Sheep seem to have two aims in life, escape and escape and get themselves killed.
I hear this often and it still just sums up sheep
The escape accusation is certainly true of some sheep: in my experience when they get killed, it's usually been the fault of a dog-owner or vehicle driver. I used to have sympathy with the accusation that one of their hobbies is finding ways to die but as the years go by I am more inclined to be critical of the dead sheep's keeper (myself included).
I've known many good escapers over the years, but usually their skills have been acquired because they have come across fences that are in poor condition and easy to get under or over. Once a sheep realises that fences can have weaknesses it will cease to respect them and look for a way past them wherever possible.
But, back on topic. Your 6 foot high bank
may work as a barrier
if it is perfectly vertical and smooth. In my experience sheep are perfectly happy to wade round to the greener grass on the other side of the fence ( unsurprising, given our climate ) and water cannot be relied upon as a barrier unless it is always deep, even in dry spells, and even then you will need to extend the fence out across the water a couple of feet (fix wooden rails to the endpost).