'Why don't they dredge the rivers?!', a comment I keep seeing on social media in response to recent #flooding. A thread:
1) Dredging is often not effective against floods. It is only useful if sediment has built up around a structure that cannot be modified or sediment is damaging a river which is currently undergoing work to return to a natural state (see reversing channel straightening).
2) The volume of water the past few days hugely exceeds any additional storage a dredged channel would have provided. Any unstable sediment from dredging would then be moved downriver and cause additional issues
3) A dredged river MAY reduce flooding in one area, but you then move the water downstream which means another community suffers. It is better to find ways to store water across a river than simply move the problem away downstream at every point.
4) Finally, dredging has huge impacts on wildlife and the river balance. If you remove tons of sediment you upset a balance and the consequences can be hard to predict. For example making river banks/walls unstable which then fail during a flood
There are many other options to dredging, such as storing water on floodplains or putting meanders back in rivers. But really the heart of the problem we need to also tackle is land use that causes water and sediment to run into rivers, and climate change driving summer storms.
5) I'd also like to add cost. Imagine the cost of dredging every local river in the UK yearly (which would be how often it was needed)! The money would be better spent on longer-lasting schemes which store or divert floodwater
