| Re: Just asking for trouble.
They can cause problems upstream, but only if they cause what’s called a ‘head loss’ i.e. it makes it harder for the water to get through and you get a back up of water in high flows and deposition of silt in low flows (which then makes the flooding worse next time there’s a flood). The problems with most flood defences are down stream with higher water levels (as the water gets down stream faster), more erosion as there’s more energy kept in the river rather than it dissipating over the flood plain and faster water speeds as flood defence channels and structures have less friction (concrete and sheet piles are smoother than vegetation) and are shorter and steeper than natural channels.
There are other problems with many flood defences as the channel size is artificial. All rivers have a natural width and depth depending on the flow and gradient and the river is always trying to get back to that natural width. In general flood defence works make rivers too wide and the river’s energy is dissipated so much that silt drops out, which them smothers all the wildlife. That why many channels in the UK have silty beds with little life. They were dug too wide, so silt drops out, then people complain that the river isn’t being cleaned out because it’s all silty, so they are dug out again and so you start again. The answer is to stop clearing the rivers (except in urban areas) so they can recover and accept that they will then flood agricultural land.
Chris  |