How Do That Repair River Banks?

Have you ever wondered how that repair the banks of the rivers, or the walls along the river?  If you have take a wonder along the Castle Mill Stream in Oxford behind Tidmarsh Lane and have a look at the work that is taking place at the moment.  These guys have been working in this area for about three weeks now, and they have a section of the stream drained so they can work in the area.  Very interesting to watch, but then again I can sit and watch other work for hours!