective and virtually maintenance free.
But floods will always happen.