I like the plastic. Liniar have some great options. The problem is where do you buy it? I can see lot's of information about it but not anywhere that sells it.
I think they might actually sell it directly. I seem to remeber that there is no minimum quantity aswell.
You can sledge them in or push them in if you are lucky and use the offcuts as ground anchors.
There is a good point about the unforseen effects downstream. It would be appropriate to put down some planting or fascines etc pegged to the base. That will slow down the flows along the edge and help prevent scour occurring and the ends of the wall which should be tied well into the banks. That will help sediment build up and vegetation to establish in the long term. Which will help shade the plastic and improve their longevity which to be fair is already pretty good.