You don't need concrete to build a waterproof dike, not even on a major river.Jacked Camry wrote:
Pretty much. And this is one of the more difficult aspects, as rivers change naturally and can be heavily influenced by manmade actions as well, including actions taken thousands of kilometers away that reduce sediment load, for example.
You can't drill to bedrock, since rivers flow in sedimentary depositions that they created, so bedrock tends to be very deep below the surface. 60m sounds about right for many places. But the issue isn't so much that your structure remains sound, it's what's happening around the structure and how you can protect against that. It's not realistic to create a watertight concrete barrier that will be bulletproof against a major river. Hence there's much research and so many different revetment techniques, the development of geotextiles, the research into biological stabilization etc. Generally speaking, the way it works is that as cities become prosperous, they'll eventually invest in long revetment works that will stabilize the banks for many kilometers to reduce these effects. But some idiot sucking up sediment nearby or a lack of drainage through the structures can still mess them up.
Fred's ideas of using natural materials are appropriate for smaller rivers and geomorphologies that include non-erosive soils or other materials. But pretending that they'd be anything other than a temporary measure that would disappear as soon as subjected to stress shows his lack of understanding of the situation.
If that were true my country wouldn't have existed.